Isolation, characterization and biological activity of a diuretic myokinin neuropeptide from the housefly, Musca domestica

Evidence for CRF-like and kinin-like peptides as neurohormones in the blood-feeding bug, Rhodnius prolixus

In Rhodnius prolixus, the rapid post-feeding diuresis is under neurohormonal control. While serotonin has been demonstrated to be a diuretic neurohormone [J Exp Biol 156 (1991) 557], a peptide is also known to be involved. Previously, we have demonstrated the presence of corticotropin releasing factor (CRF)-like and kinin-like peptides in the central nervous system (CNS) of 5th instar Rhodnius [J Exp Biol 202 (1999) 2017; Peptides 22 (2001) 161]. These peptides are present in neurohemal sites of the corpus cardiacum and are co-localized in neurohemal sites on abdominal nerves. While various CRF-like peptides have been demonstrated to increase Rhodnius Malpighian tubule secretion the kinin-like peptides do not [Peptides 23 (2002) 671]. The kinin-like peptides do however, increase hindgut contraction which may contribute to the rapid post feeding diuresis by the mixing of hemolymph and/or hindgut contents and the removal of wastes. The presence of these peptides in neurohemal sites suggests that they could be released into the hemolymph and act as neurohormones. We have used immunohistochemical techniques and radioimmunoassay (RIA) to demonstrate qualitative and quantitative changes of CRF-like and kinin-like peptides in the CNS associated with feeding. As well we have examined Malpighian tubule secretion in response to assays of hemolymph from unfed and fed insects. Hemolymph was also partially purified by Sep-Pak and HPLC and the fractions assayed for kinin-like immunoreactivity and the ability to stimulate Malpighian tubule secretion. The results suggest that both kinin-like and CRF-like peptides are neurohormones in Rhodnius, released in response to feeding.

Systematic G-protein-coupled receptor analysis in Drosophila melanogaster identifies a leucokinin receptor with novel roles

Leucokinins are insect neuropeptides that stimulate hindgut motility and renal fluid secretion. Drosophila has a single leucokinin gene, pp, encoding the longest known leucokinin, Drosokinin. To identify its receptor, a genome-wide scan for G-protein-coupled receptors was performed in silico and candidate receptors identified by similarity to known tachykinin receptors. The deduced peptides were expressed, with a transgene for the calcium reporter aequorin, in S2 cells and only one gene (CG10626) encoded a protein that responded to Drosokinin. The properties of the heterologously expressed receptor (action through intracellular calcium with an EC(50) of 4 x 10(-11) m and a t(1/2) <1 s) match closely those reported for the action of Drosokinin on Malpighian (renal) tubules. Antibodies raised against the receptor identified known sites of leucokinin action: stellate cells of the Malpighian tubule, two triplets of cells in the pars intercerebralis of the adult central nervous system, and additional cells in larval central nervous system. Western blots and reverse transcription-PCR confirmed these locations, but also identified expression in male and female gonads. These tissues also displayed elevated calcium in response to Drosokinin, demonstrating novel roles for leucokinin. A functional genomic approach has thus yielded the first complete characterization of a leucokinin receptor in an insect.

Leucokinin activates Ca(2+)-dependent signal pathway in principal cells of Aedes aegypti Malpighian tubules

The role of Ca(2+) in mediating the diuretic effects of leucokinin-VIII was studied in isolated perfused Malpighian tubules of the yellow fever mosquito, Aedes aegypti. Peritubular leucokinin-VIII (1 microM) decreased the transepithelial resistance from 11.2 to 2.6 kOmega. cm, lowered the transepithelial voltage from 42.8 to 2.7 mV, and increased transepithelial Cl(-) diffusion potentials 5.1-fold. In principal cells of the tubules, leucokinin-VIII decreased the fractional resistance of the basolateral membrane from 0.733 to 0.518. These effects were reversed by the peritubular Ca(2+)-channel blocker nifedipine, suggesting a role of peritubular Ca(2+) and basolateral Ca(2+) channels in signal transduction. In Ca(2+)-free Ringer bath, the effects of leucokinin-VIII were partial and transient but were fully restored after the bath Ca(2+) concentration was restored. Increasing intracellular Ca(2+) with thapsigargin duplicated the effects of leucokinin-VIII, provided that peritubular Ca(2+) was present. The kinetics of the effects of leucokinin-VIII is faster than that of thapsigargin, suggesting the activation of inositol-1,4,5-trisphosphate-receptor channels of intracellular stores. Store depletion may then bring about Ca(2+) entry into principal cells via nifedipine-sensitive Ca(2+) channels in the basolateral membrane.

