A Drosophila melanogaster cell line (S2) facilitates post-genome functional analysis of receptors and ion channels

Triangulation of methods using insect cell lines to investigate insecticidal mode-of-action

BACKGROUND

This study investigated three in vitro models to assist in elucidating possible mode-of-action, which could be adopted to evaluate insecticidal activity of complex, unknown, or multi-constituent formulations. We used a combination of absorbance spectrometry, confocal scanning laser microscopy and microelectrode ion flux estimation (MIFE) to provide insight into potential target sites for insecticides. This study used two insect cell lines and evaluated three pyrethroid insecticides.

RESULTS

We observed that the two cell lines produced distinctly different responses. Drosophila melanogaster D.mel-S2 cell line was a useful model to monitor ion flux changes, resulting from insecticides with neural toxicity; however, it was less useful to determine some metabolic pathway indicators of toxic stress. Conversely, the Spodoptera frugiperda Sf9 cell line produced acute reactive oxygen species (ROS) in response to insecticide treatments, but was not highly responsive in electrophysiological experiments. We also showed that the natural, multi-constituent botanical extract of pyrethrum elicited different Na⁺ , Cl^- and Ca²⁺ ion fluxes than its synthetic, single constituent analogues, α-cypermethrin and esfenvalerate. These two methods used in combination with absorbance spectrometry measuring cell growth inhibition plus cell mortality assays shed some light on cytotoxic responses in differing model cell lines.

CONCLUSION

This research highlights the importance of using multiple cell types and interdisciplinary methods to provide a better insight into mode of insecticidal action. This is especially pertinent to novel biopesticide discovery, as the underlying mechanisms for toxicity in initial screening processes are likely to be unknown.

20-Hydroxyecdysone-responsive microRNAs of insects

20-Hydroxyecdysone (20-HE) plays essential roles in coordinating developmental transitions of insects through responsive protein-coding genes and microRNAs (miRNAs). The involvement of single miRNAs in the ecdysone-signalling pathways has been extensively explored, but the interplay between ecdysone and the majority of miRNAs still remains largely unknown. Here, by small RNA sequencing, we systematically investigated the genome-wide responses of miRNAs to 20-HE in the embryogenic cell lines of Bombyx mori and Drosophila melanogaster. Over 60 and 70 20-HE-responsive miRNAs were identified in the BmE cell line and S2 cell line, respectively. The response of miRNAs to ecdysone exhibited a time-dependent pattern, and the response intensity increased with extending exposure to 20-HE. The relationship between ecdysone and the miRNAs was further explored through knockdown of ecdysone-signalling pathway genes. Specifically, ecdysone regulated the cluster miR-275 and miR-305 through the coordination of BmEcR-B and downstream BmE75B, and the interaction between BmEcR and miR-275 cluster was strengthened by the feedback regulation of BmE75B. Ecdysone induced miR-275-3p and miR-305-5p through the ecdysone response effectors (EcREs) at the upstream of the pre-miR-275 cluster. Overall, the results might help us further understand the relationship between ecdysone signalling pathways and small RNAs in the development and metamorphosis of insects.

Time-dependent transcriptomic responses of Daphnia magna exposed to metabolic disruptors that enhanced storage lipid accumulation

