Existence of four acetylcholinesterase genes in the nematodes Caenorhabditis elegans and Caenorhabditis briggsae

Characterization and functional analysis of two acetylcholinesterase genes in Bradysia odoriphaga Yang et Zhang (Diptera: Sciaridae)

Two acetylcholinesterase genes (Boace1 and Boace2) were cloned from Bradysia odoriphaga, a devastating soil pest that mainly damages Chinese chives. The Boace1 encodes BoAChE1 protein consisting of 696 amino acid residues, while Boace2 encodes BoAChE2 containing 638 amino acids. Phylogenetic analysis showed that Boace1 and Boace2 are appeared to be distinct clusters. The gene expression patterns at different development stages and various body parts tissues were examined, and their biological functions were characterized by RNA interference and analog docking prediction. The results showed that both Boace genes were expressed in all developmental stages and examined tissues. The transcript level of Boace2 was significantly higher than Boace1 in all tested samples, and Boace1 was found most abundant in the head while Boace2 was highly expressed in the fat body of B. odoriphaga. The silencing of Boace1 and Boace2 significantly decreased the AChE activity of 36.6% and 14.8% separately, and increased the susceptibility of B. odoriphaga to phoxim, with 60.8% and 44.7% mortality. Besides, overexpression and gene duplication of Boace1 were found in two field resistant populations, and two major mutations, A319S and G400V, were detected in Boace1. Moreover, the docking results revealed that BoAChE1 had a higher affinity towards organophosphorus than BoAChE2. It is concluded that Boace2 is the most abundant ace type in B. odoriphaga, while both Boace play vital roles. Boace1 might play a major neurological function and more likely be the prime target for insecticides, while Boace2 might play some important unidentified roles.

Identification, characterization and expression analyses of cholinesterases genes in Yesso scallop (Patinopecten yessoensis) reveal molecular function allocation in responses to ocean acidification

Cholinesterases are key enzymes in central and peripheral cholinergic nerve system functioning on nerve impulse transmission in animals. Though cholinesterases have been identified in most vertebrates, the knowledge about the variable numbers and multiple functions of the genes is still quite meagre in invertebrates, especially in scallops. In this study, the complete cholinesterase (ChE) family members have been systematically characterized in Yesso scallop (Patinopecten yessoensis) via whole-genome scanning through in silico analysis. Ten ChE family members in the genome of Yesso scallop (designated PyChEs) were identified and potentially acted to be the largest number of ChE in the reported species to date. Phylogenetic and protein structural analyses were performed to determine the identities and evolutionary relationships of these genes. The expression profiles of PyChEs were determined in all developmental stages, in healthy adult tissues, and in mantles under low pH stress (pH 6.5 and 7.5). Spatiotemporal expression suggested the ubiquitous functional roles of PyChEs in all stages of development, as well as general and tissue-specific functions in scallop tissues. Regulation expressions revealed diverse up- and down-regulated expression patterns at most time points, suggesting different functional specialization of gene superfamily members in response to ocean acidification (OA). Evidences in gene number, phylogenetic relationships and expression patterns of PyChEs revealed that functional innovations and differentiations after gene duplication may result in altered functional constraints among PyChEs gene clusters. Collectively, our results provide the potential clues that the selection pressures coming from the environment were the potential inducement leading to function allocation of ChE family members in scallop.

Expression and evolutionary analyses of three acetylcholinesterase genes (Mi-ace-1, Mi-ace-2, Mi-ace-3) in the root-knot nematode Meloidogyne incognita

The full cDNA of Mi-ace-3 encoding an acetylcholinesterase (AChE) in Meloidogyne incognita was cloned and characterized. Mi-ace-3 had an open reading frame of 1875 bp encoding 624 amino acid residues. Key residues essential to AChE structure and function were conserved. The deduced Mi-ACE-3 protein sequence had 72% amino acid similarity with that of Ditylenchus destructor Dd-AChE-3. Phylogenetic analyses using 41 AChEs from 24 species showed that Mi-ACE-3 formed a cluster with 4 other nematode AChEs. Our results revealed that the Mi-ace-3 cloned in this study, which is orthologous to Caenorhabditis elegans AChE, belongs to the nematode ACE-3/4 subgroup. There was a significant reduction in the number of galls in transgenic tobacco roots when Mi-ace-1, Mi-ace-2, and Mi-ace-3 were knocked down simultaneously, whereas little or no effect were observed when only one or two of these genes were knocked down. This is an indication that the functions of these three genes are redundant.

Natural genomic amplification of cholinesterase genes in animals

Tight control of the concentration of acetylcholine at cholinergic synapses requires precise regulation of the number and state of the acetylcholine receptors, and of the synthesis and degradation of the neurotransmitter. In particular, the cholinesterase activity has to be controlled exquisitely. In the genome of the first experimental models used (man, mouse, zebrafish and drosophila), there are only one or two genes coding for cholinesterases, whereas there are more genes for their closest relatives the carboxylesterases. Natural amplification of cholinesterase genes was first found to occur in some cancer cells and in insect species subjected to evolutionary pressure by insecticides. Analysis of the complete genome sequences of numerous representatives of the various metazoan phyla show that moderate amplification of cholinesterase genes is not uncommon in molluscs, echinoderms, hemichordates, prochordates or lepidosauria. Amplification of acetylcholinesterase genes is also a feature of parasitic nematodes or ticks. In these parasites, over-production of cholinesterase-like proteins in secreted products and the saliva are presumed to have effector roles related to host infection. These amplification events raise questions about the role of the amplified gene products, and the adaptation processes necessary to preserve efficient cholinergic transmission. This is an article for the special issue XVth International Symposium on Cholinergic Mechanisms.

