Pleiotropic effects of the neuropeptides CCAP and myosuppressin in the beetle, Tenebrio molitor L

Nitric oxide produced by periostial hemocytes modulates the bacterial infection-induced reduction of the mosquito heart rate

The circulatory and immune systems of mosquitoes are functionally integrated. An infection induces the migration of hemocytes to the dorsal vessel, and specifically, to the regions surrounding the ostia of the heart. These periostial hemocytes phagocytose pathogens in the areas of the hemocoel that experience the highest hemolymph flow. Here, we investigated whether a bacterial infection affects cardiac rhythmicity in the African malaria mosquito, Anopheles gambiae We discovered that infection with Escherichia coli, Staphylococcus aureus and Staphylococcus epidermidis, but not Micrococcus luteus, reduces the mosquito heart rate and alters the proportional directionality of heart contractions. Infection does not alter the expression of genes encoding crustacean cardioactive peptide (CCAP), FMRFamide, corazonin, neuropeptide F or short neuropeptide F, indicating that they do not drive the cardiac phenotype. Infection upregulates the transcription of two superoxide dismutase (SOD) genes, catalase and a glutathione peroxidase, but dramatically induces upregulation of nitric oxide synthase (NOS) in both the heart and hemocytes. Within the heart, nitric oxide synthase is produced by periostial hemocytes, and chemically inhibiting the production of nitric oxide using l-NAME reverses the infection-induced cardiac phenotype. Finally, infection induces the upregulation of two lysozyme genes in the heart and other tissues, and treating mosquitoes with lysozyme reduces the heart rate in a manner reminiscent of the infection phenotype. These data demonstrate an exciting new facet of the integration between the immune and circulatory systems of insects, whereby a hemocyte-produced factor with immune activity, namely nitric oxide, modulates heart physiology.

The Insect Circulatory System: Structure, Function, and Evolution

Although the insect circulatory system is involved in a multitude of vital physiological processes, it has gone grossly understudied. This review highlights this critical physiological system by detailing the structure and function of the circulatory organs, including the dorsal heart and the accessory pulsatile organs that supply hemolymph to the appendages. It also emphasizes how the circulatory system develops and ages and how, by means of reflex bleeding and functional integration with the immune system, it supports mechanisms for defense against predators and microbial invaders, respectively. Beyond that, this review details evolutionary trends and novelties associated with this system, as well as the ways in which this system also plays critical roles in thermoregulation and tracheal ventilation in high-performance fliers. Finally, this review highlights how novel discoveries could be harnessed for the control of vector-borne diseases and for translational medicine, and it details principal knowledge gaps that necessitate further investigation.

Function of myosuppressin in regulating digestive function in the two-spotted cricket, Gryllus bimaculatus

Myosuppressin is one of essential peptides controlling biological processes including feeding behavior. Here we identified and characterized the cDNAs that encode myosuppressin precursor and its receptor in the two-spotted cricket Gryllus bimaculatus. The presence of the mature peptide (Grybi-MS) was confirmed by direct measurement of adult brain. RT-PCR revealed the tissue distribution of these transcripts; myosuppressin is expressed predominantly in the brain and central nervous system, whereas its receptor is ubiquitously expressed in the cricket body. To address the function of Grybi-MS, we performed several bioassays to test concerning feeding behavior and digestive function upon exposure to Grybi-MS. Administration of synthetic Grybi-MS resulted in increased feeding motivation, accompanied by an increase in food intake. Meanwhile, the hemolymph lipid and carbohydrate titers were both elevated after Grybi-MS injection. As the intestinal contraction is significantly inhibited by the exposure to Grybi-MS, the upregulating feeding index might be complicated in the cricket body. The current data indicate that Grybi-MS modulates feeding behavior to control the physiological processes in the cricket.

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.

CCAP and FMRFamide-like peptides accelerate the contraction rate of the antennal accessory pulsatile organs (auxiliary hearts) of mosquitoes

Insects rely on specialized accessory pulsatile organs (APOs), also known as auxiliary hearts, to propel hemolymph into their antennae. In most insects, this is accomplished via the pulsations of a pair of ampulla located in the head, each of which propels hemolymph across an antenna via an antennal vessel. Once at the distal end of the appendage, hemolymph returns to the head via the antennal hemocoel. Although the structure of the antennal hearts has been elucidated in various insect orders, their hormonal modulation has only been studied in cockroaches and other hemimetabolous insects within the superorder Polyneoptera, where proctolin and FMRFamide-like peptides accelerate the contraction rate of these auxiliary hearts. Here, we assessed the hormonal modulation of the antennal APOs of mosquitoes, a group of holometabolous (Endopterygota) insects within the order Diptera. We show that crustacean cardioactive peptide (CCAP), FMRFamide and SALDKNFMRFamide increase the contraction rate of the antennal APOs and the heart of Anopheles gambiae. Both antennal hearts are synchronously responsive to these neuropeptides, but their contractions are asynchronous with the contraction of the heart. Furthermore, we show that these neuropeptides increase the velocity and maximum acceleration of hemolymph within the antennal space, suggesting that each contraction is also more forceful. To our knowledge, this is the first report demonstrating that hormones of a holometabolous insect modulate the contraction dynamics of an auxiliary heart, and the first report that shows that the hormones of any insect accelerate the velocity of hemolymph in the antennal space.