Neuropeptides in the nervous system of Drosophila and other insects: multiple roles as neuromodulators and neurohormones

Neuropeptides in insects act as neuromodulators in the central and peripheral nervous system and as regulatory hormones released into the circulation. The functional roles of insect neuropeptides encompass regulation of homeostasis, organization of behaviors, initiation and coordination of developmental processes and modulation of neuronal and muscular activity. With the completion of the sequencing of the Drosophila genome we have obtained a fairly good estimate of the total number of genes encoding neuropeptide precursors and thus the total number of neuropeptides in an insect. At present there are 23 identified genes that encode predicted neuropeptides and an additional seven encoding insulin-like peptides in Drosophila. Since the number of G-protein-coupled neuropeptide receptors in Drosophila is estimated to be around 40, the total number of neuropeptide genes in this insect will probably not exceed three dozen. The neuropeptides can be grouped into families, and it is suggested here that related peptides encoded on a Drosophila gene constitute a family and that peptides from related genes (orthologs) in other species belong to the same family. Some peptides are encoded as multiple related isoforms on a precursor and it is possible that many of these isoforms are functionally redundant. The distribution and possible functions of members of the 23 neuropeptide families and the insulin-like peptides are discussed. It is clear that each of the distinct neuropeptides are present in specific small sets of neurons and/or neurosecretory cells and in some cases in cells of the intestine or certain peripheral sites. The distribution patterns vary extensively between types of neuropeptides. Another feature emerging for many insect neuropeptides is that they appear to be multifunctional. One and the same peptide may act both in the CNS and as a circulating hormone and play different functional roles at different central and peripheral targets. A neuropeptide can, for instance, act as a coreleased signal that modulates the action of a classical transmitter and the peptide action depends on the cotransmitter and the specific circuit where it is released. Some peptides, however, may work as molecular switches and trigger specific global responses at a given time. Drosophila, in spite of its small size, is now emerging as a very favorable organism for the studies of neuropeptide function due to the arsenal of molecular genetics methods available.

Diuretic and myotropic activities of N-terminal truncated analogs of Musca domestica kinin neuropeptide

Musca kinin (Musdo-K; NTVVLGKKQRFHSWG-NH(2)) and N-terminal truncated analogs of 4-14 residues in length were assayed for diuretic and myotropic activity on housefly Malpighian tubules and hindgut, respectively. The pentapeptide was the minimum sequence required for biological activity, but it was > 5 orders of magnitude less potent than the intact peptide. The pharmacological profiles of the different analogs in the two assays were very similar, suggesting the same receptor is present on both tissues. Potency was little affected by the deletion of Asn(1), but was reduced > 10-fold after the removal of Thr(2). Deletion of the next 5 residues had relatively little effect, but after the second lysyl residue (Lys(8)) was removed potency fell by one to two orders of magnitude. There was a similar drop in potency after the removal of Arg(10), and at 100 microM the pentapeptide had only 20% of the diuretic activity of the intact peptide. The importance of Arg(10) was confirmed by comparing dose-response curves for Musdo-K [6-15] and Acheta kinin-V (AFSHWG-NH(2)) in the diuretic assay; the substitution of arginine by alanine produced a significant reduction in potency and some loss of activity.

cis-peptide bond mimetic tetrazole analogs of the insect kinins identify the active conformation

