RNA interference of insulin-related peptide and neuroparsins affects vitellogenesis in the desert locust Schistocerca gregaria

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.

Neuropeptidergic regulation of reproduction in insects

Successful animal reproduction depends on multiple physiological and behavioral processes that take place in a timely and orderly manner in both mating partners. It is not only necessary that all relevant processes are well coordinated, they also need to be adjusted to external factors of abiotic and biotic nature (e.g. population density, mating partner availability). Therefore, it is not surprising that several hormonal factors play a crucial role in the regulation of animal reproductive physiology. In insects (the largest class of animals on planet Earth), lipophilic hormones, such as ecdysteroids and juvenile hormones, as well as several neuropeptides take part in this complex regulation. While some peptides can affect reproduction via an indirect action (e.g. by influencing secretion of juvenile hormone), others exert their regulatory activity by directly targeting the reproductive system. In addition to insect peptides with proven activities, several others were suggested to also play a role in the regulation of reproductive physiology. Because of the long evolutionary history of many insect orders, it is not always clear to what extent functional data obtained in a given species can be extrapolated to other insect taxa. In this paper, we will review the current knowledge concerning the neuropeptidergic regulation of insect reproduction and situate it in a more general physiological context.

In vivo effect of Neuropeptide F on ecdysteroidogenesis in adult female desert locusts (Schistocerca gregaria)

Neuropeptides are important regulatory factors that mediate key life processes, both in vertebrates and invertebrates. Many insect neuropeptides display pleiotropic activities, which means that they can influence multiple aspects of insect physiology. In the fruit fly, Drosophila melanogaster, Neuropeptide F (NPF) mediates diverse physiological processes, such as learning, stress responses, feeding and male courtship behavior. In locusts, only a truncated form of the predicted "full-length" NPF, the nonapeptide "trNPF", has been isolated. This nonapeptide previously proved to be biologically active, since it was shown to influence food intake and weight increase, as well as oocyte growth in adult female desert locusts (Schistocerca gregaria [Forskål]). In the present study, we have further analyzed the effect of trNPF on female reproductive physiology in S. gregaria. We confirmed that daily trNPF injections in adult females elicit an increase of oocyte size. In addition, an RNAi-mediated knockdown of the Schgr-NPF precursor transcript in adult female locusts resulted in the opposite effect, i.e. significantly smaller oocytes. Moreover, we discovered that daily injections of trNPF in adult female S. gregaria, caused higher ecdysteroid titers in the ovaries and accelerated the appearance of ecdysteroid peaks in the hemolymph of these animals. The RNAi-based knockdown of the Schgr-NPF precursor transcript clearly resulted in reduction of both hemolymph and ovarian ecdysteroid concentrations, confirming the stimulatory effects of trNPF injections on adult female ecdysteroid levels. The observed results are discussed in relation to previous reports on NPF activities in locusts and other insects.

Advances in RNA interference technology and its impact on nutritional improvement, disease and insect control in plants

This review highlights the advances in the knowledge of RNA interference (RNAi) and discusses recent progress on the functionality of different components RNAi machinery operating in the organisms. The silencing of genes by RNA interference has become the technology of choice for investigation of gene functions in different organisms. The refinement in the knowledge of the endogenous RNAi pathways in plants along with the development of new strategies and applications for the improvement of nutritional value of important agricultural crops through suppression of genes in different plants have opened new vistas for nutritional security. The improvement in the nutritional status of the plants and reduction in the level of toxins or antinutrients was desired for long, but the available technology was not completely successful in achieving the tissue specific regulation of some genes. In the recent years, a number of economically important crop plants have been tested successfully for improving plant nutritional value through metabolic engineering using RNAi. The implications of this technology for crop improvement programs, including nutritional enrichment, reduction of antinutrients, disease, and insect control have been successfully tested in variety of crops with commercial considerations. The enhancement of the nutraceutical traits for the desired health benefits in common crop plants through manipulation of gene expression has been elaborated in this article. The tremendous potential with RNAi technology is expected to revolutionize the modern agriculture for meeting the growing challenges is discussed.

Tissue-dependence and sensitivity of the systemic RNA interference response in the desert locust, Schistocerca gregaria

We report on a comprehensive study of the systemic RNAi-response in the desert locust, Schistocerca gregaria. Upon intra-abdominal injection of dsRNA for two housekeeping genes (alpha-tubulin 1a and gapdh) in the range of pg amounts of dsRNA per mg tissue, a potent reduction of their corresponding mRNA was obtained. Moreover, the observed transcript knockdown significantly increased in at least a 10 days period and eventually resulted in high mortality upon silencing of the alpha-tubulin 1a gene. A more moderate RNAi-response was however observed in the reproductive systems. Analysis of the tissue-dependent transcript level profile of several putative RNAi-genes indicated reduced levels of two genes, namely sg-dicer-2 and sg-argonaute-2, in the reproductive systems. By silencing these components, we confirmed their importance in the RNAi-process and suggest that their expression levels are determinant for tissue-dependent differences in the potency of RNAi in the desert locust.