Neuropeptide receptor transcript expression levels and magnitude of ionic current responses show cell type-specific differences in a small motor circuit

We studied the relationship between neuropeptide receptor transcript expression and current responses in the stomatogastric ganglion (STG) of the crab, Cancer borealis. We identified a transcript with high sequence similarity to crustacean cardioactive peptide (CCAP) receptors in insects and mammalian neuropeptide S receptors. This transcript was expressed throughout the nervous system, consistent with the role of CCAP in a range of different behaviors. In the STG, single-cell qPCR showed expression in only a subset of neurons. This subset had previously been shown to respond to CCAP with the activation of a modulator-activated inward current (IMI), with one exception. In the one cell type that showed expression but no IMI responses, we found CCAP modulation of synaptic currents. Expression levels within STG neuron types were fairly variable, but significantly different between some neuron types. We tested the magnitude and concentration dependence of IMI responses to CCAP application in two identified neurons, the lateral pyloric (LP) and the inferior cardiac (IC) neurons. LP had several-fold higher expression and showed larger current responses. It also was more sensitive to low CCAP concentrations and showed saturation at lower concentrations, as sigmoid fits showed smaller EC50 values and steeper slopes. In addition, occlusion experiments with proctolin, a different neuropeptide converging onto IMI, showed that saturating concentrations of CCAP activated all available IMI in LP, but only approximately two-thirds in IC, the neuron with lower receptor transcript expression. The implications of these findings for comodulation are discussed.

FlyNap (triethylamine) increases the heart rate of mosquitoes and eliminates the cardioacceleratory effect of the neuropeptide CCAP

FlyNap (triethylamine) is commonly used to anesthetize Drosophila melanogaster fruit flies. The purpose of this study was to determine whether triethylamine is a suitable anesthetic agent for research into circulatory physiology and immune competence in the mosquito, Anopheles gambiae (Diptera: Culicidae). Recovery experiments showed that mosquitoes awaken from traditional cold anesthesia in less than 7 minutes, but that recovery from FlyNap anesthesia does not begin for several hours. Relative to cold anesthesia, moderate exposures to FlyNap induce an increase in the heart rate, a decrease in the percentage of the time the heart contracts in the anterograde direction, and a decrease in the frequency of heartbeat directional reversals. Experiments employing various combinations of cold and FlyNap anesthesia then showed that cold exposure does not affect basal heart physiology, and that the differences seen between the cold and the FlyNap groups are due to a FlyNap-induced alteration of heart physiology. Furthermore, exposure to FlyNap eliminated the cardioacceleratory effect of crustacean cardioactive peptide (CCAP), and reduced a mosquito’s ability to survive a bacterial infection. Together, these data show that FlyNap is not a suitable substitute to cold anesthesia in experiments assessing mosquito heart function or immune competence. Moreover, these data also illustrate the intricate biology of the insect heart. Specifically, they confirm that the neurohormone CCAP modulates heart rhythms and that it serves as an anterograde pacemaker.

Cardioacceleratory function of the neurohormone CCAP in the mosquito Anopheles gambiae

Crustacean cardioactive peptide (CCAP) is a highly conserved arthropod neurohormone that is involved in ecdysis, hormone release and the modulation of muscle contractions. Here, we determined the CCAP gene structure in the malaria mosquito Anopheles gambiae, assessed the developmental expression of CCAP and its receptor and determined the role that CCAP plays in regulating mosquito cardiac function. RACE sequencing revealed that the A. gambiae CCAP gene encodes a neuropeptide that shares 100% amino acid identity with all sequenced CCAP peptides, with the exception of Daphnia pulex. Quantitative RT-PCR showed that expression of CCAP and the CCAP receptor displays a bimodal distribution, with peak mRNA levels in second instar larvae and pupae. Injection of CCAP revealed that augmenting hemocoelic CCAP levels in adult mosquitoes increases the anterograde and retrograde heart contraction rates by up to 28%, and increases intracardiac hemolymph flow velocities by up to 33%. Partial CCAP knockdown by RNAi had the opposite effect, decreasing the mosquito heart rate by 6%. Quantitative RT-PCR experiments showed that CCAP mRNA is enriched in the head region, and immunohistochemical experiments in newly eclosed mosquitoes detected CCAP in abdominal neurons and projections, some of which innervated the heart, but failed to detect CCAP in the abdomens of older mosquitoes. Instead, in older mosquitoes CCAP was detected in the pars lateralis, the subesophageal ganglion and the corpora cardiaca. In conclusion, CCAP has a potent effect on mosquito circulatory physiology, and thus heart physiology in this dipteran insect is under partial neuronal control.