Identification and Expression of Acetylcholinesterase in Octopus vulgaris Arm Development and Regeneration: a Conserved Role for ACHE?

Acetylcholinesterase (ACHE) is a glycoprotein with a key role in terminating synaptic transmission in cholinergic neurons of both vertebrates and invertebrates. ACHE is also involved in the regulation of cell growth and morphogenesis during embryogenesis and regeneration acting through its non-cholinergic sites. The mollusk Octopus vulgaris provides a powerful model for investigating the mechanisms underlying tissue morphogenesis due to its high regenerative power. Here, we performed a comparative investigation of arm morphogenesis during adult arm regeneration and embryonic arm development which may provide insights on the conserved ACHE pathways. In this study, we cloned and characterized O. vulgaris ACHE, finding a single highly conserved ACHE hydrophobic variant, characterized by prototypical catalytic sites and a putative consensus region for a glycosylphosphatidylinositol (GPI)-anchor attachment at the COOH-terminus. We then show that its expression level is correlated to the stage of morphogenesis in both adult and embryonic arm. In particular, ACHE is localized in typical neuronal sites when adult-like arm morphology is established and in differentiating cell locations during the early stages of arm morphogenesis. This possibility is also supported by the presence in the ACHE sequence and model structure of both cholinergic and non-cholinergic sites. This study provides insights into ACHE conserved roles during processes of arm morphogenesis. In addition, our modeling study offers a solid basis for predicting the interaction of the ACHE domains with pharmacological blockers for in vivo investigations. We therefore suggest ACHE as a target for the regulation of tissue morphogenesis.

Three acetylcholinesterases of the pinewood nematode, Bursaphelenchus xylophilus: insights into distinct physiological functions

Acetylcholinesterase (AChE) plays a key role in postsynaptic transmission in most animals. Nematodes encode multiple AChEs, implying its functional diversity. To explore physiological functions of multiple AChEs, three distinct AChEs (BxACE-1, BxACE-2, and BxACE-3) were identified and characterized from the pinewood nematode. Sequencing comparison with Torpedo AChE and Caenorhabditis elegans ACEs identified choline-binding site, catalytic triad functional site, three internal disulfide bonds and aromatic residues for the catalytic gorge. Transcriptional profiling by quantitative real-time PCR revealed that BxACE-3 is more actively transcribed than BxACE-1 (2-3 times) and BxACE-2 (9-18 times) in both propagative and dispersal stages. The three BxACEs were functionally expressed using baculovirus system. Kinetic analysis of in vitro-expressed BxACEs revealed that the substrate specificity was highest in BxACE-1 whereas the catalytic efficiency was highest in BxACE-2. In inhibition assay, BxACE-3 showed the lowest inhibition rate. Taken together, it appears that both BxACE-1 and BxACE-2 play common but non-overlapping roles in synaptic transmission, whereas BxACE-3 may have non-neuronal functions. The current findings should provide valuable insights into the evolutionary process and various physiological roles of AChE.

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

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

A network of G-protein signaling pathways control neuronal activity in C. elegans

The Caenorhabditis elegans neuromuscular junction (NMJ) is one of the best studied synapses in any organism. A variety of genetic screens have identified genes required both for the essential steps of neurotransmitter release from motorneurons as well as the signaling pathways that regulate rates of neurotransmitter release. A number of these regulatory genes encode proteins that converge to regulate neurotransmitter release. In other cases genes are known to regulate signaling at the NMJ but how they act remains unknown. Many of the proteins that regulate activity at the NMJ participate in a network of heterotrimeric G-protein signaling pathways controlling the release of synaptic vesicles and/or dense-core vesicles (DCVs). At least four heterotrimeric G-proteins (Galphaq, Galpha12, Galphao, and Galphas) act within the motorneurons to control the activity of the NMJ. The Galphaq, Galpha12, and Galphao pathways converge to control production and destruction of the lipid-bound second messenger diacylglycerol (DAG) at sites of neurotransmitter release. DAG acts via at least two effectors, MUNC13 and PKC, to control the release of both neurotransmitters and neuropeptides from motorneurons. The Galphas pathway converges with the other three heterotrimeric G-protein pathways downstream of DAG to regulate neuropeptide release. Released neurotransmitters and neuropeptides then act to control contraction of the body-wall muscles to control locomotion. The lipids and proteins involved in these networks are conserved between C. elegans and mammals. Thus, the C. elegans NMJ acts as a model synapse to understand how neuronal activity in the human brain is regulated.

Functional genomics of nematode acetylcholinesterases

Acetylcholine is the major excitatory neurotransmitter controlling motor activities in nematodes, and the enzyme which hydrolyses and inactivates acetylcholine, acetylcholinesterase, is thus essential for regulation of cholinergic transmission. Different forms of acetylcholinesterase are encoded by multiple genes in nematodes, and analysis of the pattern of expression of these genes in Caenorhabditis elegans suggests that they perform non-redundant functions. In addition, many parasitic species which colonise host mucosal surfaces secrete hydrophilic variants of acetylcholinesterase, although the function of these enzymes is still unclear. Acetylcholinesterases have a history as targets for therapeutic agents against helminth parasites, but anti-cholinesterases have been used much more extensively as pesticides, for example to control crop damage and ectoparasitic infestation of livestock. The toxicity associated with these compounds (generally organophosphates and carbamates) has led to legislation to withdraw them from the market or restrict their use in many countries. Nevertheless, acetylcholinesterases provide a good example of a neuromuscular target enzyme in helminth parasites, and it may yet be possible to develop more selective inhibitors. In this article, we describe what is known about the structure and function of vertebrate cholinesterases, illustrate the molecular diversity and tissue distribution of these enzymes in C. elegans, and discuss to what extent this may represent a paradigm for nematodes in general.

