Acetylcholinesterase 1 of Rhipicephalus (Boophilus) annulatus (Say) [Acari: Ixodidae]

The cattle fever ticks Rhipicephalus microplus (Canestrini, 1888) and Rhipicephalus annulatus (Say, 1821) transmit pathogens responsible for bovine babesiosis and anaplasmosis. Although eradicated from the United States, these ticks and the pathogens they transmit are endemic in Mexico and other countries, and pose a serious risk to the U.S. cattle industry. Cattle imported into the U.S. are inspected and treated with an acaricide, coumaphos, to prevent tick reentry into the U.S. Coumaphos is an organophosphate (OP) that kills ticks by inactivating acetylcholinesterase (AChE), an enzyme vital to the tick central nervous system. Previous research has extensively studied the AChEs of R. microplus, however, there has been much less study in R. annulatus. The present work reports the cloning, sequencing and baculoviral expression of recombinant AChE1s of R. annulatus, including the discovery of three different RannAChE1 isoforms that are produced by alternative splicing of a single locus. Similar AChE kinetic activities were observed for the three isoforms, and these were comparable to AChE1 of R. microplus. Further, incorporation of a G170S substitution, homologous to the G119S mutation in mosquitoes that affords resistance to OP inhibition, resulted in reduced sensitivity of RannAChE1-1 to inhibition by oxon forms of OPs. This underscores the complexity of AChEs in cattle fever ticks and provides further evidence of the key role of cholinergic systems in tick physiology and development.