Drosophila acetylcholinesterase: demonstration of a glycoinositol phospholipid anchor and an endogenous proteolytic cleavage

Biochemical properties, expression profiles, and tissue localization of orthologous acetylcholinesterase-2 in the mosquito, Anopheles gambiae

Acetylcholinesterases (AChEs) catalyze the hydrolysis of acetylcholine, a neurotransmitter for cholinergic neurotransmission in animals. Most insects studied so far possess two AChE genes: ace-1 paralogous and ace-2 orthologous to Drosophila melanogaster ace. We characterized the catalytic domain of Anopheles gambiae AChE1 in a previous study (Jiang et al., 2009) and report here biochemical properties of A. gambiae AChE2 expressed in Sf9 cells. An unknown protease in the expression system cleaved the recombinant AChE2 next to Arg(110), yielding two non-covalently associated polypeptides. A mixture of the intact and cleaved AChE2 had a specific activity of 72.3 U/mg, much lower than that of A. gambiae AChE1 (523 U/mg). The order of V(max)/K(M) values for the model substrates was acetylthiocholine > propionylthiocholine ≈ acetyl-(β-methyl)thiocholine > butyrylthiocholine. The IC(50)'s for eserine, carbaryl, BW284C51, paraoxon and malaoxon were 1.32, 13.6, 26.8, 192 and 294 nM, respectively. A. gambiae AChE2 bound eserine and carbaryl stronger than paraoxon and malaoxon, whereas eserine and malaoxon modified the active site Ser(232) faster than carbaryl or paraoxon did. Consequently, the k(i)'s were 1.173, 0.245, 0.029 and 0.018 μM(-1)min(-1) for eserine, carbaryl, paraoxon and malaoxon, respectively. Quantitative polymerase chain reactions showed a similar pattern of ace-1 and ace-2 expression. Their mRNAs were abundant in early embryos, greatly decreased in late embryos, larvae, pupae, and pharate adult, and became abundant again in adults. Both transcripts were higher in head and abdomen than thorax of adults and higher in male than female mosquitoes. Transcript levels of ace-1 were 1.9- to 361.8-fold higher than those of ace-2, depending on developmental stages and body parts. Cross-reacting polyclonal antibodies detected AChEs in adult brains, thoracic ganglia, and genital/rectal area. Activity assays, immunoblotting, and tandem mass spectrometric analysis indicated that A. gambiae AChE1 is responsible for most of acetylthiocholine hydrolysis in the head extracts. Taken together, these data indicate that A. gambiae AChE2 may play a less significant role than AChE1 does in the mosquito nervous system.

Genomic structure, organization and localization of the acetylcholinesterase locus of the olive fruit fly, Bactrocera oleae

Acetylcholinesterase (AChE), encoded by the ace gene, is a key enzyme of cholinergic neurotransmission. Insensitive acetylcholinesterase (AChE) has been shown to be responsible for resistance to OPs and CBs in a number of arthropod species, including the most important pest of olives trees, the olive fruit fly Bactrocera oleae. In this paper, the organization of the B. oleae ace locus, as well as the structural and functional features of the enzyme, are determined. The organization of the gene was deduced by comparison to the ace cDNA sequence of B. oleae and the organization of the locus in Drosophila melanogaster. A similar structure between insect ace gene has been found, with conserved exon-intron positions and junction sequences. The B. oleae ace locus extends for at least 75 kb, consists of ten exons with nine introns and is mapped to division 34 of the chromosome arm IIL. Moreover, according to bioinformatic analysis, the Bo AChE exhibits all the common features of the insect AChE. Such structural and functional similarity among closely related AChE enzymes may implicate similarities in insecticide resistance mechanisms.

Genome organization, phylogenies, expression patterns, and three-dimensional protein models of two acetylcholinesterase genes from the red flour beetle

Since the report of a paralogous acetylcholinesterase (AChE, EC3.1.1.7) gene in the greenbug (Schizaphis graminum) in 2002, two different AChE genes (Ace1 and Ace2) have been identified in each of at least 27 insect species. However, the gene models of Ace1 and Ace2, and their molecular properties have not yet been comprehensively analyzed in any insect species. In this study, we sequenced the full-length cDNAs, computationally predicted the corresponding three-dimensional protein models, and profiled developmental stage and tissue-specific expression patterns of two Ace genes from the red flour beetle (Tribolium castaneum; TcAce1 and TcAce2), a globally distributed major pest of stored grain products and an emerging model organism. TcAce1 and TcAce2 encode 648 and 604 amino acid residues, respectively, and have conserved motifs including a choline-binding site, a catalytic triad, and an acyl pocket. Phylogenetic analysis show that both TcAce genes are grouped into two insect Ace clusters and TcAce1 is completely diverged from TcAce2, suggesting that these two genes evolve from their corresponding Ace gene lineages in insect species. In addition, TcAce1 is located on chromosome 5, whereas TcAce2 is located on chromosome 2. Reverse transcription polymerase chain reaction (PCR) and quantitative real-time PCR analyses indicate that both genes are virtually transcribed in all the developmental stages and predominately expressed in the insect brain. Our computational analyses suggest that the TcAce1 protein is a robust acetylcholine (ACh) hydrolase and has susceptibility to sulfhydryl agents whereas the TcAce2 protein is not a catalytically efficient ACh hydrolase.