The analysis of lipid disruption in invertebrates is limited by our poor knowledge of their lipidomes and of the associated metabolic pathways. For example, the mechanism by which exposure of the crustacean Daphnia magna to tributyltin, juvenoids, or bisphenol A increase the accumulation of storage lipids into lipid droplets is largely unknown/presently unclear. Here we analyze transcriptome changes subsequent to this lipid accumulation effect induced by either the pesticide pyriproxyfen (a juvenoid agonist), the plasticizer bisphenol A, or the antifouling agent tributyltin. Changes in the whole transcriptome were assessed after 8 and 24 h of exposure, the period showing the greatest variation in storage lipid accumulation. The three compounds affected similarly to a total of 1388 genes (965 overexpressed and 423 underexpressed transcripts), but only after 24 h of exposure. In addition, 225 transcripts became up-regulated in samples exposed to tributyltin for both 8 h and 24 h. Using D. melanogaster functional annotation, we determined that upregulated genes were enriched in members of KEGG modules implicated in fatty acid, glycerophospholipid, and glycerolipid metabolic pathways, as well as in genes related to membrane constituents and to chitin and cuticle metabolic pathways. Conversely, down-regulated genes appeared mainly related to visual perception and to oocyte development signaling pathways. Many tributyltin specifically upregulated genes were related to neuro-active ligand receptor interaction signaling pathways. These changes were consistent with the phetotypic effects reported in this and in previous studies that exposure of D. magna to the tested compounds increased lipid accumulation and reduced egg quantity and quality.

G protein-coupled receptors in arthropod vectors: omics and pharmacological approaches to elucidate ligand-receptor interactions and novel organismal functions

Regulation of many physiological processes in animals, certainly those controlled by neuropeptide hormones, involves G protein-coupled receptors (GPCRs). Our work focusing on endocrine regulation of diuresis and water balance in mosquitoes and ticks started in 1997 with the kinin receptor, at the dawn of the omics era. After the genomic revolution, we began work on the endocrinology of reproduction in the red imported fire ant. We will use the template of this comparative work to summarize key points about GPCRs and signaling, and emphasize the most recent developments in the pharmacology of arthropod neuropeptide GPCRs. We will discuss omics' contributions to the advancement of this field, and its influence on peptidomimetic design while emphasizing work on blood feeding arthropods.

An Ixodes scapularis cell line with a predominantly neuron-like phenotype

The Ixodes scapularis embryo-derived cell line ISE6 is the most widely utilized tick-derived cell line due to its susceptibility to a wide variety of tick- and non-tick-vectored pathogens. Little is known about its tissue origin or biological background. Protein expression of ISE6 cells was compared with that of another I. scapularis-derived cell line, IDE12, and dissected tick synganglia. Results demonstrated the presence of a neuronal marker protein, type 3 β-tubulin, in all three samples, as well as other shared and unique neuronal and immune response-associated proteins. Of neuronal proteins shared between the two cell lines, ISE6 expressed several in significantly greater quantities than IDE12. Stimulation of ISE6 cells by in vivo exposure to the hemocoel environment in unfed larval and molting nymphal ticks, but not unfed nymphal ticks, resulted in the development of neuron-like morphologic characteristics in the implanted cells.

Induction and inhibition of an apparent neuronal phenotype in Spodoptera frugiperda insect cells (Sf21) by chemical agents

The goal of this research was to induce neuron-like properties in Sf21 cells, an insect ovarian cell line, which could lead to a new high-throughput insecticide screening method and a way to mass produce insect neuronal material for basic research. This study applied differentiation agents to produce viable neuron-like cells. In the presence of the molting hormone 20-hydroxyecdysone (20-HE), or insulin, in the growth medium, a maximum of ca. 30 % of Sf21 cells expressed an apparent neuronal morphology of unipolar, bipolar, or multipolar axon-like processes within 2-3 days. Maximal differentiation occurred after 2 days in the presence of 50 μM 20-HE or 3 days in 10 μM insulin. Both 20-HE and insulin displayed time- and concentration-dependent differentiation with biphasic curves, suggesting that two binding sites or processes were contributing to the observed effects. In addition, combinations of 20-HE and insulin produced apparent synergistic effects on differentiation. Caffeine, a central nervous system stimulant, inhibited induction of elongated processes by 20-HE and/or insulin, with an IC(50) of 9 nM for 20-HE, and the inhibition was incomplete, resulting in about one-quarter of the differentiated cells remaining, even at high concentrations (up to 1 mM). The ability to induce a neural phenotype simplifies the studies of insect cells, compared to either the use of primary nervous tissue or genetic engineering techniques. The presence of ion channels or receptors in the differentiated cells remains to be determined.

