Benralizumab for eosinophilic gastritis: a single-site, randomised, double-blind, placebo-controlled, phase 2 trial

BACKGROUND

In eosinophilic gastrointestinal diseases, the role of eosinophils in disease pathogenesis and the effect of eosinophil depletion on patient outcomes are unclear. Benralizumab, an eosinophil-depleting monoclonal antibody that targets the interleukin-5 receptor α, might eliminate gastric tissue eosinophils and improve outcomes in eosinophilic gastritis. We aimed to assess the efficacy and safety of benralizumab in patients with eosinophilic gastritis.

METHODS

We conducted a single-site, randomised, double-blind, placebo-controlled, phase 2 trial at Cincinnati Children's Hospital Medical Center (Cincinnati, OH, USA). Individuals aged 12-60 years with symptomatic, histologically active eosinophilic gastritis (peak gastric eosinophil count ≥30 eosinophils per high-power field [eos/hpf] in at least five hpfs) and blood eosinophilia (>500 eosinophils per μL [eos/μL]) were randomly assigned (1:1, block size of four) to benralizumab 30 mg or placebo, stratified by the use of glucocorticoids for gastric disease. Investigators, study staff, and study participants were masked to treatment assignment; statisticians were unmasked when analysing data. Treatments were administered subcutaneously once every 4 weeks for a 12-week double-blind period (three total injections). The primary endpoint was the proportion of patients who achieved histological remission (peak gastric eosinophil count <30 eos/hpf) at week 12. Key secondary endpoints were the changes from baseline to week 12 in peak gastric eosinophil count, blood eosinophil count, eosinophilic gastritis histology (total, inflammatory, and structural feature scores), Eosinophilic Gastritis Endoscopic Reference System (EG-REFS) score, and patient-reported outcome symptom measures (Severity of Dyspepsia Assessment [SODA] and Patient-Reported Outcome Measurement Information System [PROMIS] short-form questionnaire). After the 12-week double-blind period, patients were eligible for entry into two open-label extension (OLE) periods up to week 88, in which all patients received benralizumab. Efficacy was analysed in the intention-to-treat (ITT) population and safety was assessed in all patients who received at least one dose of study drug. The trial was registered on ClinicalTrials.gov, NCT03473977, and is completed.

FINDINGS

Between April 23, 2018, and Jan 13, 2020, 34 patients were screened, and 26 were subsequently randomly assigned to benralizumab (n=13) or placebo (n=13) and included in the ITT and safety populations (mean age 19·5 years [SD 7·3]; 19 [73%] male patients and seven [27%] female patients). At week 12, ten (77% [95% CI 50 to 92]) of 13 patients who received benralizumab and one (8% [1 to 33]) of 13 who received placebo achieved histological remission (difference 69 percentage points [95% CI 32 to 85]; p=0·0010). Changes from baseline to week 12 were significantly greater in the benralizumab group versus the placebo group for peak gastric eosinophil counts (mean -137 eos/hpf [95% CI -186 to -88] vs -38 eos/hpf [-94 to 18]; p=0·0080), eosinophilic gastritis histology total score (mean -0·31 [-0·42 to -0·20] vs -0·02 [-0·16 to 0·12]; p=0·0016), histology inflammatory score (mean -0·46 [-0·60 to -0·31] vs -0·04 [-0·22 to 0·13]; p=0·0006), and blood eosinophil counts (median -1060 eos/μL [IQR -1740 to -830] vs -160 eos/μL [-710 to 120]; p=0·0044). Changes were not significantly different between the groups for eosinophilic gastritis histology structural score (mean -0·07 [95% CI -0·19 to 0·05] vs 0·03 [-0·09 to 0·15]; p=0·23), EG-REFS score (mean -1·0 [-2·3 to 0·3] vs -0·5 [-2·0 to 1·0]; p=0·62), or in patient-reported outcomes (SODA and PROMIS). During the double-blind period, treatment-emergent adverse events occurred in 11 (85%) of 13 patients in the benralizumab group and six (46%) of 13 in the placebo group; the most common treatment-emergent adverse events were headache (six [46%] vs two [15%] patients), nausea (three [23%] vs two [15%]), and vomiting (two [15%] vs three [23%]). There were no treatment-related deaths. Two patients had serious adverse events (dizziness and rhabdomyolysis in one patient; aspiration in one patient) during the OLE periods, which were considered unrelated to study treatment.

INTERPRETATION

Benralizumab treatment induced histological remission, as defined by absence of tissue eosinophilia, in most patients with eosinophilic gastritis. However, the persistence of histological, endoscopic, and other features of the disease suggest a co-existing, eosinophil-independent pathogenic mechanism and the need for broader targeting of type 2 immunity.

FUNDING

AstraZeneca and the Division of Intramural Research (National Institute of Allergy and Infectious Diseases, US National Institutes of Health).

Food Allergy Characteristics Associated With Coexisting Eosinophilic Esophagitis in FARE Registry Participants

BACKGROUND

Eosinophilic esophagitis (EoE) can coexist in individuals with food allergy.

OBJECTIVE

To evaluate the characteristics of food-allergic patients with and without coexisting EoE using a large food allergy patient registry.

METHODS

Data were derived from 2 Food Allergy Research & Education, Inc, Patient Registry surveys. A series of multivariable regression models were used to evaluate associations between demographic, comorbidity, and food allergy characteristics and the likelihood of reporting EoE.

RESULTS

Five percent (n = 309) of registry participants (n = 6074; ages <1 year->80 years, mean, 20.20 ± 15.37 years) reported having EoE. The odds of having EoE were significantly greater in male participants (adjusted odds ratio [aOR], 1.3; 95% CI, 1.04-1.72) and those with comorbid asthma (aOR, 2.0; 95% CI, 1.55-2.49), allergic rhinitis (aOR, 1.8; 95% CI, 1.37-2.22), oral allergy syndrome (aOR, 2.8; 95% CI, 2.09-3.70), food protein-induced enterocolitis syndrome (aOR, 2.5; 95% CI, 1.34-4.84), and hyper-IgE syndrome (aOR, 7.6; 95% CI, 2.93-19.92), though not atopic dermatitis (aOR, 1.3; 95% CI, 0.99-1.59), when adjusting for demographics (sex, age, race, ethnicity, and geographic location). Those with a greater number of food allergies (aOR, 1.3; 95% CI, 1.23-1.32), more frequent food-related allergic reactions (aOR, 1.2; 95% CI, 1.11-1.24), previous anaphylaxis (aOR, 1.5; 95% CI, 1.15-1.83), and health care utilization for food-related allergic reactions (aOR, 1.3; 95% CI, 1.01-1.67)-specifically intensive care unit admission (aOR, 1.2; 95% CI, 1.07-1.33)-were more likely to have EoE after controlling for demographics. However, no significant difference in ever using epinephrine for food-related allergic reactions was detected.

