Physiological and molecular characterization of methotrexate transport by Malpighian tubules of adult Drosophila melanogaster

Hsp27 over expression protect against cadmium induced nephrotoxicity in Drosophila melanogaster

Cadmium (Cd) exposure to the animals including humans is reported as nephrotoxic compounds i.e., disturbing redox status (increase oxidative stress), mitochondrial dysfunction, renal cell death and altered transporters in the renal system. Hsp27 (a small heat shock protein) has been shown as one of the modulators in the renal dysfunction and increased against the Cd induced toxicity. However, no studies are reported on the genetic modulation of stress protein against the Cd-induced nephrotoxicity. The current study aimed to examine the protective role of hsp27 overexpression against the Cd-induced nephrotoxicity using Drosophila melanogaster as an animal model. D. melanogaster renal system includes nephrocytes and Malpighian tubules (MTs) that show the functional similarity with mammalian kidney nephron. Overexpression of the hsp27 was found to reduce the Cd induced oxidative stress, rescue cell death in MTs of Cd exposed D. melanogaster larvae. The rescued GSH level, NADPH level and glucose 6 phosphate dehydrogenase (G6PD) activity were also observed in the MTs of the Cd exposed organism. Function (efflux activity and fluid secretion rate) of the MTs was restored in Cd exposed hsp27 overexpressed larvae. Further, results were confirmed by restored brush border microvilli density and reduced uric acid level. Tissue specific knockdown of hsp27 developed Cd like phenotypes in MTs and the phenotypes enhanced in Cd exposed condition. The present study clearly shows the role of hsp27 overexpression in restoration of the MTs function and protection against the Cd induced renal toxicity.

A single-cell transcriptomic atlas of complete insect nervous systems across multiple life stages

Molecular profiles of neurons influence neural development and function but bridging the gap between genes, circuits, and behavior has been very difficult. Here we used single cell RNAseq to generate a complete gene expression atlas of the Drosophila larval central nervous system composed of 131,077 single cells across three developmental stages (1 h, 24 h and 48 h after hatching). We identify 67 distinct cell clusters based on the patterns of gene expression. These include 31 functional mature larval neuron clusters, 1 ring gland cluster, 8 glial clusters, 6 neural precursor clusters, and 13 developing immature adult neuron clusters. Some clusters are present across all stages of larval development, while others are stage specific (such as developing adult neurons). We identify genes that are differentially expressed in each cluster, as well as genes that are differentially expressed at distinct stages of larval life. These differentially expressed genes provide promising candidates for regulating the function of specific neuronal and glial types in the larval nervous system, or the specification and differentiation of adult neurons. The cell transcriptome Atlas of the Drosophila larval nervous system is a valuable resource for developmental biology and systems neuroscience and provides a basis for elucidating how genes regulate neural development and function.

Characterization of a novel pesticide transporter and P-glycoprotein orthologues in Drosophila melanogaster

Pesticides remain one of the most effective ways of controlling agricultural and public health insects, but much is still unknown regarding how these compounds reach their targets. Specifically, the role of ABC transporters in pesticide absorption and excretion is poorly understood, especially compared to the detailed knowledge about mammalian systems. Here, we present a comprehensive characterization of pesticide transporters in the model insect Drosophila melanogaster. An RNAi screen was performed, which knocked down individual ABCs in specific epithelial tissues and examined the subsequent changes in sensitivity to the pesticides spinosad and fipronil. This implicated a novel ABC drug transporter, CG4562, in spinosad transport, but also highlighted the P-glycoprotein orthologue Mdr65 as the most impactful ABC in terms of chemoprotection. Further characterization of the P-glycoprotein family was performed via transgenic overexpression and immunolocalization, finding that Mdr49 and Mdr50 play enigmatic roles in pesticide toxicology perhaps determined by their different subcellular localizations within the midgut. Lastly, transgenic Drosophila lines expressing P-glycoprotein from the major malaria vector Anopheles gambiae were used to establish a system for in vivo characterization of this transporter in non-model insects. This study provides the basis for establishing Drosophila as a model for toxicology research on drug transporters.

The transcriptome of anal papillae of Aedes aegypti reveals their importance in xenobiotic detoxification and adds significant knowledge on ion, water and ammonia transport mechanisms

The anal papillae of mosquito larvae are osmoregulatory organs in direct contact with the external aquatic environment that actively sequester ions and take up water in dilute freshwater. In the disease vector Aedes aegypti mechanisms of ion, water and ammonia transport have only been partially resolved. Furthermore, A. aegypti larvae are known to reside in high ammonia sewage and high salt brackish waters, and understanding of anal papillae function in these conditions is in its infancy. The objective of this study was to identify the complement of ion and water transport genes expressed by the anal papillae of freshwater larvae by sequencing their transcriptome, and comparing their expression in anal papillae of larvae abruptly transferred to brackish water for 24 h. Results identified a number of ion and water transport proteins, ammonia detoxifying enzymes, a full suite of xenobiotic detoxifying enzymes and transporters, and G-protein coupled receptors of specific hormones. We identified a marked increase in transcript and protein abundance of aquaporin AaAQP2 in the anal papillae with abrupt transfer to brackish water. We present an updated and more comprehensive model for ion and water transport with additional putative transporters for Na⁺ and Cl^- uptake in the anal papillae. These are organs which are actively engaged in Na⁺, Cl^- and water uptake and regulation when the aquatic larvae encounter fluctuating salinities over the course of their development. Furthermore the transcriptome of the anal papillae includes a full set of xenobiotic detoxification genes suggesting that these are important detoxification organs which is particularly important when larvae reside in polluted water.

