Enhancement of insect susceptibility and larvicidal efficacy of Cry4Ba toxin by calcofluor

Somphob Leetachewa,Narumol Khomkhum,Saengduen Moonsom,Somsri Sakdee,Ping Wang

doi:10.1186/s13071-018-3110-3

Abstract

BackgroundMosquitoes transmit many vector-borne infectious diseases including malaria, dengue, chikungunya, yellow fever, filariasis, and Japanese encephalitis. The insecticidal δ-endotoxins Cry4, Cry11, and Cyt produced from Bacillus thuringiensis have been used for bio-control of mosquito larvae. Cry δ-endotoxins are synthesised as inactive protoxins in the form of crystalline inclusions in which they are processed to active toxins in larval midgut lumen. Previously, we demonstrated that the activated Cry4Ba toxin has to alter the permeability of the peritrophic membrane (PM), allowing toxin passage across PM to reach specific receptors on microvilli of larval midgut epithelial cells, where the toxin undergoes conformational changes, followed by membrane insertion and pore formation, resulting in larval death. A peritrophic membrane (PM)-binding calcofluor has been proposed to inhibit chitin formation and enhance baculovirus infection of lepidopteran Trichoplusia ni.MethodsIn this study, Aedes aegypti larvae were fed with the calcofluor and Cry4Ba toxin to investigate the effect of this agent on the toxicity of the Cry4Ba toxin.ResultsCalcofluor displayed an enhancing effect when co-fed with the Cry4Ba wild-type toxin. The agent could restore the killing activity of the partially active Cry4Ba mutant E417A/Y455A toward Ae. aegypti larvae. PM destruction was observed after larval challenge with calcofluor together with the toxin. Interestingly, calcofluor increased Cry4Ba toxin susceptibility toward semi-susceptible Culex quinquefasciatus larvae. However, calcofluor alone or in combination with the toxin showed no mortality effect on non-susceptible fresh-water fleas, Moina macrocopa.ConclusionsOur results suggest that PM may contribute to the resistance of the mosquito larvae to Cry4Ba toxin. The PM-permeability alternating calcofluor might be a promising candidate for enhancing insect susceptibility, which will consequently improve Cry4Ba efficacy in field settings in the future.

Highlights

Mosquitoes transmit many vector-borne infectious diseases including malaria, dengue, chikungunya, yellow fever, filariasis, and Japanese encephalitis
We previously reported that peritrophic membrane (PM) permeability alteration by Cry4Ba was required for toxin passage across PM of Ae. aegypti larvae [28]
Our results demonstrated that calcofluor increased PM permeability to Cry4Ba toxin and altered the gross morphology of the PM

Summary

Introduction

Mosquitoes transmit many vector-borne infectious diseases including malaria, dengue, chikungunya, yellow fever, filariasis, and Japanese encephalitis. Cry toxins have been used worldwide as bio-insecticide and genetically modified crops because of their environment-friendly properties and killing specificity toward a narrow spectrum of insect larvae. Cry1A and Cry2A are toxic to lepidopterans, Cry is noxious to coleopterans, whereas Cry and Cry are lethal to Aedes, Anopheles and Culex larvae in the family Culicidae [1]. Mutagenesis in the receptor binding domains of Cry1Ab and Cry3A has resulted in significant increase in their toxicities toward lepidopteran and coleopteran insect larvae [12, 13]. A 28-kDa APN fragment, AgAPN2tb, showed significant enhancement effect on Cry11Ba toxicity to Anopheles gambiae [15]

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