The insect kinin neuropeptides have been implicated in the regulation of water balance, digestive organ contraction, and energy mobilization in a number of insect species. A previous solution conformation study of an active, restricted-conformation cyclic analog, identified two possible turn conformations as the likely active conformation adopted by the insect kinins at the receptor site. These were a cisPro type VI beta-turn over C-terminal pentapeptide core residues 1-4 and a transPro type I-like beta-turn over core residues 2-5, present in a ratio of 60:40. Synthesis and evaluation of the diuretic activity of insect kinin analogs incorporating a tetrazole moiety, which mimics a cis peptide bond, identifies the active conformation as the former. The discovery of a receptor interaction model can lead to the development of potent agonist and antagonist analogs of the insect kinins. Indeed, in this study a tetrazole analog with D stereochemistry has been shown to demonstrate partial antagonism of the diuretic activity of natural insect kinins, providing a lead for more potent and effective antagonists of this critical neuropeptide family. The future development of mimetic agonists and antagonists of insect kinin neuropeptides will provide important tools to neuroendocrinologists studying the mechanisms by which they operate and to researchers developing new, environmentally friendly pest insect control strategies.

Enhanced in vivo activity of peptidase-resistant analogs of the insect kinin neuropeptide family

The diuretic/myotropic insect kinin neuropeptides, which share the common C-terminal pentapeptide core FX(1)X(2)WG-NH(2), reveal primary (X(2)-W) and secondary (N-terminal to F) sites of susceptibility to peptidases bound to corn earworm (H. zea) Malpighian tubule tissue. Analogs designed to enhance resistance to tissue-bound peptidases, and pure insect neprilysin and ACE, demonstrate markedly enhanced in vivo activity in a weight gain inhibition assay in H. zea, and strong in vivo diuretic activity in the housefly (M. domestica). The peptidase-resistant insect kinin analog pQK(pQ)FF[Aib]WG-NH(2) demonstrates a longer internal residence time in the housefly than the native muscakinin (MK), and despite a difference of over 4 orders of magnitude in an in vitro Malpighian tubule fluid secretion assay, is equipotent with MK in an in vivo housefly diuretic assay. Aminohexanoic acid (Ahx) is shown to function as a surrogate for N-terminal Lys, while at the same time providing enhanced resistance to aminopeptidase attack. Peptidaese-resistant insect kinin analogs demonstrate enhanced inhibition of weight gain in larvae of the agriculturally destructive corn earworm moth. Potent peptidase resistant analogs of the insect kinins, coupled with an increased understanding of related regulatory factors, offer promise in the development of new, environmentally friendly pest insect control measures.

The biological activity of diuretic factors in Rhodnius prolixus

The rapid post-feeding diuresis of Rhodnius prolixus is under neurohormonal control and involves the integrated activity of the crop, Malpighian tubules and hindgut. One of the factors which is involved in this rapid diuresis is serotonin, however a peptide(s) is also considered to be involved. In other insects, corticotropin releasing factor (CRF)-like and kinin-like, calcitonin-like peptides and CAP(2b) have been demonstrated to be diuretic factors/hormones. In the present study, serotonin and CRF-like peptides increased secretion rate and cAMP content of Rhodnius Malpighian tubules, while the kinin-like peptides tested did not increase secretion rate or cAMP content of the tubules. Extracts of the CNS were processed and several HPLC fractions revealed kinin-like immunoreactivity but these fractions did not increase secretion rate when tested on Malpighian tubules. However, these same fractions did possess activity when tested on the hindgut contraction assay. In addition, material eluting at higher acetonitrile concentrations from the HPLC increased secretion and cAMP content of Rhodnius Malpighian tubules. This material eluted at concentrations of acetonitrile consistent with the elution time of CRF-like peptide standards. Synergism was demonstrated using the pharmacological agent forskolin and serotonin, tested on the rate of secretion of Rhodnius Malpighian tubules, in agreement with data of Maddrell et al. As well, synergism could be demonstrated using mesothoracic ganglionic mass (MTGM) homogenates and serotonin at some concentrations of serotonin. However, combinations of CRF-like material and serotonin increased secretion additively, not synergistically. Kinin-like peptides, tested along with CRF-like material and serotonin, at low concentrations, did not increase secretion above that of those factors tested alone.