The regulation of cardiac activity by nitric oxide (NO) in the Vietnamese stick insect, Baculum extradentatum

This study examines the role of the unconventional gaseous signaling molecule nitric oxide (NO) on the regulation of heart rate in the Vietnamese stick insect, Baculum extradentatum. Using nicotinamide dinucleotide hydrogen phosphate (NADPH)-diaphorase histochemistry, as well as immunohistochemistry and Western blotting with an antibody against NO synthetase (NOS), we identified the presence of NOS in hemocytes present throughout the lumen of the dorsal vessel. We propose that NO is delivered to heart muscle tissue via hemocytes circulating within the hemolymph. In the present study, stimulation of NO levels by the application of the NO donor MAHMA-NONOate and l-arginine led to a dose-dependent decrease in heart rate. Treatment of tissues with the NOS inhibitor, L-NAME, in equimolar concentrations with l-arginine, led to a recovery of heart rate, without modifying heart rate on its own. Finally guanosine 3',5'-cyclic monophosphate (cGMP) analog, 8-bromo-cGMP, elicited similar inhibitory effects on stick insect heart rate as did the guanylate cyclase activator, YC-1, and the phosphodiesterase inhibitor, dipyridamole, indicating that cGMP is most likely the second messenger in the stick insect NO signaling pathway. Contrary to the cardioexcitatory effect of NO on other insect hearts, we have found that NO inhibits stick insect heart rate independently from any nervous system input, in a similar inhibitory fashion as that of vertebrate hearts.

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.

Leucomyosuppressin modulates cardiac rhythm in the cockroach Blattella germanica

Several lines of evidence point to leucomyosuppressin (LMS) and myosuppressin-related peptides as inhibitory modulators of heartbeat frequency in arthropods. Previous studies in Blattella germanica demonstrated that heartbeat frequency decreases after ootheca formation, and remains low during the period of ootheca transport. Subsequent work in this cockroach resulted in the characterization of LMS and the cloning and sequencing of its precursor. The present paper describes the activity of LMS on modulation of heartbeat in B. germanica. Assays using semi-isolated heart preparations revealed that LMS reduces heartbeat frequency in a dose dependent manner, at physiological concentrations. Additional experiments showed that LMS inhibits heartbeat rates in vivo. Finally, injection of dsRNA for LMS elicited a decrease in LMS mRNA to virtually undetectable levels and heartbeat frequency increased significantly in females carrying oothecae. These data suggest that LMS contributes to the modulation of cardiac rhythm in B. germanica during the reproductive cycle.

Functions of duplicated genes encoding CCAP receptors in the red flour beetle, Tribolium castaneum

Crustacean cardioactive peptide (CCAP) is a nonapeptide originally isolated from the shore crab, Carcinus maenas, based on its cardioacceleratory activity. This peptide is highly conserved in insects and other arthropods. In insects CCAP also has an essential role in ecdysis behavior. We previously identified two homologous genes, ccapr-1 and ccapr-2, encoding putative CCAP receptors in the red flour beetle, Tribolium castaneum. In contrast, some insects, including Drosophila melanogaster, carry only one gene encoding a CCAP receptor. Phylogenetic analysis of putative CCAP receptor orthologs reveals a number of independent gene duplications in several insect lineages. In this study, we confirmed that CCAP activates both putative T. castaneum receptors in a heterologous expression system. RNA interference (RNAi) of ccapr-1 and ccapr-2 revealed that ccapr-2 is essential for eclosion behavior in T. castaneum, while RNAi for ccapr-1 did not result in any abnormal phenotype. In vivo cardioacceleratory activity of exogenously applied CCAP was abolished by RNAi of ccapr-2, but not by that of ccapr-1. Thus, only ccapr-2 mediates the cardioacceleratory function, ccapr-1 having apparently lost both functions for eclosion behavior and for cardioacceleration since the recent gene duplication event.