Altered GPI modification of insect AChE improves tolerance to organophosphate insecticides

The olive fruit fly Bactrocera oleae is the most destructive and intractable pest of olives. The management of B. oleae has been based on the use of organophosphate (OP) insecticides, a practice that induced resistance. OP-resistance in the olive fly was previously shown to be associated with two mutations in the acetylcholinesterase (AChE) enzyme that, apparently, hinder the entrance of the OP into the active site. The search for additional mutations in the ace gene that encodes AChE revealed a short deletion of three glutamines (Δ3Q) from a stretch of five glutamines, in the C-terminal peptide that is normally cleaved and substituted by a GPI anchor. We verified that AChEs from B. oleae and other Dipterans are actually GPI-anchored, although this is not predicted by the "big-PI" algorithm. The Δ3Q mutation shortens the unusually long hydrophilic spacer that follows the predicted GPI attachment site and may thus improve the efficiency of GPI anchor addition. We expressed the wild type B. oleae AChE, the natural mutant Δ3Q and a constructed mutant lacking all 5 consecutive glutamines (Δ5Q) in COS cells and compared their kinetic properties. All constructs presented identical K(m) and k(cat) values, in agreement with the fact that the mutations did not affect the catalytic domain of the enzyme. In contrast, the mutants produced higher AChE activity, suggesting that a higher proportion of the precursor protein becomes GPI-anchored. An increase in the number of GPI-anchored molecules in the synaptic cleft may reduce the sensitivity to insecticides.

Functional analysis and molecular characterization of two acetylcholinesterases from the German cockroach, Blattella germanica

Two acetylcholinesterases (AChEs; BgAChE1 and BgAChE2) from Blattella germanica were functionally expressed using the baculovirus system. Kinetic analysis demonstrated that BgAChE2 had higher catalytic efficiency but lower substrate specificity than BgAChE1. With the exceptions of paraoxon and propoxur, BgAChE1 was generally less sensitive to inhibitors than BgAChE2. Western blot analysis using anti-BgAChE antibodies revealed that BgAChE1 was far more abundant in all examined tissues compared to BgAChE2, which is only present in the central nervous system. Both BgAChEs existed in dimeric form, covalently connected via a disulphide bridge under native conditions. Most fractions of BgAChE1 had a glycophosphatidylinositol (GPI) anchor, but a small fraction comprised a collagen-like tail. BgAChE2 appeared to have a collagen-GPI-fused tail. Based on the kinetic and molecular properties, tissue distribution and abundance, BgAChE1 was confirmed to play a major role in postsynaptic transmission.

Acetylcholinesterase cDNA sequencing and identification of mutations associated with organophosphate resistance in Cochliomyia hominivorax (Diptera: Calliphoridae)

Altered acetylcholinesterase (AChE) has been identified in numerous arthropod species resistant to organophosphate (OP) and carbamate insecticides. The New World screwworm (NWS) Cochliomyia hominivorax (Coquerel), one of the most important myiasis-causing flies in the Neotropics, has been controlled mainly by the application of OP insecticides in its current geographical distribution. However, few studies have investigated insecticide resistance in this species. Based on previous studies about mutations conferring OP resistance in related dipteran species, AChE cDNA was sequenced allowing a survey for mutations (I298V, G401A, F466Y) in NWS populations. In addition, the G137D mutation in the carboxylesterase E3 gene, also associated with OP resistance, was analyzed in the same NWS populations. Only 2/135 individuals presented an altered AChE gene (F466Y). In contrast, a high frequency of the G137D mutation in the E3 gene was found in some localities of Brazil and Uruguay, while the mutant allele was not found in Cuba, Venezuela or Colombia. These findings suggest that the alteration in the carboxylesterase E3 gene may be one of the main resistance mechanisms selected in this ectoparasite. The knowledge of the frequency of these resistance-associated mutations in the NWS natural populations may contribute to the selection of appropriate chemicals for control as part of pest management strategies.

Molecular characterization and inhibition analysis of the acetylcholinesterase gene from the silkworm maggot, Exorista sorbillans

Several organophosphorus (OP) insecticides can selectively kill the silkworm maggot, Exorista sorbillans (Es) (Diptera: Tachinidae), while not obviously affecting the host (Bombyx mori) larvae, but the mechanism is not yet clear. In this study, the cDNA encoding an acetylcholinesterase (AChE) from the field Es was isolated. One point mutation (Gly353Ala) was identified. The Es-353G AChE and Es-353A AChE were expressed in baculovirus- insect cell system, respectively. The inhibition results showed that for eserine and Chlorpyrifos, Es-353A AChE was significantly less sensitive than Es-353G AChE. Meanwhile, comparison of the I(50) values of eserine, dichlorvos, Chlorpyrifos and omethoate of recombinant Es AChEs with its host (Bombyx mori) AChEs indicated that, both Es AChEs are more sensitive than B. mori AChEs. The results give an insight of the mechanism that some OP insecticides can selectively kills Es while without distinct effect on its host, B. mori.