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

The diuretic helicokinins YFSPWG-amide (Hez KI), VRFSPWG-amide (Hez KII) and KVKFSAWG-amide (Hez KIII) are potent contractants of the isolated gut of the caterpillar Spodoptera frugiperda at doses ranging from 0.1 to 10nM. In comparison, the pentapeptide FSPWG-amide was a full agonist with greatly reduced potency while SPWG-amide and PWG-amide were weak partial agonists. Substitution of individual amino acids in Hez KI with alanine revealed that replacement of the [phenylalanine(2)] residue caused a large fall in potency while replacement of [tryptophan(5)] residue caused complete loss of myogenic activity. The striking fall in potency of YASPWG-amide and the lack of activity of YFSPAG-amide confirm the requirement for aromatic groups in positions 2 and 3 of the core pentapeptide as well as supporting the ideas that the active core of these peptides adopts a beta-turn when interacting with receptors, bringing together the [Phe] and [Trp] residues that are critical for activity. Neither the pentapeptide proctolin nor the potent mammalian gut contractant Substance P were able to cause contraction when applied to caterpillar gut tissue. Incubation of isolated gut tissue in the phosphodiesterase inhibitor theophylline (10-100&mgr;M) caused significant potentiation of the response to applied Hez KI. Conversely, in the presence of the L-type Ca(2+) channel blocker verapamil (10&mgr;M-1mM) or Co(2+) (1-50mM) the contractile effects of Hez KI were attentuated significantly. These data suggest that the gut of S. frugiperda contains G-protein-linked kinin receptors that utilise cyclic AMP as their second messenger system and cause contraction by promoting the entry of extracellular Ca(2+).

Insect G protein-coupled receptors and signal transduction

G protein-coupled receptors (GPCRs) are seven-transmembrane proteins (7-TM) that transduce extracellular signals into cellular physiological responses through the activation of heterotrimeric guanine nucleotide binding proteins (alpha beta gamma subunits). Their general properties are remarkably well conserved during evolution. Despite this general resemblance, a large variety of different signals are mediated via this category of receptors. Several GPCR-(sub)families have an ancient origin that is situated before the divergence of Protostomian and Deuterostomian animals. Nevertheless, an enormous diversification has occurred since then. The availability of novel sequence information is growing very rapidly as a result of molecular cloning experiments and of metazoan genome (Caenorhabditis elegans, Drosophila melanogaster, Homo sapiens) and EST (expressed sequence tags) sequencing projects. The Drosophila Genome Sequencing Project will certainly have an important impact on insect signal transduction and receptor research. In parallel, convenient expression systems and functional assay procedures will be needed to investigate insect receptor properties and to monitor the effects of natural and artificial ligands. The study of the evolutionary aspects of G protein-coupled receptors and of their signaling pathways will probably reveal insect-specific features. More insight into these features may result in novel methods and practical applications. Arch.

The Drosophila melanogaster homologue of an insect calcitonin-like diuretic peptide stimulates V-ATPase activity in fruit fly Malpighian tubules

