The monoclonal antibody 2G8 is carbohydrate-specific and distinguishes between different forms of vertebrate cholinesterases

Different sensitivities of rat skeletal muscles and brain to novel anti-cholinesterase agents, alkylammonium derivatives of 6-methyluracil (ADEMS)

BACKGROUND AND PURPOSE

The rat respiratory muscle diaphragm has markedly lower sensitivity than the locomotor muscle extensor digitorum longus (EDL) to the new acetylcholinesterase (AChE) inhibitors, alkylammonium derivatives of 6-methyluracil (ADEMS). This study evaluated several possible reasons for differing sensitivity between the diaphragm and limb muscles and between the muscles and the brain.

EXPERIMENTAL APPROACH

Increased amplitude and prolonged decay time of miniature endplate currents were used to assess anti-cholinesterase activity in muscles. In hippocampal slices, induction of synchronous network activity was used to follow cholinesterase inhibition. The inhibitor sensitivities of purified AChE from the EDL and brain were also estimated.

KEY RESULTS

The intermuscular difference in sensitivity to ADEMS is partly explained caused by a higher level of mRNA and activity of 1,3-bis[5(diethyl-o-nitrobenzylammonium)pentyl]-6-methyluracildibromide (C-547)-resistant BuChE in the diaphragm. Moreover, diaphragm AChE was more than 20 times less sensitive to C-547 than that from the EDL. Sensitivity of the EDL to C-547 dramatically decreased after treadmill exercises that increased the amount of PRiMA AChE(G4), but not ColQ AChE(A12) molecular forms. The A12 form present in muscles appeared more sensitive to C-547. The main form of AChE in brain, PRiMA AChE(G4), was apparently less sensitive because brain cholinesterase activity was almost three orders of magnitude more resistant to C-547 than that of the EDL.

CONCLUSIONS AND IMPLICATIONS

Our findings suggest that ADEMS compounds could be used for the selective inhibition of AChEs and as potential therapeutic tools.

The assembly of proline-rich membrane anchor (PRiMA)-linked acetylcholinesterase enzyme: glycosylation is required for enzymatic activity but not for oligomerization

Acetylcholinesterase (AChE) anchors onto cell membranes by a transmembrane protein PRiMA (proline-rich membrane anchor) as a tetrameric form in vertebrate brain. The assembly of AChE tetramer with PRiMA requires the C-terminal "t-peptide" in AChE catalytic subunit (AChE(T)). Although mature AChE is well known N-glycosylated, the role of glycosylation in forming the physiologically active PRiMA-linked AChE tetramer has not been studied. Here, several lines of evidence indicate that the N-linked glycosylation of AChE(T) plays a major role for acquisition of AChE full enzymatic activity but does not affect its oligomerization. The expression of the AChE(T) mutant, in which all N-glycosylation sites were deleted, together with PRiMA in HEK293T cells produced a glycan-depleted PRiMA-linked AChE tetramer but with a much higher K(m) value as compared with the wild type. This glycan-depleted enzyme was assembled in endoplasmic reticulum but was not transported to Golgi apparatus or plasma membrane.

A novel butyrylcholinesterase from serum of Leporinus macrocephalus, a Neotropical fish

We show here that serum of piaussu, a Neotropical characin fish, has the highest butyrylcholinesterase activity so far described for humans and fish. To clarify whether this cholinesterase could protect piaussu against anticholinesterase pesticides by scavenging organophosphates, we purified it 1700-fold, with a yield of 80%. Augmenting concentrations (from 0.01 to 20 mM) of butyrylthiocholine activated it. The pure enzyme was highly inhibited by chlorpyriphos-oxon (ki=10,434x10(6) M-1 min-1) and by the specific butyrylcholinesterase inhibitor, isoOMPA (ki=45.7x10(6) M-1 min-1). Electrophoresis of total serum and 2-D electrophoresis of the purified cholinesterase showed that some enzyme molecules could circulate in piaussu serum as heterogeneously glycosylated dimers. The enzyme's N-terminal sequence was similar to sequences found for butyrylcholinesterase from sera of other vertebrates. Altogether, our data present a novel butyrylcholinesterase with the potential of protecting a fish from poisoning by organophosphates.

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.

