Molecular characterization of genes encoding inward rectifier potassium (Kir) channels in the bed bug (Cimex lectularius)

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.

Updates on ion and water transport by the Malpighian tubule

The Malpighian (renal) tubule is capable of transporting fluid at remarkable rates. This review will focus on recent insights into the mechanisms by which these high rates are achieved and controlled, with particular reference to the tubules of Drosophila melanogaster, in which the combination of physiology and genetics has led to particularly rapid progress. Like many vertebrate epithelia, the Drosophila tubule has specialized cell types, with active cation transport confined to a large, metabolically active principal cell; whereas the smaller intercalated stellate cell controls chloride and water shunts to achieve net fluid secretion. Recently, the genes underlying many of these processes have been identified, functionally validated and localized within the tubule. The imminent arrival of new types of post-genomic data (notably single cell sequencing) will herald an exciting era of new discovery.

Flonicamid and knockdown of inward rectifier potassium channel gene CsKir2B adversely affect the feeding and development of Chilo suppressalis

BACKGROUND

The selective insecticide flonicamid shows highly insecticidal activities against piercing-sucking insects and has been widely used for the control of Hemipteran insect pests, whereas its effects on Lepidopteran insect pests remain largely unknown. Recently, inward rectifier potassium (Kir) channel has been verified to be a target of flonicamid, however, functional characterization of Lepidopteran Kir genes is still lacking.

RESULTS

Flonicamid shows no insecticidal toxicity against Chilo suppressalis larvae. However, the feeding and growth of larvae were reversibly inhibited by flonicamid (50-1200 mg L^-1 ). Flonicamid treatment also remarkably reduced and delayed the pupation and eclosion of Chilo suppressalis. Additionally, five distinct Kir channel genes (CsKir1, CsKir2A, CsKir2B, CsKir3A and CsKir3B) were cloned from Chilo suppressalis. Expression profiles analysis revealed that CsKir2A was predominately expressed in the hindgut of larvae, whereas CsKir2B had high expressions in the Malpighian tubules and hindgut. RNA interference (RNAi)-mediated knockdown of CsKir2B significantly reduced the growth and increased the mortalities of larvae, whereas silencing of CsKir2A had no obvious effects on Chilo suppressalis.

CONCLUSION

Flonicamid exhibits adverse effects on the growth and development of Chilo suppressalis. CsKir2B might be involved in the feeding behavior of Chilo suppressalis. These results provide valuable information on the effects of flonicamid on non-target insects as well as the function of insect Kir channels, and are helpful in developing new insecticide targeting insect Kir channels. © 2020 Society of Chemical Industry.

Identification and Expression of Inward-Rectifying Potassium Channel Subunits in Plutella xylostella

In insects, inward-rectifying potassium (Kir) channels regulate vital physiological functions, such as feeding behavior, silk secretion, renal excretion, and immune function. Therefore, they offer promising potential as targets for insecticides. Three types of Kir subunits have been identified in Diptera and Hemiptera, but the Kir subunits of Lepidoptera still remain unclear. This study identified five Kir subunit genes (pxkir1, pxkir2, pxkir3A, pxkir3B, and pxkir4) in the transcriptome of Plutella xylostella. Phylogenetic analysis identified pxkir1, pxkir2, pxkir3A, and pxkir3B as orthologous genes of kir1–3 in other insects. Interestingly, pxkir4 may be encoding a new class of Kir subunit in Lepidoptera that has not been reported to date. To identify further Kir channel subunits of P. xylostella, the gene expression profiles of five pxkir genes were studied by quantitative real-time PCR. These pxkir genes are expressed throughout the development of P. xylostella. pxkir1 and pxkir2 were highly expressed in thoraxes and legs, while pxkir3 (3A and 3B) and pxkir4 had high expression levels in the midgut and Malpighian tubules. This study identified the composition and distribution of Kir subunits in P. xylostella for the first time, and provides useful information for the further study of Kir channel subunits in Lepidoptera.

