Assessment of neuropeptide binding sites and the impact of biostable kinin and CAP2b analogue treatment on aphid (Myzus persicae and Macrosiphum rosae) stress tolerance

A fluorescently-tagged tick kinin neuropeptide triggers peristalsis and labels tick midgut muscles

Ticks are blood-feeding arthropods that require heme for their successful reproduction. During feeding they also acquire pathogens that are subsequently transmitted to humans, wildlife and/or livestock. Understanding the regulation of tick midgut is important for blood meal digestion, heme and nutrient absorption processes and for aspects of pathogen biology in the host. We previously demonstrated the activity of tick kinins on the cognate G protein-coupled receptor. Herein we uncovered the physiological role of the kinin receptor in the tick midgut. A fluorescently-labeled kinin peptide with the endogenous kinin 8 sequence (TMR-RK8), identical in the ticks Rhipicephalus microplus and R. sanguineus, activated and labeled the recombinant R. microplus receptor expressed in CHO-K1 cells. When applied to the live midgut the TMR-RK8 labeled the kinin receptor in muscles while the labeled peptide with the scrambled-sequence of kinin 8 (TMR-Scrambled) did not. The unlabeled kinin 8 peptide competed TMR-RK8, decreasing confocal microscopy signal intensity, indicating TMR-RK8 specificity to muscles. TMR-RK8 was active, inducing significant midgut peristalsis that was video-recorded and evaluated with video tracking software. The TMR-Scrambled peptide used as a negative control did not elicit peristalsis. The myotropic function of kinins in eliciting tick midgut peristalsis was established.

Transcriptome of Excretory Organs Revealed Potential Targets for the Control of Nilaparvata lugens

The excretory organs of insects offer potential physiological targets for insect control. In this study, RNA-seq was utilized to identify a set of transporter and receptor genes enriched in the excretory organs of the brown planthopper (BPH), Nilaparvata lugens, which is considered the most important phloem-feeding insect pest in rice. A total of 1565 and 1084 transcripts were upregulated in the excretory organs, Malpighian tubules, and hindgut, respectively, compared to the midgut, which was enriched for transport activity and oxidoreductase activity. Eight potentially important genes were selected for the exploration of biological function, including one sodium/potassium-ATPase (NKA) subunit (ATP1A1), five aquaporins (AQPs), and two neuropeptide receptors. RNA interference (RNAi) assays showed that the knockdown of ATP1A1 and two AQP genes in BPH resulted in significant lethal phenotypes (corrected mortalities = 42.9-63.6%, 7 days after injection) and significantly reduced honeydew amounts. Our findings suggest that several genes enriched in excretory organs were important for BPH survival, which could be new insect control targets.

Insecticidal efficacy and risk assessment of different neuropeptide analog combinations against the peach-potato aphid following topical exposure

BACKGROUND

Insect neuropeptides control essential physiological metabolic activities. In our previous studies, Capability/CAP2b (PK/CAPA) analog 1895 applied alone or as a combination of CAPA analogs (1895 + 2315) was reported to decrease aphid fitness. While this was obtained with the combination of two peptide analogs of the same neuropeptide class, the effect of combining peptide analogs of different neuropeptide classes has not been explored so far.

RESULTS

In this study, we assessed the effect of combinations of the PK/CAPA analog 1895 with neuropeptide analogs of four different classes [adipokinetic hormone (AKH) analog: 2271; myosuppressin analog: 2434; kinin analog: 2460; tachykinin-related peptide analog: 2463] on the fitness of aphids. We found that the combination of 1895 and AKH analog 2271 was the most effective one to control Myzus persicae. The triple combination 1895 + 2271 + 2315 provided a synergistic effect by further increasing aphid mortality and reducing reproduction relative to 1895 + 2315. Additionally, a biosafety assessment of the combination 1895 + 2271 + 2315 showed no significant lethal nor sub-lethal effects on survival rates and food intake for the pollinator (Bombus terrestris) and the two representative natural enemies (Harmonia axyridis and Nasonia vitripennis).

CONCLUSION

These results could facilitate establishment of the triple combination 1895 + 2271 + 2315, and/or inclusion of second generation analogs, as alternatives to broad spectrum and less friendly insecticides. © 2022 Society of Chemical Industry.

Efficacy and biosafety assessment of neuropeptide CAPA analogues against the peach-potato aphid (Myzus persicae)

Insect CAPA neuropeptidesare considered to affect water and ion balance by mediating the physiological metabolism activities of the Malpighian tubules. In previous studies, the CAPA-PK analogue 1895 (2Abf-Suc-FGPRLamide) was reported to decrease aphid fitness when administered through microinjection or via topical application. However, a further statistically significant decrease in the fitness of aphids and an increased mortality could not be established with pairwise combinations of 1895 with other CAPA analogue. In this study, we assessed the topical application of new combinations of 1895 with five CAPA-PVK analogues on the fitness of aphids. We found that 1895 and CAPA-PVK analogue 2315 (ASG-[β³ L]-VAFPRVamide) was statistically the most effective combination to control the peach potato aphid Myzus persicae nymphs via topical application, leading to 72% mortality. Additionally, the combination (1895+2315) was evaluated against a selection of beneficial insects, that is, a pollinator (Bombus terrestris) and three natural enemies (Chrysoperla carnea, Nasonia vitripennis, and Adalia bipunctata). We found no significant influence on food intake, weight increase, and survival for the pollinator and the three representative natural enemies. These results could facilitate to further establish and generate CAPA analogues as alternatives to broad spectrum and less friendly insecticides.

