Sequencing and biological effects of an adipokinetic/hypertrehalosemic peptide in the stick insect, Baculum extradentatum

Oxidation Products of Tryptophan and Proline in Adipokinetic Hormones-Artifacts or Post-Translational Modifications?

BACKGROUND

Adipokinetic hormones (AKHs) regulate important physiological processes in insects. AKHs are short peptides with blocked termini and Trp in position 8. Often, proline occupies position 6. Few post-translational modifications have been found, including hydroxyproline ([Hyp6]) and kynurenine. Our recent data suggest that the Hyp- and Kyn-containing AKHs occur more often than originally thought and we here investigate if they are natural or artifactual.

METHODS

From crude extracts of the corpora cardiaca (CC) of various insect species, AKHs were analyzed using liquid chromatography coupled to high-resolution mass spectrometry (LC-MS). Synthetic [Hyp6]-AKHs were tested in an in vivo metabolic assay. Freshly dissected Periplaneta americana and Blaberus atropos CCs (with precautions taken against oxidation) were analyzed. B. atropos CC were placed into a depolarizing saline and the released AKHs were measured.

RESULTS

Hyp was detected in several decapeptides from cockroaches. The modified form accompanied the AKH at concentrations below 7%. The [Hyp6]-AKHs of B. atropos were present in fresh CC preparations and were shown to be releasable from the CC ex vivo. Synthetic [Hyp6]-containing peptides tested positively in a hypertrehalosemic bioassay. Hydroxyprolination was also detected for Manto-CC from the termite Kalotermes flavicollis and for Tetsu-AKH of the grasshopper, Tetrix subulata. Oxidized Trp-containing forms of Nicve-AKH were found in species of the burying beetle genus Nicrophorus.

CONCLUSIONS

Trp oxidation is known to occur easily during sample handling and is likely the reason for the present findings. For hydroxyprolination, however, the experimental evidence suggests endogenous processes.

The unique C-mannosylated hypertrehalosemic hormone of Carausius morosus: Identity, release, and biological activity

Previous studies had shown that the corpora cardiaca (CC) of the Indian stick insect, Carausius morosus, synthesizes two hypertrehalosemic hormones (HrTHs)-decapeptides which differ in the way that the chromatographically less-hydrophobic form, code-named Carmo-HrTH-I, is modified by an unique C-mannosylated tryptophan residue at position 8. The availability of milligram amounts of this modified peptide in synthetic form now makes it possible to study physico-chemical and physiological properties. This study revealed that the synthetic peptide co-elutes with the natural peptide from the CC chromatographically, is heat stable for at least 30 min at 100°C, and causes hyperlipemia in acceptor locusts (a heterologous bioassay) and hypertrehalosemia in ligated stick insects (conspecific bioassay). In vitro incubation of Carmo-HrTH-I together with stick insect hemolymph (a natural source of peptidases) demonstrated clearly via chromatographic separation that the C-mannosylated Trp bond is stable and is not broken down to Carmo-HrTH-II (the more-hydrophobic decapeptide with an unmodified Trp residue). This notwithstanding, breakdown of Carmo-HrTH-I did take place, and the half-life of the compound was calculated as about 5 min. Finally, the natural peptide is releasable when CC are treated in vitro with a depolarizing saline (high potassium concentration) suggesting its role as true HrTHs in the stick insect. In conclusion, the results indicate that Carmo-HrTH-I which is synthesized in the CC is released into the hemolymph, binds to a HrTH receptor in the fat body, activates the carbohydrate metabolism pathway and is quickly inactivated in the hemolymph by (an) as yet unknown peptidase(s).