CONCLUSIONS

These self-reported data showed that coexisting EoE is associated with an increased number of food allergies, food-related allergic reactions per year, and measures of reaction severity, calling attention to the likely increased health care needs of food-allergic patients with EoE.

B cell repertoire in children with skin barrier dysfunction supports altered IgE maturation associated with allergic food sensitization

The skin is a major immune organ and skin barrier dysfunction is a major risk factor for the development of the inappropriate immune response seen in allergic disease. Skin barrier disruption alters the landscape of antigens experienced by the immune system and the downstream impacts on the antibody repertoire remain poorly characterized, particularly for the IgE isotype responsible for allergic specificity and in early life, when allergic disease is developing. In this study, we sequenced antibody gene repertoires from a large and well-characterized cohort of children with atopic dermatitis and found that food sensitization was associated with lower mutation frequencies in the IgE compartment. This trend was abrogated in children living with pets during the first year of life. These results elucidate potential molecular mechanisms underlying the protective effects of pet ownership and non-antiseptic environs reported for allergic disease, and the hygiene hypothesis more broadly. We also observed increased IgE diversity and increased isotype-switching to the IgE isotype, suggesting that B cell development, particularly isotype-switching, is heavily altered in the those with food allergen sensitizations relative to those without food allergen sensitizations. Unlike for food antigens, aeroallergen sensitization exhibited no effect on IgE mutation or diversity. Consistent patterns of antibody rearrangement were associated with food allergen sensitization in subjects with atopic dermatitis. Thus, we propose the Immune Repertoire in Atopic Disease (IRAD) score, to quantify this repertoire shift and to aid clinically in patient diagnosis and risk stratification.

Epigenetic and transcriptional dysregulation in CD4+ T cells in patients with atopic dermatitis

Atopic dermatitis (AD) is one of the most common skin disorders among children. Disease etiology involves genetic and environmental factors, with 29 independent AD risk loci enriched for risk allele-dependent gene expression in the skin and CD4+ T cell compartments. We investigated the potential epigenetic mechanisms responsible for the genetic susceptibility of CD4+ T cells. To understand the differences in gene regulatory activity in peripheral blood T cells in AD, we measured chromatin accessibility (an assay based on transposase-accessible chromatin sequencing, ATAC-seq), nuclear factor kappa B subunit 1 (NFKB1) binding (chromatin immunoprecipitation with sequencing, ChIP-seq), and gene expression levels (RNA-seq) in stimulated CD4+ T cells from subjects with active moderate-to-severe AD, as well as in age-matched non-allergic controls. Open chromatin regions in stimulated CD4+ T cells were highly enriched for AD genetic risk variants, with almost half of the AD risk loci overlapping AD-dependent ATAC-seq peaks. AD-specific open chromatin regions were strongly enriched for NF-κB DNA-binding motifs. ChIP-seq identified hundreds of NFKB1-occupied genomic loci that were AD- or control-specific. As expected, the AD-specific ChIP-seq peaks were strongly enriched for NF-κB DNA-binding motifs. Surprisingly, control-specific NFKB1 ChIP-seq peaks were not enriched for NFKB1 motifs, but instead contained motifs for other classes of human transcription factors, suggesting a mechanism involving altered indirect NFKB1 binding. Using DNA sequencing data, we identified 63 instances of altered genotype-dependent chromatin accessibility at 36 AD risk variant loci (30% of AD risk loci) that might lead to genotype-dependent gene expression. Based on these findings, we propose that CD4+ T cells respond to stimulation in an AD-specific manner, resulting in disease- and genotype-dependent chromatin accessibility alterations involving NFKB1 binding.

Approach to primary immunodeficiency

Primary immunodeficiency diseases are inherited defects of the innate or adaptive arms of the immune system that lead to an increase in the incidence, frequency, or severity of infections and/or immune dysregulation. There may be defects in the adaptive arm of the immune system, including combined immunodeficiencies and antibody deficiency syndromes, or abnormalities in innate immunity, such as defects of phagocytes, the complement pathway, or toll-like receptor mediated signaling. Recurrent sinopulmonary infections with encapsulated bacteria such as Haemophilus influenzae type B or Streptococcus pneumoniae may be characteristic of an antibody deficiency syndrome. Frequent viral, fungal, or protozoal infections may suggest T lymphocyte impairment. Multiple Staphylococcus skin infections and fungal infections may imply neutrophil dysfunction or the Hyper-IgE syndrome, and recurrent Neisseria infection is a characteristic manifestation of late complement component (C5-9, or the membrane attack complex) defects. Recurrent viral or pyogenic bacterial infections, often without the presence of a significant inflammatory response, suggest a defect in toll-like receptor signaling. Mycobacterial infections are characteristic of defects in the interleukin (IL) 12/interferon γ pathway. Screening of newborns for T-cell lymphopenia by using polymerase chain reaction to amplify T-cell receptor excision circles, which are formed when a T cell rearranges the variable region of its receptor, serves as a surrogate for newly synthesized naive T cells. Because of very low numbers of T-cell receptor excision circles, severe combined immunodeficiency, 22q11.2 syndrome, and other causes of T-cell lymphopenia have been identified in newborns.

Prevention of food allergy

Primary prevention and secondary prevention in the context of food allergy refer to prevention of the development of sensitization (i.e., the presence of food-specific immunoglobulin E (IgE) as measured by skin-prick testing and/or laboratory testing) and sensitization plus the clinical manifestations of food allergy, respectively. Until recently, interventions that target the prevention of food allergy have been limited. Although exclusive breast-feeding for the first 6 months of life has been a long-standing recommendation due to associated health benefits, recommendations regarding complementary feeding in infancy have significantly changed over the past 20 years. There now is evidence to support early introduction of peanut into the diet of infants with egg allergy, severe atopic dermatitis, or both diagnoses, defined as high risk for peanut allergy, to try to prevent development of peanut allergy. Although guideline-based recommendations are not available for early introduction of additional allergenic foods, this topic is being actively studied. There is no evidence to support additional dietary modification of the maternal or infant diet for the prevention of food allergy. Similarly, there is no conclusive evidence to support maternal avoidance diets for the prevention of food allergy.

