Identification, localization and function of glutamate-gated chloride channel receptors in the honeybee brain

Behavioral and genetic correlates of heterogeneity in learning performance in individual honeybees, Apis mellifera

Learning an olfactory discrimination task leads to heterogeneous results in honeybees with some bees performing very well and others at low rates. Here we investigated this behavioral heterogeneity and asked whether it was associated with particular gene expression patterns in the bee's brain. Bees were individually conditioned using a sequential conditioning protocol involving several phases of olfactory learning and retention tests. A cumulative score was used to differentiate the tested bees into high and low performers. The rate of CS+ odor learning was found to correlate most strongly with a cumulative performance score extracted from all learning and retention tests. Microarray analysis of gene expression in the mushroom body area of the brains of these bees identified a number of differentially expressed genes between high and low performers. These genes are associated with diverse biological functions, such as neurotransmission, memory formation, cargo trafficking and development.

Oral administration of phytochemicals protects honey bees against cognitive and motor impairments induced by the insecticide fipronil

Pollution produced by exposure to pesticides is a major concern for food security because the negative impacts on pollinators. Fipronil, an insecticide broadly used around the globe has been associated with the ongoing decline of bees. With a characteristic neuroactive toxicodynamic, fipronil leads to cognitive and motor impairments at sublethal dosages. Despite of regional bans, multilevel strategies are necessary for the protection of pollinators. Recent evidence suggests that specific nutrients in the diets of bees may induce protection against insecticides. Here, we evaluated whether the administration of three phytochemicals, namely rutin, kaempferol and p-coumaric acid provide protection to the Africanized honey bee Apis mellifera against oral administration of realistic dosages of fipronil. We tested the potential impairment produced by fipronil and the protection induced by the phytochemicals in learning, 24h memory, sucrose sensitivity and motor control. We found that the administration of fipronil induced a concentration-dependent impairment in learning and motor control, but not 24h memory or sucrose sensitivity across a 24h window. We also found that the administration of rutin, p-coumaric acid, kaempferol and the mixture was innocuous and generally offered protection against the impairments induced by fipronil. Overall, our results indicate that bees can be prophylactically protected against insecticides via nutrition, providing an alternative to the ongoing conflict between the use of insecticides and the decline of pollinators. As the studied phytochemicals are broadly present in nectar and pollen, our results suggest that the nutritional composition, and not only its production, should be considered when implementing strategies of conservation via gardens and co-cropping.

Expression and function of opsin genes associated with phototaxis in Zeugodacus cucurbitae Coquillett (Diptera: Tephritidae)

BACKGROUND

Zeugodacus cucuribitae is a major agricultural pest that causes significant damage to varieties of plants. Vision plays a critical role in phototactic behavior of herbivorous insects. However, the effect of opsin on the phototactic behavior in Z. cucuribitae remains unknown. The aim of this research is to explore the key opsin genes that associate with phototaxis behavior of Z. cucurbitae.

RESULTS

Five opsin genes were identified and their expression patterns were analyzed. The relative expression levels of ZcRh1, ZcRh4 and ZcRh6 were highest in 4-day-old larvae, ZcRh2 and ZcRh3 were highest in 3rd-instar larvae and 5-day-old pupae, respectively. Furthermore, five opsin genes had the highest expression levels in compound eyes, followed by the antennae and head, whereas the lower occurred in other tissues. The expression of the long-wavelength-sensitive (LW) opsins first decreased and then increased under green light exposure. In contrast, the expression of ultraviolet-sensitive (UV) opsins first increased and then decreased with the duration of UV exposure. Silencing of LW opsin (dsZcRh1, dsZcRh2, and dsZcRh6) and UV opsin (dsZcRh3 and dsZcRh4) reduced the phototactic efficiency of Z. cucurbitae to green light by 52.27%, 60.72%, and 67.89%, and to UV light by 68.59% and 61.73%, respectively.

CONCLUSION

The results indicate that RNAi inhibited the expression of opsin, thereby inhibiting the phototaxis of Z. cucurbitae. This result provides theoretical support for the physical control of Z. cucurbitae and lays the foundation for further exploration of the mechanism of insect phototaxis. © 2023 Society of Chemical Industry.