Inward rectifier potassium (Kir) channels in the soybean aphid Aphis glycines: Functional characterization, pharmacology, and toxicology

Inward rectifier K+ (Kir) channels contribute to a variety of physiological processes in insects and are emerging targets for insecticide development. Previous studies on insect Kir channels have primarily focused on dipteran species (e.g., mosquitoes, fruit flies). Here we identify and functionally characterize Kir channel subunits in a hemipteran insect, the soybean aphid Aphis glycines, which is an economically important insect pest and vector of soybeans. From the transcriptome and genome of Ap. glycines we identified two cDNAs, ApKir1 and ApKir2, encoding Kir subunits that were orthologs of insect Kir1 and Kir2, respectively. Notably, a gene encoding a Kir3 subunit was absent from the transcriptome and genome of Ap. glycines, similar to the pea aphid Acyrthosiphon pisum. Heterologous expression of ApKir1 and ApKir2 in Xenopus laevis oocytes enhanced K+-currents in the plasma membrane; these currents were inhibited by barium and the small molecule VU041. Compared to ApKir2, ApKir1 mediated currents that were larger in magnitude, more sensitive to barium, and less inhibited by small molecule VU041. Moreover, ApKir1 exhibited stronger inward rectification compared to ApKir2. Topical application of VU041 in adult aphids resulted in dose-dependent mortality within 24 h that was more efficacious than flonicamid, an established insecticide also known to inhibit Kir channels. We conclude that despite the apparent loss of Kir3 genes in aphid evolution, Kir channels are important to aphid survival and represent a promising target for the development of new insecticides.

Microsphere-Based Nanoindentation for the Monitoring of Cellular Cortical Stiffness Regulated by MT1-MMP

Biophysical properties are intimately connected to metastatic functions and aggressiveness in cancers. Especially, cellular stiffness is regarded as a biomarker for the understanding of metastatic potential and drug sensitivity. Here, protease-mediated changes of cortical stiffness are identified due to the deformation of cytoskeleton alignment at a cortex. For the past few decades, membrane type 1-matrix metalloproteinase (MT1-MMP) has been well known as a kernel protease enriched in podosomes during metastasis for extracellular matrix degradation. However, the biophysical significance of MT1-MMP expressing cancer cells is still unknown. Therefore, the nanomechanics of cancer cells is analyzed by a nanoindentation using a microsphere-attached cantilever of atomic force microscopy (AFM). In conclusion, the results suggest that MT1-MMP has contributed as a key regulator in cytoskeletal deformation related with cancer metastasis. Particularly, the AFM-based nanoindentation system for the monitoring of cortical nanomechanics will be crucial to understand molecular networks in cancers.

Block of Kir channels by flonicamid disrupts salivary and renal excretion of insect pests

Flonicamid is a selective insecticide for the control of sap-sucking insects; it exerts toxic effects by inhibiting insect feeding. However, its molecular target remains elusive. In this study, we functionally characterized NlKir1 channels of the brown planthopper (Nilaparvata lugens) in HEK293 cells. Homomeric NlKir1 channels generated inward-rectifying K⁺ currents. Flonicamid inhibited NlKir1 channels at nanomolar concentrations. Furthermore, flonicamid inhibited honeydew and salivary secretions of planthoppers, and reduced the renal excretion of female mosquitoes in a dose-dependent manner. The inhibitory effect of flonicamid on fluid secretion of isolated Malpighian tubules from Culex pipiens pullens was comparable to that of the selective Kir1 inhibitor. The observed physiological alterations by flonicamid are likely mediated by Kir1 channels and could lead to the disruption of feeding behaviors and eventually lethality. Our study establishes the Kir1 channel as the target of flonicamid and provided new insights into the mode of action of flonicamid.

ATP-sensitive inwardly rectifying potassium channel regulation of viral infections in honey bees

Honey bees are economically important pollinators of a wide variety of crops that have attracted the attention of both researchers and the public alike due to unusual declines in the numbers of managed colonies in some parts of the world. Viral infections are thought to be a significant factor contributing to these declines, but viruses have proven a challenging pathogen to study in a bee model and interactions between viruses and the bee antiviral immune response remain poorly understood. In the work described here, we have demonstrated the use of flock house virus (FHV) as a model system for virus infection in bees and revealed an important role for the regulation of the bee antiviral immune response by ATP-sensitive inwardly rectifying potassium (KATP) channels. We have shown that treatment with the KATP channel agonist pinacidil increases survival of bees while decreasing viral replication following infection with FHV, whereas treatment with the KATP channel antagonist tolbutamide decreases survival and increases viral replication. Our results suggest that KATP channels provide a significant link between cellular metabolism and the antiviral immune response in bees.