Acetylcholinesterase genes within the Diptera: takeover and loss in true flies

It has recently been reported that the synaptic acetylcholinesterase (AChE) in mosquitoes is encoded by the ace-1 gene, distinct and divergent from the ace-2 gene, which performs this function in Drosophila. This is an unprecedented situation within the Diptera order because both ace genes derive from an old duplication and are present in most insects and arthropods. Nevertheless, Drosophila possesses only the ace-2 gene. Thus, a secondary loss occurred during the evolution of Diptera, implying a vital function switch from one gene (ace-1) to the other (ace-2). We sampled 78 species, representing 50 families (27% of the Dipteran families) spread over all major subdivisions of the Diptera, and looked for ace-1 and ace-2 by systematic PCR screening to determine which taxonomic groups within the Diptera have this gene change. We show that this loss probably extends to all true flies (or Cyclorrhapha), a large monophyletic group of the Diptera. We also show that ace-2 plays a non-detectable role in the synaptic AChE in a lower Diptera species, suggesting that it has non-synaptic functions. A relative molecular evolution rate test showed that the intensity of purifying selection on ace-2 sequences is constant across the Diptera, irrespective of the presence or absence of ace-1, confirming the evolutionary importance of non-synaptic functions for this gene. We discuss the evolutionary scenarios for the takeover of ace-2 and the loss of ace-1, taking into account our limited knowledge of non-synaptic functions of ace genes and some specific adaptations of true flies.

Misexpression of acetylcholinesterases in the C. elegans pha-2 mutant accompanies ultrastructural defects in pharyngeal muscle cells

pha-2 is the Caenorhabditis elegans homolog of the vertebrate homeobox gene Hex. Embryonic expression of pha-2 is mostly pharyngeal and the only described mutant allele of pha-2 results in a severe pharyngeal defect in which certain muscle cells (pm5 cells) and neurons are grossly deformed. Here, we performed a detailed characterization of the pha-2 phenotype using cell-type-specific reporters, physical manipulation of the nuclei in pharyngeal muscle cells using "optical tweezers", electron microscopy, staining of the actin cytoskeleton as well as phenotypic rescue and ectopic expression experiments. The main findings of the present study are (i) the pha-2 (ad472) mutation specifically impairs the pharyngeal expression of pha-2; (ii) in the pha-2 mutant, the cytoskeleton of the pm5 cells is measurably weaker than in normal cells and is severely disrupted by large tubular structures and organelles; (iii) the pm5 cells of the pha-2 mutant fail to express the acetylcholinesterase genes ace-1 and ace-2; (iv) ectopic expression of pha-2 can induce ectopic expression of ace-1 and ace-2; and (v) the anc-1 mutant with mislocalized pm5 cell nuclei occasionally shows an isthmus phenotype similar to that of pha-2 worms.

Nematode acetylcholinesterases are encoded by multiple genes and perform non-overlapping functions

Nematodes are unusual in that diverse molecular forms of acetylcholinesterase are the product of distinct genes. This is best characterised in the free living organism Caenorhabditis elegans, in which 3 genes are known to give rise to distinct enzymes, with a fourth likely to be non-functional. ACE-1 is an amphiphilic tetramer associated with a hydrophobic non-catalytic subunit, analogous to vertebrate T enzymes, whereas ACE-2 and ACE-3 are glycosylphosphatidylinositol-linked amphiphilic dimers. The different ace genes show distinct anatomical patterns of expression in muscles, sensory neurons and motor neurons, with only a few examples of coordinated expression. Clear homologues of ace-1 and ace-2 have now been isolated from a variety of parasitic nematodes, and the predicted proteins have very similar C-terminal amino acid sequences, implying an analogous means of anchorage to membranes. In addition to these membrane-bound enzymes, many parasitic nematodes which colonise mucosal surfaces secrete acetylcholinesterases to the external (host) environment. These hydrophilic enzymes are separately encoded in the genome, so that some parasites may thus have a total complement of six ace genes. The secretory enzymes have been characterised from the intestinal nematode Nippostrongylus brasiliensis and the lungworm Dictyocaulus viviparus. These show a number of common features, including a truncated C-terminus and an insertion at the molecular surface, when compared to other nematode acetylcholinesterases. Although the function of these enzymes has not been determined, they most likely alter host physiological responses to promote survival of the parasite.

Are there non-catalytic functions of acetylcholinesterases? Lessons from mutant animal models

Acetylcholinesterase (AChE) hydrolyses acetylcholine (ACh) ensuring the fast clearance of released neurotransmitter at cholinergic synapses. Many studies led to the hypothesis that AChE and the closely related enzyme butyrylcholinesterase (BChE) may play other, non-hydrolytic roles during development. In this review, we compare data from in vivo studies performed on invertebrate and vertebrate genetic models. The loss of function of ache in these systems is responsible for the appearance of several phenotypes. In all aspects so far studied, the phenotypes can be explained by an excess of the undegraded substrate, ACh, leading to misfunction and pathological alterations. Thus, the lack of AChE catalytic activity in the mutants appears to be solely responsible for the observed phenotypes. None of them appears to require the postulated adhesive or other non-hydrolytic functions of AChE.