Characterization of Glutamate-Gated Chloride Channel in Tribolium castaneum

The glutamate-gated chloride channels (GluCls) play essential roles in signal transduction by regulating fast inhibitory synaptic transmission in the nervous system of invertebrates. While there is only one GluCl subunit in the insect, the diversity of insect GluCls is broadened by alternative splicing. In the present study, three TcGluCl variant genes were cloned from the red flour beetle Tribolium castaneum. Analysis of the characteristics of TcGluCls including sequence features, genomic structures, and alternative splicing revealed that TcGluCls had the typical structural features of GluCls and showed high homologies with the GluCls from other insect orders. The TcGluCl-encoding gene consists of nine exons and three variants (TcGluCl-3a, TcGluCl-3b, and TcGluCl-3c) were generated by the alternative splicing of exon 3, which was a highly conserved alternative splicing site in insect GluCls. Homology modeling of TcGluCl-3a showed that the exon 3 coding protein located at the N-terminal extracellular domain, and there were no steric clashes encountered between the exon 3 coding region and ivermectin/glutamate binding pocket, which indicated that the alternative splicing of exon 3 might have no impact on the binding of GluCls to glutamate and insecticide. In addition to the head tissue, TcGluCl-3a and TcGluCl-3c also had high expressions in the ovary and testis of T. castaneum, whereas TcGluCl-3b showed high expression in the midgut, suggesting the diverse physiological functions of TcGluCl variants in T. castaneum. The total TcGluCl and three variants showed the highest expression levels in the early stage larvae. The expressions of TcGluCl, TcGluCl-3b, and TcGluCl-3c were significantly increased from the late-stage larvae to the early stage pupae and indicated that the TcGluCl might be involved in the growth and development of T. castaneum. These results are helpful to further understand the molecular characteristics of insect GluCls and provide foundations for studying the specific function of the GluCl variant.

A novel V263I mutation in the glutamate-gated chloride channel of Plutella xylostella (L.) confers a high level of resistance to abamectin

The frequent and extensive use of insecticides leads to the evolution of insecticide resistance, which has become one of the constraints on global agricultural production. Avermectins are microbial-derived insecticides that target a wide number of insect pests, including the diamondback moth Plutella xylostella, an important global pest of brassicaceous vegetables. However, field populations of P. xylostella have evolved serious resistance to avermectins, including abamectin, thereby threatening the efficiency of these insecticides. In this study, a novel valine to isoleucine mutation (V263I) was identified in the glutamate-gated chloride channel (GluCl) of field P. xylostella populations, which showed different levels of resistance to abamectin. Electrophysiological analysis revealed that the V263I mutation significantly reduced the sensitivity of PxGluCl to abamectin by 6.9-fold. Genome-modified Drosophila melanogaster carrying the V263I mutation exhibited 27.1-fold resistance to abamectin. Then, a knockin strain (V263I-KI) of P. xylostella expressing the homozygous V263I mutation was successfully constructed using the CRISPR/Cas9. The V263I-KI had high resistance to abamectin (106.3-fold), but significantly reduced fecundity. In this study, the function of V263I mutation in PxGluCl was verified for the first time. These findings provide a more comprehensive understanding of abamectin resistance mechanisms and lay the foundation for providing a new molecular detection method for abamectin resistance monitoring.

Harmful effects of fipronil exposure on the behavior and brain of the stingless bee Partamona helleri Friese (Hymenoptera: Meliponini)

Fipronil is a pesticide widely used to control agricultural and household insect pests. However, fipronil is highly toxic to non-target insects, including pollinators. In this study, we investigated the acute effects of fipronil on the behavior, brain morphology, antioxidant activity, and proteins related to signaling pathways on the brain of workers of the stingless bee Partamona helleri. The ingestion of fipronil increases both the walking distance and velocity and causes enlarged intercellular spaces in the Kenyon cells and intense vacuolization in the neuropils of the brain. Moreover, fipronil decreases the activity of catalase (CAT) and increases the activity of glutathione S-transferase (GST). However, there is no difference in superoxide dismutase (SOD) activity between the control and fipronil. Regarding immunofluorescence analysis, bees exposed to fipronil showed an increase in the number of cells positive for cleaved caspase-3 and peroxidase, but a reduction in the number of cells positive for ERK 1/2, JNK and Notch, suggesting neuron death and impaired brain function. Our results demonstrate that fipronil has harmful effects on the behavior and brain of a stingless bee, which may threaten the individuals and colonies of this pollinator.

