Between two acetylcholinesterases, is AChE1 always predominately expressed as the major catalytic enzyme in the Class Insecta?

Acetylcholinesterase (AChE) plays a critical role in terminating nerve impulses by hydrolyzing acetylcholine at cholinergic synapses of most animals. Different from vertebrates having only one AChE, most insect possess two AChEs, AChE1 and AChE2. Due to its higher transcription level and responsibility for insecticide resistance, it is believed that AChE1 is likely the major AChE, among two AChEs of the insects having both AChE1 and AChE2 studied to date, but the evolutionary distribution of the main catalytic AChE remains to be elucidated. To investigate the evolutionary distribution of two AChEs, the major AChE in several insect species was determined by native-polyacrylamide gel electrophoresis in conjunction with Western blotting using AChE1- and AChE2-specific antibodies. Among the 98 insect species examined, AChE1 was expressed as the main enzyme in 65 species across diverse taxa. In the remaining species, ranging from Paleoptera to Hymenoptera, however, AChE2 was predominantly expressed as the major catalytic enzyme. These findings are contrary to the common belief that AChE1 is the major enzyme in most insects, with the exception of Cyclorrhaphan flies, and further demonstrate that the specialization of AChE2 as the main enzyme or the replacement of AChE1 function with AChE2 function are rather common events, having multiple independent origins during insect evolution. It was hypothesized that the generation of multiple AChE2 isoforms via alternative splicing has allowed the loss of ace1 during the functional replacement of AChE1 with AChE2 in Cyclorrhapha. These findings should provide valuable insights into which AChE has evolved to perform synaptic functions.