Existence of two membrane-bound acetylcholinesterases in the honey bee head

Honey bee (Apis mellifera ligustica) acetylcholinesterase enzyme activity and aversive conditioning following aluminum trichloride exposure

Background

Aluminum is the third most prevalent element in the earth’s crust. In most conditions, it is tightly bound to form inaccessible compounds, however in low soil pH, the ionized form of aluminum can be taken up by plant roots and distributed throughout the plant tissue. Following this uptake, nectar and pollen concentrations in low soil pH regions can reach nearly 300 mg/kg. Inhibition of acetylcholinesterase (AChE) has been demonstrated following aluminum exposure in mammal and aquatic invertebrate species. In honey bees, behaviors consistent with AChE inhibition have been previously recorded; however, the physiological mechanism has not been tested, nor has aversive conditioning.

Results

This article presents results of ingested aqueous aluminum chloride exposure on AChE as well as acute exposure effects on aversive conditioning in an Apis mellifera ligustica hive. Contrary to previous findings, AChE activity significantly increased as compared to controls following exposure to 300 mg/L Al3+. In aversive conditioning studies, using an automated shuttlebox, there were time and dose-dependent effects on learning and reduced movement following 75 and 300 mg/L exposures.

Conclusions

These findings, in comparison to previous studies, suggest that aluminum toxicity in honey bees may depend on exposure period, subspecies, and study metrics. Further studies are encouraged at the moderate-high exposure concentrations as there may be multiple variables that affect toxicity which should be teased apart further.

Enzymatic responses in the head and midgut of Africanized Apis mellifera contaminated with a sublethal concentration of thiamethoxam

The increasing use of insecticides, promoted by the intensification of agriculture, has raised concerns about their influence on the decline of bee colonies, which play a fundamental role in pollination. Thus, it is fundamental to elucidate the effects of insecticides on bees. This study investigated the damage caused by a sublethal concentration of thiamethoxam - TMX (0.0227 ng/μL of feed) in the head and midgut of Africanized Apis mellifera, by analyzing the enzymatic biomarkers, oxidative stress, and occurrence of lipid peroxidation. The data showed that the insecticide increased acetylcholinesterase activity (AChE) and glutathione-S-transferase (GST), whereas carboxylesterase (CaE3) activity decreased in the heads. Our results indicate that the antioxidant enzymes were less active in the head because only glutathione peroxidase (GPX) showed alterations. In the midgut, there were no alkaline phosphatase (ALP) or superoxide dismutase (SOD) responses and a decrease in the activity of CaE was observed. Otherwise, there was an increase in GPX, and the TBARS (thiobarbituric acid reactive substances) assay also showed differences in the midgut. The TBARS (thiobarbituric acid reactive substances) assay also showed differences in the midgut. The results showed enzymes such as CaE3, GST, AChE, ALP, SOD, and GPX, as well as the TBARS assay, are useful biomarkers on bees. They may be used in combination as a promising tool for characterizing bee exposure to insecticides.

Physiological effects of the interaction between Nosema ceranae and sequential and overlapping exposure to glyphosate and difenoconazole in the honey bee Apis mellifera

Pathogens and pollutants, such as pesticides, are potential stressors to all living organisms, including honey bees. Herbicides and fungicides are among the most prevalent pesticides in beehive matrices, and their interaction with Nosema ceranae is not well understood. In this study, the interactions between N. ceranae, the herbicide glyphosate and the fungicide difenoconazole were studied under combined sequential and overlapping exposure to the pesticides at a concentration of 0.1 µg/L in food. In the sequential exposure experiment, newly emerged bees were exposed to the herbicide from day 3 to day 13 after emerging and to the fungicide from day 13 to day 23. In the overlapping exposure experiment, bees were exposed to the herbicide from day 3 to day 13 and to the fungicide from day 7 to day 17. Infection by Nosema in early adult life stages (a few hours post emergence) greatly affected the survival of honey bees and elicited much higher mortality than was induced by pesticides either alone or in combination. Overlapping exposure to both pesticides induced higher mortality than was caused by sequential or individual exposure. Overlapping, but not sequential, exposure to pesticides synergistically increased the adverse effect of N. ceranae on honey bee longevity. The combination of Nosema and pesticides had a strong impact on physiological markers of the nervous system, detoxification, antioxidant defenses and social immunity of honey bees.

