Ticks transmit more pathogens to humans and animals than any other arthropod. We describe the 2.1 Gbp nuclear genome of the tick, Ixodes scapularis (Say), which vectors pathogens that cause Lyme disease, human granulocytic anaplasmosis, babesiosis and other diseases. The large genome reflects accumulation of repetitive DNA, new lineages of retro-transposons, and gene architecture patterns resembling ancient metazoans rather than pancrustaceans. Annotation of scaffolds representing ∼57% of the genome, reveals 20,486 protein-coding genes and expansions of gene families associated with tick–host interactions. We report insights from genome analyses into parasitic processes unique to ticks, including host ‘questing', prolonged feeding, cuticle synthesis, blood meal concentration, novel methods of haemoglobin digestion, haem detoxification, vitellogenesis and prolonged off-host survival. We identify proteins associated with the agent of human granulocytic anaplasmosis, an emerging disease, and the encephalitis-causing Langat virus, and a population structure correlated to life-history traits and transmission of the Lyme disease agent.
Ticks transmit a large number of pathogens that cause human diseases. Here, the authors sequence the genome of the tick Ixodes scapularis and uncover expansion of genes associated with parasitic processes unique to ticks and tick-host interactions.
•The challenges encountered in automating nematode motility.•Developments in machine vision, statistical imaging and tracking approaches.•The need for automated screening in the search for new anthelmintic drugs.•A possible roadmap for the development of automated phenotypic screening is suggested.
The scale of the damage worldwide to human health, animal health and agricultural crops resulting from parasitic nematodes, together with the paucity of treatments and the threat of developing resistance to the limited set of widely-deployed chemical tools, underlines the urgent need to develop novel drugs and chemicals to control nematode parasites. Robust chemical screens which can be automated are a key part of that discovery process. Hitherto, the successful automation of nematode behaviours has been a bottleneck in the chemical discovery process. As the measurement of nematode motility can provide a direct scalar readout of the activity of the neuromuscular system and an indirect measure of the health of the animal, this omission is acute. Motility offers a useful assay for high-throughput, phenotypic drug/chemical screening and several recent developments have helped realise, at least in part, the potential of nematode-based drug screening. Here we review the challenges encountered in automating nematode motility and some important developments in the application of machine vision, statistical imaging and tracking approaches which enable the automated characterisation of nematode movement. Such developments facilitate automated screening for new drugs and chemicals aimed at controlling human and animal nematode parasites (anthelmintics) and plant nematode parasites (nematicides).
Anthelmintic drug discovery; Automated drug screening; C. elegans; Nematode parasites
Adult house flies, Musca domestica L., are mechanical vectors of more than 100 devastating diseases that have severe consequences for human and animal health. House fly larvae play a vital role as decomposers of animal wastes, and thus live in intimate association with many animal pathogens.
We have sequenced and analyzed the genome of the house fly using DNA from female flies. The sequenced genome is 691 Mb. Compared with Drosophila melanogaster, the genome contains a rich resource of shared and novel protein coding genes, a significantly higher amount of repetitive elements, and substantial increases in copy number and diversity of both the recognition and effector components of the immune system, consistent with life in a pathogen-rich environment. There are 146 P450 genes, plus 11 pseudogenes, in M. domestica, representing a significant increase relative to D. melanogaster and suggesting the presence of enhanced detoxification in house flies. Relative to D. melanogaster, M. domestica has also evolved an expanded repertoire of chemoreceptors and odorant binding proteins, many associated with gustation.
This represents the first genome sequence of an insect that lives in intimate association with abundant animal pathogens. The house fly genome provides a rich resource for enabling work on innovative methods of insect control, for understanding the mechanisms of insecticide resistance, genetic adaptation to high pathogen loads, and for exploring the basic biology of this important pest. The genome of this species will also serve as a close out-group to Drosophila in comparative genomic studies.
Electronic supplementary material
The online version of this article (doi:10.1186/s13059-014-0466-3) contains supplementary material, which is available to authorized users.
Trichuriasis is a parasitic disease caused by the human whipworm, Trichuris trichiura. It affects millions worldwide, particularly in the tropics. This nematode parasite burrows into the colonic epithelium resulting in inflammation and morbidity, especially in children. Current treatment relies mainly on general anthelmintics such as mebendazole but resistance to these drugs is increasingly problematic. Therefore, new treatments are urgently required.
The prospect of using the retinoid X receptor (RXR) antagonist HX531 as a novel anthelmintic was investigated by carrying out multiple viability assays with the mouse whipworm Trichuris muris.
HX531 reduced both the motility and viability of T. muris at its L3, L4 and adult stages. Further, bioinformatic analyses show that the T. muris genome possesses an RXR-like receptor, a possible target for HX531.
