Splice-variant- and stage-specific RNA editing of the Drosophila GABA receptor modulates agonist potency

Grapefruit-derived nootkatone potentiates GABAergic signaling and acts as a dual-action mosquito repellent and insecticide

Humanity has long battled mosquitoes and the diseases they transmit-a struggle intensified by climate change and globalization, which have expanded mosquito ranges and the spread of associated diseases.1 Additionally, widespread insecticide resistance has reduced the efficacy of current control methods, necessitating new solutions.2,3 Nootkatone, a natural compound found in grapefruit, shows promise as both a mosquito repellent and an insecticide.4,5 However, its mechanism of action remains unclear. Our study demonstrates that nootkatone acts as a potent spatial and contact repellent against multiple mosquito species. Nootkatone-induced spatial aversion, which is influenced by human odor, is in Aedes aegypti partially mediated by Orco- and ionotropic receptor (IR)-positive neurons, while contact aversion is robust and likely mediated via the proboscis and independent of TRPA1 and IRs. We further find that nootkatone potentiates γ-aminobutyric acid (GABA)-mediated signaling by modulating the broadly expressed major insect GABA-gated chloride channel resistant to dieldrin (Rdl). At low doses, the chemosensory-mediated spatial and contact repellency is likely strengthened by nootkatone's disruption of synaptic transmission in select mosquito sensory neurons. At higher doses, nootkatone induces paralysis and death, presumably through broad-range synaptic transmission disruption. These findings reveal nootkatone's unique mode of action and highlight its potential as an effective mosquito control agent. Its dual role as a repellent and an insecticide, combined with low-to-no toxicity to humans and a pleasant smell, underscores nootkatone's promise as a future tool in mosquito control efforts.

A highly conserved A-to-I RNA editing event within the glutamate-gated chloride channel GluClα is necessary for olfactory-based behaviors in Drosophila

A-to-I RNA editing is a cellular mechanism that generates transcriptomic and proteomic diversity, which is essential for neuronal and immune functions. It involves the conversion of specific adenosines in RNA molecules to inosines, which are recognized as guanosines by cellular machinery. Despite the vast number of editing sites observed across the animal kingdom, pinpointing critical sites and understanding their in vivo functions remains challenging. Here, we study the function of an evolutionary conserved editing site in Drosophila, located in glutamate-gated chloride channel (GluClα). Our findings reveal that flies lacking editing at this site exhibit reduced olfactory responses to odors and impaired pheromone-dependent social interactions. Moreover, we demonstrate that editing of this site is crucial for the proper processing of olfactory information in projection neurons. Our results highlight the value of using evolutionary conservation as a criterion for identifying editing events with potential functional significance and paves the way for elucidating the intricate link between RNA modification, neuronal physiology, and behavior.

The Fine Art of Writing a Message: RNA Metabolism in the Shaping and Remodeling of the Nervous System

Neuronal morphogenesis, integration into circuits, and remodeling of synaptic connections occur in temporally and spatially defined steps. Accordingly, the expression of proteins and specific protein isoforms that contribute to these processes must be controlled quantitatively in time and space. A wide variety of post-transcriptional regulatory mechanisms, which act on pre-mRNA and mRNA molecules contribute to this control. They are thereby critically involved in physiological and pathophysiological nervous system development, function, and maintenance. Here, we review recent findings on how mRNA metabolism contributes to neuronal development, from neural stem cell maintenance to synapse specification, with a particular focus on axon growth, guidance, branching, and synapse formation. We emphasize the role of RNA-binding proteins, and highlight their emerging roles in the poorly understood molecular processes of RNA editing, alternative polyadenylation, and temporal control of splicing, while also discussing alternative splicing, RNA localization, and local translation. We illustrate with the example of the evolutionary conserved Musashi protein family how individual RNA-binding proteins are, on the one hand, acting in different processes of RNA metabolism, and, on the other hand, impacting multiple steps in neuronal development and circuit formation. Finally, we provide links to diseases that have been associated with the malfunction of RNA-binding proteins and disrupted post-transcriptional regulation.

Novel alternative splicing of GABA receptor RDL exon 9 from Laodelphax striatellus modulates agonist potency

The resistance to dieldrin gene (RDL) encodes the primary subunit of the insect ionotropic γ-aminobutyric acid (GABA) receptor (GABAR), which is the target of phenylpyrazole and isoxazoline insecticides. The splice variants in exons 3 and 6 of RDL, which have been widely explored in many insects, modulate the agonist potency of the homomeric RDL GABAR and potentially play an important role in the development of insects. In the present study, four splice variants of exon 9 were identified in RDL of the small brown planthopper, Laodelphax striatellus (LsRDL), resulting in LsRDL-9a, LsRDL-9a', LsRDL-9b, and LsRDL-9c. LsRDL-9a has one more amino acid (E, glutamic acid) compared with LsRDL-9a', and LsRDL-9b lacked two amino acids and had seven different amino acids compared with LsRDL-9c. Two-electrode voltage-clamp recording on LsRDLs expressed in Xenopus oocytes showed that alternative splicing of exon 9 has significant impact on LsRDL sensitivity to the agonists GABA and β-alanine, whereas no significant difference was observed in the potencies of the non-competitive antagonists (NCAs) ethiprole and fluralaner on the splice variants. Our results suggest that alternative splicing of RDL exon 9 broadens functional capabilities of the GABAR in L. striatellus by influencing the action of GABA.

The molecular targets of ivermectin and lotilaner in the human louse Pediculus humanus humanus: New prospects for the treatment of pediculosis

Control of infestation by cosmopolitan lice (Pediculus humanus) is increasingly difficult due to the transmission of parasites resistant to pediculicides. However, since the targets for pediculicides have no been identified in human lice so far, their mechanisms of action remain largely unknown. The macrocyclic lactone ivermectin is active against a broad range of insects including human lice. Isoxazolines are a new chemical class exhibiting a strong insecticidal potential. They preferentially act on the γ-aminobutyric acid (GABA) receptor made of the resistant to dieldrin (RDL) subunit and, to a lesser extent on glutamate-gated chloride channels (GluCls) in some species. Here, we addressed the pediculicidal potential of isoxazolines and deciphered the molecular targets of ivermectin and the ectoparasiticide lotilaner in the human body louse species Pediculus humanus humanus. Using toxicity bioassays, we showed that fipronil, ivermectin and lotilaner are efficient pediculicides on adult lice. The RDL (Phh-RDL) and GluCl (Phh-GluCl) subunits were cloned and characterized by two-electrode voltage clamp electrophysiology in Xenopus laevis oocytes. Phh-RDL and Phh-GluCl formed functional homomeric receptors respectively gated by GABA and L-glutamate with EC₅₀ values of 16.0 μM and 9.3 μM. Importantly, ivermectin displayed a super agonist action on Phh-GluCl, whereas Phh-RDL receptors were weakly affected. Reversally, lotilaner strongly inhibited the GABA-evoked currents in Phh-RDL with an IC₅₀ value of 40.7 nM, whereas it had no effect on Phh-GluCl. We report here for the first time the insecticidal activity of isoxazolines on human ectoparasites and reveal the mode of action of ivermectin and lotilaner on GluCl and RDL channels from human lice. These results emphasize an expected extension of the use of the isoxazoline drug class as new pediculicidal agents to tackle resistant-louse infestations in humans.

