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

Covalent inhibition of hAChE by organophosphates causes homodimer dissociation through long-range allosteric effects

Acetylcholinesterase (EC 3.1.1.7), a key acetylcholine-hydrolyzing enzyme in cholinergic neurotransmission, is present in a variety of states in situ, including monomers, C-terminally disulfide-linked homodimers, homotetramers, and up to three tetramers covalently attached to structural subunits. Could oligomerization that ensures high local concentrations of catalytic sites necessary for efficient neurotransmission be affected by environmental factors? Using small-angle X-ray scattering (SAXS) and cryo-EM, we demonstrate that homodimerization of recombinant monomeric human acetylcholinesterase (hAChE) in solution occurs through a C-terminal four-helix bundle at micromolar concentrations. We show that diethylphosphorylation of the active serine in the catalytic gorge or isopropylmethylphosphonylation by the R_P enantiomer of sarin promotes a 10-fold increase in homodimer dissociation. We also demonstrate the dissociation of organophosphate (OP)-conjugated dimers is reversed by structurally diverse oximes 2PAM, HI6, or RS194B, as demonstrated by SAXS of diethylphosphoryl-hAChE. However, binding of oximes to the native ligand-free hAChE, binding of high-affinity reversible ligands, or formation of an S_P-sarin-hAChE conjugate had no effect on homodimerization. Dissociation monitored by time-resolved SAXS occurs in milliseconds, consistent with rates of hAChE covalent inhibition. OP-induced dissociation was not observed in the SAXS profiles of the double-mutant Y337A/F338A, where the active center gorge volume is larger than in wildtype hAChE. These observations suggest a key role of the tightly packed acyl pocket in allosterically triggered OP-induced dimer dissociation, with the potential for local reduction of acetylcholine-hydrolytic power in situ. Computational models predict allosteric correlated motions extending from the acyl pocket toward the four-helix bundle dimerization interface 25 Å away.

Cholinergic Capsules and Academic Admonitions

Herein, I intend to capture highlights shared with my academic and research colleagues over the 60 years I devoted initially to my graduate and postdoctoral training and then to academic endeavors starting as an assistant professor in a new medical school at the University of California, San Diego (UCSD). During this period, the Department of Pharmacology emerged from a division within the Department of Medicine to become the first basic science department, solely within the School of Medicine at UCSD in 1979. As part of the school's plans to reorganize and to retain me at UCSD, I was appointed as founding chair. Some years later in 2002, faculty, led largely within the Department of Pharmacology and by practicing pharmacists within UCSD Healthcare, started the independent Skaggs School of Pharmacy and Pharmaceutical Sciences with a doctor of pharmacy (PharmD) program, where I served as the founding dean. My career pathway, from working at my family-owned pharmacy to chairing a department in a school of medicine and then becoming the dean of a school of pharmacy at a research-intensive, student-centered institution, involved some risky decisions. But the academic, curricular, and accreditation challenges posed were met by a cadre of creative faculty colleagues. I offer my experiences to individuals confronted with a multiplicity of real or imagined opportunities in academic health sciences, the related pharmaceutical industry, and government oversight agencies.

Classics in Chemical Neuroscience: Donepezil

The discovery of acetylcholine and acetylcholinesterase provided the first insight into the intricacies of chemical signal transduction and neuronal communication. Further elucidation of the underlying mechanisms led to an attendant leveraging of this knowledge via the synthesis of new therapeutics designed to control aberrant biochemical processes. The central role of the cholinergic system within human memory and learning, as well as its implication in Alzheimer's disease, has made it a point of focus within the neuropharmacology and medicinal chemistry communities. This review is focused on donepezil and covers the background, synthetic routes, structure-activity relationships, binding interactions with acetylcholinesterase, pharmacokinetics and metabolism, efficacy, adverse effects, and historical importance of this leading therapeutic in the treatment of Alzheimer's disease and true Classic in Chemical Neuroscience.

Competitive Inhibition Mechanism of Acetylcholinesterase without Catalytic Active Site Interaction: Study on Functionalized C60 Nanoparticles via in Vitro and in Silico Assays

Acetylcholinesterase (AChE) activity regulation by chemical agents or, potentially, nanomaterials is important for both toxicology and pharmacology. Competitive inhibition via direct catalytic active sites (CAS) binding or noncompetitive inhibition through interference with substrate and product entering and exiting has been recognized previously as an AChE-inhibition mechanism for bespoke nanomaterials. The competitive inhibition by peripheral anionic site (PAS) interaction without CAS binding remains unexplored. Here, we proposed and verified the occurrence of a presumed competitive inhibition of AChE without CAS binding for hydrophobically functionalized C₆₀ nanoparticles (NPs) by employing both experimental and computational methods. The kinetic inhibition analysis distinguished six competitive inhibitors, probably targeting the PAS, from the pristine and hydrophilically modified C₆₀ NPs. A simple quantitative nanostructure-activity relationship (QNAR) model relating the pocket accessible length of substituent to inhibition capacity was then established to reveal how the geometry of the surface group decides the NP difference in AChE inhibition. Molecular docking identified the PAS as the potential binding site interacting with the NPs via a T-shaped plug-in mode. Specifically, the fullerene core covered the enzyme gorge as a lid through π-π stacking with Tyr72 and Trp286 in the PAS, while the hydrophobic ligands on the fullerene surface inserted into the AChE active site to provide further stability for the complexes. The modeling predicted that inhibition would be severely compromised by Tyr72 and Trp286 deletions, and the subsequent site-directed mutagenesis experiments proved this prediction. Our results demonstrate AChE competitive inhibition of NPs without CAS participation to gain further understanding of both the neurotoxicity and the curative effect of NPs.

