Inventory and analysis of ATP-binding cassette (ABC) systems in Brugia malayi

Strain-level variations of Dirofilaria immitis microfilariae in two biochemical assays

BACKGROUND

The increase in reports of resistance to macrocyclic lactones in the canine heartworm, Dirofilaria immitis is alarming. While DNA based tests have been well-validated, they can be expensive. In a previous study, we showed that two biochemical tests adapted to a 96- well plate format and read in a spectrophotometer could detect differences among lab validated D. immitis isolates. The two tests- Resazurin reduction and Hoechst 33342 efflux-detect metabolism and P-glycoprotein activity respectively in microfilariae isolated from infected dog blood.

METHODS

Our objective was to optimize the two assays further by testing various assay parameters in D. immitis isolates not tested previously. We tested microfilarial seeding density, incubation time and the effect of in vitro treatment with ivermectin and doxycycline in five other D. immitis isolates-JYD-34, Big Head, Berkeley, Georgia III and LOL. All assays were performed in 3 technical replicates and 2-4 biological replicates. To understand the molecular basis of the assays, we also performed qPCR for selected drug metabolism and elimination associated genes of the ABC transporter and cytochrome P450 gene families.

RESULTS

Metabolism and ABC transporter activity as detected by these assays varied between strains. Anthelmintic status (resistant or susceptible) did not correlate with metabolism or P-gp efflux. Basal transcriptional variations were found between strains in ABC transporter and cytochrome P450 genes.

CONCLUSIONS

These assays provide a greater understanding of the biochemical variation among isolates of D. immitis, which can be exploited in the future to develop in vitro diagnostic tests capable of differentiating susceptible and resistant isolates.

Importance of ABC Transporters in the Survival of Parasitic Nematodes and the Prospect for the Development of Novel Control Strategies

ABC transporters, a family of ATP-dependent transmembrane proteins, are responsible for the active transport of a wide range of molecules across cell membranes, including drugs, toxins, and nutrients. Nematodes possess a great diversity of ABC transporters; however, only P-glycoproteins have been well-characterized compared to other classes. The ABC transport proteins have been implicated in developing resistance to various classes of anthelmintic drugs in parasitic nematodes; their role in plant and human parasitic nematodes still needs further investigation. Therefore, ABC transport proteins offer a potential opportunity to develop nematode control strategies. Multidrug resistance inhibitors are becoming more attractive for controlling nematodes due to their potential to increase drug efficacy in two ways: (i) by limiting drug efflux from nematodes, thereby increasing the amount of drug that reaches its target site, and (ii) by reducing drug excretion by host animals, thereby enhancing drug bioavailability. This article reviews the role of ABC transporters in the survival of parasitic nematodes, including the genes involved, their regulation and physiological roles, as well as recent developments in their characterization. It also discusses the association of ABC transporters with anthelmintic resistance and the possibility of targeting them with next-generation inhibitors or nutraceuticals (e.g., polyphenols) to control parasitic infections.

The P-glycoprotein repertoire of the equine parasitic nematode Parascaris univalens

P-glycoproteins (Pgp) have been proposed as contributors to the widespread macrocyclic lactone (ML) resistance in several nematode species including a major pathogen of foals, Parascaris univalens. Using new and available RNA-seq data, ten different genomic loci encoding Pgps were identified and characterized by transcriptome-guided RT-PCRs and Sanger sequencing. Phylogenetic analysis revealed an ascarid-specific Pgp lineage, Pgp-18, as well as two paralogues of Pgp-11 and Pgp-16. Comparative gene expression analyses in P. univalens and Caenorhabditis elegans show that the intestine is the major site of expression but individual gene expression patterns were not conserved between the two nematodes. In P. univalens, PunPgp-9, PunPgp-11.1 and PunPgp-16.2 consistently exhibited the highest expression level in two independent transcriptome data sets. Using RNA-Seq, no significant upregulation of any Pgp was detected following in vitro incubation of adult P. univalens with ivermectin suggesting that drug-induced upregulation is not the mechanism of Pgp-mediated ML resistance. Expression and functional analyses of PunPgp-2 and PunPgp-9 in Saccharomyces cerevisiae provide evidence for an interaction with ketoconazole and ivermectin, but not thiabendazole. Overall, this study established reliable reference gene models with significantly improved annotation for the P. univalens Pgp repertoire and provides a foundation for a better understanding of Pgp-mediated anthelmintic resistance.