Toward a Consensus in the Repertoire of Hemocytes Identified in Drosophila

The catalog of the Drosophila immune cells was until recently limited to three major cell types, based on morphology, function and few molecular markers. Three recent single cell studies highlight the presence of several subgroups, revealing a large diversity in the molecular signature of the larval immune cells. Since these studies rely on somewhat different experimental and analytical approaches, we here compare the datasets and identify eight common, robust subgroups associated to distinct functions such as proliferation, immune response, phagocytosis or secretion. Similar comparative analyses with datasets from different stages and tissues disclose the presence of larval immune cells resembling embryonic hemocyte progenitors and the expression of specific properties in larval immune cells associated with peripheral tissues.

Development of a Drosophila melanogaster based model for the assessment of cadmium and mercury mediated renal tubular toxicity

Xenobiotic mediated renal toxicity is one of the major health concerns to the organisms, including humans. New chemicals with nephrotoxic potential are continuously being added to the list of existing nephrotoxicants. To predict the nephrotoxicity of these new chemicals, reliable and cost-effective alternative animal models are required. It is a prerequisite for the identification and assessment of these compounds as potential nephrotoxicants to prevent renal toxicity in the exposed population. Drosophila melanogaster, a genetically tractable invertebrate animal model, has a renal system functionally analogous to humans. The Malpighian tubules (MTs) of D. melanogaster are similar to the tubular part of nephron of the human kidney. Besides, it recapitulates the renal toxicity hallmark with mammals when exposed to known nephrotoxicants. In this study, first instar larvae of D. melanogaster (Oregon R) were exposed to different concentrations of two well-known nephrotoxicants, cadmium (Cd) and mercury (Hg). Akin to higher organisms, Cd and Hg exposure to D. melanogaster produce similar phenotypes. MTs of exposed D. melanogaster larvae exhibited increased oxidative stress, activated cellular antioxidant defense mechanism, GSH depletion, increased cleaved caspase-3 expression, increased DEVDase activity and increased cell death. The functional status of MTs was assessed by fluid secretion rate (FSR), efflux activity of transporter protein, mitochondrial membrane potential (MMP), ATP level and expression of junctional protein (Dlg). All the phenotypes observed in MTs of D. melanogaster larvae recapitulate the phenotypes observed in higher organisms. Increased uric acid level, the hallmark of renal dysfunction, was also observed in exposed larvae. Taken together, the study suggests that MTs of D. melanogaster may be used as a functional model to evaluate xenobiotic mediated nephrotoxicity.

The Drosophila Malpighian tubule as a model for mammalian tubule function

PURPOSE OF REVIEW

Studies of the genetic model organism, Drosophila melanogaster, have unraveled molecular pathways relevant to human physiology and disease. The Malpighian tubule, the Drosophila renal epithelium, is described here, including tools available to study transport; conserved transporters, channels, and the signaling pathways regulating them; and fly models of kidney stone disease.

RECENT FINDINGS

Tools to measure Malpighian tubule transport continue to advance, including use of a transgenic sensor to quantify intracellular pH and proton fluxes. A recent study generated an RNA-sequencing-based atlas of tissue-specific gene expression, with resulting insights into Malpighian tubule gene expression of transporters and channels. Advances have been made in understanding the molecular physiology of the With No Lysine kinase-Ste20-related proline/alanine rich kinase/oxidative stress response kinase cascade that regulates epithelial ion transport in flies and mammals. New studies in Drosophila kidney stone models have characterized zinc transporters and used Malpighian tubules to study the efficacy of a plant metabolite in decreasing stone burden.

SUMMARY

Study of the Drosophila Malpighian tubule affords opportunities to better characterize the molecular physiology of epithelial transport mechanisms relevant to mammalian renal physiology.

Identification of candidate ATP-binding cassette transporter gene family members in Diaphorina citri (Hemiptera: Psyllidae) via adult tissues transcriptome analysis

The ATP-binding cassette (ABC) transporters exist in all living organisms and play major roles in various biological functions by transporting a wide variety of substrates across membranes. The functions of ABC transporters in drug resistance have been extensively studied in vertebrates; however, they are rarely characterized in agricultural pests. The Asian citrus psyllid, Diaphorina citri, is one of the most damaging pests of the Citrus genus because of its transmission of Huanglongbing, also known as Yellow Dragon disease. In this study, the next-generation sequencing technique was applied to research the ABC transporters of D. citri. Fifty-three ABC transporter genes were found in the RNA-Seq data, and among these ABC transporters, 4, 4, 5, 2, 1, 4, 18 and 15 ABC proteins belonged to the ABCA-ABCH subfamilies, respectively. Different expression profiles of 52 genes between imidacloprid-resistant and imidacloprid-susceptible strains were studied by qRT-PCR; 5 ABCGs and 4 ABCHs were significantly upregulated in the imidacloprid-resistant strain. In addition, five of the nine upregulated genes were widely expressed in adult tissues in spatial expression analysis. The results suggest that these genes may play key roles in this phenotype. In general, this study contributed to our current understanding of D. citri resistance to insecticides.

In vitro transport of methotrexate by Drosophila Multidrug Resistance-associated Protein

Methotrexate (MTX) is a widely used chemotherapeutic agent, immune suppressant and antimalarial drug. It is a substrate of several human ABC proteins that confer multidrug resistance to cancer cells and determine compartmentalization of a wide range of physiological metabolites and endo or xenobiotics, by their primary active transport across biological membranes. The substrate specificity and tissue distribution of these promiscuous human ABC transporters show a high degree of redundancy, providing robustness to these key physiological and pharmacological processes, such as the elimination of toxins, e.g. methotrexate from the body. A similar network of proteins capable of transporting methotrexate has been recently suggested to exist in Drosophila melanogaster. One of the key players of this putative network is Drosophila Multidrug-resistance Associated Protein (DMRP). DMRP has been shown to be a highly active and promiscuous ABC transporter, capable of transporting various organic anions. Here we provide the first direct evidence that DMRP, expressed alone in a heterologous system lacking other, potentially functionally overlapping D. melanogaster organic anion transporters, is indeed able to transport methotrexate. Our in vitro results support the hypothesized but debated role of DMRP in in vivo methotrexate excretion.