Mixtures of an insecticide, a fungicide and a herbicide induce high toxicities and systemic physiological disturbances in winter Apis mellifera honey bees

Multiple pesticides originating from plant protection treatments and the treatment of pests infecting honey bees are frequently detected in beehive matrices. Therefore, winter honey bees, which have a long life span, could be exposed to these pesticides for longer periods than summer honey bees. In this study, winter honey bees were exposed through food to the insecticide imidacloprid, the fungicide difenoconazole and the herbicide glyphosate, alone or in binary and ternary mixtures, at environmental concentrations (0 (controls), 0.1, 1 and 10 μg/L) for 20 days. The survival of the honey bees was significantly reduced after exposure to these 3 pesticides individually and in combination. Overall, the combinations had a higher impact than the pesticides alone with a maximum mortality of 52.9% after 20 days of exposure to the insecticide-fungicide binary mixture at 1 μg/L. The analyses of the surviving bees showed that these different pesticide combinations had a systemic global impact on the physiological state of the honey bees, as revealed by the modulation of head, midgut and abdomen glutathione-S-transferase, head acetylcholinesterase, abdomen glucose-6-phosphate dehydrogenase and midgut alkaline phosphatase, which are involved in the detoxification of xenobiotics, the nervous system, defenses against oxidative stress, metabolism and immunity, respectively. These results demonstrate the importance of studying the effects of chemical cocktails based on low realistic exposure levels and developing long-term tests to reveal possible lethal and adverse sublethal interactions in honey bees and other insect pollinators.

Chronic toxicity and biochemical response of Apis cerana cerana (Hymenoptera: Apidae) exposed to acetamiprid and propiconazole alone or combined

Acetamiprid and ergosterol-inhibiting fungicide (EBI) are frequently applied to many flowering plants, while honey bees are pollinating agents or pollinators of the flowers. Hence honey bees are often exposed to these pesticides. But until now, the effects of theses combinations at field-realistic doses on honey bee health have been poorly investigated. In this study, we explore the synergistic mortality and some physiological effects in surviving honey bees after chronic oral exposure to acetamiprid and/or propiconazole in the laboratory. The results indicated that chronic combined exposure to acetamiprid and propiconazole produced a significant synergistic effect on mortality both for newly emerged bees (50% mortality in 7.2 days) and forager bees (50% mortality in 4.8 days). Honey bee weight of newly emerged bees was decreased after feeding food with a field concentration of acetamiprid and propiconazole, alone or combined for 10 days. Combination of acetamiprid and propiconazole also modulated the activities of P450s, GST and CAT in newly emerged bees and forager bees than either alone, but neither pesticide affected the activity of AChE. These results show that chronic combined exposure to pesticides of relatively low toxicity may caused severely physiological disruptions that could be potentially damaging for the honey bees.

Ohm R, Drobne D, Jemec Kokalj A. First evidence of cholinesterase-like activity in Basidiomycota

