Effects of pesticide mixtures on xenobiotic efflux transporter P‐glycoprotein in the European Honeybee

Abstract

About 35% of the world's food crops depend on insect pollinators like the European Honeybee, Apis mellifera, to reproduce. The sudden loss of a colony's worker bee population, often referred to as colony collapse disorder (CCD), has been linked to certain crop pesticide exposures. Multidrug resistance (MDR) transporters of the ATP-binding Cassette (ABC) family, such as P-glycoprotein, are the first line of defense in biological barriers, including the intestine and brain, and a key determinants of xenobiotic uptake in all organisms. Insect homologs of P-glycoprotein (P-gp) have a major function in developing resistance against broad-spectrum insecticides. However, the effects of pesticide mixtures on the protective efflux function of insect P-glycoprotein are not known, particularly mixtures of agricultural and in-hive pesticides. In this study, we cloned two variants of honeybee P-glycoprotein and quantified relative P-gp expression in head, thorax and abdomen using the C219 antibody. Honeybee P-gp was primarily detected in head and thorax that contain the brain and ganglion, indicating a key function in protecting the bee brain towards neurotoxic pesticides. To further explore the effects of pesticide exposure on bee P-gp function, we will express both P-gp variants in insect cells and determine their molecular interactions with selected pesticides using an ATPase activity assay. Purified bee P-gp will be used to determine the interactions of 15 current-use agricultural pesticides (see table 1) for the top ten bee-pollinated crops in California, including sunflowers, almonds, and avocados. We will categorize pesticide interactions into P-gp substrates (ATPase stimulation), inhibitors (ATPase inhibition), and non-interactors that do not interfere with ATPase activity. We hypothesize that binary combinations of inhibitory pesticides or pesticide substrates with competing binding sites can promote synergistic P-gp inhibition and ultimately lead to pesticide accumulation in the honeybee nervous system and subsequent neurotoxic effects. Pesticide combinations that show synergistic or antagonistic effects in the in vitro bee P-gpATPase activity assay will be subjected to an in vivo behavioral (navigation, coordination) and mortality study using live honeybees. The findings of this project will provide critical baseline data for future development of pollinator-safe pesticides and inform good agricultural practices.