Enhanced oral availability/pheromonotropic activity of peptidase-resistant topical amphiphilic analogs of pyrokinin/PBAN insect neuropeptides

Bee-safe peptidomimetic acaricides achieved by comparative genomics

The devastating Varroa mite (Varroa destructor Anderson and Trueman) is an obligatory ectoparasite of the honey bee, contributing to significant colony losses in North America and throughout the world. The limited number of conventional acaricides to reduce Varroa mites and prevent disease in honey bee colonies is challenged with wide-spread resistance and low target-site selectivity. Here, we propose a biorational approach using comparative genomics for the development of honey bee-safe and selective acaricides targeting the Varroa mite-specific neuropeptidergic system regulated by proctolin, which is lacking in the honey bee. Proctolin is a highly conserved pentapeptide RYLPT (Arg-Tyr-Leu-Pro-Thr) known to act through a G protein-coupled receptor to elicit myotropic activity in arthropod species. A total of 33 different peptidomimetic and peptide variants were tested on the Varroa mite proctolin receptor. Ligand docking model and mutagenesis studies revealed the importance of the core aromatic residue Tyr2 in the proctolin ligand. Peptidomimetics were observed to have significant oral toxicity leading to the paralysis and death of Varroa mites, while there were no negative effects observed for honey bees. We have demonstrated that a taxon-specific physiological target identified by advanced genomics information offers an opportunity to develop Varroa mite-selective acaricides, hence, expedited translational processes.

Tick CAPA propeptide cDNAs and receptor activity of endogenous tick pyrokinins and analogs: Towards discovering pyrokinin function in ticks

Pyrokinins (PKs) are pleiotropic neuropeptides with significant roles in invertebrate physiology. Although functions of PKs are known in insects, there is a lack of knowledge of PK-encoding genes and PKs functions in ticks. Herein the first tick cDNAs of the capability (capa) gene were cloned from the southern cattle tick, Rhipicephalus microplus (Acari: Ixodidae), and the blacklegged tick, Ixodes scapularis. Each cDNA encoded one periviscerokinin and five different pyrokinins. Two PKs were identical in sequence in the two species. The three PKs unique to R. microplus (Rhimi-CAPA-PK1, -PK2, and -PK5) were tested on the recombinant R. microplus pyrokinin receptor using a calcium bioluminescence assay. The Rhimi-CAPA-PKs acted as agonists with EC_50s ranging from 101-188 nM. Twenty PK analogs designed for enhanced bioavailability and biostability were tested on the receptor. Five of these were designed based on the sequences of the three unique Rhimi-CAPA-PKs. Eight PK analogs were also agonists; four of them were full agonists that exhibited comparable efficacy to the native Rhimi-CAPA-PKs, with EC₅₀ ranging from 401 nM-1.9 μM. The structure-activity relationships (SAR) of all analogs were analyzed. Our results suggested that a positively charged, basic lysine at the variable position X of the PK active core (FXPRLamide) conferred enhanced affinity to the analogs in their interaction with the tick receptor. These analogs are promising tools to elucidate the pyrokinin function in ticks in vivo as these analogs are expected to have prolonged hemolymph residence time in comparison to the native peptides.

How do oral insecticidal compounds cross the insect midgut epithelium?

The use of oral insecticidal molecules (small molecules, peptides, dsRNA) via spray or plant mediated applications represents an efficient way to manage damaging insect species. With the exception of Bt toxins that target the midgut epithelium itself, most of these compounds have targets that lie within the hemocoel (body) of the insect. Because of this, one of the greatest factors in determining the effectiveness of an oral insecticidal compound is its ability to traverse the gut epithelium and enter the hemolymph. However, for many types of insecticidal compounds, neither the pathway taken across the gut nor the specific genes which influence uptake are fully characterized. Here, we review how different types of insecticidal compounds enter or cross the midgut epithelium through passive (diffusion) or active (transporter based, endocytosis) routes. A deeper understanding of how insecticidal molecules cross the gut will help to best utilize current insecticides and also provide for more rational design of future ones.

Evaluation of Chemical Strategies for Improving the Stability and Oral Toxicity of Insecticidal Peptides

Spider venoms are a rich source of insecticidal peptide toxins. Their development as bioinsecticides has, however, been hampered due to concerns about potential lack of stability and oral bioactivity. We therefore systematically evaluated several synthetic strategies to increase the stability and oral potency of the potent insecticidal spider-venom peptide ω-HXTX-Hv1a (Hv1a). Selective chemical replacement of disulfide bridges with diselenide bonds and N- to C-terminal cyclization were anticipated to improve Hv1a resistance to proteolytic digestion, and thereby its activity when delivered orally. We found that native Hv1a is orally active in blowflies, but 91-fold less potent than when administered by injection. Introduction of a single diselenide bond had no effect on the susceptibility to scrambling or the oral activity of Hv1a. N- to C-terminal cyclization of the peptide backbone did not significantly improve the potency of Hv1a when injected into blowflies and it led to a significant decrease in oral activity. We show that this is likely due to a dramatically reduced rate of translocation of cyclic Hv1a across the insect midgut, highlighting the importance of testing bioavailability in addition to toxin stability.