Glutamate signaling from a single sensory neuron mediates experience-dependent bidirectional behavior in Caenorhabditis elegans

Orientation and navigation behaviors of animals are modulated by past experiences. However, little is known about the mechanisms by which sensory inputs are translated into multi-directional orientation behaviors in an experience-dependent manner. Here, we report a neural mechanism for bidirectional salt-concentration chemotaxis of Caenorhabditis elegans. The salt-sensing neuron ASE right (ASER) is always activated by a decrease of salt concentration, while the directionality of reorientation behaviors is inverted depending on previous salt experiences. AIB, the interneuron postsynaptic to ASER, and neurons farther downstream of AIB show experience-dependent bidirectional responses, which are correlated with reorientation behaviors. These bidirectional behavioral and neural responses are mediated by glutamate released from ASER. Glutamate acts through the excitatory glutamate receptor GLR-1 and inhibitory glutamate receptor AVR-14, both acting in AIB. These findings suggest that experience-dependent reorientation behaviors are generated by altering the magnitude of excitatory and inhibitory postsynaptic signals from a sensory neuron to interneurons.

GABA signaling affects motor function in the honey bee

GABA is the most common inhibitory neurotransmitter in both vertebrate and invertebrate nervous systems. In insects, inhibition plays important roles at the neuromuscular junction, in the regulation of central pattern generators, and in the modulation of information in higher brain processing centers. Additionally, increasing our understanding of the functions of GABA is important since GABA_A receptors are the targets of several classes of pesticides. To investigate the role of GABA in motor function, honey bee foragers were injected with GABA or with agonists or antagonists specific for either GABA_A or GABA_B receptors. Compounds that activated either type of GABA receptor decreased activity levels. Bees injected with the GABA_A receptor antagonist picrotoxin lost the ability to right themselves, whereas blockade of GABA_B receptors led to increases in grooming. Injection with antagonists of either GABA_A or GABA_B receptors resulted in an increase in extended wing behavior, during which bees kept their wings out at right angles to their body rather than folded along their back. These data suggest that the GABA receptor types play distinct roles in behavior and that GABA may affect behavior at several different levels.

Genetics in the Honey Bee: Achievements and Prospects toward the Functional Analysis of Molecular and Neural Mechanisms Underlying Social Behaviors

The European honey bee is a model organism for studying social behaviors. Comprehensive analyses focusing on the differential expression profiles of genes between the brains of nurse bees and foragers, or in the mushroom bodies—the brain structure related to learning and memory, and multimodal sensory integration—has identified candidate genes related to honey bee behaviors. Despite accumulating knowledge on the expression profiles of genes related to honey bee behaviors, it remains unclear whether these genes actually regulate social behaviors in the honey bee, in part because of the scarcity of genetic manipulation methods available for application to the honey bee. In this review, we describe the genetic methods applied to studies of the honey bee, ranging from classical forward genetics to recently developed gene modification methods using transposon and CRISPR/Cas9. We then discuss future functional analyses using these genetic methods targeting genes identified by the preceding research. Because no particular genes or neurons unique to social insects have been found yet, further exploration of candidate genes/neurons correlated with sociality through comprehensive analyses of mushroom bodies in the aculeate species can provide intriguing targets for functional analyses, as well as insight into the molecular and neural bases underlying social behaviors.

Molecular cloning, characterization and functional analysis of GluCl from the oriental armyworm, Mythimna separata Walker

Glutamate-gated chloride channels (GluCls) mediate inhibitory synaptic transmission in invertebrate nervous systems, and only one GluCl gene has been found in insects. Therefore, insect GluCls are one of the major targets of insecticides including avermectins. In the present study, a 1347 bp full-length cDNA encoding a 449-amino acid protein (named MsGluCl, GenBank ID: MK336885) was cloned from the oriental armyworm, Mythimna separata, and characterized two alternative splicing variants of MsGluCl. The protein shares 76.9-98.6% identity with other insect GluCl isoforms. Spatial and temporal expression analysis revealed that MsGluCl was highly expressed in the 3rd instar and adult head. Dietary ingestion of dsMsGluCl significantly reduced the mRNA level of MsGluCl and decreased abamectin mortality. Thus, our results reveal that MsGluCl could be the molecular target of abamectin and provide the basis for further understanding the resistance mechanism to abamectin in arthropods.