Functional characterization of Aquaporin-like genes in the human bed bug Cimex lectularius

The bed bug Cimex lectularius is a blood-feeding re-emerging annoyance pest insect that has the ability to transmit Trypanosoma cruzi under experimental laboratory conditions. Aquaporins (AQPs) are water channel proteins that are essential in biological organisms. C. lectularius are constantly exposed to water-related stress, suggesting that AQPs may offer novel control avenues. We identified and cloned four AQPs from C. lectularius, assessed tissue and lifestage-specific expression, and characterized biochemical functions in vitro and in vivo. We identified an efficient water-specific AQP (ClAQP1), two aquaglyceroporins (ClGlp1 and ClGlp2) and a homolog of Drosophila melanogaster big brain (ClBib). ClGlp1 was only functional when co-expressed with the water-specific AQP. Simultaneous RNAi gene silencing of ClAQP1 and ClGlp1 significantly reduced water and urea excretion post blood feeding. The Bib homologue was enriched in embryos, exclusively expressed in ovaries, and when silenced, dramatically increased bug fecundity. Our data demonstrate that AQPs have critical roles in excretion, water homeostasis and reproduction in C. lectularius, and could be potential targets for control in this notorious pest.

ATP-sensitive inwardly rectifying potassium channel modulators alter cardiac function in honey bees

ATP-sensitive inwardly rectifying potassium (K_ATP) channels couple cellular metabolism to the membrane potential of the cell and play an important role in a variety of tissue types, including the insect dorsal vessel, making them a subject of interest not only for understanding invertebrate physiology, but also as a potential target for novel insecticides. Most of what is known about these ion channels is the result of work performed in mammalian systems, with insect studies being limited to only a few species and physiological systems. The goal of this study was to investigate the role that K_ATP channels play in regulating cardiac function in a model social insect, the honey bee (Apis mellifera), by examining the effects that modulators of these ion channels have on heart rate. Heart rate decreased in a concentration-dependent manner, relative to controls, with the application of the K_ATP channel antagonist tolbutamide and K_ATP channel blockers barium and magnesium, whereas heart rate increased with the application of a low concentration of the K_ATP channel agonist pinacidil, but decreased at higher concentrations. Furthermore, pretreatment with barium magnified the effects of tolbutamide treatment and eliminated the effects of pinacidil treatment at select concentrations. The data presented here confirm a role for K_ATP channels in the regulation of honey bee dorsal vessel contractions and provide insight into the underlying physiology that governs the regulation of bee cardiac function.

Dynamic expression of genes encoding subunits of inward rectifier potassium (Kir) channels in the yellow fever mosquito Aedes aegypti

Inward rectifier potassium (Kir) channels play fundamental roles in neuromuscular, epithelial, and endocrine function in mammals. Recent research in insects suggests that Kir channels play critical roles in the development, immune function, and excretory physiology of fruit flies and/or mosquitoes. Moreover, our group has demonstrated that mosquito Kir channels may serve as valuable targets for the development of novel insecticides. Here we characterize the molecular expression of 5 mRNAs encoding Kir channel subunits in the yellow fever mosquito, Aedes aegypti: Kir1, Kir2A-c, Kir2B, Kir2B', and Kir3. We demonstrate that 1) Kir mRNA expression is dynamic in whole mosquitoes, Malpighian tubules, and the midgut during development from 4th instar larvae to adult females, 2) Kir2B and Kir3 mRNA levels are reduced in 4th instar larvae when reared in water containing an elevated concentration (50mM) of KCl, but not NaCl, and 3) Kir mRNAs are differentially expressed in the Malpighian tubules, midgut, and ovaries within 24h after blood feeding. Furthermore, we provide the first characterization of Kir mRNA expression in the anal papillae of 4th instar larval mosquitoes, which indicates that Kir2A-c is the most abundant. Altogether, the data provide the first comprehensive characterization of Kir mRNA expression in Ae. aegypti and offer insights into the putative physiological roles of Kir subunits in this important disease vector.