pH-responsive polymer microcapsules for targeted delivery of biomaterials to the midgut of Drosophila suzukii

Drosophila suzukii or spotted wing Drosophila is an economically important pest which can have a devastating impact on soft and stone fruit industries. Biological pesticides are being sought as alternatives to synthetic chemicals to control this invasive pest, but many are subject to degradation either in the environment or in the insect gut and as a result require protection. In this study we identified a sharp change in pH of the adult midgut from neutral to acidic (pH <3), which we then exploited to develop poly(2-vinylpyridine) (P2VP) microcapsules that respond to the change in midgut pH by dissolution and release of their cargo for uptake into the insect. First, we used labelled solid poly(methyl methacrylate) (PMMA) particles to show that microcapsules with a diameter less than 15 μm are readily ingested by the adult insect. To encapsulate water-soluble biological species in an aqueous continuous phase, a multiple emulsion template was used as a precursor for the synthesis of pH-responsive P2VP microcapsules with a fluorescent (FITC-dextran) cargo. The water-soluble agent was initially separated from the aqueous continuous phase by an oil barrier, which was subsequently polymerised. The P2VP microcapsules were stable at pH > 6, but underwent rapid dissolution at pH < 4.2. In vivo studies showed that the natural acidity of the midgut of D. suzukii also induced the breakdown of the responsive P2VP microcapsules to release FITC-dextran which was taken up into the body of the insect and accumulated in the renal tubules.

Overexpression of TAT-PTD-diapause hormone fusion protein in tobacco and its effect on the larval development of Helicoverpa armigera (Lepidoptera: Noctuidae)

BACKGROUND

The diapause hormone (DH) has been shown either to induce or to terminate diapause, depending on the insect species. In a previous study we demonstrated that the DH from Clostera anastomosis (caDH) has biological activity in Helicoverpa armigera, which prompted us to examine the potential growth-inhibiting or antiherbivory effects of the TAT-caDH fusion protein when expressed in transgenic plants.

RESULTS

In this study, we produced transgenic tobacco plants expressing either the TAT-caDH protein or a TAT-caDH-eGFP fusion version that allowed tracking of the fluorescent protein in plant tissues. Our results indicate that H. armigera larvae feeding on transgenic tobacco expressing TAT-caDH exhibited a significantly reduced survival rate and weight gain. However, larvae feeding on transgenic tobacco expressing TAT-caDH-eGFP were unaffected. While fusion of the eGFP gene influenced the bioactivity of caDH in larvae, TAT-caDH-eGFP can still penetrate the insect midgut cell membrane.

CONCLUSION

TAT-caDH increases DH stability in oral delivery. Our results may help in targeting DH-dependent physiological processes in insects for improving herbivore tolerance in economically important crops. © 2016 Society of Chemical Industry.

Development of neuropeptide analogs capable of traversing the integument: A case study using diapause hormone analogs in Helicoverpa zea

Diapause hormone and its analogs terminate pupal diapause in Helicoverpa zea when injected, but if such agents are to be used as effective diapause disruptors it will be essential to develop simple techniques for administering active compounds that can exert their effect by penetrating the insect epidermis. In the current study, we used two molecules previously shown to have high diapause-terminating activity as lead molecules to rationally design and synthesize new amphiphilic compounds with modified hydrophobic components. An assay for diapause termination identified 13 active compounds with EC50's ranging from 0.9 to 46.0 pmol per pupa. Three compounds, Decyl-1963, Dodecyl-1967, and Heptyl-1965, selected from the 13 compounds most active in breaking diapause following injection, also successfully prevented newly-formed pupae from entering diapause when applied topically. These compounds feature straight-chain, aliphatic hydrocarbons from 7 to 12 carbons in length; DH analogs with either a short-chain length of 4 or an aromatic phenethyl group failed to act topically. Compared to a high diapause incidence of 80-90% in controls, diapause incidence in pupae receiving a 10 nmole topical application of Decyl-1963, Dodecyl-1967, or Heptyl-1965 dropped to 30-45%. Decyl-1963 and Dodecyl-1967 also remained effective when topically applied at the 1 nmole level. These results suggest the feasibility of developing DH agonists that can be applied topically and suggest the identity of new lead molecules for development of additional topically-active DH analogs. The ability to penetrate the insect epidermis and/or midgut lining is critical if such agents are to be considered for future use as pest management tools.

Oral Administration of TAT-PTD-Diapause Hormone Fusion Protein Interferes With Helicoverpa armigera (Lepidoptera: Noctuidae) Development

Diapause hormone (DH), which can terminate diapause in Helicoverpa armigera Hübner (Lepidoptera: Noctuidae), has shown promise as a pest control method. However, the main challenge in using DH as an insecticide lies in achieving effective oral delivery, since the peptide may be degraded by digestive enzymes in the gut. To improve the efficacy of oral DH application, the Clostera anastomosis (L.) (Lepidoptera: Notodontidae) diapause hormone (caDH) was fused to the Protein Transduction Domain (PTD) of the human immunodeficiency virus-1 transactivator of transcription (TAT). Cellular transduction of TAT-caDH was verified with the use of a green fluorescent protein fusion, and its ability to terminate diapause was verified by injection into diapausing H. armigera pupae. Orally administered TAT-caDH resulted in larval growth inhibition. In TAT-caDH-treated insects, larval duration was delayed and the pupation rates were decreased at both development promoting conditions [27 °C, a photoperiod of 14:10(L:D) h] and diapause inducing conditions [20 °C, a photoperiod of 10:14(L:D) h]. No significant difference in diapause rate was observed between the TAT-caDH-treated and caDH-treated or control pupae maintained at diapause inducing conditions. Our results show that treatment with a recombinant TAT-caDH protein can affect larval development in H. armigera, and it suggest that TAT-DH treatment may be useful for controlling pests. This study is the first record of oral DH application in insect.