Molecular characterization of glutamate-gated chloride channel and its possible roles in development and abamectin susceptibility in the rice stem borer, Chilo suppressalis

Glutamate-gated chloride channels (GluCls) mediate fast inhibitory neurotransmission in invertebrate nervous systems, and are of considerable interest in insecticide discovery. The full length cDNA encoding CsGluCl was cloned from the rice stem borer Chilo suppressalis (Walker). Multiple cDNA sequence alignment revealed three variants of CsGluCl generated by alternative splicing of exon 3 and exon 9. While all the transcripts were predominantly expressed in both nerve cord and brain, the expression patterns of these three variants differed among other tissues and developmental stages. Specifically, the expression level of CsGluCl C in cuticle was similar to that in nerve cord and brain, and was the predominant variant in late pupae and early adult stages. Both injection and oral delivery of dsGluCl significantly reduced the mRNA level of CsGluCl. Increased susceptibility to abamectin and reduced larvae growth and pupation rate were observed in dsGluCl-treated larvae. Thus, our results provide the evidence that in addition to act as the target of abamectin, GluCls also play important physiological roles in the development of insects.

Identification of the neurotransmitter profile of AmFoxP expressing neurons in the honeybee brain using double-label in situ hybridization

BACKGROUND

FoxP transcription factors play crucial roles for the development and function of vertebrate brains. In humans the neurally expressed FOXPs, FOXP1, FOXP2, and FOXP4 are implicated in cognition, including language. Neural FoxP expression is specific to particular brain regions but FoxP1, FoxP2 and FoxP4 are not limited to a particular neuron or neurotransmitter type. Motor- or sensory activity can regulate FoxP2 expression, e.g. in the striatal nucleus Area X of songbirds and in the auditory thalamus of mice. The DNA-binding domain of FoxP proteins is highly conserved within metazoa, raising the possibility that cellular functions were preserved across deep evolutionary time. We have previously shown in bee brains that FoxP is expressed in eleven specific neuron populations, seven tightly packed clusters and four loosely arranged groups.

RESULTS

The present study examined the co-expression of honeybee FoxP (AmFoxP) with markers for glutamatergic, GABAergic, cholinergic and monoaminergic transmission. We found that AmFoxP could co-occur with any one of those markers. Interestingly, AmFoxP clusters and AmFoxP groups differed with respect to homogeneity of marker co-expression; within a cluster, all neurons co-expressed the same neurotransmitter marker, within a group co-expression varied. We also assessed qualitatively whether age or housing conditions providing different sensory and motor experiences affected the AmFoxP neuron populations, but found no differences.

CONCLUSIONS

Based on the neurotransmitter homogeneity we conclude that AmFoxP neurons within the clusters might have a common projection and function whereas the AmFoxP groups are more diverse and could be further sub-divided. The obtained information about the neurotransmitters co-expressed in the AmFoxP neuron populations facilitated the search of similar neurons described in the literature. These comparisons revealed e.g. a possible function of AmFoxP neurons in the central complex. Our findings provide opportunities to focus future functional studies on invertebrate FoxP expressing neurons. In a broader context, our data will contribute to the ongoing efforts to discern in which cases relationships between molecular and phenotypic signatures are linked evolutionary.

Diverse Factors Affecting Efficiency of RNAi in Honey Bee Viruses

Infection and transmission of honey bee viruses pose a serious threat to the pollination services of crops and wild plants, which plays a vital role in agricultural economy and ecology. RNA interference (RNAi) is an effective defense mechanism against commonly occurring viral infections of animals and plants. However, recent studies indicate that the effects of RNAi on the honey bee can induce additional impacts and might not always be effective in suppressing the virus. Moreover, the RNAi responses differed in relation to the developmental stage of the insect and the target tissue used, even though the same method of delivery was used. These results indicate that further analysis and field experiments should be performed to characterize the varying effectiveness of RNAi-based methods for treating honey bee viral infections. In this review, we provide an overview of the current knowledge and the recent progress in RNAi-based anti-viral treatments for honey bees, focusing in particular highlight the role of the dsRNA-delivery method used and its effect on RNAi efficiency and demonstrate the potential practical value of this tool for controlling the virus. We conclude studying the gene function and disease control of honey bee by RNAi technology requires a complex consideration from physiology, genetics to environment.

