Drug-resistant Drosophila indicate glutamate-gated chloride channels are targets for the antiparasitics nodulisporic acid and ivermectin

Mass drug administration of ivermectin in south-eastern Senegal reduces the survivorship of wild-caught, blood fed malaria vectors

Background

In south-eastern Senegal, malaria and onchocerciasis are co-endemic. Onchocerciasis in this region has been controlled by once or twice yearly mass drug administration (MDA) with ivermectin (IVM) for over fifteen years. Since laboratory-raised Anopheles gambiae s.s. are susceptible to ivermectin at concentrations found in human blood post-ingestion of IVM, it is plausible that a similar effect could be quantified in the field, and that IVM might have benefits as a malaria control tool.

Methods

In 2008 and 2009, wild-caught blood fed An. gambiae s.l. mosquitoes were collected from huts of three pairs of Senegalese villages before and after IVM MDAs. Mosquitoes were held in an insectary to assess their survival rate, subsequently identified to species, and their blood meals were identified. Differences in mosquito survival were statistically analysed using a Glimmix model. Lastly, changes in the daily probability of mosquito survivorship surrounding IVM MDAs were calculated, and these data were inserted into a previously developed, mosquito age-structured model of malaria transmission.

Results

Anopheles gambiae s.s. (P < 0.0001) and Anopheles arabiensis (P = 0.0191) from the treated villages had significantly reduced survival compared to those from control villages. Furthermore, An gambiae s.s. caught 1-6 days after MDA in treated villages had significantly reduced survival compared to control village collections (P = 0.0003), as well as those caught pre-MDA (P < 0.0001) and >7 days post-MDA (P < 0.0001). The daily probability of mosquito survival dropped >10% for the six days following MDA. The mosquito age-structured model of malaria transmission demonstrated that a single IVM MDA would reduce malaria transmission (R_o) below baseline for at least eleven days, and that repeated IVM MDAs would result in a sustained reduction in malaria R_o.

Conclusions

Ivermectin MDA significantly reduced the survivorship of An. gambiae s.s. for six days past the date of the MDA, which is sufficient to temporarily reduce malaria transmission. Repeated IVM MDAs could be a novel and integrative malaria control tool in areas with seasonal transmission, and which would have simultaneous impacts on neglected tropical diseases in the same villages.

The effect of oral anthelmintics on the survivorship and re-feeding frequency of anthropophilic mosquito disease vectors

In the Tropics, there is substantial temporal and spatial overlap of diseases propagated by anthropophilic mosquito vectors (such as malaria and dengue) and human helminth diseases (such as onchocerciasis and lymphatic filariasis) that are treated though mass drug administrations (MDA). This overlap will result in mosquito vectors imbibing significant quantities of these drugs when they blood feed on humans. Since many anthelmintic drugs have broad anti-invertebrate effects, the possibility of combined helminth control and mosquito-borne disease control through MDA is apparent. It has been previously shown that ivermectin can reduce mosquito survivorship when administered in a blood meal, but more detailed examinations are needed if MDA is to ever be developed into a tool for malaria or dengue control. We examined concentrations of drugs that follow human pharmacokinetics after MDA and that matched with mosquito feeding times, for effects against the anthropophilic mosquito vectors Anopheles gambiae s.s. and Aedes aegypti. Ivermectin was the only human-approved MDA drug we tested that affected mosquito survivorship, and only An. gambiae s.s. were affected at concentrations respecting human pharmacokinetics at indicated doses. Ivermectin also delayed An. gambiae s.s. re-feeding frequency and defecation rates, and two successive ivermectin-spiked blood meals following human pharmacokinetic concentrations compounded mortality effects compared to controls. These findings suggest that ivermectin MDA in Africa may be used to decrease malaria transmission if MDAs were administered more frequently. Such a strategy would broaden the current scope of polyparasitism control already afforded by MDAs, and which is needed in many African villages simultaneously burdened by many parasitic diseases.

Recent progress in understanding the interaction between avermectins and ligand-gated ion channels: putting the pests to sleep

Avermectins and milbemycins are an important family of anthelmintics, insecticides and acaricides. Their mode of action is as positive allosteric modulators of ligand-gated chloride channels, and at higher concentrations, they gate some channels directly. Though it has long been known that the avermectins do not compete for the ligand binding site, the actual site at which they interact with their receptors has been unclear. Recent data demonstrate the importance to drug binding of amino acid residues predicted to line a water-filled pocket in the channel domain. This pocket acts as the binding site for anaesthetics and other modulators of mammalian GABA(A) and glycine receptors, suggesting similarities in the mode of action between these drugs and the avermectins/milbemycins.

Sarcoptes scabiei var. canis refractory to ivermectin treatment in two dogs

A 10-year-old castrated male Shih Tzu presented with severe generalized pruritus. Skin scrapings revealed the presence of Sarcoptes scabiei var. canis. A Yorkshire terrier in the same household simultaneously developed pruritus due to scabies. Both dogs were treated with 300 μg/kg ivermectin, at first orally and then subcutaneously at 14 day intervals. However, live mites were still found on day 35, and the skin condition deteriorated in both dogs. These findings suggested that the S. scabiei in these dogs was clinically refractory to ivermectin. The pruritus in both dogs rapidly and completely disappeared following topical fipronil administration. This appears to be the first report of canine scabies refractory to ivermectin treatment.