Functional characterisation of a cyst nematode acetylcholinesterase gene using Caenorhabditis elegans as a heterologous system

Migration of plant-parasitic nematode infective larval stages through soil and invasion of roots requires perception and integration of sensory cues culminating in particular responses that lead to root penetration and parasite establishment. Components of the chemoreceptive neuronal circuitry involved in these responses are targets for control measures aimed at preventing infection. Here we report, to our knowledge, the first isolation of cyst nematode ace-2 genes encoding acetylcholinesterase (AChE). The ace-2 genes from Globodera pallida (Gp-ace-2) and Heterodera glycines (Hg-ace-2) show homology to ace-2 of Caenorhabditis elegans (Ce-ace-2). Gp-ace-2 is expressed most highly in the infective J2 stage with lowest expression in the early parasitic stages. Expression and functional analysis of the Globodera gene were carried out using the free-living nematode C. elegans in order to overcome the refractory nature of the obligate parasite G. pallida to many biological studies. Caenorhabditis elegans transformed with a GFP reporter construct under the control of the Gp-ace-2 promoter exhibited specific and restricted GFP expression in neuronal cells in the head ganglia. Gp-ACE-2 protein can functionally complement its C. elegans homologue. A chimeric construct containing the Ce-ace-2 promoter region and the Gp-ace-2 coding region and 3' untranslated region was able to restore a normal phenotype to the uncoordinated C. elegans double mutant ace-1;ace-2. This study demonstrates conservation of AChE function and expression between free-living and plant-parasitic nematode species, and highlights the utility of C. elegans as a heterologous system to study neuronal aspects of plant-parasitic nematode biology.

Identification and characterization of a cDNA encoding the acetylcholinesterase ofHaematobia irritans(L.) (Diptera: Muscidae)

A 2217-nucleotide cDNA presumptively encoding acetylcholinesterase (AChE) of the horn fly, Haematobia irritans (L.) was sequenced. The open reading frame (ORF) encoded a 91 amino acid secretion signal peptide and a 613 amino acid mature protein with 95% identity and 98% similarity to the AChE of Musca domestica (L.). Structural features characteristic of the M. domestica and Drosophila melanogaster AChEs are conserved in the H. irritans AChE. The M. domestica and D. melanogaster AChEs are target sites for organophosphate inhibition as previously shown (Walsh et al. 2001. Biochem. J. 359: 175-181, Kozaki et al. 2002. Appl. Entomol. Zool. 37: 213-218), suggesting that this H. irritans AChE2 may be the target site for organophosphate.

Inhibition of Drosophila melanogaster acetylcholinesterase by high concentrations of substrate

Acetylcholine hydrolysis by acetylcholinesterase is inhibited at high substrate concentrations. To determine the residues involved in this phenomenon, we have mutated most of the residues lining the active-site gorge but mutating these did not completely eliminate hydrolysis. Thus, we analyzed the effect of a nonhydrolysable substrate analogue on substrate hydrolysis and on reactivation of an analogue of the acetylenzyme. Analyses of various models led us to propose the following sequence of events: the substrate initially binds at the rim of the active-site gorge and then slides down to the bottom of the gorge where it is hydrolyzed. Another substrate molecule can bind to the peripheral site: (a) when the choline is still inside the gorge - it will thereby hinder its exit; (b) after choline has dissociated but before deacetylation occurs - binding at the peripheral site increases deacetylation rate but (c) if a substrate molecule bound to the peripheral site slides down to the bottom of the active-site before the catalytic serine is deacetylated, its new position will prevent the approach of water, thus blocking deacetylation.

Mapping and sequencing of acetylcholinesterase genes from the platyhelminth blood fluke Schistosoma

Acetylcholinesterase (AChE) on the surface of the parasitic blood fluke Schistosoma is the likely target for schistosomicidal anticholinesterases. Determination of the molecular structure of this drug target is key for the development of improved anticholinesterase drugs and potentially a novel vaccine. We have recently cloned the cDNA encoding the AChE from the human parasite Schistosoma haematobium and succeeded in expressing functional recombinant protein. We now describe the cloning and molecular characterisation of homologues from two other schistosome species-Schistosoma mansoni and Schistosoma bovis, which are important parasites of man and cattle, respectively, but which differ in their sensitivity to the therapeutic anticholinesterase metrifonate. Comparison of the deduced amino acid sequences revealed that the AChE from all three species posses a high degree of identity, with conservation of all of the residues known to be important for substrate binding and catalytic activity. Also conserved is a unique C-terminal domain which is unusual in that it lacks the consensus for GPI modification, even though the native protein is considered to be GPI-anchored. We have also established the AChE gene structures for all three species and cloned the complete gene for S. haematobium AChE. The gene structure is relatively complex, comprising nine coding exons; the location of the splice sites is identical in all three species, but the size of the introns varies considerably. The two C-terminal splicing sites that are conserved in all species are also present in Schistosoma, but a third C-terminal conserved splicing site which is located 11-13 amino acids upstream of the histidine of the catalytic triad in all invertebrate AChE genes characterised to date is absent. We discuss our findings in the context of the molecular phylogeny of the AChE genes and the potential application to the control of schistosomiasis.