Genomic and functional studies of Drosophila sex hierarchy regulated gene expression in adult head and nervous system tissues

The Drosophila sex determination hierarchy controls all aspects of somatic sexual differentiation, including sex-specific differences in adult morphology and behavior. To gain insight into the molecular-genetic specification of reproductive behaviors and physiology, we identified genes expressed in the adult head and central nervous system that are regulated downstream of sex-specific transcription factors encoded by doublesex (dsx) and fruitless (fru). We used a microarray approach and identified 54 genes regulated downstream of dsx. Furthermore, based on these expression studies we identified new modes of DSX-regulated gene expression. We also identified 90 and 26 genes regulated in the adult head and central nervous system tissues, respectively, downstream of the sex-specific transcription factors encoded by fru. In addition, we present molecular-genetic analyses of two genes identified in our studies, calphotin (cpn) and defective proboscis extension response (dpr), and begin to describe their functional roles in male behaviors. We show that dpr and dpr-expressing cells are required for the proper timing of male courtship behaviors.

The fruit fly Drosophila is an excellent model system to use to understand the molecular-genetic basis of male courtship behavior, as the potential for this behavior is specified by a well-understood genetic regulatory hierarchy, called the sex determination hierarchy. The sex hierarchy consists of a pre-mRNA splicing cascade that culminates in the production of sex-specific transcription factors, encoded by doublesex (dsx) and fruitless (fru). dsx specifies all the anatomical differences between the sexes, and fru is required for all aspects of male courtship behavior. In this study, we measure gene expression differences between males and females, and between sex hierarchy mutants and wild-type animals, to identify genes that underlie the differences between males and females. We have performed these studies on adult head and nervous system tissues, as these tissues are important for establishing the potential for behaviors. We have identified several genes regulated downstream of dsx and fru and more extensively characterized two genes that are more highly expressed in males. One gene regulated downstream of dsx is expressed in the retina and is known to have a function in visual transduction. The other gene, regulated downstream of fru, plays a role in the timing of male courtship behavior.

Genetic approaches to studying adenosine-to-inosine RNA editing

Increasing proteomic diversity via the hydrolytic deamination of adenosine to inosine (A-to-I) in select mRNA templates appears crucial to the correct functioning of the nervous system in several model organisms, including Drosophila, Caenorabditis elegans, and mice. The genome of the fruitfly, Drosophila melanogaster, contains a single gene encoding the enzyme responsible for deamination, termed ADAR (for adenosine deaminase acting on RNA). The mRNAs that form the substrates for ADAR primarily function in neuronal signaling, and, correspondingly, deletion of ADAR leads to severe nervous system defects. While several ADAR enzymes are present in mice, the presence of a single ADAR in Drosophila, combined with the diverse genetic toolkit available to researchers and the wide range of ADAR target mRNAs identified to date, make Drosophila an ideal organism to study the genetic basis of A-to-I RNA editing. This chapter describes a variety of methods for genetically manipulating Drosophila A-to-I editing both in time and space, as well as techniques to study the molecular basis of ADAR-mRNA interactions. A prerequisite for experiments in this field is the ability to quantify the levels of editing in a given mRNA. Therefore, several commonly used methods for the quantification of editing levels will also be described.

Molecular basis of the CRAC channel

Ca(2+) release-activated Ca(2+) (CRAC) channels, located in the plasma membrane, are opened upon release of Ca(2+) from intracellular stores, permitting Ca(2+) entry and sustained [Ca(2+)](i) signaling that replenishes the store in numerous cell types. This mechanism is particularly important in T lymphocytes of the immune system, providing the missing link in the signal transduction cascade that is initiated by T cell receptor engagement and leads to altered expression of genes that results ultimately in the production of cytokines and cell proliferation. In the past three years, RNA interference screens together with over-expression and site-directed mutagenesis have identified the triggering molecule (Stim) that links store depletion to CRAC channel-mediated Ca(2+) influx and the pore subunit (Orai) of the CRAC channel that allows highly selective entry of Ca(2+) ions into cells.

