delta2-Avermectin B1a

Avermectin B1a (AVM B1a), a widely used insecticide and acaricide, is reported to both activate and inhibit gamma-aminobutyric acidA (GABA(A)) receptor function in mammalian brain. This study attempts to resolve these seemingly contradictory results by examining the binding properties of AVM B1a and its effects on the GABA-gated chloride channel with primary cultures of rat cerebellar granule neurons as a model system. Specific binding of [3H]AVM B1a in intact neuron cultures is time- and concentration-dependent and is displaceable by AVM analogs. Scatchard analysis of [3H]AVM B1a binding reveals high- and low-affinity sites with K(D) values of 5 and 815 nM, respectively. AVM B1a alters the binding of [3H]ethynylbicycloorthobenzoate at the noncompetitive blocker site in a biphasic manner; activation is evident with 10 to 300 nM AVM B1a after 5 to 10 min incubation and inhibition with an IC50 of 866 nM after 60 min incubation. Consistent with this observation, 36Cl- influx is stimulated by AVM B1a at 3 to 100 nM and inhibited at 1 to 3 microM. GABA-stimulated 36Cl- influx is completely blocked by both [3H]ethynylbicycloorthobenzoate and 12-ketoendrin (two GABA-gated chloride channel blockers) and AVM B1a at 1 to 1.5 microM. Also, 36Cl- influx induced by AVM B1a at 10 nM is suppressed by the two channel blockers. Thus, AVM B1a binds to two different sites in the GABA-gated chloride channel with dual effects, i.e., activating the channel on binding to the high-affinity site and blocking it on further binding to the low-affinity site.

Avermectin B1a stimulates high-affinity binding of [3H]-gamma-aminobutyric acid (GABA) to receptors in washed rat brain membranes. Scatchard analysis of the data indicates that the drug does not significantly alter the apparent dissociation constant of GABA binding, but increases the detectable number of binding sites from 3.2 to 5.1 pmol/mg protein, (+)-Bicuculline completely blocks control and avermectin B1a-stimulated GABA binding, whereas picrotoxin antagonizes specifically the avermectin B1a-stimulated GABA binding. The avermectin B1a-stimulated GABA binding is also chloride ion-dependent, whereas GABA binding in the control is not. These observations suggest that the mechanism of avermectin B1a stimulation of GABA binding may involve the chloride ion channel.

Avermectins (AVEs) are economically potent anthelmintic agents produced by Streptomyces avermitilis. Among eight AVE components, B1a exhibits the highest insecticidal activity. The purpose of this study was to enhance B1a production, particularly in the high-yielding industrial strain A229, by a combination strategy involving the following steps. (i) aveC gene was engineered to increase B1a:B2a ratio. Three aveC variants (aveC2m, aveC5m, and aveC8m, respectively encoding two, five, and eight amino acid mutations) were synthesized by fusion PCR. B1a:B2a ratio in A229 derivative having kasOp*-controlled aveC8m reached 1.33 (B1a and B2a titers were 8120 and 6124 μg/mL). Corresponding values in A229 were 0.99 and 6447 and 6480 μg/mL. (ii) β-oxidation pathway genes fadD and fadAB were overexpressed in wild-type (WT) strain and A229 to increase supply of acyl-CoA precursors for AVE production. The resulting strains all showed increased B1a titer. Co-overexpression of pkn5p-driven fadD and fadAB in A229 led to B1a titer of 8537 μg/mL. (iii) Genes bicA and ecaA involved in cyanobacterial CO2-concentrating mechanism (CCM) were introduced into WT and A229 to enhance carboxylation velocity of acetyl-CoA and propionyl-CoA carboxylases, leading to increased supply of malonyl- and methylmalonyl-CoA precursors and increased B1a titer. Co-expression of bicA and ecaA in A229 led to B1a titer of 8083 μg/mL. (iv) aveC8m, fadD-fadAB, and bicA-ecaA were co-overexpressed in A229, resulting in maximal B1a titer (9613 μg/mL; 49.1% increase relative to A229). Our findings demonstrate that the combination strategy we provided here is an efficient approach for improving B1a production in industrial strains.Key points· aveC mutation increased avermectin B1a:B2a ratio and B1a titer.· Higher levels of acyl-CoA precursors contributed to enhanced B1a production.· B1a titer in an industrial strain was increased by 49.1% via a combination strategy.