Molecular cloning and construction of the coding region for human acetylcholinesterase reveals a G + C-rich attenuating structure

Acetylcholinesterase in Alzheimer's disease

Since the discovery of the cholinergic deficit in Alzheimer disease (AD), acetylcholinesterase (AChE) has been widely investigated in tissues involved in the disease. These studies showed modifications in AChE activity and changes in its polymorphism in brain as well as in cerebro-spinal fluid (CSF) and blood. The co-localization of the enzyme in the senile plaque provided evidence of its anomalous features. It has been also shown that AChE forms a stable complex with senile plaque components through its peripheral anionic site. Moreover, the neurotoxicity of amyloid components is increased by the presence of AChE. The occurrence of an altered glycosylation of some AChE forms in AD is closely related to the presence of amyloid formations. Literature on expression, relationships and modifications in the molecular polymorphism of AChE, in brain, CSF and blood in AD is reviewed.

Mutation of residues 423 (Met/Ile), 444 (Thr/Met), and 506 (Asn/Ser) confer cholesteryl esterase activity on rat lung carboxylesterase. Ser-506 is required for activation by cAMP-dependent protein kinase

Site-directed mutagenesis is used to identify amino acid residues that dictate reported differences in substrate specificity between rat hepatic neutral cytosolic cholesteryl ester hydrolase (hncCEH) and rat lung carboxylesterase (LCE), proteins differing by only 4 residues in their primary sequences. Beginning with LCE, the substitution Met(423) --> Ile(423) alone or in combination with other mutations increased activity with p-nitrophenylcaprylate (PNPC) relative to more hydrophilic p-nitrophenylacetate (PNPA), typical of hncCEH. The substitution Thr(444) --> Met(444) was necessary but not sufficient for expression of cholesteryl esterase activity in COS-7 cells. The substitution Asn(506) --> Ser(506), creating a potential phosphorylation site, uniformly increased activity with both PNPA and PNPC, was necessary but not sufficient for expression of cholesteryl esterase activity and conferred susceptibility to activation by cAMP-dependent protein kinase, a property of hncCEH. The 3 mutations in combination were necessary and sufficient for expression of cholesteryl esterase activity by the mutated LCE. The substitution Gln(186) --> Arg(186) selectively reduced esterase activity with PNPA and PNPC but was not required for cholesteryl esterase activity. Homology modeling from x-ray structures of acetylcholinesterases is used to propose three-dimensional models for hncCEH and LCE that provide insight into the effects of these mutations on substrate specificity.

Effect of chemical modification of recombinant human acetylcholinesterase by polyethylene glycol on its circulatory longevity

Post-translational modifications were recently shown to be responsible for the short circulatory mean residence time (MRT) of recombinant human acetylcholinesterase (rHuAChE) [Kronman, Velan, Marcus, Ordentlich, Reuveny and Shafferman (1995) Biochem. J. 311, 959--967; Chitlaru, Kronman, Zeevi, Kam, Harel, Ordentlich, Velan and Shafferman (1998) Biochem. J. 336, 647--658; Chitlaru, Kronman, Velan and Shafferman (2001) Biochem. J. 354, 613--625], which is one of the major obstacles to the fulfilment of its therapeutic potential as a bioscavenger. In the present study we demonstrate that the MRT of rHuAChE can be significantly increased by the controlled attachment of polyethylene glycol (PEG) side chains to lysine residues. Attachment of as many as four PEG molecules to monomeric rHuAChE had minimal effects, if any, on either the catalytic activity (K(m)=0.09 mM and k(cat)=3.9 x 10(5) min(-1)) or the reactivity of the modified enzyme towards active-centre inhibitors, such as edrophonium and di-isopropyl fluorophosphate, or to peripheral-site ligands, such as propidium, BW284C51 and even the bulky snake-venom toxin fasciculin-II. The increase in MRT of the PEG-modified monomeric enzyme is linearly dependent, in the tested range, on the number of attached PEG molecules, as well as on their size. It appears that even low level PEG-conjugation can overcome the deleterious effect of under-sialylation on the pharmacokinetic performance of rHuAChE. At the highest tested ratio of attached PEG-20000/rHuAChE (4:1), an MRT of over 2100 min was attained, a value unmatched by any other known form of recombinant or native serum-derived AChE reported to date.

Pressure- and heat-induced inactivation of butyrylcholinesterase: evidence for multiple intermediates and the remnant inactivation process

The inactivation process of native (N) human butyrylcholinesterase (BuChE) by pressure and/or heat was found to be multi-step. It led to irreversible formation of an active intermediate (I) state and a denatured state. This series-inactivation process was described by expanding the Lumry-Eyring [Lumry, R. and Eyring, H. (1954) J. Phys. Chem. 58, 110-120] model. The intermediate state (I) was found to have a K(m) identical with that of the native state and a turnover rate (k(cat)) twofold higher than that of the native state with butyrylthiocholine as the substrate. The increased catalytic efficiency (k(cat)/K(m)) of I can be explained by a conformational change in the active-site gorge and/or restructuring of the water-molecule network in the active-site pocket, making the catalytic steps faster. However, a pressure/heat-induced covalent modification of native BuChE, affecting the catalytic machinery, cannot be ruled out. The inactivation process of BuChE induced by the combined action of pressure and heat was found to continue after interruption of pressure/temperature treatment. This secondary inactivation process was termed 'remnant inactivation'. We hypothesized that N and I were in equilibrium with populated metastable N' and I' states. The N' and I' states can either return to the active forms, N and I, or develop into inactive forms, N(')(in) and I(')(in). Both active N' and I' intermediate states displayed different rates of remnant inactivation depending on the pressure and temperature pretreatments and on the storage temperature. A first-order deactivation model describing the kinetics of the remnant inactivation of BuChE is proposed.

Modifying the cellular transport of DNA-based vaccines alters the immune response to hantavirus nucleocapsid protein

Puumala virus is a member of the hantavirus genus (family Bunyaviridae) and is one of the causative agents of hemorrhagic fever with renal syndrome (HFRS) in Europe. A genetic vaccination approach was conducted to investigate if the immune response could be modulated using different cellular secretion and/or localisation signals, and the immune responses were analysed in BALB/c mice and in a bank vole infectious model. Rodents vaccinated with DNA constructs encoding the antigen fused to an amino-terminal secretion signal raised significantly higher antibody levels when compared to using constructs lacking secretion signals. Furthermore, the ratios of the IgG subclasses (IgG2a/IgG1) were raised by the use of cellular localisation signals, indicating a more pronounced Th1-type of immune response. The majority of the mice, or bank voles, immunised with DNA encoding a secreted form of the antigen showed a positive lymphoproliferative response and were protected against challenge with Puumala virus (strain Kazan-wt).

Synthetic antigenic decapeptides of human brain acetylcholinesterase cross-immunoreact with peptide-specific antibodies against Torpediniformes narcine timlei acetylcholinesterase

Antigenic decapeptides of human brain acetylcholinesterase (AChE) were investigated for immunoreactivity with the rabbit anti-Torpediniformes narcine timlei AChE polyclonal antibody (anti-narcine AChE polyclonal antibody). The decapeptides were synthesized using the multipin combinatorial chemical synthesis technique and biotinylated at N-terminals. Rabbit anti-narcine AChE polyclonal antibodies were purified by Protein A-Sepharose CL 4B column chromatography. Enzyme-linked immunosorbent assay (ELISA) was used for the assay of the reaction between the antigen and the antibody. Seven of 11 antigenic synthetic decapeptides of human brain AChE showed obvious immunoreactivity with the rabbit anti-narcine AChE polyclonal antibodies. The similarity of the AChE sequences of humans and Torpedo species were compared thereby with the epitopes indicated. The results indicate that the epitopes of human brain AChE and Torpedo AChEs have been highly conserved during evolution. In view of this, no N-glycosylation site was found in the antigenic decapeptides tested, they all belong to oligopeptide epitopes.

