Small-Molecule Inhibitors of Inward Rectifier Potassium (Kir) Channels Reduce Bloodmeal Feeding and Have Insecticidal Activity Against the Horn Fly (Diptera: Muscidae)

Comparative assays revealed distinct toxicity characterizations between pymetrozine and flonicamid

Flonicamid is an insecticide with a unique mode of action, primarily inhibiting the feeding behavior of sap-sucking insects. However, its molecular target remains controversial. Reports indicate that both flonicamid and pymetrozine affect insect behavior and mobility. To investigate further, we compared the susceptibility of three insect species (Myzus persicae, Nilaparvata lugens, and Drosophila melanogaster) to flonicamid and pymetrozine under two different feeding postures. Results showed that feeding posture did not affect the survival curves or mean lifespans of the three insects. However, insect lifespans were significantly reduced under exposure to either insecticide compared to untreated controls. The effects of the two insecticides on susceptibility and lifespan under different feeding postures were markedly different: feeding posture significantly influenced the sensitivity of aphids, planthoppers, and fruit flies to pymetrozine but had minimal effects on the sensitivity of aphids and planthoppers to flonicamid. Flonicamid had only a minor impact on fruit flies' sensitivity. Survival curve analysis also revealed significant differences under pymetrozine stress, while no such differences were observed under flonicamid stress. These findings indicate that flonicamid exerts different toxicological effects on target insects (aphids and planthoppers) versus non-target insects (fruit flies), depending on feeding posture. This suggests that the toxicological mechanisms of flonicamid differ substantially between target and non-target organisms, with implications that findings in non-target insects may not directly apply to target pests. Our study provides valuable insights and directions for further research into flonicamid's mode of action, aiding in elucidating its underlying mechanisms.

Molecular Insights into Pharmacological Mechanism of Insect Kir Channels and the Toxicity of Kir Inhibitors on Hemipteran Insects

Inwardly rectifying potassium channels (Kir) play a key role in regulating various physiological processes. However, the structural and pharmacological mechanisms of insect Kir channels remain unclear. In this study, we show that coexpression of different Kir subunits in the same tissue did not affect the rectification properties of strongly rectifying Kir. The Kir inhibitor VU041, along with the insecticide flonicamid and its metabolite flumetnicam, were tested for their inhibitory effects on the homotetrameric Kir1 and Kir2 channels. Both Kir1 and Kir2 channels from the two insect species showed similar pharmacological responses to VU041, flonicamid, and flumetnicam. However, VU041 demonstrated significantly higher inhibitory activity than both insecticides across all four Kir channels, while flumetnicam exhibited the weakest inhibition. Molecular docking analyses indicate that the binding site of VU041 is not the same as that of flonicamid, and flumetnicam. flonicamid, and flumetnicam have binding sites similar to the ATP binding sites in cytoplasmic region of human Kir6.2, whereas VU041 is located in the pore of the ion channel, and serves as a pore blocker that inhibits Kir channels. Mutation analysis confirmed the essential roles of these residues in channel function and binding affinity. Finally, the toxicities of the three inhibitors were evaluated in N. lugens and M. persicae. VU041, a potent inhibitor of the insect Kir channel, showed lower toxicity compared to the other two inhibitors, whereas flumethoxan, which is less active on the Kir1 channel, showed higher toxicity, probably related to the different bioavailability of the different compounds. These findings suggest that the potential of targeting Kir channels as insecticidal strategies requires further evaluation.

Involvement of Inwardly Rectifying Potassium (Kir) Channels in the Toxicity of Flonicamid to Drosophila melanogaster

Inwardly rectifying potassium (Kir) channels regulate essential physiological processes in insects and have been identified as potential targets for developing new insecticides. Flonicamid has been reported to inhibit Kir channels, disrupting the functions of salivary glands and renal tubules. However, the precise molecular target of flonicamid remains debated. It is unclear whether flonicamid directly targets Kir channels or acts on other sites involved in the activation of transient receptor potential vanilloid (TRPV) channels. In this study, we observed that flonicamid is more toxic to flies than its metabolite, flumetnicam. This higher toxicity is difficult to reconcile if nicotinamidase is the active target, as flonicamid does not inhibit nicotinamidase. An alternative explanation is that flonicamid and flumetnicam may have distinct targets or act on multiple targets. Furthermore, reducing the expression of three individual Kir genes in the salivary glands of D. melanogaster significantly decreased the flies' susceptibility to both flonicamid and flumetnicam. The double knockdown of Kir1 with Kir3 or Kir2 with Kir3 further reduced the flies' sensitivity to both compounds. These findings confirm the involvement of Kir channels in mediating the toxic effects of flonicamid on flies. Overall, this study offers new insights into the physiological roles of insect Kir channels and flonicamid toxicity.