Human cosmopolitan lice are responsible for pediculosis, which represent a significant public health concern. Resistant lice against insecticides and lack of safety of the treatments for human and environment is a growing issue worldwide. Here we investigated the efficacy on lice of the classical macrocyclic lactone drug, ivermectin, and of the isoxazoline drug, lotilaner. This study was done to decipher their mode of action at the molecular and functional levels in order to propose new strategies to control lice infestation. Our bioassay results indicate that ivermectin and lotilaner were potent at killing human adult lice, with lotilaner showing a higher efficacy than ivermectin. Furthermore, we identified and pharmacologically characterized the first glutamate- and GABA-gated chloride channels ever described in human lice yet. Mechanistically, our molecular biology and electrophysiology findings demonstrate that ivermectin acted preferentially at glutamate channels, while lotilaner specifically targeted GABA channels. These results provide new insights in the understanding of the insecticide mode of action and highlight the potential of isoxazolines as a new alternative for the treatment of human lice.

Increased RNA editing in maternal immune activation model of neurodevelopmental disease

The etiology of major neurodevelopmental disorders such as schizophrenia and autism is unclear, with evidence supporting a combination of genetic factors and environmental insults, including viral infection during pregnancy. Here we utilized a mouse model of maternal immune activation (MIA) with the viral mimic PolyI:C infection during early gestation. We investigated the transcriptional changes in the brains of mouse fetuses following MIA during the prenatal period, and evaluated the behavioral and biochemical changes in the adult brain. The results reveal an increase in RNA editing levels and dysregulation in brain development-related gene pathways in the fetal brains of MIA mice. These MIA-induced brain editing changes are not observed in adulthood, although MIA-induced behavioral deficits are observed. Taken together, our findings suggest that MIA induces transient dysregulation of RNA editing at a critical time in brain development.

A unique primary structure of RDL (resistant to dieldrin) confers resistance to GABA-gated chloride channel blockers in the two-spotted spider mite Tetranychus urticae Koch

The primary structure of the second transmembrane (M2) segment of resistant to dieldrin (RDL), an ionotropic γ-aminobutyric acid receptor (GABAR) subunit, and the structure-function relationships in RDL are well conserved among insect species. An amino acid substitution at the 2' position in the M2 segment (Ala to Ser or Gly) confers resistance to non-competitive antagonists (NCAs) of GABARs. Here, a cDNA encoding RDL was cloned from the two-spotted spider mite Tetranychus urticae Koch. Unlike insect homologs, native TuRDL has His at the 2' position (H305) and Ile at 6' (I309) in the M2 segment and is insensitive to NCAs. Single and multiple mutations were introduced in the M2 segment of TuRDL, and the mutant proteins were expressed in Xenopus oocytes and examined for the restoration of sensitivity to NCAs. The sensitivity of a double mutant (H305A and I309T in the M2 segment) was greatly increased but was still considerably lower than that of insect RDLs. We therefore constructed chimeric RDLs consisting of TuRDL and Drosophila melanogaster RDL and examined their sensitivities to NCAs. The results show that the N-terminal region containing the Cys-loop as well as the M2 segment confers functional specificity; thus, our current understanding of the mechanism underlying NCA binding to GABARs requires reappraisal.

RNA editing: an overlooked source of fine-scale adaptation in insect vectors?

RNA editing is a source of molecular diversity that regulates the functional repertoire of animal transcriptomes. Multiple studies in Drosophila have revealed that conserved editing events can be a source of evolutionary adaptations, and there is a solid body of evidence linking editing and the fine-tuning of neural genes, which are often targeted by insecticides used in vector control. Yet, despite these suggestive connections, genome-wide analyses of editing in insect vectors are conspicuously lacking. Future advances will require complementing the growing wealth of vector genomes with targeted transcriptome analyses. Here, we review recent investigations of the genetic footprints of adaptive RNA editing in insects and provide an overview of new methodologies applicable to studies of RNA editing in insect vectors.

Heterogeneous expression of GABA receptor-like subunits LCCH3 and GRD reveals functional diversity of GABA receptors in the honeybee Apis mellifera

BACKGROUND AND PURPOSE

Despite a growing awareness, annual losses of honeybee colonies worldwide continue to reach threatening levels for food safety and global biodiversity. Among the biotic and abiotic stresses probably responsible for these losses, pesticides, including those targeting ionotropic GABA receptors, are one of the major drivers. Most insect genomes include the ionotropic GABA receptor subunit gene, Rdl, and two GABA-like receptor subunit genes, Lcch3 and Grd. Most studies have focused on Rdl which forms homomeric GABA-gated chloride channels, and a complete analysis of all possible molecular combinations of GABA receptors is still lacking.

EXPERIMENTAL APPROACH

We cloned the Rdl, Grd, and Lcch3 genes of Apis mellifera and systematically characterized the resulting GABA receptors expressed in Xenopus oocytes, using electrophysiological assays, fluorescence microscopy and co-immunoprecipitation techniques.

KEY RESULTS

The cloned subunits interacted with each other, forming GABA-gated heteromeric channels with particular properties. Strikingly, these heteromers were always more sensitive than AmRDL homomer to all the pharmacological agents tested. In particular, when expressed together, Grd and Lcch3 form a non-selective cationic channel that opens at low concentrations of GABA and with sensitivity to insecticides similar to that of homomeric Rdl channels.

CONCLUSION AND IMPLICATIONS

For off-target species like the honeybee, chronic sublethal exposure to insecticides constitutes a major threat. At these concentration ranges, homomeric RDL receptors may not be the most pertinent target to study and other ionotropic GABA receptor subtypes should be considered in order to understand more fully the molecular mechanisms of sublethal toxicity to insecticides.

The cell line A-to-I RNA editing catalogue

Adenosine-to-inosine (A-to-I) RNA editing is a common post transcriptional modification. It has a critical role in protecting against false activation of innate immunity by endogenous double stranded RNAs and has been associated with various regulatory processes and diseases such as autoimmune and cardiovascular diseases as well as cancer. In addition, the endogenous A-to-I editing machinery has been recently harnessed for RNA engineering. The study of RNA editing in humans relies heavily on the usage of cell lines as an important and commonly-used research tool. In particular, manipulations of the editing enzymes and their targets are often developed using cell line platforms. However, RNA editing in cell lines behaves very differently than in normal and diseased tissues, and most cell lines exhibit low editing levels, requiring over-expression of the enzymes. Here, we explore the A-to-I RNA editing landscape across over 1000 human cell lines types and show that for almost every editing target of interest a suitable cell line that mimics normal tissue condition may be found. We provide CLAIRE, a searchable catalogue of RNA editing levels across cell lines available at http://srv00.recas.ba.infn.it/atlas/claire.html, to facilitate rational choice of appropriate cell lines for future work on A-to-I RNA editing.

Potential of Competitive Antagonists of Insect Ionotropic γ-Aminobutyric Acid Receptors as Insecticides

Ionotropic γ-aminobutyric acid (GABA) receptors (GABARs) represent an important insecticide target. Currently used GABAR-targeting insecticides are non-competitive antagonists (NCAs) of these receptors. Recent studies have demonstrated that competitive antagonists (CAs) of GABARs have functions of inhibiting insect GABARs similar to NCAs and that they also exhibit insecticidal activity. CAs have different binding sites and different mechanisms of action compared to those of NCAs. Therefore, GABAR CAs should have the potential to be developed into novel insecticides, which could be used to overcome the developed resistance of insect pests to conventional NCA insecticides. Although research on insect GABAR CAs has lagged behind that on mammalian GABAR CAs, research on the CAs of insect ionotropic GABARs has made great progress in recent years, and several series of heterocyclic compounds, such as 3-isoxazolols and 6-iminopyridazines, have been identified as insect GABAR CAs. In this review, we briefly summarize the design strategies, structures, and biological activities of the novel GABAR CAs that have been found in the past decade. Updated information about GABAR CAs may benefit the design and development of novel GABAR-targeting insecticides.