The study suggested that Trichuris-specific RXR antagonists may be a source of much-needed novel anthelmintic candidates for the treatment of trichuriasis. The identification of an RXR-like sequence in the T. muris genome also paves the way for further research based on this new anthelmintic lead compound.
RXR; HX531; Trichuris muris; Nematode; Viability; Anthelmintic
The human Aβ peptide causes progressive paralysis when expressed in the muscles of the nematode worm, C. elegans. We have exploited this model of Aβ toxicity by carrying out an RNAi screen to identify genes whose reduced expression modifies the severity of this locomotor phenotype. Our initial finding was that none of the human orthologues of these worm genes is identical with the genome-wide significant GWAS genes reported to date (the “white zone”); moreover there was no identity between worm screen hits and the longer list of GWAS genes which included those with borderline levels of significance (the “grey zone”). This indicates that Aβ toxicity should not be considered as equivalent to sporadic AD. To increase the sensitivity of our analysis, we then considered the physical interactors (+1 interactome) of the products of the genes in both the worm and the white+grey zone lists. When we consider these worm and GWAS gene lists we find that 4 of the 60 worm genes have a +1 interactome overlap that is larger than expected by chance. Two of these genes form a chaperonin complex, the third is closely associated with this complex and the fourth gene codes for actin, the major substrate of the same chaperonin.
•A nicotinic acetylcholine receptor α-subunit (Rsanα1) was identified in Rhipicephalus sanguineus.•Rsanα1 was not restricted to the synganglion (“brain”).•Rsanα1 was functionally expressed in Xenopus oocytes.•Rsanα1 responded to acetylcholine, nicotine and choline.•Rsanα1 was unresponsive to imidacloprid and spinosad.
Ticks and tick-borne diseases have a major impact on human and animal health worldwide. Current control strategies rely heavily on the use of chemical acaricides, most of which target the CNS and with increasing resistance, new drugs are urgently needed. Nicotinic acetylcholine receptors (nAChRs) are targets of highly successful insecticides. We isolated a full-length nAChR α subunit from a normalised cDNA library from the synganglion (brain) of the brown dog tick, Rhipicephalus sanguineus. Phylogenetic analysis has shown this R. sanguineus nAChR to be most similar to the insect α1 nAChR group and has been named Rsanα1. Rsanα1 is distributed in multiple tick tissues and is present across all life-stages. When expressed in Xenopus laevis oocytes Rsanα1 failed to function as a homomer, with and without the addition of either Caenorhabditis elegans resistance-to-cholinesterase (RIC)-3 or X. laevis RIC-3. When co-expressed with chicken β2 nAChR, Rsanα1 evoked concentration-dependent, inward currents in response to acetylcholine (ACh) and showed sensitivity to nicotine (100 μM) and choline (100 μM). Rsanα1/β2 was insensitive to both imidacloprid (100 μM) and spinosad (100 μM). The unreliable expression of Rsanα1 in vitro suggests that additional subunits or chaperone proteins may be required for more robust expression. This study enhances our understanding of nAChRs in arachnids and may provide a basis for further studies on the interaction of compounds with the tick nAChR as part of a discovery process for novel acaricides.
Rhipicephalus sanguineus; Tick; Ion channel; Acaricide; Nicotinic acetylcholine receptor; Xenopus oocytes; Imidacloprid
Anthelmintic resistance is a major problem in livestock farming, especially of small ruminants, but our understanding of it has been limited by the difficulty in carrying out functional genetic studies on parasitic nematodes. An important nematode infecting sheep and goats is Haemonchus contortus; in many parts of the world this species is resistant to almost all the currently available drugs, including ivermectin. It is extremely polymorphic and to date it has proved impossible to relate any sequence polymorphisms to its ivermectin resistance status. Expression of candidate drug-resistance genes in Caenorhabditis elegans could provide a convenient means to study the effects of polymorphisms found in resistant parasites, but may be complicated by differences between the gene families of target and model organisms. We tested this using the glutamate-gated chloride channel (GluCl) gene family, which forms the ivermectin drug target and are candidate resistance genes. We expressed GluCl subunits from C. elegans and H. contortus in a highly resistant triple mutant C. elegans strain (DA1316) under the control of the avr-14 promoter; expression of GFP behind this promoter recapitulated the pattern previously reported for avr-14. Expression of ivermectin-sensitive subunits from both species restored drug sensitivity to transgenic worms, though some quantitative differences were noted between lines. Expression of an ivermectin-insensitive subunit, Hco-GLC-2, had no effect on drug sensitivity. Expression of a previously uncharacterised parasite-specific subunit, Hco-GLC-6, caused the transgenic worms to become ivermectin sensitive, suggesting that this subunit also encodes a GluCl that responds to the drug. These results demonstrate that both orthologous and paralogous subunits from C. elegans and H. contortus are able to rescue the ivermectin sensitivity of mutant C. elegans, though some quantitative differences were observed between transgenic lines in some assays. C. elegans is a suitable system for studying parasitic nematode genes that may be involved in drug resistance.