Effect of human acetylcholinesterase subunit assembly on its circulatory residence

Sialylated recombinant human acetylcholinesterase (rHuAChE), produced by stably transfected cells, is composed of a mixed population of monomers, dimers and tetramers and manifests a time-dependent circulatory enrichment of the higher-order oligomeric forms. To investigate this phenomenon further, homogeneous preparations of rHuAChE differing in their oligomerization statuses were generated: (1) monomers, represented by the oligomerization-impaired C580A-rHuAChE mutant, (2) wild-type (WT) dimers and (3) tetramers of WT-rHuAChE generated in vitro by complexation with a synthetic ColQ-derived proline-rich attachment domain ('PRAD') peptide. Three different series of each of these three oligoform preparations were produced: (1) partly sialylated, derived from HEK-293 cells; (2) fully sialylated, derived from engineered HEK-293 cells expressing high levels of sialyltransferase; and (3) desialylated, after treatment with sialidase to remove sialic acid termini quantitatively. The oligosaccharides associated with each of the various preparations were extensively analysed by matrix-assisted laser desorption ionization-time-of-flight MS. With the enzyme preparations comprising the fully sialylated series, a clear linear relationship between oligomerization and circulatory mean residence time (MRT) was observed. Thus monomers, dimers and tetramers exhibited MRTs of 110, 195 and 740 min respectively. As the level of sialylation decreased, this differential behaviour became less pronounced; eventually, after desialylation all oligoforms had the same MRT (5 min). These observations suggest that multiple removal systems contribute to the elimination of AChE from the circulation. Here we also demonstrate that by the combined modulation of sialylation and tetramerization it is possible to generate a rHuAChE displaying a circulatory residence exceeding that of all other known forms of native or recombinant human AChE.

Determination of the DNA sequences of acetylcholinesterase and butyrylcholinesterase from cat and demonstration of the existence of both in cat plasma

Cat serum contains 0.5 mg/L of butyrylcholinesterase (BChE, EC 3.1.1. 8) and 0.3 mg/L of acetylcholinesterase (AChE, EC 3.1.1.7); this can be compared with 5 mg/mL and < 0.01 mg/L, respectively, in human serum. Cat BChE differed from human BChE in the steady-state turnover of butyrylthiocholine, having a 3-fold higher k(cat) and 2-fold higher K(m) and K(ss) values. Sequencing of the cat BCHE cDNA revealed 70 amino acid differences between cat and human BChE, three of which could account for these kinetic differences. These amino acids, which were located in the region of the active site, were Phe398Ile, Pro285Leu, and Ala277Leu (where the first amino acid was found in human and the second in cat). Sequencing genomic DNA for cat and human ACHE demonstrated that there were 33 amino acid differences between the cat and human AChE enzymes, but that there were no differences in the active site region. In addition, a polymorphism in intron 3 of the human ACHE gene was detected, as well as a silent polymorphism at Y116 of the cat ACHE gene.

Epitopes of human brain acetylcholinesterase

The main purpose of the present work was to identify B-cell epitopes on human brain acetylcholinesterase (AChE) by the synthetic peptide approach. Five hundred and seventy-four decapeptides comprising amino acids No. n to n+9 (where n denotes the residue number of the 583 amino acids in the primary structure of human brain AChE and is an integer in the range 1-574) were synthesized, using the multipin combinatorial chemical synthesis technique, and biotinylated. Epitopes of human brain AChE were detected by enzyme-linked immunosorbent assay (ELISA) and compared with the predicted epitopes of human AChE by 'Goldkey' software. Among 574 synthetic decapeptides, 47 decapeptides at 11 antigenic regions showed immunoreactivity with mouse anti-human brain AChE polyclonal antibodies. The minimum sequence of epitope was defined at every antigenic region explored. The locations and sequences of the former ten continuous epitopes at the 11 antigenic regions of the human brain AChE had been identified as follows: TPVLVWIY (112-119), RTVLVSMNY (143-151), LLDQRLALQW (173-182), RRATQLAH (246-253), VFRFSFVPV (294 approximately 302), KDEGSYFLVY (332-341), RVYA (424-427), LMRY (476-479), KAPQWPPY (496-503), GLRAQACAFW (523-532). The rate of hits of the predicted epitopes from the software came out at 33%. In our work, the epitopes of human AChE have been mapped by purified polyclonal antibody at eleven distinct sites in the primary structure.

Autoantibodies to acetylcholinesterase revisited

A sensitive and specific enzyme linked immunosorbent assay (ELISA) utilizing human recombinant acetylcholinesterase has been employed for the detection of human antibodies to human acetylcholinesterase. The method can detect allogenic antibodies to the Yt(a) form of human erythrocyte AChE. Adaptation of this ELISA method allowed the IgG subclass typing of IgG anti-AChE antibodies, which could help to determine the possible role of these antibodies in the aetiology of any neurological conditions. Routine serological investigations established the AChE phenotype of each of the patients recruited, to determine whether anti-AChE antibodies were allogenic or autogenic in origin. These techniques were used to determine the incidence of autoantibodies to AChE in patients with neurological conditions, including the subtypes of motor neuron disease. The data presented are not consistent with earlier reports of a high incidence of autoantibodies to AChE in amyotrophic lateral sclerosis and progressive muscular atrophy.

Why so many forms of acetylcholinesterase?

Acetylcholinesterase is a key molecule in the control of cholinergic transmission. In the mammalian neuromuscular junction (NMJ), the efficiency of this phenomenon depends on the enzyme location, between the presynaptic site where acetylcholine is released and the postsynaptic membrane where the acetylcholine receptors are packed. Various molecular forms of the enzyme that possess the same catalytic activity are expressed. The relative amounts of these forms are tissue-specific. At the subcellular level, this panoply of forms allows the enzyme to be attached to the membrane or to the basal lamina. Analysis of the forms secreted and their position in the cytoarchitecture of the NMJ is essential to understand the functioning of this synapse. This review will consider the origin of the enzyme polymorphism and its physiological implication.

Comparative expression of homologous proteins. A novel mode of transcriptional regulation by the coding sequence folding compatibility of chimeras

Recombinant acetylcholinesterases (AChE) are produced at systematically different levels, depending on the enzyme species. To identify the cause of this difference, we designed expression vectors that differed only by the central region of the coding sequence, encoding Torpedo, rat, and Bungarus AChEs and two reciprocal rat/Bungarus and Bungarus/rat chimeras. We found that folding is a limiting factor in the case of Torpedo AChE and the chimeras, for which only a limited fraction of the synthesized polypeptides becomes active and is secreted. In contrast, the fact that rat AChE is less well produced than Bungarus AChE reflects the levels of their respective mRNAs, which seem to be controlled by their transcription rates. A similar difference was observed in the coding and noncoding orientations; it seems to depend on multiple cis-elements. Using CAT constructs, we found that a DNA fragment from the Bungarus AChE gene stimulates expression of the reporter protein, whereas a homologous fragment from the rat AChE gene had no influence. This stimulating effect appears different from that of classical enhancers, although its mechanism remains unknown. In any case, the present results demonstrate that the coding region contributes to control the level of gene expression.

Acetylcholinesterase inhibitors for potential use in Alzheimer's disease: molecular modeling, synthesis and kinetic evaluation of 11H-indeno-[1,2-b]-quinolin-10-ylamine derivatives

Continuing our work on tetracyclic tacrine analogues, we synthesized a series of acetylcholinesterase (AChE) inhibitors of 11H-indeno-[1,2-b]-quinolin-10-ylaminic structure. Selected substituents were placed in synthetically accessible positions of the tetracyclic nucleus, in order to explore the structure-activity relationships (SAR) and the mode of action of this class of anticholinesterases. A molecular modeling investigation of the binding interaction of the lead compound (1a) with the AChE active site was performed, from which it resulted that, despite the rather wide and rigid structure of 1a, there may still be the possibility to introduce some small substituent in some positions of the tetracycle. However, from the examination of the experimental IC50 values, it derived that the indenoquinoline nucleus probably represents the maximum allowable molecular size for rigid compounds binding to AChE. In fact, only a fluorine atom in position 2 maintains the AChE inhibitory potency of the parent compound, and, actually, increases the AChE-selectivity with respect to the butyrylcholinesterase inhibition. By studying the kinetics of AChE inhibition for two representative compounds of the series, it resulted that the lead compound (1a) shows an inhibition of mixed type, binding to both the active and the peripheral sites, while the more sterically hindered analogue 2n seems to interact only at the external binding site of the enzyme. This finding seems particularly important in the context of Alzheimer's disease research in the light of recent observations showing that peripheral AChE inhibitors might decrease the aggregating effects of the enzyme on the beta-amyloid peptide (betaA).