QSAR Models towards Cholinesterase Inhibitors for the Treatment of Alzheimer's Disease

Alzheimer's Disease (AD) is a multifactorial neurological syndrome with the combination of aging, genetic, and environmental factors triggering the pathological decline. Interestingly, the importance of the Acetylcholinesterase (AChE) enzyme has increased due to its involvement in the ß-amyloid peptide fibril formation during AD pathogenesis. In silico technique, QSAR has proven its usefulness in pharmaceutical research for the design/optimization of new chemical entities. Further, QSAR method advanced the scope of rational drug design and the search for the mechanism of drug action. It is a well-established fact that the chemical and pharmaceutical effects of a compound are closely related to its physico-chemical properties, which can be calculated by various methods from the compound structure. This chapter focuses on different Quantitative Structure-Activity Relationship (QSAR) studies carried out for a variety of cholinesterase inhibitors for the treatment of AD. These predictive models will be potentially used for further designing better and safer drugs against AD.

Alzheimer's disease: Targeting the Cholinergic System

Acetylcholine (ACh) has a crucial role in the peripheral and central nervous systems. The enzyme choline acetyltransferase (ChAT) is responsible for synthesizing ACh from acetyl-CoA and choline in the cytoplasm and the vesicular acetylcholine transporter (VAChT) uptakes the neurotransmitter into synaptic vesicles. Following depolarization, ACh undergoes exocytosis reaching the synaptic cleft, where it can bind its receptors, including muscarinic and nicotinic receptors. ACh present at the synaptic cleft is promptly hydrolyzed by the enzyme acetylcholinesterase (AChE), forming acetate and choline, which is recycled into the presynaptic nerve terminal by the high-affinity choline transporter (CHT1). Cholinergic neurons located in the basal forebrain, including the neurons that form the nucleus basalis of Meynert, are severely lost in Alzheimer’s disease (AD). AD is the most ordinary cause of dementia affecting 25 million people worldwide. The hallmarks of the disease are the accumulation of neurofibrillary tangles and amyloid plaques. However, there is no real correlation between levels of cortical plaques and AD-related cognitive impairment. Nevertheless, synaptic loss is the principal correlate of disease progression and loss of cholinergic neurons contributes to memory and attention deficits. Thus, drugs that act on the cholinergic system represent a promising option to treat AD patients.

Probing the role of amino acids in oxime-mediated reactivation of nerve agent-inhibited human acetylcholinesterase

In this study, we employed site-directed mutagenesis to understand the role of amino acids in the gorge in oxime-induced reactivation of nerve agent-inhibited human (Hu) acetylcholinesterase (AChE). The organophosphorus (OP) nerve agents studied included GA (tabun), GB (sarin), GF (cyclosarin), VX, and VR. The kinetics of reactivation were examined using both the mono-pyridinium oxime 2-PAM and bis-pyridinium oximes MMB4, HI-6, and HLö-7. The second-order reactivation rate constants were used to compare reactivation of nerve agent-inhibited wild-type (WT) and mutant enzymes. Residues including Y72, Y124 and W286 were found to play important roles in reactivation by bis-pyridinium, but not by mono-pyridinium oximes. Residue Y124 also was found to play a key role in reactivation by HI-6 and HLö-7, while E202 was important for reactivation by all oximes. Residue substitutions of F295 by Leu and Y337 by Ala showed enhanced reactivation by bis-pyridinium oximes MMB4, HI-6, and HLö-7, possibly by providing more accessibility of the OP moiety associated at the active-site serine to the oxime. These results are similar to those observed previously with bovine AChE and demonstrate that there is significant similarity between human and bovine AChEs with regard to oxime reactivation.

Acetylcholinesterase: from 3D structure to function

By rapid hydrolysis of the neurotransmitter, acetylcholine, acetylcholinesterase terminates neurotransmission at cholinergic synapses. Acetylcholinesterase is a very fast enzyme, functioning at a rate approaching that of a diffusion-controlled reaction. The powerful toxicity of organophosphate poisons is attributed primarily to their potent inhibition of acetylcholinesterase. Acetylcholinesterase inhibitors are utilized in the treatment of various neurological disorders, and are the principal drugs approved thus far by the FDA for management of Alzheimer's disease. Many organophosphates and carbamates serve as potent insecticides, by selectively inhibiting insect acetylcholinesterase. The determination of the crystal structure of Torpedo californica acetylcholinesterase permitted visualization, for the first time, at atomic resolution, of a binding pocket for acetylcholine. It also allowed identification of the active site of acetylcholinesterase, which, unexpectedly, is located at the bottom of a deep gorge lined largely by aromatic residues. The crystal structure of recombinant human acetylcholinesterase in its apo-state is similar in its overall features to that of the Torpedo enzyme; however, the unique crystal packing reveals a novel peptide sequence which blocks access to the active-site gorge.

Parameters for Carbamate Pesticide QSAR and PBPK/PD Models for Human Risk Assessment

Our interest in providing parameters for the development of quantitative structure physiologically based pharmacokinetic/pharmacodynamic (QSPBPK/PD) models for assessing health risks to carbamates (USEPA 2005) comes from earlier work with organophosphorus (OP) insecticides (Knaak et al. 2004). Parameters specific to each carbamate are needed in the construction of PBPK/PD models along with their metabolic pathways. Parameters may be obtained by (1) development of QSAR models, (2) collecting pharmacokinetic data, and (3) determining pharmacokinetic parameters by fitting to experimental data. The biological parameters are given in Table 1 (Blancato et al. 2000). Table 1 Biological Parameters Required for Carbamate Pesticide Physiologically Based Pharmacokinetic/Pharmacodynamic (PBPK/PD) Models.(a).