Mass Spectrometric Proof of Predicted Peptides: Novel Adipokinetic Hormones in Insects

The importance of insects in our ecosystems is undeniable. The indiscriminate use of broad-spectrum insecticides is a factor in the decline in insect biomass. We identify and sequence a prominent neuropeptide hormone in insects with an overarching goal to elucidate relatedness and create a database of bioactive peptides that could inform possible cross-activity in biological assays for the identification of a biorational lead compound. The major task of an adipokinetic hormone (AKH) in an insect is the regulation of metabolic events, such as carbohydrate and lipid breakdown in storage tissue during intense muscular work. From genomic and/or transcriptomic information one may predict the genes encoding neuropeptides such as the AKHs of insects. Definite elucidation of the primary structure of the mature peptide with putative post-translational modifications needs analytical chemical methods. Here we use high-resolution mass spectrometry coupled with liquid chromatography to identify unequivocally the AKHs of five insect species (one cockroach, two moths, and two flies) of which either genomic/transcriptomic information was available or sequences from related species. We confirm predicted sequences and discover novel AKH sequences, including one with a post-translational hydroxyproline modification. The additional sequences affirm an evolutionary pattern of dipteran AKHs and a conserved pattern in crambid moths.

Structure-Activity Studies on the Hypertrehalosemic Hormone II of the Stick Insect Carausius morosus (Phasmatodea): Carbohydrate-Mobilization and Cardio-Stimulatory Activities

The corpora cardiaca of the Indian stick insect, Carausius morosus, synthesize two decapeptide neuropeptides of the adipokinetic hormone (AKH) family, both of which can increase the trehalose levels in the hemolymph when the stick insect is ligated between the head and the thorax. Here, we use two biological assays to assess the potencies of 19 AKH analogs in ligated C. morosus: the carbohydrate-mobilizing assay measures the change in the levels of circulating carbohydrates following injection of a substance, while the semi-exposed heart assay measures a change in heart beat rate after the peptide is applied onto the heart. With the endogenous AKH (Carmo-HrTH-II) as lead peptide, we report here on seven naturally-occurring AKH peptides (bioanalogs) selected for testing because of a single or double amino acid replacement, or for being octapeptides. Single amino acid substitutions by an alanine residue at all positions of Carmo-HrTH-II, as well as analogs modified at the termini were also investigated to give a comprehensive view of ligand-receptor interaction at the physiological level in a hemimetabolous insect that practices thanatosis (feigning death). Only small changes are elicited in the bioassays, but the results from the two tests are comparable bar one or two anomalies. Results show that analogs modified at the termini have no or reduced activity. Regarding structural requirements of a ligand, the C. morosus AKH receptor appears to be strict: octapeptides are not preferred and many of the decapeptide analogs failed to reach 50% activity relative to Carmo-HrTH-II. The data implies that the AKH receptor in C. morosus mostly does not tolerate shorter peptides and single amino acid replacements in most places of the native AKH peptide. This information is important if environmentally friendly insect-specific pesticides are made based on an insect AKH as lead peptide: stick insects that are normally not viewed as pest insects may not be easily targeted by cross-reactive AKH mimetics directed at harmful insects, due to the very specific amino acid requirements to activate the C. morosus AKH receptor.

Insect heart rhythmicity is modulated by evolutionarily conserved neuropeptides and neurotransmitters

Insects utilize an open circulatory system to transport nutrients, waste, hormones and immune factors throughout the hemocoel. The primary organ that drives hemolymph circulation is the dorsal vessel, which is a muscular tube that traverses the length of the body and is divided into an aorta in the head and thorax, and a heart in the abdomen. The dorsal vessel is myogenic, but its rhythmicity is modulated by neuropeptides and neurotransmitters. This review summarizes how neuropeptides such as crustacean cardioactive peptide (CCAP), FMRFamide-like peptides, proctolin, allatotropin and allatostatin modulate the heart contraction rate and the directionality of heart contractions. Likewise, it discusses how neurotransmitters such as serotonin, octopamine, glutamate and nitric oxide influence the heart rate, and how transcriptomic and proteomic approaches are advancing our understanding of insect circulatory physiology. Finally, this review argues that the immune system may modulate heart rhythmicity, and discusses how the myotropic activity of cardioactive factors extends to the accessory pulsatile organs, such as the auxiliary hearts of the antennae.