The transcription factor SKN-1 and detoxification gene ugt-22 alter albendazole efficacy in Caenorhabditis elegans

Parasitic nematodes infect over 1/4 th of the human population and are a major burden on livestock and crop production. Benzimidazole class anthelmintics are widely used to treat infections, but resistance is a widespread problem. Mutation of genes encoding the benzimidazole target β-tubulin is a well-established mechanism of resistance, but recent evidence suggests that metabolism of the drugs may also occur. Our objective was to investigate contributions of the detoxification-response transcription factor SKN-1 to anthelmintic drug resistance using C. elegans. We find that skn-1 mutations alter EC₅₀ of the common benzimidazole albendazole in motility assays by 1.5–1.7 fold. We also identify ugt-22 as a detoxification gene associated with SKN-1 that influences albendazole efficacy. Mutation and overexpression of ugt-22 alter albendazole EC₅₀ by 2.3–2.5-fold. The influence of a nematode UGT on albendazole efficacy is consistent with recent studies demonstrating glucose conjugation of benzimidazoles.

P-glycoprotein-9 and macrocyclic lactone resistance status in selected strains of the ovine gastrointestinal nematode, Teladorsagia circumcincta

The Teladorsagia circumcincta P-glycoprotein-9 (Tci-pgp-9) gene has previously been implicated in multiple-anthelmintic resistance in this parasite. Here we further characterise genetic diversity in Tci-pgp-9 and its possible role in ivermectin (IVM) and multi-drug resistance using two UK field isolates of T. circumcincta, one susceptible to anthelmintics (MTci2) and the other resistant to most available anthelmintics including IVM (MTci5). A comparison of full-length Tci-pgp-9 cDNA transcripts from the MTci2 and MTci5 isolates (∼3.8 kb in both cases) indicated that they shared 95.6% and 99.5% identity at the nucleotide and amino acid levels, respectively. Nine non-synonymous SNPs were found in the MTci5 sequences relative to their MTci2 counterparts. Twelve genomic sequence variants of the first internucleotide binding domain of Tci-pgp-9 were identified and up to 10 of these were present in some individual worms, strongly supporting previous evidence that amplification of this gene has occurred in T. circumcincta. On average, fewer distinct sequence variants of Tci-pgp-9 were present in individual worms of the MTci5 isolate than in those of the MTci2 isolate. A further reduction in the number of sequence variants was observed in individuals derived from an IVM-treated sub-population of MTci5. These findings suggest that Tci-pgp-9 was under purifying selection in the face of IVM treatment in T. circumcincta, with some sequence variants being selected against.

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Tci-pgp-9 cDNA sequences from the MTci2 and MTci5 isolates shared 95.6% identity.

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Comparison of Tci-pgp-9 sequences from MTci5 and MTci2 showed 9 non-synonymous SNPs.

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Multiple Tci-pgp-9-IBDA sequence variants occurred in the majority of individual larvae.

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Tci-pgp-9 appears to be under purifying selection after anthelmintics are applied.