In silico identification and assessment of insecticide target sites in the genome of the small hive beetle, Aethina tumida

BACKGROUND

The small hive beetle, Aethina tumida, is a rapidly emerging global pest of honey bee colonies. Small hive beetle infestation can be extremely destructive, which may cause honey bees to abscond and render colony infrastructure unusable. Due to the impacts small hive beetles have on honey bees, a wide variety of physical, cultural, and chemical control measures have been implemented to manage small hive beetle infestations. The use of insecticides to control small hive beetle populations is an emerging management tactic. Currently, very little genomic information exists on insecticide target sites in the small hive beetle. Therefore, the objective of this study is to utilize focused in silico comparative genomics approaches to identify and assess the potential insecticide sensitivity of the major insecticide target sites in the small hive beetle genome.

RESULTS

No previously described resistance mutations were identified in any orthologs of insecticide target sites. Alternative exon use and A-to-I RNA editing were absent in AtumSC1. The ryanodine receptor in small hive beetle (Atum_Ryr) was highly conserved and no previously described resistance mutations were identified. A total of 12 nAChR subunits were identified with similar alternative exon use in other insects. Alternative exon use and critical structural features of the GABA-gated chloride channel subunits (Atum_RDL, Atum_GRD, and Atum_LCCH3) were conserved. Five splice variants were found for the glutamate-gated chloride channel subunit. Exon 3c of Atum_GluCl may be a beetle-specific alternative exon. The co-occurrence of exons 9a and 9b in the pH-sensitive chloride channel (Atum_pHCl) is a unique combination that introduces sites of post-translational modification. The repertoire and alternative exon use for histamine-gated chloride channels (Atum-HisCl), octopamine (Atum_OctR) and tyramine receptors (Atum_TAR) were conserved.

CONCLUSIONS

The recently published small hive beetle genome likely serves as a reference for insecticide-susceptible versions of insecticide target sites. These comparative in silico studies are the first step in discovering targets that can be exploited for small hive beetle-specific control as well as tracking changes in the frequency of resistance alleles as part of a resistance monitoring program. Comparative toxicity alongside honey bees is required to verify these in silico predictions.

Spatiotemporally different expression of alternatively spliced GABA receptor subunit transcripts in the housefly Musca domestica

Insect γ-aminobutyric acid (GABA) receptors are important as major inhibitory neurotransmitter receptors and targets for insecticides. The housefly GABA receptor subunit gene MdRdl is alternatively spliced at exons 3 (a or b) and 6 (c or d) to yield the variants of ac, ad, bc, and bd combinations. In the present study, the expression of the MdRdl transcript in the body parts and in the developmental stages of the housefly Musca domestica was examined by quantitative polymerase chain reaction using specific primers that amplify the combinations of alternative exons. The results indicated that the transcripts of MdRdl, including four combinations, were highly expressed in the adult stage. MdRdl_bd was the most abundant in the adult head. The expression pattern did not change in the adult stage over 7 days after eclosion. The expression level of the MdRdl _bd transcript in the female head was similar to that of the male head. In contrast, MdRdl _bc was the predominant transcript in the pupal head and the adult leg. Because the homomeric Rdl _bc GABA receptor has a high affinity for GABA, our results provide grounds for designing agonist or competitive-antagonist insecticides that target the orthosteric site of the GABA receptor containing this Rdl variant.

RNA editing is abundant and correlates with task performance in a social bumblebee

Colonies of the bumblebee Bombus terrestris are characterized by wide phenotypic variability among genetically similar full-sister workers, suggesting a major role for epigenetic processes. Here, we report a high level of ADAR-mediated RNA editing in the bumblebee, despite the lack of an ADAR1-homolog. We identify 1.15 million unique genomic sites, and 164 recoding sites residing in 100 protein coding genes, including ion channels, transporters, and receptors predicted to affect brain function and behavior. Some edited sites are similarly edited in other insects, cephalopods and even mammals. The global editing level of protein coding and non-coding transcripts weakly correlates with task performance (brood care vs. foraging), but not affected by dominance rank or juvenile hormone known to influence physiology and behavior. Taken together, our findings show that brain editing levels are high in naturally behaving bees, and may be regulated by relatively short-term effects associated with brood care or foraging activities.

Illuminating spatial A-to-I RNA editing signatures within the Drosophila brain

A fundamental question in contemporary neuroscience is how the remarkable cellular diversity required for the intricate function of the nervous system is achieved. Here, we bridge the gap between a cellular machinery that is known to diversify the transcriptome and the existence of distinct neuronal populations that compose the Drosophila brain. Adenosine-to-inosine (A-to-I) RNA editing is a ubiquitous mechanism that generates transcriptomic diversity in cells by recoding certain adenosines within the pre-mRNA sequence into inosines. We present a spatial map of RNA editing across different neuronal populations in Drosophila brain. Each neuronal population has a distinct editing signature, with the majority of differential editing occurring in highly conserved regions of transcripts that encode ion channels and other essential neuronal genes.

Adenosine-to-inosine (A-to-I) RNA editing, catalyzed by ADAR enzymes, is a ubiquitous mechanism that generates transcriptomic diversity. This process is particularly important for proper neuronal function; however, little is known about how RNA editing is dynamically regulated between the many functionally distinct neuronal populations of the brain. Here, we present a spatial RNA editing map in the Drosophila brain and show that different neuronal populations possess distinct RNA editing signatures. After purifying and sequencing RNA from genetically marked groups of neuronal nuclei, we identified a large number of editing sites and compared editing levels in hundreds of transcripts across nine functionally different neuronal populations. We found distinct editing repertoires for each population, including sites in repeat regions of the transcriptome and differential editing in highly conserved and likely functional regions of transcripts that encode essential neuronal genes. These changes are site-specific and not driven by changes in Adar expression, suggesting a complex, targeted regulation of editing levels in key transcripts. This fine-tuning of the transcriptome between different neurons by RNA editing may account for functional differences between distinct populations in the brain.

Cloning and Functional Characterisation of the Duplicated RDL Subunits from the Pea Aphid, Acyrthosiphon pisum

The insect GABA receptor, RDL (resistance to dieldrin), is a cys-loop ligand-gated ion channel (cysLGIC) that plays a central role in neuronal signaling, and is the target of several classes of insecticides. Many insects studied to date possess one Rdl gene; however, there is evidence of two Rdls in aphids. To characterise further this insecticide target from pests that cause millions of dollars' worth of crop damage each year, we identified the complete cysLGIC gene superfamily of the pea aphid, Acyrthosiphon pisum, using BLAST analysis. This confirmed the presence of two Rdl-like genes (RDL1 and RDL2) that likely arose from a recent gene duplication. When expressed individually in Xenopus laevis oocytes, both subunits formed functional ion channels gated by GABA. Alternative splicing of RDL1 influenced the potency of GABA, and the potency of fipronil was different on the RDL1_bd splice variant and RDL2. Imidacloprid and clothianidin showed no antagonistic activity on RDL1, whilst 100 μM thiacloprid reduced the GABA responses of RDL1 and RDL2 to 55% and 62%, respectively. It was concluded that gene duplication of Rdl may have conferred increased tolerance to natural insecticides, and played a role in the evolution of insect cysLGICs.

Species specific RNA A-to-I editing of mosquito RDL modulates GABA potency and influences agonistic, potentiating and antagonistic actions of ivermectin

The insect GABA receptor, RDL, is the target of several classes of pesticides. The peptide sequences of RDL are generally highly conserved between diverse insects. However, RNA A-to-I editing can effectively alter amino acid residues of RDL in a species specific manner, which can affect the potency of GABA and possibly insecticides. We report here that RNA A-to-I editing alters the gene products of Rdl in three mosquito disease vectors, recoding five amino acid residues in RDL of Aedes aegypti and six residues in RDLs of Anopheles gambiae and Culex pipiens, which is the highest extent of editing in RDL observed to date. Analysis of An. gambiae Rdl cDNA sequences identified 24 editing isoforms demonstrating a considerable increase in gene product diversity. RNA editing influenced the potency of the neurotransmitter, GABA, on An. gambiae RDL editing isoforms expressed in Xenopus laevis oocytes, as demonstrated by EC₅₀s ranging from 5 ± 1 to 246 ± 41 μM. Fipronil showed similar potency on different editing isoforms, with IC₅₀s ranging from 0.18 ± 0.08 to 0.43 ± 0.09 μM. In contrast, editing of An. gambiae RDL affected the activating, potentiating and inhibiting actions of ivermectin. For example, ivermectin potentiated currents induced by GABA at the EC₂₀ concentration in the unedited isoform but not in the fully edited variant. Editing of a residue in the first transmembrane domain or the cys-loop influenced this potentiation, highlighting residues involved in the allosteric mechanisms of cys-loop ligand-gated ion channels. Understanding the interactions of ivermectin with molecular targets may have relevance to mosquito control in areas where people are administered with ivermectin to treat parasitic diseases.