ATP-sensitive inward rectifier potassium channels regulate secretion of pro-feeding salivary proteins in the lone star tick (Amblyomma americanum)

Understanding the physiological and molecular regulation of tick feeding is necessary for developing intervention strategies to curb disease transmission by ticks. Pharmacological activation of ATP-gated inward rectifier potassium (KATP) channels reduced fluid secretion from isolated salivary gland and blood feeding in the lone star tick, Amblyomma americanum, yet the temporal expression pattern of KATP channel proteins remained unknown. KATP channels were highly expressed in type II and III acini in off-host stage and early feeding phase ticks, yet expression was reduced in later stages of feeding. We next assessed KATP channel regulation of the secreted proteome of tick saliva. LC-MS/MS analysis identified 40 differentially secreted tick saliva proteins after exposure to KATP activators or inhibitors. Secretion of previously validated tick saliva proteins that promote tick feeding, AV422, AAS27, and AAS41 were significantly reduced by upwards of 8 log units in ticks exposed to KATP channel activators when compared to untreated ticks. Importantly, activation of KATP channels inhibited tick feeding and vice versa for KATP channel inhibitors. Data indicate KATP channels regulate tick feeding biology by controlling secretion of pro-feeding proteins that are essential during early feeding phases, which provides insights into physiological and molecular regulation of tick feeding behavior.

Potassium ion channels as a molecular target to reduce virus infection and mortality of honey bee colonies

Declines in managed honey bee populations are multifactorial but closely associated with reduced virus immunocompetence and thus, mechanisms to enhance immune function are likely to reduce viral infection rates and increase colony viability. However, gaps in knowledge regarding physiological mechanisms or 'druggable' target sites to enhance bee immunocompetence has prevented therapeutics development to reduce virus infection. Our data bridge this knowledge gap by identifying ATP-sensitive inward rectifier potassium (KATP) channels as a pharmacologically tractable target for reducing virus-mediated mortality and viral replication in bees, as well as increasing an aspect of colony-level immunity. Bees infected with Israeli acute paralysis virus and provided KATP channel activators had similar mortality rates as uninfected bees. Furthermore, we show that generation of reactive oxygen species (ROS) and regulation of ROS concentrations through pharmacological activation of KATP channels can stimulate antiviral responses, highlighting a functional framework for physiological regulation of the bee immune system. Next, we tested the influence of pharmacological activation of KATP channels on infection of 6 viruses at the colony level in the field. Data strongly support that KATP channels are a field-relevant target site as colonies treated with pinacidil, a KATP channel activator, had reduced titers of seven bee-relevant viruses by up to 75-fold and reduced them to levels comparable to non-inoculated colonies. Together, these data indicate a functional linkage between KATP channels, ROS, and antiviral defense mechanisms in bees and define a toxicologically relevant pathway that can be used for novel therapeutics development to enhance bee health and colony sustainability in the field.

Catalyzing systemic movement of inward rectifier potassium channel inhibitors for antifeedant activity against the cotton aphid, Aphis gossypii (Glover)

BACKGROUND

The cotton aphid, Aphis gossypii Glover (Hemiptera: Aphididae), is a destructive agricultural pest, capable of photosynthate removal and plant virus transmission. Therefore, we aimed to test the antifeedant properties of small-molecule inhibitors of inward rectifier potassium (Kir) channels expressed in insect salivary glands and develop an approach for enabling systemic movement of lipophilic Kir inhibitors.

RESULTS

Two Kir channel inhibitors, VU041 and VU730, reduced the secretory activity of the aphid salivary glands by 3.3-fold and foliar applications of VU041 and VU730 significantly (P < 0.05) increased the time to first probe, total probe duration, and nearly eliminated phloem salivation and ingestion. Next, we aimed to facilitate systemic movement of VU041 and VU730 through evaluation of a novel natural product based solubilizer containing rubusoside that was isolated from Chinese sweet leaf (Rubus suavissimus) plants. A single lower leaf was treated with Kir inhibitor soluble liquid (KI-SL) and systemic movement throughout the plant was verified via toxicity bioassays and changes to feeding behavior through the electrical penetration graph (EPG) technique. EPG data indicate KI-SL significantly reduced ability to reach E1 (phloem salivation) and E2 (phloem ingestion) waveforms and altered plant probing behavior when compared to the untreated control. High-performance liquid chromatography (HPLC) analysis indicated the presence of VU041 and VU730 in the upper leaf tissue of these plants. Together, these data provide strong support that incorporation of rubusoside with Kir inhibitors enhanced translaminar and translocation movement through the plant tissue.