Describing the role of Drosophila melanogaster ABC transporters in insecticide biology using CRISPR-Cas9 knockouts

ABC transporters have a well-established role in drug resistance, effluxing xenobiotics from cells and tissues within the organism. More recently, research has been dedicated to understanding the role insect ABC transporters play in insecticide toxicity, but progress in understanding the contribution of specific transporters has been hampered by the lack of functional genetic tools. Here, we report knockouts of three Drosophila melanogaster ABC transporter genes, Mdr49, Mdr50, and Mdr65, that are homologous to the well-studied mammalian ABCB1 (P-glycoprotein). Each knockout mutant was created in the same wild type background and tested against a panel of insecticides representing different chemical classes. Mdr65 knockouts were more susceptible to all neuroactive insecticides tested, but Mdr49 and Mdr50 knockouts showed increased susceptibility or resistance depending on the insecticide used. Mdr65 was chosen for further analysis. Calculation of LC₅₀ values for the Mdr65 knockout allowed the substrate specificity of this transporter to be examined. No obvious distinguishing structural features were shared among MDR65 substrates. A role for Mdr65 in insecticide transport was confirmed by testing the capacity of the knockout to synergize with the ABC inhibitor verapamil and by measuring the levels of insecticide retained in the body of knockout flies. These data unambiguously establish the influence of ABC transporters on the capacity of wild type D. melanogaster to tolerate insecticide exposure and suggest that both tissue and substrate specificity underpin this capacity.

Proteomic and metabolomic analysis reveals rapid and extensive nicotine detoxification ability in honey bee larvae

Despite potential links between pesticides and bee declines, toxicology information on honey bee larvae (Apis mellifera) is scarce and detoxification mechanisms in this development stage are virtually unknown. Larvae are exposed to natural and synthetic toxins present in pollen and nectar through consumption of brood food. Due to the characteristic intensive brood care displayed by honey bees, which includes progressive feeding throughout larval development, it is generally assumed that larvae rely on adults to detoxify for them and exhibit a diminished detoxification ability. We found the opposite. We examined the proteomic and metabolomic responses of in vitro reared larvae fed nicotine (an alkaloid found in nectar and pollen) to understand how larvae cope on a metabolic level with dietary toxins. Larvae were able to effectively detoxify nicotine through an inducible detoxification mechanism. A coordinated stress response complemented the detoxification processes, and we detected significant enrichment of proteins functioning in energy and carbohydrate metabolism, as well as in development pathways, suggesting that nicotine may promote larval growth. Further exploration of the metabolic fate of nicotine using targeted mass spectrometry analysis demonstrated that, as in adult bees, formation of 4-hydroxy-4-(3-pyridyl) butanoic acid, the result of 2'C-oxidation of nicotine, is quantitatively the most significant pathway of nicotine metabolism. We provide conclusive evidence that larvae are capable of effectively catabolising a dietary toxin, suggesting that increased larval sensitivity to specific toxins is not due to diminished detoxification abilities. These findings broaden the current understanding of detoxification biochemistry at different organizational levels in the colony, bringing us closer to understanding the capacity of the colony as a superorganism to tolerate and resist toxic compounds, including pesticides, in the environment.

Multidrug transporters and organic anion transporting polypeptides protect insects against the toxic effects of cardenolides

In the struggle against dietary toxins, insects are known to employ target site insensitivity, metabolic detoxification, and transporters that shunt away toxins. Specialized insects across six taxonomic orders feeding on cardenolide-containing plants have convergently evolved target site insensitivity via specific amino acid substitutions in the Na/K-ATPase. Nonetheless, in vitro pharmacological experiments have suggested a role for multidrug transporters (Mdrs) and organic anion transporting polypeptides (Oatps), which may provide a basal level of protection in both specialized and non-adapted insects. Because the genes coding for these proteins are evolutionarily conserved and in vivo genetic evidence in support of this hypothesis is lacking, here we used wildtype and mutant Drosophila melanogaster (Drosophila) in capillary feeder (CAFE) assays to quantify toxicity of three chemically diverse, medically relevant cardenolides. We examined multiple components of fitness, including mortality, longevity, and LD50, and found that, while the three cardenolides each stimulated feeding (i.e., no deterrence to the toxin), all decreased lifespan, with the most apolar cardenolide having the lowest LD50 value. Flies showed a clear non-monotonic dose response and experienced high levels of toxicity at the cardenolide concentration found in plants. At this concentration, both Mdr and Oatp knockout mutant flies died more rapidly than wildtype flies, and the mutants also experienced more adverse neurological effects on high-cardenolide-level diets. Our study further establishes Drosophila as a model for the study of cardenolide pharmacology and solidifies support for the hypothesis that multidrug and organic anion transporters are key players in insect protection against dietary cardenolides.

Characterization and expression profiling of ATP-binding cassette transporter genes in the diamondback moth, Plutella xylostella (L.)