The simple nematode, Caenorhabditis elegans, possesses the most extensive known gene family of nicotinic acetylcholine receptor (nAChR)-like subunits. Whilst all show greatest similarity with nAChR subunits of both invertebrates and vertebrates, phylogenetic analysis suggests that just over half of these (32) may represent other members of the cys-loop ligand-gated ion channel superfamily. We have introduced a novel nomenclature system for these ‘Orphan’ subunits, designating them as lgc genes (ligand-gated ion channels of the cys-loop superfamily), which can also be applied in future to unnamed and uncharacterised members of the cys-loop ligand-gated ion channel superfamily. We present here the resulting updated version of the C. elegans nAChR gene family and related ligand-gated ion channel (lgc) genes.
Caenorhabditis elegans; gene family; ion channel; nematode; nicotinic acetylcholine receptor
The power of fruit fly genetics is being deployed against some of the most intractable and economically significant problems in modern medicine, the neurodegenerative diseases. Fly models of Alzheimer's disease can be exposed to the rich diversity of biological techniques that are available to the community and are providing new insights into disease mechanisms, and assisting in the identification of novel targets for therapy. Similar approaches might also help us to interpret the results of genome-wide association studies of human neurodegenerative diseases by allowing us to triage gene “hits” according to whether a candidate risk factor gene has a modifying effect on the disease phenotypes in fly model systems.
The "thrashing assay", in which nematodes are placed in liquid and the frequency of lateral swimming ("thrashing") movements estimated, is a well-established method for measuring motility in the genetic model organism Caenorhabditis elegans as well as in parasitic nematodes. It is used as an index of the effects of drugs, chemicals or mutations on motility and has proved useful in identifying mutants affecting behaviour. However, the method is laborious, subject to experimenter error, and therefore does not permit high-throughput applications. Existing automation methods usually involve analysis of worm shape, but this is computationally demanding and error-prone. Here we present a novel, robust and rapid method of automatically counting the thrashing frequency of worms that avoids morphometry but nonetheless gives a direct measure of thrashing frequency. Our method uses principal components analysis to remove the background, followed by computation of a covariance matrix of the remaining image frames from which the interval between statistically-similar frames is estimated.
We tested the performance of our covariance method in measuring thrashing rates of worms using mutations that affect motility and found that it accurately substituted for laborious, manual measurements over a wide range of thrashing rates. The algorithm used also enabled us to determine a dose-dependent inhibition of thrashing frequency by the anthelmintic drug, levamisole, illustrating the suitability of the system for assaying the effects of drugs and chemicals on motility. Furthermore, the algorithm successfully measured the actions of levamisole on a parasitic nematode, Haemonchus contortus, which undergoes complex contorted shapes whilst swimming, without alterations in the code or of any parameters, indicating that it is applicable to different nematode species, including parasitic nematodes. Our method is capable of analyzing a 30 s movie in less than 30 s and can therefore be deployed in rapid screens.
We demonstrate that a covariance-based method yields a fast, reliable, automated measurement of C. elegans motility which can replace the far more time-consuming, manual method. The absence of a morphometry step means that the method can be applied to any nematode that swims in liquid and, together with its speed, this simplicity lends itself to deployment in large-scale chemical and genetic screens.
Parasitic nematodes are of medical and veterinary importance, adversely affecting human health and animal welfare. Ascaris suum is a gastrointestinal parasite of pigs; in addition to its veterinary significance it is a good model of the human parasite Ascaris lumbricoides, estimated to infect ∼1.4 billion people globally. Anthelmintic drugs are essential to control nematode parasites, and nicotinic acetylcholine receptors (nAChRs) on nerve and muscle are the targets of cholinergic anthelmintics such as levamisole and pyrantel. Previous genetic analyses of nematode nAChRs have been confined to Caenorhabditis elegans, which is phylogenetically distinct from Ascaris spp. and many other important parasites. Here we report the cloning and expression of two nAChR subunit cDNAs from A. suum. The subunits are very similar in sequence to C. elegans UNC-29 and UNC-38, are expressed on muscle cells and can be expressed robustly in Xenopus oocytes to form acetylcholine-, nicotine-, levamisole- and pyrantel-sensitive channels. We also demonstrate that changing the stoichiometry of the receptor by injecting different ratios of the subunit cRNAs can reproduce two of the three pharmacological subtypes of nAChR present in A. suum muscle cells. When the ratio was 5∶1 (Asu-unc-38∶Asu-unc-29), nicotine was a full agonist and levamisole was a partial agonist, and oocytes responded to oxantel, but not pyrantel. At the reverse ratio (1∶5 Asu-unc-38∶Asu-unc-29), levamisole was a full agonist and nicotine was a partial agonist, and the oocytes responded to pyrantel, but not oxantel. These results represent the first in vitro expression of any parasitic nicotinic receptor and show that their properties are substantially different from those of C. elegans. The results also show that changing the expression level of a single receptor subunit dramatically altered the efficacy of some anthelmintic drugs. In vitro expression of these subunits may permit the development of parasite-specific screens for future anthelmintics.