Distribution of choline acetyltransferase (ChAT) immunoreactivity in the brain of the adult trout and tract-tracing observations on the connections of the nuclei of the isthmus

The distribution of cholinergic neurons and fibers was studied in the brain and rostral spinal cord of the brown trout and rainbow trout by using an antiserum against the enzyme choline acetyltransferase (ChAT). Cholinergic neurons were observed in the ventral telencephalon, preoptic region, habenula, thalamus, hypothalamus, magnocellular superficial pretectal nucleus, optic tectum, isthmus, cranial nerve motor nuclei, and spinal cord. In addition, new cholinergic groups were detected in the vascular organ of the lamina terminalis, the parvocellular and magnocellular parts of the preoptic nucleus, the anterior tuberal nucleus, and a mesencephalic tegmental nucleus. The presence of ChAT in the magnocellular neurosecretory system of trout suggests that acetylcholine is involved in control of hormone release by neurosecretory terminals. In order to characterize the several cholinergic nuclei observed in the isthmus of trout, their projections were studied by application of 1,1;-dioctadecyl-3,3,3;, 3;-tetramethylindocarbocyanine perchlorate (DiI) to selected structures of the brain. The secondary gustatory nucleus projected mainly to the lateral hypothalamic lobes, whereas the nucleus isthmi projected to the optic tectum and parvocellular superficial pretectal nucleus, as previously described in other teleost groups. In addition, other isthmic cholinergic nuclei of trout may be homologs of the mesopontine system of mammals. We conclude that the cholinergic systems of teleosts show many primitive features that have been preserved during evolution, together with characteristics exclusive to the group.

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.

Structure of glycan moieties responsible for the extended circulatory life time of fetal bovine serum acetylcholinesterase and equine serum butyrylcholinesterase

Cholinesterases are serine hydrolases that can potentially be used as pretreatment drugs for organophosphate toxicity, as drugs to alleviate succinylcholine-induced apnea, and as detoxification agents for environmental toxins such as heroin and cocaine. The successful application of serum-derived cholinesterases as bioscavengers stems from their relatively long residence time in the circulation. To better understand the relationship between carbohydrate structure and the stability of cholinesterases in circulation, we determined the monosaccharide composition, the distribution of various oligosaccharides, and the structure of the major asparagine-linked oligosaccharides units present in fetal bovine serum acetylcholinesterase and equine serum butyrylcholinesterase. Our findings indicate that 70-80% of the oligosaccharides in both enzymes are negatively charged. This finding together with the molar ratio of galactose to sialic acid clearly suggests that the beta-galactose residues are only partially capped with sialic acid, yet they displayed a long duration in circulation. The structures of the two major oligosaccharides from fetal bovine serum acetylcholinesterase and one major oligosaccharide from equine serum butyrylcholinesterase were determined. The three carbohydrate structures were of the biantennary complex type, but only the ones from fetal bovine serum acetylcholinesterase were fucosylated on the innermost N-acetylglucosamine residue of the core. Pharmacokinetic studies with native, desialylated, and deglycosylated forms of both enzymes indicate that the microheterogeneity in carbohydrate structure may be responsible, in part, for the multiphasic clearance of cholinesterases from the circulation of mice.

Characterization of monoclonal antibodies that strongly inhibit Electrophorus electricus acetylcholinesterase

In this study, we describe three different monoclonal antibodies (mAbs Elec-403, Elec-408, and Elec-410) directed against Electrophorus electricus acetylcholinesterase (AChE) which were selected as inhibitors for this enzyme. Two of these antibodies (Elec-403 and Elec-410), recognized overlapping but different epitopes, competed with snake venom toxin fasciculin for binding to the enzyme, and thus apparently recognized the peripheral site of AChE. In addition, the binding of Elec-403 was antagonized by 1,5-bis(4-allyldimethylammoniumphenyl)pentan-3-one dibromide (BW284C51) and propidium, indicating that the corresponding epitope encompassed the anionic site involved in the binding of these low-molecular-mass inhibitors. The third mAb (Elec-408), was clearly bound to another site on the AChE molecule, and its inhibitory effect was cumulative with those of Elec-403, Elec-410, and fasciculin. All mAbs bound AChE with high affinity and were as strong inhibitors with an apparent Ki values less than 0.1 nM. Elec-403 was particularly efficient with an inhibitory activity similar to that of fasciculin. Inhibition was observed with both charged (acetylthiocholine) and neutral substrates (o-nitrophenyl acetate) and had the characteristics of a non-competitive process. Elec-403 and Elec-410 probably exert their effect by triggering allosteric transitions from the peripheral site to the active site. The epitope recognized by mAb Elec-408 has not been localized, but it may correspond to a new regulatory site on AChE.

Production and characterization of separate monoclonal antibodies to human brain and erythrocyte acetylcholinesterases

Four murine monoclonal antibodies (MAbs) of the IgM class were raised against human acetylcholinesterase (AChE; Ec 3.1.1.7). The MAbs BMS-3E4, BMS-7G10, and BMS-9F4 all recognized human erythrocyte AChE, while BMS-6D6 bound specifically to human soluble brain AChE, on the basis of immunobinding assays. Dose-response studies gave an ELISA ED50 titer of 4.5 x 10(-4) M for BMS-6D6, while BMS-3E4 gave the best titer at 8.8 x 10(-4) M. Sucrose density gradients demonstrated sedimentation of antigen-antibody complexes, consistent with earlier findings (i.e., BMS-6D6 bound with brain AChE while BMS-3E4 preferred erythrocyte (AChE). No cross-reactivity between the two MAbs against the two antigens was noted.