Active diuretic peptidomimetic insect kinin analogs that contain β-turn mimetic motif 4-aminopyroglutamate and lack native peptide bonds

The multifunctional 'insect kinins' of arthropods share the evolutionarily conserved C-terminal pentapeptide core sequence Phe-X(1)-X(2)-Trp-Gly-NH(2), where X(1)=His, Asn, Ser, or Tyr and X(2)=Ser, Pro, or Ala. Insect kinins regulate diuresis in many species of insects, including the house cricket, Acheta domesticus. Insect kinins, however, are susceptible to fast enzymatic degradation by endogenous peptidases that severely limit their potential use as tools for pest control or for endocrinological studies. To enhance resistance to peptidases, the core insect kinin sequence was structurally modified in this study to replace native peptide bonds susceptible to proteolytic degradation. These modifications include incorporation of two stereochemical variants of the β-turn mimetic motif 4-aminogutamate in place of the X(1)-X(2) residues, insertion of a reduced peptide bond between residues Trp-Gly, and replacement of the Phe residue with a hydrocinnamyl group. The resulting biostable, peptidomimetic analogs contain no native peptide bonds and yet retain significant diuretic activity in an in vitro cricket Malpighian tubule fluid secretion assay, matching the efficacy of a native A. domesticus kinin (Achdo-KI). These novel analogs represent ideal new tools for endocrinologists studying arthropod kinin regulated processes in vivo, and provide leads in the development of novel, environmentally friendly pest insect management agents capable of disruption of the critical processes that kinins regulate.

The CPP Tat enhances eGFP cell internalization and transepithelial transport by the larval midgut of Bombyx mori (Lepidoptera, Bombycidae)

Cell-Penetrating Peptides (CPPs) are short peptides that are able to translocate across the cell membrane a wide range of cargoes. In the past decade, different mammalian cell lines have been used to clarify the mechanism of CPPs penetration and to characterize the internalization process, which has been described either as an energy-independent direct penetration through the plasma membrane, or as endocytic uptake. Whatever the mechanism involved, the cell penetration properties of these peptides make their use very attractive as vector for promoting the cellular uptake of coupled bioactive macromolecules, such as peptides, proteins and oligonucleotides. Here we demonstrate, for the first time in insect, that cultured columnar cells from the larval midgut of Bombyx mori more readily internalize eGFP (enhanced Green Fluorescent Protein) when fused to CPP Tat. Tat-eGFP translocates across the plasma membrane of absorptive cells in an energy-independent and non-endocytic manner, since no inhibition of the fusion protein uptake is exerted by metabolic inhibitors and by drugs that interfere with the endocytic uptake. Moreover, the CPP Tat enhances the internalization of eGFP in the columnar cells of intact midgut tissue, mounted in a suitable perfusion apparatus, and the transepithelial flux of the protein. These results open new perspectives for effective delivery of insecticidal macromolecules targeting receptors located both within the insect gut epithelium and behind the gut barrier, in the hemocoel compartment.

Disruption of insect diapause using agonists and an antagonist of diapause hormone

The dormant state known as diapause is widely exploited by insects to circumvent winter and other adverse seasons. For an insect to survive, feed, and reproduce at the appropriate time of year requires fine coordination of the timing of entry into and exit from diapause. One of the hormones that regulates diapause in moths is the 24-aa neuropeptide, diapause hormone (DH). Among members of the Helicoverpa/Heliothis complex of agricultural pests, DH prompts the termination of pupal diapause. Based on the structure of DH, we designed several agonists that are much more active than DH in breaking diapause. One such agonist that we describe also prevents the entry into pupal diapause when administered to larvae that are environmentally programmed for diapause. In addition, we used the unique antagonist development strategy of incorporating a dihydroimidazole ("Jones") trans-Proline mimetic motif into one of our DH agonists, thereby converting the agonist into a DH antagonist that blocks the termination of diapause. These results suggest potential for using such agents or next-generation derivatives for derailing the success of overwintering in pest species.