The Drosophila melanogaster homologue of an insect calcitonin-like diuretic hormone was identified in a BLAST search of the Drosophila genome database. The predicted 31-residue amidated peptide (D. melanogaster DH(31); Drome-DH(31)) was synthesised and tested for activity on fruit fly Malpighian tubules. It increases tubule secretion by approximately 35 % of the response obtained with a myokinin from the housefly Musca domestica (muscakinin; Musdo-K) and has an EC(50) of 4.3 nmol l(−)(1). The diuretic activities of Drome-DH(31) and Musdo-K were additive when tested at threshold and supra-maximal concentrations, which suggests that they target different transport processes. In support of this, Drome-DH(31) increased the rate of secretion by tubules held in bathing fluid with a reduced Cl(−) concentration, whereas Musdo-K did so only in the presence of Drome-DH(31). Stimulation with Drome-DH(31) increased the lumen-positive transepithelial potential in the main secretory segment of the tubule. This was attributed to activation of an apical electrogenic proton-translocating V-ATPase in principal cells, since it was associated with hyperpolarisation of the apical membrane potential and acidification of secreted urine by 0.25 pH units. Exogenous 8-bromo-cyclic AMP and cyclic GMP increased tubule secretion to the same extent as Drome-DH(31) and, when tested together with the diuretic peptide, their activities were not additive. Stimulation with Drome-DH(31) resulted in a dose-dependent increase in cyclic AMP production by tubules incubated in saline containing 0.5 mmol l(−)(1) 3-isobutyl-1-methylxanthine, whereas cyclic GMP production was unchanged. Taken together, the data are consistent with Drome-DH(31) activating an apical membrane V-ATPase via cyclic AMP. Since the K(+) concentration of the secreted urine was unchanged, it is likely that Drome-DH(31) has an equal effect on K(+) and Na(+) entry across the basolateral membrane.

Partial purification of a novel insect antidiuretic factor from the Colorado potato beetle, Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae), which acts on Malpighian tubules

In this study, we report for the first time the presence of an antidiuretic factor in the head of the Colorado potato beetle (Leptinotarsa decemlineata (Say)) which acts directly on Malpighian tubules. Biologically active fractions were isolated from the head and separated using molecular weight filtration and high-performance liquid chromatography (HPLC). The resulting fractions were tested for their antidiuretic activity on single isolated Malpighian tubules. Antidiuretic activity was found in the 25% acetonitrile Sep-Pak fraction and the Cn-2 (3000-10,000 MW) and Cn-3 (<3000 MW) fractions, suggesting that the antidiuretic factor was probably a peptide of 25 to 50 amino acids. The antidiuretic factor was very potent, since after five successive fractionations on two different HPLC columns, a high level of inhibition (63%) of fluid secretion by Malpighian tubules could be observed at low dose (0.14 head-equivalent/microl). The antidiuretic factor isolated from the head of the Colorado potato beetle was not affected by repeated freezing and thawing but was sensitive to heat. The differences observed between the Colorado potato beetle antidiuretic factor and other insect diuretic and antidiuretic factors may indicate the possibility of a novel family of water regulation hormones in insects.

Leucokinin and the modulation of the shunt pathway in Malpighian tubules

Transepithelial secretion in Malpighian tubules of the yellow fever mosquito (Aedes aegypti) is mediated by active transport of Na(+) and K(+) through principal cells and passive Cl(-) transport through the shunt. Permeation through the shunt was assessed by measuring transepithelial halide diffusion potentials in isolated perfused Malpighian tubules, after first inhibiting active transport with dinitrophenol. Diffusion potentials were small under control conditions, revealing Eisenman selectivity sequence I (I(-)>Br(-)>Cl(-)>F(-)) which is the halide mobility sequence in free solution. Accordingly, electrical field strengths of the shunt are small, selecting halides for passage on the basis of hydrated size. Leucokinin-VIII (LK-VIII) significantly increased the shunt conductance from 57.1 µS/cm to 250.0 µS/cm. In parallel, the shunt selectivity sequence shifted to Eisenman sequence III (Br(-)>Cl(-)>I(-)>F(-)), revealing increased electrical field strengths in the shunt, now capable of selecting small, dehydrated halides for passage. High concentrations of peritubular F(-) (142.5 mM) duplicated the effects of LK-VIII on shunt conductance and selectivity, suggesting a role for G-protein. In the presence of LK-VIII (or F(-)), coulombic interactions between the shunt and I(-) and F(-) may be strong enough to cause binding, thereby blocking the passage of Cl(-). Thus, LK-VIII increases both shunt conductance and selectivity, presumably via G-protein.