Using invertebrate model organisms for neuroscience research and training: an opportunity for Africa

Africa is faced with an increasing underrepresentation of her research progress in many fields of science including neuroscience. This underrepresentation stems from the very low investments directed towards research by African governments as these are thought to be high-priced. Scientists and researchers within the continent are left to compete highly for the very limited research grants or choose to fund research from their personal purse. Therefore, presenting a need for all possible strategies to make science and research approaches more affordable in Africa. This paper presents one of such strategy, which advocates the use of invertebrate animal models for neuroscience research in place of the commonly used vertebrate models. Invertebrates are cheaper, more available and easy to handle options and their use is on the rise, even in the developed societies of the world. Here, we investigate the current state of invertebrate neuroscience research in Africa looking at countries and institutions conducting neuroscience research with invertebrates and their publication output. We discuss the factors which impede invertebrate neuroscience research in Africa like lack of research infrastructure and adequate expert scientists and conclude by suggesting solutions to these challenges.

The evolution and nomenclature of GnRH-type and corazonin-type neuropeptide signaling systems

Gonadotropin-releasing hormone (GnRH) was first discovered in mammals on account of its effect in triggering pituitary release of gonadotropins and the importance of this discovery was recognized forty years ago in the award of the 1977 Nobel Prize for Physiology or Medicine. Investigation of the evolution of GnRH revealed that GnRH-type signaling systems occur throughout the chordates, including agnathans (e.g. lampreys) and urochordates (e.g. sea squirts). Furthermore, the discovery that adipokinetic hormone (AKH) is the ligand for a GnRH-type receptor in the arthropod Drosophila melanogaster provided evidence of the antiquity of GnRH-type signaling. However, the occurrence of other AKH-like peptides in arthropods, which include corazonin and AKH/corazonin-related peptide (ACP), has complicated efforts to reconstruct the evolutionary history of this family of related neuropeptides. Genome/transcriptome sequencing has revealed that both GnRH-type receptors and corazonin-type receptors occur in lophotrochozoan protostomes (annelids, mollusks) and in deuterostomian invertebrates (cephalochordates, hemichordates, echinoderms). Furthermore, peptides that act as ligands for GnRH-type and corazonin-type receptors have been identified in mollusks. However, what has been lacking is experimental evidence that distinct GnRH-type and corazonin-type peptide-receptor signaling pathways occur in deuterostomes. Importantly, we recently reported the identification of two neuropeptides that act as ligands for either a GnRH-type receptor or a corazonin-type receptor in an echinoderm species - the common European starfish Asterias rubens. Discovery of distinct GnRH-type and corazonin-type signaling pathways in this deuterostomian invertebrate has demonstrated for the first time that the evolutionarily origin of these paralogous systems can be traced to the common ancestor of protostomes and deuterostomes. Furthermore, lineage-specific losses of corazonin signaling (in vertebrates, urochordates and nematodes) and duplication of the GnRH signaling system in arthropods (giving rise to the AKH and ACP signaling systems) and quadruplication of the GnRH signaling system in vertebrates (followed by lineage-specific losses or duplications) accounts for the phylogenetic distribution of GnRH/corazonin-type peptide-receptor pathways in extant animals. Informed by these new insights, here we review the history of research on the evolution of GnRH/corazonin-type neuropeptide signaling. Furthermore, we propose a standardized nomenclature for GnRH/corazonin-type neuropeptides wherein peptides are either named "GnRH" or "corazonin", with the exception of the paralogous GnRH-type peptides that have arisen by gene duplication in the arthropod lineage and which are referred to as "AKH" (or red pigment concentrating hormone, "RCPH", in crustaceans) and "ACP".

