Structure-activity relationship of contractile effects induced by helicokinins in the isolated gut of the lepidopteran caterpillar Spodoptera frugiperda

Role of peptide hormones in insect gut physiology

Nutrient uptake and digestion are essential for optimal growth and development. In insects, these processes are regulated by the gut-brain axis, which is a neurohumoral communication system for maintaining gut homeostasis. The insect gut is a complex organ consisting of three distinct structures, denominated foregut, midgut and hindgut, each with their specific specializations. These specializations are tightly regulated by the interplay of several neuropeptides: a versatile group of signalling molecules involved in a multitude of processes including gut physiology. Neuropeptides take part in the regulation of gut processes ranging from digestive enzyme release to muscle activity and satiety. Some neuropeptide mimetics are a promising strategy for ecological pest management. This review focuses on a selection of neuropeptides that are well-known for their role in gut physiology, and neuropeptides for which the mode of action is yet to be unravelled.

Helicokinin alters ion transport in the secondary cell-containing region of the Malpighian tubule of the larval cabbage looper Trichoplusia ni

Excretion in insects is accomplished by the combined actions of the Malpighian tubules (MTs) and hindgut, which together form the functional kidney. MTs of many insect groups consist of principal cells (PC) and secondary cells (SC). In most insect groups SCs are reported to secrete ions from haemolymph into the tubule lumen. Paradoxically, SCs in the MTs of the lepidopteran cabbage looper T. ni are used to reabsorb Na⁺ and K⁺ back into haemolymph. The current study was designed to investigate the effects and mode of action of the lepidopteran kinin, Helicokinin (HK), on ion transport in the SC-containing region of MT of T. ni. We identified a HK receptor (HK-R) homologue in T. ni and detected its expression in the SC-containing region of the MTs. The mRNA abundance of hk-r altered in response to changes in dietary K⁺ and Na⁺ content. HK-R immunolocalized to both PCs and SCs. Ramsay assays of preparations of the isolated distal ileac plexus (DIP) indicated that [HK] = 10^-8 M: (i) decreased fluid secretion rate in unstimulated and serotonin-stimulated preparations, and (ii) increased [Na⁺]/[K⁺] ratio in the secreted fluid. Scanning ion-selective electrode technique measurements revealed that HK reduced: (i) K⁺ secretion by the PCs, and (ii) Na⁺ reabsorption by the SCs in intact tubules. In vitro incubation of the DIP with HK resulted in reduced mRNA abundance of hk-r as well as Na⁺/K⁺-ATPase subunit α (NKAα), Na⁺/K⁺/Cl^- co-transporter (nkcc), Na⁺/H⁺ exchangers (nhe) 7 and 8, and aquaporin (aqp) 1. Taken together, results of the current study suggest that HK is capable of altering fluid secretion rate and [Na⁺]/[K⁺] ratio of the fluid, and that HK targets both PCs and SCs in the DIP of T. ni.

Neuropeptide signaling sequences identified by pyrosequencing of the American dog tick synganglion transcriptome during blood feeding and reproduction

Ticks are important vectors of numerous pathogens that impact human and animal health. The tick central nervous system represents an understudied area in tick biology and no tick synganglion-specific transcriptome has been described to date. Here we characterize whole or partial cDNA sequences of fourteen putative neuropeptides (allatostatin, insulin-like peptide, ion-transport peptide, sulfakinin, bursicon alpha/beta, eclosion hormone, glycoprotein hormone alpha/beta, corazonin, four orcokinins) and five neuropeptide receptors (gonadotropin receptor, leucokinin-like receptor, sulfakinin receptor, calcitonin receptor, pyrokinin receptor) translated from cDNA synthesized from the synganglion of unfed, partially fed and replete female American dog ticks, Dermacentor variabilis. Their homology to the same neuropeptides in other taxa is discussed. Many of these neuropeptides such as an allatostatin, insulin-like peptide, eclosion hormone, bursicon alpha and beta and glycoprotein hormone alpha and beta have not been previously described in the Chelicerata. An insulin-receptor substrate protein was also found indicating that an insulin signaling network is present in ticks. A putative type-2 proprotein processing convertase was also sequenced that may be involved in cleavage at monobasic and dibasic endoproteolytic cleavage sites in prohormones. The possible physiological role of the proteins discovered in adult tick blood feeding and reproduction will be discussed.