An amino acid substitution attributable to insecticide-insensitivity of acetylcholinesterase in a Japanese encephalitis vector mosquito, Culex tritaeniorhynchus

A cDNA sequence encoding a Drosophila Ace-paralogous acetylcholinesterase (AChE) precursor of 701 amino acid residues was identified as the second AChE gene (Ace2) transcript from Culex tritaeniorhynchus. The Ace2 gene is tightly linked to organophosphorus insecticide (OP)-insensitivity of AChE on chromosome 2. The cDNA sequences were compared between an insecticide-susceptible strain and the resistant strain, TYM, that exhibits a 870-fold decrease in fenitroxon-sensitivity of AChE. Two amino acid substitutions were present in TYM mosquitoes. One is F455W whose homologous position in Torped AChE (Phe331) is located in the vicinity of the catalytic His in the acyl pocket of the active site gorge. The other substitution is located to a C-terminal Ile697 position that apparently seems to be excluded from the mature protein and is irrelevant to catalytic activity. The F455W replacement in the Ace2 gene is solely responsible for the insecticide-insensitivity of AChE in TYM mosquitoes.

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

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

Karyotype and genome characterization in four cartilaginous fishes

Different approaches can be used to elucidate the unsolved questions concerning taxonomic evolution in cartilaginous fish. The study of the karyological characteristics of these vertebrates by combining molecular and traditional techniques of chromosome preparation and banding has been demonstrated to be a very effective method. In this paper we studied the localization and the composition of the constitutive heterochromatin by using C- and restriction endonuclease-banding in four selachian species, belonging to two of the four superorders. We also characterized two different types of repetitive genomic sequences in these species: satellite DNA and (TTAGGG)(n) telomeric sequences. Finally, we analysed the nuclear ribosomal gene to determine the number of the nucleolar organizers and their position on chromosomes by using silver staining, chromomycin A(3), and FISH (fluorescent in situ hybridization). The results showed a prevailingly telomeric localization of constitutive heterochromatin in the Galeomorphii, the presence of additional nucleolar organizer sites in Raja asterias, an exclusively telomeric localization of the (TTAGGG)(n) sequences in Scyliorhinus stellaris and both telomeric and interstitial in Taeniura lymma. These data, together with those concerning the conservation of the satellite DNA, seem to support the hypothesis that Chondrichthyes have an evolutionary history leading them to the acquisition of large genomes rich in highly repeated sequences and subjected to some selective pressures favoring the conservation of this DNA fraction.

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

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

Acetylcholinesterase genes in the nematode Caenorhabditis elegans

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

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

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

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

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

The skeletal muscle alpha-actin gene of channel catfish (Ictalurus punctatus) and its association with piscine specific SINE elements

The alpha-actin gene of channel catfish (Ictalurus punctatus) was cloned and sequenced. The gene has a similar organization and exhibited a high level of sequence similarity to those from other vertebrate animals. The upstream region of the alpha-actin gene included a TATA box, a CAAT box, three E-boxes, and a CArG box. Nested deletion segments containing these transcriptional motifs were fused to the reporter gene chloramphenicol acetyl transferase (CAT). Transfection of the clones into C2C12 cells indicated that all these motifs are required for transcriptional activities. The channel catfish alpha-actin gene is associated with two distinct short interspersed repetitive elements (SINEs). The first SINE element showed high levels of sequence similarity to the zebrafish Mermaid element, while the second SINE element is not similar to the Mermaid element except for an 8bp sequence CCCCGTGC suggesting their evolutionary linkage. However, the second SINE element appeared to co-exist with the Mermaid element in most cases and therefore was designated as the Merman element. Approximately 9000 copies and 1200 copies of the Mermaid and Merman elements exist per haploid channel catfish genome, respectively. BLAST searches indicated that both the Mermaid and the Merman elements were frequently associated with gene sequences, mostly those of aquatic animals, suggesting their evolutionary origin in association with aquatic organisms and their function in shaping the evolution of genomes in aquatic animals.

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.

Novel messenger RNA and alternative promoter for murine acetylcholinesterase

A portion of the 5'-flanking region of murine acetylcholinesterase was cloned from genomic DNA by 5'-rapid amplification of genomic ends, identified in a mouse genomic library, and sequenced. Multiple potential binding sites for universal and tissue-specific transcription factors were suggestive of a promoter region within this DNA sequence. Potential promoter activity was confirmed by coupling the new sequence to the open reading frame of a luciferase reporter gene in transient expression experiments with nerve and muscle cells. 5'-Rapid amplification of cDNA ends with templates from multiple sources revealed a novel transcription start site (at position -626, relative to translation start), located 32 bases downstream from a TATAA sequence. This start site appeared to mark a novel exon (1a) comprising 291 base pairs between positions -335 and -626, relative to the translation start. Supporting this conclusion, polymerase chain reactions with cDNA from mouse brain, heart, and other tissues, consistently amplified a transcript containing the exon 1a sequence fused to the invariant sequence beginning at position -22 in exon 2, but lacking exon 1. Northern blot analyses confirmed the in vivo expression of exon 1a-containing transcripts, especially in heart, brain, liver, and kidney. These results indicate that the murine acetylcholinesterase gene has a functioning alternative promoter that may influence expression of acetylcholinesterase in certain tissues.