Cholinesterases (ChE), the enzymes whose primary function is the hydrolysis of choline esters, are widely expressed throughout the nature. Although they have already been found in plants and microorganisms, including ascomycete fungi, this study is the first report of ChE-like activity in fungi of the phylum Basidiomycota. This activity was detected in almost a quarter of the 45 tested aqueous fungal extracts. The ability of these extracts to hydrolyse acetylthiocholine was about ten times stronger than the hydrolytic activity towards butyrylthiocholine and propionylthiocholine. In-gel detection of ChE-like activity with acetylthiocholine indicated a great variability in the characteristics of these enzymes which are not characterized as vertebrate-like based on (i) differences in inhibition by excess substrate, (ii) susceptibility to different vertebrate acetylcholinesterase and butyrylcholinesterase inhibitors, and (iii) a lack of orthologs using phylogenetic analysis. Limited inhibition by single inhibitors and multiple activity bands using in-gel detection indicate the presence of several ChE-like enzymes in these aqueous extracts. We also observed inhibitory activity of the same aqueous mushroom extracts against insect acetylcholinesterase in 10 of the 45 samples tested; activity was independent of the presence of ChE-like activity in extracts. Both ChE-like activities with different substrates and the ability of extracts to inhibit insect acetylcholinesterase were not restricted to any fungal family but were rather present across all included Basidiomycota families. This study can serve as a platform for further research regarding ChE activity in mushrooms.

Different response of acetylcholinesterases in salt- and detergent-soluble fractions of honeybee haemolymph, head and thorax after exposure to diazinon

Organophosphate pesticide diazinon is a specific inhibitor of acetylcholinesterase (AChE), which is a common neurotoxicity biomarker in environmental studies. In honeybees, AChE exists in two forms having different physiological roles, one existing as a soluble form and the other as membrane-bound. In most studies AChE activity has been analysed without paying considerable attention to different forms of AChE. In this study, we exposed honeybees Apis mellifera carnica for 10days to diazinon via oral exposure and analysed the total AChE activities in salt soluble (SS) and detergent soluble (DS) fractions. We assumed that SS fraction would preferentially contain the soluble AChE, but the DS fraction would contain only membrane AChE. On the contrary, our results showed that SS and DS fractions both contain soluble and membrane AChE and the latter has considerably higher activity. Despite this we obtained a differential response of AChE activity in SS and DS fractions when exposed to diazinon. The head/thorax AChE activity in DS fraction decreased, while the head/thorax AChE activity in SS fraction increased at sublethal concentrations. The AChE activity in honeybee hemolymph shown here for the first time is a salt soluble enzyme. Its activity remained unaltered after diazinon treatment. In conclusion, we provide evidence that varying results regarding AChE activity alterations upon stressor exposure are obtained when extracted through different procedures. In further environmental studies with honeybees this differential response of AChE activity should be given considerable attention because this affects the outcome of ecotoxicity study.

Biochemical response of the Africanized honeybee exposed to fipronil

Bees are recognized worldwide for their social, economic, and environmental value. In recent decades they have been seriously threatened by diseases and high levels of pesticide use. The susceptibility of bees to insecticides makes them an important terrestrial model for assessing environmental quality, and various biomarkers have been developed for such assessments. The present study aimed to evaluate the activity of the enzymes acetylcholinesterase (AChE), carboxylesterase (CaE), and glutathione-S-transferase (GST) in Africanized honeybees exposed to fipronil. The results showed that fipronil at a sublethal dose (0.01 ng/bee) modulates the activity of CaE in all isoforms analyzed (CaE-1, CaE-2, and CaE-3) in both newly emerged and aged bees, and does not affect the activity of AChE or GST. The recovery of the bees after fipronil exposure was also investigated, and these results demonstrated that even the cessation of fipronil ingestion might not lead to complete recovery of individual bees. Even at low doses, fipronil was shown to cause changes in the activity of key enzymes in bees. The possible consequences of these changes are discussed. Environ Toxicol Chem 2017;36:1652-1660. © 2016 SETAC.