Cloning and sequencing of putative acetylcholinesterase cDNAs from the American dog tick, Dermacentor variabilis, and the brown dog tick, Rhipicephalus sanguineus (Acari: Ixodidae)

Two putative cDNAs of acetylcholinesterase (AChE), one from Dermacentor variabilis, and the other from Rhipicephalus sanguineus, were amplified and sequenced. The deduced amino acid sequences have high amino acid identities (between 70 and 94%) to known tick AChE sequences deposited in GenBank. Furthermore, these two AChEs also possess common features in their primary AChE structure such as catalytic active sites. A 2,220-bp contiguous sequence, containing a 1,791-bp open reading frame encoding an AChE precursor with 596 amino acid residues, was obtained from D. variabilis. The deduced proteins of R. sanguineus are different in size by 6 amino acids because of alternative splicing at the 5' end. A gene tree deduced from phylogenetic analysis indicates that there are at least three lineages of AChE in arthropods.

Multiple ace genes encoding acetylcholinesterases of Caenorhabditis elegans have distinct tissue expression

ace-1 and ace-2 genes encoding acetylcholinesterase in the nematode Caenorhabditis elegans present 35% identity in coding sequences but no homology in noncoding regions (introns, 5'- and 3'-untranslated regions). A 5'-region of ace-2 was defined by rescue of ace-1;ace-2 mutants. When green fluorescent protein (GFP) expression was driven by this regulatory region, the resulting pattern was distinct from that of ace-1. This latter gene is expressed in all body-wall and vulval muscle cells (Culetto et al., 1999), whereas ace-2 is expressed almost exclusively in neurons. ace-3 and ace-4 genes are located in close proximity on chromosome II (Combes et al., 2000). These two genes were first transcribed in vivo as a bicistronic messenger and thus constitute an ace-3;ace-4 operon. However, there was a very low level of monocistronic mRNA of ace-4 (the upstream gene) in vivo, and no ACE-4 enzymatic activity was ever detected. GFP expression driven by a 5' upstream region of the ace-3;ace-4 operon was detected in several muscle cells of the pharynx (pm3, pm4, pm5 and pm7) and in the two canal associated neurons (CAN cells). A dorsal row of body-wall muscle cells was intensively labelled in larval stages but no longer detected in adults. The distinct tissue-specific expression of ace-1, ace-2 and ace-3 (coexpressed only in pm5 cells) indicates that ace genes are not redundant.

A novel acetylcholinesterase gene in mosquitoes codes for the insecticide target and is non-homologous to the ace gene in Drosophila

Acetylcholinesterase (AChE) is the target of two major insecticide families, organophosphates (OPs) and carbamates. AChE insensitivity is a frequent resistance mechanism in insects and responsible mutations in the ace gene were identified in two Diptera, Drosophila melanogaster and Musca domestica. However, for other insects, the ace gene cloned by homology with Drosophila does not code for the insensitive AChE in resistant individuals, indicating the existence of a second ace locus. We identified two AChE loci in the genome of Anopheles gambiae, one (ace-1) being a new locus and the other (ace-2) being homologous to the gene previously described in Drosophila. The gene ace-1 has no obvious homologue in the Drosophila genome and was found in 15 mosquito species investigated. In An. gambiae, ace-1 and ace-2 display 53% similarity at the amino acid level and an overall phylogeny indicates that they probably diverged before the differentiation of insects. Thus, both genes are likely to be present in the majority of insects and the absence of ace-1 in Drosophila is probably due to a secondary loss. In one mosquito (Culex pipiens), ace-1 was found to be tightly linked with insecticide resistance and probably encodes the AChE OP target. These results have important implications for the design of new insecticides, as the target AChE is thus encoded by distinct genes in different insect groups, even within the Diptera: ace-2 in at least the Drosophilidae and Muscidae and ace-1 in at least the Culicidae. Evolutionary scenarios leading to such a peculiar situation are discussed.

An overview of insecticide resistance

Insecticide resistance poses a serious threat to current malaria control efforts. The Anopheles gambiae genome will enable identification of new resistance genes and will provide new molecular targets for the design of more effective insecticides.

Molecular cloning and characterization of a greenbug (Schizaphis graminum) cDNA encoding acetylcholinesterase possibly evolved from a duplicate gene lineage

An acetylcholinesterase (AChE, EC 3.1.1.7) cDNA was cloned and characterized from a greenbug (Schizaphis graminum (Rondani)) cDNA library. The complete cDNA (3283 bp) contains a 2028-bp open reading frame encoding 676 amino acid residues. The putative AChE preproenzyme has a 17 amino acid signal peptide, a 78 amino acid activation peptide and a mature enzyme of 581 amino acid residues. The first nine amino acid residues (YTSDDPLII) that were determined by sequencing the N-terminus of a 72-kDa AChE purified from the greenbug matched the nine residues deduced from the cDNA. The key amino acid residues, including the three residues Ser206 (200 in Torpedo), Glu332 (327) and His446 (440) forming a catalytic triad, three pairs of cysteine putatively forming intrachain disulfide bonds, and 10 out of the 14 aromatic residues lining the active site gorge of the Torpedo AChE, are conserved. However, Ser336 (Phe331) in the greenbug substituted an aromatic amino acid residue that is conserved in all other known AChEs. Northern blot analysis of mRNA revealed a 3.7-kb transcript, and Southern blot analysis suggested a single copy of this gene in the greenbug. The deduced amino acid sequence is most similar to AChE1 of the nematodes Caenorhabditis briggsae and C. elegans with 43% identity. Phylogenetic analysis showed that the greenbug AChE formed a cluster with those of nematodes, a squid and ticks, and grouped out of the insect cluster. This result suggests that the cloned gene evolved from a different duplicate gene lineage of insect AChEs.