Ascarid nematodes are major pathogens of humans and livestock. The major method of control is by the use of anthelmintic drugs, many of which target the nervous system. Drugs such as levamisole, pyrantel and oxantel target the nicotinic acetylcholine receptors present on muscle. Nematodes have several such receptors, and until now these have been best understood in the model species Caenorhabditis elegans. We have started to characterise the nicotinic receptors of Ascaris suum, and find that the genetics and pharmacology of the A. suum receptors differ from C. elegans. In both species, nicotine and levamisole preferentially activate different forms of the nicotinic receptor, the N- and L-type, respectively. In C. elegans, the L-type receptor is made up of five subunits, whereas the N-type is a homomer of a sixth subunit. We can recapitulate many of the properties of the A. suum N- and L-type receptors, including their sensitivity to two other important anthelmintics, pyrantel and oxantel, by expressing just two subunits at varying ratios. This has implications for the use of drug combinations and for cross-resistance between nicotinic anthelmintics. It may start to give an explanation for the varying effectiveness of nicotinic drugs against different parasites.
Spinal muscular atrophy is the most common genetic cause of infant mortality and is characterized by degeneration of lower motor neurons leading to muscle wasting. The causative gene has been identified as survival motor neuron (SMN). The invertebrate model organism Caenorhabditis elegans contains smn-1, the ortholog of human SMN. Caenorhabditis elegans smn-1 is expressed in various tissues including the nervous system and body wall muscle, and knockdown of smn-1 by RNA interference is embryonic lethal. Here we show that the smn-1(ok355) deletion, which removes most of smn-1 including the translation start site, produces a pleiotropic phenotype including late larval arrest, reduced lifespan, sterility as well as impaired locomotion and pharyngeal activity. Mutant nematodes develop to late larval stages due to maternal contribution of the smn-1 gene product that allows to study SMN-1 functions beyond embryogenesis. Neuronal, but not muscle-directed, expression of smn-1 partially rescues the smn-1(ok355) phenotype. Thus, the deletion mutant smn-1(ok355) provides a useful platform for functional analysis of an invertebrate ortholog of the human SMN protein.
Neonicotinoid insecticides, which act on nicotinic acetylcholine receptors (nAChRs) in a variety of ways, have extremely low mammalian toxicity, yet the molecular basis of such actions is poorly understood. To elucidate the molecular basis for nAChR–neonicotinoid interactions, a surrogate protein, acetylcholine binding protein from Lymnaea stagnalis (Ls-AChBP) was crystallized in complex with neonicotinoid insecticides imidacloprid (IMI) or clothianidin (CTD). The crystal structures suggested that the guanidine moiety of IMI and CTD stacks with Tyr185, while the nitro group of IMI but not of CTD makes a hydrogen bond with Gln55. IMI showed higher binding affinity for Ls-AChBP than that of CTD, consistent with weaker CH–π interactions in the Ls-AChBP–CTD complex than in the Ls-AChBP–IMI complex and the lack of the nitro group-Gln55 hydrogen bond in CTD. Yet, the NH at position 1 of CTD makes a hydrogen bond with the backbone carbonyl of Trp143, offering an explanation for the diverse actions of neonicotinoids on nAChRs.
Acetylcholine binding protein (Lymnaea stagnalis); Crystal structures; Neonicotinoids; Nicotinic acetylcholine receptors; Ion channels
The nematode, Caenorhabditis elegans, possesses the most extensive known superfamily of cys-loop ligand-gated ion channels (cys-loop LGICs) consisting of 102 subunit-encoding genes. Less than half of these genes have been functionally characterised which include cation-permeable channels gated by acetylcholine (ACh) and γ-aminobutyric acid (GABA) as well as anion-selective channels gated by ACh, GABA, glutamate and serotonin. Following the guidelines set for genetic nomenclature for C. elegans, we have designated unnamed subunits as lgc genes (ligand-gated ion channels of the cys-loop superfamily). Phylogenetic analysis shows that several of these lgc subunits form distinct groups which may represent novel cys-loop LGIC subtypes.