Polymorphism in ABC transporter genes of Dirofilaria immitis

Dirofilaria immitis, a filarial nematode, causes dirofilariasis in dogs, cats and occasionally in humans. Prevention of the disease has been mainly by monthly use of the macrocyclic lactone (ML) endectocides during the mosquito transmission season. Recently, ML resistance has been confirmed in D. immitis and therefore, there is a need to find new classes of anthelmintics. One of the mechanisms associated with ML resistance in nematodes has been the possible role of ATP binding cassette (ABC) transporters in reducing drug concentrations at receptor sites. ABC transporters, mainly from sub-families B, C and G, may contribute to multidrug resistance (MDR) by active efflux of drugs out of the cell. Gene products of ABC transporters may thus serve as the targets for agents that may modulate susceptibility to drugs, by inhibiting drug transport. ABC transporters are believed to be involved in a variety of physiological functions critical to the parasite, such as sterol transport, and therefore may also serve as the target for drugs that can act as anthelmintics on their own. Knowledge of polymorphism in these ABC transporter genes in nematode parasites could provide useful information for the process of drug design. We have identified 15 ABC transporter genes from sub-families A, B, C and G, in D. immitis, by comparative genomic approaches and analyzed them for polymorphism. Whole genome sequencing data from four ML susceptible (SUS) and four loss of efficacy (LOE) pooled populations were used for single nucleotide polymorphism (SNP) genotyping. Out of 231 SNPs identified in those 15 ABC transporter genes, 89 and 75 of them were specific to the SUS or LOE populations, respectively. A few of the SNPs identified may affect gene expression, protein function, substrate specificity or resistance development and may be useful for transporter inhibitor/anthelmintic drug design, or in order to anticipate resistance development.

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In the D. immitis genome, all ABC-A, -B, -C and -G transporter genes were identified.

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Within 15 ABC transporter genes identified in D. immitis, 231 SNP loci were found.

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Four exonic SNPs caused changes in predicted secondary structure of ABC proteins.

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D. immitis populations have low genetic variability among ABC transporter genes.

ABC-B transporter genes in Dirofilaria immitis

Dirofilaria immitis is a filarial nematode causing infection and heartworm disease in dogs and other canids, cats, and occasionally in humans. Prevention with macrocyclic lactones (ML) is recommended during the mosquito transmission season. Recently, ML resistance has been reported. ABC-B transporter genes are thought to be involved in the mechanism of ML resistance in other nematodes. This study aimed to identify all the ABC-B transporter genes in D. immitis using as a reference the nDi.2.2 D. immitis whole genome, which is not completely annotated. Using bioinformatic tools and PCR amplification on pooled D. immitis genomic DNA and on pooled cDNA, nine ABC transporter genes including one pseudogene were characterized. Bioinformatic and phylogenetic analyses allowed identification of three P-glycoproteins (Pgps) (Dim-pgp-3 Dim-pgp-10, Dim-pgp-11), of two ABC-B half transporter genes (one ortholog of Cel-haf-4 and Cel-haf-9; and one ortholog of Cel-haf-1 and Cel-haf-3), of one ABC half transporter gene (ortholog of Cel-haf-5) that contained an ABC-C motif, and of one additional half transporter that would require functional study for characterization. The number of ABC-B transporter genes identified was lower than in Caenorhabditis elegans and Haemonchuscontortus. Further studies are needed to understand their possible role in ML resistance in D. immitis. These ABC transporters constitute a base for ML resistance investigation in D. immitis and advance our understanding of the molecular biology of this parasite.

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Identification of ABC-B full and half transporter genes in Dirofilaria immitis.

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Phylogenetic analysis of the D. immitis ABC-B transporter genes.

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Lower number of ABC-B transporter genes in D. immitis compared with Clade V nematodes.

