An atypical residue in the pore of Varroa destructor GABA-activated RDL receptors affects picrotoxin block and thymol modulation

• Published in: Insect biochemistry and molecular biology Vol. 55; pp. 19 - 25
• Main Authors: Price, Kerry L., Lummis, Sarah C.R.
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01-12-2014; Elsevier Science
• Subjects: Amino Acid Sequence; Animals; Antagonist; Cys-loop; Drosophila; Drosophila Proteins - chemistry; Drosophila Proteins - genetics; Drosophila Proteins - metabolism; Female; GABA; Ligand-gated; Molecular Sequence Data; Patch-Clamp Techniques; Picrotoxin - antagonists & inhibitors; Receptors, GABA-A - chemistry; Receptors, GABA-A - genetics; Receptors, GABA-A - metabolism; Thymol - toxicity; Varroa; Varroa destructor; Varroidae - chemistry; Varroidae - metabolism; Xenopus laevis; Ligand-gated; ELIC; Drosophila; ACh; AChBP; nACh; Varroa; PTX; GABA; Antagonist; RDL; 5-HT; Cys-loop; GLIC
• ISSN: 0965-1748; 1879-0240
• DOI: 10.1016/j.ibmb.2014.10.002

GABA-activated RDL receptors are the insect equivalent of mammalian GABAA receptors, and play a vital role in neurotransmission and insecticide action. Here we clone the pore lining M2 region of the Varroa mite RDL receptor and show that it has 4 atypical residues when compared to M2 regions of most other insects, including bees, which are the major host of Varroa mites. We create mutant Drosophila RDL receptors containing these substitutions and characterise their effects on function. Using two electrode voltage clamp electrophysiology we show that one substitution (T6′M) ablates picrotoxin inhibition and increases the potency of GABA. This mutation also alters the effect of thymol, which enhances both insect and mammalian GABA responses, and is widely used as a miticide. Thymol decreases the GABA EC50 of WT receptors, enhancing responses, but in T6′M-containing receptors it is inhibitory. The other 3 atypical residues have no major effects on either the GABA EC50, the picrotoxin potency or the effect of thymol. In conclusion we show that the RDL 6′ residue is important for channel block, activation and modulation, and understanding its function also has the potential to prove useful in the design of Varroa-specific insecticidal agents.
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•The Varroa mite RDL receptor has 4 atypical M2 residues: -7′H, 6′M, 20′S and 21′Q.•A Drosophila T6′M RDL receptor has a decreased GABA EC50 and is resistant to picrotoxin block.•The T6′M mutation ablates thymol enhancement of RDL responses.•N-7′H, A20′S and A21′Q do not modify GABA, picrotoxin or thymol effects.•The data could be useful in the design of Varroa-specific insecticidal agents.