Pediatric asthma: Principles and treatment

Approximately one-half of children with asthma present with symptoms before 3 years of age. The typical history describes recurrent episodes of wheezing and/or cough triggered by a viral upper respiratory infection (URI), activity, or changes in weather. When symptoms occur after a viral URI, children with asthma often take longer than the usual week to fully recover from their respiratory symptoms. Wheezing and coughing during exercise or during laughing or crying, and episodes triggered in the absence of infection suggest asthma. A trial of bronchodilator medication should show symptomatic improvement. The goal of asthma therapy is to keep children "symptom free" by preventing chronic symptoms, maintaining lung function, and allowing for normal daily activities. Avoidance of triggers identified by a history, such as second-hand cigarette smoke exposure, and allergens identified by skin-prick testing can significantly reduce symptoms. According to the 2007 National Asthma Education and Prevention Program (NAEPP) report, if impairment symptoms are present for >2 days/week or 2 nights/month, then the disease process is characterized as persistent, and, in all age groups, inhaled corticosteroids (ICS) are recommended as the preferred daily controller therapy. Montelukast is approved for children ages ≥ 12 months and is often used for its ease of daily oral dosing. Long-acting beta-2 adrenergic agonists should only be used in combination with an ICS. For more-severe or difficult-to-control phenotypes, biologic therapy has been developed, which targets the type of inflammation present.

Elevated blood pressure in adolescent girls: correlation to body size and composition

Background

To improve understanding of the pathophysiology of hypertension in adolescents and pave the way for risk stratification, studies have sought to determine the correlates of blood pressure (BP). Inconsistencies in dependent and independent variables have resulted in an elusive consensus. The aim of this report is to examine an inclusive array of correlates of BP, as a continuous (systolic and diastolic BP) and a dichotomous variable.

Methods

Subjects were a school-based sample of 730 urban, mostly African American, non-referred 6th and 7th grade girls. To find independent correlates of SBP/DBP, we used a stepwise model selection method based on the Schwarz Bayesian Information Criterion, enabling selection of a parsimonious model among highly correlated covariates. Candidate variables were: age, stature, heart rate, pubertal development, BMI, BMI z-score, waist circumference, waist-to-height ratio (WHtR), body surface area, fat mass (by bioelectrical impedance analysis), fat-free mass (FFM), percentage of body fat, and presence of overweight/obesity.

Results

The best-fitting models for DBP and SBP (considered separately) included fat-free mass, heart rate and, in the case of SBP, stature. The best-fitting model for high-normal/elevated blood pressure (H-N/EBP) included WHtR with no independent relation of any other variable. The prevalence of H-N/EBP tripled between a WHtR of 0.5 and 0.7.

Conclusions

The easily obtained and calculated WHtR is the strongest correlate of elevated blood pressure among available variables and is a prime candidate for longitudinal studies of predictors of the development of hypertension.

Trial registration

ClinicalTrials.gov Identifier, NCT00746083

Isomer-specific comparisons of the hydrolysis of synthetic pyrethroids and their fluorogenic analogues by esterases from the cotton bollworm Helicoverpa armigera

The low aqueous solubility and chiral complexity of synthetic pyrethroids, together with large differences between isomers in their insecticidal potency, have hindered the development of meaningful assays of their metabolism and metabolic resistance to them. To overcome these problems, Shan and Hammock (2001) [7] therefore developed fluorogenic and more water-soluble analogues of all the individual isomers of the commonly used Type 2 pyrethroids, cypermethrin and fenvalerate. The analogues have now been used in several studies of esterase-based metabolism and metabolic resistance. Here we test the validity of these analogues by quantitatively comparing their hydrolysis by a battery of 22 heterologously expressed insect esterases with the hydrolysis of the corresponding pyrethroid isomers by these esterases in an HPLC assay recently developed by Teese et al. (2013) [14]. We find a strong, albeit not complete, correlation (r = 0.7) between rates for the two sets of substrates. The three most potent isomers tested were all relatively slowly degraded in both sets of data but three esterases previously associated with pyrethroid resistance in Helicoverpa armigera did not show higher activities for these isomers than did allelic enzymes derived from susceptible H. armigera. Given their amenability to continuous assays at low substrate concentrations in microplate format, and ready detection of product, we endorse the ongoing utility of the analogues in many metabolic studies of pyrethroids.

Hydrolysis of individual isomers of fluorogenic pyrethroid analogs by mutant carboxylesterases from Lucilia cuprina

We previously showed that wild-type E3 carboxylesterase of Lucilia cuprina has high activity against Type 1 pyrethroids but much less for the bulkier, alpha-cyano containing Type 2 pyrethroids. Both Types have at least two optical centres and, at least for the Type 1 compounds, we found that wild-type E3 strongly prefers the less insecticidal configurations of the acyl group. However, substitutions to smaller residues at two sites in the acyl pocket of the enzyme substantially increased overall activity, particularly for the more insecticidal isomers. Here we extend these analyses to Type 2 pyrethroids by using fluorogenic analogs of all the diastereomers of cypermethrin and fenvalerate. Wild-type E3 hydrolysed some of these appreciably, but, again, not those corresponding to the most insecticidal isomers. Mutations in the leaving group pocket or oxyanion hole were again generally neutral or deleterious. However, the two sets of mutants in the acyl pocket again improved activity for the more insecticidal acyl group arrangements as well as for the more insecticidal configuration of the cyano moiety on the leaving group. The activities of the best mutant enzyme against the analogs of the most insecticidal isomers of cypermethrin and fenvalerate were more than ten and a hundred fold higher, respectively, than those of wild-type. The implications for resistance development are discussed.

Hydrolysis of organophosphorus insecticides by in vitro modified carboxylesterase E3 from Lucilia cuprina

Resistance of the blowfly, Lucilia cuprina, to organophosphorus (OP) insecticides is due to mutations in LcalphaE7, the gene encoding carboxylesterase E3, that enhance the enzyme's ability to hydrolyse insecticides. Two mutations occur naturally, G137D in the oxyanion hole of the esterase, and W251L in the acyl binding pocket. Previous in vitro mutagenesis and expression of these modifications to the cloned gene have confirmed their functional significance. G137D enhances hydrolysis of diethyl and dimethyl phosphates by 55- and 33-fold, respectively. W251L increases dimethyl phosphate hydrolysis similarly, but only 10-fold for the diethyl homolog; unlike G137D however, it also retains ability to hydrolyse carboxylesters in the leaving group of malathion (malathion carboxylesterase, MCE), conferring strong resistance to this compound. In the present work, we substituted these and nearby amino acids by others expected to affect the efficiency of the enzyme. Changing G137 to glutamate or histidine was less effective than aspartate in improving OP hydrolase activity and like G137D, it diminished MCE activity, primarily through increases in Km. Various substitutions of W251 to other smaller residues had a broadly similar effect to W251L on OP hydrolase and MCE activities, but at least two were quantitatively better in kinetic parameters relating to malathion resistance. One, W251G, which occurs naturally in a malathion resistant hymenopterous parasitoid, improved MCE activity more than 20-fold. Mutations at other sites near the bottom of the catalytic cleft generally diminished OP hydrolase and MCE activities but one, F309L, also yielded some improvements in OP hydrolase activities. The results are discussed in relation to likely steric effects on enzyme-substrate interactions and future evolution of this gene.