BACKGROUND

ATP-binding cassette (ABC) transporters are one of the major transmembrane protein families found in all organisms and play important roles in transporting a variety of compounds across intra and extra cellular membranes. In some species, ABC transporters may be involved in the detoxification of substances such as insecticides. The diamondback moth, Plutella xylostella (L.), a destructive pest of cruciferous crops worldwide, is an important species to study as it is resistant to many types of insecticides as well as biological control Bacillus thuringiensis toxins.

RESULTS

A total of 82 ABC genes were identified from our published P. xylostella genome, and grouped into eight subfamilies (ABCA-H) based on phylogenetic analysis. Genes of subfamilies ABCA, ABCC and ABCH were found to be expanded in P. xylostella compared with those in Bombyx mori, Manduca sexta, Heliconius melpomene, Danaus plexippus, Drosophila melanogaster, Tetranychus urticae and Homo sapiens. Phylogenetic analysis indicated that many of the ABC transporters in P. xylostella are orthologous to the well-studied ABC transporter genes in the seven other species. Transcriptome- and qRT-PCR-based analysis elucidated physiological effects of ABC gene expressions of P. xylostella which were developmental stage- and tissue-specific as well as being affected by whether or not the insects were from an insecticide-resistant strain. Two ABCC and one ABCA genes were preferentially expressed in midgut of the 4th-instar larvae of a susceptible strain (Fuzhou-S) suggesting their potential roles in metabolizing plant defensive chemicals. Most of the highly expressed genes in insecticide-resistant strains were also predominantly expressed in the tissues of Malpighian tubules and midgut.

CONCLUSIONS

This is the most comprehensive study on identification, characterization and expression profiling of ABC transporter genes in P. xylostella to date. The diversified features and expression patterns of this gene family may be associated with the evolutionary capacity of this species to develop resistance to a wide range of insecticides and biological toxins. Our findings provide a solid foundation for future functional studies on specific ABC transporter genes in P. xylostella, and for further understanding of their physiological roles and regulatory pathways in insecticide resistance.

MOLECULAR CLONING, EXPRESSION PATTERN OF MULTIDRUG RESISTANCE ASSOCIATED PROTEIN 1 (MRP1, ABCC1) GENE, AND THE SYNERGISTIC EFFECTS OF VERAPAMIL ON TOXICITY OF TWO INSECTICIDES IN THE BIRD CHERRY-OAT APHID

The ATP-binding cassette (ABC) transporters are important transmembrane proteins encoded by a supergene family. The majority of ABC proteins are primary active transporters that bind and hydrolyze ATP to mediate the efflux of a diverse range of substrates across lipid membranes. In this study, we cloned and characterized a putative multidrug resistance associated protein 1 (MRP1) from Rhopalosiphum padi encoded by ABCC1. Structural analysis showed that this protein has structural features typical of the ABC transporter family. Phylogenetic analysis indicated that the amino acid sequence was highly similar that of the corresponding protein from Acyrthosiphon pisum. Real-time quantitative polymerase chain reaction (PCR) analysis showed that ABCC1 was expressed throughout all R. padi developmental stages, with the highest level of expression in the fourth larval instar. We also examined ABCC1 expression in four different tissue types and found that it was most highly expressed in the midgut. Exposing R. padi to imidacloprid and chlorpyrifos increased ABCC1 expression. Furthermore, ABCC1 expression was higher in the imidacloprid-resistant (IR) and chlorpyrifos-resistant (CR) strains than in an insecticide-susceptible strain (SS) of R. padi. Exposing R. padi to verapamil in combination with insecticides significantly increased the toxicity of the insecticides. The respective synergy factor of CR and IR R. padi strain was 1.33 and 1.26, which was lower than that (2.72 and 1.64, respectively) of the SS. Our results clarify the biological function of ABCC1 in R. padi, particularly its role in insecticide resistance, and suggest novel strategies for pest management that use ABC transporter inhibitors to increase the effectiveness of insecticides.

Multi-Drug Resistance Transporters and a Mechanism-Based Strategy for Assessing Risks of Pesticide Combinations to Honey Bees

Annual losses of honey bee colonies remain high and pesticide exposure is one possible cause. Dangerous combinations of pesticides, plant-produced compounds and antibiotics added to hives may cause or contribute to losses, but it is very difficult to test the many combinations of those compounds that bees encounter. We propose a mechanism-based strategy for simplifying the assessment of combinations of compounds, focusing here on compounds that interact with xenobiotic handling ABC transporters. We evaluate the use of ivermectin as a model substrate for these transporters. Compounds that increase sensitivity of bees to ivermectin may be inhibiting key transporters. We show that several compounds commonly encountered by honey bees (fumagillin, Pristine, quercetin) significantly increased honey bee mortality due to ivermectin and significantly reduced the LC₅₀ of ivermectin suggesting that they may interfere with transporter function. These inhibitors also significantly increased honey bees sensitivity to the neonicotinoid insecticide acetamiprid. This mechanism-based strategy may dramatically reduce the number of tests needed to assess the possibility of adverse combinations among pesticides. We also demonstrate an in vivo transporter assay that provides physical evidence of transporter inhibition by tracking the dynamics of a fluorescent substrate of these transporters (Rhodamine B) in bee tissues. Significantly more Rhodamine B remains in the head and hemolymph of bees pretreated with higher concentrations of the transporter inhibitor verapamil. Mechanism-based strategies for simplifying the assessment of adverse chemical interactions such as described here could improve our ability to identify those combinations that pose significantly greater risk to bees and perhaps improve the risk assessment protocols for honey bees and similar sensitive species.