Comparison of standard chromatographic procedures for the optimal purification of soluble human brain acetylcholinesterase

With the view of purifying soluble human brain acetylcholinesterase (AChE) into its separate isoforms, various preparative chromatographic procedures were compared. Chromatofocusing of cerebrospinal fluid (CSF) AChE revealed two major activity peaks, whilst that of caudate nucleus AChE showed one major peak. Both CSF and caudate nucleus AChE eluted at isoelectric points (pI) of between 5.5 and 5.2. Chromatofocusing failed to separate AChE into its individual isoforms, based on qualitative isoelectric focusing. Preparative purification by affinity chromatography showed a better AChE yield with the use of procainamide as a ligand, vis-à-vis acridinium. Maximum recovery for CSF and caudate nucleus AChE was 10 and 43% using acridinium and procainamide, respectively. Qualitative analysis by SDS-PAGE of affinity-purified AChE revealed four major bands between 50 and 62 kDa, corresponding to the catalytic subunits of AChE as verified by an anti-AChE polyclonal antibody. A size-exclusion column also allowed brain AChE purification, with the latter eluting at a putative molecular mass of 310 kDa. Unfortunately, cation-exchange using the state-of-the-art SMART system failed to separate AChE into its isoforms. AChE aggregation is given as one major obstacle precluding good resolution of isoforms.

The membrane form of acetylcholinesterase from rat brain contains a 20 kDa hydrophobic anchor

Rat brain acetylcholinesterase (AChE, EC 3.1.1.7) consists of about 80% amphiphilic detergent-soluble (DS-) AChE and 20% hydrophilic salt-soluble (SS-) AChE. DS-AChE contains about 65% tetrameric, 20% dimeric and 10% monomeric, SS-AChE about 40% tetrameric and 60% monomeric forms. N-terminal sequencing of DS- and SS-AChE gave identical N-termini corresponding to the published cDNA sequence of the mature enzyme. The band pattern on SDS-gels is similar to that of AChE from human and bovine brain. SDS-PAGE of hydrophobically labeled DS-AChE revealed the presence of a disulfide bonded hydrophobic membrane anchor of about 20 kDa. Monoclonal antibodies (mAbs) recognizing the anchor-containing subunits of mammalian brain DS-AChE, crossreacted with rat brain DS-AChE but not with SS-AChE. DS- and SS-AChE also reacted with antibodies raised against a peptide comprising the last 10 amino acids of the sequence of bovine brain AChE. Our results led us to conclude that both DS- and SS-AChE from rat brain contain T-type catalytic subunits, and DS-AChE in addition a P-type hydrophobic anchor similar to other mammalian brain DS-AChE.

N-glycosylation of human acetylcholinesterase: effects on activity, stability and biosynthesis

The role of N-glycosylation in the function of human acetylcholinesterase (HuAChE) was examined by site-directed mutagenesis (Asn to Gln substitution) of the three potential N-glycosylation sites Asn-265, Asn-350 and Asn-464. Analysis of HuAChE mutants, defective in a single or multiple N-glycosylation sites, by expression in transiently or stably transfected human embryonal 293 kidney cells suggests the following. (a) All three AChE glycosylation signals are utilized, but not all the secreted molecules are fully glycosylated. (b) Glycosylation at all sites is important for effective biosynthesis and secretion; extracellular AChE levels in mutants defective in one, two or all three sites amounted to 20-30%, 2-4% and about 0.5% of wild-type level respectively. (c) Some glycosylation mutants display impaired stability, as reflected by increased susceptibility to heat inactivation; substitution of Asn-464 has the most pronounced effect on thermostability. (d) Abrogation of N-glycosylation has no detectable effect on the enzyme activity of HuAChE; all glycosylation mutants, including the triple mutant, hydrolyse acetylthiocholine efficiently, displaying Km, kcat. and kcat./Km values similar to those of the wild-type enzyme. (e) In most mutants, inhibition profiles with edrophonium and bisquaternary ammonium ligands are identical with those of wild-type enzyme; the Asn-350 mutants, however, exhibit a slight decrease in their affinity towards these ligands. (f) Elimination of oligosaccharide side chains has no detectable effect on the surface-related 'peripheral-site' functions; like the wild-type enzyme, all mutants were inhibited by propidium and by increased concentrations of acetylthiocholine.