Molecular characterization of an acetylcholinesterase implicated in the regulation of glucose scavenging by the parasite Schistosoma

Acetylcholinesterase (AChE) present on the surface of the trematode blood fluke Schistosoma has been implicated in the regulation of glucose scavenging from the host blood. Determination of the molecular structure and functional characteristics of this molecule is a crucial first step in understanding the novel function for AChE and in evaluating the potential of schistosome AChE as a target of new parasite control methods. We have determined the primary structure of acetylcholinesterase from Schistosoma haematobium. Immunolocalization studies confirmed that the enzyme was present on the parasite surface as well as in the muscle. The derived amino acid sequence possesses features common to acetylcholinesterases: the catalytic triad, six cysteines that form three intramolecular disulphide bonds, and aromatic residues lining the catalytic gorge. An unusual feature is that the fully processed native enzyme exists as a glycoinositol phospholipid (GPI)-anchored dimer, but the sequence of the C?terminus does not conform to the current consensus for GPI modification. The enzyme expressed in Xenopus oocytes showed conventional substrate specificity and sensitivity to established inhibitors of AChE, although it is relatively insensitive to the peripheral site inhibitor propidium iodide. Distinctions between host and parasite AChEs will allow the rational design of schistosome-specific drugs and vaccines.

The acetylcholinesterase gene and organophosphorus resistance in the Australian sheep blowfly, Lucilia cuprina

Acetylcholinesterase (AChE), encoded by the Ace gene, is the primary target of organophosphorous (OP) and carbamate insecticides. Ace mutations have been identified in OP resistants strains of Drosophila melanogaster. However, in the Australian sheep blowfly, Lucilia cuprina, resistance in field and laboratory generated strains is determined by point mutations in the Rop-1 gene, which encodes a carboxylesterase, E3. To investigate the apparent bias for the Rop-1/E3 mechanism in the evolution of OP resistance in L. cuprina, we have cloned the Ace gene from this species and characterized its product. Southern hybridization indicates the existence of a single Ace gene in L. cuprina. The amino acid sequence of L. cuprina AChE shares 85.3% identity with D. melanogaster and 92.4% with Musca domestica AChE. Five point mutations in Ace associated with reduced sensitivity to OP insecticides have been previously detected in resistant strains of D. melanogaster. These residues are identical in susceptible strains of D. melanogaster and L. cuprina, although different codons are used. Each of the amino acid substitutions that confer OP resistance in D. melanogaster could also occur in L. cuprina by a single non-synonymous substitution. These data suggest that the resistance mechanism used in L. cuprina is determined by factors other than codon bias. The same point mutations, singly and in combination, were introduced into the Ace gene of L. cuprina by site-directed mutagenesis and the resulting AChE enzymes expressed using a baculovirus system to characterise their kinetic properties and interactions with OP insecticides. The K(m) of wild type AChE for acetylthiocholine (ASCh) is 23.13 microM and the point mutations change the affinity to the substrate. The turnover number of Lucilia AChE for ASCh was estimated to be 1.27x10(3) min(-1), similar to Drosophila or housefly AChE. The single amino acid replacements reduce the affinities of the AChE for OPs and give up to 8.7-fold OP insensitivity, while combined mutations give up to 35-fold insensitivity. However, other published studies indicate these same mutations yield higher levels of OP insensitivity in D. melanogaster and A. aegypti. The inhibition data indicate that the wild type form of AChE of L. cuprina is 12.4-fold less sensitive to OP inhibition than the susceptible form of E3, suggesting that the carboxylesterases may have a role in the protection of AChE via a sequestration mechanism. This provides a possible explanation for the bias towards the evolution of resistance via the Rop-1/E3 mechanism in L. cuprina.

