Biostable and PEG polymer-conjugated insect pyrokinin analogs demonstrate antifeedant activity and induce high mortality in the pea aphid Acyrthosiphon pisum (Hemiptera: Aphidae)

Effects of alloferon and its analogues on reproduction and development of the Tenebrio molitor beetle

As the most numerous group of animals on Earth, insects are found in almost every ecosystem. Their useful role in the environment is priceless; however, for humans, their presence may be considered negative or even harmful. For years, people have been trying to control the number of pests by using synthetic insecticides, which eventually causes an increased level of resistance to applied compounds. The effects of synthetic insecticides have encouraged researchers to search for alternatives and thus develop safe compounds with high specificity. Using knowledge about the physiology of insects and the functionality of compounds of insect origin, a new class of bioinsecticides called peptidomimetics, which are appropriately modified insect analogues, was created. One promising compound that might be successfully modified is the thirteen amino acid peptide alloferon (HGVSGHGQHGVHG), which is obtained from the hemolymph of the blue blowfly Calliphora vicinia. Our research aimed to understand the physiological properties of alloferon and the activity of its peptidomimetics, which will provide the possibility of using alloferon or its analogues in the pharmaceutical industry, as a drug or adjuvant, or in agriculture as a bioinsecticide. We used alloferon and its three peptidomimetics, which are conjugates of the native peptide with three unsaturated fatty acids with various chain lengths: caprylic, myristic, and palmitic. We tested their effects on the morphology and activity of the reproductive system and the embryogenesis of the Tenebrio molitor beetle. We found that the tested compounds influenced the growth and maturation of ovaries and the expression level of the vitellogenin gene. The tested compounds also influenced the process of egg laying, embryogenesis, and offspring hatching, showing that alloferon might be a good peptide for the synthesis of effective bioinsecticides or biopharmaceuticals.

A Perspective on Current Therapeutic Molecule Screening Methods Against 'Candidatus Liberibacter asiaticus', the Presumed Causative Agent of Citrus Huanglongbing

Huanglongbing (HLB), referred to as citrus greening disease, is a bacterial disease impacting citrus production worldwide and is fatal to young trees and mature trees of certain varieties. In some areas, the disease is devastating the citrus industry. A successful solution to HLB will be measured in economics: citrus growers need treatments that improve tree health, fruit production, and most importantly, economic yield. The profitability of citrus groves is the ultimate metric that truly matters when searching for solutions to HLB. Scientific approaches used in the laboratory, greenhouse, or field trials are critical to the discovery of those solutions and to estimate the likelihood of success of a treatment aimed at commercialization. Researchers and the citrus industry use a number of proxy evaluations of potential HLB solutions; understanding the strengths and limitations of each assay, as well as how best to compare different assays, is critical for decision-making to advance therapies into field trials and commercialization. This perspective aims to help the reader compare and understand the limitations of different proxy evaluation systems based on the treatment and evaluation under consideration. The researcher must determine the suitability of one or more of these metrics to identify treatments and predict the usefulness of these treatments in having an eventual impact on citrus production and HLB mitigation. As therapies advance to field trials in the next few years, a reevaluation of these metrics will be useful to guide future research efforts on strategies to mitigate HLB and vascular bacterial pathogens in other perennial crops.

Myotropic Activities of Tick Pyrokinin Neuropeptides and Analog in Feeding Tissues of Hard Ticks (Ixodidae)

Neuropeptides regulate many important physiological processes in animals. The G protein-coupled receptors of corresponding small neuropeptide ligands are considered promising targets for controlling arthropod pests. Pyrokinins (PKs) are pleiotropic neuropeptides that, in some insect species, stimulate muscle contraction and modulate pheromone biosynthesis, embryonic diapause, and feeding behavior. However, their function remains unknown in ticks. In this study, we reported the myotropic activity of tick endogenous PKs and a PK agonist analog, PK-PEG₈ (MS[PEG₈]-YFTPRLa), on feeding tissues of two tick species representing the family Ixodidae lineages, namely, Prostriata (Ixodes scapularis) and Metastriata (Rhipicephalus sanguineus). First, we predicted the sequences of two periviscerokinins (PVK), one with a derived ending RNa and five PKs encoded by the CAPA peptide precursor from R. sanguineus and found the encoded PKs were identical to those of R. microplus identified previously. The pharynx-esophagus of both tick species responded with increased contractions to 10 μM of the endogenous PK as well as to PK-PEG₈ but not to the scrambled PK peptide, as expected. A dose-dependent myotropic activity of the PK-PEG₈ was found for both tick species, validating the analog activity previously found in the pyrokinin recombinant receptor assay. In agreement with the tissue activity elicited, we quantified the relative transcript abundance of R. sanguineus PK receptor in unfed female ticks and found it was the highest in the feeding tissues extracted from the capitulum and lowest in the reproductive tissue. This is the first report of the activity of pyrokinins in ticks. These findings strongly indicate the potential role of PKs in regulating tick blood feeding and therefore, making the tick PK receptor a potential target for interference.