Oral activity of FMRFamide-related peptides on the pea aphid Acyrthosiphon pisum (Hemiptera: Aphididae) and degradation by enzymes from the aphid gut

Insect myosuppressins and myosuppressin analogues were tested for oral toxicity against the pea aphid Acyrthosiphon pisum (Harris) by incorporation into an artificial diet. Acyrthosiphon pisum myosuppressin (Acypi-MS) and leucomyosuppressin (LMS) had significant dose-dependent effects (0.1-0.5μg peptide/μl diet) on feeding suppression, mortality, reduced growth and fecundity compared with control insects, but Acypi-MS was more potent than LMS. One hundred percent of aphids had died after 10days of feeding on 0.5μg Acypi-MS/μl diet whereas 40% of aphids feeding on 0.5μg LMS/μl diet were still alive after 13days. Myosuppressins were degraded by aphid gut enzymes; degradation was most likely due to a carboxypeptidase-like protease, an aminopeptidase and a cathepsin L cysteine protease. The estimated half-life of Acypi-MS in a gut extract was 30min, whereas LMS was degraded more slowly (t½=54min). No toxicity was observed when the analogues δR(9) LMS and citrolline(9) Acypi-MS or FMRFamide were fed to the pea aphid. These findings not only help to better understand the biological effects of myosuppressins in aphids but also demonstrate the potential use of myosuppressins in a strategy to control aphid pests.

Neuropeptide physiology in insects

In a search for more environmentally benign alternatives to chemical pesticides, insect neuropeptides have been suggested as ideal candidates. Neuropeptides are neuromodulators and/or neurohormones that regulate most major physiological and behavioral processes in insects. The major neuropeptide structures have been identified through peptide purification in insects (peptidomics) and insect genome projects. Neuropeptide receptors have been identified and characterized in Drosophila and similar receptors are being targeted in other insects considered to be economically detrimental pests in agriculture and forestry. Defining neuropeptide action in different insect systems has been more challenging and as a consequence, identifying unique targets for potential pest control is also a challenge. In this chapter, neuropeptide biosynthesis as well as select physiological processes are examined with a view to pest control targets. The application of molecular techniques to transform insects with neuropeptide or neuropeptide receptor genes, or knockout genes to identify potential pest control targets, is a relatively new area that offers promise to insect control. Insect immune systems may also be manipulated through neuropeptides which may aid in compromising the insects ability to defend against foreign invasion.

Effects of Manduca sexta allatostatin and an analogue on the peach-potato aphid Myzus persicae (hemiptera: aphididae) and degradation by enzymes in the aphid gut

The oral toxicity of the C-type allatostatin, Manduca sexta allatostatin (Manse-AS) and the analogue δR³δR⁵Manse-AS, where R residues were replaced by their D-isomers, were tested against the peach-potato aphid Myzus persicae by incorporation into an artificial diet. Both peptides had significant dose-dependent effects on mortality, growth, and fecundity compared with control insects. The analogue, δR³δR⁵Manse-AS, had an estimated LC₅₀ of 0.31 µg/µl diet and was more potent than Manse-AS (estimated LC₅₀ of 0.58 µg/µl diet). At a dose of 0.35 µg δR³δR⁵Manse-AS/µl diet, 76% of the aphids were dead after 6 days and all were dead after 10 days. In comparison, three times the dose of Manse-AS was required to achieve 74% mortality after 8 days and 98% mortality after 16 days. The degradation of both peptides by extracts prepared from the gut of M. persicae was investigated. The estimated half-life of Manse-AS, when incubated with the gut extract from M. persicae, was 31 min. Degradation was due to a cathepsin L-like cysteine protease, carboxypeptidase-like activity, endoprotease activity with glutamine specificity, pyroglutamate aminopeptidase activity, and possibly trypsin-like proteases. The half-life of the δR³δR⁵ Manse-AS analogue was enhanced (73 min) with the D-isomers of R appearing to prevent cleavage around the R residues by cathepsin L-like cysteine proteases or from trypsin-like proteases. The greater stability of the analogue may explain its increased potency in M. persicae. This work demonstrates the potential use of Manse-AS and analogues, with greater resistance to enzymatic attack, in aphid control strategies.

Effects of Manduca sexta allatostatin and an analog on the pea aphid Acyrthosiphon pisum (Hemiptera: Aphididae) and degradation by enzymes from the aphid gut

The C-type allatostatin, Manduca sexta allatostatin (Manse-AS) and the analog delta R(3)delta R(5)Manse-AS, where R residues were replaced by their d-isomers, were tested for oral toxicity against the pea aphid Acyrthosiphon pisum (Harris) by incorporation into an artificial diet. Both peptides had significant dose-dependent feeding suppression effects, resulting in mortality, reduced growth and fecundity compared with control insects. The delta R(3)delta R(5)Manse-AS analog had an estimated LC(50) of 0.18 microg/microl diet, and was more potent than Manse-AS. At a dose of 0.35 microg delta R(3)delta R(5)Manse-AS/microl diet, 98% of aphids were dead within 3 days, at a rate similar to those aphids that had been starved (no diet controls). On comparison, it required 13 days and three times the dose of Manse-AS fed to aphids to attain 96% mortality. It is possible that the feeding suppression effects of Manse-AS on aphids are due to the inhibition of gut motility. The estimated half-life of Manse-AS when incubated with a gut extract from A. pisum was 54 min. Degradation was most likely due to cathepsin L cysteine and/or trypsin-like proteases, by an unidentified glutamine-specific protease and by a carboxypeptidase-like enzyme. The d-isomers of R in the Manse-AS analog appeared to prevent hydrolysis by cathepsin L cysteine and trypsin-like enzymes, and enhance its half-life (145 min). However delta R(3)delta R(5)Manse-AS was cleaved by enzymes with carboxypeptidase-like and chymotrypsin-like activity. The increased stability of the Manse-AS analog may explain its enhanced feeding suppression effects when continually fed to aphids, and demonstrates the potential use of Manse-AS in a strategy to control aphid pests.