Involvement of deacylation in activation of substrate hydrolysis by Drosophila acetylcholinesterase

Insect acetylcholinesterase (AChE), an enzyme whose catalytic site is located at the bottom of a gorge-like structure, hydrolyzes its substrate over a wide range of concentrations (from 2 microm to 300 mm). AChE is activated at low substrate concentrations and inhibited at high substrate concentrations. Several rival kinetic models have been developed to try to describe and explain this behavior. One of these models assumes that activation at low substrate concentrations partly results from an acceleration of deacetylation of the acetylated enzyme. To test this hypothesis, we used a monomethylcarbamoylated enzyme, which is considered equivalent to the acylated form of the enzyme and a non-hydrolyzable substrate analog, 4-oxo-N,N,N-trimethylpentanaminium iodide. It appears that this substrate analog increases the decarbamoylation rate by a factor of 2.2, suggesting that the substrate molecule bound at the activation site (K(d) = 130 +/- 47 microm) accelerates deacetylation. These two kinetic parameters are consistent with our analysis of the hydrolysis of the substrate. The location of the active site was investigated by in vitro mutagenesis. We found that this site is located at the rim of the active site gorge. Thus, substrate positioning at the rim of the gorge slows down the entrance of another substrate molecule into the active site gorge (Marcel, V., Estrada-Mondaca, S., Magné, F., Stojan, J., Klaébé, A., and Fournier, D. (2000) J. Biol. Chem. 275, 11603-11609) and also increases the deacylation step. This results in an acceleration of enzyme turnover.

Exploration of the Drosophila acetylcholinesterase substrate activation site using a reversible inhibitor (Triton X-100) and mutated enzymes

Cholinesterases are activated at low substrate concentration, and this is followed by inhibition as the level of substrate increases. However, one of these two components is sometimes lacking. In Drosophila acetylcholinesterase, the two phases are present, allowing both phenomena to be studied. Several kinetic schemes can explain this complex kinetic behavior. Among them, one model assumes that activation results from the binding of a substrate molecule to a non-productive site affecting the entrance of a substrate molecule into the active site. To test this hypothesis, we looked for an inhibitor competitive for activation and we found Triton X-100. Using organophosphates or carbamates as hemisubstrates, we showed that Triton X-100 inhibits or increases phosphorylation or carbamoylation of the enzyme. In vitro mutagenesis of the residues lining the active site gorge allowed us to locate the Triton X-100 binding site at the rim of the gorge with glutamate 107 playing the major role. These results led to the hypothesis that substrate binding at this site affects the entrance of another substrate molecule into the active site cleft.

Why are there so few resistance-associated mutations in insecticide target genes?

The genes encoding the three major targets of conventional insecticides are: Rdl, which encodes a gamma-aminobutyric acid receptor subunit (RDL); para, which encodes a voltage-gated sodium channel (PARA); and Ace, which encodes insect acetylcholinesterase (AChE). Interestingly, despite the complexity of the encoded receptors or enzymes, very few amino acid residues are replaced in different resistant insects: one within RDL, two within PARA and three or more within AChE. Here we examine the possible reasons underlying this extreme conservation by looking at the aspects of receptor and/or enzyme function that may constrain replacements to such a limited number of residues.

Stabilization of recombinant Drosophila acetylcholinesterase

The uses of pure and stable acetylcholinesterase can range from simple basic research to applications in environment quality assessment. In order to satisfy some of these needs its recombinant expression is routinely performed. Affinity-purified recombinant Drosophila melanogaster acetylcholinesterase proved to be instable; an apparent cause of this seemed to be the presence of contaminants with protease activity as evidenced by SDS-PAGE. The elimination of these accompanying products was achieved by anion-exchange, hydrophobic interaction, and cibacron blue affinity chromatography applied downstream from procainamide affinity chromatography. The utilization of a parallel affinity acting via an engineered histidine tail permitted the elimination of the copurified proteases as well. Despite the elimination of the contaminants, the apparently pure extracts were still unstable. It is shown that such instability can be counterbalanced by provoking protein-protein interactions, either between enzyme molecules or with other molecules such as bovine serum albumin. Another way to reduce instability is the addition of a reversible inhibitor or polyethylene glycol 3350.

Drosophila acetylcholinesterase: effect of post-translational [correction of post-traductional] modifications on the production in the baculovirus system and substrate metabolization

Acetylcholinesterase cDNAs from Drosophila melanogaster modified on its primary sequence were cloned into baculovirus and were expressed in Sf9 cells with the aim to identify a mutant form that produces the enzyme at a high level. Directed mutagenesis was used in order to independently knockout different sites of post-translational modifications: exchange of the C-terminal hydrophobic peptide for a glycolipid molecule, dimerization by disulfide bridge, N-linked glycosylation at the five accessible sites, and subunit formation by proteolytic cleavage of a hydrophilic peptide found in the precursor. Another mutation involved the elimination of a free cysteine in the mature protein. All mutations involving post-translational modifications resulted in lower recoveries, suggesting that they are useful for maintaining high amounts of protein in the synapse. By contrast, elimination of a free cysteine in the mature protein permitted an increase in the level of production of the enzyme. These mutations did not affect specific activity of the enzyme at substrate concentrations ranging from 3 microM to 200 mM, suggesting that activation and inhibition of the enzyme activity does not originate from a polymorphism in post-translational modifications.

