GABA receptors in insects

Decoding bee cleptoparasitism through comparative transcriptomics of Coelioxoides waltheriae and its host Tetrapedia diversipes

Cleptoparasitism, also known as brood parasitism, is a widespread strategy among bee species in which the parasite lays eggs into the nests of the host species. Even though this behavior has significant ecological implications for the dynamics of several species, little is known about the molecular pathways associated with cleptoparasitism. To shed some light on this issue, we used gene expression data to perform a comparative analysis between two solitary neotropical bees: Coelioxoides waltheriae, an obligate parasite, and their specific host Tetrapedia diversipes. We found that ortholog genes involved in signal transduction, sensory perception, learning, and memory formation were differentially expressed between the cleptoparasite and the host. We hypothesize that these genes and their associated molecular pathways are engaged in cleptoparasitism-related processes and, hence, are appealing subjects for further investigation into functional and evolutionary aspects of cleptoparasitism in bees.

Gamma-aminobutyric acid (GABA) in flower nectar and its possible physiological and behavioral effects on insect pollinators

Floral nectar, a vital nutrition source for pollinators, contains diverse chemical compounds, including γ-Aminobutyric acid (GABA), a prevalent non-proteinogenic amino acid. While GABA's physiological role is known and well-studied, its ecological significance in plant-pollinator interactions remains unclear. Recent studies on GABA's effects on pollinators' preference, consumption, survival, physiology, and behavior show varying outcomes according to the species, indicating a complex relationship. GABA consumption impacts motor function and cognitive abilities, potentially influencing pollination efficiency. Future research addressing diverse concentrations, species, and behavioral aspects is crucial for comprehensively understanding GABA's ecological role in plant-pollinator interactions.

Biochemical alterations in cotton leafworm, Spodoptera littoralis (Boisd.) related to emamectin benzoate and fipronil compared to their joint action

Cotton leafworm, Spodoptera littoralis (Boisduval), is one of the major destructive pests of ornamental, industrial, and vegetable crops. The efficacy of technical emamectin benzoate (EMB) and fipronil (FPR) was assessed against the 4th larval instar using leaf-dip bioassay method. EMB was more efficient than FPR based on 96 h LC50 values of 0.004 and 0.023 μg/ml, respectively. Joint toxic action of the dual exposure in sequence with time interval 24 h and in mix were evaluated at LC10:LC10, LC25:LC25 and LC50:LC50 after 96 h posttreatment, as well. Their impacts on detoxification enzymes, esterases (ESTs); alkaline phosphatase (ALP); and glutathione S-transferase (GST) as well as acetylcholine esterase (AChE) were also determined. The sequential exposure of EMB after FPR (S1) produced antagonism, potentiation, and potentiation effects, respectively while sequential exposure of FPR after EMB (S2) interacted as addition, potentiation, and potentiation respectively. The rest of binary mixtures (Mix) revealed antagonistic effect regardless of concentration. Orthogonal contrast analysis showed that the highest elevations of AChE, α-EST, β- EST and ALP enzymes were obtained from Mix at LC50:LC50 (181.6%, 288.4, 229.2 and 460.9%, respectively), LC25:LC25 (131.5%, 252.8, 205.60 and 252.0, respectively) and LC10:LC10 (106.6%, 215.6%, 201.8% and 170.0%, respectively). Differently, the greatest elevation of GST activity (157.7%) resulted from S1 at LC50:LC50, while it was significantly lower at LC25:LC25 and LC10:LC10 as well as Mix and S2 at all concentrations than corresponding concentrations of FPR. These findings shed some light on the role of GST in FPR toxicity and clarified the risk of these dual exposures in elevating detoxification enzymes dangerously compared to their individual insecticides. These dual exposures should be carefully handled. Although rotational exposure at low concentrations may enhance performance and mitigate resistance risk, rotational exposure at high concentrations and Mix may indirectly contribute to the evolution of cross-resistance to other insecticides.

Permethrin resistance in Aedes aegypti: Genomic variants that confer knockdown resistance, recovery, and death