A glycine residue essential for high ivermectin sensitivity in Cys-loop ion channel receptors

Ivermectin exerts its anthelmintic effect by activating nematode Cys-loop glutamate-gated receptors. Here we show that a glycine residue at a specific transmembrane domain location is essential for high ivermectin sensitivity in both glycine- and glutamate-gated Cys-loop receptors. We also show that ivermectin sensitivity can be conferred on an ivermectin-insensitive receptor by introducing a glycine at this position. Furthermore, comparison of amino acid sequences of ivermectin-sensitive and -resistant receptors reveals that the presence of a glycine reliably predicts ivermectin sensitivity. By providing a means of identifying ivermectin-sensitive receptors, this finding should help in characterising ivermectin-resistance mechanisms and identifying new anthelmintic targets.

A point mutation in a glutamate-gated chloride channel confers abamectin resistance in the two-spotted spider mite, Tetranychus urticae Koch

The molecular mechanisms and genetics of abamectin resistance mediated by target site insensitivity in the two-spotted spider mite, Tetranychus urticae, were investigated by comparing two isogenic abamectin-susceptible (AbaS) and abamectin-resistant (AbaR) strains. Cloning and sequencing of full-length cDNA fragments of gamma-amino butyric acid (GABA)-gated chloride channel genes revealed no polymorphisms between the two strains. However, sequence comparison of the full-length cDNA fragment of a T. urticae glutamate-gated chloride channel gene (TuGluCl) identified a G323D point mutation as being tentatively related with abamectin resistance. In individual F(2) progenies obtained by backcrossing, the G323D genotype was confirmed to correlate with abamectin resistance. Bioassays using progeny from reciprocal crossings revealed that the abamectin resistance trait resulting from TuGluCl insensitivity is incompletely recessive.

Glutamate-activated chloride channels: Unique fipronil targets present in insects but not in mammals

Selectivity to insects over mammals is one of the important characteristics for a chemical to become a useful insecticide. Fipronil was found to block cockroach GABA receptors more potently than rat GABA(A) receptors. Furthermore, glutamate-activated chloride channels (GluCls), which are present in cockroaches but not in mammals, were very sensitive to the blocking action of fipronil. The IC(50)s of fipronil block were 30 nM in cockroach GABA receptors and 1600 nM in rat GABA(A) receptors. Moreover, GluCls of cockroach neurons had low IC(50)s for fipronil. Two types of glutamate-induced chloride current were obswerved: desensitizing and non-desensitizing, with fipronil IC(50)s of 800 and 10 nM, respectively. We have developed methods to separately record these two types of GluCls. The non-desensitizing and desensitizing currents were selectively inhibited by trypsin and polyvinylpyrrolidone, respectively. In conclusion, in addition to GABA receptors, GluCls play a crucial role in selectivity of fipronil to insects over mammals. GluCls form the basis for development of selective and safe insecticides.

Cys-Loop Ligand-Gated Chloride Channels in Dorsal Unpaired Median Neurons of Locusta migratoria

In insects, inhibitory neurotransmission is generally associated with members of the cys-loop ligand-gated anion channels, such as the glutamate-gated chloride channel (GluCl), the GABA-gated chloride channels (GABACl), and the histamine-gated chloride channels (HisCl). These ionotropic receptors are considered established target sites for the development of insecticides, and therefore it is necessary to obtain a better insight in their distribution, structure, and functional properties. Here, by combining electrophysiology and molecular biology techniques, we identified and characterized GluCl, GABACl, and HisCl in dorsal unpaired median (DUM) neurons of Locust migratoria. In whole cell patch-clamp recordings, application of glutamate, GABA, or histamine induced rapidly activating ionic currents. GluCls were sensitive to ibotenic acid and blocked by picrotoxin and fipronil. The pharmacological profile of the L. migratoria GABACl fitted neither the vertebrate GABAA nor GABAC receptor and was similar to the properties of the cloned Drosophila melanogaster GABA receptor subunit (Rdl). The expression of Rdl-like subunit-containing GABA receptors was shown at the molecular level using RT-PCR. Sequencing analysis indicated that the orthologous GABACl of D. melanogaster CG10357-A is expressed in DUM neurons of L. migratoria. Histamine-induced currents exhibited a fast onset and desensitized completely on continuous application of histamine. In conclusion, within the DUM neurons of L. migratoria, we identified three different cys-loop ligand-gated anion channels that use GABA, glutamate, or histamine as their neurotransmitter.