A novel role of Drosophila cytochrome P450-4e3 in permethrin insecticide tolerance

The exposure of insects to xenobiotics, such as insecticides, triggers a complex defence response necessary for survival. This response includes the induction of genes that encode key Cytochrome P450 monooxygenase detoxification enzymes. Drosophila melanogaster Malpighian (renal) tubules are critical organs in the detoxification and elimination of these foreign compounds, so the tubule response induced by dietary exposure to the insecticide permethrin was examined. We found that expression of the gene encoding Cytochrome P450-4e3 (Cyp4e3) is significantly up-regulated by Drosophila fed on permethrin and that manipulation of Cyp4e3 levels, specifically in the principal cells of the Malpighian tubules, impacts significantly on the survival of permethrin-fed flies. Both dietary exposure to permethrin and Cyp4e3 knockdown cause a significant elevation of oxidative stress-associated markers in the tubules, including H2O2 and lipid peroxidation byproduct, HNE (4-hydroxynonenal). Thus, Cyp4e3 may play an important role in regulating H2O2 levels in the endoplasmic reticulum (ER) where it resides, and its absence triggers a JAK/STAT and NF-κB-mediated stress response, similar to that observed in cells under ER stress. This work increases our understanding of the molecular mechanisms of insecticide detoxification and provides further evidence of the oxidative stress responses induced by permethrin metabolism.

A Survey of the ATP-Binding Cassette (ABC) Gene Superfamily in the Salmon Louse (Lepeophtheirus salmonis)

Salmon lice, Lepeophtheirus salmonis (Krøyer, 1837), are fish ectoparasites causing significant economic damage in the mariculture of Atlantic salmon, Salmo salar Linnaeus, 1758. The control of L. salmonis at fish farms relies to a large extent on treatment with anti-parasitic drugs. A problem related to chemical control is the potential for development of resistance, which in L. salmonis is documented for a number of drug classes including organophosphates, pyrethroids and avermectins. The ATP-binding cassette (ABC) gene superfamily is found in all biota and includes a range of drug efflux transporters that can confer drug resistance to cancers and pathogens. Furthermore, some ABC transporters are recognised to be involved in conferral of insecticide resistance. While a number of studies have investigated ABC transporters in L. salmonis, no systematic analysis of the ABC gene family exists for this species. This study presents a genome-wide survey of ABC genes in L. salmonis for which, ABC superfamily members were identified through homology searching of the L. salmonis genome. In addition, ABC proteins were identified in a reference transcriptome of the parasite generated by high-throughput RNA sequencing (RNA-seq) of a multi-stage RNA library. Searches of both genome and transcriptome allowed the identification of a total of 33 genes / transcripts coding for ABC proteins, of which 3 were represented only in the genome and 4 only in the transcriptome. Eighteen sequences were assigned to ABC subfamilies known to contain drug transporters, i.e. subfamilies B (4 sequences), C (11) and G (2). The results suggest that the ABC gene family of L. salmonis possesses fewer members than recorded for other arthropods. The present survey of the L. salmonis ABC gene superfamily will provide the basis for further research into potential roles of ABC transporters in the toxicity of salmon delousing agents and as potential mechanisms of drug resistance.

Transcriptome-based identification of ABC transporters in the western tarnished plant bug Lygus hesperus

ATP-binding cassette (ABC) transporters are a large superfamily of proteins that mediate diverse physiological functions by coupling ATP hydrolysis with substrate transport across lipid membranes. In insects, these proteins play roles in metabolism, development, eye pigmentation, and xenobiotic clearance. While ABC transporters have been extensively studied in vertebrates, less is known concerning this superfamily in insects, particularly hemipteran pests. We used RNA-Seq transcriptome sequencing to identify 65 putative ABC transporter sequences (including 36 full-length sequences) from the eight ABC subfamilies in the western tarnished plant bug (Lygus hesperus), a polyphagous agricultural pest. Phylogenetic analyses revealed clear orthologous relationships with ABC transporters linked to insecticide/xenobiotic clearance and indicated lineage specific expansion of the L. hesperus ABCG and ABCH subfamilies. The transcriptional profile of 13 LhABCs representative of the ABCA, ABCB, ABCC, ABCG, and ABCH subfamilies was examined across L. hesperus development and within sex-specific adult tissues. All of the transcripts were amplified from both reproductively immature and mature adults and all but LhABCA8 were expressed to some degree in eggs. Expression of LhABCA8 was spatially localized to the testis and temporally timed with male reproductive development, suggesting a potential role in sexual maturation and/or spermatozoa protection. Elevated expression of LhABCC5 in Malpighian tubules suggests a possible role in xenobiotic clearance. Our results provide the first transcriptome-wide analysis of ABC transporters in an agriculturally important hemipteran pest and, because ABC transporters are known to be important mediators of insecticidal resistance, will provide the basis for future biochemical and toxicological studies on the role of this protein family in insecticide resistance in Lygus species.

The ABC gene family in arthropods: comparative genomics and role in insecticide transport and resistance

About a 100 years ago, the Drosophila white mutant marked the birth of Drosophila genetics. The white gene turned out to encode the first well studied ABC transporter in arthropods. The ABC gene family is now recognized as one of the largest transporter families in all kingdoms of life. The majority of ABC proteins function as primary-active transporters that bind and hydrolyze ATP while transporting a large diversity of substrates across lipid membranes. Although extremely well studied in vertebrates for their role in drug resistance, less is known about the role of this family in the transport of endogenous and exogenous substances in arthropods. The ABC families of five insect species, a crustacean and a chelicerate have been annotated in some detail. We conducted a thorough phylogenetic analysis of the seven arthropod and human ABC protein subfamilies, to infer orthologous relationships that might suggest conserved function. Most orthologous relationships were found in the ABCB half transporter, ABCD, ABCE and ABCF subfamilies, but specific expansions within species and lineages are frequently observed and discussed. We next surveyed the role of ABC transporters in the transport of xenobiotics/plant allelochemicals and their involvement in insecticide resistance. The involvement of ABC transporters in xenobiotic resistance in arthropods is historically not well documented, but an increasing number of studies using unbiased differential gene expression analysis now points to their importance. We give an overview of methods that can be used to link ABC transporters to resistance. ABC proteins have also recently been implicated in the mode of action and resistance to Bt toxins in Lepidoptera. Given the enormous interest in Bt toxicology in transgenic crops, such findings will provide an impetus to further reveal the role of ABC transporters in arthropods.