Structural roles of acetylcholinesterase variants in biology and pathology

Apart from its catalytic function in hydrolyzing acetylcholine, acetylcholinesterase (AChE) affects cell proliferation, differentiation and responses to various insults, including stress. These responses are at least in part specific to the three C-terminal variants of AChE which are produced by alternative splicing of the single ACHE gene. 'Synaptic' AChE-S constitutes the principal multimeric enzyme in brain and muscle; soluble, monomeric 'readthrough' AChE-R appears in embryonic and tumor cells and is induced under psychological, chemical and physical stress; and glypiated dimers of erythrocytic AChE-E associate with red blood cell membranes. We postulate that the homology of AChE to the cell adhesion proteins, gliotactin, glutactin and the neurexins, which have more established functions in nervous system development, is the basis of its morphogenic functions. Competition between AChE variants and their homologs on interactions with the corresponding protein partners would inevitably modify cellular signaling. This can explain why AChE-S exerts process extension from cultured amphibian, avian and mammalian glia and neurons in a manner that is C-terminus-dependent, refractory to several active site inhibitors and, in certain cases, redundant to the function of AChE-like proteins. Structural functions of AChE variants can explain their proliferative and developmental roles in blood, bone, retinal and neuronal cells. Moreover, the association of AChE excess with amyloid plaques in the degenerating human brain and with progressive cognitive and neuromotor deficiencies observed in AChE-transgenic animal models most likely reflects the combined contributions of catalytic and structural roles.

Human erythrocyte but not brain acetylcholinesterase hydrolyses heroin to morphine

1. In human blood, heroin is rapidly hydrolysed by sequential deacylation of two ester bonds to yield first 6-monoacetylmorphine (6-MAM), then morphine. 2. Serum butyrylcholinesterase (BuChE) hydrolyses heroin to 6-MAM with a catalytic efficiency of 4.5/min per mumol/L, but does not proceed to produce morphine. 3. In vitro, human erythrocyte acetylcholinesterase (AChE) hydrolyses heroin to 6-MAM, with a catalytic efficiency of 0.5/min per mumol/L under first-order kinetics. Moreover, erythrocyte AChE, but not BuChE is capable of further hydrolysing 6-MAM to morphine, albeit at a considerably slower rate. 4. Both hydrolysis steps by erythrocyte AChE were totally blocked by the selective AChE inhibitor BW284c51 but were not blocked by the BuChE-specific inhibitor, iso-OMPA (tetraisopropylpyrophosphoramide). 5. The brain synaptic form of AChE, which differs from the erythrocyte enzyme in its C-terminus, was incapable of hydrolysing heroin. 6. Heroin suppressed substrate hydrolysis by antibody-immobilized erythrocyte but not by brain AChE. 7. These findings reveal a new metabolic role for erythrocyte AChE, the biological function of which is as yet unexplained, and demonstrate distinct biochemical properties for the two AChE variants, which were previously considered catalytically indistinguishable.

Molecular isoform distribution and glycosylation of acetylcholinesterase are altered in brain and cerebrospinal fluid of patients with Alzheimer's disease

The glycosylation of acetylcholinesterase (AChE) in CSF was analyzed by lectin binding. AChE from Alzheimer's disease (AD) patients was found to bind differently to two lectins, concanavalin A and wheat germ agglutinin, than AChE from controls. As multiple isoforms of AChE are present in both CSF and brain, we examined whether the abnormal glycosylation of AD AChE was due to changes in a specific molecular isoform. Globular amphiphilic dimeric (G2a) and monomeric (G1a) isoforms of AChE were found to be differentially glycosylated in AD CSF. Glycosylation of AChE was also altered in AD frontal cortex but not in cerebellum and was also associated with an increase in the proportion of light (G2 and G1) isoforms. This study demonstrates that the glycosylation of AChE is altered in the AD brain and that changes in AChE glycosylation in AD CSF may reflect changes in the distribution of brain isoforms. The study also suggests that glycosylation of AChE may be a useful diagnostic marker for AD.

Cholinesterases in neural development: new findings and toxicologic implications

Developing animals are more sensitive than adults to acute cholinergic toxicity from anticholinesterases, including organophosphorus pesticides, when administered in a laboratory setting. It is also possible that these agents adversely affect the process of neural development itself, leading to permanent deficits in the architecture of the central and peripheral nervous systems. Recent observations indicate that organophosphorus exposure can affect DNA synthesis and cell survival in neonatal rat brain. New evidence that acetylcholinesterase may have a direct role in neuronal differentiation provides additional grounds for interest in the developmental toxicity of anticholinesterases. For example, correlative anatomic studies show that transient bursts of acetylcholinesterase expression often coincide with periods of axonal outgrowth in maturing avian, rodent, and primate brain. Some selective cholinesterase inhibitors effectively suppress neurite outgrowth in model systems like differentiating neuroblastoma cells and explanted sensory ganglia. When enzyme expression is altered by genetic engineering, acetylcholinesterase levels on the outer surface of transfected neurons correlate with ability to extend neurites. Certain of these "morphogenic" effects may depend on protein-protein interactions rather than catalytic acetylcholinesterase activity. Nonetheless, it remains possible that some pesticides interfere with important developmental functions of the cholinesterase enzyme family.

Acetylcholinesterase inhibitors: synthesis and structure-activity relationships of omega-[N-methyl-N-(3-alkylcarbamoyloxyphenyl)- methyl]aminoalkoxyheteroaryl derivatives

Acetylcholinesterase (AChE) inhibitors are one of the most actively investigated classes of compounds in the search for an effective treatment of Alzheimer's disease. This work describes the synthesis, AChE inhibitory activity, and structure-activity relationships of some compounds related to a recently discovered series of AChE inhibitors: the omega-[N-methyl-N-(3-alkylcarbamoyloxyphenyl)methyl]aminoalkoxy xanthen-9-ones. The influence of structural variations on the inhibitory potency was carefully investigated by modifying different parts of the parent molecule, and a theoretical model of the binding of one representative compound to the enzyme was developed. The biological properties of the series were investigated in some detail by considering not only the activity on isolated enzyme but the selectivity with respect to butyrylcholinesterase (BuChE) and the in vitro inhibitory activity on rat cerebral cortex as well. Some of the newly synthesized derivatives, when tested on isolated and/or AChE-enriched rat brain cortex fraction, displayed a selective inhibitory activity and were more active than physostigmine. In particular, compound 13, an azaxanthone derivative, displayed the best rat cortex AChE inhibition (190-fold higher than physostigmine), as well as a high degree of enzyme selectivity (over 60-fold more selective for AChE than for BuChE). When tested in the isolated enzyme, compound 13 was less active, suggesting some differences either in drug availability/biotransformation or in the inhibitor-sensitive residues of the enzyme when biologically positioned in rat brain membranes.

Bovine acetylcholinesterase: cloning, expression and characterization

The bovine acetylcholinesterase (BoAChE) gene was cloned from genomic DNA and its structure was determined. Five exons coding for the AChE T-subunit and the alternative H-subunit were identified and their organization suggests high conservation of structure in mammalian AChE genes. The deduced amino acid sequence of the bovine T-subunit is highly similar to the human sequence, showing differences at 34 positions only. However, the cloned BoAChE sequence differs from the published amino acid sequence of AChE isolated from fetal bovine serum (FBS) by: (1) 13 amino acids, 12 of which are conserved between BoAChE and human AChE, and (2) the presence of four rather than five potential N-glycosylation sites. The full coding sequence of the mature BoAChE T-subunit was expressed in human embryonal kidney 293 cells (HEK-293). The catalytic properties of recombinant BoAChE and its reactivity towards various inhibitors were similar to those of the native bovine enzyme. Soluble recombinant BoAChE is composed of monomers, dimers and tetramers, yet in contrast to FBS-AChE, tetramer formation is not efficient. Comparative SDS/PAGE analysis reveals that all four potential N-glycosylation sites identified by DNA sequencing appear to be utilized, and that recombinant BoAChE comigrates with FBS-AChE. A major difference between the recombinant enzyme and the native enzyme was observed when clearance from circulation was examined. The HEK-293-derived enzyme was cleared from the circulation at a much faster rate than FBS-AChE. This difference in behaviour, together with previous studies on the effect of post-translation modification on human AChE clearance [Kronman, Velan, Marcus, Ordentlich, Reuveny and Shafferman (1995) Biochem. J. 311, 959-967] suggests that cell-dependent glycosylation plays a key role in AChE circulatory residence.