The Repellent DEET Potentiates Carbamate Effects via Insect Muscarinic Receptor Interactions: An Alternative Strategy to Control Insect Vector-Borne Diseases

Insect vector-borne diseases remain one of the principal causes of human mortality. In addition to conventional measures of insect control, repellents continue to be the mainstay for personal protection. Because of the increasing pyrethroid-resistant mosquito populations, alternative strategies to reconstitute pyrethroid repellency and knock-down effects have been proposed by mixing the repellent DEET (N,N-Diethyl-3-methylbenzamide) with non-pyrethroid insecticide to better control resistant insect vector-borne diseases. By using electrophysiological, biochemichal, in vivo toxicological techniques together with calcium imaging, binding studies and in silico docking, we have shown that DEET, at low concentrations, interacts with high affinity with insect M1/M3 mAChR allosteric site potentiating agonist effects on mAChRs coupled to phospholipase C second messenger pathway. This increases the anticholinesterase activity of the carbamate propoxur through calcium-dependent regulation of acetylcholinesterase. At high concentrations, DEET interacts with low affinity on distinct M1/M3 mAChR site, counteracting the potentiation. Similar dose-dependent dual effects of DEET have also been observed at synaptic mAChR level. Additionally, binding and in silico docking studies performed on human M1 and M3 mAChR subtypes indicate that DEET only displays a low affinity antagonist profile on these M1/M3 mAChRs. These results reveal a selective high affinity positive allosteric site for DEET in insect mAChRs. Finally, bioassays conducted on Aedes aegypti confirm the synergistic interaction between DEET and propoxur observed in vitro, resulting in a higher mortality of mosquitoes. Our findings reveal an unusual allosterically potentiating action of the repellent DEET, which involves a selective site in insect. These results open exciting research areas in public health particularly in the control of the pyrethroid-resistant insect-vector borne diseases. Mixing low doses of DEET and a non-pyrethroid insecticide will lead to improvement in the efficiency treatments thus reducing both the concentration of active ingredients and side effects for non-target organisms. The discovery of this insect specific site may pave the way for the development of new strategies essential in the management of chemical use against resistant mosquitoes.

Mutation and duplication of arthropod acetylcholinesterase: Implications for pesticide resistance and tolerance

A series of common/shared point mutations in acetylcholinesterase (AChE) confers resistance to organophosphorus and carbamate insecticides in most arthropod pests. However, the mutations associated with reduced sensitivity to insecticides usually results in the reduction of catalytic efficiency and leads to a fitness disadvantage. To compensate for the reduced catalytic activity, overexpression of neuronal AChE appears to be necessary, which is achieved by a relatively recent duplication of the AChE gene (ace) as observed in the two-spotted spider mite and other insects. Unlike the cases with overexpression of neuronal AChE, the extensive generation of soluble AChE is observed in some insects either from a distinct non-neuronal ace locus or from a single ace locus via alternative splicing. The production of soluble AChE in the fruit fly is induced by chemical stress. Soluble AChE acts as a potential bioscavenger and provides tolerance to xenobiotics, suggesting its role in chemical adaptation during evolution.

Time-dependent kinetic complexities in cholinesterase-catalyzed reactions

Cholinesterases (ChEs) display a hysteretic behavior with certain substrates and inhibitors. Kinetic cooperativity in hysteresis of ChE-catalyzed reactions is characterized by a lag or burst phase in the approach to steady state. With some substrates damped oscillations are shown to superimpose on hysteretic lags. These time dependent peculiarities are observed for both butyrylcholinesterase and acetylcholinesterase from different sources. Hysteresis in ChE-catalyzed reactions can be interpreted in terms of slow transitions between two enzyme conformers E and E'. Substrate can bind to E and/or E', both Michaelian complexes ES and Ε'S can be catalytically competent, or only one of them can make products. The formal reaction pathway depends on both the chemical structure of the substrate and the type of enzyme. In particular, damped oscillations develop when substrate exists in different, slowly interconvertible, conformational, and/or micellar forms, of which only the minor form is capable of binding and reacting with the enzyme. Biphasic pseudo-first-order progressive inhibition of ChEs by certain carbamates and organophosphates also fits with a slow equilibrium between two reactive enzyme forms. Hysteresis can be modulated by medium parameters (pH, chaotropic and kosmotropic salts, organic solvents, temperature, osmotic pressure, and hydrostatic pressure). These studies showed that water structure plays a role in hysteretic behavior of ChEs. Attempts to provide a molecular mechanism for ChE hysteresis from mutagenesis studies or crystallographic studies failed so far. In fact, several lines of evidence suggest that hysteresis is controlled by the conformation of His438, a key residue in the catalytic triad of cholinesterases. Induction time may depend on the probability of His438 to adopt the operative conformation in the catalytic triad. The functional significance of ChE hysteresis is puzzling. However, the accepted view that proteins are in equilibrium between preexisting functional and non-functional conformers, and that binding of a ligand to the functional form shifts equilibrium towards the functional conformation, suggests that slow equilibrium between two conformational states of these enzymes may have a regulatory function in damping out the response to certain ligands and irreversible inhibitors. This is particularly true for immobilized (membrane bound) enzymes where the local substrate and/or inhibitor concentrations depend on influx in crowded organellar systems, e.g. cholinergic synaptic clefts. Therefore, physiological or toxicological relevance of the hysteretic behavior and damped oscillations in ChE-catalyzed reactions and inhibition cannot be ruled out.