How Complex Can Resistance to Dieldrin, the Insect γ-Aminobutyric Acid Receptor, Get?

Recently, Taylor-Wells et al published evidence that the γ-aminobutyric acid (GABA) receptor, resistance to dieldrin (RDL), from mosquitoes undergoes RNA A-to-I editing to generate an extraordinarily large range of isoforms. This editing was found to affect GABA receptor pharmacology, as it influenced the potency of GABA and ivermectin. This highlights RNA editing as a species-specific mechanism to fine-tune receptor function as well as possibly increase tolerance of mosquitoes to certain insecticides. This commentary also considers novel findings from analysis of Rdl transcripts from individual mosquitoes taken from different geographical areas.

Splice Variants of pH-Sensitive Chloride Channel Identify a Key Determinant of Ivermectin Sensitivity in the Larvae of the Silkworm Bombyx mori

The pH-sensitive chloride channels (pHCls) are broadly expressed in insects, but little is known about their physiologic role, diversity, and sensitivity to insecticides acting on relevant chloride channels. Here we have sequenced 50 transcripts of the pHCl-1 gene from the brain, third thoracic ganglion (T3G), and midgut of larvae of silkworm Bombyx mori It was found that >50 variants were expressed with distinct splicing in the T3G compared with the brain and midgut. Of the variants detected, variant 9, which was expressed most abundantly in the larvae, was reconstituted in Xenopus laevis oocytes to characterize its pH and ivermectin sensitivity. Variant 9 formed a functional pHCl with half-maximal activation at a pH of 7.87, and was activated by ivermectin irrespective of the extracellular pH. This was in contrast to variant 1, which was activated more profoundly at acidic rather than basic pH. To identify a key determinant for such differential ivermectin sensitivity, different amino acids in variants 1 and 9 were swapped, and the effects of the mutations on ivermectin sensitivity were investigated. The V275S mutation of variant 1 enhanced ivermectin sensitivity, whereas the S275V mutation of variant 9 caused a reduction in sensitivity. In homology models of the Bombyx pHCls, Val275 of variant 1 interacted more strongly with Ala273 than Ser275 of variant 9 at the channel gate. This interaction is likely to prevent ivermectin-induced opening of the channel, accounting, at least partially, for the differential macrolide action on the two variants.

Expression pattern and pharmacological characterisation of two novel alternative splice variants of the glutamate-gated chloride channel in the small brown planthopper Laodelphax striatellus

BACKGROUND

Glutamate-gated chloride channels (GluCl) mediate fast inhibitory neurotransmission in invertebrate nervous systems. Although only one GluCl gene was presented in insects, it showed diverse alternative splicing that was speculated could affect channel function and pharmacology.

RESULTS

In this study, we isolated GluCl cDNAs from adults of the small brown planthopper (SBPH) Laodelphax striatellus and showed that six L. striatellus GluCl variants (LsGluCl-AS, LsGluCl-BS, LsGluCl-CS, LsGluCl-AL, LsGluCl-BL, LsGluCl-CL) were present in the SBPH. The expression patterns of six variants differed among developmental stages (egg, first- to fifth-instar nymphs, male and female adults) and among the body parts (head, thorax, abdomen, leg) of the female adult SBPH. All the transcripts were abundant in the head of the adult. When expressed in African clawed frog, Xenopus laevis, oocytes, the two functional variants (LsGluCl-AS, LsGluCl-AL) had similar EC₅₀ and IC₅₀ values for L-glutamate and channel blockers picrotoxinin and fipronil.

CONCLUSION

This study represents a comprehensive molecular, expression and pharmacological characterisation of GluCl in the SBPH. These findings should be useful in providing more opportunities to discover novel insect control chemicals. © 2016 Society of Chemical Industry.

Ion channels as insecticide targets

Ion channels remain the primary target of most of the small molecule insecticides. This review examines how the subunit composition of heterologously expressed receptors determines their insecticide-specific pharmacology and how the pharmacology of expressed receptors differs from those found in the insect nervous system. We find that the insecticide-specific pharmacology of some receptors, like that containing subunits of the Rdl encoded GABA receptor, can be reconstituted with very few of the naturally occurring subunits expressed. In contrast, workers have struggled even to express functional insect nicotinic acetylcholine receptors (nAChRs), and work has therefore often relied upon the expression of vertebrate receptor subunits in their place. We also examine the extent to which insecticide-resistance-associated mutations, such as those in the para encoded voltage-gated sodium channel, can reveal details of insecticide-binding sites and mode of action. In particular, we examine whether mutations are present in the insecticide-binding site and/or at sites that allosterically affect the drug preferred conformation of the receptor. We also discuss the ryanodine receptor as a target for the recently developed diamides. Finally, we examine the lethality of the genes encoding these receptor subunits and discuss how this might determine the degree of conservation of the resistance-associated mutations found.

Sequence variation between 462 human individuals fine-tunes functional sites of RNA processing

Recent advances in the cost-efficiency of sequencing technologies enabled the combined DNA- and RNA-sequencing of human individuals at the population-scale, making genome-wide investigations of the inter-individual genetic impact on gene expression viable. Employing mRNA-sequencing data from the Geuvadis Project and genome sequencing data from the 1000 Genomes Project we show that the computational analysis of DNA sequences around splice sites and poly-A signals is able to explain several observations in the phenotype data. In contrast to widespread assessments of statistically significant associations between DNA polymorphisms and quantitative traits, we developed a computational tool to pinpoint the molecular mechanisms by which genetic markers drive variation in RNA-processing, cataloguing and classifying alleles that change the affinity of core RNA elements to their recognizing factors. The in silico models we employ further suggest RNA editing can moonlight as a splicing-modulator, albeit less frequently than genomic sequence diversity. Beyond existing annotations, we demonstrate that the ultra-high resolution of RNA-Seq combined from 462 individuals also provides evidence for thousands of bona fide novel elements of RNA processing-alternative splice sites, introns, and cleavage sites-which are often rare and lowly expressed but in other characteristics similar to their annotated counterparts.

The emerging role of RNA editing in plasticity

All true metazoans modify their RNAs by converting specific adenosine residues to inosine. Because inosine binds to cytosine, it is a biological mimic for guanosine. This subtle change, termed RNA editing, can have diverse effects on various RNA-mediated cellular pathways, including RNA interference, innate immunity, retrotransposon defense and messenger RNA recoding. Because RNA editing can be regulated, it is an ideal tool for increasing genetic diversity, adaptation and environmental acclimation. This review will cover the following themes related to RNA editing: (1) how it is used to modify different cellular RNAs, (2) how frequently it is used by different organisms to recode mRNA, (3) how specific recoding events regulate protein function, (4) how it is used in adaptation and (5) emerging evidence that it can be used for acclimation. Organismal biologists with an interest in adaptation and acclimation, but with little knowledge of RNA editing, are the intended audience.