Neuropeptide signaling in insects

Neuropeptides represent the largest single class of signal compounds and are involved in regulation of development, growth, reproduction, metabolism and behavior of insects. Over the last few years there has been a tremendous increase in our knowledge of neuropeptide signaling due to genome sequencing, peptidomics, gene micro arrays, receptor characterization and targeted gene interference combined with physiological and behavior analysis. In this chapter we review the current knowledge of structure and distribution of insect neuropeptides and their receptors, as well as their diverse functions. We also discuss peptide biosynthesis, processing and expression, as well as classification of insect neuropeptides. Special attention is paid to the role insect neuropeptides play as potential targets for pest management and as a basis for development of insect control agents employing the rational/structural design approaches.

Bioavailability of beta-amino acid and C-terminally derived PK/PBAN analogs

The ability of linear beta-amino acid substituted peptides (PK-betaA-1: Ac-YFT[beta(3)P]RLa; PK-betaA-2: Ac-Y[beta(3)homoF]TPRLa; PK-betaA-3: Ac-Y[beta(3)F]TPRLa; PK-betaA-4: Ac-[beta(3)F]FT[beta(3)P]RLa) and unsubstituted analogs (Ac-YFTPRLa and YFTPRLa) of the pyrokinin(PK)/pheromone biosynthesis-activating neuropeptide (PBAN) family to penetrate the insect cuticle and exert biological activity (i.e., stimulate sex pheromone biosynthesis), was tested by topical application on Heliothis peltigera moths. The present results clearly indicate that small linear synthetic peptides can penetrate the cuticle very efficiently by contact application and activate their target organ. The time responses of the peptides applied in DDW and DMSO were tested and the activities of topically applied and injected peptides were compared. The results clearly indicate that PK-betaA-4 and PK-betaA-3 exhibited high bioavailability (ability to penetrate through the cuticle and exertion of bioactivity) with the latter showing longer persistence in both solvents than any other analog in the study; indicative that incorporation of a beta-amino acid at the Phe(2) position can enhance longevity in topical PK/PBAN analogs. PK-betaA-4 was significantly more active in DMSO than in DDW, and significantly more active than the parent peptide LPK in DMSO. PK-betaA-1 and PK-betaA-2 exhibited negligible activity. Interestingly, Ac-YFTPRLa was highly potent in both solvents; its activity in DDW did not differ from that of PK-betaA-4 and PK-betaA-3, and was higher than that of LPK. Even the unacylated peptide YFTPRLa was active in both solvents, at a similar level to LPK. Topically applied PK-betaA-4 and Ac-YFTPRLa exhibited significantly higher activity than the injected peptides. PK-betaA-3 and YFTPRLa were equally potent in both routes of administration.

Bioavailability of insect neuropeptides: the PK/PBAN family as a case study

The ability of unmodified linear peptides to penetrate the insect cuticle and exert bioactivity (e.g., stimulation of sex pheromone biosynthesis) was tested by topical application onto Heliothis peltigera moths of four insect neuropeptides (Nps) of the pyrokinin (PK)/pheromone biosynthesis activating neuropeptide (PBAN) family: Helicoverpa zea PBAN (Hez-PBAN), Pseudaletia (Mythimna) separata pheromonotropin (PT), Leucophaea maderae PK (LPK) and Locusta migratoria myotropin (Lom-MT-II). The time kinetic of the peptides applied in double distilled water (DDW) or dimethylsulfoxide (DMSO) was tested and the activities of topically applied and injected peptides were compared. The results clearly indicated that all four peptides were highly potent but with differing activities in the two solvents: PBAN was most active in water, and PT in DMSO. The activity of PBAN in DDW lasted up to 8h post-application and its activity in this solvent showed a faster onset and a longer persistence than in DMSO. LPK and MT differed less in their kinetics between the two solvents. Topically applied PBAN at 1 nmol exhibited an equivalent or even significantly higher potency than the injected peptide at several different times post-treatment. Similar results were obtained with topically applied and injected LPK. The present results add important information on the bioavailability of unmodified linear peptides in moths, clearly indicate that linear hydrophilic peptides can penetrate the cuticle by contact application in aqueous solutions and in organic solvents very efficiently, reach their target organ and activate it.

Conformational aspects and hyperpotent agonists of diapause hormone for termination of pupal diapause in the corn earworm

Diapause hormone (DH) is a peptide well known to induce embryonic diapause in the commercial silkmoth Bombyx mori. More recently, this same neuropeptide was reported to break diapause in pupae of the agriculturally important Heliothis/Helicoverpa complex. In this study we examine the efficacy and potency of a select group of structural analogs of the native hormone in Helicoverpa zea and report the structures of several analogs that are considerably more potent than DH in breaking diapause. Among the most potent analogs (PK-Etz, PK-2Abf, 901) were those with structural components that enhance resistance to peptidases that degrade and inactivate the native peptide in vivo, which may account, at least in part, for the observed increase in potency for these analogs. Analog 901 was previously demonstrated to both enhance biostablility and bioavailability properties in adult heliothines and thus may be a potential candidate for topical application as a diapause-terminating agent. The significant activity observed for two restricted conformation analogs is consistent with an active conformation for diapause hormone that features a transPro within a type I beta-turn in the C-terminal region. DH is also known to successfully break diapause only within a fairly narrow temperature range. While DH is effective at 21 degrees C, it is not effective at 18 degrees C. Likewise, the analogs were effective at 21 degrees C but not at 18 degrees C. By contrast, 20-hydroxyecdysone, a steroid hormone that is also capable of breaking diapause is effective at both temperatures, thus suggesting that DH and the ecdysteroids act through different mechanisms to terminate diapause.