Effects of in vitro exposure to ivermectin and levamisole on the expression patterns of ABC transporters in Haemonchus contortus larvae

This study investigated the interaction of ATP binding cassette (ABC) transport proteins with ivermectin (IVM) and levamisole (LEV) in larvae of susceptible and resistant isolates of Haemonchus contortus in vitro by measuring transcription patterns following exposure to these anthelmintics. Furthermore, we studied the consequences of drug exposure by measuring the sensitivity of L₃ to subsequent exposure to higher drug concentrations using larval migration assays. The most highly transcribed transporter genes in both susceptible and resistant L₃ were pgp-9.3, abcf-1, mrp-5, abcf-2, pgp-3, and pgp-10. The resistant isolate showed significantly higher transcription of pgp-1, pgp-9.1 and pgp-9.2 compared to the susceptible isolate. Five P-gp genes and the haf-6 gene showed significantly higher transcription (up to 12.6-fold) after 3 h exposure to IVM in the resistant isolate. Similarly, five P-gp genes, haf-6 and abcf-1 were transcribed at significantly higher levels (up to 10.3-fold) following 3 h exposure to LEV in this isolate. On the other hand, there were no significant changes in transcriptional patterns of all transporter genes in the susceptible isolate following 3 and 6 h exposure to IVM or LEV. In contrast to these isolate-specific transcription changes, both isolates showed an increase in R-123 efflux following exposure to the drugs, suggesting that the drugs stimulated activity of existing transporter proteins in both isolates. Exposure of resistant larvae to IVM or LEV resulted, in some instances, in an increase in the proportion of the population able to migrate at the highest IVM concentrations in subsequent migration assays. The significant increase in transcription of some ABC transporter genes following 3 h exposure to both IVM and LEV in the resistant isolate only, suggests that an ability to rapidly upregulate protective pathways in response to drugs may be a component of the resistance displayed by this isolate.

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We studied interaction of anthelmintics with ABC transporters in Haemonchus contortus.

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pgp-1, 2, -9.1, -10, and -11 and haf-6 up-regulated after 3 h exposure to ivermectin.

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Up-regulation occurred in a drug-resistant isolate but not in a -susceptible isolate.

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A proportion of the drug exposed larvae showed tolerance to subsequent drug treatment.

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Rapid up-regulation of transporters may be component of resistance in parasitic nematodes.

Host ABC transporter proteins may influence the efficacy of ivermectin and possibly have broader implications for the development of resistance in parasitic nematodes

ABC transporter proteins function to extrude compounds from the cell. These proteins present an obstacle for treatment and for overcoming drug resistance as they are expressed by both host and parasite, and function similarly. The contribution of host ABC proteins to drug efficacy was examined using ivermectin and a Brugia malayi model system. Parallel in vitro and in vivo experiments were conducted using equal concentrations of ivermectin. The motilities and fecundity of B. malayi exposed to ivermectin in vitro were significantly lower than those treated in vivo. The higher motilities were correlated with low concentrations of ivermectin in worms extracted from treated hosts. The expression of ABC proteins was significantly higher in worms treated in vitro compared to those treated in vivo as well as in gerbils treated with ivermectin than in non-treated controls. The results suggest that host ABC transporters may influence the efficacy of ivermectin.

Resistance to the macrocyclic lactone moxidectin is mediated in part by membrane transporter P-glycoproteins: Implications for control of drug resistant parasitic nematodes

Our objective was to determine if the resistance mechanism to moxidectin (MOX) is similar of that to ivermectin (IVM) and involves P-glycoproteins (PGPs). Several Caenorhabditis elegans strains were used: an IVM and MOX sensitive strain, 13 PGP deletion strains and the IVM-R strain which shows synthetic resistance to IVM (by creation of three point mutations in genes coding for α-subunits of glutamate gated chloride channels [GluCls]) and cross-resistance to MOX. These strains were used to compare expression of PGP genes, measure motility and pharyngeal pumping phenotypes and evaluate the ability of compounds that inhibit PGP function to potentiate sensitivity or reverse resistance to MOX. The results suggest that C. elegans may use regulation of PGPs as a response mechanism to MOX. This was indicated by the over-expression of several PGPs in both drug sensitive and IVM-R strains and the significant changes in phenotype in the IVM-R strain in the presence of PGP inhibitors. However, as the inhibitors did not completely disrupt expression of the phenotypic traits in the IVM-R strain, this suggests that there likely are multiple avenues for MOX action that may include receptors other than GluCls. If MOX resistance was mediated solely by GluCls then exposure of the IVM-R strain to PGP inhibitors should not have affected sensitivity to MOX. Targeted gene deletions showed that protection of C. elegans against MOX involves complex mechanisms and depends on the PGP gene family, particularly PGP-6. While the results presented are similar to others using IVM, there were some important differences observed with respect to PGPs which may play a role in the disparities seen in the characteristics of resistance to IVM and MOX. The similarities are of concern as parasites resistant to IVM show some degree but not complete cross-resistance to MOX; this could impact nematodes that are resistant to IVM.