Transformation of Drosophila cell lines: an alternative approach to exogenous protein expression

Techniques and experimental applications are described for exogenous protein expression in Drosophila cell lines. Ways in which the Drosophila cell lines and the baculovirus expression vector system differ in their applications are emphasized.

NovelDrosophilatwo-pore domain K+channels: rescue of channel function by heteromeric assembly

Ten genes with essential structural features of two-pore domain potassium channels were identified in the genome of Drosophila melanogaster. Two Drosophila two-pore domain potassium subunits displayed substantial amino acid similarity to human TWIK-related acid-sensitive K(+) (TASK) channels (38-43%), whereas all others were less than 26% similar to any human homolog. The cDNAs of Drosophila TASK (dTASK)-6 and dTASK-7 channels were isolated from adult fruit flies. In Northern blots dTASK transcripts were found predominantly in the head fraction of adult flies and whole-mount brain in situ hybridizations showed strongly overlapping expression patterns of both dTASK isoforms in the antennal lobes. When heterologously expressed in Drosophila Schneider 2 cells, dTASK-6 gave rise to rapidly activating K(+)-selective currents that steeply depended on external pH. Structural elements in the extracellular M1-P1 loop of dTASK-6 were found to be involved in proton sensation. In contrast to mammalian TASK channels, the pH sensitivity was independent of extracellular histidines adjacent to the GYG selectivity filter (His98). As revealed by mutational analysis, functional expression of dTASK-7 was prevented by two nonconserved amino acids (Ala92-Met93) in the pore domain. When these two residues were replaced by conserved Thr92-Thr93, typical K(+)-selective leak currents were generated that were insensitive to changes in external pH. Nonfunctional wildtype dTASK-7 channels appeared to form heteromeric assemblies with dTASK-6. Following cotransfection of dTASK-6 and wildtype dTASK-7 (or when engineered as concatemers), K(+) currents were observed that were smaller in amplitude, harbored slower activation kinetics and were considerably less inhibited by local anesthetics as compared with dTASK-6. Thus, pore-loop residues in dTASK-7 changed functional and pharmacological properties in heteromeric dTASK channels.

Functional genomics: applying calcium imaging and RNA interference to Drosophila cell lines to identify new roles for gene products

The selective knockdown of genes using RNA interference, and the analysis of the resultant phenotype using calcium imaging, is proving to be a powerful combination for discovering new genes involved in calcium signalling. This technical note describes the application of this approach to the Drosophila S2 cell line and its derivatives.

Ion channels: molecular targets of neuroactive insecticides

Many of the insecticides in current use act on molecular targets in the insect nervous system. Recently, our understanding of these targets has improved as a result of the complete sequencing of an insect genome, i.e., Drosophila melanogaster. Here we examine the recent work, drawing on genetics, genomics and physiology, which has provided evidence that specific receptors and ion channels are targeted by distinct chemical classes of insect control agents. The examples discussed include, sodium channels (pyrethroids, p,p'-dichlorodiphenyl-trichloroethane (DDT), dihydropyrazoles and oxadiazines); nicotinic acetylcholine receptors (cartap, spinosad, imidacloprid and related nitromethylenes/nitroguanidines); gamma-aminobutyric acid (GABA) receptors (cyclodienes, gamma-BHC and fipronil) and L-glutamate receptors (avermectins). Finally, we have examined the molecular basis of resistance to these molecules, which in some cases involves mutations in the molecular target, and we also consider the future impact of molecular genetic technologies in our understanding of the actions of neuroactive insecticides.