Acetylcholinesterase cDNA of the cattle tick, Boophilus microplus: characterisation and role in organophosphate resistance

Acetylcholinesterase is the target of organophosphate and carbamate pesticides. Organophosphate resistance is widespread in the cattle tick, Boophilus microplus, in Australia. We have isolated a cDNA of acetylcholinesterase from B. microplus and show that it would encode a protein 62 kDa in size. The predicted amino acid sequence contains all the residues characteristic of an acetylcholinesterase. Alternative splicing of the transcript was detected at both the 5' and 3' ends. Alternative splicing at the 5' end would result in two proteins differing by six amino acids. This is the first report of alternative splicing of the N-terminal coding region in a cholinesterase. No point mutations were detected in the acetylcholinesterase gene from organophosphate resistant strains of B. microplus. Alternative explanations for resistance to organophosphates in B. microplus are discussed.

Functional characteristics of the oxyanion hole in human acetylcholinesterase

The contribution of the oxyanion hole to the functional architecture and to the hydrolytic efficiency of human acetylcholinesterase (HuAChE) was investigated through single replacements of its elements, residues Gly-121, Gly-122 and the adjacent residue Gly-120, by alanine. All three substitutions resulted in about 100-fold decrease of the bimolecular rate constants for hydrolysis of acetylthiocholine; however, whereas replacements of Gly-120 and Gly-121 affected only the turnover number, mutation of residue Gly-122 had an effect also on the Michaelis constant. The differential behavior of the G121A and G122A enzymes was manifested also toward the transition state analog m-(N,N, N-trimethylammonio)trifluoroacetophenone (TMTFA), organophosphorous inhibitors, carbamates, and toward selected noncovalent active center ligands. Reactivity of both mutants toward TMTFA was 2000-11, 000-fold lower than that of the wild type HuAChE; however, the G121A enzyme exhibited a rapid inhibition pattern, as opposed to the slow binding kinetics shown by the G122A enzyme. For both phosphates (diethyl phosphorofluoridate, diisopropyl phosphorofluoridate, and paraoxon) and phosphonates (sarin and soman), the decrease in inhibitory activity toward the G121A enzyme was very substantial (2000-6700-fold), irrespective of size of the alkoxy substituents on the phosphorus atom. On the other hand, for the G122A HuAChE the relative decline in reactivity toward phosphonates (500-460-fold) differed from that toward the phosphates (12-95-fold). Although formation of Michaelis complexes with substrates does not seem to involve significant interaction with the oxyanion hole, interactions with this motif are a major stabilizing element in accommodation of covalent inhibitors like organophosphates or carbamates. These observations and molecular modeling suggest that replacements of residues Gly-120 or Gly-121 by alanine alter the structure of the oxyanion hole motif, abolishing the H-bonding capacity of residue at position 121. These mutations weaken the interaction between HuAChE and the various ligands by 2.7-5.0 kcal/mol. In contrast, variations in reactivity due to replacement of residue Gly-122 seem to result from steric hindrance at the active center acyl pocket.

Purification, molecular characterization and catalytic properties of a Pseudomonas fluorescens enzyme having cholinesterase-like activity

An enzyme with a cholinesterase (ChE) activity, produced by Pseudomonas fluorescens, was purified to homogeneity in a three-step procedure. Analysis by non-denaturing and SDS-PAGE, and by isoelectric focusing, indicated that the enzyme was a monomer of 43 kDa, with a pI of 6.1. The N-terminal sequence, AEPLKAVGAGEGQLDIVAWPGYIEA, showed some similarities with proteins of the ChE family and a strong similarity with a protein from Escherichia coli with unknown structure and function. Cholinesterase activity at pH 7.0 and 25 degreesC was maximum with propionylthiocholine as substrate (kcat,app=670 min-1), followed by acetylthiocholine, and significantly lower with butyrylthiocholine. Catalytic specificity (kcat/Km) was the same for propionylthiocholine and acetylthiocholine, but was two orders of magnitude lower for butyrylthiocholine. Kinetics of thiocholine ester hydrolysis showed inhibition by excess substrate which was ascribed to binding of a second substrate molecule, leading to non-productive ternary complex (Km=35 microM, KSS=0.49 mM with propionylthiocholine). There was low or no reactivity with organophosphates and carbamates. The enzyme inhibited by echothiophate (kII=0.44x102 M-1 min-1) was not reactivated by pralidoxime methiodide. However, the P. fluorescens enzyme had affinity for procainamide and decamethonium, two reversible ChE inhibitors used as affinity chromatography ligand and eluant, respectively. Although similarity of the N-terminal amino acid sequence of the enzyme with an internal sequence of ChEs is weak, its catalytic activity towards thiocholine esters, and its affinity for positively charged ligands supports the contention that this enzyme may belong to the ChE family. However, we cannot rule out that the enzyme belongs to another structural family of proteins having cholinesterase-like properties. The reaction of the enzyme with organophosphates suggests that it is a serine esterase, and currently this enzyme may be termed as having a cholinesterase-like activity.

H2-M3 restricted presentation of a Listeria-derived leader peptide

Protective immunity to infection by many intracellular pathogens requires recognition by cytotoxic T lymphocytes (CTLs) of antigens presented on major histocompatibility complex (MHC) class I molecules. To be presented for recognition by pathogen-specific CTLs, these antigens must gain access to the host cell class I processing pathway. In the case of intracellular bacterial pathogens, the majority of bacterial proteins are retained within the bacterial membrane and therefore remain inaccessible to the host cell for antigen processing. We have isolated a CTL clone from a C57BL/6 mouse infected with the intracellular gram-positive bacterium Listeria monocytogenes (LM) and have identified the source of the antigen. Using a genomic expression library, we determined that the clone recognizes an antigenic N-formyl peptide presented by the nonpolymorphic murine MHC class Ib molecule, H2-M3. Several lengths of this peptide were able to sensitize cells for lysis by this CTL clone. The source of this antigenic peptide is a 23-amino acid polypeptide encoded at the start of a polycistronic region. Analysis of mRNA secondary structure of this region suggests that this polypeptide may be a leader peptide encoded by a transcriptional attenuator.

Identification of a novel type of alternatively spliced exon from the acetylcholinesterase gene of Bungarus fasciatus. Molecular forms of acetylcholinesterase in the snake liver and muscle

The venom of the snake Bungarus fasciatus contains a hydrophilic, monomeric species of acetylcholinesterase (AChE), characterized by a C-terminal region that does not resemble the alternative T- or H-peptides. Here, we show that the snake contains a single gene for AChE, possessing a novel alternative exon (S) that encodes the C-terminal region of the venom enzyme, located downstream of the T exon. Alternative splicing generates S mRNA in the venom gland and S and T mRNAs in muscle and liver. We found no evidence for the presence of an H exon between the last common "catalytic" exon and the T exon, where H exons are located in Torpedo and in mammals. Moreover, COS cells that were transfected with AChE expression vectors containing the T exon with or without the preceding genomic region produced exclusively AChET subunits. In the snake tissues, we could not detect any glycophosphatidylinositol-anchored AChE form that would have derived from H subunits. In the liver, the cholinesterase activity comprises both AChE and butyrylcholinesterase components; butyrylcholinesterase corresponds essentially to nonamphiphilic tetramers and AChE to nonamphiphilic monomers (G1na). In muscle, AChE is largely predominant: it consists of globular forms (G1a and G4a) and trace amounts of asymmetric forms (A8 and A12), which derive from AChET subunits. Thus, the Bungarus AChE gene possesses alternatively spliced T and S exons but no H exon; the absence of an H exon may be a common feature of AChE genes in reptiles and birds.