Caenorhabditis elegans; Ion channel; Acetylcholine; GABA; Glutamate; Serotonin
Members of the cys-loop ligand-gated ion channel (cys-loop LGIC) superfamily mediate chemical neurotransmission and are studied extensively as potential targets of drugs used to treat neurological disorders such as Alzheimer's disease. Insect cys-loop LGICs are also of interest as they are targets of highly successful insecticides. The red flour beetle, Tribolium castaneum, is a major pest of stored agricultural products and is also an important model organism for studying development.
As part of the T. castaneum genome sequencing effort, we have characterized the beetle cys-loop LGIC superfamily which is the third insect superfamily to be described after those of Drosophila melanogaster and Apis mellifera, and also the largest consisting of 24 genes. As with Drosophila and Apis, Tribolium possesses ion channels gated by acetylcholine, γ-amino butyric acid (GABA), glutamate and histamine as well as orthologs of the Drosophila pH-sensitive chloride channel subunit (pHCl), CG8916 and CG12344. Similar to Drosophila and Apis, Tribolium cys-loop LGIC diversity is broadened by alternative splicing although the beetle orthologs of RDL and GluCl possess more variants of exon 3. Also, RNA A-to-I editing was observed in two Tribolium nicotinic acetylcholine receptor subunits, Tcasα6 and Tcasβ1. Editing in Tcasα6 is evolutionarily conserved with D. melanogaster, A. mellifera and Heliothis virescens, whereas Tcasβ1 is edited at a site so far only observed in the beetle.
Our findings reveal that in diverse insect species the cys-loop LGIC superfamily has remained compact with only minor changes in gene numbers. However, alternative splicing, RNA editing and the presence of divergent subunits broadens the cys-loop LGIC proteome and generates species-specific receptor isoforms. These findings on Tribolium castaneum enhance our understanding of cys-loop LGIC functional genomics and provide a useful basis for the development of improved insecticides that target an important agricultural pest.
Nicotinic acetylcholine receptors (nAChRs) are pentameric proteins that are important drug targets for a variety of diseases including Alzheimer's, schizophrenia and various forms of epilepsy. One of the most intensively studied nAChR subunits in recent years has been α7. This subunit can form functional homomeric pentamers (α7)5, which can make interpretation of physiological and structural data much simpler. The growing amount of structural, pharmacological and physiological data for these receptors indicates the need for a dedicated and accurate database to provide a means to access this information in a coherent manner.
A7DB is a new relational database of manually curated experimental physiological data associated with the α7 nAChR. It aims to store as much of the pharmacology, physiology and structural data pertaining to the α7 nAChR. The data is accessed via web interface that allows a user to search the data in multiple ways: 1) a simple text query 2) an incremental query builder 3) an interactive query builder and 4) a file-based uploadable query. It currently holds more than 460 separately reported experiments on over 85 mutations.
A7DB will be a useful tool to molecular biologists and bioinformaticians not only working on the α7 receptor family of proteins but also in the more general context of nicotinic receptor modelling. Furthermore it sets a precedent for expansion with the inclusion of all nicotinic receptor families and eventually all cys-loop receptor families.
The GABA-modulating and GABA-mimetic activities of the monoterpenoid thymol were explored on human GABAA and Drosophila melanogaster homomeric RDLac GABA receptors expressed in Xenopus laevis oocytes, voltage-clamped at −60 mV. The site of action of thymol was also investigated.Thymol, 1–100 μM, resulted in a dose-dependent potentiation of the EC20 GABA response in oocytes injected with either α1β3γ2s GABAA subunit cDNAs or the RDLac subunit RNA. At 100 μM thymol, current amplitudes in response to GABA were 416±72 and 715±85% of controls, respectively. On both receptors, thymol, 100 μM, elicited small currents in the absence of GABA.The EC50 for GABA at α1β3γ2s GABAA receptors was reduced by 50 μM thymol from 15±3 to 4±1 μM, and the Hill slope changed from 1.35±0.14 to 1.04±0.16; there was little effect on the maximum GABA response.Thymol (1–100 μM) potentiation of responses to EC20 GABA for α1β1γ2s, α6β3γ2s and α1β3γ2s human GABAA receptors was almost identical, arguing against actions at benzodiazepine or loreclezole sites.Neither flumazenil, 3-hydroxymethyl-β-carboline (3-HMC), nor 5α-pregnane-3α, 20α-diol (5α-pregnanediol) affected thymol potentiation of the GABA response at α1β3γ2s receptors, providing evidence against actions at the benzodiazepine/β-carboline or steroid sites. Thymol stimulated the agonist actions of pentobarbital and propofol on α1β3γ2s receptors, consistent with a mode of action distinct from that of either compound. These data suggest that thymol potentiates GABAA receptors through a previously unidentified binding site.