Influence of aminergic and peptidergic substances on heart beat frequency in the stick insect Carausius morosus (Insecta, Phasmatodea)

The dorsal heart of the Indian stick insect, Carausius morosus, is responsible for the anterograde flow of hemolymph to the aorta and into the body cavity. The contraction frequency of the insect heart is known to be influenced by several substances of neural source. Here, a semi-exposed heart assay was employed to study the effect of an aminergic substance (octopamine) and three neuropeptides (C. morosus hypertrehalosemic hormone [Carmo-HrTH], crustacean cardioactive peptide [CCAP], and proctolin) on heart contraction. The contraction frequency was measured as beats per minute in adults ligated between the head and the prothorax. All three investigated neuropeptides had a stimulatory effect on heart contraction that lasted approximately 6 min, after which the normal heart beat rate was restored. Proctolin and CCAP stimulated the rate of heart beat also in unligated stick insects, whereas Carmo-HrTH was active only in ligated insects. The latter could suggest that when the stick insect is not ligated, a competing substance may be released from the head of C. morosus; the competing substance is, apparently, not physiologically active but it binds or blocks access to the receptor of Carmo-HrTH-II, thereby rendering the HrTH peptide "not active." In ligated stick insects, 6.7 × 10^-8  M Carmo-HrTH-II significantly increased the heart beat rate; higher doses resulted in no further increase, suggesting the saturation of the HrTH receptor. Octopamine inhibited the rate at which the heart contracted in a dose-dependent manner; inhibition was achieved with 10^-4  M of octopamine.

Two symbiotic bacteria of the entomopathogenic nematode Heterorhabditis spp. against Galleria mellonella

The entomopathogenic nematode Heterorhabditis spp. is considered a promising agent in the biocontrol of injurious insects of agriculture. However, different symbiotic bacteria associated with the nematode usually have different specificity and virulence toward their own host. In this study, two symbiotic bacteria, LY2W and NK, were isolated from the intestinal canals of two entomopathogenic nematode Heterorhabditis megidis 90 (PDSj1 and PDSj2) from Galleria mellonela, separately. To determine their species classification, we carried out some investigations on morphology, culture, biochemistry, especially 16S rDNA sequence analyses. As a result, both of them belong to Enterobacter spp., showing the closest relatedness with Enterobacter gergoviae (LY2W) and Enterobacter cloacae (NK), respectively. Moreover, the toxicity to Galleria mellonella was examined using both the metabolites and washed cells (primary and secondary) of these two strains. The results indicated both metabolites and cells of the primary-type bacteria could cause high mortalities (up to 97%) to Galleria mellonella, while those of the primary-type bacteria only killed 20%. These findings would provide new symbiotic bacteria and further references for biological control of the agricultural pest.

Adipokinetic hormone signalling system in the Chagas disease vector, Rhodnius prolixus

Neuropeptides and their G protein-coupled receptors are widespread throughout Metazoa and in several cases, clear orthologues can be identified in both protostomes and deuterostomes. One such neuropeptide is the insect adipokinetic hormone (AKH), which is related to the mammalian gonadotropin-releasing hormone. AKH has been studied extensively and is known to mobilize lipid, carbohydrates and proline for energy-consuming activities such as flight. In order to determine the possible roles for this signalling system in Rhodnius prolixus, we isolated the cDNA sequences encoding R. prolixus AKH (Rhopr-AKH) and its receptor (Rhopr-AKHR). We also examined their spatial expression pattern using quantitative PCR. Our expression analysis indicates that Rhopr-AKH is only expressed in the corpus cardiacum of fifth-instars and adults. Rhopr-AKHR, by contrast, is expressed in several peripheral tissues including the fat body. The expression of the receptor in the fat body suggests that AKH is involved in lipid mobilization, which was confirmed by knockdown of Rhopr-AKHR via RNA interference. Adult males that had been injected with double-stranded RNA (dsRNA) for Rhopr-AKHR exhibited increased lipid content in the fat body and decreased lipid levels in the haemolymph. Moreover, injection of Rhopr-AKH in Rhopr-AKHR dsRNA-treated males failed to elevate haemolymph lipid levels, confirming that this is indeed the receptor for Rhopr-AKH.

