Predictive 3D modelling of the interactions of pyrethroids with the voltage-gated sodium channels of ticks and mites.

BACKGROUND: The pyrethroid insecticides are a very successful group of compounds that target invertebrate voltage-gated sodium channels and are widely used in the control of insects, ticks and mites. It is well established that some pyrethroids are good insecticides whereas others are more effective as acaricides. This species specificity is advantageous for controlling particular pest(s) in the presence of another non-target invertebrate, for example controlling the Varroa mite in honeybee colonies. RESULTS: We applied in silico techniques to compare the voltage-gated sodium channels of insects versus ticks and mites and their interactions with a range of pyrethroids and DDT analogues. We identified a single amino acid difference within the pyrethroid binding pocket of ticks/mites that may have significant impact on the effectiveness of pyrethroids as acaricides. Other individual amino acid differences within the binding pocket in distinct tick and mite species may provide a basis for future acaricidal selectivity. CONCLUSIONS: Three-dimensional modelling of the pyrethroid/DDT receptor site has led to a new hypothesis to explain the preferential binding of acaricidal pyrethroids to the sodium channels of ticks/mites. This is important for understanding pyrethroid selectivity and the potential effects of mutations that can give rise to resistance to pyrethroids in commercially-important pest species.

Association of neonicotinoid insensitivity with a conserved residue in the loop d binding region of the tick nicotinic acetylcholine receptor.

Neonicotinoid insecticides target nicotinic acetylcholine receptors (nAChR) in the nervous system of insects but are largely ineffective against ticks. This study aimed to identify the molecular basis for this insensitivity. A homology model of the nAChR binding domain was generated on the basis of the crystal structure of an acetylcholine-binding protein with the insecticide imidacloprid bound. We hypothesized that tick ß-subunits would differ at a critical residue (Arg81) in their D loops. To test this, we sequenced nAChR genes from five tick species and found that instead of the conserved arginine found in insects, a glutamine was present in all the tick sequences.

The synergistic action of imidacloprid and flumethrin and their release kinetics from collars applied for ectoparasite control in dogs and cats.

BACKGROUND: The control of tick and flea burdens in dogs and cats has become essential to the control of important and emerging vector borne diseases, some of which are zoonoses. Flea worry and flea bite hypersensitivity are additionally a significant disease entity in dogs and cats. Owner compliance in maintaining the pressure of control measures has been shown to be poor. For these reasons efforts are continuously being made to develop ectoparasiticides and application methods that are safe, effective and easy to apply for pet owners. A new polymer matrix collar has recently been developed which is registered for 8 months use in cats and dogs. The basic properties of this collar have been investigated in several in vitro and in vivo studies. METHODS: The effects of imidacloprid, flumethrin and the combination were evaluated in vitro by means of whole cell voltage clamp measurement experiments conducted on isolated neuron cells from Spodoptera frugiperda. The in vitro efficacy of the two compounds and the combination against three species of ticks and their life stages and fleas were evaluated in a dry surface glass vial assay. The kinetics of the compounds over time in the collar were evaluated by the change in mass of the collar and measurement of the surface concentrations and concentrations of the actives in the collar matrix by HPLC. Hair clipped from collar treated dogs and cats, collected at various time points, was used to assess the acaricidal efficacy of the actives ex vivo. RESULTS: An in vitro isolated insect nerve model demonstrated the synergistic neurotoxic effects of the pyrethroid flumethrin and the neonicotinoid imidacloprid. An in vitro glass vial efficacy and mortality study against various life stages of the ticks Ixodes ricinus, Rhipicephalus sanguineus and Dermacentor reticulatus and against the flea (Ctenocephalides felis) demonstrated that the combination of these products was highly effective against these parasites. The release kinetics of these actives from a neck collar (compounded with 10% imidacloprid and 4.5% flumethrin) was extensively studied in dogs and cats under laboratory and field conditions. Acaricidal concentrations of the actives were found to be consistently released from the collar matrix for 8 months. None of the collar studies in dogs or cats were associated with any significant collar related adverse event. CONCLUSION: Here we demonstrated the synergism between the pyrethroid flumethrin and the neonicotinoid imidacloprid, both provided in therapeutically relevant doses by a slow release collar matrix system over 8 months. This collar is therefore a convenient and safe tool for a long-term protection against ectoparasites.