New physiological activities of myosuppressin, sulfakinin and NVP-like peptide in Zophobas atratus beetle

Three neuropeptides Zopat-MS-2 (pEDVDHVFLRFa), Zopat-SK-1 (pETSDDYGHLRFa) and Zopat-NVPL-4trunc. (GRWGGFA), recently isolated from the neuroendocrine system of the Zophobas atratus beetle, were tested for their myotropic and hyperglycaemic activities in this species. These peptides exerted differentiated dose-dependent and tissue specific physiological effects. Zopat-MS-2 inhibited contractions of the isolated heart, ejaculatory duct, oviduct and hindgut of adult beetles and induced bimodal effects in the heart contractile activity of pupae in vivo. It also increased the haemolymph free sugar level in larvae of this species, apart from myotropic activity. Zopat-SK-1 showed myostimulatory action on the isolated hindgut of the adult beetles, but it decreased contractions of the heart, ejaculatory duct and oviduct. Injections of this peptide at a dose of 2 μg also caused delayed cardioinhibitory effects on the heartbeat of the pupae. Together with the ability to increase free sugar level in the haemolymph of larvae these were new physiological activities of sulfakinins in insects. Zopat-NVPL-4trunc. inhibited the muscle contractions of the two organs: hindgut and ejaculatory duct but it was inactive on the oviduct and the heart of the adult beetles. This peptide also increased free sugar level concentration in the haemolymph of Z. atratus larvae. These physiological actions are the first biological activities discovered for this group of the insect peptides. The present work showed pleiotropic activity of three neuropeptides and indicates that the visceral muscle contractions and the haemolymph sugar homeostasis in Z. atratus are regulated by complex mechanisms.

Human RFamide-related peptide-1 diminishes cellular and integrated cardiac contractile performance

Peptides influence cardiac dysfunction; however, peptidergic modulation of contractile performance remains relatively uncharacterized. We identified a novel human peptide that modulates mammalian contractile performance. Members of the FMRFamide-related peptide (FaRP) family contain a C-terminal RFamide but structurally variant N-terminal extension. We report human RFamide-related peptide-1 (hRFRP-1) and rat RFRP-1 rapidly and reversibly decreased shortening and relaxation in isolated mammalian cardiac myocytes in a dose dependent manner. The mammalian FaRP, 26RFa, structurally related to RFRP-1 by only an RFamide did not influence myocyte contractile function. The protein kinase C (PKC) inhibitor bisindolylmaleimide-1 blocked hRFRP-1 activity. Pretreatment with pertussis toxin (PTX) did not diminish hRFRP-1 influence on contractile function. In addition, intravenous injection of hRFRP-1 in mice decreased heart rate, stroke volume, ejection fraction, and cardiac output. Collectively these findings are consistent with the conclusion RFRP-1 is an endogenous signaling molecule that activates PKC and acts through a PTX-insensitive pathway to modulate cardiac contractile function. Taken together these negative chronotropic, inotropic, and lusitropic effects of hRFRP-1 are significant; they suggest direct acute cellular and organ-level responses in mammalian heart. This is the first known study to identify a mammalian FaRP with cardio-depressant effects, opening a new area of research on peptidergic modulation of contractile performance. The high degree of RFRP structure conservation from amphibians to mammals, and similarity to invertebrate cardioinhibitory peptides suggests RFRP-1 is involved in important physiological functions. Elucidation of mechanisms involved in hRFRP-1 synthesis, release, and signaling may aid the development of strategies to prevent or attenuate cardiac dysfunction.

An unusual peptide from Conus villepinii: synthesis, solution structure, and cardioactivity

The venom of marine cone snails contains a variety of conformationally constrained peptides utilized by the animal to capture prey. Besides numerous conotoxins, which are characterized by complex disulfide patterns, other peptides with only a single disulfide bridge were isolated from different conus species. Here, we report the synthesis, structure elucidation and biological evaluation of the novel C-terminally amidated decapeptide CCAP-vil, PFc[CNSFGC]YN-NH(2), from Conus villepinii. The linear precursor peptide was generated by standard solid phase synthesis. Oxidation of the cysteine residues to yield the disulfide-bridged peptide was investigated under different conditions, including several ionic liquids (ILs) as new biocompatible reaction media. Among the examined ILs, 1-ethyl-3-methylimidazolium tosylate ([C(2)mim][OTs]) was most efficient for CCAP-vil oxidative folding, since oxidation occurred without any byproduct formation. The structure of CCAP-vil was determined by NMR methods in aqueous solution and revealed a loop structure adopting a type(I) beta-turn between residues 4-7 imposed by the flanking disulfide bridge. The amino acid side chains of Pro(1), Phe(2), Phe(6) and Tyr(9) point in three directions away from the cyclic core into the solvent creating a rather hydrophobic surface of the molecule. Based on sequence homology to cardioactive peptides (CAPs) from gastropods and arthropods, such as PFc[CNAFTGC]-NH(2) (CCAP), the influence of CCAP-vil on heart rate using zebrafish embryos was investigated. CCAP-vil reduced the heart rate immediately upon injection into the heart as well as upon indirect application indicating an opposite effect to the cardioaccelerating CCAP.