Characterization of acetylcholinesterases, and their genes, from the hemipteran species Myzus persicae (Sulzer), Aphis gossypii (Glover), Bemisia tabaci (Gennadius) and Trialeurodes vaporariorum (Westwood)

Gene sequences encoding putative acetylcholinesterases have been reported for four hemipteran insect species. Although acetylcholinesterase insensitivity occurs in insecticide-resistant populations of each of these species, no mutations were detected in the gene sequences from the resistant insects. This, coupled with a series of experiments using novel reversible inhibitors to compare the biochemical characteristics of acetylcholinesterase from a range of insect species, showed that the cloned cDNA fragments are unlikely to encode the hemipteran synaptic acetylcholinesterases, and there is likely to be a second ace locus.

Mutations of the para Sodium Channel of Drosophila melanogaster Identify Putative Binding Sites for Pyrethroids

The effects of two pyrethroids on recombinant wild-type and mutant (pyrethroid-resistant) Na+ channels of Drosophila melanogaster have been studied. Three mutations that confer resistance (kdr/superkdr) to pyrethroids were inserted, either individually or in combination, into the para Na+ channel of D. melanogaster: L1014F in domain IIS6, M918T in the IIS4-S5 linker, and T929I in domain IIS5. Channels were expressed in Xenopus laevis oocytes and the effects of the pyrethroids permethrin (type I) and deltamethrin (type II) on Na+ currents were investigated using voltage clamp. The Na+ channels deactivated slowly after deltamethrin treatment, the resultant "tail" currents being used to quantify the effects of this pyrethroid. The Hill slope of 2 for deltamethrin action on the wild-type channel and the mutant L1014F channel is indicative of cooperative binding at two or more sites on these channels. In contrast, binding to the mutants M918T and T929I is noncooperative. Tail currents for the wild-type channel and L1014F channel decayed biphasically, whereas those for M918T and T929I mutants decayed monophasically. The L1014F mutant was approximately 20-fold less sensitive than the wild-type to deltamethrin. Surprisingly, the sensitivity of the double mutant M918T+L1014F to deltamethrin was similar to that of M918T alone, whereas the sensitivity of T929I+L1014F was >30,000-fold lower than that of T929I. Permethrin was less potent than deltamethrin, and its binding to all channel types was noncooperative. The decays of permethrin-induced tail currents were exclusively monophasic. These findings are discussed in terms of the properties and possible locations of pyrethroid binding sites on the D. melanogaster Na+ channel.

The molecular interactions of pyrethroid insecticides with insect and mammalian sodium channels

Recent progress in the cloning of alpha (para) and beta (TipE) Na channel sub-units from Drosophila melanogaster (fruit fly) and Musca domestica (housefly) have facilitated functional expression studies of insect Na channels in Xenopus laevis oocytes, assayed by voltage clamp techniques. The effects of Type I and Type III pyrethroids on the biophysical properties of these channels are critically reviewed. Pyrethroid resistance mutations (termed kdr and super-kdr) that reduce the sensitivity of the insect Na channel to pyrethroids have been identified in a range of insect species. Some of these mutations (e.g. L1014F, M918T and T929I) have been incorporated into the para Na channel of Drosophila, either individually or in combination, to investigate their effects on the sensitivity of this channel to pyrethroids. The kdr mutation (L1014F) shifts the voltage dependence of both activation and steady-state inactivation by approximately 5 mV towards more positive potentials and facilitates Na channel inactivation. Incorporation of the super-kdr mutation (M918T) into the Drosophila Na channel also increases channel inactivation and causes a > 100-fold reduction in deltamethrin sensitivity. These effects are shared by T929I, an alternative mutation that confers super-kdr-like resistance. Parallel studies have been undertaken using the rat IIA Na channel to investigate the molecular basis for the low sensitivity of mammalian brain Na channels to pyrethroids. Rat IIA channels containing the mutation L1014F exhibit a shift in their mid-point potential for Na activation, but their overall sensitivity to permethrin remains similar to that of the wild-type rat channel (i.e. both are 1000-fold less sensitive than the wild-type insect channel). Mammalian neuronal Na channels have an isoleucine rather than a methionine at the position (874) corresponding to the super-kdr (M918) residue of the insect channel. Replacement of the isoleucine of the wild-type rat IIA Na channel with a methionine (I874M) increases deltamethrin sensitivity 100-fold. In this way, studies of wild-type and mutant Na channels of insects and mammals are providing a molecular understanding of kdr and super-kdr resistance in insects, and of the low pyrethroid sensitivity of most mammalian Na channels. They are also giving valuable insights into the binding sites for pyrethroids on these channels.

Identification and characterization of mutations in housefly (Musca domestica) acetylcholinesterase involved in insecticide resistance

Acetylcholinesterase (AChE) insensitive to organophosphate and carbamate insecticides has been identified as a major resistance mechanism in numerous arthropod species. However, the associated genetic changes have been reported in the AChE genes from only three insect species; their role in conferring insecticide insensitivity has been confirmed, using functional expression, only for those in Drosophila melanogaster. The housefly, Musca domestica, was one of the first insects shown to have this mechanism; here we report the occurrence of five mutations (Val-180-->Leu, Gly-262-->Ala, Gly-262-->Val, Phe-327-->Tyr and Gly-365-->Ala) in the AChE gene of this species that, either singly or in combination, confer different spectra of insecticide resistance. The baculovirus expression of wild-type and mutated housefly AChE proteins has confirmed that the mutations each confer relatively modest levels of insecticide insensitivity except the novel Gly-262-->Val mutation, which results in much stronger resistance (up to 100-fold) to certain compounds. In all cases the effects of mutation combinations are additive. The mutations introduce amino acid substitutions that are larger than the corresponding wild-type residues and are located within the active site of the enzyme, close to the catalytic triad. The likely influence of these substitutions on the accessibility of the different types of inhibitor and the orientation of key catalytic residues are discussed in the light of the three-dimensional structures of the AChE protein from Torpedo californica and D. melanogaster.

Cloning, heterologous expression and co-assembly of Mpbeta1, a nicotinic acetylcholine receptor subunit from the aphid Myzus persicae

Nicotinic acetylcholine receptors (nAChRs) play a major role in excitatory synaptic transmission in insects and are also the target site for chloronicotinyl insecticides such as imidacloprid. Here we report the cloning and characterization of a novel nAChR beta subunit, Mpbeta1, from the aphid Myzus persicae, an economically important pest species. Sequence analysis has identified an open reading frame of 509 amino acids with features typical of nAChR subunits. The Mpbeta1 gene is expressed as a single major transcript of 4.6 kb, considerably larger than the predicted length of the Mpbeta1 open reading frame (1527 bp). By heterologous expression in Drosophila S2 cells, the Mpbeta1 subunit has been shown to co-assemble with the previously cloned nAChR subunits Mpalpha1 and Mpalpha2. In contrast, no co-assembly of Mpbeta1 could be detected with either Mpalpha3 or Mpalpha4. With the aim of gaining a clearer insight into the influence of subunit composition upon assembly, the ability of M. persicae nAChR subunits to co-assemble with vertebrate nAChR subunits has also been examined.