Selection of an ivermectin-resistant strain of Rhipicephalus microplus (Acari: Ixodidae) in Brazil

Resistance to ivermectin (IVM) in field populations of Rhipicephalus microplus of Brazil has been observed since 2001. In this work, four selection methods (infestations with: (1) IVM-treated larvae; (2) larvae from IVM-treated adult female ticks; (3) larvae from IVM-treated adult female ticks on an IVM-treated host; and (4) larvae obtained from IVM-treated females that produced eggs with a high eclosion rate) were used on a field population with an initial ivermectin (IVM) resistance ratio at LC50 (RR50) of 1.37 with the objective to obtain experimentally a highly-resistant strain. After ten generations, using these methods combined, the final RR50 was 8.06. This work shows for the first time that it was possible to increase IVM resistance in R. microplus in laboratory conditions. The establishment of a drug resistant R. microplus strain is a fundamental first step for further research into the mechanisms of ivermectin-resistance in R. microplus and potentially methods to control this resistance.

Comparative pharmacology and computational modelling yield insights into allosteric modulation of human alpha7 nicotinic acetylcholine receptors

The human alpha7 nicotinic acetylcholine receptor (nAChR) subunit and its Caenorhabditis elegans homolog, ACR-16, can generate functional recombinant homomeric receptors when expressed in Xenopus laevis oocytes. Both nAChRs express robustly in the presence of the co-injected chaperone, RIC-3, and show striking differences in the actions of a type I positive allosteric modulator (PAM), ivermectin (IVM). Type I PAMs are characterised by an increase in amplitude only of the response to acetylcholine (ACh), whereas type II PAMs exhibit, in addition, changes in time-course/desensitization of the ACh response. The type I PAMs, ivermectin, 5-hydroxyindole (5-HI), NS-1738 and genistein and the type II PAM, PNU-120596, are all active on human alpha7 but are without PAM activity on ACR-16, where they attenuate the amplitude of the ACh response. We used the published structure of avermectin B1a to generate a model of IVM, which was then docked into the candidate transmembrane allosteric binding site on alpha7 and ACR-16 in an attempt to gain insights into the observed differences in IVM actions. The new pharmacological findings and computational approaches being developed may inform the design of novel PAM drugs targeting major neurological disorders.

Altered ivermectin pharmacology and defective visual system in Drosophila mutants for histamine receptor HCLB

The Drosophila gene hclB encodes a histamine-gated chloride channel, which can be activated by the neurotoxin ivermectin when expressed in vitro. We have identified two novel hclB mutants, carrying either a missense mutation (P293S, allele hclB (T1)) or a putative null mutation (W111*, allele hclB (T2)), as well as a novel splice form of the gene. In survival studies, hclB (T1) mutants were more sensitive to ivermectin than wild-type, whereas hclB (T2) were more resistant. Electroretinogram recordings from the two mutants exhibited enlarged peak amplitudes of the transient components, indicating altered synaptic transmission between retinal photoneurons and their target cells. Ivermectin treatment severely affected or completely suppressed these transient components in an allele-specific manner. This suppression of synaptic signals by ivermectin was dose-dependent. These results identify HCLB as an important in vivo target for ivermectin in Drosophila melanogaster, and demonstrate the involvement of this protein in the visual pathway.

Inhibitory neurotransmission and olfactory memory in honeybees

In insects, gamma-aminobutyric acid (GABA) and glutamate mediate fast inhibitory neurotransmission through ligand-gated chloride channel receptors. Both GABA and glutamate have been identified in the olfactory circuit of the honeybee. Here we investigated the role of inhibitory transmission mediated by GABA and glutamate-gated chloride channels (GluCls) in olfactory learning and memory in honeybees. We combined olfactory conditioning with injection of ivermectin, an agonist of GluCl receptors. We also injected a blocker of glutamate transporters (L-trans-PDC) or a GABA analog (TACA). We measured acquisition and retention 1, 24 and 48 h after the last acquisition trial. A low dose of ivermectin (0.01 ng/bee) impaired long-term olfactory memory (48 h) while a higher dose (0.05 ng/bee) had no effect. Double injections of ivermectin and L-trans-PDC or TACA had different effects on memory retention, depending on the doses and agents combined. When the low dose of ivermectin was injected after Ringer, long-term memory was again impaired (48 h). Such an effect was rescued by injection of both TACA and L-trans-PDC. A combination of the higher dose of ivermectin and TACA decreased retention at 48 h. We interpret these results as reflecting the involvement of both GluCl and GABA receptors in the impairment of olfactory long-term memory induced by ivermectin. These results illustrate the diversity of inhibitory transmission and its implication in long-term olfactory memory in honeybees.

A randomized, double-blind, multicenter clinical trial on the efficacy of ivermectin against intestinal nematode infections in China

To assess the efficacy of ivermectin against intestinal nematode infections, a randomized, double-blind, multicenter clinical trial was carried out in a total of 816 human individuals infected with different nematodes from three counties in China. The subjects were randomly assigned into experimental and control groups and orally given a single dose of 0.1, 0.2, 0.2 and 0.2mg/kg ivermectin against Ascaris lumbricoides, hookworm, Trichuris trichiura and Enterobius vermicularis, respectively. Parallel control groups to each of the ivermectin groups were given a single oral dose of 6.7 mg/kg albendazole. The cure rates with ivermectin and albendazole were 100% (102/102) and 99.0% (101/102) for Ascaris, and 66.7% (68/102) and 67.7% (69/102) for Trichuris, respectively, with no significant difference (P>0.05) between the two treatments. The parasitological cure rates of albendazole were 69.6% (71/102) for hookworm and 94.1% (96/102) for Enterobius, which were significantly higher than ivermectin (33.3% and 52.9%, respectively, P<0.0001). The expulsion of worm in the feces reached its peak 1-2 days after ivermectin treatment. The study showed that ivermectin, with few side effects, could be used as an additional treatment tool for intestinal nematodes, especially for the treatment of Ascaris and Trichuris infections in China.

