Species specific RNA A-to-I editing of mosquito RDL modulates GABA potency and influences agonistic, potentiating and antagonistic actions of ivermectin.

The insect GABA receptor, RDL, is the target of several classes of pesticides. The peptide sequences of RDL are generally highly conserved between diverse insects. However, RNA A-to-I editing can effectively alter amino acid residues of RDL in a species specific manner, which can affect the potency of GABA and possibly insecticides. We report here that RNA A-to-I editing alters the gene products of Rdl in three mosquito disease vectors, recoding five amino acid residues in RDL of Aedes aegypti and six residues in RDLs of Anopheles gambiae and Culex pipiens, which is the highest extent of editing in RDL observed to date. Analysis of An. gambiae Rdl cDNA sequences identified 24 editing isoforms demonstrating a considerable increase in gene product diversity. RNA editing influenced the potency of the neurotransmitter, GABA, on An. gambiae RDL editing isoforms expressed in Xenopus laevis oocytes, as demonstrated by EC50s ranging from 5 ± 1 to 246 ± 41 µM. Fipronil showed similar potency on different editing isoforms, with IC50s ranging from 0.18 ± 0.08 to 0.43 ± 0.09 µM. In contrast, editing of An. gambiae RDL affected the activating, potentiating and inhibiting actions of ivermectin. For example, ivermectin potentiated currents induced by GABA at the EC20 concentration in the unedited isoform but not in the fully edited variant. Editing of a residue in the first transmembrane domain or the cys-loop influenced this potentiation, highlighting residues involved in the allosteric mechanisms of cys-loop ligand-gated ion channels. Understanding the interactions of ivermectin with molecular targets may have relevance to mosquito control in areas where people are administered with ivermectin to treat parasitic diseases.

A P-glycoprotein gene serves as a component of the protective mechanisms against 2-tridecanone and abamectin in Helicoverpa armigera.

P-glycoprotein (P-gp) exists in animals, fungi and bacteria and likely evolved as a defense mechanism against harmful substances. Here a cDNA (4054bp) encoding a putative P-glycoprotein gene from Helicoverpa armigera was cloned and named HaPgp1. This putative HaPgp1 sequence encoded a protein of 1253 amino acids with a molecular mass of approximately 137kDa. qPCR analyses demonstrated that the expression of HaPgp1 was significantly higher in 4th instar larvae when compared to other developmental stages. HaPgp1 transcripts were more abundant in the head and fat bodies than in other tissues. Compared with the control, the expression of HaPgp1 reach a peak at 12h after the treatment by 2-tridecanone in all tissues. However, the expression of HaPgp1 increased from 12h to 48h after treatment with abamectin in all tissues. Immunohistochemistry analyses also verified that 2-tridecanone and abamectin can induce the increase of HaPgp1 expression. RNAi of HaPgp1 significantly raised the mortality rate of larvae treated by 2-tridecanone and abamectin, as compared to control larvae fed with GFP dsRNA. These results illustrate the possible involvement of HaPgp1 as a component of the protective mechanisms to plant secondary chemicals such as 2-tridecanone and to certain classes of insecticides, like abamectin.

AvaR2, a pseudo γ-butyrolactone receptor homologue from Streptomyces avermitilis, is a pleiotropic repressor of avermectin and avenolide biosynthesis and cell growth.

Avermectins produced by Streptomyces avermitilis are effective anthelmintic agents. The autoregulatory signalling molecule that triggers avermectin biosynthesis is a novel butenolide-type molecule, avenolide, rather than common γ-butyrolactones (GBLs). We identified AvaR2, a pseudo GBL receptor homologue, as an important repressor of avermectin and avenolide biosynthesis and cell growth. AvaR2 directly repressed transcription of aveR (the ave cluster-situated activator gene), aco (a key gene for avenolide biosynthesis), its own gene (avaR2) and two other GBL receptor homologous genes (avaR1 and avaR3) by binding to their promoter regions. The aveR promoter had the highest affinity for AvaR2. A consensus 18 bp ARE (autoregulatory element)-like sequence was found in the AvaR2-binding regions of these five target genes. Eleven novel AvaR2 targets were identified, including genes involved in primary metabolism, ribosomal protein synthesis, and stress responses. AvaR2 bound and responded to endogenous avenolide and exogenous antibiotics jadomycin B (JadB) and aminoglycosides to modulate its DNA-binding activity. Our findings help to clarify the roles of pseudo GBL receptors as pleiotropic regulators and as receptors for new type autoregulator and exogenous antibiotic signal. A pseudo GBL receptor-mediated antibiotic signalling transduction system may be a common strategy that facilitates Streptomyces interspecies communication and survival in complex environments.

Ophiobolin M and analogues, noncompetitive inhibitors of ivermectin binding with nematocidal activity.

A series of ophiobolins were isolated from a fungal extract based on their nematocidal activity. These compounds are non-competitive inhibitors of ivermectin binding to membranes prepared from the free-living nematode, Caenorhabditis elegans, with an inhibition constant of 15 microM. The ophiobolins which were most potent in the biological assays, ophiobolin C and ophiobolin M, were also the most potent compounds when evaluated in a C. elegans motility assay. These data suggest that the nematocidal activity of the ophiobolins is mediated via an interaction with the ivermectin binding site. The isolation, structure and biological activity of ophiobolins have been described.

Clinical signs of ivermectin toxicity and the efficacy of antigabaergic convulsants as antidotes for ivermectin poisoning in epileptic chickens.

The clinical signs of ivermectin toxicity were determined in 6 groups of 10 epileptic and 8 non-epileptic chickens for 72 h after dosing with sc injections of 5.0, 7.5, 10.0, 12.5 or 15.0 mg ivermectin/kg bw. At the 5.0 mg/kg dose, mild diarrhea developed 4 h post-dosing and lasted until the end of the 72-h monitoring period. With higher doses of ivermectin body weight, egg production and feed and water consumption were markedly reduced. Severe diarrhea, mydriasis, bradypnea, ataxia, sedation, coma and death occurred with the highest dose of ivermectin. No differences in the signs of ivermectin toxicity were observed between epileptic and non-epileptic chickens. To assess the efficacy of the antiGABAergic convulsants, methyl-beta carboline-carboxylate (beta-CCM), picrotoxin and pentylenetetrazol (PTZ), as antidotes for ivermectin toxicity, 8 epileptic and 6 non-epileptic chickens/treatment group were given dosages of each convulsant which previously induced convulsions in 50% (ED50) and again in 100% (ED100) of treated chickens. These convulsants were given 6 h after dosing with 15.0 mg ivermectin/kg. The ED100 dosages of picrotoxin and PTZ alleviated mydriasis and sedation, but did not reduce the diarrhea. The ED50 dose convulsants were not effective in reducing or alleviating ivermectin toxicity, nor was alleviation of any sign of ivermectin toxicity obtained with any dosage of beta-CCM. Although the dosages of these antiGABAergic convulsants used normally produced convulsions in epileptic and non-epileptic chickens, no convulsions were observed in chickens with ivermectin toxicity.(ABSTRACT TRUNCATED AT 250 WORDS)