Optimising the signal peptide for glycosyl phosphatidylinositol modification of human acetylcholinesterase using mutational analysis and peptide-quantitative structure-activity relationships

Glycosyl phosphatidylinositol (GPI)-modified proteins have a C-terminal signal peptide (GPIsp) that mediates the addition of a GPI-anchor to an amino acid residue at the cleavage and modification site (omega-site). Within the GPIsp, a stretch of hydrophilic amino acid residues are found which constitutes the spacer region that separates the omega-site residue from a hydrophobic C-terminus. Deletions and insertions into the spacer region of human acetylcholinesterase (AChE) show that the length of this spacer region is very important for efficient GPI-modification. Surprisingly, the natural length of the spacer region in human AChE was not optimal for the highest degree of GPI modification. The importance of the two adjacent residues downstream of the omega-site, the omega+1 and omega+2 residues, was investigated by peptide-quantitative structure-activity relationships (Peptide-QSAR). A model was made that predicts the efficiency of the GPI modification when these residues are substituted with others, and suggests important features for these residues. The most preferred omega+1 and omega+2 residues, predicted by the model, in combination with an ideal spacer length resulted in an optimised GPIsp. This mutant protein is more efficiently GPI-modified than any mutant AChE tested thus far.

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.

Human endplate acetylcholinesterase deficiency caused by mutations in the collagen-like tail subunit (ColQ) of the asymmetric enzyme

In skeletal muscle, acetylcholinesterase (AChE) exists in homomeric globular forms of type T catalytic subunits (ACHET) and heteromeric asymmetric forms composed of 1, 2, or 3 tetrameric ACHET attached to a collagenic tail (ColQ). Asymmetric AChE is concentrated at the endplate (EP), where its collagenic tail anchors it into the basal lamina. The ACHET gene has been cloned in humans; COLQ cDNA has been cloned in Torpedo and rodents but not in humans. In a disabling congenital myasthenic syndrome, EP AChE deficiency (EAD), the normal asymmetric species of AChE are absent from muscle. EAD could stem from a defect that prevents binding of ColQ to ACHET or the insertion of ColQ into the basal lamina. In six EAD patients, we found no mutations in ACHET. We therefore cloned human COLQ cDNA, determined the genomic structure and chromosomal localization of COLQ, and then searched for mutations in this gene. We identified six recessive truncation mutations of COLQ in six patients. Coexpression of each COLQ mutant with wild-type ACHET in SV40-transformed monkey kidney fibroblast (COS) cells reveals that a mutation proximal to the ColQ attachment domain for ACHET prevents association of ColQ with ACHET; mutations distal to the attachment domain generate a mutant approximately 10.5S species of AChE composed of one ACHET tetramer and a truncated ColQ strand. The approximately 10.5S species lack part of the collagen domain and the entire C-terminal domain of ColQ, or they lack only the C-terminal domain, which is required for formation of the triple collagen helix, and this likely prevents their insertion into the basal lamina.

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.

Cloning and expression of acetylcholinesterase from Electrophorus. Splicing pattern of the 3' exons in vivo and in transfected mammalian cells

We cloned and expressed a cDNA encoding acetylcholinesterase (AChE) of type T from Electrophorus electricus organs. When expressed in COS, HEK, and Chinese hamster ovary cells, the AChET subunits generated dimers and tetramers. The cells produced more activity at 27 than at 37 degrees C. The kinetic parameters of a recombinant enzyme, produced in the yeast Pichia pastoris, were close to those of the natural AChE. Analysis of genomic clones showed that the coding sequence is interrupted by an intron that does not exist in Torpedo and differs in its location from that observed in the mouse. This intron is preceded by a sequence encoding a non-conserved 29-amino acid peptide, which does not exist in Torpedo or mammalian AChEs. According to a three-dimensional model, this non-conserved peptide is located at the surface of the protein, opposite from the entry of the catalytic gorge; its deletion did not modify the catalytic parameters. Sequence analyses and expression of various constructs showed that the gene does not contain any H exon. We also found that splicing of transcripts in mammalian cells reveals cryptic donor sites in exons and acceptor sites in introns, which do not appear to be used in vivo.

Cloning and characterization of a repetitive DNA detected by HindIII in the genome of Raja montagui (Batoidea, Chondrichthyes)

A repetitive HindIII fragment of DNA from Raja montagui (Rajiformes) was cloned and sequenced for the first time in cartilaginous fishes. This element, which comprises approximately 5% of the whole genome of the spotted ray, is absent in long tandem arrays, being typical of satellite DNA. It appeared constituted by 311 AT-rich bp (61%). The clone was hybridized to the genomic DNA of species with varying phyletic distances, revealing a high degree of conservation.

Ciliary neurotrophic factor: regulation of acetylcholinesterase in skeletal muscle and distribution of messenger RNA encoding its receptor in synaptic versus extrasynaptic compartments