Thymol; thyme essential oil; monoterpenoids; ionotropic GABA receptor; Drosophila melanogaster; allosteric modulation
Neonicotinoid insecticides are agonists of insect nicotinic acetylcholine receptors (AChRs) and show selective toxicity for insects over vertebrates. To elucidate the molecular basis of the selectivity, amino acid residues influencing neonicotinoid sensitivity were investigated by site-directed mutagenesis of the chicken α7 nicotinic AChR subunit, based on the crystal structure of an ACh binding protein (AChBP).In the ligand binding site of AChBP, Q55 in loop D is close to Y164 in loop F that corresponds to G189 of the α7 nicotinic receptor. Since Q55 of AChBP is preserved as Q79 in the α7 nicotinic receptor and the G189D and G189E mutations have been found to reduce the neonicotinoid sensitivity, we investigated effects of Q79E, Q79K and Q79R mutations on the neonicotinoid sensitivity of the α7 receptor expressed in Xenopus laevis oocytes to evaluate contributions of the glutamine residue to nicotinic AChR–neonicotinoid interactions.The Q79E mutation markedly reduced neonicotinoid sensitivity of the α7 nicotinic AChR whereas the Q79K and Q79R mutations increased sensitivity, suggesting electronic interactions of the neonicotinoids with the added residues.By contrast, the Q79E mutation scarcely influenced responses of the α7 nicotinic receptor to ACh, (−)-nicotine and desnitro–imidacloprid (DN–IMI), an imidacloprid derivative lacking the nitro group, whereas the Q79K and Q79R mutations reduced the sensitivity to these ligands. The results indicate that the glutamine residue of the α7 nicotinic receptor is likely to be located close to the nitro group of the insecticides in the nicotinic receptor–insecticide complex.
Imidacloprid; nitenpyram; neonicotinoid; desnitro-imidacloprid; nicotinic acetylcholine receptor; chicken α7 subunit; loop D; loop F; Xenopus laevis oocyte; two-electrode voltage-clamp
We report the cloning and expression of a novel Caenorhabditis elegans polypeptide, GLC-3, with high sequence identity to previously cloned L-glutamate-gated chloride channel subunits from nematodes and insects.Expression of glc-3 cRNA in Xenopus oocytes resulted in the formation of homo-oligomeric L-glutamate-gated chloride channels with robust and rapidly desensitizing currents, an EC50 of 1.9±0.03 mM and a Hill coefficient of 1.5±0.1. GABA, glycine, histamine and NMDA all failed to activate the GLC-3 homo-oligomer at concentrations of 1 mM. The anthelminthic, ivermectin, directly and irreversibly activated the L-glutamate-gated channel with an EC50 of 0.4±0.02 μM.The GLC-3 channels were selective for chloride ions, as shown by the shift in the reversal potential for L-glutamate-gated currents after the reduction of external Cl− from 107.6 to 62.5 mM.Picrotoxinin failed to inhibit L-glutamate agonist responses at concentrations up to 1 mM. The polycyclic dinitrile, 3,3-bis-trifluoromethyl-bicyclo[2,2,1]heptane-2,2-dicarbonitrile (BIDN), completely blocked L-glutamate-induced chloride currents recorded from oocytes expressing GLC-3 with an IC50 of 0.2±0.07 μM. The phenylpyrazole insecticide, fipronil, reversibly inhibited L-glutamate-gated currents recorded from the GLC-3 receptor with an IC50 of 11.5±0.11 μM.In this study, we detail the unusual antagonist pharmacology of a new GluCl subunit from C. elegans. Unlike all other native and recombinant nematode GluCl reported to date, the GLC-3 receptor is insensitive to picrotoxinin, but is sensitive to two other channel blockers, BIDN and fipronil. Further study of this receptor may provide insights into the molecular basis of non-competitive antagonism by these compounds.