Biostable beta-amino acid PK/PBAN analogs: agonist and antagonist properties

The pyrokinin/pheromone biosynthesis activating neuropeptide (PK/PBAN) family plays a significant role in a multifunctional array of important physiological processes in insects. PK/PBAN analogs incorporating beta-amino acids were synthesized and evaluated in a pheromonotropic assay in Heliothis peltigera, a melanotropic assay in Spodoptera littoralis, a pupariation assay in Neobellieria bullata, and a hindgut contractile assay in Leucophaea maderae. Two analogs (PK-betaA-1 and PK-betaA-4) demonstrate greatly enhanced resistance to the peptidases neprilysin and angiotensin converting enzyme that are shown to degrade the natural peptides. Despite the changes to the PK core, analog PK-betaA-4 represents a biostable, non-selective agonist in all four bioassays, essentially matching the potency of a natural PK in pupariation assay. Analog PK-betaA-2 is a potent agonist in the melanotropic assay, demonstrating full efficacy at 1pmol. In some cases, the structural changes imparted to the analogs modify the physiological responses. Analog PK-betaA-3 is a non-selective agonist in all four bioassays. The analog PK-betaA-1 shows greater selectivity than parent PK peptides; it is virtually inactive in the pupariation assay and represents a biostable antagonist in the pheromonotropic and melanotropic assays, without the significant agonism of the parent hexapeptide. These analogs provide new, and in some cases, biostable tools to endocrinologists studying similarities and differences in the mechanisms of the variety of PK/PBAN mediated physiological processes. They also may provide leads in the development of PK/PBAN-based, insect-specific pest management agents.

An amphiphilic, PK/PBAN analog is a selective pheromonotropic antagonist that penetrates the cuticle of a heliothine insect

A linear pyrokinin (PK)/pheromone biosynthesis activating neuropeptide (PBAN) antagonist lead (RYF[dF]PRLa) was structurally modified to impart amphiphilic properties to enhance its ability to transmigrate the hydrophobic cuticle of noctuid moth species and yet retain aqueous solubility in the hemolymph to reach target PK/PBAN receptors within the internal insect environment. The resulting novel PK/PBAN analog, Hex-Suc-A[dF]PRLa (PPK-AA), was synthesized and evaluated as an antagonist in a pheromonotropic assay in Heliothis peltigera against 4 natural PK/PBAN peptide elicitors (PBAN; pheromonotropin, PT; myotropin, MT; leucopyrokinin, LPK) and in a melanotropic assay in Spodoptera littoralis against 3 natural PK/PBAN peptide elicitors (PBAN, PT, LPK). The analog proved to be a potent and efficacious inhibitor of sex pheromone biosynthesis elicited by PBAN (84% at 100 pmol) and PT (54% at 100 pmol), but not by MT and LPK. PPK-AA is a selective pure antagonist (i.e., does not exhibit any agonistic activity) as it failed to inhibit melanization elicited by any of the natural PK/PBAN peptides. The analog was shown to transmigrate isolated cuticle dissected from adult female Heliothis virescens moths to a high extent of 25-30% (130-150 pmol), representing physiologically significant quantities. PPK-AA represents a significant addition to the arsenal of tools available to arthropod endocrinologists studying the endogenous mechanisms of PK/PBAN regulated processes, and a prototype for the development of environmentally friendly pest management agents capable of disrupting the critical process of reproduction.

Biological activity and identification of neuropeptides in the neurosecretory complexes of the cabbage pest insect, Mamestra brassicae (Noctuidae; Lepidoptera)

The need for more environmentally sound strategies of plant protection has become a driving force in physiological entomology to combat insect pests more efficiently. Since neuropeptides regulate key biological processes, these "special agents" or their synthetic analogues, mimetics, agonists or antagonists may be useful tools. We examined brain-suboesophageal ganglia and corpora cardiaca-corpora allata complexes of the cabbage moth, Mamestra brassicae, in order to obtain clues about possible peptide candidates which may be appropriate for the biological control of this pest. With the aid of bioassays, reversed phase high performance liquid chromatography, and mass spectrometry, five neuropeptides were unequivocally identified and the presence of a further three were inferred solely by comparing mass spectra with known peptides. Only one neuropeptide with adipokinetic capability was identified in M. brassicae. Data from the established homologous bioassay indicated that the cabbage moths rely on a lipid-based metabolism which is aided by an adipokinetic hormone (viz. Manse-AKH) that had previously been isolated in many different lepidopterans. Other groups of neuropeptides identified in this study are: FLRFamides, corazonin, allatostatin and pheromonotropic peptide.

