Cloning, recombinant expression and inhibitor profiles of dihydrofolate reductase from the Australian sheep blow fly, Lucilia cuprina

Molecular targets of insecticides and herbicides - Are there useful overlaps?

New insecticide modes of action are needed for insecticide resistance management strategies. The number of molecular targets of commercial herbicides and insecticides are fewer than 35 for both. Few commercial insecticide targets are found in plants, but ten targets of commercial herbicides are found in insects. For several of these commonly held targets, some compounds kill both plants and insects. For example, herbicidal inhibitors of p-hydroxyphenylpyruvate dioxygenase are effective insecticides on blood-fed insects. The glutamine synthetase-inhibiting herbicide glufosinate is insecticidal by the same mechanism of action, inhibition of glutamine synthetase. These and other examples of shared activities of commercial herbicides with insecticides through the same target site are discussed. Compounds with novel herbicide targets shared by insects that are not commercialized as pesticides (such as statins) are also discussed. Compounds that are both herbicidal and insecticidal can be used for insect pests not associated with crops or with crops made resistant to the compounds.

Reduction Chemistry of Natural Pyrethrins and Preliminary Insecticidal Activity of Reduced Pyrethrins

The natural extract pyrethrum is an insecticidal oil derived from Tanacetum cinerariifolium that is commonly used in domestic and agricultural pesticides. The major constituents of the extract are the Pyrethrins, six esters that provide pyrethrum with its insecticidal properties. These Pyrethrins readily degrade through several environmental means and as such, there can be significant Pyrethrin losses during processing and long-term storage of pyrethrum-based insecticides. This work attempts to alleviate the effect of these degradative processes through the pursuit of stabilised Pyrethrins by chemically removing oxidatively sensitive functionality. Several reduced Pyrethrin analogues were produced and a method to convert the more sensitive Pyrethrins present in the pyrethrum concentrate into their respective more stable jasmolin counterparts, as a mixture with the over-reduced tetrahydropyrethrins, was developed. All other reduction processes abolished insecticidal activity against Lucilia cuprina larvae, whereas some isomerised analogues showed comparable potency with the individual natural pyrethrin esters. This work has revealed new insights into the structure–activity relationships in this unique class of insecticide.

The Effects of Diets and Long-term Laboratory Rearing on Reproduction, Behavior, and Morphology of Lucilia cuprina (Diptera: Calliphoridae)

The Australian sheep blow fly, Lucilia cuprina (Wiedemann), is commonly reared in the laboratory for many sequential generations on simple, fixed diets, so it can be used in veterinary, medical, and forensic studies. To investigate the effect of diet and long-term laboratory rearing on L. cuprina, flies were fed with two different diets (sugar and milk-sugar) over a year and F1, F6, and F11 generations were used for comparisons based on the number of eggs, attraction to wool and liver, and wing size. The results showed that the number of eggs of gravid flies, and the attractiveness of wool and liver did not differ significantly between diets and generations, but gravid flies were more attracted to wool and liver than non-gravid flies (P < 0.05). Moreover, in the F1 generation, thorax length and wing aspect ratio were significantly longer than in the F6 and F11 generations (P < 0.05), and the wing length was significantly longer than in the F11 generation (P < 0.05). It was concluded that neither diet nor long-term laboratory rearing affect potential fecundity or the behavioral responses of L. cuprina, but the gravidity of flies affects their behavioral response, and long-term laboratory rearing significantly affects fly morphology, apparently explaining a loss in flight performance.

Molecular Characteristics and Serodiagnostic Potential of Dihydrofolate Reductase from Echinococcus granulosus

The larval stage of Echinococcus granulosus causes cystic echinococcosis (CE), a neglected tropical disease that leads to morbidity and mortality in humans and livestock worldwide. Here, we identified and characterized dihydrofolate reductase (Eg-DHFR) from E. granulosus, and evaluated its potential as a diagnostic antigen for sheep CE. Comparison between mammalian (host) DHFR and Eg-DHFR indicates that 45.7% of the 35 active site residues are different. Immunolocalisation analysis showed that native Eg-DHFR was widely distributed in all life-cycle stages of E. granulosus. Recombinant Eg-DHFR (rEg-DHFR) showed typical DHFR enzymatic parameters towards substrate, and was very sensitive to inhibition by methotrexate (IC₅₀ = 27.75 ± 1.03 nM) and aminopterin (IC₅₀ = 63.67 ± 6.76 nM). However, inhibition of DHFR exhibited little protoscolicidal effect in vitro. As there is no reliable method to monitor sheep CE, the immunogenicity of rEg-DHFR was detected, and we developed an indirect ELISA (iELISA) for CE serodiagnosis. The iELISA exhibited diagnostic specificity of 89.58%, diagnostic sensitivity of 95.83%, and the diagnostic accuracy was 91.67% compared with necropsy. Cross-reactivity assay showed analytical specificity of 85.7%. These suggest that rEg-DHFR is an effective antigen for the diagnosis of sheep CE.