Acetylcholinesterase enhances neurite growth and synapse development through alternative contributions of its hydrolytic capacity, core protein, and variable C termini

Accumulated indirect evidence suggests nerve growth-promoting activities for acetylcholinesterase (AChE). To determine unequivocally whether such activities exist, whether they are related to the capacities of this enzyme to hydrolyze acetylcholine and enhance synapse development, and whether they are associated with alternative splicing variants of AChEmRNA, we used four recombinant human AChEDNA vectors. When Xenopus laevis embryos were injected with a vector expressing the synapse-characteristic human AChE-E6, which contains the exon 6-encoded C terminus, cultured spinal neurons expressing this enzyme grew threefold faster than co-cultured control neurons. Similar enhancement occurred in neurons expressing an insertion-inactivated human AChE-E6-IN protein, containing the same C terminus, and displaying indistinguishable immunochemical and electrophoretic migration properties from AChE-E6, but incapable of hydrolyzing acetylcholine. In contrast, the nonsynaptic secretory human AChE-I4, which contains the pseudointron 4-derived C terminus, did not affect neurite growth. Moreover, no growth promotion occurred in neurons expressing the catalytically active C-terminally truncated human AChE-E4, demonstrating a dominant role for the E6-derived C terminus in neurite extension. Also, AChE-E6 was the only active enzyme variant to be associated with Xenopus membranes. However, postsynaptic length measurements demonstrated that both AChE-E6 and AChE-E4 enhanced the development of neuromuscular junctions in vivo, unlike the catalytically inert AChE-E6-IN and the nonsynaptic AChE-I4. These findings demonstrate an evolutionarily conserved synaptogenic activity for AChE that depends on its hydrolytic capacity but not on its membrane association. Moreover, this synaptogenic effect differs from the growth-promoting activity of AChE, which is unrelated to its hydrolytic capacity yet depends on its exon 6-mediated membrane association.

Acetylcholinesterase enhances neurite growth and synapse development through alternative contributions of its hydrolytic capacity, core protein, and variable C termini

Accumulated indirect evidence suggests nerve growth-promoting activities for acetylcholinesterase (AChE). To determine unequivocally whether such activities exist, whether they are related to the capacities of this enzyme to hydrolyze acetylcholine and enhance synapse development, and whether they are associated with alternative splicing variants of AChEmRNA, we used four recombinant human AChEDNA vectors. When Xenopus laevis embryos were injected with a vector expressing the synapse-characteristic human AChE-E6, which contains the exon 6-encoded C terminus, cultured spinal neurons expressing this enzyme grew threefold faster than co-cultured control neurons. Similar enhancement occurred in neurons expressing an insertion-inactivated human AChE-E6-IN protein, containing the same C terminus, and displaying indistinguishable immunochemical and electrophoretic migration properties from AChE-E6, but incapable of hydrolyzing acetylcholine. In contrast, the nonsynaptic secretory human AChE-I4, which contains the pseudointron 4-derived C terminus, did not affect neurite growth. Moreover, no growth promotion occurred in neurons expressing the catalytically active C-terminally truncated human AChE-E4, demonstrating a dominant role for the E6-derived C terminus in neurite extension. Also, AChE-E6 was the only active enzyme variant to be associated with Xenopus membranes. However, postsynaptic length measurements demonstrated that both AChE-E6 and AChE-E4 enhanced the development of neuromuscular junctions in vivo, unlike the catalytically inert AChE-E6-IN and the nonsynaptic AChE-I4. These findings demonstrate an evolutionarily conserved synaptogenic activity for AChE that depends on its hydrolytic capacity but not on its membrane association. Moreover, this synaptogenic effect differs from the growth-promoting activity of AChE, which is unrelated to its hydrolytic capacity yet depends on its exon 6-mediated membrane association.

Differentiation intensifies the susceptibility of pheochromocytoma cells to antisense oligodeoxynucleotide-dependent suppression of acetylcholinesterase activity

To investigate the effect of neuronal differentiation on the capacity of antisense oligonucleotides (AS-ODNs) to suppress the production of acetylcholinesterase (AChE) in rat pheochromocytoma cells, we tested seven 3'-phosphorothioated AS-ODNs targeted to ACHEmRNA and two control ODNs. Three different administration protocols were used: oligonucleotides were added at 1 microM for 24 hours to nondifferentiated PC12 cells, together with nerve growth factor (NGF) or 24 hours following NGF-induced cholinergic differentiation. The content of free thiol groups in lysed cells was measured to evaluate cell number, therefore, survival, and the rate of acetylthiocholine hydrolysis was the measure of AChE activity. Among nondifferentiated cells, over 95% survived treatment with 8 of 9 of the ODNs. Moreover, two AS-ODN suppressed AChE activity in non-differentiated PC12 cells by 16%-20% as compared with 10% suppression by control ODNs (P < or = 0.01). When added concurrently with NGF, one other AS-ODN suppressed AChE activity significantly better (28%) than the control ODNs (16%). Moreover, when added following NGF treatment, which induced a significant increase in AChE activity, four different AS-ODNs but not the control ODNs suppressed 20%-35% of the enhanced AChE activity (p < or = 0.01). Reduced levels of AChE mRNA but no difference in actin mRNA levels were observed by following the kinetics of RT-PCR amplification in differentiated PC12 cells treated with these four AS-ODNs, as compared with control cells. Our findings demonstrate a differentiation-related increase in the susceptibility of PC12 cells to inhibition by specific AS-ODNs, suggesting the use of this model system to select AS-ODNs for suppression of AChE levels in the treatment of neurodegenerative diseases associated with cholinergic malfunction.

Normal and atypical butyrylcholinesterases in placental development, function, and malfunction

1. In utero exposure to poisons and drugs (e.g., anticholinesterases, cocaine) is frequently associated with spontaneous absorption and placental malfunction. The major protein interacting with these compounds is butyrylcholinesterase (BuChE), which attenuates the effects of such xenobiotics by their hydrolysis or sequestration. Therefore, we studied BuChE expression during placental development. 2. RT-PCR revealed both BuChEmRNA and acetylcholinesterase (AChE) mRNA throughout gestation. However, cytochemical staining detected primarily BuChE activity in first-trimester placenta but AChE activity in term placenta. 3. As the atypical variant of BuChE has a narrower specificity for substrates and inhibitors than the normal enzyme, we investigated its interactions with alpha-solanine and cocaine, and sought a correlation between the occurrence of this variant and placental malfunction. 4. Atypical BuChE of serum or recombinant origin presented > 10-fold weaker affinities than normal BuChE for cocaine and alpha-solanine. However, BuChE in the serum of the heterozygote and a homozygous normal were similar in their drug affinities. Therefore, heterozygous serum or placenta can protect the fetus from drug or poison exposure, unlike homozygous atypical serum or placenta. 5. Genotype analyses revealed that heterozygous carriers of atypical BuChE were threefold less frequent among 49 patients with placental malfunction than among 76 controls of the entire Israeli population. These observations exclude heterozygote carriers of atypical BuChE from being at high risk for placental malfunction under exposure to anticholinesterases.

Immature Human Megakaryocytes Produce Nuclear-Associated Acetylcholinesterase

Acetylcholinesterase (AChE) is expressed in murine megakaryocytes (MK), where its antisense inhibition suppresses differentiation, yet was never detected in human MK. Here, we report that AChE is produced in normal human bone marrow MK and in cell lines derived thereof. Reverse transcriptase-polymerase chain reaction (RT-PCR) amplification showed two ACHEmRNA forms in human megakaryoblastic DAMI cells. In situ hybridization demonstrated ACHEmRNA surrounding the nucleus of small DAMI cells and the nuclear lobes of large, polyploid cells. Differentiation induction with phorbol ester and exposure to recombinant human thrombopoietin suppressed both ACHEmRNA and AChE activity. The residual AChE in mature differentiated cells acquired higher stability and detergent-sensitivity as compared with AChE in small proliferating cells. AChE activity was primarily associated with nuclei of both DAMI cells and small (10 μm) primary proliferating human bone marrow MK identified with GPIIb/IIIa antibodies. This activity was significantly reduced in medium size MK (10 to 25 μm) and was almost undetectable in large MK (<25 μm), yet was twofold more abundant in some large MK from idiopathic thrombocytopenia purpura (ITP) patients with accelerated MK maturation. The loss of AChE activity at the transition from proliferating to differentiating MK highlights species-specific differences in its expression, suggesting a distinct role for AChE in human MK development.