Genetic variants and increased expression of Parascaris equorum P-glycoprotein-11 in populations with decreased ivermectin susceptibility

Macrocyclic lactones (MLs) represent the major drug class for control of parasitic infections in humans and animals. However, recently reports of treatment failures became more frequent. In addition to human and ruminant parasitic nematodes this also is the case for the horse-nematode Parascaris equorum. Nevertheless, to date the molecular basis of ML resistance is still not understood. Unspecific resistance mechanisms involving transporters such as P-glycoproteins (Pgps) are expected to contribute to ML resistance in nematodes. Here, complete sequences of two P. equorum Pgps were cloned and identified as orthologs of Caenorhabditis elegans Ppg-11 and an unnamed Caenorhabditis briggsae Pgp designated as Pgp-16 using phylogenetic analysis. Quantitative real-time PCR was used to compare expression between tissues. Significantly higher PeqPgp-11 expression was found in the gut for both genders, whereas for PeqPgp-16 the body wall was identified as predominant expression site. Furthermore, Pgps were analyzed regarding their participation in resistance development. Using SeqDoC analyses, Pgp-sequences of P. equorum populations with different ML susceptibility were compared. This approach revealed three single nucleotide polymorphisms (SNPs) causing missense mutations in the PeqPgp-11 sequence which correlated with decreased ML susceptibility. However, no resistance associated differences in mRNA expression levels were detected between embryonated eggs of these populations. In contrast, comparison of two pre-adult groups with different ivermectin (IVM) susceptibility revealed the presence of the three SNPs and in addition statistically significant PeqPgp-11 overexpression in the group of worms with reduced susceptibility. These results indicate that Pgp-11 might be involved in IVM resistance in P. equorum as it shows increased expression in an IVM exposed life-cycle stage of an IVM resistant population as well as occurrence of putatively resistance associated SNPs in populations with reduced IVM susceptibility. These SNPs are promising diagnostic candidates for detection of ML resistance with potential also for other parasitic nematode species.

Gene expression analysis of ABC transporters in a resistant Cooperia oncophora isolate following in vivo and in vitro exposure to macrocyclic lactones

Members of the ATP-binding cassette (ABC) transporter family (P-glycoproteins, Half-transporters and Multidrug Resistant Proteins) potentially play a role in the development of anthelmintic resistance. The aim of this study was to investigate the possible involvement of ABC transporters in anthelmintic resistance in the bovine parasite, Cooperia oncophora. Partial sequences of 15 members of the ABC transporter protein family were identified, by mining a transcriptome dataset combined with a degenerate PCR approach. Reverse transcriptase PCR showed that most of the ABC transporters identified were constitutively transcribed throughout the life cycle of C. oncophora. Constitutive differences in gene transcript levels between a susceptible and resistant isolate were only observed for Con-haf-9 and Con-mrp-1 in eggs of the resistant isolate, while no differences were observed in L3 or the adult life stage. Analysis of resistant adult worms, collected from calves 14 days after treatment with either ivermectin or moxidectin, showed a significant 3- to 5-fold increase in the transcript levels of Con-pgp-11 compared to non-exposed worms. Interestingly, a 4-fold transcriptional up-regulation of Con-pgp-11 was also observed in L3 of the resistant isolate, after in vitro exposure to different concentrations of ivermectin, whereas this effect was not observed in exposed L3 of the susceptible isolate. The results suggest that the worms of this particular resistant isolate have acquired the ability to up-regulate Con-pgp-11 upon exposure to macrocyclic lactones. Further work is needed to understand the genetic basis underpinning this process and the functional role of PGP-11.