The distribution of a kinin-like peptide and its co-localization with a CRF-like peptide in the blood-feeding bug, Rhodnius prolixus

Rhodnius prolixus, a blood-feeding hemipteran insect, ingests large meals which are followed by rapid diuresis to eliminate excess water and salt. In Rhodnius, serotonin and an unidentified peptide(s) [33,34] have been shown to act as neurohormonal diuretic factors. In other insects, two families of diuretic peptides, the corticotropin releasing factor (CRF)-like, and kinin peptides [9], have been identified and sequenced. Recently, we demonstrated the presence of a CRF-like diuretic peptide in the CNS and digestive system of Rhodnius [47] using immunohistochemistry and bioassay. In this study, combining immunohistochemistry and radioimmunoassay (RIA) techniques, we show the presence of leucokinin-like peptide(s) in the CNS and digestive system of Rhodnius 5th instar. Additionally, double-label immunohistochemistry demonstrates that the leucokinin-like and CRF-like peptides are co-localized in the posterior lateral neurosecretory cells of the mesothoracic ganglionic mass (MTGM) and in neurohaemal areas on abdominal nerves one and two, suggesting the possibility of co-release of the peptides into the hemolymph.Partially purified extracts of the CNS and neurohaemal tissue were tested in vitro on Malpighian tubule secretion and cAMP assays. The factors eluting with increasing acetonitrile percentages from Sep-Pak cartridges were assayed in the presence or absence of ketanserin, a serotonin antagonist which blocks the effects of serotonin on Malpighian tubules. The results indicate activity of serotonin and a CRF-like diuretic peptide on Rhodnius Malpighian tubules, but fail to demonstrate activity of the leucokinin-like peptide(s). The rapid diuresis following feeding is a highly coordinated event, requiring the movement of water and salt across the epithelial cells of the crop into the hemolymph, and from the hemolymph across the cells of the Malpighian tubules. The urine then travels along the Malpighian tubules into the hindgut in order to be expelled. The presence of a leucokinin-like peptide(s) in the CNS and digestive system, which co-localizes with a CRF-like peptide(s), suggests that kinins may play a role in the rapid diuresis, although possibly not directly on the Malpighian tubules.

Diuresis in the housefly (Musca domestica) and its control by neuropeptides

Diuresis was studied in vivo by measuring the loss of tritiated water. The basal rate of water loss (5 nl/min) represents respiratory and cuticular losses, whereas higher rates reflect urine output, which reaches 20 nl/min after injection of 1 microl distilled water. This response to hypervolemia involves release of a diuretic hormone(s) into the hemolymph. However, housefly diuretic peptides increased urine output to a maximum of only 7 nl/min, and higher rates may require fluid reabsorption from the hindgut to be reduced. Diuresis is partially blocked by injected anti-muscakinin antibodies, providing evidence of a hormonal function for this insect myokinin.

Characterization of a leucokinin binding protein in Aedes aegypti (Diptera: Culicidae) Malpighian tubule

The insect myokinin (leucokinin-like) neuropeptide family includes peptides that have different physiological effects such as the induction of hindgut myotropic activity and stimulation of urine production. The C-terminal pentamer of myokinins Phe-X-(Ser/Pro/Ala)-Trp-Gly-amide [X=Phe, His, Asn, Ser or Tyr], had been previously determined as the minimum fragment able to elicit a functional response. The receptor(s) for these insect neuropeptides has not yet been identified. In order to characterize the Malpighian tubule leucokinin-like peptide receptor(s) from the yellow fever mosquito (Aedes aegypti), a leucokinin photoaffinity analogue (LPA) of sequence dAla-dTyr-Bpa-dLys-Phe-Phe-Ser-Trp-Gly-amide was designed based on structure/activity relationships for leucokinins. LPA caused depolarization of the transepithelial voltage (TEV) in female Malpighian tubule, confirming the activity of the peptide. The effective concentration to give half the maximum depolarization (EC(50)) was 17 nM. The (125)I-LPA was then used to characterize leucokinin binding proteins in female Malpighian tubule membranes. It specifically labeled and saturated a protein(s) of about 54 kDa as shown by SDS-PAGE/autoradiography and by competition experiments with excess unlabeled leucokinin analogues. (125)I-LPA bound to the 54 kDa protein(s) with a K(d) value of 13+/-3 nM in agreement with the EC(50) for the TEV bioassay. Altogether these data suggest that the 54 kDa protein is an Aedes-leucokinin receptor. This is the first characterization of an insect leucokinin receptor and reveals that LPA is a powerful tool to label insect myokinin receptors.