Glutamate-gated chloride channel; Caenorhabditis elegans; ivermectin; BIDN; fipronil; picrotoxinin
Decarbomethoxyllated JW062 (DCJW), the active component of a new oxadiazine insecticide DPX-JW062 (Indoxacarb), was tested on action potentials and the inward sodium current recorded from short-term cultured dorsal unpaired median neurones of the cockroach Periplaneta americana.Under whole-cell current-clamp conditions, 100 nM DCJW reduced the amplitude of action potentials and induced a large hyperpolarization of the resting membrane potential associated with a 41% increase in input resistance.In voltage-clamp, DCJW resulted in a dose-dependent inhibition (IC50 28 nM) of the peak sodium current. Based on IC50 values, the effect of DCJW was about 10 fold less potent than tetrodotoxin (TTX) but 1000 fold more potent than the local anaesthetic lidocaine. DCJW (100 nM) was without effect on activation properties of the sodium current, reversal potential, voltage dependence of sodium conductance and on both fast and slow steady-state inactivations.TTX (2 nM) resulted in 48% inhibition of the peak inward sodium current. Co-application of TTX (2 nM) with various concentrations of DCJW produced an additional inhibition of the peak inward current, indicating that the blocking actions of DCJW and TTX were distinct. Co-application of lidocaine (IC50 30 μM) with various concentrations of DCJW produced a reduction of the apparent potency of DCJW, suggesting that DCJW and lidocaine acted at the same site.DCJW (100 nM) did not affect inward calcium or outward potassium currents.This study describes, for the first time, the action on insect neuronal voltage-dependent sodium channels of Indoxacarb, a new class of insecticides.
Insect; DUM neurones; oxadiazine; insecticide; local anaesthetic; neuronal sodium channels
Single channel recordings were obtained from a Drosophila S2 cell line stably expressing the wild-type RDLac Drosophila melanogaster homomer-forming ionotropic GABA receptor subunit, a product of the resistance to dieldrin gene, Rdl. GABA (50 μM) was applied by pressure ejection to outside-out patches from S2-RDL cells at a holding potential of −60 mV. The resulting inward current was completely blocked by 100 μM picrotoxin (PTX). The unitary current-voltage relationship was linear at negative potentials but showed slight inward rectification at potentials more positive than 0 mV. The reversal potential of the current (EGABA=−1.4 mV) was close to the calculated chloride equilibrium potential.The single channel conductance elicited by GABA was 36 pS. A 71 pS conductance channel was also observed when the duration of the pulse, used to eject GABA, was longer than 80 ms. The mean open time distribution of the unitary events was fitted best by two exponential functions suggesting two open channel states.When either 1 μM fipronil or 1 μM BIDN was present in the external saline, the GABA-gated channels were completely blocked. When BIDN or fipronil was applied at a concentration close to the IC50 value for suppression of open probability (281 nM, BIDN; 240 nM, fipronil), the duration of channel openings was shortened. In addition, the blocking action of BIDN resulted in the appearance of a novel channel conductance (17 pS).The effects of co-application of BIDN and fipronil were examined. Co-application of BIDN (300 nM) with various concentrations (100–1000 nM) of fipronil resulted in an additional BIDN-induced dose-dependent reduction of the maximum Po value.Thus both BIDN and fipronil shorten the duration of wild-type RDLac GABA receptor channel openings but appear to act at distinct sites.
GABA receptor; BIDN; fipronil; Drosophila melanogaster; stable cell line; single channel
The nitroguanidine insecticide imidacloprid along with a second generation of related compounds including nitenpyram, all nicotinic acetylcholine (ACh) receptor ligands, are used increasingly in many countries. Site-directed mutagenesis and heterologous expression in Xenopus laevis oocytes have been deployed to investigate mutants (G189D and G189E) of the chicken α7 homomer-forming nicotinic receptor subunit which are predicted to enhance the negative charge at the negative subsite (loop D) of the ACh binding site.Xenopus oocytes expressing wild-type α7 nicotinic receptors respond to imidacloprid with rapid inward currents. Imidacloprid and nitenpyram are partial agonists, whereas ACh, (−)-nicotine and (+)-epibatidine are full agonists.Compared to wild-type α7, the mutant G189D and G189E receptors are much less sensitive to the insecticides, whereas their sensitivity to (−)-nicotine, ACh and (+)-epibatidine is only slightly reduced. In contrast, G189N and G189Q mutants are sensitive not only to ACh, (−)-nicotine and (+)-epibatidine, but also to the two insecticides. Thus reduction of the insecticide-sensitivity by the mutations G189D and G189E are attributed to an increase in negativity of loop D. Desnitro-imidacloprid (DN-IMI), an imidacloprid derivative lacking the nitro group is a potent agonist on the G189D and G189E mutants suggesting an important role of loop D in nicotinic receptor interactions with the nitro group of nitroguanidine insecticides.