A single amino acid change makes a rat neuronal sodium channel highly sensitive to pyrethroid insecticides

Two amino acid substitutions in a housefly sodium channel, L1014F in domain IIS6 and M918T in the IIS4-S5 linker, have been identified in kdr and super-kdr pyrethroid-resistant phenotypes, respectively. Unlike their native insect counterparts, mammalian sodium channels are only weakly sensitive to pyrethroids. Do the sodium channels of mammal and pyrethroid-resistant housefly share similar structural characteristics that account for their low pyrethroid sensitivities? We report here that substitution of isoleucine for methionine at position 874 (equivalent to the super-kdr site 918 in the housefly) in the rat IIA alpha-subunit causes a 100-fold increase in sensitivity.

Amplification and methylation of an esterase gene associated with insecticide-resistance in greenbugs, Schizaphis graminum (Rondani) (Homoptera: Aphididae)

The greenbug aphid, Schizaphis graminum (Rondani) has developed resistance to organophosphorus insecticides by the over-production of esterases that have been classified as Type I and Type II. The first twenty N-terminal amino acids of the Type I esterase were determined and used to design an oligonucleotide, which in conjunction with an active site primer derived from conserved sequences of other insect esterases and two internal primers specific for esterases from another aphid species resulted in a 0.85 kb genomic DNA fragment from resistant greenbugs. This was extended by 5' RACE which provided approximately 1.2 kb of the 5' end of the esterase gene. The 5' DNA sequence corresponded to 19 of the 20 known amino acids of the Type I esterase, with the last needing only a one base change (probably resulting from a PCR artifact). Furthermore, the sequence showed very close similarity to the amplified E4/FE4 esterase genes of Myzus persicae (Sulzer). A comparison of sequences suggested that the S. graminum gene has introns in the same positions as the first two introns of E4/FE4, with the second intron being considerably larger in S. graminum. Probing of Southern blots with the 0.85 kb esterase fragment showed that the gene encoding the Type I esterase is amplified 4- to 8-fold in resistant S. graminum and that the amplified sequences contain 5-methylcytosine at MspI/HpaII sites, again in agreement with previous findings for M. persicae genes.

The pharmacological flexibility of the insect voltage gated sodium channel: toxicity of AaIT to knockdown resistant (kdr) flies

AaIT is an insect selective neurotoxic polypeptide shown to affect insect neuronal sodium conductance by binding to excitable sodium channels. In the present study the paralytic potency of AaIT to wild type and various mutant strains of houseflies (Musca domestica) and fruitflies (Drosophila melanogaster) was examined and it has been shown that: On the basis of body weight when compared to published data on Sarcophaga falculata blowflies, the Musca and Drosophila flies reveal at least two orders of magnitude decreased susceptibility to the AaIT. When compared to wild type flies the toxicity of AaIT is greatly altered in knockdown resistant fly strains which are mutated in their para gene encoding the voltage gated sodium channel. Several strains, with genetically mapped para mutations conferring pyrethroid resistance, exhibited opposing response to AaIT. The para ts2 Drosophila strain, with a point of mutation in domain I of the para gene conferring a 6-fold resistance to deltamethrin also showed about 15-fold tolerance to AaIT. On the other hand the Musca kdr and super-kdr flies, with a single or a double point mutation, respectively in domain II of the para gene, are about 9- and 14-fold more susceptible to AaIT, respectively. The above data are interpreted in terms of the pharmacological diversity and flexibility ("allosteric coupling") of voltage gated sodium channels and their implications for the management of pesticide resistance are discussed.

A sodium channel point mutation is associated with resistance to DDT and pyrethroid insecticides in the peach-potato aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae)

The voltage-gated sodium channel is the primary target site of DDT and pyrethroid insecticides, and point mutations in the domain II region of the channel protein have been implicated in the knockdown resistant (kdr ) phenotype of several insect species. Here, we report that one of these mutations, a leucine-to-phenylalanine replacement in transmembrane segment IIS6, is also found in certain insecticide-resistant clones of the peach-potato aphid, Myzus persicae. The mutation was present in four clones with amplified E4 esterase genes, but was absent from both susceptible clones and those with amplified FE4 genes. The inferred presence of kdr-type resistance in the four E4 clones was subsequently confirmed by bioassays that showed this to be the primary mechanism of resistance to deltamethrin and DDT, although the esterase-based mechanism also contributes to the overall level of deltamethrin resistance. The kdr mutation on its own conferred 35-fold resistance to deltamethrin and this was enhanced up to 540-fold when it was present in a high (E4) esterase background. The esterase (FE4) mechanism was far less effective without the kdr mutation, conferring just 3-4-fold resistance to deltamethrin. These findings, and the linkage disequilibrium of the kdr mutation within clones overproducing the E4 esterase, have important implications for the evolution of resistance in this insect and for the use of pyrethroid sprays in the management of M. persicae populations in the field.

Kinetic microplate-based assays for inhibitors of mitochondrial NADH:ubiquinone oxidoreductase (complex I) and succinate:cytochrome c oxidoreductase

Kinetic microplate-based assays for both mitochondrial NADH:ubiquinone oxidoreductase (complex I) and succinate:cytochrome c oxidoreductase using insect submitochondrial particles as the source of the enzyme activities have been developed. These assays have been used to design high-throughput screens for inhibitors of these mitochondrial electron transfer activities to assess their intrinsic in vitro efficacies as potential pesticides. These methods can be used to test up to 60 compounds per day without the use of automated sample handling and diluting technology. The accuracy, specificity, and reproducibility of the microplate methods compared well with conventional spectrophotometer-based assays.

Molecular characterization and imidacloprid selectivity of nicotinic acetylcholine receptor subunits from the peach-potato aphid Myzus persicae

The recent introduction of the chloronicotinyl insecticide imidacloprid, targeting insect nicotinic acetylcholine receptors (nAChRs), emphasises the importance of a detailed molecular characterisation of these receptors. We are investigating the molecular diversity of insect nAChR subunit genes in an important agricultural pest, the peach-potato aphid Myzus persicae. Two M. persicae alpha-subunit cDNAs, Mp alpha1 and Mp alpha2, have been cloned previously. Here we report the isolation of three novel alpha-subunit genes (Mp alpha3-5) with overall amino acid sequence identities between 43 and 76% to characterised insect nAChR subunits. Alignment of their amino acid sequences with other invertebrate and vertebrate nAChR subunits suggests that the insect alpha subunits evolved in parallel to the vertebrate neuronal nAChRs and that the insect non-alpha subunits are clearly different from vertebrate neuronal beta and muscle non-alpha subunits. The discovery of novel subtypes in M. persicae is a further indicator of the complexity of the insect nAChR gene family. Heterologous co-expression of M. persicae nAChR alpha-subunit cDNAs with the rat beta2 in Drosophila S2 cells resulted in high-affinity binding of nicotinic radioligands. The affinity of recombinant nAChRs for [3H]imidacloprid was influenced strongly by the alpha subtype. This is the first demonstration that imidacloprid selectively acts on Mp alpha2 and Mp alpha3 subunits, but not Mp alpha1, in M. persicae.