Immature Human Megakaryocytes Produce Nuclear-Associated Acetylcholinesterase

Acetylcholinesterase (AChE) is expressed in murine megakaryocytes (MK), where its antisense inhibition suppresses differentiation, yet was never detected in human MK. Here, we report that AChE is produced in normal human bone marrow MK and in cell lines derived thereof. Reverse transcriptase-polymerase chain reaction (RT-PCR) amplification showed two ACHEmRNA forms in human megakaryoblastic DAMI cells. In situ hybridization demonstrated ACHEmRNA surrounding the nucleus of small DAMI cells and the nuclear lobes of large, polyploid cells. Differentiation induction with phorbol ester and exposure to recombinant human thrombopoietin suppressed both ACHEmRNA and AChE activity. The residual AChE in mature differentiated cells acquired higher stability and detergent-sensitivity as compared with AChE in small proliferating cells. AChE activity was primarily associated with nuclei of both DAMI cells and small (10 μm) primary proliferating human bone marrow MK identified with GPIIb/IIIa antibodies. This activity was significantly reduced in medium size MK (10 to 25 μm) and was almost undetectable in large MK (<25 μm), yet was twofold more abundant in some large MK from idiopathic thrombocytopenia purpura (ITP) patients with accelerated MK maturation. The loss of AChE activity at the transition from proliferating to differentiating MK highlights species-specific differences in its expression, suggesting a distinct role for AChE in human MK development.

Expression of the cholinergic signal-transduction pathway components during embryonic rat heart development

BACKGROUND

Previous studies showed that acetylcholinesterase (AChE) activity is present in the downstream (arterial) part of the embryonic chick and rat heart, but its functional significance was unclear. To establish whether other components of a cholinergic signal-transduction pathway are present in the embryonic heart, we localised the mRNAs encoding choline acetyltransferase (ChAT), acetylcholinesterase (AChE), and the muscarinic receptor isoforms (mAChRs; m1-m5).

METHODS

Messenger RNA detection and localisation by in situ hybridisation and reverse transcriptase-polymerase chain reaction were employed.

RESULTS

Expression of ChAT and AChE mRNAs was observed from 15 embryonic days onward in the neural tissue covering the dorsocranial wall of the atria. Muscarinic receptors (m1, m2, m4) were observed at the same localisation as AChE and ChAT mRNAs, both during embryogenesis and after birth. In addition, m1 and m4 mAChRs showed a low level of expression in the atrial myocardium during the fetal period. No expression of the m3 or the m5 mAChRs was observed in or near the embryonic hearts. ChAT, AChE, and mAChRs (m1, m2, m4) mRNAs always colocalised in the cardiac ganglia. However, none of these mRNAs was found at a detectable level in the outflow tract and/or the ventricular trabeculations.

CONCLUSIONS

The AChE activity in the arterial part of the embryonic heart is probably synthesised elsewhere and subserves a function different from the hydrolysis of locally produced acetylcholine.

Definitive evidence for the acute sarin poisoning diagnosis in the Tokyo subway

A new method was developed to detect sarin hydrolysis products from erythrocytes of four victims of sarin (isopropylmethylphosphonofluoridate) poisoning resulting from the terrorist attack on the Tokyo subway. Sarin-bound acetylcholinesterase (AChE) was solubilized from erythrocyte membranes of sarin victims, digested with trypsin, the sarin hydrolysis products bound to AChE were released by alkaline phosphatase digestion, and the digested sarin hydrolysis products were subjected to trimethylsilyl derivatization and detected by gas chromatography-mass spectrometry. Isopropylmethylphosphonic acid, which is a sarin hydrolysis product, was detected in all sarin poisoning, victims we examined and methylphosphonic acid, which is a sarin and soman hydrolysis product, was determined in all victims. Postmortem examinations revealed no macroscopic and microscopic findings specific to sarin poisoning and sarin and its hydrolysis products were almost undetectable in their blood. We think that the procedure described below will be useful for the forensic diagnosis of acute sarin poisoning.

A novel fusidic acid resistance gene from Streptomyces lividans 66 encodes a highly specific esterase

Resistance to fusidic acid in Streptomyces lividans is due to secretion of an extracellular enzyme (FusH) that converts the steroid antibiotic into an inactive derivative. NH2-terminal and several internal amino acid sequences were prepared from the purified enzyme. Using one of the deduced oligonucleotides to probe a subgenomic DNA library, the fusH gene was cloned and sequenced. Sequence analysis located an ORF which, owing to the presence of two putative start codons, indicates a predicted protein with 520 or 509 amino acids. A signal peptide was identified by aligning the deduced amino acids with the N-terminal sequence determined for the mature enzyme. The C-terminal part of the deduced FusH contains a region of three tandemly repeated stretches of 50 amino acids, which is preceded and followed by amino acids showing high homology with the repeats. FusH was found to share a GDS motif with some deduced esterases. S. lividans transformants carrying fusH on a multicopy vector synthesized high levels of FusH. Purified FusH cleaved equally well an acetyl, a thioacetyl or a formyl group from the 16 beta-position of fusidic acid and its derivatives. However, a propionyl group at the 16 beta-position was attacked with difficulty and a 16 alpha-acetyl group was not hydrolysed at all. These data indicate that FusH is a highly specific esterase. The fusH gene is widely distributed among streptomycetes that modify fusidic acid to its inactive lactone derivative.

Two cholinesterase activities and genes are present in amphioxus

To obtain information about the evolution of the cholinesterases, acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) in the vertebrates, we investigated the cholinesterase (ChE) activity of the cephalochordate amphioxus (Branchiostoma floridae and Branchiostoma lanceolatum). On the basis of evidence from enzymology, pharmacology, and molecular biology, we conclude that amphioxus possesses two ChE activities and two ChE genes. Two covalent inhibitors of cholinesterases were able to pharmacologically isolate the two activities as drug-sensitive ChE and drug-resistant ChE. Kinetically, in terms of substrate specificity, the drug-sensitive ChE resembles vertebrate AChE, and the drug-resistant ChE resembles the BuChE of cartilaginous and bony fish or the intermediate ChE of protostome invertebrates. We also used the polymerase chain reaction with degenerate oligonucleotide primers and genomic DNA to obtain clones of 1,574 and 1,011 bp corresponding to two cholinesterase genes from amphioxus, which we designated as ChE1 and ChE2. ChE2 codes for an enzyme with an acyl-binding pocket sequence, a portion of the protein that plays an important role in determining substrate specificity, typical of invertebrate ChE. ChE1, which contains a 503-bp intron, encodes a protein with a novel acyl binding site. Phylogenetic analysis of the sequences suggests that the two genes are a result of a duplication event in the lineage leading to amphioxus. We discuss the relevance of our results to the evolution of the cholinesterases in the chordates. Previously, we reported that amphioxus contained a single cholinesterase activity with properties intermediate to AChE and BuChE (Pezzementi et al. [1991] In: Cholinesterases: Structure, Function, Mechanism, Genetics and Cell Biology. J. Massoulié et al., eds. ACS: Washington, D.C., pp. 24-31).

Structural modifications of the omega loop in human acetylcholinesterase

Conformational mobility of the surface omega loop (Cys-69-Cys-96) in human acetylcholinesterase (HuAChE) was recently implicated in substrate accessibility to the active center and in the mechanism of allosteric modulation of enzymatic activity. We therefore generated and kinetically evaluated the following modifications or replacements in HuAChE: (a) residues at the loop ends, (b) residues involved in putative hydrogen-bond interactions within the loop and between the loop and the protein core, (c) ChEs conserved proline residues within the loop and (d) a deletion of a conserved segment of 5 residues. All the residue replacements, including those of the prolines, had either limited or no effect on enzyme reactivity. These results suggest that unlike the case of lipase, the omega loop in the HuAChE is not involved in large lid-like displacements. In cases where modifications of the loop sequence had some effect on reactivity, the effects could be attributed to an altered position of residue Trp-86 supporting the proposed coupling between the structure of the omega loop and the positioning of the Trp-86 indole moiety, in catalytic activity and in allosterism.

