Molecular biology of tick acetylcholinesterases

Ticks vector many pathogens with major health and economic impacts and have developed resistance to most acaricides used for tick control. Organophosphate (OP) acaricides target acetylcholinesterase (AChE) critical to tick central nervous system function. Mutations producing tick AChEs resistant to OPs were characterized; but tick OP-resistance is not fully elucidated, due to remarkable complexity of tick cholinergic systems. Three paralogous tick AChEs exhibiting differences in primary structure and biochemical kinetics are encoded by amplified genes with developmentally regulated expression. Gene silencing data suggest tick AChEs are functional complements in vivo, and transcriptomic and genomic data suggest existence of additional tick AChEs. Cholinergic systems are crucial in neural transmission and are also regulators of vertebrate immune function. Ticks exhibit prolonged intimate host contact, suggesting adaptive functions for tick cholinergic system complexity. AChE was recently reported in tick saliva and a role in manipulation of host immune responses was hypothesized. Physiological roles and genetic control of multiple tick AChEs requires further elucidation and may provide unique opportunities to understand and manipulate cholinergic involvement in biological systems.