Pyrethroids are one of the few classes of insecticides available to control Aedes aegypti, the major vector of dengue, chikungunya, and Zika viruses. Unfortunately, evolving mechanisms of pyrethroid resistance in mosquito populations threaten our ability to control disease outbreaks. Two common pyrethroid resistance mechanisms occur in Ae. aegypti: 1) knockdown resistance, which involves amino acid substitutions at the pyrethroid target site-the voltage-gated sodium channel (VGSC)-and 2) enhanced metabolism by detoxification enzymes. When a heterogeneous population of mosquitoes is exposed to pyrethroids, different responses occur. During exposure, a proportion of mosquitoes exhibit immediate knockdown, whereas others are not knocked-down and are designated knockdown resistant (kdr). When these individuals are removed from the source of insecticide, the knocked-down mosquitoes can either remain in this status and lead to dead or recover within a few hours. The proportion of these phenotypic responses is dependent on the pyrethroid concentration and the genetic background of the population tested. In this study, we sequenced and performed pairwise genome comparisons between kdr, recovered, and dead phenotypes in a pyrethroid-resistant colony from Tapachula, Mexico. We identified single-nucleotide polymorphisms (SNPs) associated with each phenotype and identified genes that are likely associated with the mechanisms of pyrethroid resistance, including detoxification, the cuticle, and insecticide target sites. We identified high association between kdr and mutations at VGSC and moderate association with additional insecticide target site, detoxification, and cuticle protein coding genes. Recovery was associated with cuticle proteins, the voltage-dependent calcium channel, and a different group of detoxification genes. We provide a list of detoxification genes under directional selection in this field-resistant population. Their functional roles in pyrethroid metabolism and their potential uses as genomic markers of resistance require validation.

Genomic evidence for gene flow between monarchs with divergent migratory phenotypes and flight performance

Monarch butterflies are known for their spectacular annual migration in eastern North America, with millions of monarchs flying up to 4,500 km to overwintering sites in central Mexico. Monarchs also live west of the Rocky Mountains, where they travel shorter distances to overwinter along the Pacific Coast. It is often assumed that eastern and western monarchs form distinct evolutionary units, but genomic studies to support this notion are lacking. We used a tethered flight mill to show that migratory eastern monarchs have greater flight performance than western monarchs, consistent with their greater migratory distances. However, analysing more than 20 million SNPs in 43 monarch genomes, we found no evidence for genomic differentiation between eastern and western monarchs. Genomic analysis also showed identical and low levels of genetic diversity, and demographic analyses indicated similar effective population sizes and ongoing gene flow between eastern and western monarchs. Gene expression analysis of a subset of candidate genes during active flight revealed differential gene expression related to nonmuscular motor activity. Our results demonstrate that eastern and western monarchs maintain migratory differences despite ongoing gene flow, and suggest that migratory differences between eastern and western monarchs are not driven by select major-effects alleles. Instead, variation in migratory distance and destination may be driven by environmentally induced differential gene expression or by many alleles of small effect.

Zika virus-induced hyper excitation precedes death of mouse primary neuron

BACKGROUND

Zika virus infection in new born is linked to congenital syndromes, especially microcephaly. Studies have shown that these neuropathies are the result of significant death of neuronal progenitor cells in the central nervous system of the embryo, targeted by the virus. Although cell death via apoptosis is well acknowledged, little is known about possible pathogenic cellular mechanisms triggering cell death in neurons.

METHODS

We used in vitro embryonic mouse primary neuron cultures to study possible upstream cellular mechanisms of cell death. Neuronal networks were grown on microelectrode array and electrical activity was recorded at different times post Zika virus infection. In addition to this method, we used confocal microscopy and Q-PCR techniques to observe morphological and molecular changes after infection.

RESULTS

Zika virus infection of mouse primary neurons triggers an early spiking excitation of neuron cultures, followed by dramatic loss of this activity. Using NMDA receptor antagonist, we show that this excitotoxicity mechanism, likely via glutamate, could also contribute to the observed nervous system defects in human embryos and could open new perspective regarding the causes of adult neuropathies.

CONCLUSIONS

This model of excitotoxicity, in the context of neurotropic virus infection, highlights the significance of neuronal activity recording with microelectrode array and possibility of more than one lethal mechanism after Zika virus infection in the nervous system.

Neurotropism and behavioral changes associated with Zika infection in the vector Aedes aegypti

Understanding Zika virus infection dynamics is essential, as its recent emergence revealed possible devastating neuropathologies in humans, thus causing a major threat to public health worldwide. Recent research allowed breakthrough in our understanding of the virus and host pathogenesis; however, little is known on its impact on its main vector, Aedes aegypti. Here we show how Zika virus targets Aedes aegypti’s neurons and induces changes in its behavior. Results are compared to dengue virus, another flavivirus, which triggers a different pattern of behavioral changes. We used microelectrode array technology to record electrical spiking activity of mosquito primary neurons post infections and discovered that only Zika virus causes an increase in spiking activity of the neuronal network. Confocal microscopy also revealed an increase in synapse connections for Zika virus-infected neuronal networks. Interestingly, the results also showed that mosquito responds to infection by overexpressing glutamate regulatory genes while maintaining virus levels. This neuro-excitation, possibly via glutamate, could contribute to the observed behavioral changes in Zika virus-infected Aedes aegypti females. This study reveals the importance of virus-vector interaction in arbovirus neurotropism, in humans and vector. However, it appears that the consequences differ in the two hosts, with neuropathology in human host, while behavioral changes in the mosquito vector that may be advantageous to the virus.