RNAi-mediated knockdown of Shade negatively affects ecdysone-20-hydroxylation in the desert locust, Schistocerca gregaria

A major breakthrough in elucidating the ecdysteroid biosynthetic pathway in insects was realized with the molecular identification and further functional characterization of the 'Halloween' genes. These genes were found to encode cytochrome P450 enzymes catalysing the final steps of ecdysteroid biosynthesis in the dipteran, Drosophila melanogaster, and in the Lepidoptera, Manduca sexta and Bombyx mori. A recent report focused on the identification of Halloween orthologs in the desert locust, Schistocerca gregaria, a member of the hemimetabolous insect order of the Orthoptera. In the present study, an additional Halloween gene Shade, is identified in the desert locust. In Diptera and Lepidoptera, this gene encodes a 20-hydroxylase, catalysing the conversion of ecdysone (E) to 20-hydroxyecdysone (20E). However, this enzymatic function has previously been suggested for CYP6H1 in another locust species, the migratory locust, Locusta migratoria. Using q-RT-PCR, the spatial and temporal transcript profiles of S. gregaria orthologs for Shade as well as CYP6H1 were analysed in last larval stage desert locusts. An RNA interference (RNAi)-based approach was employed to study whether these genes could possibly encode a functional 20-hydroxylase in the desert locust.

RNA interference mortality points to noncircadian functions for the clock gene in the desert locust Schistocerca gregaria

One of the core genes in the circadian regulation network is clock (clk). By forming a heterodimer with CYCLE (CYC) that binds on an E-box in the promoter region, it induces the transcription of other elements in the circadian transcriptional feedback loops and different clock output genes. In contrast to other insects, a clk double-stranded RNA (dsRNA) treatment is lethal in adults and fifth instar nymphs of the desert locust, Schistocerca gregaria, in a dose-dependent manner. Clk knock down fifth instar nymphs are able to undergo their imaginal moult but, depending on the amount of dsRNA, it takes them longer than the controls to reach adulthood. As adults, clk knock down animals do not develop their fat body and ovaries like the control animals. Therefore, we tested the expression of different genes involved in energy metabolism and reproduction to see the effect of the clk RNA interference knock down. Surprisingly, the expression of the vitellogenin gene was up-regulated in the clk knock down females who did not appear to invest their energy in egg development. Taken together, our results point out that the clk gene in the desert locust has an additional function in development besides its established role in maintaining the circadian rhythms in the brain.

Endocrine archeology: do insects retain ancestrally inherited counterparts of the vertebrate releasing hormones GnRH, GHRH, TRH, and CRF?

Vertebrate releasing hormones include gonadotropin releasing hormone (GnRH), growth hormone releasing hormone (GHRH), corticotropin releasing hormone (CRF), and thyrotropin-releasing hormone (TRH). They are synthesized in the hypothalamus and stimulate the release of pituitary hormones. Here we review the knowledge on hormone releasing systems in the protostomian lineage. We address the question: do insects have peptides that may be phylogenetically related to an ancestral GnRH, GHRH, TRH, and CRF? Such endocrine archeology has become possible thanks to the growing list of fully sequenced genomes as well as to the continuously improving bioinformatic tool set. It has recently been shown that the ecdysozoan (nematodes and arthropods) adipokinetic hormones (AKHs), the lophotrochozoan (annelids and mollusks) GnRHs as well as the protochordate GnRHs are structurally related. The adipokinetic hormone precursor-related peptides (APRPs), in locusts encoded by the same gene that contains the AKH-coding region, have been forwarded as the structural counterpart of GHRH of vertebrates. CRF is relatively well conserved in insects, in which it functions as a diuretic hormone. Members of TRH-receptor family seem to have been conserved in some arthropods, but other elements of the thyroid hormone signaling system are not. A challenging idea is that in insects the functions of the thyroid hormones were taken over by juvenile hormone (JH). Our reconstruction suggests that, perhaps, the ancestral releasing hormone precursors played a role in controlling energy metabolism and water balance, and that releasing hormone functions as present in extant vertebrates were probably secondarily acquired.