Use of non-mammalian alternative models for neurotoxicological study

The field of neurotoxicology needs to satisfy two opposing demands: the testing of a growing list of chemicals, and resource limitations and ethical concerns associated with testing using traditional mammalian species. National and international government agencies have defined a need to reduce, refine or replace mammalian species in toxicological testing with alternative testing methods and non-mammalian models. Toxicological assays using alternative animal models may relieve some of this pressure by allowing testing of more compounds while reducing expense and using fewer mammals. Recent advances in genetic technologies and the strong conservation between human and non-mammalian genomes allow for the dissection of the molecular pathways involved in neurotoxicological responses and neurological diseases using genetically tractable organisms. In this review, applications of four non-mammalian species, zebrafish, cockroach, Drosophila, and Caenorhabditis elegans, in the investigation of neurotoxicology and neurological diseases are presented.

Scabies: molecular perspectives and therapeutic implications in the face of emerging drug resistance

Limited effective treatments, coupled with recent observations of emerging drug resistance to oral ivermectin and 5% permethrin, raise concerns regarding the future control of scabies, especially in severe cases and in endemic areas where repeated community treatment programs are in place. There is consequently an urgent need to define molecular mechanisms of drug resistance in scabies mites and to develop and assess alternative therapeutic options, such as tea tree oil, in the event of increasing treatment failure. Molecular studies on scabies mites have, until recently, been restricted; however, recent advances are providing new insights into scabies mite biology and genetic mechanisms underlying drug resistance. These may assist in overcoming many of the current difficulties in monitoring treatment efficacy and allow the development of more sensitive tools for monitoring emerging resistance.

Nematode ligand-gated chloride channels: an appraisal of their involvement in macrocyclic lactone resistance and prospects for developing molecular markers

Ligand-gated chloride channels, including the glutamate-(GluCl) and GABA-gated channels, are the targets of the macrocyclic lactone (ML) family of anthelmintics. Changes in the sequence and expression of these channels can cause resistance to the ML in laboratory models, such as Caenorhabditis elegans and Drosophila melanogaster. Mutations in multiple GluCl subunit genes are required for high-level ML resistance in C. elegans, and this can be influenced by additional mutations in gap junction and amphid genes. Parasitic nematodes have a different complement of channel subunit genes from C. elegans, but a few genes, including avr-14, are widely present. A polymorphism in an avr-14 orthologue, which makes the subunit less sensitive to ivermectin and glutamate, has been identified in Cooperia oncophora, and polymorphisms in several subunits have been reported from resistant isolates of Haemonchus contortus. This has led to suggestions that ML resistance may be polygenic. Possible reasons for this, and its consequences for the development of molecular tests for resistance, are explored.

Molecular characterisation of a pH-gated chloride channel from Sarcoptes scabiei

Reports of ivermectin resistance in scabies mites raise concerns regarding the sustainability of mass intervention programs for scabies worldwide and for the treatment of crusted scabies. Ligand gated ion channels (LGICs) are the primary targets of ivermectin in invertebrates. We report the molecular characterisation of SsCl--a novel LGIC from Sarcoptes scabiei var. hominis. While SsCl shows sequence similarity to other LGICs, phylogenetic analysis does not suggest strong homology to conventional glutamate, histamine or GABA gated channels. Instead, it is most similar to Drosophila pH-sensitive and group 1 clades. When expressed in Xenopus oocytes, SsCl forms a homomeric, pH-gated chloride channel that is irreversibly activated by ivermectin. These results provide the first confirmation that this group of LGIC exists in arachnids, and suggest that SsCl may be an in vivo target of ivermectin in S. scabiei.

Dorsal Unpaired Median Neurons ofLocusta migratoriaExpress Ivermectin- and Fipronil-Sensitive Glutamate-Gated Chloride Channels

Together with type A GABA and strychnine-sensitive glycine receptors, glutamate-gated chloride channels (GluCl) are members of the Cys-loop family of ionotropic receptors, which mediate fast inhibitory neurotransmission. To date, GluCls are found in invertebrates only and therefore represent potential specific targets for insecticides, such as ivermectin and fipronil. In this study, we identified the functional expression of GluCls in dorsal unpaired median (DUM) neurons of the metathoracic ganglion of Locusta migratoria using electrophysiological and molecular biological techniques. In whole cell patch-clamped DUM neurons, glutamate-induced changes in both their membrane potentials (current-clamp) and currents (voltage-clamp) were dependent on the chloride equilibrium potential. On continuous application of glutamate, the glutamate-elicited current response became rapidly and completely desensitized. Application of glutamate in the presence of 10 μM fipronil or 100 μM picrotoxin reversibly decreased GluCl-mediated currents by 87 and 39%, respectively. Furthermore, 1 μM ivermectin induced a persistent chloride current, suggesting the expression of ivermectin-sensitive GluCl α subunits. A degenerate PCR/RACE strategy was used to clone the full-length L. migratoria LmGlClα subunit. Finally, RT-PCR experiments demonstrated the presence of LmGluClα transcripts in locust DUM neurons. Our results provide the first direct evidence of a functional ivermectin-sensitive GluCl channel on the cell surface of DUM neurons of L. migratoria.