Several recent studies have shown that the ciliary neurotrophic factor exerts myotrophic effects in addition to its well-characterized neurotrophic actions on various neuronal populations. Since expression of acetylcholinesterase in skeletal muscle has been shown to be regulated by putative yet unknown nerve-derived trophic factors, we tested the hypothesis that the ciliary neurotrophic factor is a neurotrophic agent capable of influencing expression of acetylcholinesterase in adult rat skeletal muscle in vivo. To this end, we first determined the impact of daily ciliary neurotrophic factor administration on expression of acetylcholinesterase in both intact and denervated rat soleus muscles. The results of our experiments indicate that although chronic administration of ciliary neurotrophic factor partially counteracted the atrophic response of soleus muscles to surgical denervation, thus confirming its myotrophic effects, it failed to either increase acetylcholinesterase expression in intact muscles or prevent the decrease normally occurring in seven-day denervated muscles. In fact, acetylcholinesterase messenger RNA and enzyme levels were further reduced by ciliary neurotrophic factor treatment in denervated muscles without significant modifications in the pattern of acetylcholinesterase molecular forms. Conversely, transcript levels of the epsilon subunit of the acetylcholine receptor in intact and denervated soleus muscles treated with the ciliary neurotrophic factor were similar to those observed in their respective counterparts from vehicle-treated animals. In addition, we also determined whether transcripts encoding the receptor for the ciliary neurotrophic factor selectively accumulate in junctional domains of rat skeletal muscle fibres. In contrast to the preferential localization of transcripts encoding acetylcholinesterase and the epsilon subunit of the acetylcholine receptor within the postsynaptic sarcoplasm, messenger RNAs for the ciliary neurotrophic factor receptor appeared homogeneously distributed between junctional and extra-junctional compartments of both diaphragm and extensor digitorum longus muscle fibres, with no compelling evidence for a selective accumulation within the postsynaptic sarcoplasm. These data show that the ciliary neurotrophic factor exerts an inhibitory influence on expression of acetylcholinesterase in muscle fibres. Furthermore, the lack of an effect on expression of the epsilon acetylcholine receptor transcripts indicates that treatment with ciliary neurotrophic factor does not lead to general adaptations in the expression of all synaptic proteins. Given the distribution of transcripts encoding the ciliary neurotrophic factor receptor along multinucleated muscle fibres, we propose a model whereby the ciliary neurotrophic factor, or a related unknown molecule that also utilizes the receptor for the ciliary neurotrophic factor, contributes to the maintenance of low levels of enzyme activity in extrajunctional regions of muscle fibres by acting as a repressor of acetylcholinesterase expression that functions directly or indirectly via a pretranslational regulatory mechanism. Accordingly, these results further highlight the complexity of the regulatory mechanisms presiding over acetylcholinesterase expression in vivo.

Non-classical actions of cholinesterases: role in cellular differentiation, tumorigenesis and Alzheimer's disease

The cholinesterases are members of the serine hydrolase family, which utilize a serine residue at the active site. Acetylcholinesterase (AChE) is distinguished from butyrylcholinesterase (BChE) by its greater specificity for hydrolysing acetylcholine. The function of AChE at cholinergic synapses is to terminate cholinergic neurotransmission. However, AChE is expressed in tissues that are not directly innervated by cholinergic nerves. AChE and BChE are found in several types of haematopoietic cells. Transient expression of AChE in the brain during embryogenesis suggests that AChE may function in the regulation of neurite outgrowth. Overexpression of cholinesterases has also been correlated with tumorigenesis and abnormal megakaryocytopoiesis. Acetylcholine has been shown to influence cell proliferation and neurite outgrowth through nicotinic and muscarinic receptor-mediated mechanisms and thus, that the expression of AChE and BChE at non-synaptic sites may be associated with a cholinergic function. However, structural homologies between cholinesterases and adhesion proteins indicate that cholinesterases could also function as cell-cell or cell-substrate adhesion molecules. Abnormal expression of AChE and BChE has been detected around the amyloid plaques and neurofibrillary tangles in the brains of patients with Alzheimer's disease. The function of the cholinesterases in these regions of the Alzheimer brain is unknown, but this function is probably unrelated to cholinergic neurotransmission. The presence of abnormal cholinesterase expression in the Alzheimer brain has implications for the pathogenesis of Alzheimer's disease and for therapeutic strategies using cholinesterase inhibitors.

Residues in Torpedo californica acetylcholinesterase necessary for processing to a glycosyl phosphatidylinositol-anchored form

Acetylcholinesterase from Torpedo californica (TcAChE) can be found as a glycosyl phosphatidylinositol (GPI)-anchored, membrane associated form. The C-terminal amino-acid sequence of the precursor protein resembles the signal peptide sequence found in proteins and enzymes destined for GPI-modification. Characteristics of such a signal peptide are a relatively polar stretch of amino acids, separating a cleavage- and modification-site (omega-site) residue from a hydrophobic C-terminus. We have introduced mutations, both at putative omega-sites and in the hydrophobic region, and analysed their effects on GPI-anchoring of TcAChE. Our results show that substitution of all three Ser residues in the region Ser542-Ser544 prevents GPI-modification and membrane anchoring. Individual substitution of each of these residues resulted in no or only a minor effect on the modification. We therefore conclude that more than one residue within this sequence can be utilised as the omega-site. Our analyses of double substitutions indicated that Ser543 and Ser544 are the preferred residues for GPI-modification. Moreover, the hydrophobic region is shown to be essential for GPI-anchoring of TcAChE.

Thyroid hormones stabilize acetylcholinesterase mRNA in neuro-2A cells that overexpress the beta 1 thyroid receptor

We investigated the intracellular events involved in the 3,3',5-triiodo-L-thyronine (T3)-induced accumulation in acetylcholinesterase (AChE) activity in neuroblastoma cells (neuro-2a) that overexpress the human thyroid receptor beta 1 (hTR beta 1). Treatment of these cells with T3 increased AChE activity and its mRNAs after a lag period of 24-48 h, and these levels increased through stabilization of the transcripts by T3. T3 had no effect on the transcriptional rate or processing of AChE transcripts. The protein kinase inhibitor H7 inhibited T3-induced accumulation in AChE activity and its mRNAs, whereas okadaic acid (a potent inhibitor of phosphatases 1 and 2A) potentiated the effect of T3. Okadaic acid and H7 have no effect on the binding of hTR beta 1 to T3 or the transcriptional rate of the AChE gene. Finally, treatment of cells with T3 stimulated cytosolic serine/threonine, but not tyrosine kinase, activities. The time course analysis reveals that the increase in serine/threonine activity precedes the effect of T3 on AChE mRNAs. These results suggest that activation of a serine/threonine protein kinase pathway might be a link between nuclear thyroid hormone receptor activation and stabilization of AChE mRNA.