Transepithelial flux of an allatostatin and analogs across the anterior midgut of Manduca sexta larvae in vitro

The transepithelial flux of cydiastatin 4 and analogs across flat sheet preparations of the anterior midgut of larvae of the tobacco hawkmoth moth, Manduca sexta, was investigated using a combination of reversed-phase high-performance liquid chromatography (RP-HPLC), enzyme-linked immunosorbent assay (ELISA) and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). The lumen to hemolymph (L-H) flux of cydiastatin 4 was dose and time-dependent, with a maximum rate of flux of c. 178 pmol/cm2/h) measured after a 60-min incubation with 100 micromol/l of peptide in the lumen bathing fluid. The rates of flux, L-H and H-L, across the isolated gut preparations were not significantly different. These data suggest that uptake across the anterior midgut of larval M. sexta is via a paracellular route. Cydiastatin 4 was modified to incorporate a hexanoic acid (Hex) moiety at the N-terminus, the N-terminus extended with 5 P residues and/or the substitution of G7 with Fmoc-1-amino-cyclopropylcarboxylic acid (Acpc). The incorporation of hexanoic acid enhanced the uptake of these amphiphilic analogs compared to the native peptide. Analogs were also more resistant to enzymes in hemolymph and gut preparations from larval M. sexta. A modified N-terminus gave protection against aminopeptidase-like activity and incorporation of Acpc inhibited endopeptidase-like activity. Although analogs were stable in the hemolymph, they were susceptible to amidase-like activity in the gut, which appears to convert the C-terminal amide group to a free carboxylic acid, identified by an increase in 1 mass unit of the peptide analog.

The movement of proteins across the insect and tick digestive system

The movement of intact proteins across the digestive system was shown in a number of different blood-feeding and non-blood-feeding insects in the orders Blattaria, Coleoptera, Diptera, Hemiptera, Lepidoptera, Orthoptera, Neuroptera and Siphonaptera, as well as in two tick families Ixodidae and Argasidae. Protein movement was observed for both normal dietary and xenobiotic proteins, which suggest that the mechanism for transfer is not substrate specific. The number of studies on the mechanism of movement is limited. The research so far suggests that movement can occur by either a transcellular or an intercellular pathway in the ventriculus with most of the research describing the former. Transfer is by continuous diffusion with no evidence of pinocytosis or vesicular transport common in mammalian systems. Proteins can move across the digestive system without modification of their primary or multimeric structure and with retention of their functional characteristics. Accumulation in the hemolymph is the result of the protein degradation rate in the gut and hemolymph and transfer rate across the digestive system and can be highly variable depending on species. Research on the development of delivery systems to enhance protein movement across the insect digestive system is in its infancy. The approaches so far considered with some success include the use of lipophilic-polyethylene glycol (PEG) polymers, the development of fusion proteins with lectins, reduced gut protease activity and the development of amphiphilic peptidic analogs. Additional research on understanding the basic mechanisms of protein delivery across the insect digestive system, the importance of structure activity in this transfer and the development of technology to improve movement across the gut could be highly significant to the future of protein and nucleic acid-based insecticide development as well as traditional chemical insecticidal technologies.

Metabolism of cydiastatin 4 and analogues by enzymes associated with the midgut and haemolymph of Manduca sexta larvae

The degradation of synthetic cydiastatin 4 (ARPYSFGL-amide) and cydiastatin 4 analogues cydiastatin 4alpha (PPPPPARPYSFGL-amide) and cydiastatin 4beta (PPPPPARPYSF[Acpc]L-amide) by enzymes associated with the midgut and/or haemolymph of the tobacco hawkmoth moth, Manduca sexta was investigated using reversed-phase high performance liquid chromatography (RP-HPLC) combined with matrix assisted laser desorption ionisation-time of flight mass spectrometry (MALDI-TOF MS). Cydiastatin 4 had an estimated half-life of c. 16.5min when incubated with midgut tissue in vitro and c. 2.5min with midgut lumen contents. Two degradation products were identified; cydiastatin(1-6), due to cleavage of the C-terminal di-peptide GL-amide, and cydiastatin(2-8), due to cleavage of the N-terminal A residue. Both cydiastatin 4alpha and cydiastatin 4beta had increased stability to gut and haemolymph enzymes, and full biological activity, but reduced potency compared to cydiastatin 4 when assayed on foregut peristalsis. The P-extended N-terminus of both analogues prevented hydrolysis by aminopeptidases and the replacement of the susceptible G residue with cyclopropylalanine ([Acpc]) counteracted carboxypeptidase activity. However, both analogues were susceptible to amidase-like activity giving an increase in one mass unit presumably due to the conversion of the C-terminal amide group to the free carboxylic acid. No metabolism of cydiastatin 4beta occurred when incubated with larval M. sexta haemolymph over a 90min period.