Inhibition of P-glycoprotein enhances sensitivity of Caenorhabditis elegans to ivermectin

In vertebrates, the function of P-glycoprotein (PGP) is to protect against toxic compounds through active efflux of the toxin from target tissues. In clinical oncology, the overexpression of PGP confers drug resistance. The function(s) of PGP in nematode physiology or in conferring drug resistance is less understood. The objective of this study was to determine the role of PGP in drug resistance in nematodes using Caenorhabditis elegans and ivermectin (IVM) as the model system. The IVM sensitive wild-type Bristol N2 strain, seven PGP deletion strains and a triple IVM receptor (avr-14/avr-15/glc-1) knock-out strain showing synthetic resistance to IVM (IVM-R) were used to (1) compare the gene expression signatures of 15 PGPs in the wild-type and resistant strains following treatment; (2) measure motility and pharyngeal pumping phenotypes in the wild-type, IVM-R and PGP deletion strains before and after treatment; and (3) quantify the phenotypic responses of the wild-type and IVM-R strains to IVM or IVM co-administered with 12 chemosensitizers that interfere with PGP function. IVM induced changes in both amplitude and timing of gene expression for the 15 PGP genes. Following IVM treatment, the most significant effects were observed in the IVM-R strain for those PGP genes expressed in the neurons, pharynx and intestine. Inactivation of pgp-2, pgp-5, pgp-6, pgp-7, pgp-12 and pgp-13 resulted in increased sensitivity to IVM compared with the wild-type. The phenotypic responses of the IVM-R strain differed from those of the wild-type strain when exposed to IVM alone, or IVM co-administered with chemosensitizers. The phenotypic responses to the co-administration of chemosensitizers varied with the concentration of IVM used, suggesting that the action of PGP's is influenced by the concentration of IVM. Verapamil restored sensitivity to IVM in the IVM-R strain. Our results demonstrate that PGPs play a role in protecting C. elegans from IVM toxicity and inhibition of PGP enhances susceptibility to IVM. PGP may be a mechanism for multidrug resistance (MDR) in parasitic nematodes.

Unique structure and regulation of the nematode detoxification gene regulator, SKN-1: implications to understanding and controlling drug resistance

Nematodes parasitize an alarming number of people and agricultural animals globally and cause debilitating morbidity and mortality. Anthelmintics have been the primary tools used to control parasitic nematodes for the past several decades, but drug resistance is becoming a major obstacle. Xenobiotic detoxification pathways defend against drugs and other foreign chemicals in diverse organisms, and evidence is accumulating that they play a role in mediating resistance to anthelmintics in nematodes. Related antioxidation pathways may also provide filarial parasites with protection against host free-radical-mediated immune responses. Upstream regulatory pathways have received almost no attention in nematode parasites, despite their potential to coregulate multiple detoxification and antioxidation genes. The nuclear eurythroid 2-related factor 2 (NRF2) transcription factor mediates inducible detoxification and antioxidation defenses in mammals, and recent studies have demonstrated that it promotes multidrug resistance in some human tumors. Recent studies in the free-living model nematode, Caenorhabditis elegans, have defined the homologous transcription factor, SKN-1, as a master regulator of detoxification and antioxidation genes. Despite similar functions, SKN-1 and NRF2 have important differences in structure and regulatory pathways. Protein alignment and phylogenetic analyses indicate that these differences are shared among many nematodes, making SKN-1 a candidate for specifically targeting nematode detoxification and antioxidation.