STIM1, an essential and conserved component of store-operated Ca2+ channel function

Store-operated Ca2+ (SOC) channels regulate many cellular processes, but the underlying molecular components are not well defined. Using an RNA interference (RNAi)-based screen to identify genes that alter thapsigargin (TG)-dependent Ca2+ entry, we discovered a required and conserved role of Stim in SOC influx. RNAi-mediated knockdown of Stim in Drosophila S2 cells significantly reduced TG-dependent Ca2+ entry. Patch-clamp recording revealed nearly complete suppression of the Drosophila Ca2+ release-activated Ca2+ (CRAC) current that has biophysical characteristics similar to CRAC current in human T cells. Similarly, knockdown of the human homologue STIM1 significantly reduced CRAC channel activity in Jurkat T cells. RNAi-mediated knockdown of STIM1 inhibited TG- or agonist-dependent Ca2+ entry in HEK293 or SH-SY5Y cells. Conversely, overexpression of STIM1 in HEK293 cells modestly enhanced TG-induced Ca2+ entry. We propose that STIM1, a ubiquitously expressed protein that is conserved from Drosophila to mammalian cells, plays an essential role in SOC influx and may be a common component of SOC and CRAC channels.

A store-operated calcium channel in Drosophila S2 cells

Using whole-cell recording in Drosophila S2 cells, we characterized a Ca(2+)-selective current that is activated by depletion of intracellular Ca2+ stores. Passive store depletion with a Ca(2+)-free pipette solution containing 12 mM BAPTA activated an inwardly rectifying Ca2+ current with a reversal potential >60 mV. Inward currents developed with a delay and reached a maximum of 20-50 pA at -110 mV. This current doubled in amplitude upon increasing external Ca2+ from 2 to 20 mM and was not affected by substitution of choline for Na+. A pipette solution containing approximately 300 nM free Ca2+ and 10 mM EGTA prevented spontaneous activation, but Ca2+ current activated promptly upon application of ionomycin or thapsigargin, or during dialysis with IP3. Isotonic substitution of 20 mM Ca2+ by test divalent cations revealed a selectivity sequence of Ba2+ > Sr2+ > Ca2+ >> Mg2+. Ba2+ and Sr2+ currents inactivated within seconds of exposure to zero-Ca2+ solution at a holding potential of 10 mV. Inactivation of Ba2+ and Sr2+ currents showed recovery during strong hyperpolarizing pulses. Noise analysis provided an estimate of unitary conductance values in 20 mM Ca2+ and Ba2+ of 36 and 420 fS, respectively. Upon removal of all external divalent ions, a transient monovalent current exhibited strong selectivity for Na+ over Cs+. The Ca2+ current was completely and reversibly blocked by Gd3+, with an IC50 value of approximately 50 nM, and was also blocked by 20 microM SKF 96365 and by 20 microM 2-APB. At concentrations between 5 and 14 microM, application of 2-APB increased the magnitude of Ca2+ currents. We conclude that S2 cells express store-operated Ca2+ channels with many of the same biophysical characteristics as CRAC channels in mammalian cells.

Gene silencing of selected calcium-signalling molecules in a Drosophila cell line using double-stranded RNA interference

Using the Drosophila melanogaster S2 cell line, stably expressing a cloned muscarinic acetylcholine receptor (AChR), DM1, we have applied gene silencing by double-stranded RNA interference (RNAi) to knock down gene products involved in DM1-mediated calcium signalling. We have shown that RNAi knock down of either the inositol 1,4,5-trisphosphate receptor (Ins(1,4,5)P(3)R), or the SERCA calcium pump in the S2-DM1 cells blocks the increase in intracellular calcium concentration ([Ca(2+)](i)) resulting from activation of the DM1 receptor by 100 microM carbamylcholine (CCh). When RNAi designed to knock down the ryanodine receptor (RyR) was tested, there was no change in the calcium increase detected in response to CCh, consistent with a failure to detect RyRs in S2-DM1 cells using RT-PCR. A combination of RNAi and calcium imaging has provided a direct demonstration of key roles for the Ins(1,4,5)P(3)R and the SERCA pump in the response to DM1 receptor activation.Thus, we show that silencing of individual genes by RNAi in a well characterised Drosophila S2 cell line offers experimental opportunities for cell-signalling studies. Future investigations with RNAi libraries taking full advantage of the wealth of new information available from sequencing the Drosophila genome, may help identify novel components of cell-signalling pathways and functionally linked gene products.