Relationship between amount of esterase and gene copy number in insecticide-resistant Myzus persicae (Sulzer)

Overproduction of the insecticide-degrading esterases, E4 and FE4, in peach-potato aphids, Myzus persicae (Sulzer), depends on both gene amplification and transcriptional control, the latter being associated with changes in DNA methylation. The structure and function of the aphid esterase genes have been studied but the determination of their copy number has proved difficult, a common problem with gene amplification. We have now used a combination of pulsed-field gel electrophoresis and quantitative competitive PCR to determine relative esterase gene copy numbers in aphid clones with different levels of insecticide resistance (R1, R2 and R3). There are approx. 4-fold increases between susceptible, R1, R2 and R3 aphids, reaching a maximum of approx. 80 times more genes in R3; this gives proportionate increases in esterase protein relative to susceptible aphids. Thus there is no overexpression of the amplified genes, in contrast with what was thought previously. For E4 genes, the loss of 5-methylcytosine is correlated with a loss of expression, greatly decreasing the amount of enzyme relative to the copy number.

Molecular characterization of pyrethroid knockdown resistance (kdr) in the major malaria vector Anopheles gambiae s.s

Pyrethroid-impregnated bednets are playing an increasing role for combating malaria, especially in stable malaria areas. More than 90% of the current annual malaria incidence (c. 500 million clinical cases with up to 2 million deaths) is in Africa where the major vector is Anopheles gambiae s.s. As pyrethroid resistance has been reported in this mosquito, reliable and simple techniques are urgently needed to characterize and monitor this resistance in the field. In insects, an important mechanism of pyrethroid resistance is due to a modification of the voltage-gated sodium channel protein recently shown to be associated with mutations of the para-type sodium channel gene. We demonstrate here that one of these mutations is present in certain strains of pyrethroid resistant A. gambiae s.s. and describe a PCR-based diagnostic test allowing its detection in the genome of single mosquitoes. Using this test, we found this mutation in six out of seven field samples from West Africa, its frequency being closely correlated with survival to pyrethroid exposure. This diagnostic test should bring major improvement for field monitoring of pyrethroid resistance, within the framework of malaria control programmes.

Evidence that the E4 and FE4 esterase genes responsible for insecticide resistance in the aphid Myzus persicae (Sulzer) are part of a gene family

The amplification of genes encoding the esterases E4 and FE4 is a widespread mechanism of insecticide resistance in the peach-potato aphid, Myzus persicae (Sulzer). We present evidence that in susceptible aphids the two genes are adjacent to each other in a head-to-tail arrangement with E4 upstream of FE4 and with approx. 19 kb of intervening sequence. There are also at least two other closely related sequences which might come from other members of an esterase gene family, in line with reports of other insect gene families encoding detoxifying enzymes. The close identity between E4 and FE4 genes indicates a recent duplication and divergence. The subsequent amplifications giving multiple copies of either E4 or FE4 must have involved two separate events, each probably occurring once and then being selected by insecticide exposure and spread by migration. The cloning of sequences upstream of the FE4 gene suggest, by comparison with E4, that the two genes are regulated in different ways. FE4 has sequences corresponding to a conventional promoter (TATA box and CAP site) that are not present in E4; on the other hand, FE4 lacks the CpG island present 5' of E4 genes that may control expression through changes in DNA methylation. The differences are likely to have occurred by the duplication event that gave rise to E4 and FE4 leading to different 5' sequences.

Characterization of the acetylcholinesterase gene from insecticide-resistant houseflies (Musca domestica)

Acetylcholinesterase (AChE) is the target site for the organophosphates and carbamates in insects. Widespread use of these two classes of insecticides has led to the selection of resistance. Target modification was regarded as a molecular mechanism in some resistance species. The altered AChEs with reduced sensitivity to inhibition are related to this resistance. AChE genes from two insecticide-resistant housefly (Musca domestica) strains D3 and Kash were isolated and sequenced using RT-PCR and streptavidin-linked magnetic bead techniques. The cDNAs have a 2082-bp open reading frame from which the complete amino acid sequence of AChE has been deduced. Some differences in nucleotide sequence and four-point mutations of amino acid were found compared to a susceptible strain, i.e., the Cooper strain. Three substitutions are likely to confer insecticide insensitivity, which seems that D3 and Kash belong to CH2 pattern of resistance.

Functional analysis of a rat sodium channel carrying a mutation for insect knock-down resistance (kdr) to pyrethroids

Pyrethroid insensitivity in resistant (kdr) insects has been correlated with a leucine to phenylalanine replacement in the S6 transmembrane segment of domain II of the axonal sodium channel alpha(para)-subunit. An alpha-subunit of rat brain type II sodium channel containing this mutation has been expressed and its sensitivity to permethrin compared with that of the wild-type channel. The steady-state activation curve of the mutant was shifted 14 mV in the depolarizing direction. We propose that an equivalent shift of the sodium current activation curve in kdr insects could account for their low sensitivity to permethrin toxicity.

Structure and organization of amplicons containing the E4 esterase genes responsible for insecticide resistance in the aphid Myzus persicae (Sulzer)

Insecticide resistance in the aphid Myzus persicae results primarily from the amplification of genes encoding the insecticide-detoxifying esterase, E4. Here we report the analysis of flanking DNA co-amplified with the E4 gene. The 5' end of this gene has an untranslated leader sequence interspersed by two introns, and the promoter region lacks TATA and CAAT boxes. The DNA breakpoint involved in the generation of the amplification is just upstream (approx. 250 bp) of the putative E4 transcription start site; thus the E4 gene is very close to the 5' end of the approx. 24 kb amplicon. PCR primers specific to the 'novel joint' generated during the amplification have been used to show that a wide range of aphid clones have the same amplicons, arranged as a series of head-to-tail direct repeats. Long-distance mapping has revealed the structure of these repeats. This has important implications for understanding both the generation of the amplified genes and the origin and spread of insecticide resistance in M. persicae.