The role of several ABC transporter genes in ivermectin resistance in Caenorhabditis elegans

The functions of nine ATP-binding cassette (ABC) transporter genes, mrp-1, mrp-4, mrp-6, pgp-2, pgp-3, pgp-4, pgp-5, haf-2 and haf-9, in an ivermectin (IVM) resistant strain of Caenorhabditis elegans were screened by comparing transcription levels between the resistant (IVR10) and wild-type (Bristol N2) strains, and by measuring the effects of RNA interference (RNAi) on the IVM resistant strain, on motility, pharyngeal pumping, egg production and death in the presence or varying concentrations of IVM (0-20 ng/ml). mRNA levels of mrp-1, 2, 4, 5, 6, 7, pgp-1, 2, 4, 12, 14, haf-1, 2 and 3 were significantly increased in IVR10 compared with the N2 strain. At 15 or 20 ng/ml IVM, down regulation of mrp-1, pgp-4, haf-2 and haf-9 significantly increased the effect of IVM to reduce egg production. At low to moderate IVM concentrations, down regulation of mrp-1 and haf-2 reduced the motility of C. elegans. However, at high IVM concentrations motility was increased by down regulation of transcription of pgp-3, pgp-4 and haf-9. Down regulation of expression of mrp-1, pgp-2 and pgp-5 resulted in reduced pharyngeal pumping in the presence of varying concentrations of IVM, while down regulation of mrp-6 and haf-2 increased pharyngeal pumping of the resistant strain irrespective of the IVM concentration used. Although the IVR10 strain was markedly resistant to IVM, compared with the unselected N2 strain, IVM led to the death of the C. elegans in a concentration dependent manner. However, differences in the IVM induced death rate, following RNAi, were not significantly different from the IVR10 strain without RNAi. The study shows that different ABC transporter genes may play a role in modulating the effects of IVM on pharyngeal pumping, motility and egg production, with down regulation of mrp-1 and haf-2 perhaps having the greatest effects. However, down regulation of expression of no individual ABC transporter gene profoundly affected the effect of IVM on mortality in the IVR10 strain. This suggests that some of these ABC transporter genes and their products may play a role in modulating the effects of IVM, but are not, individually, the critical gene responsible for IVM resistance. This study provides a model that may help to understand drug resistance in parasitic nematodes.

P-glycoproteins and other multidrug resistance transporters in the pharmacology of anthelmintics: Prospects for reversing transport-dependent anthelmintic resistance

Parasitic helminths cause significant disease in animals and humans. In the absence of alternative treatments, anthelmintics remain the principal agents for their control. Resistance extends to the most important class of anthelmintics, the macrocyclic lactone endectocides (MLs), such as ivermectin, and presents serious problems for the livestock industries and threatens to severely limit current parasite control strategies in humans. Understanding drug resistance is important for optimizing and monitoring control, and reducing further selection for resistance. Multidrug resistance (MDR) ABC transporters have been implicated in ML resistance and contribute to resistance to a number of other anthelmintics. MDR transporters, such as P-glycoproteins, are essential for many cellular processes that require the transport of substrates across cell membranes. Being overexpressed in response to chemotherapy in tumour cells and to ML-based treatment in nematodes, they lead to therapy failure by decreasing drug concentration at the target. Several anthelmintics are inhibitors of these efflux pumps and appropriate combinations can result in higher treatment efficacy against parasites and reversal of resistance. However, this needs to be balanced against possible increased toxicity to the host, or the components of the combination selecting on the same genes involved in the resistance. Increased efficacy could result from modifying anthelmintic pharmacokinetics in the host or by blocking parasite transporters involved in resistance. Combination of anthelmintics can be beneficial for delaying selection for resistance. However, it should be based on knowledge of resistance mechanisms and not simply on mode of action classes, and is best started before resistance has been selected to any member of the combination. Increasing knowledge of the MDR transporters involved in anthelmintic resistance in helminths will play an important role in allowing for the identification of markers to monitor the spread of resistance and to evaluate new tools and management practices aimed at delaying its spread.