Irreversible thermal denaturation of Torpedo californica acetylcholinesterase

Thermal denaturation of Torpedo californica acetylcholinesterase, a disulfide-linked homodimer with 537 amino acids in each subunit, was studied by differential scanning calorimetry. It displays a single calorimetric peak that is completely irreversible, the shape and temperature maximum depending on the scan rate. Thus, thermal denaturation of acetylcholinesterase is an irreversible process, under kinetic control, which is described well by the two-state kinetic scheme N-->D, with activation energy 131 +/- 8 kcal/mol. Analysis of the kinetics of denaturation in the thermal transition temperature range, by monitoring loss of enzymic activity, yields activation energy of 121 +/- 20 kcal/mol, similar to the value obtained by differential scanning calorimetry. Thermally denatured acetylcholinesterase displays spectroscopic characteristics typical of a molten globule state, similar to those of partially unfolded enzyme obtained by modification with thiol-specific reagents. Evidence is presented that the partially unfolded states produced by the two different treatments are thermodynamically favored relative to the native state.

Engineering of human cholinesterases explains and predicts diverse consequences of administration of various drugs and poisons

The acetylcholine hydrolyzing enzyme, acetylcholinesterase, primarily functions in nerve conduction, yet it appears in several guises, due to tissue-specific expression, alternative mRNA splicing and variable aggregation modes. The closely related enzyme, butyrylcholinesterase, most likely serves as a scavenger of toxins to protect acetylcholine binding proteins. One or both of the cholinesterases probably also plays a non-catalytic role(s) as a surface element on cells to direct intercellular interactions. The two enzymes are subject to inhibition by a wide variety of synthetic (e.g., organophosphorus and carbamate insecticides) and natural (e.g., glycoalkaloids) anticholinesterases that can compromise these functions. Butyrylcholinesterase may function, as well, to degrade several drugs of interest, notably aspirin, cocaine and cocaine-like local anesthetics. The widespread occurrence of butyrylcholinesterase mutants with modified activity further complicates this picture, in ways that are only now being dissected through the use of site-directed mutagenesis and heterologous expression of recombinant cholinesterases.

Patients with congenital myasthenia associated with end-plate acetylcholinesterase deficiency show normal sequence, mRNA splicing, and assembly of catalytic subunits

A congenital myasthenic condition has been described in several patients characterized by a deficiency in end-plate acetylcholinesterase (AChE). The characteristic form of AChE in the end-plate basal lamina has the catalytic subunits disulfide linked to a collagen-like tail unit. Southern analysis of the gene encoding the catalytic subunits revealed no differences between patient and control DNA. Genomic DNA clones covering exon 4 and the alternatively spliced exons 5 and 6 were analyzed by nuclease protection and sequencing. Although allelic differences were detected between controls, we found no differences in exonic and intronic areas that might yield distinctive splicing patterns in patients and controls. The ACHE gene was cloned from genomic libraries from a patient and a control. Transfection of the cloned genes revealed identical species of mRNA and expressed AChE. Cotransfection of the genes expressing the catalytic subunits with a cDNA from Torpedo encoding the tail unit yielded asymmetric species that require assembly of catalytic subunits and tail unit. thus the catalytic subunits of AChE expressed in the congenital myasthenic syndrome appear identical in sequence, arise from similar splicing patterns, and assemble normally with a tail unit to form a heteromeric species.

Residues important for folding and dimerisation of recombinant Torpedo californica acetylcholinesterase

The three-dimensional crystal structure of the glycosyl phosphatidylinositol (GPI)-modified form of Torpedo acetylcholinesterase reveals the participation of Arg-44 and Glu-92 in a salt bridge and a hydrogen bond between Asp-93 and Tyr-96. To investigate the biological significance of these interactions, we have made amino acid replacements in this form of AChE: R44E, R44K, E92Q, E92L, D93N, and D93V. None of the introduced mutations affected the production of the acetylcholinesterase polypeptide significantly. However, the mutations introduced at position 92, as well as the D93V and R44E mutations, resulted in a total loss of surface located, active acetylcholinesterase. Replacement of Asp-93 with Asn resulted in a reduced amount of active enzyme. This mutant enzyme was indistinguishable from the wild-type enzyme regarding catalytic constants, but was more sensitive to thermal inactivation. The results show that the salt bridge and hydrogen bond involving residues Arg-44, Glu-92, and Asp-93 have important structural roles and are needed for correct folding, required for transport to the cell surface of TcAChE. The GPI-modified form of acetylcholinesterase is a disulfide bonded dimer. Cys-537 is shown to be required for the formation of the intersubunit disulfide bond in the dimer. Replacement with Ser resulted in the production of an enzyme, that migrates as a monomer upon non-reducing SDS-PAGE and has a lower stability compared to the dimeric wild-type enzyme.