CONCLUSION

These data further support hemipteran Kir channels as a target to prevent feeding and induce toxicity. Further, these studies highlight a novel delivery approach for generating plant systemic activity of lipophilic insecticides. © 2022 Society of Chemical Industry.

ATP-sensitive inward rectifier potassium channels reveal functional linkage between salivary gland function and blood feeding in the mosquito, Aedes aegypti

Reducing saliva secretions into the vertebrate host reduces feeding efficacy by most hematophagous arthropods. However, seminal studies suggested saliva is not a prerequisite for blood feeding in Aedes aegypti. To test this paradigm, we manually transected the salivary duct of female A. aegypti and an inability to salivate was correlated to an inability to imbibe blood. These data justified testing the relevance of inwardly rectifying potassium (Kir) channels in the A. aegypti salivary gland as an antifeedant target site. Pharmacological activation of ATP-gated Kir (K_ATP) channels reduced the secretory activity of the salivary gland by 15-fold that led to near elimination of blood ingestion during feeding. The reduced salivation and feeding success nearly eliminated horizontal transmission and acquisition of Dengue virus-2 (DENV2). These data suggest mosquito salivation is a prerequisite for blood feeding and provide evidence that K_ATP channels are critical for salivation, feeding, and vector competency.

The salivary gland of Aedes aegypti is needed for efficient blood feeding, and disruption of ATP-gated Kir channels prevents salivation and blood feeding in A. aegypti as well as horizontal transmission and acquisition of Dengue virus2.

The Molecular Physiology and Toxicology of Inward Rectifier Potassium Channels in Insects

Inward rectifier K⁺ (Kir) channels have been studied extensively in mammals, where they play critical roles in health and disease. In insects, Kir channels have recently been found to be key regulators of diverse physiological processes in several tissues. The importance of Kir channels in insects has positioned them to serve as emerging targets for the development of insecticides with novel modes of action. In this article, we provide the first comprehensive review of insect Kir channels, highlighting the rapid progress made in understanding their molecular biology, physiological roles, pharmacology, and toxicology. In addition, we highlight key gaps in our knowledge and suggest directions for future research to advance our understanding of Kir channels and their roles in insect physiology. Further knowledge of their functional roles will also facilitate their exploitation as targets for controlling arthropod pests and vectors of economic, medical, and/or veterinary relevance.

Phylogenomics of Tick Inward Rectifier Potassium Channels and Their Potential as Targets to Innovate Control Technologies

This study was conducted to enhance the identification of novel targets to develop acaricides that can be used to advance integrated tick-borne disease management. Drivers for the emergence and re-emergence of tick-borne diseases affecting humans, livestock, and other domestic animals in many parts of the world include the increased abundance and expanded geographic distribution of tick species that vector pathogens. The evolution of resistance to acaricides among some of the most important tick vector species highlights the vulnerability of relying on chemical treatments for tick control to mitigate the health burden of tick-borne diseases. The involvement of inward rectifier potassium (Kir) channels in homeostasis, diuresis, and salivary gland secretion in ticks and other pests identified them as attractive targets to develop novel acaricides. However, few studies exist on the molecular characteristics of Kir channels in ticks. This bioinformatic analysis described Kir channels in 20 species of hard and soft ticks. Summarizing relevant investigations on Kir channel function in invertebrate pests allowed the phylogenomic study of this class of ion channels in ticks. How this information can be adapted to innovate tick control technologies is discussed.

AC-DC electropenetrography: fundamentals, controversies, and perspectives for arthropod pest management

Studying the intimate association of arthropods with their physical substrate is both important and challenging. It is important because substrate is a key determinant for organism fitness; challenging because the intricacies of this association are dynamic, and difficult to record and resolve. The advent of electropenetrography (EPG) and subsequent developments allowed researchers to overcome this challenge. Nonetheless, EPG research has been historically restricted to piercing-sucking hemipteran plant pests. Recently, its potential use has been greatly broadened for additional pests with instrument advances. Thus, blood-feeding arthropods and chewing feeders, as well as non-feeding behaviors like oviposition by both pests and parasitoids, are novel new targets for EPG research, with critical consequences for integrated pest management. EPG can explain mechanisms of crop damage, plant or animal pathogen transmission, and the effects of insecticides, antifeedants, repellents, or transgenic plants and animals, on specific behaviors of damage or transmission. This review broadly covers the principles and development of EPG technology, emphasizing controversies and challenges remaining with suggested research to overcome them. In addition, it summarizes 60+ years of basic and applied EPG research, and previews future directions for pest management. The goal is to stimulate new applications for this unique enabling technology. Published 2020. This article is a U.S. Government work and is in the public domain in the USA.