ABC transporters influence sensitivity of Brugia malayi to moxidectin and have potential roles in drug resistance

Some ABC transporters play a significant role in human health and illness because they confer multidrug resistance (MDR) through their overexpression. Compounds that inhibit the drug efflux mechanism can improve efficacy or reverse resistance. Of the eight described ABC transporter subfamilies, those proteins conferring MDR in humans are in subfamilies A, B, C, and G. In nematodes, transporters in subfamilies B and C are suggested to confer resistance to ivermectin. The Brugia malayi ABC transporter superfamily was examined to assess their potential to influence sensitivity to moxidectin. There was an increase in expression of ABC transporters in subfamilies A, B, C, and G following treatment. Co-administration of moxidectin with inhibitors of ABC transporter function did not enhance sensitivity to moxidectin in males; however, sensitivity was significantly enhanced in females and microfilariae. The work suggests that ABC transporters influence sensitivity to moxidectin and have a potential role in drug resistance.

P-glycoproteins of Haemonchus contortus: development of real-time PCR assays for gene expression studies

P-glycoproteins (P-gps) are proteins that function as efflux pumps, removing lipophilic xenobiotic compounds from cells. There is evidence that P-gps play a role in the resistance of parasitic nematodes to anthelmintic drugs such as benzimidazoles and macrocyclic lactones. As anthelmintic resistance becomes more common, it is important to identify candidate resistance genes with the aim of understanding the molecular basis of resistance, and of developing assays to detect these resistance-associated changes. We identified several sequences from the genome of the parasite Haemonchus contortus with convincing homology to the known P-gp coding genes of the model nematode Caenorhabditis elegans. Nine of these sequences were successfully amplified by polymerase chain reaction (PCR) and shown to be most similar to the C. elegans sequences for pgp-1, pgp-2, pgp-3, pgp-4, pgp-9, pgp-10, pgp-11, pgp-12 and pgp-14. These partial P-gp sequences from H. contortus were used to design and optimize a quantitative real-time PCR assay to investigate potential changes in the expression levels of P-gp transcripts associated with drug resistance. No significant changes in P-gp mRNA expression levels were found in a rapidly selected ivermectin-resistant parasite isolate compared to its drug-sensitive parent, but the assay has the potential to be used on other isolates in the future to further investigate resistance-associated changes in P-gp gene expression.

The role of Brugia malayi ATP-binding cassette (ABC) transporters in potentiating drug sensitivity

ATP binding cassette (ABC) systems are a diverse group of proteins that have been identified in every organism, from bacteria to humans. Analysis of nematode genomes indicates that the number and arrangement of ABC systems are similar to other organisms, with the majority being ABC transporters. There are few functional studies of ABC transporters in parasitic nematodes; most reports have been on their identification or use as genetic markers to monitor drug resistance. In eukaryotes, some ABC transporters function in tissue defense by actively removing drugs, thus preventing their accumulation. The overexpression of ABC transporters that function as efflux pumps, such as P-glycoprotein (PGP) and the multidrug resistance associated protein (MRP) are known to confer resistance. Drug sensitivity can be restored by administration of PGP interfering or MDR reversal agents. The objective of this study was to determine if ABC systems in filarioid nematodes function similarly to those of other organisms. The relative expression of 33 ABC systems identified in Brugia malayi was quantified following exogenous exposure to the commonly used drug ivermectin (IVM). Following exposure of adults and microfilariae to IVM, there was a significant increase in the transcriptional profiles of a number of ABC systems, mostly within the PGP and MRP subgroups. Coadministration of PGP-interfering and MDR-reversal agents with IVM potentiated sensitivity to the drug in adults and microfilariae. The results suggest that B. malayi ABC transporters function similarly to those in other organisms and are a factor in determining drug sensitivity.