The novel isoxazoline ectoparasiticide fluralaner: selective inhibition of arthropod γ-aminobutyric acid- and L-glutamate-gated chloride channels and insecticidal/acaricidal activity

Isoxazolines are a novel class of parasiticides that are potent inhibitors of γ-aminobutyric acid (GABA)-gated chloride channels (GABACls) and L-glutamate-gated chloride channels (GluCls). In this study, the effects of the isoxazoline drug fluralaner on insect and acarid GABACl (RDL) and GluCl and its parasiticidal potency were investigated. We report the identification and cDNA cloning of Rhipicephalus (R.) microplus RDL and GluCl genes, and their functional expression in Xenopus laevis oocytes. The generation of six clonal HEK293 cell lines expressing Rhipicephalus microplus RDL and GluCl, Ctenocephalides felis RDL-A285 and RDL-S285, as well as Drosophila melanogaster RDLCl-A302 and RDL-S302, combined with the development of a membrane potential fluorescence dye assay allowed the comparison of ion channel inhibition by fluralaner with that of established insecticides addressing RDL and GluCl as targets. In these assays fluralaner was several orders of magnitude more potent than picrotoxinin and dieldrin, and performed 5-236 fold better than fipronil on the arthropod RDLs, while a rat GABACl remained unaffected. Comparative studies showed that R. microplus RDL is 52-fold more sensitive than R. microplus GluCl to fluralaner inhibition, confirming that the GABA-gated chloride channel is the primary target of this new parasiticide. In agreement with the superior RDL on-target activity, fluralaner outperformed dieldrin and fipronil in insecticidal screens on cat fleas (Ctenocephalides felis), yellow fever mosquito larvae (Aedes aegypti) and sheep blowfly larvae (Lucilia cuprina), as well as in acaricidal screens on cattle tick (R. microplus) adult females, brown dog tick (Rhipicephalus sanguineus) adult females and Ornithodoros moubata nymphs. These findings highlight the potential of fluralaner as a novel ectoparasiticide.

The actions of chloride channel blockers, barbiturates and a benzodiazepine on Caenorhabditis elegans glutamate- and ivermectin-gated chloride channel subunits expressed in Xenopus oocytes

The pharmacology of Caenorhabditis elegans glutamate-gated chloride (GluCl) channels was determined by making intracellular voltage-clamp recordings from Xenopus oocytes expressing GluCl subunits. As previously reported (Cully et al. 1994), GluClalpha1beta responded to glutamate (in a picrotoxin sensitive manner) and ivermectin, while GluClbeta responded only to glutamate and GluClalpha1 only to ivermectin. This assay was used to further investigate the action of chloride channel compounds. The arylaminobenzoate, NPPB, reduced the action of glutamate on the heteromeric GluClalpha1beta channel (IC(50) 6.03 +/- 0.81 microM). The disulphonate stilbene, DNDS, blocked the effect of both glutamate and ivermectin on GluClalpha1beta channels, the action of glutamate on GluClbeta subunits, and the effect of ivermectin on GluClalpha1 subunits (IC(50)s 1.58-3.83 microM). Surprisingly, amobarbital and pentobarbital, otherwise known as positive allosteric modulators of ligand-gated chloride channels, acted as antagonists. Both compounds reduced the action of glutamate on the GluClalpha1beta heteromer (IC(50)s of 2.04 +/- 0.5 and 17.56 +/- 2.16 microM, respectively). Pentobarbital reduced the action of glutamate on the GluClbeta homomeric subunit with an IC(50) of 0.59 +/- 0.09 microM, while reducing the responses to ivermectin on both GluClalpha1beta and GluClalpha1 with IC(50)s of 8.7 +/- 0.5 and 12.9 +/- 2.5 microM, respectively. For all the antagonists, the mechanism is apparently non-competitive. The benzodiazepine, flurazepam had no apparent effect on these glutamate- and ivermectin-gated chloride channel subunits. Thus, arylaminobenzoates, disulphonate stilbenes, and barbiturates are non-competitive antagonists of C. elegans GluCl channels.

