Cry toxins from Bacillus thuringiensis subsp. israelensis (Bti) have multiple targets in Aedes aegypti mosquitoes

Aedes aegypti is an important mosquito vector of dengue and yellow fever, diseases that are of increasing concern worldwide. Formulations using Bacillus thuringiensis subsp. israelensis (Bti), a larvicide, are frequently used worldwide for the control of this insect vector. Bti produces four major insecticidal proteins (Cry4Aa, Cry4Ba, Cry10Aa and Cry11Aa), but their protein receptors and mechanisms of action have not been fully elucidated. Due to the low toxicity of Cry10A, this study mainly focuses on other three major Cry toxins. It has been shown that cadherin, APN and ALP proteins are involved in the Cry11A toxin action, whose mechanism of action is similar to the sequential binding mode observed with Cry1A toxins. However, bioassays using transgenic mosquitoes with RNAi-mediated AeaCad gene knockdown, demonstrated that the toxicity of Cry4A and Cry4B is not affected in these transgenic mosquitoes, suggesting AeaCad is not employed as a receptor for Cry4A and Cry4B. Additionally, Cry11A-resistant mosquito strain selected in the lab shows cross-resistance with Cry4A toxins, but not with Cry4B toxins, which indicates Cry4A and Cry4B could also have different receptors although their protein structures are highly similar. Attempts are presently underway to identify the receptors for the Cry4 toxins.  In sum, complex mechanisms and multiple targets used by these Cry toxins in Bti could partially explain the extremely low frequency of Bti-resistant strains observed in the field even though it has been widely used for Aedes biological control in the world for several decades.