Molecular cloning and characterization of GABA receptor and GluCl subunits in the western flower thrips, Frankliniella occidentalis

To understand the role of target site insensitivity in abamectin resistance in the western flower thrips (WFT), Frankliniella occidentalis (Pergande), cDNAs encoding gamma-aminobutyric acid receptor subunit (FoRdl) and glutamate-gated chloride channel (FoGluCl) were cloned from WFT, and both single nucleotide polymorphisms (SNPs) and mRNA expression levels of FoRdl and FoGluCl were detected in a susceptible strain (ABA-S) and a laboratory selected strain (ABA-R) displaying 45.5-fold resistance to abamectin. Multiple cDNA sequence alignment revealed three alternative splicing variants of FoRdl and two alternative splicing variants of FoGluCl generated by alternative splicing of exon 3. While sequence comparison of FoRdl and FoGluCl in ABA-S and ABA-R strains identified no resistance-associated mutations, the expression level of FoGluCl in ABA-R strain was 2.63-fold higher than that in ABA-S strain. Thus, our preliminary results provide the evidence that the increased mRNA expression of FoGluCl could be an important factor in FoGluCl-mediated target site insensitivity in WFT.

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.

Expression of Cys-loop receptor subunits and acetylcholine binding protein in the mechanosensory neurons, glial cells, and muscle tissue of the spider Cupiennius salei

The central and peripheral nervous system transcriptomes of the spider Cupiennius salei have 15 Cys-loop receptor subunits and an acetylcholine-binding protein (AChBP). Twelve subunits are predicted to form anion channels gated by γ-aminobutyric acid (GABA), glutamate, histamine, or changes in pH, and three are putative ACh-gated cation channels. Spiders have a variety of mechanosensilla and proprioceptive organs that are innervated by efferents in their peripherally located parts, and efferents also innervate muscle fibers. We investigated Cys-loop gene expression in muscle tissue by qPCR and localized this expression in mechanosensilla via in situ hybridization. The cuticular mechanosensory neurons had only CsGABArdl and CspHCl2 subunits, whereas the muscle tissue expressed a wider variety of subunits, especially CsGABAgrd, CsGABA_A β, CsGluCl1 and CspHCl, but very low levels of the CsGABArdl or CsnACh subunits. An nACh non-α subunit was expressed in a group of unidentified cells in the hypodermis and at low level in the muscle tissue, but the physiological function of this subunit is unknown. The CsnAChα subunit was not expressed in sensory neurons and was expressed at extremely low level in the muscle tissue. None of the probes gave signals in proprioceptive joint receptors, suggesting that efferent innervation to this sense organ employs other receptor types. CsAChBP and a glia-specific homeodomain CsREPO were both expressed in glial cells that surround sensory neurons and also in muscle tissue, probably around the nerve endings of the neuromuscular junction. These locations have large numbers of synapses, suggesting that AChBP may have a function in modulating synaptic transmission. J. Comp. Neurol. 525:1139-1154, 2017. © 2016 Wiley Periodicals, Inc.

Effects of a neonicotinoid pesticide on thermoregulation of African honey bees (Apis mellifera scutellata)

Thiamethoxam is a widely used neonicotinoid pesticide that, as agonist of the nicotinic acetylcholine receptors, has been shown to elicit a variety of sublethal effects in honey bees. However, information concerning neonicotinoid effects on honey bee thermoregulation is lacking. Thermoregulation is an essential ability for the honey bee that guarantees the success of foraging and many in-hive tasks, especially brood rearing. We tested the effects of acute exposure to thiamethoxam (0.2, 1, 2ng/bee) on the thorax temperatures of foragers exposed to low (22°C) and high (33°C) temperature environments. Thiamethoxam significantly altered honey bee thorax temperature at all doses tested; the effects elicited varied depending on the environmental temperature and pesticide dose to which individuals were exposed. When bees were exposed to the high temperature environment, the high dose of thiamethoxam increased their thorax temperature 1-2h after exposure. When bees were exposed to the low temperature, the higher doses of the neonicotinoid reduced bee thorax temperatures 60-90min after treatment. In both experiments, the neonicotinoid decreased the temperature of bees the day following the exposure. After a cold shock (5min at 4°C), the two higher doses elicited a decrease of the thorax temperature, while the lower dose caused an increase, compared to the control. These alterations in thermoregulation caused by thiamethoxam may affect bee foraging activity and a variety of in-hive tasks, likely leading to negative consequences at the colony level. Our results shed light on sublethal effect of pesticides which our bees have to deal with.

Knockdown of juvenile hormone acid methyl transferase severely affects the performance of Leptinotarsa decemlineata (Say) larvae and adults

BACKGROUND

Juvenile hormone (JH) plays a critical role in the regulation of metamorphosis in Leptinotarsa decemlineata, a notorious defoliator of potato. JH acid methyltransferase (JHAMT) is involved in one of the final steps of JH biosynthesis.