Analysis of the sequence and expression of a second putative acetylcholinesterase cDNA from organophosphate-susceptible and organophosphate-resistant cattle ticks

The cattle tick, Boophilus microplus, is a major pest of cattle in Australia, Central and South America, and parts of Africa and Asia. Control of ticks with organophosphates (OPs) and carbamates, which target acetylcholinesterases (AChE), led to evolution of resistance to these pesticides. Alleles at the locus studied here, AChE2, from OP-susceptible female ticks from Australia and Mexico differed at 46 of 1689 nucleotide positions (20 putative amino acid differences) whereas alleles from three strains of OP-resistant ticks from Australia differed with the allele from the Australian susceptible ticks at six to 13 nucleotide positions (three to six putative amino acid differences). However, the role, if any, of these polymorphisms in the OP-resistance phenotype is unknown. Certainly none of the polymorphisms correspond to sites in AChE that are involved in catalysis or binding of acetylcholine in other organisms. Both of the AChE loci of B. microplus, AChE1 and AChE2, are apparently expressed in synganglia; AChE1 is also expressed in salivary glands and ovaries, in OP-susceptible and OP-resistant ticks. This seems to contradict studies of enzyme kinetics, which indicated that only one form of AChE was present in the synganglia, the site of the action of OPs, in this species of tick.

Acetylcholinesterase genes in the nematode Caenorhabditis elegans

Acetylcholinesterase (AChE, EC 3.1.1.7) is responsible for the termination of cholinergic nerve transmission. It is the target of organophosphates and carbamates, two types of chemical pesticides being used extensively in agriculture and veterinary medicine against insects and nematodes. Whereas there is usually one single gene encoding AChE in insects, nematodes are one of the rare phyla where multiple ace genes have been unambiguously identified. We have taken advantage of the nematode Caenorhabditis elegans model to identify the four genes encoding AChE in this species. Two genes, ace-1 and ace-2, encode two major AChEs with different pharmacological properties and tissue repartition: ace-1 is expressed in muscle cells and a few neurons, whereas ace-2 is mainly expressed in motoneurons. ace-3 represents a minor proportion of the total AChE activity and is expressed only in a few cells, but it is able to sustain double null mutants ace-1; ace-2. It is resistant to usual cholinesterase inhibitors. ace-4 was transcribed but the corresponding enzyme was not detected in vivo.

Immune responses in hookworm infections

Hookworms infect perhaps one-fifth of the entire human population, yet little is known about their interaction with our immune system. The two major species are Necator americanus, which is adapted to tropical conditions, and Ancylostoma duodenale, which predominates in more temperate zones. While having many common features, they also differ in several key aspects of their biology. Host immune responses are triggered by larval invasion of the skin, larval migration through the circulation and lungs, and worm establishment in the intestine, where adult worms feed on blood and mucosa while injecting various molecules that facilitate feeding and modulate host protective responses. Despite repeated exposure, protective immunity does not seem to develop in humans, so that infections occur in all age groups (depending on exposure patterns) and tend to be prolonged. Responses to both larval and adult worms have a characteristic T-helper type 2 profile, with activated mast cells in the gut mucosa, elevated levels of circulating immunoglobulin E, and eosinophilia in the peripheral blood and local tissues, features also characteristic of type I hypersensitivity reactions. The longevity of adult hookworms is determined probably more by parasite genetics than by host immunity. However, many of the proteins released by the parasites seem to have immunomodulatory activity, presumably for self-protection. Advances in molecular biotechnology enable the identification and characterization of increasing numbers of these parasite molecules and should enhance our detailed understanding of the protective and pathogenetic mechanisms in hookworm infections.

Regulation of cell fate in Caenorhabditis elegans by a novel cytoplasmic polyadenylation element binding protein

The fog-1 gene of Caenorhabditis elegans specifies that germ cells differentiate as sperm rather than as oocytes. We cloned fog-1 through a combination of transformation rescue experiments, RNA-mediated inactivation, and mutant analyses. Our results show that fog-1 produces two transcripts, both of which are found in germ cells but not in the soma. Furthermore, two deletion mutants alter these transcripts and are likely to eliminate fog-1 activity. The larger transcript is expressed under the control of sex-determination genes, is necessary for fog-1 activity, and is sufficient to rescue a fog-1 mutant. This transcript encodes a novel member of the CPEB family of RNA-binding proteins. Because CPEB proteins in Xenopus and Drosophila regulate gene expression at the level of translation, we propose that FOG-1 controls germ cell fates by regulating the translation of specific messenger RNAs.

Zebrafish acetylcholinesterase is encoded by a single gene localized on linkage group 7. Gene structure and polymorphism; molecular forms and expression pattern during development

We cloned and sequenced the acetylcholinesterase gene and cDNA of zebrafish, Danio rerio. We found a single gene (ache) located on linkage group LG7. The relative organization of ache, eng2, and shh genes is conserved between zebrafish and mammals and defines a synteny. Restriction fragment length polymorphism analysis was allowed to identify several allelic variations. We also identified two transposable elements in non-coding regions of the gene. Compared with other vertebrate acetylcholinesterase genes, ache gene contains no alternative splicing at 5' or 3' ends where only a T exon is present. The translated sequence is 60-80% identical to acetylcholinesterases of the vertebrates and exhibits an extra loop specific to teleosts. Analysis of molecular forms showed a transition, at the time of hatching, from the globular G4 form to asymmetric A12 form that becomes prominent in adults. In situ hybridization and enzymatic activity detection on whole embryos confirmed early expression of the acetylcholinesterase gene in nervous and muscular tissues. We found no butyrylcholinesterase gene or activity in Danio. These findings make zebrafish a promising model to study function of acetylcholinesterase during development and regulation of molecular forms assembly in vivo.