Maintenance of GABA receptor function of small-diameter cockroach neurons by adenine nucleotides

Small diameter (<20 microm) neurons from the sixth abdominal ganglion of the American cockroach, Periplaneta americana, were enzymatically isolated and responses to exogenously applied gamma-aminobutyric acid (GABA) were recorded using the whole-cell patch clamp technique. With a minimal intracellular medium, responses to repeated applications of GABA decreased to zero within a few minutes. The rate of rundown of GABA responses was decreased by the intracellular inclusion of the phosphatase inhibitors microcystin and okadaic acid, suggesting that phosphorylation is necessary for the maintenance of cockroach GABA receptor function. ATP (5 mM) prevented GABA response rundown. ADP (5 mM) also slowed GABA response rundown, but responses stabilized at a level about half that seen with ATP. In the presence of protein kinase A inhibitory peptide (PKI), ATP was only as efficacious as ADP in slowing rundown. PKI had no effect on the ability of ADP to slow rundown, suggesting that the beta-phosphate of ADP is not involved in PKA-dependent phosphorylation of the GABA receptor. These results suggest that in cockroach neurons, GABA receptor function is maintained intracellularly by adenine nucleotides, not only by phosphorylation, but also possibly by an interaction with a nucleotide recognition site unrelated to PKA-dependent phosphorylation.

G-protein-coupled receptors in insect cells

The main classes of transmembrane signaling receptor proteins are well conserved during evolution and are encountered in vertebrates as well as in invertebrates. All members of the G-protein-coupled receptor superfamily share a number of basic structural and functional characteristics. In both insects and mammals, this receptor class is involved in the perception and transduction of many important extracellular signals, including a great deal of paracrine, endocrine, and neuronal messengers and visual, olfactory and gustatory stimuli. Therefore, most of the receptor subclasses appear to have originated several hundred million years ago, before the divergence of the major animal Phyla took place. Nevertheless, many insect-specific molecular interactions are encountered and these could become interesting tools for future applications, e.g., in insect pest control. Insect cell lines are well suited for large-scale expression and characterization of cloned receptor genes. Furthermore, novel methods for the production of stably transformed insect cells may form a major breakthrough for insect signal transduction research.

The molecular and population genetics of cyclodiene insecticide resistance

Cyclodiene resistance has accounted for over 60% of reported cases of insecticide resistance. Understanding of this resistance can therefore help us answer questions relating to the mechanism and origin of representative resistance-associated mutations, questions fundamental to the molecular and populations genetics of pesticide resistance. The cyclodiene resistance gene Rdl (resistance to dieldrin) was cloned from a mutant of the model insect Drosophila resistant to cyclodienes and picrotoxinin. Rdl codes for a subunit of a novel class of GABA gated chloride ion channels and resistance is correlated with replacement of the same amino acid residue in a wide range of species from different insect orders. This single amino acid replacement Ala302 > Ser, within the proposed lining of the chloride ion channel, also confers insensitivity to the blocking action of cyclodienes and picrotoxinin on GABA gated chloride ion channels expressed in Xenopus oocytes. The resistance mechanism involves both changes in cyclodiene binding site affinity and also a change in the rate of receptor desensitization which destabilizes the cyclodiene-favored conformation. Documentation of the resistance associated mutation has allowed for the design of a PCR based molecular monitoring technique. This technique gives more accurate estimates of resistance gene frequency from smaller sample sizes and has shown the frequency of resistance in apparently unselected populations of Drosophila to be as high as 1%. We are still uncertain as to why resistance persists in the apparent absence of selection pressure and any severe reduction in the fitness of resistant strains, besides a paralytic phenotype at high temperature, remains undocumented.

A neural gene from Drosophila melanogaster with homology to vertebrate and invertebrate glutamate decarboxylases

Cross-species hybridization has been used to isolate a second Drosophila gene, with homology to a feline glutamate decarboxylase (Gad) cDNA. The gene differs in sequence, chromosomal location, and spatial expression from the previously reported Drosophila Gad gene, but both encode proteins of 58 kDa. The derived amino acid sequence reveals a typical pyridoxal phosphate binding site and sequence homology consistent with a glutamate decarboxylase function. The protein includes an amino-terminal polyasparagine sequence, and a beta-pleated sheet region, with regularly spaced glutamine and arginine residues, not found in other decarboxylases. Expression in the adult is limited to the neuropil of the first optic ganglion and to regions of the thoracic musculature that may correspond to the location of motor neuron axons. This is consistent with a glial localization for the transcript. There is no overlap with the reported expression of Drosophila Gad. Although the molecular evidence suggests that this gene encodes a pyridoxal phosphate-dependent decarboxylase, glutamate decarboxylase activity associated with this gene could not be demonstrated, and the in vivo substrate is unknown. It is possible that the protein encoded by this gene is novel, not only in sequence and spatial expression, but also in substrate specificity.