Thermal nociception in adult Drosophila: behavioral characterization and the role of the painless gene

Nociception, warning of injury that should be avoided, serves an important protective function in animals. In this study, we show that adult Drosophila avoids noxious heat by a jump response. To quantitatively analyze this nociceptive behavior, we developed two assays. In the CO2 laser beam assay, flies exhibit this behavior when a laser beam heats their abdomens. The consistency of the jump latency in this assay meets an important criterion for a good nociceptive assay. In the hot plate assay, flies jump quickly to escape from a hot copper plate (>45 degrees C). Our results demonstrate that, as in mammals, the latency of the jump response is inversely related to stimulus intensity, and innoxious thermosensation does not elicit this nociceptive behavior. To explore the genetic mechanisms of nociception, we examined several mutants in both assays. Abnormal nociceptive behavior of a mutant, painless, indicates that painless, a gene essential for nociception in Drosophila larvae, is also required for thermal nociception in adult flies. painless is expressed in certain neurons of the peripheral nervous system and thoracic ganglia, as well as in the definite brain structures, the mushroom bodies. However, chemical or genetic insults to the mushroom bodies do not influence the nociceptive behavior, suggesting that different painless-expressing neurons play diverse roles in thermal nociception. Additionally, no-bridge(KS49), a mutant that has a structural defect in the protocerebral bridge, shows defective response to noxious heat. Thus, our results validate adult Drosophila as a useful model to study the genetic mechanisms of thermal nociception.

Two splice variants derived from a Drosophila melanogaster candidate ClC gene generate ClC-2-type Cl- channels

Members of the ClC family of membrane proteins have been found in a variety of species and they can function as Cl- channels or Cl-/H+ antiporters. Three potential ClC genes are present in the Drosophila melanogaster genome. Only one of them shows homology with a branch of the mammalian ClC genes that encode plasma membrane Cl- channels. The remaining two are close to mammalian homologues coding for intracellular ClC proteins. Using RT-PCR we have identified two splice variants showing highest homology (41% residue identity) to the mammalian ClC-2 chloride channel. One splice variant (DmClC-2S) is expressed in the fly head and body and an additional, larger variant (DmClC-2L) is only present in the head. Both putative Drosophila channels conserve key features of the ClC channels cloned so far, including residues conforming the selectivity filter and C-terminus CBS domains. The splice variants differ in a stretch of 127 aa at the intracellular C-terminal portion separating cystathionate beta synthase (CBS) domains. Expression of either Drosophila ClC-2 variant in HEK-293 cells generated inwardly rectifying Cl- currents with similar activation and deactivation characteristics. There was great similarity in functional characteristics between DmClC-2 variants and their mammalian counterpart, save for slower opening kinetics and faster closing rate. As CBS domains are believed to be sites of regulation of channel gating and trafficking, it is suggested that the extra amino acids present between CBS domains in DmClC-2L might endow the channel with a differential response to signals present in the fly cells where it is expressed.

Cloning and characterization of genes encoding alpha and beta subunits of glutamate-gated chloride channel protein in Cylicocyclus nassatus

The invertebrate glutamate-gated chloride channels (GluCls) are receptor molecules and targets for the avermectin-milbemycin (AM) group of anthelmintics. Mutations in GluCls are associated with ivermectin resistance in the soil dwelling nematode Caenorhabditis elegans and the parasitic nematode Cooperia oncophora. In this study, full-length cDNAs encoding alpha and beta subunits of GluCl were cloned and sequenced in Cylicocyclus nassatus, a common and important cyathostomin nematode parasite of horses. Both genes possess the sequence characteristics typical of GluCls, and phylogenetic analysis confirms that these genes are evolutionarily closely related to GluCls of other nematodes and flies. Complete coding sequences of C. nassatus GluCl-alpha and GluCl-beta were subcloned into pTL1 mammalian expression vector, and proteins were expressed in COS-7 cells. Ivermectin-binding characteristics were determined by incubating COS-7 cell membranes expressing C. nassatus GluCl-alpha and GluCl-beta proteins with [(3)H]ivermectin. In competitive binding experiments, fitting the data to a one site competition model, C. nassatus GluCl-alpha was found to bind [(3)H]ivermectin with a high amount of displaceable binding (IC(50)=208 pM). Compared to the mock-transfected COS-7 cells, the means of [(3)H]ivermectin binding were significantly different for C. nassatus GluCl-alpha and the Haemonchus contortus GluCl (HcGluCla) (p=0.018 and 0.023, respectively) but not for C. nassatus GluCl-beta (p=0.370). This is the first report of orthologs of GluCl genes and in vitro expression of an ivermectin-binding protein in a cyathostomin species. These data suggest the likelihood of a similar mechanism of action of AM drugs in these parasites, and suggest that mechanisms of resistance may also be similar.