RESULTS

A putative JHAMT cDNA (LdJHAMT) was cloned. Two double-stranded RNAs (dsRNAs) (dsJHAMT1 and dsJHAMT2) against LdJHAMT were constructed and bacterially expressed. Experiments were conducted to investigate the effectiveness of RNAi in both second- and fourth-instar larvae. Dietary introduction of dsJHAMT1 and dsJHAMT2 successfully knocked down the target gene, lowered JH titre in the haemolymph and reduced the transcript of Krüppel homologue 1 gene. Ingestion of dsJHAMT caused larval death and weight loss, shortened larval developmental period and impaired pupation. Moreover, the dsJHAMT-fed pupae exhibited lower adult emergence rates. The resulting adults weighed an average of 50 mg less than the control group, and the females did not deposit eggs. Application of pyriproxyfen to the dsJHAMT-fed insects rescued all the negative effects.

CONCLUSIONS

LdJHAMT expresses functional JHAMT enzyme. The RNAi targeting LdJHAMT could be used for control of L. decemlineata. © 2015 Society of Chemical Industry.

CaMKII knockdown affects both early and late phases of olfactory long-term memory in the honeybee

Honeybees are able to solve complex learning tasks and memorize learned information for long time periods. The molecular mechanisms mediating long-term memory (LTM) in the honeybee Apis mellifera are, to a large part, still unknown. We approached this question by investigating the potential function of the calcium/calmodulin-dependent protein kinase II (CaMKII), an enzyme known as a 'molecular memory switch' in vertebrates. CaMKII is able to switch to a calcium-independent constitutively active state, providing a mechanism for a molecular memory and has further been shown to play an essential role in structural synaptic plasticity. Using a combination of knockdown by RNA interference and pharmacological manipulation, we disrupted the function of CaMKII during olfactory learning and memory formation. We found that learning, memory acquisition and mid-term memory were not affected, but all manipulations consistently resulted in an impaired LTM. Both early LTM (24 h after learning) and late LTM (72 h after learning) were significantly disrupted, indicating the necessity of CaMKII in two successive stages of LTM formation in the honeybee.

De novo assembly and characterization of central nervous system transcriptome reveals neurotransmitter signaling systems in the rice striped stem borer, Chilo suppressalis

Background

Neurotransmitter signaling systems play crucial roles in multiple physiological and behavioral processes in insects. Genome wide analyses of de novo transcriptome sequencing and gene specific expression profiling provide rich resources for studying neurotransmitter signaling pathways. The rice striped stem borer, Chilo suppressalis is a destructive rice pest in China and other Asian countries. The characterization of genes involved in neurotransmitter biosynthesis and transport could identify potential targets for disruption of the neurochemical communication and for crop protection.

Results

Here we report de novo sequencing of the C. suppressalis central nervous system transcriptome, identification and expression profiles of genes putatively involved in neurotransmitter biosynthesis, packaging, and recycling/degradation. A total of 54,411 unigenes were obtained from the transcriptome analysis. Among these unigenes, we have identified 32 unigenes (31 are full length genes), which encode 21 enzymes and 11 transporters putatively associated with biogenic aminergic signaling, acetylcholinergic signaling, glutamatergic signaling and GABAergic signaling. RT-PCR and qRT-PCR results indicated that 12 enzymes were highly expressed in the central nervous system and all the transporters were expressed at significantly high levels in the central nervous system. In addition, the transcript abundances of enzymes and transporters in the central nervous system were validated by qRT-PCR. The high expression levels of these genes suggest their important roles in the central nervous system.

Conclusions

Our study identified genes potentially involved in neurotransmitter biosynthesis and transport in C. suppressalis and these genes could serve as targets to interfere with neurotransmitter production. This study presents an opportunity for the development of specific and environmentally safe insecticides for pest control.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1742-7) contains supplementary material, which is available to authorized users.