Site-directed mutagenesis of an acetylcholinesterase gene from the yellow fever mosquito Aedes aegypti confers insecticide insensitivity

Insecticide resistance is a serious problem facing the effective control of insect vectors of disease. Insensitive acetylcholinesterase (AChE) confers resistance to organophosphorus (OP) and carbamate insecticides and is a widespread resistance mechanism in vector mosquitoes. Although the point mutations that underlie AChE insensitivity have been described from Drosophila, the Colorado potato beetle, and house flies, no resistance associated mutations have been documented from mosquitoes to date. We are therefore using a cloned acetylcholinesterase gene from the yellow fever mosquito Aedes aegypti as a model in which to perform site directed mutagenesis in order to understand the effects of potential resistance associated mutations. The same resistance associated amino-acid replacements as found in other insects also confer OP and carbamate resistance to the mosquito enzyme. Here we describe the levels of resistance conferred by different combinations of these mutations and the effects of these mutations on the kinetics of the AChE enzyme. Over-expression of these constructs in baculovirus will facilitate purification of each of the mutant enzymes and a more detailed analysis of their associated inhibition kinetics.

Analysis of molecular forms and pharmacological properties of acetylcholinesterase in several mosquito species

Two acetylcholinesterases (AChE1 and AChE2) have recently been characterized in the common mosquito Culex pipiens. This situation appeared to be an exception among insects, where only one acetylcholinesterase gene had previously been repeatedly reported. In the present study, acetylcholinesterase was studied in five mosquito species: Aedes aegypti, Anopheles gambiae, Anopheles stephensi, Culiseta longeareolata and Culex hortensis, in order to test whether or not two different acetylcholinesterase enzymes could be detected as occurs in C. pipiens. Molecular forms and catalytic properties of the enzyme show that only one enzyme species was detected in the five species. This suggests that a duplication of a single locus Ace probably occurred recently in the phylogeny tree leading to C. pipiens, and produced two distinct acetylcholinesterases: AchE1 and AChE2.

Alkaline unfolding and salt-induced folding of yeast alcohol dehydrogenase under high pH conditions

The conformational changes of yeast alcohol dehydrogenase during unfolding at alkaline pH have been followed by fluorescence emission and circular dichroism spectra. A result of comparison of inactivation and conformational changes shows that much lower values of alkaline pH are required to bring about inactivation than significant conformational change of the enzyme molecule. At pH 9.5, although the enzyme has been completely inactivated, no marked conformational changes can be observed. Even at pH 12, the apparently fully unfolded enzyme retains some ordered secondary structure. After removal of Zn2+ from the enzyme molecule, the conformational stability decreased. At pH 12 by adding the salt, the relatively unfolded state of denatured enzyme changes into a compact conformational state by hydrophobic collapsing. Folded states induced by salt bound ANS strongly, indicating the existence of increased hydrophobic surface. More extensive studies showed that although apo-YADH and holo-YADH exhibited similar behavior, the folding cooperative ability of apo-enzyme was lower than that of holo-enzyme. The above results suggest that the zinc ion plays an important role in helping the folding of YADH and in stabilizing its native conformation.

Replacement of the glycoinositol phospholipid anchor of Drosophila acetylcholinesterase with a transmembrane domain does not alter sorting in neurons and epithelia but results in behavioral defects

Drosophila has a single glycoinositol phospholipid (GPI)-anchored form of acetylcholinesterase (AChE) encoded by the Ace locus. To assess the role that GPI plays in the physiology, of AChE, we have replaced the wild-type GPI-AChE with a chimeric transmembrane form (TM-AChE) in the nervous system of the fly. Ace null alleles provided a genetic background completely lacking in endogenous GPI-AChE, and Ace minigene P transposon constructs were used to express both GPI- and TM-AChE forms in the tissues where AChE is normally expressed. Control experiments with the GPI-AChE minigene demonstrated a threshold between 9 and 12% of normal AChE activity for adult viability. Ace mutant flies were rescued by GPI-AChE minigene lines that expressed 12-40% of normal activity and were essentially unchanged from wild-type flies in behavior. TM-AChE minigene lines were able to rescue Ace null alleles, although with a slightly higher threshold than that for GPI-AChE. Although rescued flies expressing GPI-AChE at a level of 12% of normal activity were viable, flies expressing 13-16% of normal activity from the TM-AChE transgene died shortly after eclosion. Flies expressing TM-AChE at about 30% of normal levels were essentially unchanged from wild-type flies in gross behavior but had a reduced lifespan secondary to subtle coordination defects. These flies also showed reduced locomotor activity and performed poorly in a grooming assay. However, light level and electron microscopic immunocytochemistry showed no differences in the localization of GPI- and TM-AChE. Furthermore, endogenous and ectopic-induced expression of both AChEs in epithelial tissues of the adult and embryo, respectively, showed that they were sorted identically. Most epithelial cells sorted GPI- and TM-AChE to the apical surface, but cuticle-secreting epithelia sorted both proteins basolaterally. Our data suggest that rather than having a primary role in protein sorting, the GPI anchor or AChE plays some other more subtle cellular role in neuronal physiology.