Phylogenetic shadowing of a histamine-gated chloride channel involved in insect vision

A recently identified gene, hclA (synonym: ort), codes for an ionotrophic histamine receptor subunit in Drosophila melanogaster, and known hclA mutations lead to defects in the visual system, neurologic disorders and changed responsiveness to neurotoxins. To investigate whether this novel class of receptors is common across the Insecta, we analysed the genomes of 15 other insect species (Diptera, Hymenoptera, Coleoptera, Lepidoptera) and revealed orthologs of hclA in all of them. The predicted receptor domain of HCLA is extensively conserved (86-100% of identity) among the 16 proteins. Minor changes in the amino acid sequence that includes the putative transmembrane domains (TMs) 1-3 were found in non-drosophilid species only. Substantial amino acid variability was observed in the signal polypeptides, the intracellular loop domains and in TM4, in good accordance with known data on sequence variations in ligand-gated ion channels. Pairwise comparisons revealed three consensus sequences for N-glycosylation, conserved in HCLAs of all species studied, as well as a drosophilid-specific putative phosphorylation site. Real-time PCR analysis demonstrated that hclA-mRNA is abundant in heads of adult Drosophila. However, species- and sex-specific variations of the hclA expression levels were also observed.

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.

Molecular structure and function of the glycine receptor chloride channel

The glycine receptor chloride channel (GlyR) is a member of the nicotinic acetylcholine receptor family of ligand-gated ion channels. Functional receptors of this family comprise five subunits and are important targets for neuroactive drugs. The GlyR is best known for mediating inhibitory neurotransmission in the spinal cord and brain stem, although recent evidence suggests it may also have other physiological roles, including excitatory neurotransmission in embryonic neurons. To date, four alpha-subunits (alpha1 to alpha4) and one beta-subunit have been identified. The differential expression of subunits underlies a diversity in GlyR pharmacology. A developmental switch from alpha2 to alpha1beta is completed by around postnatal day 20 in the rat. The beta-subunit is responsible for anchoring GlyRs to the subsynaptic cytoskeleton via the cytoplasmic protein gephyrin. The last few years have seen a surge in interest in these receptors. Consequently, a wealth of information has recently emerged concerning GlyR molecular structure and function. Most of the information has been obtained from homomeric alpha1 GlyRs, with the roles of the other subunits receiving relatively little attention. Heritable mutations to human GlyR genes give rise to a rare neurological disorder, hyperekplexia (or startle disease). Similar syndromes also occur in other species. A rapidly growing list of compounds has been shown to exert potent modulatory effects on this receptor. Since GlyRs are involved in motor reflex circuits of the spinal cord and provide inhibitory synapses onto pain sensory neurons, these agents may provide lead compounds for the development of muscle relaxant and peripheral analgesic drugs.

Breed distribution and history of canine mdr1-1Delta, a pharmacogenetic mutation that marks the emergence of breeds from the collie lineage

A mutation in the canine multidrug resistance gene, MDR1, has previously been associated with drug sensitivities in two breeds from the collie lineage. We exploited breed phylogeny and reports of drug sensitivity to survey other purebred populations that might be genetically at risk. We found that the same allele, mdr1-1Delta, segregated in seven additional breeds, including two sighthounds that were not expected to share collie ancestry. A mutant haplotype that was conserved among affected breeds indicated that the allele was identical by descent. Based on breed histories and the extent of linkage disequilibrium, we conclude that all dogs carrying mdr1-1Delta are descendants of a dog that lived in Great Britain before the genetic isolation of breeds by registry (ca. 1873). The breed distribution and frequency of mdr1-1Delta have applications in veterinary medicine and selective breeding, whereas the allele's history recounts the emergence of formally recognized breeds from an admixed population of working sheepdogs.

First documentation of in vivo and in vitro ivermectin resistance in Sarcoptes scabiei

Ivermectin is increasingly being used to treat scabies, especially crusted (Norwegian) scabies. However, treatment failures, recrudescence, and reinfection can occur, even after multiple doses. Ivermectin resistance has been documented for some intestinal helminths in animals with intensive ivermectin exposure. Ivermectin resistance has also been induced in arthropods in laboratory experiments but, to date, has not been documented among arthropods in nature. We report clinical and in vitro evidence of ivermectin resistance in 2 patients with multiple recurrences of crusted scabies who had previously received 30 and 58 doses of ivermectin over 4 and 4.5 years, respectively. As predicted, ivermectin resistance in scabies mites can develop after intensive ivermectin use.