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.

Molecular and Functional Characterization of Pyrokinin-Like Peptides in the Western Tarnished Plant Bug Lygus hesperus (Hemiptera: Miridae)

Simple Summary

Neuropeptides regulate most insect biological functions. One such group of peptides, the pyrokinins (PKs), are distinguished by a C-terminal FXPRLamide. While widely distributed in most insects, they are poorly characterized in plant bugs. To address this limitation, we identified the PK transcript in the western tarnished plant bug (Lygus hesperus) and examined its expression. The Lygus PK transcript is predicted to yield three PK-like peptides but only two (LyghePKa and LyghePKb) have the characteristic C-terminal amide. The transcript is expressed throughout development and is most abundant in heads. A custom FXPRLamide antibody revealed immunoreactive cells throughout the Lygus central nervous system consistent with typical neuropeptide expression. To assess potential functional roles of the peptides, a fluorescence-based Ca²⁺ influx assay using cultured insect cells stably expressing a moth PK receptor was performed. LyghePKa was unable to stimulate receptor activation, whereas LyghePKb triggered a robust response. The in vivo pheromonotropic activity of the two peptides was likewise assessed using three different moth species. Like the cell culture system, only the LyghePKb peptide was active. The study suggests evolutionary divergence of the PK gene in plant bugs and provides critical insights into likely biological functions in the western tarnished plant bug.

Abstract

The pyrokinin (PK) family of insect neuropeptides, characterized by C termini consisting of either WFGPRLamide (i.e., PK1) or FXPRLamide (i.e., PK2), are encoded on the capa and pk genes. Although implicated in diverse biological functions, characterization of PKs in hemipteran pests has been largely limited to genomic, transcriptomic, and/or peptidomic datasets. The Lygus hesperus (western tarnished plant bug) PK transcript encodes a prepropeptide predicted to yield three PK2 FXPRLamide-like peptides with C-terminal sequences characterized by FQPRSamide (LyghePKa), FAPRLamide (LyghePKb), and a non-amidated YSPRF. The transcript is expressed throughout L. hesperus development with greatest abundance in adult heads. PRXamide-like immunoreactivity, which recognizes both pk- and capa-derived peptides, is localized to cells in the cerebral ganglia, gnathal ganglia/suboesophageal ganglion, thoracic ganglia, and abdominal ganglia. Immunoreactivity in the abdominal ganglia is largely consistent with capa-derived peptide expression, whereas the atypical fourth pair of immunoreactive cells may reflect pk-based expression. In vitro activation of a PK receptor heterologously expressed in cultured insect cells was only observed in response to LyghePKb, while no effects were observed with LyghePKa. Similarly, in vivo pheromonotropic effects were only observed following LyghePKb injections. Comparison of PK2 prepropeptides from multiple hemipterans suggests mirid-specific diversification of the pk gene.

Peptidomics Approaches for the Identification of Bioactive Molecules from Diaphorina citri

Huanglongbing (HLB), a deadly citrus disease, is primarily associated with Candidatus Liberibacter asiaticus (CLas) and spread by the hemipteran insect Diaphorina citri. Control strategies to combat HLB are urgently needed. In this work, we developed and compared workflows for the extraction of the D. citri peptidome, a dynamic set of polypeptides produced by proteolysis and other cellular processes. High-resolution mass spectrometry revealed bias among methods reflecting the physiochemical properties of the peptides: while TCA/acetone-based methods resulted in enrichment of C-terminally amidated peptides, a modification characteristic of bioactive peptides, larger peptides were overrepresented in the aqueous phase of chloroform/methanol extracts, possibly indicative of reduced co-analytical degradation during sample preparation. Parallel reaction monitoring (PRM) was used to validate the structure and upregulation of peptides derived from hemocyanin, a D. citri immune system protein, in insects reared on healthy and CLas-infected trees. Mining of the data sets also revealed 122 candidate neuropeptides, including PK/PBAN family neuropeptides and kinins, biostable analogs of which have known insecticidal properties. Taken together, this information yields new, in-depth insights into peptidomics methodology. Additionally, the putative neuropeptides identified may lead to psyllid mortality if applied to or expressed in citrus, consequently blocking the spread of HLB disease in citrus groves.