The neonicotinoid imidacloprid, and the pyrethroid deltamethrin, are antagonists of the insect Rdl GABA receptor

A mutation in the second transmembrane domain of the GABA receptor subunit, Rdl, is associated with resistance to insecticides such as dieldrin and fipronil. Molecular cloning of Rdl cDNA from a strain of the malaria mosquito, Anopheles gambiae, which is highly resistant to dieldrin revealed this mutation (A296G) as well as another mutation in the third transmembrane domain (T345M). Wild-type, A296G, T345M and A296G + T345M homomultimeric Rdl were expressed in Xenopus laevis oocytes and their sensitivities to fipronil, deltamethrin, 1,1,1-trichloro-2,2-bis(p-chlorophenyl) ethane (DDT), imidacloprid and spinosad were measured using two-electrode voltage-clamp electrophysiology. Spinosad and DDT had no agonist or antagonist actions on Rdl. However, fipronil, deltamethrin and imidacloprid decreased GABA-evoked currents. These antagonistic actions were either reduced or abolished with the A296G and the A296G + T345M mutations while T345M alone appeared to have no significant effect. In conclusion, this study identifies another mutation in the mosquito Rdl that is associated with insecticide resistance. While T345M itself does not affect insecticide sensitivity, it may serve to offset the structural impact of A296G. The present study also highlights Rdl as a potential secondary target for neonicotinoids and pyrethroids. We show for the first time that deltamethrin (a pyrethroid insecticide) and imidacloprid (a neonicotinoid insecticide) act directly on the insect GABA receptor, Rdl. Our findings highlight Rdl as a potential secondary target of pyrethroids and neonicotinoids mutations in which may contribute to resistance to these widely used insecticides.

4,5-Substituted 3-Isoxazolols with Insecticidal Activity Act as Competitive Antagonists of Housefly GABA Receptors

The insect GABA receptor (GABAR), which is composed of five RDL subunits, represents an important target for insecticides. A series of 4,5-disubstituted 3-isoxazolols, including muscimol analogues, were synthesized and examined for their activities against four splice variants (ac, ad, bc, and bd) of housefly GABARs expressed in Xenopus oocytes. Muscimol was a more potent agonist than GABA in all four splice variants, whereas synthesized analogues did not exhibit agonism but rather antagonism in housefly GABARs. The introduction of bicyclic aromatic groups at the 4-position of muscimol and the simultaneous replacement of the aminomethyl group with a carbamoyl group at the 5-position to afford six 4-aryl-5-carbamoyl-3-isoxazolols resulted in compounds that exhibited significantly enhanced antagonism with IC50 values in the low micromolar range in the ac variant. The inhibition of GABA-induced currents by 100 μM analogues was approximately 1.5-4-fold greater in the ac and bc variants than in the ad and bd variants. 4-(3-Biphenylyl)-5-carbamoyl-3-isoxazolol displayed competitive antagonism, with IC50 values of 30, 34, 107, and 96 μM in the ac, bc, ad, and bd variants, respectively, and exhibited moderate insecticidal activity against houseflies, with an LD50 value of 5.6 nmol/fly. These findings suggest that these 3-isoxazolol analogues are novel lead compounds for the design and development of insecticides that target the orthosteric site of housefly GABARs.

Comparison between the modes of action of novel meta-diamide and macrocyclic lactone insecticides on the RDL GABA receptor

Macrocyclic lactones, avermectins, and milbemycins are widely used to control arthropods, nematodes, and endo- and ectoparasites in livestock and pets. Their main targets are glutamate-gated chloride channels. Furthermore, macrocyclic lactones reportedly interact with insect RDL γ-aminobutyric acid (GABA) receptors, but their modes of action on insect RDL GABA receptors remain unknown. In this study, we attempted to better understand the modes of action of macrocyclic lactones on RDL GABA receptors. We observed that ivermectin and milbemectin behaved as allosteric agonists of the Drosophila RDL GABA receptor. G336A, G336S, and G336T mutations had profound effects on the activities of ivermectin and milbemectin, and a G336M mutation abolished the allosteric agonist and antagonist activities of these macrocyclic lactones. These results suggest that G336 in TM3 of the Drosophila RDL GABA receptor is important for the binding of macrocyclic lactones. Recently, it has been suggested that a novel RDL GABA receptor antagonist, 3-benzamido-N-(2-bromo-4-perfluoroisopropyl-6-(trifluoromethyl)phenyl)-2-fluorobenzamide (meta-diamide 7), binds to the transmembrane intersubunit pocket near G336 in the Drosophila RDL GABA receptor. Thus, we compared the effects of mutations around G336 and A302 mutations in TM2 on the activities of macrocyclic lactone and meta-diamide 7. The effects of L281C, V340Q, V340N, A302S, and A302N mutations on the activity of meta-diamide 7 differed from those on ivermectin and milbemectin. Molecular modeling studies showed that macrocyclic lactones docked in the intersubunit pocket near G336 in the Drosophila RDL GABA receptor in the open state. In contrast, meta-diamide 7 docked into the Drosophila RDL GABA receptor in the closed state. This suggests that the modes of action of macrocyclic lactone binding to the wild-type Drosophila RDL GABA receptor differ from those of meta-diamide binding.

Temporal integration of cholinergic and GABAergic inputs in isolated insect mushroom body neurons exposes pairing-specific signal processing

GABAergic modulation of neuronal activity plays a crucial role in physiological processes including learning and memory in both insects and mammals. During olfactory learning in honeybees (Apis mellifera) and Drosophila melanogaster the temporal relation between excitatory cholinergic and inhibitory GABAergic inputs critically affects learning. However, the cellular mechanisms of temporal integration of these antagonistic inputs are unknown. To address this question, we use calcium imaging of isolated honeybee and Drosophila Kenyon cells (KCs), which are targets of cholinergic and GABAergic inputs during olfactory learning. In the whole population of honeybee KCs we find that pairing of acetylcholine (ACh) and γ-aminobutyric acid (GABA) Comment: Please use the greek letter for gamma reduces the ACh-induced calcium influx, and depending on their temporal sequence, induces different forms of neuronal plasticity. After ACh-GABA pairing the calcium influx of a subsequent excitatory stimulus is increased, while GABA-ACh pairing affects the decay time leading to elevated calcium levels during the late phase of a subsequent excitatory stimulus. In an exactly defined subset of Drosophila KCs implicated in learning we find similar pairing-specific differences. Specifically the GABA-ACh pairing splits the KCs in two functional subgroups: one is only weakly inhibited by GABA and shows no neuronal plasticity and the other subgroup is strongly inhibited by GABA and shows elevated calcium levels during the late phase of a subsequent excitatory stimulus. Our findings provide evidence that insect KCs are capable of contributing to temporal processing of cholinergic and GABAergic inputs, which provides a neuronal mechanism of the differential temporal role of GABAergic inhibition during learning.

Large-scale detection and analysis of adenosine-to-inosine RNA editing during development in Plutella xylostella

Adenosine-to-inosine (A-to-I) RNA editing is site-specific modification of RNAs that increases the diversity of the transcriptome and proteome. Most insects undergo complete metamorphosis, including four life cycle stages: egg, larva, pupa and adult. Many previous studies have confirmed that RNA-editing events occur in a development-specific manner; in other words, RNA-editing levels change during metamorphosis. Here, we describe an effort to identify the developmental specificity of RNA-editing events using a large-scale computational analysis of RNA-seq data derived from four developmental stages of the diamondback moth, Plutella xylostella. One thousand one hundred and eighty-seven A-to-I RNA-editing sites were predicted to be developmental stage specific (false-discovery rate <0.01) and 1,094 of these sites were located in protein-coding regions. Editing of 152 sites resulted in an altered amino acid residue. A putative adult-specific A-to-I RNA-editing site was verified by comparing cDNA sequences with its corresponding genomic locus at different stages of the P. xylostella life cycle. Our findings will help elucidate the role of A-to-I RNA editing in the regulation of metamorphosis. Further studies detailing changes in the extent of editing are needed to establish how as yet unknown regulatory factors are involved in the editing mechanism and what biological functions' editing serves.