Identification of mutations in the housefly para-type sodium channel gene associated with knockdown resistance (kdr) to pyrethroid insecticides

We report the isolation of cDNA clones containing the full 6.3-kb coding sequence of the para-type sodium channel gene of the housefly, Musca domestica. This gene has been implicated as the site of knockdown resistance (kdr), an important resistance mechanism that confers nerve insensitivity to DDT and pyrethroid insecticides. The cDNAs predict a polypeptide of 2108 amino acids with close sequence homology (92% identity) to the Drosophila para sodium channel, and around 50% homology to vertebrate sodium channels, Only one major splice form of the housefly sodium channel was detected, in contrast to the Drosophila para transcript which has been reported to undergo extensive alternative splicing. Comparative sequence analysis of housefly strains carrying kdr or the more potent super-kdr factor revealed two amino acid mutations that correlate with these resistance phenotypes. Both mutations are located in domain II of the sodium channel. A leucine to phenylalanine replacement in the hydro-phobic IIS6 transmembrane segment was found in two independent kdr strains and six super-kdr strains of diverse geographic origin, while an additional methionine to threonine replacement within the intracellular IIS4-S5 loop was found only in the super-kdr strains. Neither mutation was present in five pyrethroid-sensitive strains. The mutations suggest a binding site for pyrethroids at the intracellular mouth of the channel pore in a region known to be important for channel inactivation.

Polymerase chain reaction-based identification of insecticide resistance genes and DNA methylation in the aphid Myzus persicae (Sulzer)

The ability of a peach-potato aphid (Myzus persicae) to resist insecticides can depend on both the presence of amplified esterase genes and on differences in their expression. Here we report a polymerase chain reaction-restriction enzyme (PCR-REN) technique which can detect the presence of amplified esterase genes and distinguish between the two possible types of amplified gene (E4 and FE4) and a homologous fragment from susceptible aphids. The technique is quick and sensitive enough to be used on a fraction of an aphid or an individual embryo. Furthermore, it can be combined with a pre-PCR digestion using a methylation-sensitive enzyme (Hpall) to determine whether or not the esterase genes contain 5-methylcytosine, the presence or absence of which is known to correlate with changes in gene expression.

Insensitive Acetylcholinesterase and Resistance to Thiodicarb in Australian Helicoverpa armigera Hübner (Lepidoptera: Noctuidae)

Thiodicarb resistance was diagnosed in a field strain of Helicoverpa armigera from Tamworth, New South Wales in March 1993. This is the first record of H. armigera resistance to thiodicarb. Bioassays after selection in the laboratory indicated that the thiodicarb resistance factor was greater than 40-fold and was associated with cross resistance to methomyl. Insensitive acetylcholinesterase was identified as a resistance mechanism. Potential resistance management tactics and opportunities for biochemical resistance monitoring are discussed.

Biochemical evidence of haplodiploidy in the whitefly Bemisia tabaci

Polymorphic esterase and acetylcholinesterase alleles in the whitefly Bemisia tabaci were studied using electrophoretic and colorimetric assays. The segregation of these alleles between parental and F1 generations provided unequivocal evidence of haplodiploidy in this pest species. Unmated females, heterozygous at a polymorphic locus, produced a 1:1 ratio of haploid males expressing either of the maternal alleles. Although male offspring were produced by both virgin and mated females, the segregation of alleles showed they were always haploid (hemizygous) for the marker enzymes. Females only arose from fertilized eggs and invariably expressed paternal and maternal alleles.

Analysis of amplicons containing the esterase genes responsible for insecticide resistance in the peach-potato aphid Myzus persicae (Sulzer)

The amplification of genes encoding an insecticide-detoxifying esterase (E4) in the peach-potato aphid Myzus persicae is one of the few examples where this genetic phenomenon has been shown to be involved in the response of an intact higher organism to artificial selection. Here we report quantitative and qualitative studies of the repeat units (amplicons) containing the E4 genes in a highly resistant aphid clone. Initial studies to quantify esterase sequences showed a 5-11-fold increase in resistant aphids compared with susceptible aphids, suggesting the presence of 10-22 gene copies per diploid genome. A more incisive analysis by pulsed-field gel electrophoresis confirmed the presence of about 12 copies of the E4 gene and showed them to be on about 24 kb amplicons, arranged as a tandem array of direct repeats. This, together with previous results from crossing experiments and with recent in situ hybridization studies, confirms that the E4 gene amplification in this aphid clone is heterozygous at a single locus. However, these data show that the gene amplification alone cannot account for the approx. 60 times higher levels of E4 protein and its mRNA present in this aphid clone, and therefore resistance must involve changes in both esterase gene copy number and gene expression.

Changes in the methylation of amplified esterase DNA during loss and reselection of insecticide resistance in peach-potato aphids, Myzus persicae

Insecticide resistance in peach-potato aphids, Myzus persicae, results from the amplification of genes encoding an esterase that hydrolyses and sequesters insecticides. Resistance is normally stable, but highly resistant aphid clones sometimes lose resistance when insecticidal selection pressure is removed. This loss of resistance, termed reversion, arises from a loss of elevated esterase enzyme through transcriptional control, i.e. without loss of the amplified esterase DNA sequences. We have shown that loss of the elevated enzyme occurred simultaneously with loss of methylation at CCGG sites in the amplified DNA sequences. During reselection of resistance in these revertant clones, enzyme levels increased, but there was no corresponding return of methylation to DNA sequences. Thus, although DNA methylation is closely correlated with expression of the amplified esterase genes during reversion, it may not be a factor in the reverse process.

Biochemical identification of interbreeding between B-type and non B-type strains of the tobacco whitefly Bemisia tabaci

Polyacrylamide gel electrophoresis and a kinetic microplate assay were used to detect heterozygotes resulting from a cross between B-type and non B-type strains of the whitefly Bemisia tabaci. Both strains were homozygous for different esterase and acetylcholinesterase (AChE) enzymes, and heterozygotes were produced in one of two crosses between B-type females (diploid) and non B-type males (haploid). In reciprocal crosses, however, no female offspring were produced, indicating that fertilization had not occurred. Despite the identification of individuals heterozygous for the esterase and AChE markers, there was clearly a significant degree of reproductive incompatibility between the two strains. The biochemical assays provided a vital component of this study and the advantages of their use are discussed.

The peach-potato aphid Myzus persicae and the tobacco aphid Myzus nicotianae have the same esterase-based mechanisms of insecticide resistance

Biochemical and molecular studies have established that in the peach-potato aphid, Myzus persicae, insecticide resistance is conferred by amplification of genes encoding the insecticide-detoxifying esterases E4 or FE4. Here we report that two insecticide-resistant clones of the closely related tobacco aphid Myzus nicotianae have elevated esterases indistinguishable from E4 and FE4 and amplified esterase DNA sequences, and flanking regions, with identical restriction maps to the M. persicae genes. Furthermore, the DNA sequences of c. 630 bp fragments of the E4 and FE4 genes of M. persicae are different from each other but identical to the fragment from corresponding M. nicotianae clones. The existence of apparently identical insecticide resistance genes in the two species can be best explained by the selection of the amplified genes in M. persicae, transfer to hybrids of M. persicae and M. nicotianae by sexual reproduction and subsequent spread through M. nicotianae populations.