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.

Leucokinins: Multifunctional Neuropeptides and Hormones in Insects and Other Invertebrates

Leucokinins (LKs) constitute a neuropeptide family first discovered in a cockroach and later identified in numerous insects and several other invertebrates. The LK receptors are only distantly related to other known receptors. Among insects, there are many examples of species where genes encoding LKs and their receptors are absent. Furthermore, genomics has revealed that LK signaling is lacking in several of the invertebrate phyla and in vertebrates. In insects, the number and complexity of LK-expressing neurons vary, from the simple pattern in the Drosophila larva where the entire CNS has 20 neurons of 3 main types, to cockroaches with about 250 neurons of many different types. Common to all studied insects is the presence or 1-3 pairs of LK-expressing neurosecretory cells in each abdominal neuromere of the ventral nerve cord, that, at least in some insects, regulate secretion in Malpighian tubules. This review summarizes the diverse functional roles of LK signaling in insects, as well as other arthropods and mollusks. These functions include regulation of ion and water homeostasis, feeding, sleep-metabolism interactions, state-dependent memory formation, as well as modulation of gustatory sensitivity and nociception. Other functions are implied by the neuronal distribution of LK, but remain to be investigated.

Activity of native tick kinins and peptidomimetics on the cognate target G protein-coupled receptor from the cattle fever tick, Rhipicephalus microplus (Acari: Ixodidae)

BACKGROUND

Kinins are multifunctional neuropeptides that regulate key insect physiological processes such as diuresis, feeding, and ecdysis. However, the physiological roles of kinins in ticks are unclear. Furthermore, ticks have an expanded number of kinin paracopies in the kinin gene. Silencing the kinin receptor (KR) in females of Rhipicephalus microplus reduces reproductive fitness. Thus, it appears the kinin signaling system is important for tick physiology and its disruption may have potential for tick control.

RESULTS

We determined the activities of endogenous kinins on the KR, a G protein-coupled receptor, and identified potent peptidomimetics. Fourteen predicted R. microplus kinins (Rhimi-K), and 11 kinin analogs containing aminoisobutyric acid (Aib) were tested. The latter incorporated tick kinin sequences and/or were modified for enhanced resistance to arthropod peptidases. A high-throughput screen using a calcium fluorescence assay in 384-well plates was performed. All tested kinins and Aib analogs were full agonists. The most potent kinin and two kinin analogs were equipotent. Analogs 2414 ([Aib]FS[Aib]WGa) and 2412 ([Aib]FG[Aib]WGa) were the most active with EC₅₀ values of 0.9 and 1.1 nM, respectively, matching the EC₅₀ of the most potent tick kinin, Rhimi-K-14 (QDSFNPWGa) (EC₅₀  = 1 nM). The potent analog 2415 ([Aib]FR[Aib]WGa, EC₅₀  = 6.8 nM) includes both Aib molecules for resistance to peptidases and a positively charged residue, R, for enhanced water solubility and amphiphilic character.

CONCLUSION

These tick kinins and pseudopeptides expand the repertoire of reagents for tick physiology and toxicology towards finding novel targets for tick management. © 2019 Society of Chemical Industry.

Assessment of insecticidal effects and selectivity of CAPA-PK peptide analogues against the peach-potato aphid and four beneficial insects following topical exposure

BACKGROUND

Insect Capability neuropeptides (CAP2b/CAPA-PKs) play a critical role in modulating different physiologies and behavior in insects. In a previous proof-of-concept study, the CAP2b analogues 1895 (2Abf-Suc-FGPRLamide) and 2129 (2Abf-Suc-ATPRIamide) were reported to reduce aphid fitness when administered by injection. In the current study, the insecticidal efficacy of 1895 and 2129 on the peach potato aphid Myzus persicae was analyzed by topical application, simulating a spray application scenario in the field. Additionally, the selectivity of the tested analogues was evaluated against a selection of beneficial insects, namely three natural enemies (Adalia bipunctata, Chrysoperla carnea and Nasonia vitripennis) and a pollinator (Bombus terrestris).

RESULTS

Within 3-5 days post topical exposure of aphids to 1895, higher mortality (33%) was observed, as was the case for the treatment with 2129 (17%) and the mixture of 1895 + 2129 (47%) compared to the control (3%). 1895 and the mix 1895 + 2129 showed the strongest and comparable insecticidal effects. Additionally, surviving aphids treated with 1895 showed a reduction in total lifetime reproduction (GRR) of 30%, 19% with 2129 and 39% with the mix 1895 + 2129. Of interest from a biosafety perspective is that by using the same delivery method and dose, no significant effects on survival, weight increase and food intake was observed for the representative natural enemies and the pollinator.