The integrative analysis of microRNA and mRNA expression in Apis mellifera following maze-based visual pattern learning

The honeybee (Apis mellifera) is a social insect with strong sensory capacity and diverse behavioral repertoire and is recognized as a good model organism for studying the neurobiological basis of learning and memory. In this study, we analyzed the changes in microRNA (miRNA) and messenger RNA (mRNA) following maze-based visual learning using next-generation small RNA sequencing and Solexa/lllumina Digital Gene Expression tag profiling (DGE). For small RNA sequencing, we obtained 13 367 770 and 13 132 655 clean tags from the maze and control groups, respectively. A total of 40 differentially expressed known miRNAs were detected between these two samples, and all of them were up-regulated in the maze group compared to the control group. For DGE, 5 681 320 and 5 939 855 clean tags were detected from the maze and control groups, respectively. There were a total of 388 differentially expressed genes between these two samples, with 45 genes up-regulated and 343 genes down-regulated in the maze group, compared to the control group. Additionally, the expression levels of 10 differentially expressed genes were confirmed by quantitative reverse transcription polymerase chain reaction (qRT-PCR) and the expression trends of eight of them were consistent with the DGE result, although the degree of change was lower in amplitude. The integrative analysis of miRNA and mRNA expression showed that, among the 40 differentially expressed known miRNAs and 388 differentially expressed genes, 60 pairs of miRNA/mRNA were identified as co-expressed in our present study. These results suggest that both miRNA and mRNA may play a pivotal role in the process of learning and memory in honeybees. Our sequencing data provide comprehensive miRNA and gene expression information for maze-based visual learning, which will facilitate understanding of the molecular mechanisms of honeybee learning and memory.

Molecular mechanisms underlying formation of long-term reward memories and extinction memories in the honeybee (Apis mellifera)

The honeybee (Apis mellifera) has long served as an invertebrate model organism for reward learning and memory research. Its capacity for learning and memory formation is rooted in the ecological need to efficiently collect nectar and pollen during summer to ensure survival of the hive during winter. Foraging bees learn to associate a flower's characteristic features with a reward in a way that resembles olfactory appetitive classical conditioning, a learning paradigm that is used to study mechanisms underlying learning and memory formation in the honeybee. Due to a plethora of studies on appetitive classical conditioning and phenomena related to it, the honeybee is one of the best characterized invertebrate model organisms from a learning psychological point of view. Moreover, classical conditioning and associated behavioral phenomena are surprisingly similar in honeybees and vertebrates, suggesting a convergence of underlying neuronal processes, including the molecular mechanisms that contribute to them. Here I review current thinking on the molecular mechanisms underlying long-term memory (LTM) formation in honeybees following classical conditioning and extinction, demonstrating that an in-depth analysis of the molecular mechanisms of classical conditioning in honeybees might add to our understanding of associative learning in honeybees and vertebrates.

Honeybee Kenyon cells are regulated by a tonic GABA receptor conductance

The higher cognitive functions of insects are dependent on their mushroom bodies (MBs), which are particularly large in social insects such as honeybees. MB Kenyon cells (KCs) receive multisensory input and are involved in associative learning and memory. In addition to receiving sensory input via excitatory nicotinic synapses, KCs receive inhibitory GABAergic input from MB feedback neurons. Cultured honeybee KCs exhibit ionotropic GABA receptor currents, but the properties of GABA-mediated inhibition in intact MBs are currently unknown. Here, using whole cell recordings from KCs in acutely isolated honeybee brain, we show that KCs exhibit a tonic current that is inhibited by picrotoxin but not by bicuculline. Bath application of GABA (5 μM) and taurine (1 mM) activate a tonic current in KCs, but l-glutamate (0.1-0.5 mM) has no effect. The tonic current is strongly potentiated by the allosteric GABAA receptor modulator pentobarbital and is reduced by inhibition of Ca(2+) channels with Cd(2+) or nifedipine. Noise analysis of the GABA-evoked current gives a single-channel conductance value for the underlying receptors of 27 ± 3 pS, similar to that of resistant to dieldrin (RDL) receptors. The amount of injected current required to evoke action potential firing in KCs is significantly lower in the presence of picrotoxin. KCs recorded in an intact honeybee head preparation similarly exhibit a tonic GABA receptor conductance that reduces neuronal excitability, a property that is likely to contribute to the sparse coding of sensory information in insect MBs.