Functional characterisation of the Anopheles leucokinins and their cognate G-protein coupled receptor

Identification of the Anopheles gambiae leucokinin gene from the completed A. gambiae genome revealed that this insect species contains three leucokinin peptides, named Anopheles leucokinin I-III. These peptides are similar to those identified in two other mosquito species, Aedes aegypti and Culex salinarius. Additionally, Anopheles leucokinin I displays sequence similarity to Drosophila melanogaster leucokinin. Using a combination of computational and molecular approaches, a full-length cDNA for a candidate leucokinin-like receptor was isolated from A. stephensi, a close relative of A. gambiae. Alignment of the known leucokinin receptors--all G protein-coupled receptors (GPCRs)--with this receptor, identified some key conserved regions within the receptors, notably transmembrane (TM) domains I, II, III, VI and VII. The Anopheles leucokinins and receptor were shown to be a functional receptor-ligand pair. All three Anopheles leucokinins caused a dose-dependent rise in intracellular calcium ([Ca2+]i) when applied to S2 cells co-expressing the receptor and an aequorin transgene, with a potency order of I>II>III. Drosophila leucokinin was also found to activate the Anopheles receptor with a similar EC50 value to Anopheles leucokinin I. However, when the Anopheles peptides were applied to the Drosophila receptor, only Anopheles leucokinin I and II elicited a rise in [Ca2+]i. This suggests that the Anopheles receptor has a broader specificity for leucokinin ligands than the Drosophila receptor. Antisera raised against the Anopheles receptor identified a doublet of approx. 65 and 72 kDa on western blots, consistent with the presence of four N-glycosylation sites within the receptor sequence, and the known glycosylation of the receptor in Drosophila. In Anopheles tubules, as in Drosophila, the receptor was localised to the stellate cells. Thus we provide the first identification of Anopheles mosquito leucokinins (Anopheles leucokinins) and a cognate leucokinin receptor, characterise their interaction and show that Dipteran leucokinin signalling is closely conserved between Drosophila and Anopheles.

Neuropeptides associated with the frontal ganglion of larval Lepidoptera

The occurrence of neuropeptides in the frontal ganglia of larvae of the tobacco hawkmoth, Manduca sexta, the tomato moth, Lacanobia oleracea and the cotton leafworm, Spodoptera littoralis was investigated using reversed-phase high performance liquid chromatography (RP-HPLC), matrix-assisted laser desorption time of flight mass spectrometry (MALDI-TOF MS) and enzyme-linked immunosorbent assay (ELISA). Only three types of peptides could be identified or assigned from frontal ganglion extracts; M. sexta allatostatin (Manse-AS), M. sexta allatotropin (Manse-AT), and F/YXFGL-NH2 allatostatins. The peptide profiles of frontal ganglion of L. oleracea and S. littoralis were similar, with ten identical [M+H]+ ions, seven of which could be assigned to known lepidopteran peptides (Manse-AT, cydiastatin 2, 3, 4 and helicostatin 1, 5, 9). In addition, mass ions corresponding to helicostatin 7 (which was confirmed by MALDI-post source decay analysis) and Manse-AS were present in frontal ganglia of L. oleracea and helicostatin 6 in frontal ganglia of S. littoralis. Only four mass ions from M. sexta frontal ganglia corresponded to known peptides, cydiastatin 3 and 4, helicostatin 1, and Manse-AT. The only difference between the profiles of frontal ganglia from different stages of L. oleracea were mass ions which could not be assigned, and no differences were observed in the allatoregulatory peptides present. In HPLC fractions of M. sexta frontal ganglia, F/YXFGL-NH2 allatostatin-like immunoreactivity was widespread suggesting that more allatostatins were present than were identified.

Myotropic effect of helicokinins, tachykinin-related peptides and Manduca sexta allatotropin on the gut of Heliothis virescens (Lepidoptera: Noctuidae)

Different insect neuropeptides (helicokinins, tachykinin-related and allatoregulating peptides) were investigated with regard to their myostimulatory effects using whole-gut preparations isolated from fifth instar Heliothis virescens larvae. The experiments demonstrated that representatives of all three peptide families are able to induce and amplify gut contractions in this species in a dose-dependent manner. Structure-activity studies (alanine scan, D-amino acid scan and truncated analogues) with the helicokinin Hez-K1 supported the finding, that the core sequence for biological activity of kinins is the amidated C-terminal pentapeptide (FSPWG-amide). Similar investigations with insect tachykinin isolated from Leucophaea madera (Lem-TRP1) revealed that the minimum sequence evoking a physiological gut response in H. virescens is the amidated hexapeptide (GFLGVR-amide), which represents the conserved amino acid sequence for Leucophaea TRPs in general. The peptide concentration causing a half-maximal gut contraction (EC(50)) for Lem-TRP1 was about 26 nM. Although the potency of Lem-TRP1 was 9-fold lower compared with Hez-KI (EC(50): 3 nM), the maximal tension of the gut obtained with Lem-TRP1 was 1.7-fold higher compared with Hez-KI. The EC(50) of Manduca sexta allatotropin (Mas-AT; 79 nM) was of lowest potency among all three peptides tested. In a pharmacological study, co-incubation experiments with Lem-TRP1, Hez-KI or Mas-AT and compounds interfering with signal transduction pathways were employed to investigate the mode of action of the myotropic effects of these peptides. Cadmium and the protein kinase C (PKC) inhibitor tamoxifen attenuated the contractile effects of all three peptides tested. The data suggest that in the gut muscle of H. virescens the myotropic peptides bind to G-protein-coupled receptors that cause contraction by promoting the entry of extracellular calcium mediated by a PKC involved pathway.