Cloning, sequencing and functional expression of an acetylcholinesterase gene from the yellow fever mosquito Aedes aegypti

A degenerate PCR strategy was used to isolate a fragment of the acetylcholinesterase gene (Ace) homolog from Aedes aegypti and screen for a cDNA clone containing the complete open reading frame of the gene. The predicted amino acid sequence of the Aedes gene shares 64% identity with Ace from Drosophila and 87% identity with the acetylcholinesterase gene from another mosquito species Anopheles stephensi. High levels of expression of the Aedes gene were achieved by infection of Sf21 cells with a recombinant baculovirus containing the Aedes Ace cDNA. The catalytic properties and sensitivity of the recombinant enzyme to insecticide inhibition are described and discussed in relation to the role of insensitive AChE in conferring resistance to organophosphorus and carbamate insecticides.

Acetylcholinesterases of the nematode Steinernema carpocapsae. Characterization of two types of amphiphilic forms differing in their mode of membrane association

We analyzed the molecular forms of acetylcholinesterase (AChE) in the nematode Steinernema carpocapsae. Two major AChEs are involved in acetylcholine hydrolysis. The first class of AChE is highly sensitive to eserine (IC50 = 0.05 microM). The corresponding molecular forms are: an amphiphilic 14S form converted into a hydrophilic 14.5S form by mild proteolysis and two hydrophilic 12S and 7S forms. Reduction of the amphiphilic 14S form with 10 mM dithiothreitol produces hydrophilic 7S and 4S forms, indicating that it is an oligomer of hydrophilic catalytic subunits linked by disulfide bond(s) to a hydrophobic structural element that confers the amphiphilicity to the complex. Sedimentation coefficients suggest that 4S, 7S, 12S forms correspond to hydrophilic monomer, dimer, tetramer and that the 14S form is also a tetramer linked to one structural element. The second class of AChE is less sensitive to eserine (IC50 = 0.1 mM). Corresponding molecular forms are hydrophilic and amphiphilic 4S forms (monomers) and a major amphiphilic 7S form converted into a hydrophilic dimer by Bacillus thuringiensis phosphatidylinositol-specific phospholipase C. This amphiphilic 7S form thus possesses a glycolipid anchor. It appears that Steinernema (a very primitive invertebrate) presents AChEs with two types of membrane association that closely resemble those described for amphiphilic G2 and G4 forms of AChE in more evolved animals.

Drosophila acetylcholinesterase. Expression of a functional precursor in Xenopus oocytes

In insects, acetylcholinesterase is mainly found in the central nervous system. It is expressed in the synapse where it hydrolyzes the neurotransmitter acetylcholine. Maturation of this protein involves several post-translational modifications. The precursor polypeptide is cut at three sites; the N-terminal signal peptide is removed, the C-terminal hydrophobic polypeptide is clipped off and replaced by a glycolipid anchor and the resulting peptide is cut into two polypeptides, corresponding to active subunits. Two of these active subunits are associated to form the final active glycosylated protein. We have expressed the protein via microinjection of an expression vector into Xenopus oocyte nuclei. When the complete cDNA is injected, the acetylcholinesterase formed is biochemically similar to the Drosophila-head acetylcholinesterase. However, the hydrophobic C-terminal peptide is not replaced by a glycolipid anchor. As a consequence, the enzyme is no longer externalized, the proteolytic cutting of the main peptide does not occur and a new polymerization form occurs. Although incompletely processed, this protein is enzymatically active. When a cDNA lacking the coding region of the C-terminal hydrophobic peptide is injected, the resulting acetylcholinesterase is hydrophilic, cleaved into two subunits and secreted into the incubation medium free of contaminants.

Cholinesterase-like domains in enzymes and structural proteins: functional and evolutionary relationships and identification of a catalytically essential aspartic acid

Primary sequences of cholinesterases and related proteins have been systematically compared. The cholinesterase-like domain of these proteins, about 500 amino acids, may fulfill a catalytic and a structural function. We identified an aspartic acid residue that is conserved among esterases and lipases (Asp-397 in Torpedo acetylcholinesterase) but that had not been considered to be involved in the catalytic mechanism. Site-directed mutagenesis demonstrated that this residue is necessary for activity. Analysis of evolutionary relationships shows that the noncatalytic members of the family do not constitute a separate subgroup, suggesting that loss of catalytic activity occurred independently on several occasions, probably from bifunctional molecules. Cholinesterases may thus be involved in cell-cell interactions in addition to the hydrolysis of acetylcholine. This would explain their specific expression in well-defined territories during embryogenesis before the formation of cholinergic synapses and their presence in noncholinergic tissues.