Insect Mouthpart Transcriptome Unveils Extension of Cuticular Protein Repertoire and Complex Organization

Insects have developed intriguing cuticles with very specific structures and functions, including microstructures governing their interactions with transmitted microbes, such as in aphid mouthparts harboring virus receptors within such microstructures. Here, we provide the first transcriptome analysis of an insect mouthpart cuticle (“retort organs” [ROs], the stylets' precursors). This analysis defined stylets as a complex composite material. The retort transcriptome also allowed us to propose an algorithmic definition of a new cuticular protein (CP) family with low complexity and biased amino acid composition. Finally, we identified a differentially expressed gene encoding a pyrokinin (PK) neuropeptide precursor and characterizing the mandibular glands. Injection of three predicted synthetic peptides PK1/2/3 into aphids prior to ecdysis caused a molt-specific phenotype with altered head formation. Our study provides the most complete description to date of the potential protein composition of aphid stylets, which should improve the understanding of the transmission of stylet-borne viruses.

•

First transcriptome of aphid retort glands and stylet cuticular protein composition

•

A pyrokinin transcript is mandibular gland specific at the onset of adult moult

•

Stylet cuticle is of higher protein complexity than other insect cuticles

•

A new class of low-complexity cuticular proteins is predicted

Bioactivity-guided isolation of rosmarinic acid as the principle bioactive compound from the butanol extract of Isodon rugosus against the pea aphid, Acyrthosiphon pisum

Aphids are agricultural pest insects that transmit viruses and cause feeding damage on a global scale. Current pest control practices involving the excessive use of synthetic insecticides over many years have resulted in aphid resistance to a number of pesticides. In nature, plants produce secondary metabolites during their interaction with insects and these metabolites can act as toxicants, antifeedants, anti-oviposition agents and deterrents towards the insects. In a previous study, we demonstrated that the butanol fraction from a crude methanolic extract of an important plant species, Isodon rugosus showed strong insecticidal activity against the pea aphid, Acyrthosiphon pisum. To further explore this finding, the current study aimed to exploit a bioactivity-guided strategy to isolate and identify the active compound in the butanol fraction of I. rugosus. As such, reversed-phase flash chromatography, acidic extraction and different spectroscopic techniques were used to isolate and identify the new compound, rosmarinic acid, as the bioactive compound in I. rugosus. Insecticidal potential of rosmarinic acid against A. pisum was evaluated using standard protocols and the data obtained was analyzed using qualitative and quantitative statistical approaches. Considering that a very low concentration of this compound (LC90 = 5.4 ppm) causes significant mortality in A. pisum within 24 h, rosmarinic acid could be exploited as a potent insecticide against this important pest insect. Furthermore, I. rugosus is already used for medicinal purposes and rosmarinic acid is known to reduce genotoxic effects induced by chemicals, hence it is expected to be safer compared to the current conventional pesticides. While this study highlights the potential of I. rugosus as a possible biopesticide source against A. pisum, it also provides the basis for further exploration and development of formulations for effective field application.

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.

Neuropeptides in the regulation of Rhodnius prolixus physiology

In the kissing bug Rhodnius prolixus, events such as diuresis, antidiuresis, development and reproduction are triggered by blood feeding. Hence, these events can be accurately timed, facilitating physiological experiments. This, combined with its relatively big size, makes R. prolixus an excellent model in insect neuroendocrinological studies. The importance of R. prolixus as a Chagas' disease vector as much as an insect model has motivated the sequencing of its genome in recent years, facilitating genetic and molecular studies. Most crucial physiological processes are regulated by the neuroendocrine system, composed of neuropeptides and their receptors. The identification and characterization of neuropeptides and their receptors could be the first step to find targets for new insecticides. The sequences of 41 neuropeptide precursor genes and the receptors for most of them were identified in the R. prolixus genome. Functional information about many of these molecules was obtained, whereas many neuroendocrine systems are still unstudied in this model species. This review addresses the knowledge available to date regarding the structure, distribution, expression and physiological effects of neuropeptides in R. prolixus, and points to future directions in this research field.