Effect of helicokinins and ACE inhibitors on water balance and development of Heliothis virescens larvae

The diuretic activity of the helicokinins I (YFSPWG-amide), II (VRFSPWG-amide) and III (KVKFSAWG-amide) was tested on Heliothis virescens larvae. All three kinins increased fluid secretion in isolated Malpighian tubules in a dose-dependent manner. Injections into the haemolymph caused a significant reduction in weight gain after 24 h and, in the case of helicokinin I, led to an increased mortality of 43% within 6 days. When truncated analogues of helicokinin I were tested in vitro, only the pentapeptide (FSPWG-amide) stimulated fluid secretion. Tested in vivo the pentapeptide did not influence normal development of the larvae. An alanine scan of helicokinin I showed that the substitution of phenylalanine, tryptophan and glycine led to a massive decrease or even loss of diuretic activity. The substitution of the other amino acids had no effect in vitro. The ACE inhibitors captopril, enalapril-maleate and lisinopril were tested for their influence on the development of the larvae. In combination with one of the helicokinins the in vivo injection of the ACE inhibitors led to increased rates of mortality and/or reductions in pupal weight.

Cloning and transcriptional expression of a leucokinin-like peptide receptor from the southern cattle tick, Boophilus microplus (Acari: Ixodidae)

Leucokinins are invertebrate neuropeptides that exhibit myotropic and diuretic activity. Only one leucokinin-like peptide receptor is known, the lymnokinin receptor from the mollusc Lymnaea stagnalis. A cDNA encoding a leucokinin-like peptide receptor was cloned from the Southern cattle tick, Boophilus microplus, a pest of cattle world-wide. This is the first neuropeptide receptor known from the Acari and the second known in the subfamily of leucokinin-like peptide G-protein-coupled receptors. The deduced amino acid sequence exhibits 40% identity to the lymnokinin receptor. The receptor transcript is present in all tick life stages as determined by semiquantitative reverse transcription polymerase chain reaction. We also propose that the sequence AAF50775.1 from the Drosophila melanogaster genome (CG10626) encodes the first identified insect leucokinin receptor.

Expression and functional characterization of a Drosophila neuropeptide precursor with homology to mammalian preprotachykinin A

Peptides structurally related to mammalian tachykinins have recently been isolated from the brain and intestine of several insect species, where they are believed to function as both neuromodulators and hormones. Further evidence for the signaling role of insect tachykinin-related peptides was provided by the cloning and characterization of cDNAs for two tachykinin receptors from Drosophila melanogaster. However, no endogenous ligand has been isolated for the Drosophila tachykinin receptors to date. Analysis of the Drosophila genome allowed us to identify a putative tachykinin-related peptide prohormone (prepro-DTK) gene. A 1.5-kilobase pair cDNA amplified from a Drosophila head cDNA library contained an 870-base pair open reading frame, which encodes five novel Drosophila tachykinin-related peptides (called DTK peptides) with conserved C-terminal FXGXR-amide motifs common to other insect tachykinin-related peptides. The tachykinin-related peptide prohormone gene (Dtk) is both expressed and post-translationally processed in larval and adult midgut endocrine cells and in the central nervous system, with midgut expression starting at stage 17 of embryogenesis. The predicted Drosophila tachykinin peptides have potent stimulatory effects on the contractions of insect gut. These data provide additional evidence for the conservation of both the structure and function of the tachykinin peptides in the brain and gut during the course of evolution.