Insecticidal activities of histone deacetylase inhibitors against a dipteran parasite of sheep, Lucilia cuprina

Histone deacetylase inhibitors (HDACi) are being investigated for the control of various human parasites. Here we investigate their potential as insecticides for the control of a major ecto-parasite of sheep, the Australian sheep blowfly, Lucilia cuprina. We assessed the ability of HDACi from various chemical classes to inhibit the development of blowfly larvae in vitro, and to inhibit HDAC activity in nuclear protein extracts prepared from blowfly eggs. The HDACi prodrug romidepsin, a cyclic depsipeptide that forms a thiolate, was the most potent inhibitor of larval growth, with equivalent or greater potency than three commercial blowfly insecticides. Other HDACi with potent activity were hydroxamic acids (trichostatin, CUDC-907, AR-42), a thioester (KD5170), a disulphide (Psammaplin A), and a cyclic tetrapeptide bearing a ketone (apicidin). On the other hand, no insecticidal activity was observed for certain other hydroxamic acids, fatty acids, and the sesquiterpene lactone parthenolide. The structural diversity of the 31 hydroxamic acids examined here revealed some structural requirements for insecticidal activity; for example, among compounds with flexible linear zinc-binding extensions, greater potency was observed in the presence of branched capping groups that likely make multiple interactions with the blowfly HDAC enzymes. The insecticidal activity correlated with inhibition of HDAC activity in blowfly nuclear protein extracts, indicating that the toxicity was most likely due to inhibition of HDAC enzymes in the blowfly larvae. The inhibitor potencies against blowfly larvae are different from inhibition of human HDACs, suggesting some selectivity for human over blowfly HDACs, and a potential for developing compounds with the inverse selectivity. In summary, these novel findings support blowfly HDAC enzymes as new targets for blowfly control, and point to development of HDAC inhibitors as a promising new class of insecticides.

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We measured the insecticidal effects of histone deacetylase inhibitors against the sheep blowfly.

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Insecticidal activity correlated with inhibition of HDAC enzyme activity in nuclear extracts.

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Romidepsin showed equivalent or greater potency than commercial blowfly insecticides.

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Some insights gained into structural requirements for insecticidal HDAC inhibitors.

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Potential for HDAC inhibitors as insecticides.

Histone deacetylase enzymes as drug targets for the control of the sheep blowfly, Lucilia cuprina

The Australian sheep blowfly, Lucilia cuprina, is an ecto-parasite that causes significant economic losses in the sheep industry. Emerging resistance to insecticides used to protect sheep from this parasite is driving the search for new drugs that act via different mechanisms. Inhibitors of histone deacetylases (HDACs), enzymes essential for regulating eukaryotic gene transcription, are prospective new insecticides based on their capacity to kill human parasites. The blowfly genome was found here to contain five HDAC genes corresponding to human HDACs 1, 3, 4, 6 and 11. The catalytic domains of blowfly HDACs 1 and 3 have high sequence identity with corresponding human and other Dipteran insect HDACs (Musca domestica and Drosophila melanogaster). On the other hand, HDACs 4, 6 and 11 from the blowfly and the other Dipteran species showed up to 53% difference in catalytic domain amino acids from corresponding human sequences, suggesting the possibility of developing HDAC inhibitors specific for insects as desired for a commercial insecticide. Differences in transcription patterns for different blowfly HDACs through the life cycle, and between the sexes of adult flies, suggest different functions in regulating gene transcription within this organism and possibly different vulnerabilities. Data that supports HDACs as possible new insecticide targets is the finding that trichostatin A and suberoylanilide hydroxamic acid retarded growth of early instar blowfly larvae in vitro, and reduced the pupation rate. Trichostatin A was 8-fold less potent than the commercial insecticide cyromazine in inhibiting larval growth. Our results support further development of inhibitors of blowfly HDACs with selectivity over human and other mammalian HDACs as a new class of prospective insecticides for sheep blowfly.

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Lucilia cuprina genome contains five histone deacetylase genes.

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Significant amino acid differences between insect and human HDACs 4,6 and 11.

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Trichostatin highly toxic towards blowfly larvae.

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Potential for HDAC inhibitors as insecticides.