Verticilide, a new ryanodine-binding inhibitor, produced by Verticillium sp. FKI-1033.

A new ryanodine-binding inhibitor, verticilide, was isolated from the cultured broth of a fungus, Verticillium sp. FKI-1033. It is a 24-membered ring cyclic depsipeptide, its structure being elucidated as cyclo[(2R)-2-hydroxyheptanoyl-N-methyl- L-alanyl](4). Verticilide inhibited ryanodine binding to ryanodine receptors in the cockroach at an IC(50) value of 4.2 microM, whereas inhibition against mouse ryanodine receptors was weak (IC(50)=53.9 microM).

Molecular characterisation of nicotinic acetylcholine receptor subunits from the cat flea, Ctenocephalides felis (Siphonaptera: Pulicidae).

As part of a program to monitor the susceptibility of cat flea populations to the insecticide imidacloprid we have examined the cat flea nicotinic acetylcholine receptor, the target site protein of the neonicotinoid group of insecticides. Seven nAChR subunits (six alpha-type and one beta-type) were identified in cat flea using a degenerate PCR-based strategy. Five of these were expressed in vitro by creating chimeras containing the N-terminal ligand-binding domain of the cat flea subunits and the C-terminal region of the Drosophila Dalpha2 (SAD) subunit. Two of the five chimeric subunits, Cfalpha1/Dalpha2 and Cfalpha3/Dalpha2, when co-expressed with rat beta2 in Drosophila S2 cells, showed high-affinity binding of both epibatidine (Kd=1.6+/-0.6 and 0.13+/-0.06nM, respectively), and imidacloprid (Ki=142+/-34 and 28.7+/-2.4nM, respectively). It is likely therefore that Cfalpha1 and Cfalpha3 contribute to nAChR populations in vivo that are sensitive to imidacloprid. The identification of cat flea nAChR subunits that have a high affinity for imidacloprid presents candidate genes in which to look for resistance-associated mutations if target-site resistance to imidacloprid arises in domestic pet flea populations.

Identification of the Rdl mutation in laboratory and field strains of the cat flea, Ctenocephalides felis (Siphonaptera: Pulicidae).

In many insect species, resistance to cyclodiene insecticides is caused by amino acid substitutions at a single residue (A302) within the M2 transmembrane region of the gamma-aminobutyric acid (GABA) receptor sub-unit termed Rdl (resistance to dieldrin). These mutations (A302S and A302G) have also been shown to confer varying levels of cross-resistance to fipronil, a phenylpyrazole insecticide with a similar mode of action to cyclodienes. To investigate the possible occurrence of these mutations in the cat flea, Ctenocephalides felis (Bouché), a 176-bp fragment of the cat flea Rdl gene, encompassing the mutation site, was PCR amplified and sequenced from nine laboratory flea strains. The A302S mutation was found in eight of the nine strains analysed, although the relative frequency of the mutant allele varied between strains. Only one strain (R6) was found to be homozygous for the S302 allele in all the individuals tested, and this correlated with previous reports of low-level fipronil resistance in this strain. A PCR-based diagnostic assay, capable of screening individual fleas for this mutation, was developed and used to survey a range of fleas collected at random from veterinary clinics in the UK and USA. The A302S mutation was present at a high frequency in these domestic pet populations.

Identification of mutations associated with pyrethroid resistance in the para-type sodium channel of the cat flea, Ctenocephalides felis.

