A point mutation in the β-adrenergic-like octopamine receptor: possible association with amitraz resistance

Amitraz is a unique formamidine-class acaricide/insecticide that effectively controls ticks, mites, and insect pests. However, the recent emergence of amitraz-resistant cattle ticks is a serious problem that requires an urgent solution. A nonsynonymous single nucleotide polymorphism (A181T) leading to an amino acid substitution (I61F) in the β-adrenergic-like (β-AL) octopamine receptor (OAR) of amitraz-resistant southern cattle ticks (Rhipicephalus microplus) (RmβAOR) was proposed to be a cause of the amitraz resistance. However, it remains unclear whether this substitution exerts any functional effect on the action of amitraz. To make this clear, the functional role of this mutation was examined using an orthologous OAR (BmOAR2) from the silkworm (Bombyx mori).

Both amitraz and its metabolite N² -(2,4-dimethylphenyl)-N¹ -methyformamidine (DPMF) elevated intracellular cyclic AMP levels as orthosteric OAR agonists in HEK-293 cells stably expressing BmOAR2. The I45F mutant of BmOAR2 (equivalent to I61F in RmβAOR) was generated and tested for its sensitivity to amitraz and DPMF. The assay result showed that the I45F mutation reduces the potency of DPMF to a level similar to that of the endogenous agonist (R)-OA in wild-type BmOAR2.

The amino acid substitution found in the first transmembrane segment of RmβAOR most likely causes target-site insensitivity to DPMF, which might contribute to the resistance of R. microplus to amitraz. This needs to be further confirmed using RmβAOR. © 2020 Society of Chemical Industry.