Rapid analysis of glycolipid anchors in amphiphilic dimers of acetylcholinesterases

1. We describe two simple procedures for the rapid identification of certain structural features of glycolipid anchors in acetylcholinesterases (AChEs). 2. Treatment with alkaline hydroxylamine (that cleaves ester-linked acyl chains but not ether-linked alkyl chains) converts molecules possessing a diacylglycerol, but not those with an alkylacylglycerol, into hydrophilic derivatives. AChEs in human and bovine erythrocytes possess an alkylacylglycerol (Roberts et al., J. Biol. Chem. 263:18766-18775, 1988; Biochem. Biophys. Res. Commun. 150:271-277, 1988) and are not converted to hydrophilic dimers by alkaline hydroxylamine. Amphiphilic dimers of AChE from Drosophila, from mouse erythrocytes, and from the human erythroleukaemia cell line K562 also resist the treatment with hydroxylamine and likely possess a terminal alkylacylglycerol. This indicates that the cellular pool of free glycolipids used as precursors of protein anchors is distinct from the pool of membrane phosphatidylinositols (which contain diacylglycerols). 3. Pretreatment with alkaline hydroxylamine is required to render the amphiphilic AChE from human erythrocytes susceptible to digestion by Bacillus thuringiensis phosphatidylinositol-specific phospholipase C (PI-PLC) (Toutant et al., Eur. J. Biochem. 180:503-508, 1989). We show here that this is also the case for the AChE from mouse erythrocytes, which therefore likely possesses an additional acyl chain in the anchor that prevents the action of PI-PLC. 4. In two sublines of K562 cells (48 and 243), we observed that AChE either was directly susceptible to PI-PLC (243) or required a prior deacylation by alkaline hydroxylamine (48). This suggests that glycolipid anchors in AChE of K562-48 cells, but not those in AChE of K562-243 cells, contain the additional acylation demonstrated in AChE from human erythrocytes. These observations illustrate the cell specificity (and the lack of species-specificity) of the structure of glycolipid anchors.

The acetylcholinesterase gene of Anopheles stephensi

1. The acetylcholinesterase (AChE) gene from the important malaria vector Anopheles stephensi has been isolated by homology to the Drosophila acetylcholinesterase gene. 2. The complete sequence and intron-exon organization has been determined. The encoded protein has 69% identity to Drosophila AChE and 38 and 36% identity to Torpedo AChE and human butyrylcholinesterase, respectively.

Single gene encodes glycophospholipid-anchored and asymmetric acetylcholinesterase forms: alternative coding exons contain inverted repeat sequences

Polymorphic forms of acetylcholinesterase are tethered extracellularly either as dimers membrane-anchored by a glycophospholipid or as catalytic subunits disulfidelinked to a collagen tail that associates with the basal lamina. Genomic clones of acetylcholinesterase from T. californica revealed that individual enzyme forms are encoded within a single gene that yields multiple mRNAs. Each enzyme form is encoded in three exons: the first two exons, bases -22 to 1502 and 1503 to 1669, encode sequence common to both forms, while alternative third exons encode a hydrophobic C-terminal region, to which a glycophospholipid is added upon processing, and a nonprocessed C-terminus, yielding a catalytic subunit that disulfide-links with a collagen-like structural unit. The 3' untranslated region of each alternative exon contains tandem repeat sequences that are inverted with respect to the other exon. This may either dictate alternative exon usage by formation of cis stem-loops or affect the abundance of translatable mRNA by trans-hybridization between the alternative spliced mRNA species.

Conversion of human erythrocyte acetylcholinesterase from an amphiphilic to a hydrophilic form by phosphatidylinositol-specific phospholipase C and serum phospholipase D

Each catalytic subunit in the amphiphilic dimer of human erythrocyte acetylcholinesterase (AChE) is anchored in the plasma membrane exclusively by a glycoinositol phospholipid. In contrast to erythrocyte AChEs in other mammalian species, the human enzyme is resistant to direct cleavage by phosphatidylinositol-specific phospholipase C (PtdIns-specific PLC). The resistance is due to the existence of an additional fatty acyl chain on the inositol ring which blocks the action of PtdIns-specific PLC [Roberts et al. (1988) J. Biol. Chem. 263, 18766-18775]. In this report, nondenaturing polyacrylamide gel electrophoresis was applied to permit rapid and unambiguous distinction between amphiphilic AChE, in which each catalytic subunit binds one nonionic detergent micelle, and hydrophilic AChE, which does not interact with detergent. Deacylation of human erythrocyte AChE by an alkaline treatment with hydroxylamine rendered the amphiphilic AChE susceptible to PtdIns-specific PLC with the consequent release of hydrophilic AChE. Although serum anchor-specific phospholipase D (PLD) cleaves the intact human erythrocyte AChE anchor, this treatment, as judged by nondenaturing electrophoresis, did not release hydrophilic AChE. Hydroxylamine treatment before or after PLD digestion was necessary to achieve the conversion. These observations indicate that binding of a single detergent micelle was maintained when any of the three fatty acyl or alkyl groups in the human erythrocyte AChE anchor phospholipid were retained. For proteins that can be identified following nondenaturing gel electrophoresis, these procedures provide methods both for detecting glycoinositol phospholipid anchors resistant to PtdIns-specific PLC and for indicating fatty acyl and/or alkyl chains in these anchors.