Subunit association and glycosylation of acetylcholinesterase from monkey brain

Cercopithecus monkey brain acetylcholinesterase (AChE; EC 3.1.1.7) consists of about 15% hydrophilic, salt-soluble enzyme and 83% amphiphilic, detergent-soluble enzyme. Sucrose density gradient centrifugation showed that hydrophilic, salt-soluble AChE was composed of about 85% tetramer (10.3S) and 15% monomer (3.3S). In amphiphilic, detergent-soluble AChE, 85% tetramer (9.7S), 10% dimer (5.7S), and 5% monomer (3.2S) were seen. The enzyme is N-glycosylated, and no O-linked carbohydrate could be detected. Use of two monoclonal antibodies, one directed against the catalytic subunit and the other against the hydrophobic anchor, gave new insights into the subunit assembly of brain AChE. It is shown that in tetrameric AChE, not all of the subunits are disulfide-bonded and that two populations of tetramers exist, one carrying one and the other carrying two hydrophobic anchors.

Monoclonal antibodies against brain acetylcholinesterases which recognize the subunits bearing the hydrophobic anchor

Monoclonal antibodies were raised against amphiphilic detergent-soluble (DS) acetylcholinesterase (AChE) from human brain caudate nucleus. Three mAb, 132-4 (IgG1), 132-5 (IgG1) and 132-6 (IgG3), specific for brain DS-AChE were selected and subcloned. These mAb reacted with native as well as heat-denatured and SDS-denatured DS-AChE, indicating that the epitopes to which mAb bound are continuous determinants. The mAb cross-reacted with DS-AChE from bovine and mouse brain and with brain DS-AChE from river trout (Salmo trutta forma fario) and lake trout (Salmo trutta forma lacustris). No cross-reaction was detected with the following antigens: salt-soluble (SS) AChE from bovine brain, glycophospholipid-anchored AChE from human and bovine erythrocytes, DS-butyrylcholinesterase and SS-butyrylcholinesterase (BtChE) from the brains of human and bovine, DS-BtChE from chicken and BtChE from human serum. Deglycosylation of brain DS-AChE with N-glycosidase F did not abolish the binding of mAb to DS-AChE. After reduction of brain DS-AChE by dithiothreitol, the mAb no longer reacted with the antigen, indicating that a disulfide bridge is important for the epitope. Monomerization of brain DS-AChE by trypsin and limited proteinase K treatment also abolished the binding of mAb to DS-AChE. Sucrose-density-gradient centrifugation showed that mAb reacted only with native tetrameric forms, but not with dimeric and monomeric forms. Western blot, after SDS/PAGE under non-reducing conditions, showed that mAb reacted with those subunits carrying the hydrophobic anchor (i.e. tetramers, trimers and heavy dimers) but not with those devoid of it (light dimers or monomers). Since mAb 132-4, 132-5 and 132-6 recognized DS-AChE from fish up to mammalian brain in the evolutionary tree, it is concluded that the epitope to which these mAb bind, is conserved in nature.

Different glycosylation in acetylcholinesterases from mammalian brain and erythrocytes

Acetylcholinesterases (EC 3.1.1.7, AChE) have varying amounts of carbohydrates attached to the core protein. Sequence analysis of the known primary structures gives evidence for several asparagine-linked carbohydrates. From the differences in molecular mass determined on sodium dodecyl sulfate-polyacrylamide gel before and after deglycosylation with N-glycosidase F (EC 3.2.2.18), it is seen that dimeric AChE from red cell membranes is more heavily glycosylated than the tetrameric brain enzyme. Furthermore, dimeric and tetrameric forms of bovine AChE are more heavily glycosylated than the corresponding human enzymes. Monoclonal antibodies 2E6, 1H11, and 2G8 raised against detergent-soluble AChE from electric organs of Torpedo nacline timilei as well as Elec-39 raised against AChE from Electrophorus electricus cross-reacted with AChE from bovine and human brain but not with AChE from erythrocytes. Treatment of the enzyme with N-glycosidase F abolished binding of monoclonal antibodies, suggesting that the epitope, or part of it, consists of N-linked carbohydrates. Analysis of N-acetylglucosamine sugars revealed the presence of N-acetylglucosamine in all forms of cholinesterases investigated, giving evidence for N-linked glycosylation. On the other hand, N-acetylgalactosamine was not found in AChE from human and bovine brain or in butyrylcholinesterase (EC 3.1.1.8) from human serum, indicating that these forms of cholinesterase did not contain O-linked carbohydrates. Despite the notion that within one species, the different forms of AChE arise from one gene by different splicing, our present results show that dimeric erythrocyte and tetrameric brain AChE must undergo different postsynthetic modifications leading to differences in their glycosylation patterns.