Insect omics research coming of age1This 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

As more and more insect genomes are fully sequenced and annotated, omics technologies, including transcriptomic, proteomic, peptidomics, and metobolomic profiling, as well as bioinformatics, can be used to exploit this huge amount of sequence information for the study of different biological aspects of insect model organisms. Omics experiments are an elegant way to deliver candidate genes, the function of which can be further explored by genetic tools for functional inactivation or overexpression of the genes of interest. Such tools include mainly RNA interference and are currently being developed in diverse insect species. In this manuscript, we have reviewed how omics technologies were integrated and applied in insect biology.

CRF-like diuretic hormone negatively affects both feeding and reproduction in the desert locust, Schistocerca gregaria

Diuretic hormones (DH) related to the vertebrate Corticotropin Releasing Factor (CRF) have been identified in diverse insect species. In the migratory locust, Locusta migratoria, the CRF-like DH (CRF/DH) is localized in the same neurosecretory cells as the Ovary Maturating Parsin (OMP), a neurohormone that stimulates oocyte growth, vitellogenesis and hemolymph ecdysteroid levels in adult female locusts. In this study, we investigated whether CRF-like DH can influence feeding and reproduction in the desert locust, Schistocerca gregaria. We identified two highly similar S. gregaria CRF-like DH precursor cDNAs, each of which also encodes an OMP isoform. Alignment with other insect CRF-like DH precursors shows relatively high conservation of the CRF/DH sequence while the precursor region corresponding to OMP is not well conserved. Quantitative real-time RT-PCR revealed that the precursor transcripts mainly occur in the central nervous system and their highest expression level was observed in the brain. Injection of locust CRF/DH caused a significantly reduced food intake, while RNAi knockdown stimulated food intake. Therefore, our data indicate that CRF-like DH induces satiety. Furthermore, injection of CRF/DH in adult females retarded oocyte growth and caused lower ecdysteroid titers in hemolymph and ovaries, while RNAi knockdown resulted in opposite effects. The observed effects of CRF/DH may be part of a wider repertoire of neurohormonal activities, constituting an integrating control system that affects food intake and excretion, as well as anabolic processes like oocyte growth and ecdysteroidogenesis, following a meal. Our discussion about the functional relationship between CRF/DH and OMP led to the hypothesis that OMP may possibly act as a monitoring peptide that can elicit negative feedback effects.

Male reproduction is affected by RNA interference of period and timeless in the desert locust Schistocerca gregaria

In all living organisms, behavior, metabolism and physiology are under the regulation of a circadian clock. The molecular machinery of this clock has been conserved throughout the animal kingdom. Besides regulating the circadian timing of a variety of processes through a central oscillating mechanism in the brain, these circadian clock genes were found to have a function in peripheral tissues in different insects. Here, we provide evidence that the circadian clock genes period (per) and timeless (tim) have a role in the male locust reproduction. A knockdown of either of the two genes has no effect on male sexual maturation or behavior, but progeny output in their untreated female copulation partners is affected. Indeed, the fertilization rates of the eggs are lower for females with a per or tim RNAi copulation partner as compared to the eggs deposited by females that mated with a control male. As the sperm content of the seminal vesicles is higher in per or tim knockdown males, we suggest that this phenotype could be caused by a disturbance of the circadian regulated sperm transfer in the male reproductive organs, or an insufficient maturation of the sperm after release from the testes.

Fruitless RNAi knockdown in males interferes with copulation success in Schistocerca gregaria

In Drosophila melanogaster, the male-specific splice isoform of the fruitless gene (Fru(M)) codes for a set of transcription factors that are involved in the regulation of male courtship and copulation. Fru(M) is expressed in an interconnected neuronal circuit containing central and sensory neurons as well as motor neurons. A partial sequence from the Schistocerca gregaria fru-gene from an EST database allowed quantitative real time analysis of fru-expression in adult locusts, and revealed the highest expression in the testes, accessory glands as well as the brain (and optic lobes). Starting fru specific RNAi knockdown in the third and fourth nymphal stage resulted in a significantly lower cumulative copulation frequency of the RNAi-treated animals compared to controls after 3 h of observation. In addition, the testes of RNAi-treated males weigh less. Analysis of the egg pods resulting from a successful copulation event revealed that egg pods from females that mated with an RNAi-treated male were smaller and contained less fertilized eggs compared to egg pods from females who mated with control males. Starting injections in the fifth nymphal stage showed the complete opposite for the cumulative copulation frequency and testes weight. We conclude that already in the early nymphal phases of male desert locusts, fruitless starts to play an important role in the regulation of successful copulation in the adult. The RNAi treatment in the male has also its effects on fertility and fecundity. It remains unknown whether this effect is coming from aberrant courtship behaviour or from an altered composition of the sperm or seminal fluids.