Evolutionary conservation of redundancy between a diverged pair of forkhead transcription factor homologues

The Caenorhabditis elegans gene pes-1 encodes a transcription factor of the forkhead family and is expressed in specific cells of the early embryo. Despite these observations suggesting pes-1 to have an important regulatory role in embryogenesis, inactivation of pes-1 caused no apparent phenotype. This lack of phenotype is a consequence of genetic redundancy. Whereas a weak, transitory effect was observed upon disruption of just T14G12.4 (renamed fkh-2) gene function, simultaneous disruption of the activity of both fkh-2 and pes-1 resulted in a penetrant lethal phenotype. Sequence comparison suggests these two forkhead genes are not closely related and the functional association of fkh-2 and pes-1 was only explored because of the similarity of their expression patterns. Conservation of the fkh-2/pes-1 genetic redundancy between C. elegans and the related species C. briggsae was demonstrated. Interestingly the redundancy in C. briggsae is not as complete as in C. elegans and this could be explained by alterations of pes-1 specific to the C. briggsae ancestry. With overlapping function retained on an evolutionary time-scale, genetic redundancy may be extensive and expression pattern data could, as here, have a crucial role in characterization of developmental processes.

Four genes encode acetylcholinesterases in the nematodes Caenorhabditis elegans and Caenorhabditis briggsae. cDNA sequences, genomic structures, mutations and in vivo expression

We report the full coding sequences and the genomic organization of the four genes encoding acetylcholinesterase (AChE) in Caenorhabditis elegans and Caenorhabditis briggsae, in relation to the properties of the encoded enzymes. ace-1 and ace-2, located on chromosome X and I, respectively, encode two AChEs (ACE-1 and ACE-2) that present 35% identity. The C-terminal end of ACE-1 is homologous to the C terminus of T subunits of vertebrate AChEs. ACE-1 oligomerizes into amphiphilic tetramers. ACE-2 has a hydrophobic C terminus of H type. It associates into glycolipid-anchored dimers. In C. elegans and C. briggsae, ace-3 and ace-4 are organized in tandem on chromosome II, with only 356 nt and 369 nt, respectively, between the stop codon of ace-4 (upstream gene) and the ATG of ace-3. ace-3 produces only 5 % of the total AChE activity. It encodes an H subunit that associates into dimers of glycolipid-anchored catalytic subunits, which are highly resistant to the usual AChE inhibitors, and which hydrolyze butyrylthiocholine faster than acetylthiocholine. ACE-4 is closer to ACE-3 (54 % identity) than to ACE-1 or ACE-2. The usual sequence FGESAG surrounding the active serine residue in cholinesterases is changed to FGQSAG in ace-4. ACE-4 was not detected by our current biochemical methods, although the gene is transcribed in vivo. However the level of ace-4 mRNAs is far lower than those of ace-1, ace-2 and ace-3. The ace-2, ace-3 and ace-4 transcripts were found to be trans-spliced by both SL1 and SL2, although these genes are not included in typical operons. The molecular bases of null mutations g72 (ace-2), p1304 and dc2 (ace-3) have been identified.

Absence of protein polymorphism attributable to insecticide-insensitivity of acetylcholinesterase in the green rice leafhopper, Nephotettix cincticeps

The cDNA sequence of acetylcholinesterase (AChE) from the green rice leafhopper, Nephotettix cincticeps, was amplified, based on conserved peptide sequences of AChEs. A 2.3 kb contiguous sequence, containing an ORF encoding an AChE precursor with 677 amino acid residues was obtained. The deduced protein sequence showed the most similarity to that of AChE in the Colorado potato beetle, having common features in the primary AChE structure. cDNA sequences of individual leafhoppers from an insecticide susceptible strain and the resistant strain Nakagawara, whose methylcarbamate-insensitive AChEs show 10(2) or more I(50) ratio for propoxur, were compared. No fixed inter-strain difference was identified in the protein sequence, though amino acid substitution polymorphism was found at one position in the susceptible strain. Insecticide-insensitivity of leafhopper AChE does not result from changes in the protein primary structure that is encoded by the AChE gene sequence isolated in this study.

Mutations in the dystrophin-like dys-1 gene of Caenorhabditis elegans result in reduced acetylcholinesterase activity

Mutations of the Caenorhabditis elegans dystrophin/utrophin-like dys-1 gene lead to hyperactivity and hypercontraction of the animals. In addition dys-1 mutants are hypersensitive to acetylcholine and acetylcholinesterase inhibitors. We investigated this phenotype further by assaying acetylcholinesterase activity. Total extracts from three different dys-1 alleles showed significantly less acetylcholinesterase-specific activity than wild-type controls. In addition, double mutants carrying a mutation in the dys-1 gene plus a mutation in either of the two major acetylcholinesterase genes (ace-1 and ace-2) display locomotor defects consistent with a strong reduction of acetylcholinesterases, whereas none of the single mutants does. Therefore, in C. elegans, disruption of the dystrophin/utrophin-like dys-1 gene affects acetylcholinesterase activity.