Exon 3 splicing and mutagenesis identify residues influencing cell surface density of heterologously expressed silkworm (Bombyx mori) glutamate-gated chloride channels

Glutamate-gated chloride channels (GluCls) mediate fast inhibitory neurotransmission in invertebrate nervous systems. Insect GluCls show alternative splicing, and, to determine its impact on channel function and pharmacology, we isolated GluCl cDNAs from larvae of the silkworm (Bombyx mori). We show that six B. mori glutamate-gated chloride channel variants are generated by splicing in exons 3 and 9 and that exons 3b and 3c are common in the brain and third thoracic ganglion. When expressed in Xenopus laevis oocytes, the three functional exon 3 variants (3a, b, c) all had similar EC50 values for l-glutamate and ivermectin (IVM); however, Imax (the maximum l-glutamate- and IVM-induced response of the channels at saturating concentrations) differed strikingly between variants, with the 3c variant showing the largest l-glutamate- and IVM-induced responses. By contrast, a partial deletion detected in exon 9 had a much smaller impact on l-glutamate and IVM actions. Binding assays using [(3)H]IVM indicate that diversity in IVM responses among the GluCl variants is mainly due to the impact on channel assembly, altering receptor cell surface numbers. GluCl variants expressed in HEK293 cells show that structural differences influenced Bmax but not Kd values of [(3)H]IVM. Domain swapping and site-directed mutagenesis identified four amino acids in exon 3c as hot spots determining the highest amplitude of the l-glutamate and IVM responses. Modeling the GluCl 3a and 3c variants suggested that three of the four amino acids contribute to intersubunit contacts, whereas the other interacts with the TM2-TM3 linker, influencing the receptor response.

Regulation of Ion Channel and Transporter Function Through RNA Editing

A large proportion of the recoding events mediated by RNA editing are in mRNAs that encode ion channels and transporters. The effects of these events on protein function have been characterized in only a few cases. In even fewer instances are the mechanistic underpinnings of these effects understood. This review focuses on how RNA editing affects protein function and higher order physiology. In mammals, particular attention is given to the GluA2, an ionotropic glutamate receptor subunit, and K(v) 1.1, a voltage-dependent K+ channel, because they are particularly well understood. In K(v) addition, work on cephalopod K+ channels and Na+/K+-ATPases has also provided important clues on the rules used by RNA editing to regulate excitability. Finally, we discuss some of the emerging targets for editing and how this process may be used to regulate nervous function in response to a variable environment.

Competitive antagonism of insect GABA receptors by 4-substituted 5-(4-piperidyl)-3-isothiazolols

γ-Aminobutyric acid (GABA) receptors are important targets of parasiticides/insecticides. Several 4-substituted analogs of the partial GABAA receptor agonist 5-(4-piperidyl)-3-isothiazolol (Thio-4-PIOL) were synthesized and examined for their antagonism of insect GABA receptors expressed in Drosophila S2 cells or Xenopus oocytes. Thio-4-PIOL showed weak antagonism of three insect GABA receptors. The antagonistic activity of Thio-4-PIOL was enhanced by introducing bicyclic aromatic substituents into the 4-position of the isothiazole ring. The 2-naphthyl and the 3-biphenylyl analogs displayed antagonist potencies with half maximal inhibitory concentrations in the low micromolar range. The 2-naphthyl analog induced a parallel rightward shift of the GABA concentration-response curve, suggesting competitive antagonism by these analogs. Both compounds exhibited weak insecticidal activities against houseflies. Thus, the orthosteric site of insect GABA receptors might be a potential target site of insecticides.

Molecular biology of insect sodium channels and pyrethroid resistance

Voltage-gated sodium channels are essential for the initiation and propagation of the action potential in neurons and other excitable cells. Because of their critical roles in electrical signaling, sodium channels are targets of a variety of naturally occurring and synthetic neurotoxins, including several classes of insecticides. This review is intended to provide an update on the molecular biology of insect sodium channels and the molecular mechanism of pyrethroid resistance. Although mammalian and insect sodium channels share fundamental topological and functional properties, most insect species carry only one sodium channel gene, compared to multiple sodium channel genes found in each mammalian species. Recent studies showed that two posttranscriptional mechanisms, alternative splicing and RNA editing, are involved in generating functional diversity of sodium channels in insects. More than 50 sodium channel mutations have been identified to be responsible for or associated with knockdown resistance (kdr) to pyrethroids in various arthropod pests and disease vectors. Elucidation of molecular mechanism of kdr led to the identification of dual receptor sites of pyrethroids on insect sodium channels. Many of the kdr mutations appear to be located within or close to the two receptor sites. The accumulating knowledge of insect sodium channels and their interactions with insecticides provides a foundation for understanding the neurophysiology of sodium channels in vivo and the development of new and safer insecticides for effective control of arthropod pests and human disease vectors.

Actions of agonists, fipronil and ivermectin on the predominant in vivo splice and edit variant (RDLbd, I/V) of the Drosophila GABA receptor expressed in Xenopus laevis oocytes

Ionotropic GABA receptors are the targets for several classes of insecticides. One of the most widely-studied insect GABA receptors is RDL (resistance to dieldrin), originally isolated from Drosophila melanogaster. RDL undergoes alternative splicing and RNA editing, which influence the potency of GABA. Most work has focussed on minority isoforms. Here, we report the first characterisation of the predominant native splice variant and RNA edit, combining functional characterisation with molecular modelling of the agonist-binding region. The relative order of agonist potency is GABA> muscimol> TACA> β-alanine. The I/V edit does not alter the potency of GABA compared to RDLbd. Docking calculations suggest that these agonists bind and activate RDLbdI/V through a similar binding mode. TACA and β-alanine are predicted to bind with lower affinity than GABA, potentially explaining their lower potency, whereas the lower potency of muscimol and isoguvacine cannot be explained structurally from the docking calculations. The A301S (resistance to dieldrin) mutation reduced the potency of antagonists picrotoxin, fipronil and pyrafluprole but the I/V edit had no measurable effect. Ivermectin suppressed responses to GABA of RDLbdI/V, RDLbd and RDLbdI/VA301S. The dieldrin resistant variant also showed reduced sensitivity to Ivermectin. This study of a highly abundant insect GABA receptor isoform will help the design of new insecticides.

Expression pattern and function of alternative splice variants of glutamate-gated chloride channel in the housefly Musca domestica

Glutamate-gated chloride channels (GluCls) mediate fast inhibitory neurotransmission in invertebrate nervous systems. cDNAs encoding two alternative splice variants (MdGluClB and C) of the GluCl subunit were cloned from the housefly Musca domestica. The expression patterns of three variants, including the previously reported MdGluClA, differed among the body parts (head, thorax, abdomen, and leg) of the adult housefly and among developmental stages (embryo, larva, pupa, and adult). The MdGluClA and B transcripts were abundant in the central nervous system of the adult, whereas the MdGluClC transcript was expressed in the central nervous system and as the predominant variant in the peripheral tissues. The sensitivities to the agonist glutamate and the allosteric activator ivermectin B1a did not differ between channels containing MdGluCl variants when they were singly or co-expressed in Xenopus oocytes. By contrast, MdGluClA and B channels were more sensitive to the channel blockers fipronil and picrotoxinin than was MdGluClC channels. Heteromeric channels containing different subunit variants were more sensitive to picrotoxinin than were homomeric channels. Heteromeric channels were more sensitive to fipronil than were homomeric MdGluClC channels but not than homomeric MdGluClA and B channels. These results suggest that functionally indistinguishable but pharmacologically distinct GluCls are expressed in a spatially and temporally distinct manner in the housefly.