Nicotinic acetylcholine receptor; α7 subunit; negative subsite mutations; neonicotinoids; imidacloprid; nitenpyram; (+)-epibatidine
A novel tricyclic dinitrile, KN244, blocked the wild-type (dieldrin-sensitive) homo-oligomeric γ-aminobutyric acid (GABA)-gated chloride channel of Drosophila melanogaster expressed in Xenopus oocytes. Sensitivity to the block by KN244 of the response to 30 μM GABA (IC50=41.6 nM, wild-type RDLac) was reduced abut 100 fold (IC50=4.5 μM) in the dieldrin-resistant (RDLacA302S) form of RDL.
GABA-gated Cl− channel; KN244 (a tricyclic dinitrile); Drosophila melanogaster; RDL (resistant to dieldrin) subunit
Site-directed mutagenesis was used to create an altered form of the chicken α7 nicotinic acetylcholine (ACh) receptor subunit (α7x61) in which a leucine residue was inserted between residues Leu9′ and Ser10′ in transmembrane domain 2. The properties of α7x61 receptors are distinct from those of the wild-type receptor.Oocytes expressing wild-type α7 receptors responded to 10 μM nicotine with rapid inward currents that desensitized with a time-constant of 710±409 ms (mean±s.e.mean, n=5). However in α7x61 receptors 10 μM nicotine resulted in slower onset inward currents that desensitized with a time-constant of 5684±3403 ms (mean±s.e.mean, n=4). No significant difference in the apparent affinity of nicotine or acetylcholine between mutant and wild-type receptors was observed. Dihydro-β-erythroidine (DHβE) acted as an antagonist on both receptors.Molecular modelling of the α7x61 receptor channel pore formed by a bundle of M2 α-helices suggested that three of the channel lining residues would be altered by the leucine insertion i.e.; Ser10′ would be replaced by the leucine insertion, Val13′ and Phe14′ would be replaced, by Thr and Val, respectively.When present in the LEV-1 nicotinic ACh receptor subunit from Caenorhabditis elegans the same alteration conferred resistance to levamisole anthelmintic drug. Levamisole blocked responses to nicotine of wild-type and α7x61 receptors. However, block was more dependent on membrane potential for the α7x61 receptors.We conclude that the leucine insertion in transmembrane domain 2 has the unusual effect of slowing desensitization without altering apparent agonist affinity.
Nicotinic; acetylcholine receptor (nicotinic AChR); α7 subunit; channel mutation; desensitization; agonist affinity; levamisole
The radiolabelled bicyclic dinitrile, [3H]-3,3-bis-trifluoromethyl-bicyclo[2.2.1]heptane-2,2-dicarbonitrile ([3H]-BIDN), exhibited, specific binding of high affinity to membranes of the southern corn rootworm (Diabrotica undecimpunctata howardi) and other insects. A variety of γ-aminobutyric acid (GABA) receptor convulsants, including the insecticides heptachlor (IC50, 35±3 nM) and dieldrin (IC50, 93±7 nM), displaced [3H]-BIDN from rootworm membranes. When tested at 100 μM, 1-(4-ethynylphenyl)-4-n-propyl-2,6,7-trioxabicyclo[2.2.2]octane(EBOB), 4-t-butyl-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane-1-thione (TBPS), 1-phenyl-4-t-butyl-2,6,7-trioxabicyclo[2.2.2]octane (TBOB) and picrotoxin failed to displace 50% of [3H]-BIDN binding to rootworm membranes indicating that the bicyclic dinitrile radioligand probes a site distinct from those identified by other convulsant radioligands.Dissociation studies showed that dieldrin, ketoendrin, toxaphene, heptachlor epoxide and α and β endosulphan displace bound [3H]-BIDN from rootworm membranes by a competitive mechanism.Rat brain membranes were also shown to possess a population of saturable, specific [3H]-BIDN binding sites, though of lower affinity than in rootworm and with a different pharmacological profile. Of the insecticidal GABAergic convulsants that displaced [3H]-BIDN from rootworm, cockroach (Periplaneta americana) and rat brain membranes, many were more effective in rootworm.Functional GABA-gated chloride channels of rootworm nervous system and of cockroach nerve and muscle were blocked by BIDN, whereas cockroach neuronal GABAB receptors were unaffected.Expression in Xenopus oocytes of either rat brain mRNA, or cDNA-derived RNA encoding a GABA receptor subunit (Rdl) that is expressed widely in the nervous system of Drosophila melanogaster resulted in functional, homo-oligomeric GABA receptors that were blocked by BIDN. Thus, BIDN probes a novel site on GABA-gated Cl− channels to which a number of insecticidally-active molecules bind.
Ionotropic GABA receptors; Rdl gene; [3H]-BIDN; convulsants; dieldrin