Differential involvement of glutamate-gated chloride channel splice variants in the olfactory memory processes of the honeybee Apis mellifera

Glutamate-gated chloride channels (GluCl) belong to the cys-loop ligand-gated ion channel superfamily and their expression had been described in several invertebrate nervous systems. In the honeybee, a unique gene amel_glucl encodes two alternatively spliced subunits, Amel_GluCl A and Amel_GluCl B. The expression and differential localization of those variants in the honeybee brain had been previously reported. Here we characterized the involvement of each variant in olfactory learning and memory processes, using specific small-interfering RNA (siRNA) targeting each variant. Firstly, the efficacy of the two siRNAs to decrease their targets' expression was tested, both at mRNA and protein levels. The two proteins showed a decrease of their respective expression 24h after injection. Secondly, each siRNA was injected into the brain to test whether or not it affected olfactory memory by using a classical paradigm of conditioning the proboscis extension reflex (PER). Amel_GluCl A was found to be involved only in retrieval of 1-nonanol, whereas Amel_GluCl B was involved in the PER response to 2-hexanol used as a conditioned stimulus or as new odorant. Here for the first time, a differential behavioral involvement of two highly similar GluCl subunits has been characterized in an invertebrate species.

dsRNA uptake and persistence account for tissue-dependent susceptibility to RNA interference in the migratory locust, Locusta migratoria

RNA interference (RNAi) by introducing double-stranded RNA (dsRNA) is a powerful approach to the analysis of gene function in insects; however, RNAi responses vary dramatically in different insect species and tissues, and the underlying mechanisms remain poorly understood. The migratory locust, a destructive insect pest and a hemimetabolic insect with panoistic ovaries, is considered to be a highly susceptible species to RNAi via dsRNA injection, but its ovary appears to be completely insensitive. In the present study, we showed that dsRNA persisted only briefly in locust haemolymph. The ovariole sheath was permeable to dsRNA, but injected dsRNA was not present in the follicle cells and oocytes. The lack of dsRNA uptake into the follicle cells and oocytes is likely to be the primary factor that contributes to the ineffective RNAi response in locust ovaries. These observations provide insights into tissue-dependent variability of RNAi and help in achieving successful gene silencing in insensitive tissues.

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.

Characterisation of the RNA interference response against the long-wavelength receptor of the honeybee

Targeted knock-down is the method of choice to advance the study of sensory and brain functions in the honeybee by using molecular techniques. Here we report the results of a first attempt to interfere with the function of a visual receptor, the long-wavelength-sensitive (L-) photoreceptor. RNA interference to inhibit this receptor led to a reduction of the respective mRNA and protein. The interference effect was limited in time and space, and its induction depended on the time of the day most probably because of natural daily variations in opsin levels. The inhibition did not effectively change the physiological properties of the retina. Possible constraints and implications of this method for the study of the bee's visual system are discussed. Overall this study underpins the usefulness and feasibility of RNA interference as manipulation tool in insect brain research.

Differential distribution of glutamate- and GABA-gated chloride channels in the housefly Musca domestica

l-Glutamic acid (glutamate) mediates fast inhibitory neurotransmission by affecting glutamate-gated chloride channels (GluCls) in invertebrates. The molecular function and pharmacological properties of GluCls have been well studied, but not much is known about their physiological role and localization in the insect body. The distribution of GluCls in the housefly (Musca domestica L.) was thus compared with the distribution of γ-aminobutyric acid (GABA)-gated chloride channels (GABACls). Quantitative PCR and ligand-binding experiments indicate that the GluCl and GABACl transcripts and proteins are predominantly expressed in the adult head. Intense GluCl immunostaining was detected in the lamina, leg motor neurons, and legs of adult houseflies. The GABACl (Rdl) immunostaining was more widely distributed, and was found in the medulla, lobula, lobula plate, mushroom body, antennal lobe, and ellipsoid body. The present findings suggest that GluCls have physiological roles in different tissues than GABACls.

Expression and localization of glutamate-gated chloride channel variants in honeybee brain (Apis mellifera)

Due to its specificity to invertebrate species, glutamate-gated chloride channels (GluCls) are the target sites of antiparasitic agents and insecticides, e.g. ivermectin and fipronil, respectively. In nematodes and insects, the GluCls diversity is broadened by alternative splicing. GluCl subunits have been characterized according to their sensitivity to drugs, and to their anatomical localization. In the honeybee, the GluCl gene can encode different alpha subunits due to alternative splicing of exon 3. We examined mRNA expression in brain parts and we confirmed the existence of two GluCl variants with RT-PCR, Amel_GluCl A and Amel_GluCl B. Surprisingly, a mixed isoform not yet described in insect was obtained, we called it Amel_GluCl C. We determined precise immunolocalization of peptide sequence corresponding to Amel_GluCl A and Amel_GluCl B in the honeybee brain. Amel_GluCl A is mainly located in neuropils, whereas Amel_GluCl B is mostly expressed in cell bodies. Both proteins can also be co-localized. According to their anatomical localization, different GluCl variants might be involved in olfactory and visual modalities and in learning and memory.