A burst of ABC genes in the genome of the polyphagous spider mite Tetranychus urticae

Background

The ABC (ATP-binding cassette) gene superfamily is widespread across all living species. The majority of ABC genes encode ABC transporters, which are membrane-spanning proteins capable of transferring substrates across biological membranes by hydrolyzing ATP. Although ABC transporters have often been associated with resistance to drugs and toxic compounds, within the Arthropoda ABC gene families have only been characterized in detail in several insects and a crustacean. In this study, we report a genome-wide survey and expression analysis of the ABC gene superfamily in the spider mite, Tetranychus urticae, a chelicerate ~ 450 million years diverged from other Arthropod lineages. T. urticae is a major agricultural pest, and is among of the most polyphagous arthropod herbivores known. The species resists a staggering array of toxic plant secondary metabolites, and has developed resistance to all major classes of pesticides in use for its control.

Results

We identified 103 ABC genes in the T. urticae genome, the highest number discovered in a metazoan species to date. Within the T. urticae ABC gene set, all members of the eight currently described subfamilies (A to H) were detected. A phylogenetic analysis revealed that the high number of ABC genes in T. urticae is due primarily to lineage-specific expansions of ABC genes within the ABCC, ABCG and ABCH subfamilies. In particular, the ABCC subfamily harbors the highest number of T. urticae ABC genes (39). In a comparative genomic analysis, we found clear orthologous relationships between a subset of T. urticae ABC proteins and ABC proteins in both vertebrates and invertebrates known to be involved in fundamental cellular processes. These included members of the ABCB-half transporters, and the ABCD, ABCE and ABCF families. Furthermore, one-to-one orthologues could be distinguished between T. urticae proteins and human ABCC10, ABCG5 and ABCG8, the Drosophila melanogaster sulfonylurea receptor and ecdysone-regulated transporter E23. Finally, expression profiling revealed that ABC genes in the ABCC, ABCG ABCH subfamilies were differentially expressed in multi-pesticide resistant mite strains and/or in mites transferred to challenging (toxic) host plants.

Conclusions

In this study we present the first comprehensive analysis of ABC genes in a polyphagous arthropod herbivore. We demonstrate that the broad plant host range and high levels of pesticide resistance in T. urticae are associated with lineage-specific expansions of ABC genes, many of which respond transcriptionally to xenobiotic exposure. This ABC catalogue will serve as a basis for future biochemical and toxicological studies. Obtaining functional evidence that these ABC subfamilies contribute to xenobiotic tolerance should be the priority of future research.

Multidrug resistance protein gene expression in Trichoplusia ni caterpillars

Many insect species exhibit pesticide-resistant phenotypes. One of the mechanisms capable of contributing to resistance is the overexpression of multidrug resistance (MDR) transporter proteins. Here we describe the cloning of three genes encoding MDR proteins from Trichoplusia ni: trnMDR1, trnMDR2 and trnMDR3. Real-time quantitative PCR (qPCR) detected trnMDR mRNA in the whole nervous system, midgut and Malpighian tubules of final instar T. ni caterpillars. To test whether these genes are upregulated in response to chemical challenge in this insect, qPCR was used to compare trnMDR mRNA levels in unchallenged insects with those of insects fed the synthetic pyrethroid, deltamethrin. Only limited increases were detected in a single gene, trnMDR2, which is the most weakly expressed of the three MDR genes, suggesting that increased multidrug resistance of this type is not a significant part of the response to deltamethrin exposure.

Effects of genetic knock-down of organic anion transporter genes on secretion of fluorescent organic ions by Malpighian tubules of Drosophila melanogaster

An earlier study has shown that RNAi knock-down of a single organic anion transporter (OAT) gene in the principal cells of Drosophila Malpighian tubules is associated with reductions in the expression of multiple, functionally related genes. In this study, we measured the rates of secretion of four fluorescent ions by tubules isolated from flies expressing targeted RNAi knock-down of specific OAT genes. Droplets secreted by isolated tubules set up in the Ramsay assay were collected in optically flat capillary tubes and the concentrations of fluorescent ions were determined by confocal laser scanning microscopy. Reductions in the expression of organic anion (OA) transporting polypeptide 58Dc (OATP; CG3380) were associated with reduced secretion of the OAs fluorescein and Texas Red. Reduction in the expression of Drosophila multidrug resistance associated protein (dMRP; CG6214) was correlated with reduced secretion of the P-glycoprotein substrate daunorubicin. Secretion of the organic cation quinacrine was unaffected by reduced expression of OATP, dMRP, or a multidrug efflux transporter (MET; CG30344). The results highlight the difficulties of assigning a rate-limiting role in transport of a specific OA to a single membrane transporter.