Functional and comparative analysis of two distinct ecdysteroid-responsive gene expression constructs in Drosophila S2 cells

Inducible expression systems have proven to be of major interest when analysing the function of specific genes or when expressing cytotoxic proteins. In an effort to develop inducible switches allowing for flexible fine-tuning of gene expression levels in insect cells, we have compared the induction capacities of two Drosophila minimal promoters when linked to four consecutive ecdysone response elements. These minimal promoters, either containing a TATA-box or a downstream promoter element, drove the expression of a luciferase reporter gene. Potent induction capacities were observed with the insect moulting hormone, 20-hydroxyecdysone, and with ponasterone A, a plant ecdysteroid. The developed inducible switches further expand the repertoire of molecular tools for functional expression of proteins of interest in insect cells. In addition, the combination of an ecdysone switch with promoters that possess different structural elements can provide novel insights into ecdysteroid-induced transcription in an insect cell line.

Spatiotemporal calcium signaling in a Drosophila melanogaster cell line stably expressing a Drosophila muscarinic acetylcholine receptor

A muscarinic acetylcholine receptor (mAChR), DM1, expressed in the nervous system of Drosophila melanogaster, has been stably expressed in a Drosophila S2 cell line (S2-DM1) and used to investigate spatiotemporal calcium changes following agonist activation. Carbamylcholine (CCh) and oxotremorine are potent agonists, whereas application of the vertebrate M1 mAChR agonist, McN-A-343, results in a weak response. Activation of S2-DM1 receptors using CCh resulted in an increase in intracellular calcium ([Ca(2+)](i)) that was biphasic. Two distinct calcium sources were found to contribute to calcium signaling: (1) internal stores that are sensitive to both thapsigargin and 2-aminoethoxydiphenyl borate and (2) capacitative calcium entry. Spatiotemporal imaging of individual S2-DM1 cells showed that the CCh-induced [Ca(2+)](i) transient resulted from a homogeneous calcium increase throughout the cell, indicative of calcium release from internal stores. In contrast, ionomycin induced the formation of a "calcium ring" at the cell periphery, consistent with external calcium influx.

Dimethyl disulfide exerts insecticidal neurotoxicity through mitochondrial dysfunction and activation of insect K(ATP) channels

The plant-derived insecticides have introduced a new concept in insecticide research. In response to insect attacks, some plants can release volatile sulfur compounds such as dimethyl disulfide (DMDS) in the atmosphere, which are lethal for the generalist insects. We demonstrate that DMDS induced an uncommon complex neurotoxic activity. The studies of in vivo toxicity of DMDS in three insect species and mice indicated a highest bioactivity for insects. Although DMDS did not alter the electrophysiological properties of the cockroach Periplaneta americana giant axon, it affected the synaptic transmission at the presynaptic level resulting in an inhibition of the neurotransmitter release. Whole cell patch-clamp experiments performed on cockroach cultured dorsal unpaired median (DUM) neurons revealed a dose-dependent hyperpolarization induced by DMDS associated with a decrease in the input resistance and the disappearance of action potentials. The hyperpolarization was inhibited by glibenclamide and tolbutamide, and was dependent on intracellular ATP concentration, demonstrating a neurotoxicity via the activation of KATP channels. Finally, the same effects observed with oligomycin, 2,4-dinitrophenol, and KCN together with the studies of DMDS toxicity on isolated mitochondria confirmed an unusual action occurring through an inhibition of the mitochondrial respiratory chain complex IV (cytochrome oxydase). This DMDS-induced inhibition of complex IV subsequently decreased the intracellular ATP concentration, which thereby activated neuronal KATP channels mediating membrane hyperpolarization and reduction of neuronal activity.