Reprint of "The distribution and physiological effects of three evolutionarily and sequence-related neuropeptides in Rhodnius prolixus: Adipokinetic hormone, corazonin and adipokinetic hormone/corazonin-related peptide"

We have examined the distribution and physiological effects of three evolutionarily and sequence-related neuropeptides in Rhodnius prolixus. These neuropeptides, adipokinetic hormone (RhoprAKH), corazonin (CRZ) and adipokinetic hormone/corazonin-related peptide (RhoprACP) are present in distinct, non-overlapping neuronal subsets in the central nervous system (CNS), as determined by immunohistochemistry. Corazonin-like immunoreactive cell bodies are present in the brain and ventral nerve cord, whereas ACP-like immunoreactive cell bodies are only present in the brain, and AKH-like immunoreactive cell bodies only present in the corpus cardiacum (CC). The immunoreactivity to ACP, CRZ and AKH in R. prolixus suggests that ACP and CRZ are released within the CNS, and that CRZ and AKH are released as neurohormones from the CC. Injection of RhoprAKH into adult males elevated haemolymph lipid levels, but injection of CRZ or RhoprACP failed to have any effect on haemolymph lipid levels. Corazonin stimulated an increase in heart-beat frequency in vitro, but RhoprAKH and RhoprACP failed to do so. Thus, although all three neuropeptides share sequence similarity, the AKH and CRZ receptors only respond to their own ligand.

The distribution and physiological effects of three evolutionarily and sequence-related neuropeptides in Rhodnius prolixus: Adipokinetic hormone, corazonin and adipokinetic hormone/corazonin-related peptide

We have examined the distribution and physiological effects of three evolutionarily and sequence-related neuropeptides in Rhodnius prolixus. These neuropeptides, adipokinetic hormone (RhoprAKH), corazonin (CRZ) and adipokinetic hormone/corazonin-related peptide (RhoprACP) are present in distinct, non-overlapping neuronal subsets in the central nervous system (CNS), as determined by immunohistochemistry. Corazonin-like immunoreactive cell bodies are present in the brain and ventral nerve cord, whereas ACP-like immunoreactive cell bodies are only present in the brain, and AKH-like immunoreactive cell bodies only present in the corpus cardiacum (CC). The immunoreactivity to ACP, CRZ and AKH in R. prolixus suggests that ACP and CRZ are released within the CNS, and that CRZ and AKH are released as neurohormones from the CC. Injection of RhoprAKH into adult males elevated haemolymph lipid levels, but injection of CRZ or RhoprACP failed to have any effect on haemolymph lipid levels. Corazonin stimulated an increase in heart-beat frequency in vitro, but RhoprAKH and RhoprACP failed to do so. Thus, although all three neuropeptides share sequence similarity, the AKH and CRZ receptors only respond to their own ligand.

More than two decades of research on insect neuropeptide GPCRs: an overview

This review focuses on the state of the art on neuropeptide receptors in insects. Most of these receptors are G protein-coupled receptors (GPCRs) and are involved in the regulation of virtually all physiological processes during an insect's life. More than 20 years ago a milestone in invertebrate endocrinology was achieved with the characterization of the first insect neuropeptide receptor, i.e., the Drosophila tachykinin-like receptor. However, it took until the release of the Drosophila genome in 2000 that research on neuropeptide receptors boosted. In the last decade a plethora of genomic information of other insect species also became available, leading to a better insight in the functions and evolution of the neuropeptide signaling systems and their intracellular pathways. It became clear that some of these systems are conserved among all insect species, indicating that they fulfill crucial roles in their physiological processes. Meanwhile, other signaling systems seem to be lost in several insect orders or species, suggesting that their actions were superfluous in those insects, or that other neuropeptides have taken over their functions. It is striking that the deorphanization of neuropeptide GPCRs gets much attention, but the subsequent unraveling of the intracellular pathways they elicit, or their physiological functions are often hardly examined. Especially in insects besides Drosophila this information is scarce if not absent. And although great progress made in characterizing neuropeptide signaling systems, even in Drosophila several predicted neuropeptide receptors remain orphan, awaiting for their endogenous ligand to be determined. The present review gives a précis of the insect neuropeptide receptor research of the last two decades. But it has to be emphasized that the work done so far is only the tip of the iceberg and our comprehensive understanding of these important signaling systems will still increase substantially in the coming years.