Prothoracicostatic Activity of the Ecdysis-Regulating Neuropeptide Crustacean Cardioactive Peptide (CCAP) in the Desert Locust.

Accurate control of innate behaviors associated with developmental transitions requires functional integration of hormonal and neural signals. Insect molting is regulated by a set of neuropeptides, which trigger periodic pulses in ecdysteroid hormone titers and coordinate shedding of the old cuticle during ecdysis. In the current study, we demonstrate that crustacean cardioactive peptide (CCAP), a structurally conserved neuropeptide described to induce the ecdysis motor program, also exhibits a previously unknown prothoracicostatic activity to regulate ecdysteroid production in the desert locust, Schistocerca gregaria. We identified the locust genes encoding the CCAP precursor and three G protein-coupled receptors that are activated by CCAP with EC50 values in the (sub)nanomolar range. Spatiotemporal expression profiles of the receptors revealed expression in the prothoracic glands, the endocrine organs where ecdysteroidogenesis occurs. RNAi-mediated knockdown of CCAP precursor or receptors resulted in significantly elevated transcript levels of several Halloween genes, which encode ecdysteroid biosynthesis enzymes, and in elevated ecdysteroid levels one day prior to ecdysis. Moreover, prothoracic gland explants exhibited decreased secretion of ecdysteroids in the presence of CCAP. Our results unequivocally identify CCAP as the first prothoracicostatic peptide discovered in a hemimetabolan species and reveal the existence of an intricate interplay between CCAP signaling and ecdysteroidogenesis.

Analysis of Peptide Ligand Specificity of Different Insect Adipokinetic Hormone Receptors.

Adipokinetic hormone (AKH) is a highly researched insect neuropeptide that induces the mobilization of carbohydrates and lipids from the fat body at times of high physical activity, such as flight and locomotion. As a naturally occurring ligand, AKH has undergone quite a number of amino acid changes throughout evolution, and in some insect species multiple AKHs are present. AKH acts by binding to a rhodopsin-like G protein-coupled receptor, which is related to the vertebrate gonadotropin-releasing hormone receptors. In the current study, we have cloned AKH receptors (AKHRs) from seven different species, covering a wide phylogenetic range of insect orders: the fruit fly, Drosophila melanogaster, and the yellow fever mosquito, Aedes aegypti (Diptera); the red flour beetle, Tribolium castaneum, and the large pine weevil, Hylobius abietis (Coleoptera); the honeybee, Apis mellifera (Hymenoptera); the pea aphid, Acyrthosiphon pisum (Hemiptera); and the desert locust, Schistocerca gregaria (Orthoptera). The agonistic activity of different insect AKHs, including the respective endogenous AKHs, at these receptors was tested with a bioluminescence-based assay in Chinese hamster ovary cells. All receptors were activated by their endogenous ligand in the nanomolar range. Based on our data, we can refute the previously formulated hypothesis that a functional AKH signaling system is absent in the beneficial species, Apis mellifera. Furthermore, our data also suggest that some of the investigated AKH receptors, such as the mosquito AKHR, are more selective for the endogenous (conspecific) ligand, while others, such as the locust AKHR, are more promiscuous and can be activated by AKHs from many other insects. This information will be of high importance when further analyzing the potential use of AKHRs as targets for developing novel pest control agents.

Oxytocin/vasopressin-like neuropeptide signaling in insects.

The origin of the oxytocin (OT)/vasopressin (VP) signaling system is thought to date back more than 600million years. OT/VP-like peptides have been identified in numerous invertebrate phyla including molluscs, annelids, nematodes and insects. However, to date we only have a limited understanding of the biological role(s) of this GPCR-mediated signaling system in insects. This chapter presents the current knowledge of OT/VP-like neuropeptide signaling in insects by providing a brief overview of insect OT/VP-like neuropeptides, their genetic and structural commonalities, and their experimentally tested and proposed functions. Despite their widespread occurrence across insect orders these peptides (and their endogenous receptors) appear to be absent in common insect model species, such as flies and bees. We therefore explain the known functionalities of this signaling system in three different insect model systems: beetles, locusts, and ants. Additionally, we review the phylogenetic distribution of the OT/VP signaling system in arthropods as obtained from extensive genome/transcriptome mining. Finally, we discuss the unique challenges in the development of selective OT/VP ligands for human receptors and share our perspective on the possible application of insect- and other non-mammalian-derived OT/VP-like peptide ligands in pharmacology.

Peptides in insect oogenesis.

The physiological control of reproduction in insects depends on a combination of environmental and internal cues. In the adult stage, insects become sexually mature and generate gametes. In females, the latter process is designated as oogenesis. Peptides are a versatile class of extracellular signalling molecules that regulate many processes, including oogenesis. At present, the best documented physiological control mechanism of insect oogenesis is the insulin-related peptide signalling pathway. It regulates different stages of the process and provides a functional link between nutritional status and reproduction. Several other peptides have been shown to exert gonadoregulatory activities, but in most cases their exact mode of action still has to be unravelled and their effects on oogenesis could be direct or indirect. Some regulatory peptides, such as the Drosophila sex peptide, are being transferred from the male to the female during the mating process.