Exposure to acetylcholinesterase inhibitors alters the physiology and motor function of honeybees

Cholinergic signaling is fundamental to neuromuscular function in most organisms. Sub-lethal doses of neurotoxic pesticides that target cholinergic signaling can alter the behavior of insects in subtle ways; their influence on non-target organisms may not be readily apparent in simple mortality studies. Beneficial arthropods such as honeybees perform sophisticated behavioral sequences during foraging that, if influenced by pesticides, could impair foraging success and reduce colony health. Here, we investigate the behavioral effects on honeybees of exposure to a selection of pesticides that target cholinergic signaling by inhibiting acetylcholinesterase (AChE). To examine how continued exposure to AChE inhibitors affected motor function, we fed adult foraging worker honeybees sub-lethal concentrations of these compounds in sucrose solution for 24 h. Using an assay for locomotion in bees, we scored walking, stopped, grooming, and upside down behavior continuously for 15 min. At a 10 nM concentration, all the AChE inhibitors caused similar effects on behavior, notably increased grooming activity and changes in the frequency of bouts of behavior such as head grooming. Coumaphos caused dose-dependent effects on locomotion as well as grooming behavior, and a 1 μM concentration of coumaphos induced symptoms of malaise such as abdomen grooming and defecation. Biochemical assays confirmed that the four compounds we assayed (coumaphos, aldicarb, chlorpyrifos, and donepezil) or their metabolites acted as AChE inhibitors in bees. Furthermore, we show that transcript expression levels of two honeybee AChE inhibitors were selectively upregulated in the brain and in gut tissues in response to AChE inhibitor exposure. The results of our study imply that the effects of pesticides that rely on this mode of action have subtle yet profound effects on physiological effects on behavior that could lead to reduced survival.

Molecular and kinetic properties of two acetylcholinesterases from the western honey bee, Apis mellifera

We investigated the molecular and kinetic properties of two acetylcholinesterases (AmAChE1 and AmAChE2) from the Western honey bee, Apis mellifera. Western blot analysis revealed that AmAChE2 has most of catalytic activity rather than AmAChE1, further suggesting that AmAChE2 is responsible for synaptic transmission in A. mellifera, in contrast to most other insects. AmAChE2 was predominately expressed in the ganglia and head containing the central nervous system (CNS), while AmAChE1 was abundantly observed not only in the CNS but also in the peripheral nervous system/non-neuronal tissues. Both AmAChEs exist as homodimers; the monomers are covalently connected via a disulfide bond under native conditions. However, AmAChE2 was associated with the cell membrane via the glycophosphatidylinositol anchor, while AmAChE1 was present as a soluble form. The two AmAChEs were functionally expressed with a baculovirus system. Kinetic analysis revealed that AmAChE2 has approximately 2,500-fold greater catalytic efficiency toward acetylthiocholine and butyrylthiocholine than AmAChE1, supporting the synaptic function of AmAChE2. In addition, AmAChE2 likely serves as the main target of the organophosphate (OP) and carbamate (CB) insecticides as judged by the lower IC₅₀ values against AmAChE2 than against AmAChE1. When OP and CB insecticides were pre-incubated with a mixture of AmAChE1 and AmAChE2, a significant reduction in the inhibition of AmAChE2 was observed, suggesting a protective role of AmAChE1 against xenobiotics. Taken together, based on their tissue distribution pattern, molecular and kinetic properties, AmAChE2 plays a major role in synaptic transmission, while AmAChE1 has non-neuronal functions, including chemical defense.