Determining rates of epithelial solute transport by optical measurement of fluorochrome concentration gradients in the unstirred layer

We describe a method for calculating rates of fluorochrome transport from unstirred layer (USL) concentration gradients measured using confocal microscopy. Isolated Malpighian tubules or guts of Drosophila melanogaster were secured to depression slides and bathed in saline containing a fluorescent compound. By measuring the concentration gradient of fluorescent organic anions (fluorescein, Texas Red) or the P-glycoprotein substrate daunorubicin in the USL adjacent to the epithelium we were able to calculate the transepithelial flux of the fluorochrome using Fick's equation. Dose-response curves for fluorescein and Texas Red based on USL concentration gradients near the surface of the Malpighian tubule were comparable to those based on collection and analysis of secreted fluid droplets. Rates of Texas Red and daunorubicin secretion were also calculated for the gut of second instar D. melanogaster larvae, a tissue that is too small for measurement of transport rates by other in vitro techniques such as cannulation and perfusion. Our results suggest that measurement of USL concentration gradients by confocal microscopy may be applicable to any fluorescent indicator of rapidly transported compounds.

Genetic knockdown of a single organic anion transporter alters the expression of functionally related genes in Malpighian tubules of Drosophila melanogaster

Insects excrete a wide variety of toxins via the Malpighian (renal) tubules. Previous studies have implicated three transporters in the secretion of the organic anion (OA) methotrexate (MTX) by the Drosophila Malpighian tubule: Drosophila multidrug resistance-associated protein (dMRP, CG6214), a multidrug efflux transporter (MET, CG30344), and an organic anion transporting polypeptide 58Dc (OATP58Dc, CG3380). RNA interference (RNAi) knockdown and P-element insertion mutation of single OA transporter genes were used to evaluate the importance of these three putative transporters in the secretion of MTX by the Malpighian tubules of Drosophila melanogaster. A major finding is that genetic knockdown of a single OA transporter gene leads to reductions in the expression of at least one other OA transporter gene and in secretion of MTX by Malpighian tubules isolated from flies reared on a standard diet. The pattern of changes indicates that decreases in MTX secretion do not correspond to decreases in dMRP expression in all of the RNAi lines. Genetic knockdown of a single OA transporter gene also alters the extent of upregulation of multiple OA transporter genes in the tubules in response to dietary MTX. Knockdown of dMRP is associated with a decrease in MET expression but an increase in OATP expression when flies are reared on MTX-enriched diet. Our results indicate that dMRP and MET are not the dominant MTX transporters in the tubules when flies are reared on MTX-enriched diets. At least one additional transporter, and possibly OATP, are required for MTX secretion. The implications of our results for studies using genetic knockdown techniques to identify OA transporters in whole tissues such as Malpighian tubules are discussed.

Functional correlates of positional and gender-specific renal asymmetry in Drosophila

BACKGROUND

In humans and other animals, the internal organs are positioned asymmetrically in the body cavity, and disruption of this body plan can be fatal in humans. The mechanisms by which internal asymmetry are established are presently the subject of intense study; however, the functional significance of internal asymmetry (outside the brain) is largely unexplored. Is internal asymmetry functionally significant, or merely an expedient way of packing organs into a cavity?

METHODOLOGY/PRINCIPAL FINDINGS

Like humans, Drosophila shows internal asymmetry, with the gut thrown into stereotyped folds. There is also renal asymmetry, with the rightmost pair of renal (Malpighian) tubules always ramifying anteriorly, and the leftmost pair always sitting posteriorly in the body cavity. Accordingly, transcriptomes of anterior-directed (right-side) and posterior-directed (left-side) Malpighian (renal) tubules were compared in both adult male and female Drosophila. Although genes encoding the basic functions of the tubules (transport, signalling) were uniformly expressed, some functions (like innate immunity) showed positional or gender differences in emphasis; others, like calcium handling or the generation of potentially toxic ammonia, were reserved for just the right-side or left-side tubules, respectively. These findings correlated with the distinct locations of each tubule pair within the body cavity. Well known developmental genes (like dorsocross, dachshund and doublesex) showed continuing, patterned expression in adult tubules, implying that somatic tissues maintain both left-right and gender identities throughout life. Gender asymmetry was also noted, both in defence and in male-specific expression of receptors for neuropeptide F and sex-peptide: NPF elevated calcium only in male tubules.

CONCLUSIONS/SIGNIFICANCE

Accordingly, the physical asymmetry of the tubules in the body cavity is directly adaptive. Now that the detailed machinery underlying internal asymmetry is starting to be delineated, our work invites the investigation, not just of tissues in isolation, but in the context of their unique physical locations and milieux.

First evidence of epithelial transport in tardigrades: a comparative investigation of organic anion transport

We investigated transport of the organic anion Chlorophenol Red (CPR) in the tardigrade Halobiotus crispae using a new method for quantifying non-fluorescent dyes. We compared the results acquired from the tardigrade with CPR transport data obtained from Malpighian tubules of the desert locust Schistocerca gregaria. CPR accumulated in the midgut lumen of H. crispae, indicating that organic anion transport takes place here. Our results show that CPR transport is inhibited by the mitochondrial un-coupler DNP (1 mmol l–1; 81% reduction), the Na+/K+-ATPase inhibitor ouabain (10 mmol l–1; 21% reduction) and the vacuolar H+-ATPase inhibitor bafilomycin (5 μmol l–1; 21% reduction), and by the organic anions PAH (10 mmol l–1; 44% reduction) and probenecid (10 mmol l–1; 61% reduction, concentration-dependent inhibition). Transport by locust Malpighian tubules exhibits a similar pharmacological profile, albeit with markedly higher concentrations of CPR being reached in S. gregaria. Immunolocalization of the Na+/K+-ATPase α-subunit in S. gregaria revealed that this transporter is abundantly expressed and localized to the basal cell membranes. Immunolocalization data could not be obtained from H. crispae. Our results indicate that organic anion secretion by the tardigrade midgut is transporter mediated with likely candidates for the basolateral entry step being members of the Oat and/or Oatp transporter families. From our results, we cautiously suggest that apical H+ and possibly basal Na+/K+ pumps provide the driving force for the transport; the exact coupling between electrochemical gradients generated by the pumps and transport of ions, as well as the nature of the apical exit step, are unknown. This study is, to our knowledge, the first to show active epithelial transport in tardigrades.