Suppression of Bedbug's Reproduction by RNA Interference of Vitellogenin

Recent resurgence of the bedbug Cimex lectularius is a global problem on the public health. On account of the worldwide rise of insecticide-resistant bedbug populations, exploration of new approaches to the bedbug control and management is anticipated. In this context, gene silencing by RNA interference (RNAi) has been considered for its potential application to pest control and management, because RNAi enables specific suppression of target genes and thus flexible selection of target traits to be disrupted. In this study, in an attempt to develop a control strategy targeting reproduction of the bedbug, we investigated RNAi-mediated gene silencing of vitellogenin (Vg), a major yolk protein precursor essential for oogenesis. From the bedbug transcriptomes, we identified a typical Vg gene and a truncated Vg gene, which were designated as ClVg and ClVg-like, respectively. ClVg gene was highly expressed mainly in the fat body of adult females, which was more than 100 times higher than the expression level of ClVg-like gene, indicating that ClVg gene is the primary functional Vg gene in the bedbug. RNAi-mediated suppression of ClVg gene expression in adult females resulted in drastically reduced egg production, atrophied ovaries, and inflated abdomen due to hypertrophied fat bodies. These phenotypic consequences are expected not only to suppress the bedbug reproduction directly but also to deteriorate its feeding and survival indirectly via behavioral modifications. These results suggest the potential of ClVg gene as a promising target for RNAi-based population management of the bedbug.

Two inwardly rectifying potassium channels, Irk1 and Irk2, play redundant roles in Drosophila renal tubule function

Inwardly rectifying potassium channels play essential roles in renal physiology across phyla. Barium-sensitive K(+) conductances are found on the basolateral membrane of a variety of insect Malpighian (renal) tubules, including Drosophila melanogaster. We found that barium decreases the lumen-positive transepithelial potential difference in isolated perfused Drosophila tubules and decreases fluid secretion and transepithelial K(+) flux. In those insect species in which it has been studied, transcripts from multiple genes encoding inwardly rectifying K(+) channels are expressed in the renal (Malpighian) tubule. In Drosophila melanogaster, this includes transcripts of the Irk1, Irk2, and Irk3 genes. The role of each of these gene products in renal tubule function is unknown. We found that simultaneous knockdown of Irk1 and Irk2 in the principal cell of the fly tubule decreases transepithelial K(+) flux, with no additive effect of Irk3 knockdown, and decreases barium sensitivity of transepithelial K(+) flux by ∼50%. Knockdown of any of the three inwardly rectifying K(+) channels individually has no effect, nor does knocking down Irk3 simultaneously with Irk1 or Irk2. Irk1/Irk2 principal cell double-knockdown tubules remain sensitive to the kaliuretic effect of cAMP. Inhibition of the Na(+)/K(+)-ATPase with ouabain and Irk1/Irk2 double knockdown have additive effects on K(+) flux, and 75% of transepithelial K(+) transport is due to Irk1/Irk2 or ouabain-sensitive pathways. In conclusion, Irk1 and Irk2 play redundant roles in transepithelial ion transport in the Drosophila melanogaster renal tubule and are additive to Na(+)/K(+)-ATPase-dependent pathways.

Identification of life-stage and tissue-specific splice variants of an inward rectifying potassium (Kir) channel in the yellow fever mosquito Aedes aegypti

Inward-rectifier potassium (Kir) channels play key roles in nerve, muscle, and epithelial cells in mammals, but their physiological roles in insects remain to be determined. The yellow fever mosquito (Aedes aegypti) possesses five different genes encoding Kir channel subunits: Kir1, Kir2A, Kir2B, Kir2B', and Kir3. We have recently cloned and characterized the Kir1, Kir2B, and Kir3 cDNAs in the renal (Malpighian) tubules of adult female Ae. aegypti. Here we characterize the expression of the Kir2A gene in Ae. aegypti, which was not abundantly expressed in Malpighian tubules. We find that the 1) Kir2A gene is expressed primarily in the midgut and hindgut of adult female mosquitoes, and 2) Kir2A mRNAs are alternatively spliced into three distinct variants (Kir2A-a, -b, and -c). The deduced Kir2A proteins from these splice forms share a completely conserved transmembrane domain (a pore-forming domain flanked by two transmembrane-spanning segments), but possess novel NH2-terminal and/or COOH-terminal domains. Semi-quantitative RT-PCR analyses indicate that the splice variants exhibit both developmental- and tissue-specific expression. Lastly, we provide insights into the conservation of alternative splicing among the Kir2A genes of dipterans, which may add molecular diversity that compensates for the relatively limited number of Kir channel genes in insects compared to mammals.