Development of biomarkers of exposure to xenobiotics in the honey bee Apis mellifera: application to the systemic insecticide thiamethoxam

This study describes the development of acetylcholinesterase (AChE), carboxylesterases (CaE1, CaE2, CaE3), glutathion-S-transferase (GST), alkaline phosphatase (ALP) and catalase (CAT) as enzyme biomarkers of exposure to xenobiotics such as thiamethoxam in the honey bee Apis mellifera. Extraction efficiency, stability under freezing and biological variability were studied. The extraction procedure achieved good recovery rates in one extraction step and ranged from 65 percent (AChE) to 97.3 percent (GST). Most of the enzymes were stable at -20°C, except ALP that displayed a slight but progressive decrease in its activity. Modifications of enzyme activities were considered after exposure to thiamethoxam at the lethal dose 50 percent (LD(50), 51.16 ng bee(-1)) and two sublethal doses, LD(50)/10 (5.12 ng bee(-1)) and LD(50)/20 (2.56 ng bee(-1)). The biomarker responses revealed that, even at the lowest dose used, exposure to thiamethoxam elicited sublethal effects and modified the activity of CaEs, GST, CAT and ALP. Different patterns of biomarker responses were observed: no response for AChE, an increase for GST and CAT, and differential effects for CaEs isoforms with a decrease in CaE1 and CaE3 and an increase in CaE2. ALP and CaE3 displayed contrasting variations but only at 2.56 ng bee(-1). We consider that this profile of biomarker variation could represent a useful fingerprint to characterise exposure to thiamethoxam in the honey bee A. mellifera. This battery of honey bee biomarkers might be a promising option to biomonitor the health of aerial and terrestrial ecosystems and to generate valuable information on the modes of action of pesticides.

Molecular cloning and characterization of an acetylcholinesterase cDNA in the brown planthopper, Nilaparvata lugens

A full cDNA encoding an acetylcholinesterase (AChE, EC 3.1.1.7) was cloned and characterized from the brown planthopper, Nilaparvata lugens Stål (Hemiptera: Delphacidae). The complete cDNA (2467 bp) contains a 1938-bp open reading frame encoding 646 amino acid residues. The amino acid sequence of the AChE deduced from the cDNA consists of 30 residues for a putative signal peptide and 616 residues for the mature protein with a predicted molecular weight of 69,418. The three residues (Ser242, Glu371, and His485) that putatively form the catalytic triad and the six Cys that form intra-subunit disulfide bonds are completely conserved, and 10 out of the 14 aromatic residues lining the active site gorge of the AChE are also conserved. Northern blot analysis of poly(A)+ RNA showed an approximately 2.6-kb transcript, and Southern blot analysis revealed there likely was just a single copy of this gene in N. lugens. The deduced protein sequence is most similar to AChE of Nephotettix cincticeps with 83% amino acid identity. Phylogenetic analysis constructed with 45 AChEs from 30 species showed that the deduced N. lugens AChE formed a cluster with the other 8 insect AChE2s. Additionally, the hypervariable region and amino acids specific to insect AChE2 also existed in the AChE of N. lugens. The results revealed that the AChE cDNA cloned in this work belongs to insect AChE2 subgroup, which is orthologous to Drosophila AChE. Comparison of the AChEs between the susceptible and resistant strains revealed a point mutation, Gly185Ser, is likely responsible for the insensitivity of the AChE to methamidopho in the resistant strain.