A modular treatment of molecular traffic through the active site of cholinesterase

We present a model for the molecular traffic of ligands, substrates, and products through the active site of cholinesterases (ChEs). First, we describe a common treatment of the diffusion to a buried active site of cationic and neutral species. We then explain the specificity of ChEs for cationic ligands and substrates by introducing two additional components to this common treatment. The first module is a surface trap for cationic species at the entrance to the active-site gorge that operates through local, short-range electrostatic interactions and is independent of ionic strength. The second module is an ionic-strength-dependent steering mechanism generated by long-range electrostatic interactions arising from the overall distribution of charges in ChEs. Our calculations show that diffusion of charged ligands relative to neutral isosteric analogs is enhanced approximately 10-fold by the surface trap, while electrostatic steering contributes only a 1.5- to 2-fold rate enhancement at physiological salt concentration. We model clearance of cationic products from the active-site gorge as analogous to the escape of a particle from a one-dimensional well in the presence of a linear electrostatic potential. We evaluate the potential inside the gorge and provide evidence that while contributing to the steering of cationic species toward the active site, it does not appreciably retard their clearance. This optimal fine-tuning of global and local electrostatic interactions endows ChEs with maximum catalytic efficiency and specificity for a positively charged substrate, while at the same time not hindering clearance of the positively charged products.

Comparison of acetylcholinesterase genes from cattle ticks

We describe the isolation and characterisation of two putatively new acetylcholinesterase genes from the African cattle ticks Boophilus decoloratus and Rhipicephalus appendiculatus. The nucleotide sequences of these genes had 93% homology to each other and 95% and 91% identity, respectively, to the acetylcholinesterase gene from an Australian strain of another cattle tick, Boophilus microplus. Translation of the nucleotide sequences revealed putative amino acids that are essential for acetylcholinesterase activity: the active site serine, and the histidine and glutamate residues that associate with this serine to form the catalytic triad. All known acetylcholinesterases have three sets of cysteines that form disulfide bonds; however, the acetylcholinesterase genes of these three species of ticks encode only two sets of cysteines. Acetylcholinesterases of B. microplus from South Africa, Zimbabwe, Kenya and Mexico had 98-99% identity with acetylcholinesterase from B. microplus from Australia, whereas acetylcholinesterase from B. microplus from Indonesia was identical to that from Australia. Preliminary phylogenetic analyses surprisingly indicate that the acetylcholinesterases of ticks are closer phylogenetically to acetylcholinesterases of vertebrates than they are to those of other arthropods.

Structure and promoter activity of the 5' flanking region of ace-1, the gene encoding acetylcholinesterase of class A in Caenorhabditis elegans

We report the structure and the functional activity of the promoter region of ace-1, the gene encoding acetylcholinesterase of class A in the nematode Caenorhabditis elegans. We found that ace-1 was trans -spliced to the SL1 spliced leader and that transcription was initiated at a cluster of multiple starts. There was neither a TATA nor a CAAT box at consensus distances from these starts. Interspecies sequence comparison of the 5' regions of ace-1 in C. elegans and in the related nematode Caenorhabditis briggsae identified four blocks of conserved sequences located within a sequence of 2.4 kilobases upstream from the initiator ATG. In vitro expression of CAT reporter genes in mammalian cells allowed the determination of a minimal promoter in the first 288 nucleotides. In phenotype rescue experiments in vivo, the ace-1 gene containing 2.4 kilobases of 5' flanking region of either C. elegans or C. briggsae was found to restore a coordinated mobility to the uncoordinated double mutants ace-1(-);ace-2(-)of C. elegans. This showed that the ace-1 promoter was contained in 2.4 kilobases of the 5' region, and indicated that cis -regulatory elements as well as coding sequences of ace-1 were functionally conserved between the two nematode species. The pattern of ace-1 expression was established through microinjection of Green Fluorescent Protein reporter gene constructs and showed a major mesodermal expression. Deletion analysis showed that two of the four blocks of conserved sequences act as tissue-specific activators. The distal block is a mesodermal enhancer responsible for the expression in body wall muscle cells, anal sphincter and vulval muscle cells. Another block of conserved sequence directs expression in pharyngeal muscle cells pm5 and three pairs of cephalic sensory neurons.

The polymorphism of acetylcholinesterase: post-translational processing, quaternary associations and localization

The molecular forms of acetylcholinesterase (AChE) correspond to various quaternary structures and modes of anchoring of the enzyme. In vertebrates, these molecules are generated from a single gene: the catalytic domain may be associated with several types of C-terminal peptides, that define distinct types of catalytic subunits (AChE(S), AChE(H), AChE(T)) and determine their post-translational maturation. AChE(S) generates soluble monomers, in the venom of Elapid snakes. AChE(H) generates GPI-anchored dimers, in Torpedo muscles and on mammalian blood cells. AChE(T) is the only type of catalytic subunit that exists in all vertebrate cholinesterases; it produces the major forms in adult brain and muscle. AChE(T) generates multiple structures, ranging from monomers and dimers to collagen-tailed and hydrophobic-tailed forms, in which catalytic tetramers are associated with anchoring proteins that attach them to the basal lamina or to cell membranes. In the collagen-tailed forms, AChE(T) subunits are associated with a specific collagen, ColQ, which is encoded by a single gene in mammals. ColQ contains a short peptidic motif, the proline-rich attachment domain (PRAD), that triggers the formation of AChE(T) tetramers, from monomers and dimers. The critical feature of this motif is the presence of a string of prolines, and in fact synthetic polyproline shows a similar capacity to organize AChE(T) tetramers. Although the COLQ gene produces multiple transcripts, it does not generate the hydrophobic tail. P, which anchors AChE in mammalian brain membranes. The coordinated expression of AChE(T) subunits and anchoring proteins determines the pattern of molecular forms and therefore the localization and functionality of the enzyme.