Purifying selection and demographic expansion affect sequence diversity of the ligand-binding domain of a glutamate-gated chloride channel gene of Haemonchus placei

Ninety-five genomic sequences of the ligand-binding domain of glutamate-gated chloride channel genes of three populations of the parasitic nematode H. placei were evaluated for patterns of diversity, demography, and selection. These genes code for subunits of ion channels, which are involved in the mode of action of the most commonly used antiparasitic drugs, the macrocyclic lactones. An extremely high frequency of unique segregating sites in exons and introns was observed, with significantly negative neutrality tests in each population for noncoding, synonymous, and nonsynonymous sites. Several tests indicated that support for balancing selection, positive selection, and hitchhiking was lacking. McDonald-Kreitman tests using H. contortus or C. elegans as an outgroup revealed an extreme excess of replacement polymorphism, consistent with weak purifying selection. Although these tests agree that negative selection may explain the excess of replacement changes, an alternative interpretation is required for the significantly negative Fu and Li's D statistics based on silent and noncoding sites. These include homogeneous forces such as background selection and demographic expansion. The lack of population subdivision and the negative values of Tajima's D for this outbreeding parasitic nematode render background selection less likely than demographic expansion. Comparison of D statistics based on different site types using neutral coalescent simulations supported this interpretation. Although this statistic was more negative for nonsynonymous sites than for synonymous sites, most comparisons of the D statistic were not significantly different between mutation classes. A few significant site comparisons were also consistent with demographic expansion, because the observed test statistic ( D(neutral) - D(selected)) were low relative to the neutral expectations. Finally, previous mitochondrial studies also identified a demographic expansion of this parasitic nematode species, which lends further support to a scenario involving both demographic and purifying forces in the ligand-binding domain of H. placei.

Scabies: New Future for a Neglected Disease

Scabies is a disease of global proportions in both human and animal populations, resulting from infestation of the skin with the "itch" mite Sarcoptes scabiei. Despite the availability of effective chemotherapy the intensely itching lesions engender significant morbidity primarily due to secondary sepsis and post-infective complications. Some patients experience an extreme form of the disease, crusted scabies, in which many hundreds of mites may infest the skin causin severe crusting and hyperkeratosis. Overcrowded living conditions and poverty have been identified as significant confounding factors in transmission of the mite in humans. Control is hindered by difficulties with diagnosis, the cost of treatment, evidence for emerging resistance and lack of effective vaccines. Historically research on scabies has been extremely limited because of the difficulty in obtaining sufficient quantities of the organism. Recent molecular approaches have enabled considerable advances in the study of population genetics and transmission dynamics of S. scabiei. However, the most exciting and promising development is the potential exploitation of newly available data from S. scabiei cDNA libraries and EST projects. Ultimately this knowledge may aid early identification of disease, novel forms of chemotherapy, vaccine development and new treatment possibilities for this important but neglected parasite.

Chloride channels as tools for developing selective insecticides

Ligand-gated chloride channels underlie inhibition in excitable membranes and are proven target sites for insecticides. The gamma-aminobutyric acid (GABA(1)) receptor/chloride ionophore complex is the primary site of action for a number of currently used insecticides, such as lindane, endosulfan, and fipronil. These compounds act as antagonists by stabilizing nonconducting conformations of the chloride channel. Blockage of the GABA-gated chloride channel reduces neuronal inhibition, which leads to hyperexcitation of the central nervous system, convulsions, and death. We recently investigated the mode of action of the silphinenes, plant-derived natural compounds that structurally resemble picrotoxinin. These materials antagonize the action of GABA on insect neurons and block GABA-mediated chloride uptake into mouse brain synaptoneurosomes in a noncompetitive manner. In mammals, avermectins have a blocking action on the GABA-gated chloride channel consistent with a coarse tremor, whereas at longer times and higher concentrations, activation of the channel suppresses neuronal activity. Invertebrates display ataxia, paralysis, and death as the predominant signs of poisoning, with a glutamate-gated chloride channel playing a major role. Additional target sites for the avermectins or other chloride channel-directed compounds might include receptors gated by histamine, serotonin, or acetylcholine.The voltage-sensitive chloride channels form another large gene family of chloride channels. Voltage-dependent chloride channels are involved in a number of physiological processes including: maintenance of electrical excitability, chloride ion secretion and resorption, intravesicular acidification, and cell volume regulation. A subset of these channels is affected by convulsants and insecticides in mammals, although the role they play in acute lethality in insects is unclear. Given the wide range of functions that they mediate, these channels are also potential targets for insecticide development.

Possible pathogenic pathways in the adverse clinical events seen following ivermectin administration to onchocerciasis patients

BACKGROUND: Reactions are commonly associated with the chemotherapy of onchocerciasis. However unmanageable reactions are uncommon when ivermectin (Mectizan(R)) is used for the treatment of this infection, and this drug has proved to be a great improvement over previously used agents. Serious adverse events (SAE) nevertheless have occurred, and there is considerable concern about the negative effect such events may have on mass drug administration programs.This paper reviews the basic pathogenic mechanisms that can be involved in the destruction of microfilaria by chemotherapeutic agents. A central challenge to filarial chemotherapy is the need to remove parasites from biologically sensitive tissues, a more difficult medical challenge than eliminating nematodes from the gastrointestinal tract.Explanations for the etiology of the serious adverse reactions occurring with ivermectin treatment in specific geographic areas where there is coincident heavy Loa loa infections are hampered by a lack of specific pathological case material. Ways to investigate these possibilities are reviewed. Possible pathogenic mechanisms include embolic vascular pathology accompanied by local inflammation, blood brain barrier mdr1 abnormalities, and genetic predisposition to excessive inflammatory responses. CONCLUSION: It is important to keep ivermectin, and all its associated adverse clinical events, in perspective with the many other chemotherapeutic agents in general use - many of which produce serious adverse events even more frequently than does ivermectin. Currently available evidence indicates that the pathogenesis of the Loa-associated adverse reactions are probably related to inflammatory responses to microfilariae in specific tissues. However, the possibility of genetic predispositions to pathology should also be considered.