Cloning and analysis of the esterase genes conferring insecticide resistance in the peach-potato aphid, Myzus persicae (Sulzer)

Full-length cDNA clones encoding the esterases (E4 and FE4) that confer insecticide resistance in the peach-potato aphid [Myzus persicae (Sulzer)] were isolated and characterized. The E4 cDNA contained an open reading frame of 1656 nucleotides, coding for a protein of 552 amino acids. The FE4 cDNA shared 99% identity with E4 over this region, the most important difference being a single nucleotide substitution resulting in the FE4 mRNA having an extra 36 nucleotides at the 3' end. The derived amino acid sequences for the N-terminus of E4 and FE4 were identical, with the first 23 residues being characteristic of a signal peptide and the next 40 residues being an exact match to the N-terminal sequence determined by Edman degradation of both purified proteins. The predicted molecular masses of 58.8 and 60.2 kDa for the E4 and FE4 polypeptides were consistent with those previously observed by in vitro translation of mRNA. Five potential N-linked glycosylation sites were present in both polypeptides, in accordance with earlier evidence that the native esterases are glycoproteins. Comparison of the aphid esterase protein sequences with other serine hydrolases provided evidence that their activity involves a charge-relay system with a catalytic triad the same as that found in acetylcholinesterase. Restriction mapping and sequencing of cloned genomic DNA showed that the E4 gene is spread over 4.3 kb with six introns and that the previously reported differences between the 3' ends of the E4 and FE4 genes result from single nucleotide substitutions and not gross differences in the DNA sequences.

Knockdown resistance (kdr) to DDT and pyrethroid insecticides maps to a sodium channel gene locus in the housefly (Musca domestica)

The voltage-sensitive sodium channel is generally regarded as the primary target site of dichloro-diphenyl-trichloro-ethane (DDT) and pyrethroid insecticides, and has been implicated in the widely reported mechanism of nerve insensitivity to these compounds. This phenomenon is expressed as knockdown resistance (kdr) and has been best characterised in the housefly where several putative alleles, including the more potent super-kdr factor, have been identified. We report the isolation of cDNA clones containing part of a housefly sodium channel gene, designated Msc, which show close homology to the para sodium channel of Drosophila (99% amino acid identity within the region of overlap). Using Southern blots of insect DNA, restriction fragment length polymorphisms (RFLPs) at the Msc locus were identified in susceptible, kdr and super-kdr housefly strains. These RFLPs showed tight linkage to resistance in controlled crosses involving these strains, thus providing clear genetic evidence that kdr, and hence pyrethroid mode of action, is closely associated with the voltage-sensitive sodium channel.

Molecular evidence that insecticide resistance in peach-potato aphids (Myzus persicae Sulz.) results from amplification of an esterase gene

cDNA clones for the esterase (E4) responsible for broad insecticide resistance in peach-potato aphids (Myzus persicae Sulz.) were isolated and used to study the molecular basis of resistance. Increased esterase synthesis by resistant aphids was found to be associated with amplification of the structural gene for the esterase (E4 or its closely related variant, FE4), the degree of amplification being correlated with the activity of the esterase and the level of resistance. Hybridization of the cDNA clones to genomic Southern blots showed that only some of the esterase-related restriction fragments are amplified. Qualitative differences between restriction patterns in different clones of resistant aphids correlated with the presence or absence of a specific chromosome translocation and with production of E4 or FE4.

A multiple homogenizer for rapid sample preparation in immunoassays and electrophoresis

A multiple homogenizer is described for preparing samples of small invertebrates or tissue in a flat-bottom immunoplate. Its efficiency was evaluated by immunoassay of carboxylesterase (E4), the enzyme conferring insecticide resistance in the peach potato aphid (Myzus persicae). This equipment was shown to release more enzyme, with less variability, than homogenizing individual aphids and its efficiency allows one person to analyze up to 3000 individual insects per day. It is also suitable for preparing samples for electrophoretic analysis. In the present study samples were loaded onto electrophoresis gels rapidly and accurately by using an eight-channel multipipette.

The properties of a carboxylesterase from the peach-potato aphid, Myzus persicae (Sulz.), and its role in conferring insecticide resistance

Carboxylesterases from different strains of Myzus persicae were examined to try to understand their contribution to insecticide resistance. Preliminary evidence that they are involved comes from the good correlation between the degree of resistance and the carboxylesterase and paraoxon-degrading activity in aphid homogenates. Furthermore the carboxylesterase associated with resistance could not be separated from the insecticide-degrading enzyme by electrophoresis or ion-exchange chromatography. Homogenates of resistant aphids hydrolysed paraoxon 60 times faster than did those of susceptible aphids, yet the purified enzymes from both sources had identical catalytic-centre activities towards this substrate and also towards naphth-1-yl acetate, the latter being hydrolysed by both 2x10(6) times faster than paraoxon. These observations provide evidence that the enzyme from both sources is identical, and that one enzyme hydrolyses both substrates. This was confirmed by relating the rate of paraoxon hydrolysis to the rate at which paraoxon-inhibited carboxylesterase re-activated. Both had the same first-order rate constant (0.01min(-1)), showing clearly that the hydrolysis of both substrates is brought about by the same enzyme. Its K(m) for naphth-1-yl acetate was 0.131mm, and for paraoxon 75pm. The latter very small value could not be measured directly, but was calculated from substrate-competition studies coupled with measurements of re-activation of the diethyl phosphorylated enzyme. Since the purified enzymes from resistant and susceptible aphids had the same catalytic-centre activity, the 60-fold difference between strains must be caused by different amounts of the same enzyme resulting from mutations of the regulator gene(s) rather than of the structural gene.

Studies of the acetylcholinesterase from houseflies (Musca domestica L.) resistant and susceptible to organophosphorus insecticides

Acetylcholinesterase from the heads of insecticide-resistant and -susceptible houseflies (Musca domestica L.) was studied in vitro. The enzymes could not be distinguished electrophoretically, and their behaviour on polyacrylamide-disc-gel electrophoresis was influenced by the presence of Triton X-100 in both the homogenate and the gels. In the absence of detergent, the acetylcholinesterase was heterogeneous, but behaved as a single enzyme when it was present. By analogy with studies of acetylcholinesterase from other sources, these observations were attributed to aggregation of the enzyme when not bound by membranes. The enzyme from resistant flies was more slowly inhibited than the susceptible enzyme, bimolecular rate constants (ki) differing by approx. 4-20-fold for a range of organophosphorus compounds. The kinetics of inhibition of acetylcholinesterase were consistent with the results of electrophoresis, i.e. they corresponded to those of a single enzyme in the presence of Triton X-100, but a mixture of enzymes in its absence. The susceptibility of the more sensitive components in these mixtures was determined.