Genetic analysis of the xenobiotic resistance-associated ABC gene subfamilies of the Lepidoptera

Some ATP-binding cassette (ABC) transporters of subfamilies B, C and G confer resistance to xenobiotics including insecticides. We identified genes of these subfamilies expressed by the lepidopterans Trichoplusia ni and Bombyx mori. The B. mori genome includes eight, six and 13 ABC-B, -C and -G genes, respectively, which encode P-glycoprotein, multidrug resistance protein, MRP, and breast cancer resistance protein, BCRP, homologues. Among the ABC-C and -G subfamilies, gene duplication contributes to protein diversity. We have identified three ABC-B and two ABC-C T. ni genes. Analyses of the T. ni MRP (TrnMRP) revealed unique features, including the potential for TrnMRP4 hyperglycosylation and the alternative splicing of TrnMRP1. Taken together, these attributes of moth multidrug resistance-associated ABCs may confer distinct functional capacities to xenobiotic efflux.

Expression of multidrug resistance proteins is localized principally to the Malpighian tubules in larvae of the cabbage looper moth, Trichoplusia ni

The multidrug resistance proteins (MRPs) serve a number of important roles in development, physiological homeostasis and metabolic resistance. In insects, they may also contribute to resistance against xenobiotics including insecticides and plant secondary metabolites. To investigate their contribution to xenobiotic resistance, we have examined the tissue distribution of gene and protein expression of the multidrug resistance proteins TrnMRP1 and TrnMRP4 of the lepidopteran insect, Trichoplusia ni. Using quantitative PCR and immunohistochemistry, we have identified high expression levels of both transporters in the Malpighian tubules relative to levels in other major tissues of the body, where they probably contribute to excretion of metabolic wastes or ingested xenobiotics. We have specifically located TrnMRP protein expression in a subpopulation of Malpighian tubule secondary cells. Expression of TrnMRP1 was also detected both at a high level in specific cortical neurons of larval ganglia and at a lower level throughout the cortex, where it may act in signaling or protective functions, respectively. In contrast, expression of TrnMRP4 was low to absent in larval ganglia, with the exception of single cells in the central connective. We discuss the potential implications of this TrnMRP activity on insect development and metabolic resistance.

Interactions between detoxification mechanisms and excretion in Malpighian tubules of Drosophila melanogaster

Insects have long been known to excrete toxins via the Malpighian (renal) tubules. In addition, exposure to natural or synthetic toxins is commonly associated with increases in the activity of detoxification enzymes such as the P450 monoxygenases (P450s) and the glutathione-S-transferases (GSTs). We examined the links between mechanisms for detoxification and excretion in adult Drosophila melanogaster using functional assays and measurements of changes in gene expression by quantitative reverse transcriptase PCR in response to dietary exposure to compounds known to alter activity or gene expression of P450s and GSTs. Dietary exposure to phenol, which alters gene expression for multiple GSTs after seven to 10 generations, was also associated with an increase (more than twofold) in secretion of the organic anion methotrexate (MTX) by isolated tubules. Dietary exposure to the insecticide synergist piperonyl butoxide (PBO) was associated with reduced expression of two P450 genes (Cyp4e2, Cyp4p1) and two GST genes (GstD1, GstD5) in the tubules, as well as increased expression of Cyp12d1 and GstE1. Thin layer chromatographic analysis of fluid secreted by isolated tubules indicated that dietary exposure to PBO resulted in increased levels of an MTX metabolite. In addition, exposure to PBO altered the expression of transporter genes in the tubules, including a Drosophila multidrug resistance-associated protein, and was associated with a 73% increase in MTX secretion by isolated tubules. The results suggest that exposure of Drosophila to toxins evokes a coordinated response by the Malpighian tubules, involving both alterations in detoxification pathways as well as enhanced transport.

Effects of acute or chronic exposure to dietary organic anions on secretion of methotrexate and salicylate by Malpighian tubules of Drosophila melanogaster larvae

The effects of dietary exposure to organic anions on the physiology of isolated Malpighian tubules and on tubule gene expression were examined using larvae of Drosophila melanogaster. Acute (24 h) or chronic (7 d) exposure to type I organic anions (fluorescein or salicylate) was associated with increased fluid secretion rates and increased fluxes of both salicylate and the type II organic anion methotrexate. By contrast, chronic exposure to dietary methotrexate was associated with increased fluid secretion rate and increased flux of methotrexate, but not salicylate. Exposure to methotrexate in the diet resulted in increases in the expression of a multidrug efflux transporter gene (MET; CG30344) in the Malpighian tubules. There were also increases in expression of genes for either a Drosophila multidrug resistance-associated protein (dMRP; CG6214) or an organic anion transporting polypeptide (OATP; CG3380), depending on the concentration of methotrexate in the diet. Exposure to salicylate in the diet was associated with an increase in expression of dMRP and with decreases of MET and OATP. Exposure to dietary salicylate or methotrexate was also associated with different patterns of expression of heat shock protein genes. The results suggest that exposure to specific type I or type II organic anions has multiple effects and results not only in increased organic anion transport but also in increased rates of inorganic ion transport, which drives osmotically-obliged fluid secretion. Increased fluid secretion may enhance secretion of organic anions by eliminating diffusive backflux from the tubule lumen to the hemolymph.