G protein coupled receptors as targets for next generation pesticides

There is an on-going need for the discovery and development of new pesticides due to the loss of existing products through the continuing development of resistance, the desire for products with more favourable environmental and toxicological profiles and the need to implement the principles of integrated pest management. Insect G protein coupled receptors (GPCRs) have important roles in modulating biology, physiology and behaviour, including reproduction, osmoregulation, growth and development. Modifying normal receptor function by blocking or over stimulating its actions may either result in the death of a pest or disrupt its normal fitness or reproductive capacity to reduce pest populations. Hence GPCRs offer potential targets for the development of next generation pesticides providing opportunities to discover new chemistries for invertebrate pest control. Such receptors are important targets for pharmaceutical drugs, but are under-exploited by the agro-chemical industry. The octopamine receptor agonists are the only pesticides with a recognized mode of action, as described in the classification scheme developed by the Insecticide Resistance Action Committee, that act via a GPCR. The availability of sequenced insect genomes has facilitated the characterization of insect GPCRs, but the development and utilization of screening assays to identify lead compounds has been slow. Various studies using knock-down technologies or applying the native ligands and/or neuropeptide analogues to pest insects in vivo, have however demonstrated that modifying normal receptor function can have an insecticidal effect. This review presents examples of potential insect neuropeptide receptors that are potential targets for lead compound development, using case studies from three representative pest species, Tribolium castaneum, Acyrthosiphon pisum, and Drosophila suzukii. Functional analysis studies on T. castaneum suggest that GPCRs involved in growth and development (eclosion hormone, ecdysis triggering hormone and crustacean cardioacceleratory peptide receptors) as well as the dopamine-2 like, latrophilin-like, starry night, frizzled-like, methuselah-like and the smoothened receptors may be suitable pesticide targets. From in vivo studies using native ligands and peptide analogues, receptors which appear to have a role in the regulation of feeding in the pea aphid, such as the PISCF-allatostatin and the various "kinin" receptors, are also potential targets. In Drosophila melanogaster various neuropeptides and their signalling pathways have been studied extensively. This may provide insights into potential pesticide targets that could be exploited in D. suzukii. Examples include the sex peptide receptor, which is involved in reproduction and host seeking behaviours, and those responsible for osmoregulation such as the diuretic hormone receptors. However the neuropeptides and their receptors in insects are often poorly characterized, especially in pest species. Although data from closely related species may be transferable (e.g. D. melanogaster to D. suzukii), peptides and receptors may have different roles in different insects, and hence a target in one insect may not be appropriate in another. Hence fundamental knowledge of the roles and functions of receptors is vital for development to proceed.

Genomic and peptidomic analyses of the neuropeptides from the emerging pest, Drosophila suzukii

Drosophila suzukii is a highly polyphagous invasive pest which has been recently introduced into Europe and North America, where it is causing severe economic losses through larval infestations of stone and berry fruits. The peptidome of the selected nervous tissues of adult D. suzukii was investigated as a first step in identifying potential targets for the development of novel insecticides. Through in silico analyses of the D. suzukii genome databases 28 neuropeptide families, comprising more than 70 predicted peptides were identified. Using a combination of liquid chromatography and mass spectrometry of tissue extracts, 33 predicted peptides, representing 15 different peptide families were identified by their molecular masses and a total of 17 peptide sequences were confirmed by ion fragmentation. A comparison between the peptides and precursors of D. suzukii and D. melanogaster shows they are highly conserved, with differences only identified in the amino acid sequences of the peptides encoded in the FMRFamide, hugin and ecydysis triggering hormone precursors. All other peptides predicted and identified from D. suzukii appear to be identical to those previously characterized from D. melanogaster. Adipokinetic hormone was only identified in the corpus cardiacum, other peptides present included short neuropeptide F, a pyrokinin and myosuppressin, the latter of which was the only peptide identified from the crop nerve bundle. Peptides present in extracts of the brain and/or thoracico-abdominal ganglion included allatostatins, cardioacceleratory peptide 2b, corazonin, extended FMRFamides, pyrokinins, myoinihibitory peptides, neuropeptide-like precursor 1, SIFamide, short neuropeptide F, kinin, sulfakinins and tachykinin related peptides.