Neural regulation of acetylcholinesterase mRNAs at mammalian neuromuscular synapses

We examined the role of innervation on acetylcholinesterase (AChE) gene expression within mammalian skeletal muscle fibers. First, we showed the selective accumulation of AChE mRNAs within the junctional vs extrajunctional sarcoplasm of adult muscle fibers using a quantitative reverse transcription PCR assay and demonstrated by in situ hybridization experiments that AChE transcripts are concentrated immediately beneath the postsynaptic membrane of the neuromuscular junction. Next, we determined the influence of nerve-evoked activity vs putative trophic factors on the synaptic accumulation of AChE mRNA levels in muscle fibers paralyzed by either surgical denervation or selective blockage of nerve action potentials with chronic superfusion of tetrodotoxin. Our results indicated that muscle paralysis leads to a marked decrease in AChE transcripts from the postsynaptic sarcoplasm, yet the extent of this decrease is less pronounced after tetrodotoxin inactivation than after denervation. These results suggest that although nerve-evoked activity per se appears a key regulator of AChE mRNA levels, the integrity of the synaptic structure or the release of putative trophic factors contribute to maintaining the synaptic accumulation of AChE transcripts at adult neuromuscular synapses. Furthermore, the pronounced downregulation of AChE transcripts in paralyzed muscles stands in sharp contrast to the well-documented increase in nicotinic acetylcholine receptor mRNAs under these conditions, and indicates that expression of the genes encoding these two synaptic proteins are subjected to different regulatory mechanisms in adult muscle fibers in vivo.

Tissue distribution of human acetylcholinesterase and butyrylcholinesterase messenger RNA

Cholinesterase inhibitors occur naturally in the calabar bean (eserine), green potatoes (solanine), insect-resistant crab apples, the coca plant (cocaine) and snake venom (fasciculin). There are also synthetic cholinesterase inhibitors, for example man-made insecticides. These inhibitors inactivate acetylcholinesterase and butyrylcholinesterase as well as other targets. From a study of the tissue distribution of acetylcholinesterase and butyrylcholinesterase mRNA by Northern blot analysis, we have found the highest levels of butyrylcholinesterase mRNA in the liver and lungs, tissues known as the principal detoxication sites of the human body. These results indicate that butyrylcholinesterase may be a first line of defense against poisons that are eaten or inhaled.

Cloning and analysis of chicken acetylcholinesterase transcripts from muscle and brain

We have isolated cDNA clones from an embryonic chicken muscle cDNA library which encodes the complete catalytic T subunit of acetylcholinesterase. The deduced polypeptide comprises 767 amino acids, shows approximately 60% homology to acetylcholinesterases from other vertebrates and contains a 155 amino acid sequence inserted into the middle of the peptide which is unique to the chicken enzyme. Northern blots of embryonic chicken muscle and adult brain show three transcripts approximately 4.5, 5.5, and 6.0 kb hybridizing to a cDNA fragment of AChE. The 6.0 kb transcript is expressed transiently in embryonic muscle and is a major transcript in adult brain.

Neural regulation of muscle acetylcholinesterase is exerted on the level of its mRNA

In rat muscles, AChE activity drops rapidly after denervation, and the patterns of AChE molecular forms in slow and fast muscles differ considerably. Both observations imply that muscle AChE is regulated by the motor nerve. In order to obtain a better insight into the underlying mechanism, AChE regulation in rat muscles was examined on the level of its catalytic subunit mRNA using northern blot analysis. The level of two AChE transcripts (2.4 and 3.2 kb) was much higher in the fast sternomastoid (STM) than in the slow soleus muscle, which explains the difference in AChE activity between the two types of muscles. Expression of AChE mRNA in the extrajunctional region of STM muscle is fairly high so that little difference in the level of AChE mRNAs was observed in comparison to the region rich in the neuromuscular junctions. This indicates that very high AChE activity in the neuromuscular junctions is achieved by unique posttranslational modifications and cellular processing of AChE enhancing stability of the junctional in comparison to the extrajunctional AChE. Denervation as well as botulinum toxin evoked paralysis of STM muscle caused rapid decline of AChE transcripts to almost undetectable levels both in the junctional and extrajunctional regions. The low level of AChE mRNA is therefore largely responsible for low AChE activity in denervated rat muscles. It seems that either muscle activity and/or quantal ACh release enhance the level of AChE mRNA in the junctional as well as extrajunctional regions. In rat muscles, extrajunctional mRNA level of the catalytic subunit of AChE is neurally regulated in exact opposite fashion from that of acetylcholine receptor subunits.

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.

Epitope mapping of form-specific and nonspecific antibodies to acetylcholinesterase

We have mapped the epitopes to which two monoclonal antibodies against acetylcholinesterase (AChE) from Torpedo californica are directed. One antibody, 2C9, has equivalent affinity for both the 5.6S (amphiphilic) and 11S (hydrophilic) enzyme forms; the other, 4E7, recognizes only the amphiphilic form and has been shown previously to require an N-linked oligosaccharide residue on the protein. Isolation of cyanogen bromide peptides from the amphiphilic form and assay by a competition ELISA for 2C9 and by a direct binding ELISA for 4E7 identified the same peptide, residues 44-82, as containing epitopes against both antibodies. The epitope for 4E7 includes the oligosaccharide conjugated to Asp59, an N-linked glycosylation site not present in mouse AChE. A 20-amino-acid synthetic peptide, RFRRPEPKKPWSGVWNASTY, representing residues 44-63, was synthesized and found to inhibit completely 2C9 binding to 5.6S enzyme at molar concentrations comparable to those of the cyanogen bromide peptide. It was unreactive with 4E7. Fractionation of the synthetic peptide further localized the 2C9 epitope. Peptides RFRRPEPKKPW and KPWSGVWNASTY both reacted but less so than the entire synthetic peptide at equivalent molar concentrations, whereas the peptide RPEPKKPWSGVWNASTY was as effective as the larger synthetic peptide. The crystal structure of AChE shows the peptide to be on the surface of the molecule as part of a convex hairpin loop starting before the first alpha-helix.