Characterization of 12 silent alleles of the human butyrylcholinesterase (BCHE) gene

The silent phenotype of human butyrylcholinesterase (BChE), present in most human populations in frequencies of approximately 1/100,000, is characterized by the complete absence of BChE activity or by activity <10% of the average levels of the usual phenotype. Heterogeneity in this phenotype has been well established at the phenotypic level, but only a few silent BCHE alleles have been characterized at the DNA level. Twelve silent alleles of the human butyrylcholinesterase gene (BCHE) have been identified in 17 apparently unrelated patients who were selected by their increased sensitivity to the muscle relaxant succinylcholine. All of these alleles are characterized by single nucleotide substitutions or deletions leading to distinct changes in the structure of the BChE enzyme molecule. Nine of the nucleotide substitutions result in the replacement of single amino acid residues. Three of these variants, BCHE*33C, BCHE*198G, and BCHE*201T, produce normal amounts of immunoreactive but enzymatically inactive BChE protein in the plasma. The other six amino acid substitutions, encoded by BCHE*37S, BCHE*125F, BCHE*170E, BCHE*471R, and BCHE*518L, seem to cause reduced expression of BChE protein, and their role in determining the silent phenotype was confirmed by expression in cell culture. The other four silent alleles, BCHE*271STOP, BCHE*500STOP, BCHE*FS6, and BCHE*I2E3-8G, encode BChES truncated at their C-terminus because of premature stop codons caused by nucleotide substitutions, a frame shift, or altered splicing. The large number of different silent BCHE alleles found within a relatively small number of patients shows that the heterogeneity of the silent BChE phenotype is high. The characterization of silent BChE variants will be useful in the study of the structure/function relationship for this and other closely related enzymes.

Involvement of oligomerization, N-glycosylation and sialylation in the clearance of cholinesterases from the circulation

The possible role of post-translational modifications such as subunit oligomerization, protein glycosylation and oligosaccharide processing on the circulatory life-time of proteins was studied using recombinant human acetylcholinesterase (rHuAChE). Different preparations of rHuAChE containing various amounts of tetramers, dimers and monomers are cleared at similar rates from the circulation, suggesting that oligomerization does not play an important role in determining the rate of clearance. An engineered rHuAChE mutant containing only one N-glycosylation site was cleared from the circulation more rapidly than the wild-type triglycosylated enzyme. On the other hand, hyperglycosylated mutants containing either four or five occupied N-glycosylation sites, analagous to those present on the slowly cleared fetal bovine serum acetylcholinesterase (FBS-AChE), were also cleared more rapidly from the bloodstream than the wild-type species. Furthermore, the two different tetraglycosylated mutants were cleared at different rates while the pentaglycosylated mutant exhibited the most rapid clearance profile. These results imply that though the number of N-glycosylation sites plays a role in the circulatory life-time of the enzyme, the number of N-glycan units in itself does not determine the rate of clearance. When saturating amounts of asialofetuin were administered together with rHuAChE, the circulatory half-life of the enzyme was dramatically increased (from 80 min to 19 h) and was found to be similar to that displayed by plasma-derived cholinesterases while desialylation of these enzymes caused a sharp decrease in the circulatory half-life to approximately 3-5 min. Determination of the average number of sialic acid residues per enzyme subunit of the five different N-glycosylation species generated, revealed that the rate of clearance is not a function of the absolute number of appended sialic acid moieties but rather of the number of unoccupied sialic acid attachment sites per enzyme molecule. Specifically, we demonstrate an inverse-linear relationship between the number of vacant sialic acid attachment sites and the values of the enzyme residence time within the bloodstream.

Transcription factor repression and activation of the human acetylcholinesterase gene

Acetylcholinesterase in man is encoded by a single gene, ACHE, located on chromosome 7q22. In this study, the transcription start sites and major DNA promoter elements controlling the expression of this gene have been characterized by structural and functional studies. Immediately upstream of the first untranslated exon of the gene are GC-rich sequences containing consensus binding sites for several transcription factors, including Sp1, EGR-1 and AP2. In vitro transcription studies and RNase protection analyses of mRNA isolated from human NT2/D1 teratocarcinoma cells reveal that two closely spaced transcription cap sites are located at a consensus initiator (Inr) element similar to that found in the terminal transferase gene. Transient transfection of mutant genes shows that removal of three bases of this initiator sequence reduces promoter activity by 98% in NT2/D1 cells. In vitro transcription studies and transient transfection of a series of 5' deletion mutants of the ACHE promoter linked to a luciferase reporter show an Sp1 site at -71 to be essential for promoter activity. Purified Sp1 protein protects this site from DNase cleavage during in vitro footprinting experiments. A conserved AP2 consensus binding site, located between the GC box elements and the Inr, is protected by recombinant AP2 protein in DNase footprinting experiments, induces a mobility shift with AP2 protein and AP2-containing cell extracts, and fosters inhibition of transcription by AP2 as measured by transient transfection in mouse and human cell lines and in in vitro transcription reactions. These results indicate that AP2 functions as a repressor of human ACHE and mouse Ache transcription.

Synaptic and epidermal accumulations of human acetylcholinesterase are encoded by alternative 3'-terminal exons

Tissue-specific heterogeneity among mammalian acetylcholinesterases (AChE) has been associated with 3' alternative splicing of the primary AChE gene transcript. We have previously demonstrated that human AChE DNA encoding the brain and muscle AChE form and bearing the 3' exon E6 (ACHE-E6) induces accumulation of catalytically active AChE in myotomes and neuromuscular junctions (NMJs) of 2- and 3-day-old Xenopus embryos. Here, we explore the possibility that the 3'-terminal exons of two alternative human AChE cDNA constructs include evolutionarily conserved tissue-recognizable elements. To this end, DNAs encoding alternative human AChE mRNAs were microinjected into cleaving embryos of Xenopus laevis. In contrast to the myotomal expression demonstrated by ACHE-E6, DNA carrying intron 14 and alternative exon E5 (ACHE-I4/E5) promoted punctuated staining of epidermal cells and secretion of AChE into the external medium. Moreover, ACHE-E6-injected embryos displayed enhanced NMJ development, whereas ACHE-I4/E5-derived enzyme was conspicuously absent from muscles and NMJs and its expression in embryos had no apparent effect on NMJ development. In addition, cell-associated AChE from embryos injected with ACHE-I4/E5 DNA was biochemically distinct from that encoded by the muscle-expressible ACHE-E6, displaying higher electrophoretic mobility and greater solubility in low-salt buffer. These findings suggest that alternative 3'-terminal exons dictate tissue-specific accumulation and a particular biological role(s) of AChE, associate the 3' exon E6 with NMJ development, and indicate the existence of a putative secretory AChE form derived from the alternative I4/E5 AChE mRNA.

Contribution of aromatic moieties of tyrosine 133 and of the anionic subsite tryptophan 86 to catalytic efficiency and allosteric modulation of acetylcholinesterase

Substitution of Trp-86, in the active center of human acetylcholinesterase (HuAChE), by aliphatic but not by aromatic residues resulted in a several thousandfold decrease in reactivity toward charged substrate and inhibitors but only a severalfold decrease for noncharged substrate and inhibitors. The W86A and W86E HuAChE enzymes exhibit at least a 100-fold increase in the Michaelis-Menten constant or 100-10,000-fold increase in inhibition constants toward various charged inhibitors, as compared to W86F HuAChE or the wild type enzyme. On the other hand, replacement of Glu-202, the only acidic residue proximal to the catalytic site, by glutamine resulted in a nonselective decrease in reactivity toward charged and noncharged substrates or inhibitors. Thus, the quaternary nitrogen groups of substrates and other active center ligands, are stabilized by cation-aromatic interaction with Trp-86 rather than by ionic interactions, while noncharged ligands appear to bind to distinct site(s) in HuAChE. Analysis of the Y133F and Y133A HuAChE mutated enzymes suggests that the highly conserved Tyr-133 plays a dual role in the active center: (a) its hydroxyl appears to maintain the functional orientation of Glu-202 by hydrogen bonding and (b) its aromatic moiety maintains the functional orientation of the anionic subsite Trp-86. In the absence of aromatic interactions between Tyr-133 and Trp-86, the tryptophan acquires a conformation that obstructs the active site leading, in the Y133A enzyme, to several hundredfold decrease in rates of catalysis, phosphorylation, or in affinity to reversible active site inhibitors. It is proposed that allosteric modulation of acetylcholinesterase activity, induced by binding to the peripheral anionic sites, proceeds through such conformational change of Trp-86 from a functional anionic subsite state to one that restricts access of substrates to the active center.