In vitro transport of an allatostatin across the foregut of Manduca sexta larvae and metabolism by the gut and hemolymph

The degradation of synthetic cydiastatin 4 by enzymes of the foregut and hemolymph, and transport across the foregut of larvae of the tobacco hawkmoth moth, Manduca sexta, were investigated using reversed-phase high performance liquid chromatography (RP-HPLC) together with matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). In the hemolymph in vitro, cydiastatin 4 had a half-life of ca. 30 min. Two degradation products were identified; cydiastatin 4(1-6), due to cleavage of the C-terminal di-peptide GL-amide, and cydiastatin 4(2-8), due to cleavage of the N-terminal A residue. This hydrolysis could be inhibited by up to 93% by 1,10-phenanthroline. Other protease inhibitors had lesser effects (<21% inhibition of degradation) including the aminopeptidase inhibitors amastatin and bestatin, and the chelator EDTA. When incubated with foregut extract in vitro, cydiastatin 4 had a half-life of 23 min, and the hydrolysis products detected were also cydiastatin 4(1-6) and cydiastatin 4(2-8). Similarly, 1-10 phenanthroline inhibited foregut enzyme degradation of cydiastatin 4 by ca. 80%, whereas amastatin, bestatin, and EDTA had very little effect (<10% inhibition). Cydiastatin 4 was transported, intact, from the lumen to the hemolymph side of foregut tissues that were mounted as flat sheets in modified Ussing chambers. This trans-epithelial flux of peptide was dose and time-dependent, but was <3% of the amount of cydiastatin 4 present in the lumen bathing saline. In contrast, no trans-epithelial transport of peptide was apparent across everted foregut sac preparations.

Effects of peptides, with emphasis on feeding, pain, and behavior A 5-year (1999-2003) review of publications in Peptides

Novel effects of naturally occurring peptides are continuing to be discovered, and their mechanisms of actions as well as interactions with other substances, organs, and systems have been elucidated. Synthetic analogs may have actions similar or antagonistic to the endogenous peptides, and both the native peptides and analogs have potential as drugs or drug targets. The journal Peptides publishes many leading articles on the structure-activity relationship of peptides as well as outstanding reviews on some families of peptides. Complementary to the reviews, here we extract information from the original papers published during the past five years in Peptides (1999-2003) to summarize the effects of different classes of peptides, their modulation by other chemicals and various pathophysiological states, and the mechanisms by which the effects are exerted. Special attention is given to peptides related to feeding, pain, and other behaviors. By presenting in condensed form the effects of peptides which are essential for systems biology, we hope that this summary of existing knowledge will encourage additional novel research to be presented in Peptides.

Fraenkel's pupariation factor identified at last

Thirty-five years ago, Zdarek and Fraenkel demonstrated that nervous tissue extracts influenced development by accelerating pupariation in the grey flesh fly, Neobellieria bullata. We have now identified this pupariation factor as SVQFKPRLamide, designated Neb-pyrokinin-2 (Neb-PK-2). To achieve this, the central nervous system of N. bullata wandering stage larvae, that is, preceding pupariation, were dissected and extracted before HPLC separation. Chromatographic fractions were screened with a bioassay for pupariation accelerating activity. Only one fraction showed huge pupariation activity. Mass spectrometry revealed the presence of a pyrokinin, whose primary sequence could not be unequivocally determined by tandem mass spectrometry. However, this Neb-pyrokinin appeared to be very prominent in the ring gland from which it was subsequently purified and identified. Synthetic Neb-PK-2 accelerates pupariation with a threshold dose of only 0.2 pmol and therefore, Neb-pyrokinin is considered to be the genuine pupariation factor. The immunohistochemical distribution pattern of Neb-PK-2 is very similar to that of Drosophila pyrokinin-2, from which it differs by only one amino acid residue. Hence, the recently identified G-protein coupled receptors (CG8784, CG8795) for Drosophila pyrokinin-2 might play an important role in puparium formation.

Peptidergic innervation of the crop and the effects of an ingested nonpeptidal agonist on longevity in female Musca domestica (Diptera: Muscidae)

Dromyosuppressin (DMS) immunoreactive neurons were discovered running along the crop duct and on the surface of the crop in the house fly, Musca domestica L. DMS is a myoinhibitory neuropeptide that has been shown to inhibit crop contractions, in vitro, in the blow fly, Phormia regina (Meigen), and in Drosophila melanogaster Meigen. Various concentrations of benzethonium chloride (Bztc), an agonist of DMS with shown inhibitory effects on blow fly crop contractions, were fed to adult female M. domestica. Flies exhibited a dose-dependent mortality; avoidance and subsequent dehydration are probably the cause of the low survivorship at higher Bztc concentrations.

Insect peptide hormones: a selective review of their physiology and potential application for pest control

Our knowledge on primary structure, synthesis, release, receptor binding, structure-activity relationships, mode of action and degradation of, mainly, neuropeptides from insects has increased dramatically during the last 10 years or so. Here, five case studies are presented, which deal selectively with effects on: reproduction (trypsin modulating oostatic factor in mosquito); energy metabolism, locomotion and the immune system (adipokinetic hormones); water and ion balance, and feeding behaviour (diuretic hormones, kinins, sulfakinins); sex attraction (pheromone biosynthesis activating neuropeptide); and growth and development, and muscle activity (allatostatins). The literature is reviewed in the context of how the knowledge on neuropeptides has been and can be used for the design of novel, safe and selective compounds to control pest insects in the foreseeable future.