The ADAR RNA editing enzyme controls neuronal excitability in Drosophila melanogaster

RNA editing by deamination of specific adenosine bases to inosines during pre-mRNA processing generates edited isoforms of proteins. Recoding RNA editing is more widespread in Drosophila than in vertebrates. Editing levels rise strongly at metamorphosis, and Adar^5G1 null mutant flies lack editing events in hundreds of CNS transcripts; mutant flies have reduced viability, severely defective locomotion and age-dependent neurodegeneration. On the other hand, overexpressing an adult dADAR isoform with high enzymatic activity ubiquitously during larval and pupal stages is lethal. Advantage was taken of this to screen for genetic modifiers; Adar overexpression lethality is rescued by reduced dosage of the Rdl (Resistant to dieldrin), gene encoding a subunit of inhibitory GABA receptors. Reduced dosage of the Gad1 gene encoding the GABA synthetase also rescues Adar overexpression lethality. Drosophila Adar^5G1 mutant phenotypes are ameliorated by feeding GABA modulators. We demonstrate that neuronal excitability is linked to dADAR expression levels in individual neurons; Adar-overexpressing larval motor neurons show reduced excitability whereas Adar^5G1 null mutant or targeted Adar knockdown motor neurons exhibit increased excitability. GABA inhibitory signalling is impaired in human epileptic and autistic conditions, and vertebrate ADARs may have a relevant evolutionarily conserved control over neuronal excitability.

Gene duplication in the major insecticide target site, Rdl, in Drosophila melanogaster

The Resistance to Dieldrin gene, Rdl, encodes a GABA-gated chloride channel subunit that is targeted by cyclodiene and phenylpyrazole insecticides. The gene was first characterized in Drosophila melanogaster by genetic mapping of resistance to the cyclodiene dieldrin. The 4,000-fold resistance observed was due to a single amino acid replacement, Ala(301) to Ser. The equivalent change was subsequently identified in Rdl orthologs of a large range of resistant insect species. Here, we report identification of a duplication at the Rdl locus in D. melanogaster. The 113-kb duplication contains one WT copy of Rdl and a second copy with two point mutations: an Ala(301) to Ser resistance mutation and Met(360) to Ile replacement. Individuals with this duplication exhibit intermediate dieldrin resistance compared with single copy Ser(301) homozygotes, reduced temperature sensitivity, and altered RNA editing associated with the resistant allele. Ectopic recombination between Roo transposable elements is involved in generating this genomic rearrangement. The duplication phenotypes were confirmed by construction of a transgenic, artificial duplication integrating the 55.7-kb Rdl locus with a Ser(301) change into an Ala(301) background. Gene duplications can contribute significantly to the evolution of insecticide resistance, most commonly by increasing the amount of gene product produced. Here however, duplication of the Rdl target site creates permanent heterozygosity, providing unique potential for adaptive mutations to accrue in one copy, without abolishing the endogenous role of an essential gene.

A-to-I RNA editing: effects on proteins key to neural excitability

RNA editing by adenosine deamination is a process used to diversify the proteome. The expression of ADARs, the editing enzymes, is ubiquitous among true metazoans, and so adenosine deamination is thought to be universal. By changing codons at the level of mRNA, protein function can be altered, perhaps in response to physiological demand. Although the number of editing sites identified in recent years has been rising exponentially, their effects on protein function, in general, are less well understood. This review assesses the state of the field and highlights particular cases where the biophysical alterations and functional effects caused by RNA editing have been studied in detail.

Auto-regulatory RNA editing fine-tunes mRNA re-coding and complex behaviour in Drosophila

Auto-regulatory feedback loops are a common molecular strategy used to optimize protein function. In Drosophila, many messenger RNAs involved in neuro-transmission are re-coded at the RNA level by the RNA-editing enzyme, dADAR, leading to the incorporation of amino acids that are not directly encoded by the genome. dADAR also re-codes its own transcript, but the consequences of this auto-regulation in vivo are unclear. Here we show that hard-wiring or abolishing endogenous dADAR auto-regulation dramatically remodels the landscape of re-coding events in a site-specific manner. These molecular phenotypes correlate with altered localization of dADAR within the nuclear compartment. Furthermore, auto-editing exhibits sexually dimorphic patterns of spatial regulation and can be modified by abiotic environmental factors. Finally, we demonstrate that modifying dAdar auto-editing affects adaptive complex behaviours. Our results reveal the in vivo relevance of auto-regulatory control over post-transcriptional mRNA re-coding events in fine-tuning brain function and organismal behaviour.

Antisense sequencing improves the accuracy and precision of A-to-I editing measurements using the peak height ratio method

Background

A-to-I RNA editing is found in all phyla of animals and contributes to transcript diversity that may have profound impacts on behavior and physiology. Many transcripts of genes involved in axonal conductance, synaptic transmission and modulation are the targets of A-to-I RNA editing. There are a number of methods to measure the extent of A-to-I RNA editing, but they are generally costly and time consuming. One way to determine the frequency of A-to-I RNA editing is the peak height ratio method, which compares the size of peaks on electropherograms that represent unedited and edited sites.

Findings

Sequencing of 4 editing sites of the Dα6 nicotinic acetylcholine receptor subunit with an antisense primer (which uses T/C peaks to measure unedited and edited sites, respectively) showed very accurate and precise measurements of A-to-I RNA editing. The accuracy and precision were excellent for all editing sites, including those edited with high or low frequencies. The frequency of A-to-I RNA editing was comparable to the editing frequency as measured by clone counting from the same sample. Sequencing these same sites with the sense primer (which uses A/G peaks) yielded inaccurate and imprecise measurements.

Conclusions

We have validated and improved the accuracy and precision of the peak height ratio method to measure the frequency of A-to-I RNA editing, and shown that results are primer specific. Thus, the correct sequencing primer must be utilized for the most dependable data. When compared to other methods used to measure the frequency of A-to-I RNA editing, the major benefits of the peak height ratio are that this method is inexpensive, fast, non-labor intensive and easily adaptable to many laboratory and field settings.

The genetic analysis of functional connectomics in Drosophila

Fly and vertebrate nervous systems share many organizational features, such as layers, columns and glomeruli, and utilize similar synaptic components, such as ion channels and receptors. Both also exhibit similar network features. Recent technological advances, especially in electron microscopy, now allow us to determine synaptic circuits and identify pathways cell-by-cell, as part of the fly's connectome. Genetic tools provide the means to identify synaptic components, as well as to record and manipulate neuronal activity, adding function to the connectome. This review discusses technical advances in these emerging areas of functional connectomics, offering prognoses in each and identifying the challenges in bridging structural connectomics to molecular biology and synaptic physiology, thereby determining fundamental mechanisms of neural computation that underlie behavior.

Visualizing adenosine-to-inosine RNA editing in the Drosophila nervous system

Informational recoding by adenosine-to-inosine RNA editing diversifies neuronal proteomes by chemically modifying structured mRNAs. However, techniques for analyzing editing activity on substrates in defined neurons in vivo are lacking. Guided by comparative genomics, here we reverse-engineered a fluorescent reporter sensitive to Drosophila melanogaster adenosine deaminase that acts on RNA (dADAR) activity and alterations in dADAR autoregulation. Using this artificial dADAR substrate, we visualized variable patterns of RNA-editing activity in the Drosophila nervous system between individuals. Our results demonstrate the feasibility of structurally mimicking ADAR substrates as a method to regulate protein expression and, potentially, therapeutically repair mutant mRNAs. Our data suggest variable RNA editing as a credible molecular mechanism for mediating individual-to-individual variation in neuronal physiology and behavior.