Comparison of active conformations of the insectatachykinin/tachykinin and insect kinin/Tyr-W-MIF-1 neuropeptide family pairs

A comparison of solution conformations of active, restricted-conformation analogues of two sequence-similar insect/vertebrate neuropeptide family pairs shed light on the potential existence of molecular evolutionary relationships. Analogues of the locustatachykinins and the mammalian tachykinin substance P, containing a sterically hindered Aib-NMePhe/Tyr residue block, share similar low-energy turn conformations incorporating a cis peptide bond. Conversely, restricted conformation analogues of the insect kinins and the mammalian opiate peptide Tyr-W-MIF-1, with near identical C-terminal tetrapeptide sequences, adopt different conformations. The insect kinins adopt a cisPro 1-4 beta-turn, in which the Phe1 is critical for bioactivity. Tyr-W-MIF-1 prefers a transPro 2-5 turn, and an additional N-terminal Phe severely inhibits mu-opiate receptor binding. Comparisons of the chemical/conformational requirements for receptor interaction are consistent with a distant evolutionary relationship between the insectatachykinins and tachykinins, but not between the insect kinins and Tyr-W-MIF-1. Therefore, analogues of the insect kinins with pest control potential can be readily designed to avoid mammalian interactions.

Mono- and dibasic proteolytic cleavage sites in insect neuroendocrine peptide precursors

Regulatory peptides are synthesized as part of larger precursors that are subsequently processed into the active substances. After cleavage of the signal peptide, further proteolytic processing occurs predominantly at basic amino acid residues. Rules have been proposed in order to predict which putative proteolytic processing sites are actually used, but these rules have been established for vertebrate peptide precursors and it is unclear whether they are also valid for insects. The aim of this paper is to establish the validity of these rules to predict proteolytic cleavage sites at basic amino acids in insect neuropeptide precursors. Rules describing the cleavage of mono- and dibasic potential processing sites in insect neuropeptide precursors are summarized below. Lys-Arg pairs not followed by an aliphatic or basic amino acid residue are virtually always cleaved in insect regulatory peptide precursors, but cleavages of Lys-Arg pairs followed by either an aliphatic or a basic amino acid residue are ambiguous, as is processing at Arg-Arg pairs. Processing at Arg-Lys pairs has so far not been demonstrated in insects and processing at Lys-Lys pairs appears very rare. Processing at single Arg residues occurs only when there is a basic amino acid residue in position -4, -6, or -8, usually an Arg, but Lys or His residues work also. Although the current number of such sites is too limited to draw definitive conclusions, it seems plausible that cleavage at these sites is inhibited by the presence of aliphatic residues in the +1 position. However, cleavage at single Arg residues is ambiguous. When several potential cleavage sites overlap the one most easily cleaved appears to be processed. It cannot be excluded that some of the rules formulated here will prove less than universal, as only a limited number of cleavage sites have so far been identified. It is likely that, as in vertebrates, ambiguous processing sites exist to allow differential cleavage of the same precursor by different convertases and it seems possible that the precursors of allatostatins and PBAN are differentially cleaved in different cell types. Arch. Insect Biochem. Physiol. 43:49-63, 2000.

Post-translational modifications of the insect sulfakinins: sulfation, pyroglutamate-formation and O-methylation of glutamic acid

We identified and chemically characterized the two major forms of sulfakinins from an extract of 800 corpora cardiaca/corpora allata complexes of the American cockroach, Periplaneta americana. Bioactivity during the purification was monitored by measuring heart beat frequency in a preparation in situ. By Edman degradation analysis and MS, these main forms were identified as having the primary structures Pea-SK [EQFDDY(SO(3)H)GHMRFamide] and Lem-SK-2 [pQSDDY(SO(3)H)GHMRFamide]. The sulfation was confirmed by UV, MS and peptide synthesis. In addition, post-translationally modified sulfakinins of both major forms were isolated and identified. Firstly, nonsulfated forms of these peptides are present in considerable amounts in the corpora cardiaca/allata. Secondly, the N-terminally blocked Pea-SK and the nonblocked Lem-SK-2 occur naturally in neurohaemal release sites. Thirdly, modified Pea-SK with O-methylated glutamic acid occurs which is not an artefact of peptide purification. The major forms of the sulfakinins were shown to be highly active on both the heart and hindgut with threshold concentrations of approximately 5 x 10(-10) M (heart) and 2 x 10(-9) M (hindgut).