Molecular cloning of an acetylcholinesterase gene from the plant parasitic nematodes, Meloidogyne incognita and Meloidogyne javanica

A gene encoding a protein with strong homology with Caenorhabditis elegans and C. briggsae acetylcholinesterase ACE-1 was cloned from Meloidogyne incognita and M. javanica pre-parasitic juveniles. Both cDNAs have an ORF of 1968 bp for a deduced translation product of 656 amino acid residues. The key residues essential to acetylcholinesterase (AChE) structure and function are conserved in both sequences. M. incognita and M. javanica AChE share a homology of 98.8% at the amino acid level and 97% at the nucleotide level. Phylogenetic analysis showed that Meloidogyne and Caenorhabditis AChE form a cluster among AChE of triploblastic organisms. This Meloidogyne AChE is expressed in eggs, pre-parasitic juveniles and males and AChE activity was detected in situ in amphids of pre-parasitic juveniles. The opportunity of using AChE as a target in new strategies of nematode control is discussed.

Cloning, expression, and properties of a nonneuronal secreted acetylcholinesterase from the parasitic nematode Nippostrongylus brasiliensis

We have isolated a full-length cDNA encoding an acetylcholinesterase secreted by the nematode parasite Nippostrongylus brasiliensis. The predicted protein is truncated in comparison with acetylcholinesterases from other organisms such that the carboxyl terminus aligns closely to the end of the catalytic domain of the vertebrate enzymes. The residues in the catalytic triad are conserved, as are the six cysteines which form the three intramolecular disulfide bonds. Three of the fourteen aromatic residues which line the active site gorge in the Torpedo enzyme are substituted by nonaromatic residues, corresponding to Tyr-70 (Thr), Trp-279 (Asn), and Phe-288 (Met). High level expression was obtained via secretion from Pichia pastoris. The purified enzyme behaved as a monomeric hydrophilic species. Although of invertebrate origin and possessing the above substitutions in the active site gorge residues, the enzyme efficiently hydrolyzed acetylthiocholine and showed minimal activity against butyrylthiocholine. It displayed excess substrate inhibition with acetylthiocholine at concentrations over 2. 5 mM and was highly sensitive to both active site and "peripheral" site inhibitors. Northern blot analysis indicated a progressive increase in mRNA for AChE B in parasites isolated from 6 days postinfection.

Molecular cloning and expression of a full-length cDNA encoding acetylcholinesterase in optic lobes of the squid Loligo opalescens: a new member of the cholinesterase family resistant to diisopropyl fluorophosphate

Acetylcholinesterase cDNA was cloned by screening a library from Loligo opalescens optic lobes; cDNA sequence analysis revealed an open reading frame coding for a protein of 610 amino acids that showed 20-41% amino acid identity with the acetylcholinesterases studied so far. The characteristic structure of cholinesterase (the choline binding site, the catalytic triad, and six cysteines that form three intrachain disulfide bonds) was conserved in the protein. The heterologous expression of acetylcholinesterase in COS cells gave a recovery of acetylcholinesterase activity 20-fold higher than in controls. The enzyme, partially purified by affinity chromatography, showed molecular and kinetic features indistinguishable from those of acetylcholinesterase expressed in vivo, which displays a high catalytic efficiency. Both enzymes are true acetylcholinesterase corresponding to phosphatidylinositol-anchored G2a dimers of class I, with a marked substrate specificity for acetylthiocholine. The deduced amino acid sequence may explain some particular kinetic characteristics of Loligo acetylcholinesterase, because the presence of a polar amino acid residue (S313) instead of a nonpolar one [F(288) in Torpedo] in the acyl pocket of the active site could justify the high substrate specificity of the enzyme, the absence of hydrolysis with butyrylthiocholine, and the poor inhibition by the organophosphate diisopropyl fluorophosphate.

Four acetylcholinesterase genes in the nematode Caenorhabditis elegans

Whereas a single gene encodes acetylcholinesterase (AChE) in vertebrates and most insect species, four distinct genes have been cloned and characterized in the nematode Caenorhabditis elegans. We found that ace-1 (mapped to chromosome X) is prominently expressed in muscle cells whereas ace-2 (located on chromosome I) is mainly expressed in neurons. Ace-x and ace-y genes are located in close proximity on chromosome II where they are separated by only a few hundred base pairs. The role of these two genes is still unknown.

Acetylcholinesterase: C-terminal domains, molecular forms and functional localization

Acetylcholinesterase (AChE) possesses short C-terminal peptides that are not necessary for catalytic activity. These peptides belong to different classes (R, H, T, S) and define the post-translational processing and targeting of the enzyme. In vertebrates, subunits of type H (AChEH) and of type T (AChET) are the most important: AChEH subunits produce glycolipid (GPI)-anchored dimers and AChET subunits produce hetero-oligomeric forms such as membrane-bound tetramers in the mammalian brain (containing a 20 kDa hydrophobic protein) and asymmetric collagen-tailed forms in neuromuscular junctions (containing a specific collagen, ColQ). The T peptide allows the formation of tetrameric assemblies with a proline-rich attachment domain (PRAD) of collagen ColQ. These complex molecular structures condition the functional localization of the enzyme in the supramolecular architecture of cholinergic synapses.