RdlDv, a novel GABA-gated chloride channel gene from the American dog tick Dermacentor variabilis

The American dog tick Dermacentor variabilis is a major transmitter of bacterial and viral pathogens in human and animal populations, and compounds active against this species would benefit both human and animal health. Invertebrate GABA-gated chloride channels are validated targets of commonly used insecticides and acaricides. We cloned a novel member of the invertebrate GABA-gated chloride channel gene family from Dermacentor variabilis, RdlDv. The closest homologue of the predicted gene product of RdlDv is the RDL protein encoded by the GABA-gated chloride channel gene Drosophila Rdl (Resistance to Dieldrin), with which it shares 64% amino acid identity. When expressed in Xenopus oocytes, RdlDv produces GABA-activated currents blocked by the known insecticides and RDL antagonists fipronil and picrotoxinin. These results suggest that RdlDv encodes a GABA-gated chloride channel subunit, making it a potential target for compounds active against the tick D. variabilis.

Ivermectin

Ivermectin is a potent antiparasitic drug and the first commercially available member of a new class of drugs (macrocyclic lactones) that has been approved for human use. Ivermectin has already proven to be highly effective in the elimination of river blindness as a public health burden. Side effects have been minor, and patient acceptance is good. Promising results in off-label applications for ectoparasitic infestations are increasingly important as resistance to topical therapy becomes more prevalent Ivermectin represents an advance in the therapeutic armamentarium and should be considered in appropriate cases.

Molecular approaches to the study of myiasis-causing larvae

Among arthropod diseases affecting animals, larval infections - myiases - of domestic and wild animals have been considered important since ancient times. Besides the significant economic losses to livestock worldwide, myiasis-causing larvae have attracted the attention of scientists because some parasitise humans and are of interest in forensic entomology. In the past two decades, the biology, epidemiology, immunology, immunodiagnosis and control methods of myiasis-causing larvae have been focused on and more recently the number of molecular studies have also begun to increase. The 'new technologies' (i.e. molecular biology) are being used to study taxonomy, phylogenesis, molecular identification, diagnosis (recombinant antigens) and vaccination strategies. In particular, more in depth molecular studies have now been performed on Sarcophagidae, Calliphoridae and flies of the Oestridae sister group. This review discusses the most topical issues and recent studies on myiasis-causing larvae using molecular approaches. In the first part, PCR-based techniques and the genes that have already been analysed, or are potentially useful for the molecular phylogenesis and identification of myiasis-causing larvae, are described. The second section deals with the more recent advances concerning taxonomy, phylogenetics, population studies, molecular identification, diagnosis and vaccination.

Novel animal-health drug targets from ligand-gated chloride channels

The world's three best-selling veterinary antiparasitic drugs ('parasiticides') act on ligand-gated ion channels. The sequencing of the complete genomes of the invertebrate genetic model organisms Caenorhabditis elegans and Drosophila melanogaster has led to the recent cloning of new subunits of 5-hydroxytryptamine-gated and histamine-gated chloride channels. Together with L-glutamate-gated chloride channels, which are important targets of known parasiticides, and acetylcholine-gated chloride channels, these new classes of ligand-gated chloride channels, which are known only from invertebrates, add to our understanding of inhibitory neural signalling. They could offer the prospect of being targets for a new generation of selective drugs to control nematode and insect parasites.

Ivermectin, an unconventional agonist of the glycine receptor chloride channel

The effects of the antihelmintic, ivermectin, were investigated in recombinantly expressed human alpha(1) homomeric and alpha(1)beta heteromeric glycine receptors (GlyRs). At low (0.03 microm) concentrations ivermectin potentiated the response to sub-saturating glycine concentrations, and at higher (> or =0.03 microm) concentrations it irreversibly activated both alpha(1) homomeric and alpha(1)beta heteromeric GlyRs. Relative to glycine-gated currents, ivermectin-gated currents exhibited a dramatically reduced sensitivity to inhibition by strychnine, picrotoxin, and zinc. The insensitivity to strychnine could not be explained by ivermectin preventing the access of strychnine to its binding site. Furthermore, the elimination of a known glycine- and strychnine-binding site by site-directed mutagenesis had little effect on ivermectin sensitivity, demonstrating that the ivermectin- and glycine-binding sites were not identical. Ivermectin strongly and irreversibly activated a fast-desensitizing mutant GlyR after it had been completely desensitized by a saturating concentration of glycine. Finally, a mutation known to impair dramatically the glycine signal transduction mechanism had little effect on the apparent affinity or efficacy of ivermectin. Together, these findings indicate that ivermectin activates the GlyR by a novel mechanism.