CONCLUSION

This study highlights the potential of exploiting CAP2b analogues such as 1895 (core structure FGPRL) as aphicides. Additionally, the CAP2b analogues used in this study were selective as they showed no effects when applied on four representative beneficial insects.

Synthesis, aphicidal activity and conformation of novel insect kinin analogues as potential eco-friendly insecticides

BACKGROUND

The discovery of ecofriendly insecticides through a new strategy for aphid control is important because of the substantial resistance and unexpected eco-toxicity to honeybees caused by traditional insecticides. The insect kinins, a class of multifunctional insect neuropeptides, are considered for potential application in pest control. In our previous work we developed several series of insect kinin analogues and found a promising lead II-1 with good aphicidal activity. To seek further eco-friendly aphicides, the optimization of II-1 is carried out in this study.

RESULTS

Fifteen novel Yaa³ modified analogues based on the lead II-1 were synthesized. The aphicidal tests indicated that IV-3, IV-5 and IV-10 exhibited significant activity against the soybean aphid Aphis glycines with LC₅₀ values of 0.0029, 0.0072 and 0.0086 mmol L^-1 , respectively, higher than that of lead II-1 and the commercial Pymetrozine. The molecular modeling results showed that analogues II-1, IV-3, IV-5, IV-7 and IV-10 formed a β-turn-like conformation, while the conformation of analogues IV-1, IV-2 and IV-9 seemed to be linear. Some structural elements favorable for the activity were proposed based on the conformation-activity relationship of the analogues.

CONCLUSION

Insect kinin analogues derived from lead II-1 by modifying the hydrolysis site Yaa³ with natural, sterically hindered α- and β-amino acids showed great potential as eco-friendly insecticides. Inspiringly, the most active analogue IV-3 can be a candidate for further development. The β-turn-like conformation and the orientation of the aromatic rings of the side chain of Phe² and Trp⁴ may be critical factors beneficial to activity. © 2019 Society of Chemical Industry.

Role of the Insect Neuroendocrine System in the Response to Cold Stress

Insects are the largest group of animals. They are capable of surviving in virtually all environments from arid deserts to the freezing permafrost of polar regions. This success is due to their great capacity to tolerate a range of environmental stresses, such as low temperature. Cold/freezing stress affects many physiological processes in insects, causing changes in main metabolic pathways, cellular dehydration, loss of neuromuscular function, and imbalance in water and ion homeostasis. The neuroendocrine system and its related signaling mediators, such as neuropeptides and biogenic amines, play central roles in the regulation of the various physiological and behavioral processes of insects and hence can also potentially impact thermal tolerance. In response to cold stress, various chemical signals are released either via direct intercellular contact or systemically. These are signals which regulate osmoregulation - capability peptides (CAPA), inotocin (ITC)-like peptides, ion transport peptide (ITP), diuretic hormones and calcitonin (CAL), substances related to the general response to various stress factors - tachykinin-related peptides (TRPs) or peptides responsible for the mobilization of body reserves. All these processes are potentially important in cold tolerance mechanisms. This review summarizes the current knowledge on the involvement of the neuroendocrine system in the cold stress response and the possible contributions of various signaling molecules in this process.

Physiological effects of biostable kinin and CAPA analogs in the Chagas disease vector, Rhodnius prolixus

In the Chagas disease vector Rhodnius prolixus, the kinin and CAPA family of neuropeptides are implicated in feeding and diuresis-related behaviours, with Rhopr-kinins stimulating contractions of the midgut, salivary glands, and hindgut, and RhoprCAPA-2 functioning as an anti-diuretic hormone. The current study examined the effects of kinin and CAPA neuropeptides and their analogs on feeding and diuresis, and on hindgut contractions and MT fluid secretion in R. prolixus. The biostable Aib-containing kinin analog 2139[Φ1]wp-2 was found to have antifeedant effects, and to be more potent than Rhopr-kinin 2 in stimulating hindgut contractions. The CAPA analog 2129-SP3[Φ3]wp-2 induced the intake of a larger blood meal, and increased the rate of post-prandial rapid diuresis. RhoprCAPA-2, but not its analog, potentiated hindgut contractions induced by Rhopr-kinin 2. Potentiation was observed with the CAPA analog on 5-HT-stimulated increases in frequency of hindgut contractions, whereas RhoprCAPA-2 inhibited this 5-HT-mediated stimulation. The CAPA analog induced hindgut contractions and prevented the inhibition induced by RhoprCAPA-2 on 5-HT-stimulated MT secretion. These results demonstrate novel interactions between Rhopr-kinin and RhoprCAPA-2 on the hindgut, possibly influencing post-feeding excretion. The kinin analog is a potent agonist of the kinin receptor, and the CAPA analog an antagonist of the CAPA receptor. The use of neuropeptide mimetics is a promising approach to vector control as they can disrupt behaviours, and the effects of these neuropeptide analogs highlight their value as lead compounds, given their ability to interfere with epidemiologically-relevant behaviours.