Can neuropeptide GPCRs be useful acaricide targets?: Endocrinology and the road to validate the kinin receptor from tick Rhipicephalus microplus

Rhipicephalus (Boophilus) microplus (Acari: Ixodidae) is the most important ectoparasite of cattle and a vector for the important disease babesiosis in cattle worldwide. Though eradicated from the United States, there is a continuous threat of reintroduction from Mexican pest populations. There is a constant effort to look for new pest control agents. The kinin receptor, a G-protein coupled receptor involved in water balance, was cloned and functionally expressed in Chinese Hamster Ovary (CHO-K1) cells and is being investigated as a potential target for novel pesticide development. The tick kinin receptor is activated by °¥insect kinins°¦. The insect kinins share the evolutionarily conserved C-terminal pentapeptide motif Phe1-X(1)-X(2)-Trp4-Gly-NH(2), where X(1)=His, Asn, Ser, or Tyr and X(2)=Ser, Pro, or Ala and regulate diuresis in many insects. Using bioluminescence plate assay, we found that position Phe1 and Trp4 are the most critical residues for receptor ligand activity. Insect kinins, however, are prone to enzymatic degradation by tissue bound peptidases. To enhance resistance to peptidases pseudopeptide analogs of the insect kinins incorporating ƒÒ-amino acid or bulky £\,£\-disubstituted amino acids in positions adjacent to both primary and secondary peptidase hydrolysis sites were synthesized. Single-replacement analog ƒÒ2Trp4 and double-replacement analog [ƒÒ3Phe2, ƒÒ3Pro3] of the insect kinins proved to be selective agonists for the tick receptor. One double ƒÑ-aminoisobutyric acid analog [Aib]FF[Aib]WGa was found to be five times more potent than the control agonist, FFFSWGa. These analogs represent potential leads in the development of selective, environmentally friendly arthropod pest control agents capable of disrupting critical processes.