Knockdown resistance (kdr) to pyrethroid insecticides is caused by point mutations in the pyrethroid target site, the para-type sodium channel of nerve membranes. This most commonly involves alterations within the domain II (S4-S6) region of the channel protein where five different mutation sites have been identified across a range of insect species. To investigate the incidence of this mechanism in cat fleas, we have cloned and sequenced the IIS4-IIS6 region of the para sodium channel gene from seven laboratory flea strains. Analysis of these sequences revealed two amino acid replacements at residues previously implicated in pyrethroid resistance. One is the 'common' kdr mutation, a leucine to phenylalanine substitution (equivalent to L1014F of housefly) reported previously in several other insects. The other is a threonine to valine substitution (equivalent to T929V) and is a novel variant of the T929I mutation first identified in diamondback moth. The L1014F mutation was found at varying frequency in all of the laboratory flea strains, whereas the T929V mutation was found only in the highly resistant Cottontail strain. We have developed rapid PCR-based diagnostic assays for the detection of these mutations in individual cat fleas and used them to show that both L1014F and T929V are common in UK and US flea populations. This survey revealed a significant number of fleas that carry only the V929 allele indicating that co-expression with the F1014 allele is not necessary for flea viability.

Synthesis and biological evaluation of novel 4"-alkoxy avermectin derivatives.

Novel 4"-alkoxy avermectin derivatives were synthesized via rhodium carbenoid mediated O-H insertion reaction and tested for antiparasite activity against Artemia salina and Caenorhabditis elegans.

Synthesis and biological activities of novel 4"-alkylidene avermectin derivatives.

Horner-Emmons reaction of 4"-dehydro-5-O-TBDMS-avermectin B(1a) with a variety of phosphorus ylides using LHMDS gave novel 4"-alkylidene avermectin derivatives in high yields. Further modifications led to derivatives bearing diverse functional groups. The new avermectin derivatives showed potent growth inhibitory activity against Artemia salina and Caenorhabditis elegans.

High-resolution structures of a chitinase complexed with natural product cyclopentapeptide inhibitors: mimicry of carbohydrate substrate.

Over the past years, family 18 chitinases have been validated as potential targets for the design of drugs against human pathogens that contain or interact with chitin during their normal life cycles. Thus far, only one potent chitinase inhibitor has been described in detail, the pseudotrisaccharide allosamidin. Recently, however, two potent natural-product cyclopentapeptide chitinase inhibitors, argifin and argadin, were reported. Here, we describe high-resolution crystal structures that reveal the details of the interactions of these cyclopeptides with a family 18 chitinase. The structures are examples of complexes of a carbohydrate-processing enzyme with high-affinity peptide-based inhibitors and show in detail how the peptide backbone and side chains mimic the interactions of the enzyme with chitooligosaccharides. Together with enzymological characterization, the structures explain why argadin shows an order of magnitude stronger inhibition than allosamidin, whereas argifin shows weaker inhibition. The peptides bind to the chitinase in remarkably different ways, which may explain the differences in inhibition constants. The two complexes provide a basis for structure-based design of potent chitinase inhibitors, accessible by standard peptide chemistry.

Stage-specific expression of the chitin synthase DmeChSA and DmeChSB genes during the onset of Drosophila metamorphosis.

Chitin, the major structural polysaccharide of arthropods, is an important constituent of the insect extracellular structures, cuticle and gut peritrophic matrix. Synthesis of cuticular chitin is strictly coordinated with the ecdysone-regulated molting cycle of insect development (the term "ecdysone" is used in this paper instead of "ecdysteroids" since the exact ratio of various hormonal forms changes during metamorphosis). Based on observed similarities between the fungal chitin synthases and other processive beta-glycosyltransferases, we have identified the first insect chitin synthase genes, DmeChSA and DmeChSB (Database accession numbers: EMBL/GenBank/DDBJ A83122, A83126, AJ309488, AJ309489), from Drosophila melanogaster. Chromosomal localization has identified these genes close to and on either side of the centromere of the third chromosome. Partial cDNA clones of both genes have been isolated from a pupal cDNA library. To obtain the first insight into the transcriptional regulation of chitin synthesis, we have monitored the expression of DmeChSA and DmeChSB during the periods of the late-larval and prepupal ecdysone pulses that direct metamorphosis. Transcripts of either gene are barely detected prior to and during the late-larval ecdysone pulse. Once the late-larval ecdysone pulse is ceased completely, both DmeChSA and DmeChSB genes show a remarkable up-regulation.