Structure-activity relationships of two Rhodnius prolixus calcitonin-like diuretic hormone analogs

The calcitonin-like diuretic hormone (CT/DH) in Rhodnius prolixus influences various tissues associated with feeding-related physiological events. The receptors for this peptide have also been identified and shown to be expressed in these tissues. In the present study, we have investigated the effects of two R. prolixus CT/DH analogs (full-length form and N-terminal truncated form) on hindgut contractions and in a heterologous receptor expression system. The analogs contained the amino acid methyl-homoserine in place of methionine in order to prevent them from being oxidized and thus increase their stability. The full-length form of the analog retained all of its activity in our assays when compared to the endogenous peptide. Truncated analog displayed no activity in our assays.

Essential oils nanoformulations for stored-product pest control - characterization and biological properties

The lethal and sublethal activity of poly(ethylene glycol) (PEG) nanoparticles containing essential oils (EO), also the physicochemical characterization, were determined against Tribolium castaneum and Rhizopertha dominica. The 10% ratio EO-PEG nanoparticles showed an average diameter<235 nm (PDI<0.280) and a loading efficacy>75%; after 6 month of storage their size did not change significantly and the amount of the EOs decreased 25%, approximately. Furthermore, during this period, no chemical derivates were observed. The EOs nanoparticles produced a notable increase of the residual contact toxicity apparently due to the slow and persistent release of the active terpenes. In addition, the nanoformulation enhanced the EO contact toxicity and altered the nutritional physiology of both stored product pest. The results indicated that these novel systems could be used in integrated pest management program for T. castaneum and R. dominica control.

The pro-apoptotic action of new analogs of the insect gonadoinhibiting peptide Neb-colloostatin: synthesis and structure-activity studies

Neb-colloostatin (SIVPLGLPVPIGPIVVGPR), an insect oostatic factor found in the ovaries of the flesh fly Neobellieria bullata, strongly induces apoptosis in insect haemocytes. To explain the role of Ser(1) and Pro(4) residues of Neb-colloostatin in the pro-apoptotic activity of this peptide, the synthesis of a series of analogs was performed, such as: [Ac-Ser(1)]- (1), [d-Ser(1)]- (2), [Thr(1)]- (3), [Asp(1)]- (4), [Glu(1)]- (5), [Gln(1)]- (6), [Ala(1)]- (7), [Val(1)]- (8), [d-Pro(4)]-(9), [Hyp(4)]- (10), [Acp(4)]- (11), [Ach(4)]- (12), [Ala(4)]- (13), [Ile(4)]- (14), and [Val(4)]-colloostatin (15). All peptides were bioassayed in vivo for the pro-apoptotic action on haemocytes of Tenebrio molitor. Additionally, the structural properties of Neb-colloostatin and its analogs were examined by the circular dichroism in water and methanol. Peptides 1, 4, 5, 7, 8, 10, 12, 14, and 15 strongly induce T. molitor haemocytes to undergo apoptosis and they show about 120-230% of the Neb-colloostatin activity at a dose of 1nM. The CD conformational studies show that the investigated peptides seem to prefer the unordered conformation.

Toxins for transgenic resistance to hemipteran pests

The sap sucking insects (Hemiptera), which include aphids, whiteflies, plant bugs and stink bugs, have emerged as major agricultural pests. The Hemiptera cause direct damage by feeding on crops, and in some cases indirect damage by transmission of plant viruses. Current management relies almost exclusively on application of classical chemical insecticides. While the development of transgenic crops expressing toxins derived from the bacterium Bacillus thuringiensis (Bt) has provided effective plant protection against some insect pests, Bt toxins exhibit little toxicity against sap sucking insects. Indeed, the pest status of some Hemiptera on Bt-transgenic plants has increased in the absence of pesticide application. The increased pest status of numerous hemipteran species, combined with increased prevalence of resistance to chemical insecticides, provides impetus for the development of biologically based, alternative management strategies. Here, we provide an overview of approaches toward transgenic resistance to hemipteran pests.