Endogenous butyrylcholinesterase in SV40 transformed cell lines: COS-1, COS-7, MRC-5 SV40, and WI-38 VA13

Comparison of proteins expressed by SV40 transformed cell lines and untransformed cell lines is of interest because SV40 transformed cells are immortal, whereas untransformed cells senesce after about 50 doublings. In MRC-5 SV40 cells, only seven proteins have previously been reported to shift from undetectable to detectable after transformation by SV40 virus. We report that butyrylcholinesterase is an 8th protein in this category. Butyrylcholinesterase activity in transformed MRC-5 SV40 cells increased at least 150-fold over its undetectable level in MRC-5 parental cells. Other SV40 transformed cell lines, including COS-1, COS-7, and WI-38 VA13, also expressed endogenous butyrylcholinesterase, whereas the parental, untransformed cell lines, CV-1 and WI-38, had no detectable butyrylcholinesterase activity or mRNA. Infection of CV-1 cells by SV40 virus did not result in expression of butyrylcholinesterase, showing that the butyrylcholinesterase promoter was not activated by the large T antigen of SV40. We conclude that butyrylcholinesterase expression resulted from events related to cell immortalization and did not result from activation by the large T antigen.

Acetylcholinesterase and butyrylcholinesterase expression in adult rabbit tissues and during development

A large cDNA fragment covering the complete sequence of the mature catalytic subunit of rabbit acetylcholinesterase (AChE) has been cloned and sequenced. This sequence was compared to that of rabbit butyrylcholinesterase [BChE; Jbilo, O. & Chatonnet, A. (1990) Nucleic Acids Res. 18, 3990]. Amino acid sequences of AChE and BChE have 51% identity. They both possessed a choline-binding site W84, a catalytic triad S200-H440-E327 and six cysteine residues (positions 67-94, 254-265, 402-521) in conserved sequence positions to those that form three intrachain disulfide bonds in all cholinesterases (by convention, numbering of amino acids is that used for Torpedo AChE). Rabbit AChE had a larger number of aromatic residues lining the active-site gorge than rabbit BChE (14 compared to 8, respectively) and a smaller number of potential N-glycosylation sites (3 compared to 8, respectively). Both catalytic subunits have a hydrophilic C-terminus (catalytic subunits of type T). Expression of acetylcholinesterase and butyrylcholinesterase genes (ACHE and BCHE) was studied in rabbit tissues and during development by a correlation of Northern-blot analysis and enzymic activities. This correlation was rendered difficult by the presence of an eserine-resistant esterase active on butyrylthiocholine in serum, liver and lung. When the contribution of this carboxylesterase was taken into account, brain was found as the richest source of BChE followed by lung and heart. Rabbit liver had a very low content of BChE that correlated with the low BChE activity in plasma. During development, BCHE transcripts were detected as early as day 10 post coitum, whereas ACHE transcripts appeared only on day 12.

Antisense oligonucleotide inhibition of acetylcholinesterase gene expression induces progenitor cell expansion and suppresses hematopoietic apoptosis ex vivo

To examine the role of acetylcholinesterase (EC 3.1.1.7) in hematopoietic cell proliferation and differentiation, we administered a 15-mer phosphorothioate oligonucleotide, antisense to the corresponding ACHE gene (AS-ACHE), to primary mouse bone marrow cultures. Within 2 hr of AS-ACHE addition to the culture, ACHE mRNA levels dropped by approximately 90%, as compared with those in cells treated with the "sense" oligomer, S-ACHE. Four days after AS-ACHE treatment, ACHE mRNA increased to levels 10-fold higher than in S-ACHE cultures or in fresh bone marrow. At this later time point, differential PCR display revealed significant differences between cellular mRNA transcripts in bone marrow and those in AS-ACHE- or S-ACHE-treated cultures. These oligonucleotide-triggered effects underlay considerable alterations at the cellular level: AS-ACHE but not S-ACHE increased cell counts, reflecting enhanced proliferation. In the presence of erythropoietin it also enhanced colony counts, reflecting expansion of progenitors. AS-ACHE further suppressed apoptosis-related fragmentation of cellular DNA in the progeny cells, and it diverted hematopoiesis toward production of primitive blasts and macrophages in a dose-dependent manner promoted by erythropoietin. These findings suggest that the hematopoietic role of acetylcholinesterase, anticipated to be inverse to the observed antisense effects, is to reduce proliferation of the multipotent stem cells committed to erythropoiesis and megakaryocytopoiesis and macrophage production and to promote apoptosis in their progeny. Moreover, these findings may explain the tumorigenic association of perturbations in ACHE gene expression with leukemia.

Evolution of acetylcholinesterase transcripts and molecular forms during development in the central nervous system of the quail

We studied the expression of acetylcholinesterase (AChE) in the nervous system (cerebellum, optic lobes and neuroretina) of the quail at different stages of development, from embryonic day 10 (E10) to the adult. Analyzing AChE mRNAs and AChE molecular forms, we observed variations in the following: (a) production of multiple mRNA species (4.5 kb, 5.3 kb, and 6 kb); (b) translation and/or stability of the AChE protein; (c) production of active and inactive AChE molecules; (d) production of amphiphilic and nonamphiphilic AChE forms; and (e) proportions of tetrameric G4, dimeric G2, and monomeric G1 forms. The large transcripts present distinct temporal patterns and disappear in the adult, which possesses only the 4.5-kb mRNA; these changes are unlikely to be related to those observed for the AChE protein, because all transcripts seem to encode the same catalytic subunit (type T). In addition, the levels of mRNA and AChE are not correlated in the three regions, especially at the adult stage. The proportion of inactive AChE was found to be markedly higher at the hatching period (E16) than at earlier stages (E10 and E13) or in the adult. The G4 form is predominant already at E10, and in the adult its proportion reaches 80% of the activity in the cerebellum and optic lobes, and 65-70% in the neuroretina. This form is largely nonamphiphilic in embryonic tissues, but it becomes progressively more amphiphilic with development. Thus, the different processing and maturation steps appear to be regulated in an independent manner and potentially correspond to physiologically adaptative mechanisms.

Overexpressed monomeric human acetylcholinesterase induces subtle ultrastructural modifications in developing neuromuscular junctions of Xenopus laevis embryos

Formation of a functional neuromuscular junction (NMJ) involves the biosynthesis and transport of numerous muscle-specific proteins, among them the acetylcholine-hydrolyzing enzyme acetylcholinesterase (AChE). To study the mechanisms underlying this process, we have expressed DNA encoding human AChE downstream of the cytomegalovirus promoter in oocytes and developing embryos of Xenopus laevis. Recombinant human AChE (rHAChE) produced in Xenopus was biochemically and immunochemically indistinguishable from native human AChE but clearly distinguished from the endogenous frog enzyme. In microinjected embryos, high levels of catalytically active rHAChE induced a transient state of over-expression that persisted for at least 4 days postfertilization. rHAChE appeared exclusively as nonassembled monomers in embryos at times when endogenous Xenopus AChE displayed complex oligomeric assembly. Nonetheless, cell-associated rHAChE accumulated in myotomes of 2- and 3-day-old embryos within the same subcellular compartments as native Xenopus AChE. NMJs from 3-day-old DNA-injected embryos displayed fourfold or greater overexpression of AChE, a 30% increase in postsynaptic membrane length, and increased folding of the postsynaptic membrane. These findings indicate that an evolutionarily conserved property directs the intracellular trafficking and synaptic targeting of AChE in muscle and support a role for AChE in vertebrate synaptogenesis.