Endectocides for malaria control

Systemic endectocidal drugs, used to control nematodes in humans and other vertebrates, can be toxic to Anopheles spp. mosquitoes when they take a blood meal from a host that has recently received one of these drugs. Recent laboratory and field studies have highlighted the potential of ivermectin to control malaria parasite transmission if this drug is distributed strategically and more often. There are important theoretical benefits to this strategy, as well as caveats. A better understanding of drug effects against vectors and malaria ecologies are needed. In the near future, ivermectin and other endectocides could serve as potent and novel malaria transmission control tools that are directly linked to the control of neglected tropical diseases in the same communities.

The ionotropic &gamma;-aminobutyric acid receptor gene family of the silkworm, Bombyx mori

&gamma;-Aminobutyric acid (GABA) is a very important inhibitory neurotransmitter in both vertebrate and invertebrate nervous systems. GABA receptors (GABARs) are known to be the molecular targets of a class of insecticides. Members of the GABAR gene family of the silkworm, Bombyx mori, a model insect of Lepidoptera, have been identified and characterized in this study. All putative silkworm GABAR cDNAs were cloned using the reverse transcriptase polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE). Bombyx mori appears to have the largest insect GABAR gene family known to date, including three RDL, one LCCH3, and one GRD subunit. The silkworm RDL1 gene has RNA-editing sites, and the RDL1 and RDL3 genes possess alternative splicing. These mRNA modifications enhance the diversity of the silkworm's GABAR gene family. In addition, truncated transcripts were found for the RDL1 and LCCH3 genes. In particular, the three RDL subunits may have arisen from two duplication events.

Regulation and functions of ADAR in drosophila

Drosophila melanogaster has a single Adar gene encoding a protein related to mammalian ADAR2 that edits transcripts encoding glutamate receptor subunits. We describe the structure of the Drosophila Adar locus and use ModENCODE information to supplement published data on Adar gene transcription, and splicing. We discuss the roles of ADAR in Drosophila in terms of the two main types of RNA molecules edited and roles of ADARs as RNA-binding proteins. Site-specific RNA editing events in transcripts encoding ion channel subunits were initially found serendipitously and subsequent directed searches for editing sites and transcriptome sequencing have now led to 972 edited sites being identified in 597 transcripts. Four percent of D. melanogaster transcripts are site-specifically edited and these encode a wide range of largely membrane-associated proteins expressed particularly in CNS. Electrophysiological studies on the effects of specific RNA editing events on ion channel subunits do not suggest that loss of RNA editing events in ion channels consistently produce a particular outcome such as making Adar mutant neurons more excitable. This possibility would have been consistent with neurodegeneration seen in Adar mutant fly brains. A further set of ADAR targets are dsRNA intermediates in siRNA generation, derived from transposons and from structured RNA loci. Transcripts with convergent overlapping 3' ends are also edited and the first discovered instance of RNA editing in Drosophila, in the Rnp4F transcript, is an example. There is no evidence yet to show that Adar antagonizes RNA interference in Drosophila. Evidence has been obtained that catalytically inactive ADAR proteins exert effects on microRNA generation and RNA interference. Whether all effects of inactive ADARs are due to RNA-binding or to even further roles of these proteins remains to be determined.

Perturbing A-to-I RNA editing using genetics and homologous recombination

Evidence for the chemical conversion of adenosine-to-inosine (A-to-I) in messenger RNA (mRNA) has been detected in numerous metazoans, especially those "most successful" phyla: Arthropoda, Mollusca, and Chordata. The requisite enzymes for A-to-I editing, ADARs (adenosine deaminases acting on RNA) are highly conserved and are present in every higher metazoan genome sequenced to date. The fruit fly, Drosophila melanogaster, represents an ideal model organism for studying A-to-I editing, both in terms of fundamental biochemistry and in relation to determining adaptive downstream effects on physiology and behavior. The Drosophila genome contains a single structural gene for ADAR (dAdar), yet the fruit fly transcriptome has the widest range of conserved and validated ADAR targets in coding mRNAs of any known organism. In addition, many of the genes targeted by dADAR have been genetically identified as playing a role in nervous system function, providing a rich source of material to investigate the biological relevance of this intriguing process. Here, we discuss how recent advances in the use of ends-out homologous recombination (HR) in Drosophila make possible both the precise control of the editing status for defined adenosine residues and the engineering of flies with globally altered RNA editing of the fly transcriptome. These new approaches promise to significantly improve our understanding of how mRNA modification contributes to insect physiology and ethology.

Engineered alterations in RNA editing modulate complex behavior in Drosophila: regulatory diversity of adenosine deaminase acting on RNA (ADAR) targets

Select proteins involved in electrical and chemical neurotransmission are re-coded at the RNA level via the deamination of particular adenosines to inosine by adenosine deaminases acting on RNA (ADARs). It has been hypothesized that this process, termed RNA editing, acts to "fine-tune" neurophysiological properties in animals and potentially downstream behavioral outputs. However, the extreme phenotypes resulting from deletions of adar loci have precluded investigations into the relationship between ADAR levels, target transcripts, and complex behaviors. Here, we engineer Drosophila hypomorphic for ADAR expression using homologous recombination. A substantial reduction in ADAR activity (>80%) leads to altered circadian motor patterns and abnormal male courtship, although surprisingly, general locomotor coordination is spared. The altered phenotypic landscape in our adar hypomorph is paralleled by an unexpected dichotomous response of ADAR target transcripts, i.e. certain adenosines are minimally affected by dramatic ADAR reduction, whereas editing of others is severely curtailed. Furthermore, we use a novel reporter to map RNA editing activity across the nervous system, and we demonstrate that knockdown of editing in fruitless-expressing neurons is sufficient to modify the male courtship song. Our data demonstrate that network-wide temporal and spatial regulation of ADAR activity can tune the complex system of RNA-editing sites and modulate multiple ethologically relevant behavioral modalities.

Allosteric modulation by benzodiazepines of GABA-gated chloride channels of an identified insect motor neurone

The actions of benzodiazepines were studied on the responses to GABA of the fast coxal depressor (D(f)) motor neurone of the cockroach, Periplaneta americana. Ro5-4864, diazepam and clonazepam were investigated. Responses to GABA receptors were enhanced by both Ro5-4864 and diazepam, whereas clonazepam, a potent-positive allosteric modulator of human GABA(A) receptors, was ineffective on the native insect GABA receptors of the D(f) motor neurone. Thus, clear pharmacological differences exist between insect and mammalian native GABA-gated chloride channels with respect to the actions of benzodiazepines. The results enhance our understanding of invertebrate GABA-gated chloride channels which have recently proved important in (a) comparative studies aimed at identifying human allosteric drug-binding sites and (b) understanding the actions of compounds used to control ectoparasites and insect crop pests.

Adenosine-to-inosine genetic recoding is required in the adult stage nervous system for coordinated behavior in Drosophila

Adenosine deaminases acting on RNA (ADARs) catalyze the deamination of adenosine to inosine in double-stranded RNA templates, a process known as RNA editing. In Drosophila, multiple ADAR isoforms are generated from a single locus (dAdar) via post-transcriptional modifications. Collectively, these isoforms act to edit a wide range of transcripts involved in neuronal signaling, as well as the precursors of endogenous small interfering RNAs. The phenotypic consequences of a loss of dADAR activity have been well characterized and consist of profound behavioral defects manifested at the adult stage, including extreme uncoordination, seizures, and temperature-sensitive paralysis. However, the spatio-temporal requirements of adenosine to inosine editing for correct behavior are unclear. Using transgenic RNA interference, we show that network-wide editing in the nervous system is required for normal adult locomotion. Regulated restoration of editing activity demonstrates that the neuronal requirement of dADAR activity has a significant adult stage component. Furthermore we show that in relation to behavior there are no observable genetic interactions between dAdar and several loci encoding RNA interference components, suggesting that editing of neuronal transcripts is the key mode of ADAR activity for normal behavior in Drosophila.