Expression of acetylcholinesterase messenger RNA in human brain: an in situ hybridization study

The distribution of messenger RNA coding for acetylcholinesterase was studied in human post mortem brain and rhesus monkey by in situ hybridization histochemistry and compared to the distribution of acetylcholinesterase activity. Acetylcholinesterase messenger RNA had--similar to acetylcholinesterase enzymatic activity--a widespread distribution in human bain. Acetylcholinesterase messenger RNA positive cells corresponded to perikarya rich in acetylcholinesterase activity in most but not all regions. Examples for mismatches included the inferior olive and human cerebellar cortex. The presence of hybridization signals in cerebral cortex and an enrichment in layer III and V of most isocortical areas confirmed that perikaryal acetylcholinesterase in cerebral cortex is of postsynaptic origin and not derived from cholinergic projections. In striatum the expression of high levels of acetylcholinesterase messenger RNA was restricted to a small population of large striatal neurons. In addition, low levels of expression were found in most medium sized striatal neurons. Cholinergic neurons tended to express high levels of acetylcholinesterase messenger RNA whereas in cholinoceptive neurons the levels were moderate to low. However, some noncholinergic neurons like dopaminergic cells in substantia nigra, noradrenergic cells in locus coeruleus, serotoninergic cells in raphé dorsalis, GABAergic cells in thalamic reticular nucleus, granular cells in cerebellar cortex and pontine relay neurons expressed levels comparable to cholinergic neurons in basal forebrain. It is suggested that neurons expressing high levels of acetylcholinesterase messenger RNA may synthesize acetylcholinesterase for axonal transport whereas neurons with an expression of acetylcholinesterase confined to somatodendritic regions tend to contain lower levels of acetylcholinesterase messenger RNA.

Regulated and constitutive secretion of distinct molecular forms of acetylcholinesterase from PC12 cells

PC12 cells secrete the enzyme acetylcholinesterase (AChE) while at rest, and increase the overall rate of this secretion 2-fold upon depolarization. This behavior is different from the release of other markers by the constitutive or regulated secretory pathways in PC12 cells. Both the resting and stimulated release of AChE are unchanged after treatment with a membrane-impermeable esterase inhibitor, demonstrating that it represents true secretion and not shedding from the cell surface. The stimulation release of AChE is Ca(2+)-dependent, while the unstimulated release is not. Analysis of the molecular forms of AChE secreted by PC12 cells indicates that the release of AChE actually involves two concurrent but independent secretory processes, and that the G4 form of the enzyme is secreted constitutively, while both the G2 and G4 forms are secreted in a regulated manner, presumably from regulated secretory vesicles. Compared with other regulated secretory proteins, a much smaller fraction of cellular AChE is secreted, and the intracellular localization of this enzyme differs from that of other regulated secretory proteins. The demonstration that a cell line that exhibits regulated secretion of acetylcholine (ACh) is also capable of regulated secretion of AChE provides additional evidence for the existence of multiple regulated secretory pathways within a single cell. Moreover, there appears to be a selective packaging of different molecular forms of AChE into the regulated versus the constitutive secretory pathway. Both the specificity of sorting of AChE and the regulation of its secretion suggest that AChE may play a more dynamic role in synaptic function than has been recognized previously.

Compartmentalization of acetylcholinesterase mRNA and enzyme at the vertebrate neuromuscular junction

Acetylcholinesterase (AChE) is concentrated at the vertebrate neuromuscular synapse. To determine whether increased transcript levels could underlie this selective accumulation, we employed a quantitative reverse transcription polymerase chain reaction-based assay to determine mRNA copy number in samples as small as single neuromuscular junctions (NMJs) and a microassay to measure AChE enzyme activity at single synapses. Our results show that AChE mRNA is an intermediate transcript at NMJs, whereas in noninnervated regions of muscle fibers, AChE transcripts are either undetectable or rare. In contrast, alpha-actin transcript levels in the same samples are similar in junctional and extrajunctional regions. However, compared with AChE enzyme activity and alpha-actin mRNA levels, the levels of AChE transcripts at NMJs are highly variable. These results indicate that AChE mRNA and protein expression are compartmentalized at the vertebrate NMJ and provide a direct approach toward dissecting the molecular events leading from synaptic activation to plastic changes in gene expression at single vertebrate synapses.

Chicken repeat 1 elements contain a pol-like open reading frame and belong to the non-long terminal repeat class of retrotransposons

Chicken genomes contain approximately 30,000 chicken repeat 1 (CR1) elements scattered among single-copy sequences, but no information has yet been presented to account for how these elements could have dispersed. The fact that CR1 elements have common (although atypical) 3' ends and variable 5' truncations suggested to us that they might belong to the class of non-long terminal repeat retrotransposons that encode reverse transcriptases. From an analysis of unusually large CR1 elements, we now provide evidence for the presence of such a reverse transcriptase open reading frame. CR1 elements are distantly related to previously described non-long terminal repeat retrotransposons; however, we find that frog and torpedo ray genomes contain dispersed open reading frame segments that have > 50% identity to the CR1 open reading frame. This result suggests that CR1-like elements exist in several vertebrate classes that have evolved independently for approximately 400 million years.