Quaternary associations of acetylcholinesterase. I. Oligomeric associations of T subunits with and without the amino-terminal domain of the collagen tail

Changes in the Acetylcholinesterase Enzymatic Activity in Tumor Development and Progression

Acetylcholinesterase is a well-known protein because of the relevance of its enzymatic activity in the hydrolysis of acetylcholine in nerve transmission. In addition to the catalytic action, it exerts non-catalytic functions; one is associated with apoptosis, in which acetylcholinesterase could significantly impact the survival and aggressiveness observed in cancer. The participation of AChE as part of the apoptosome could explain the role in tumors, since a lower AChE content would increase cell survival due to poor apoptosome assembly. Likewise, the high Ach content caused by the reduction in enzymatic activity could induce cell survival mediated by the overactivation of acetylcholine receptors (AChR) that activate anti-apoptotic pathways. On the other hand, in tumors in which high enzymatic activity has been observed, AChE could be playing a different role in the aggressiveness of cancer; in this review, we propose that AChE could have a pro-inflammatory role, since the high enzyme content would cause a decrease in ACh, which has also been shown to have anti-inflammatory properties, as discussed in this review. In this review, we analyze the changes that the enzyme could display in different tumors and consider the different levels of regulation that the acetylcholinesterase undergoes in the control of epigenetic changes in the mRNA expression and changes in the enzymatic activity and its molecular forms. We focused on explaining the relationship between acetylcholinesterase expression and its activity in the biology of various tumors. We present up-to-date knowledge regarding this fascinating enzyme that is positioned as a remarkable target for cancer treatment.

Natural Products for the Treatment of Neurodegenerative Diseases

Neurodegenerative diseases are a heterogeneous group of disorders characterized by the progressive degeneration of the structure and function of the central nervous system or peripheral nervous system. Alzheimer's Disease (AD), Parkinson's Disease (PD) and Spinal Cord Injury (SCI) are the common neurodegenerative diseases, which typically occur in people over the age of 60. With the rapid development of an aged society, over 60 million people worldwide are suffering from these uncurable diseases. Therefore, the search for new drugs and therapeutic methods has become an increasingly important research topic. Natural products especially those from the Traditional Chinese Medicines (TCMs), are the most important sources of drugs, and have received extensive interest among pharmacist. In this review, in order to facilitate further chemical modification of those useful natural products by pharmacists, we will bring together recent studies in single natural compound from TCMs with neuroprotective effect.

Interacting with α 7 nAChR is a new mechanism for AChE to enhance the inflammatory response in macrophages

Acetylcholine (ACh) regulates inflammation via α7 nicotinic acetylcholine receptor (α7 nAChR). Acetylcholinesterase (AChE), an enzyme hydrolyzing ACh, is expressed in immune cells suggesting non-classical function in inflammatory responses. Here, the expression of PRiMA-linked G4 AChE was identified on the surface of macrophages. In lipopolysaccharide-induced inflammatory processes, AChE was upregulated by the binding of NF-κB onto the ACHE promotor. Conversely, the overexpression of G4 AChE inhibited ACh-suppressed cytokine release and cell migration, which was in contrast to that of applied AChE inhibitors. AChEmt, a DNA construct without enzymatic activity, was adopted to identify the protein role of AChE in immune system. Overexpression of G4 AChEmt induced cell migration and inhibited ACh-suppressed cell migration. The co-localization of α7 nAChR and AChE was found in macrophages, suggesting the potential interaction of α7 nAChR and AChE. Besides, immunoprecipitation showed a close association of α7 nAChR and AChE protein in cell membrane. Hence, the novel function of AChE in macrophage by interacting with α7 nAChR was determined. Together with hydrolysis of ACh, AChE plays a direct role in the regulation of inflammatory response. As such, AChE could serve as a novel target to treat age-related diseases by anti-inflammatory responses.

The NMJ as a model synapse: New perspectives on formation, synaptic transmission and maintenance: Acetylcholinesterase at the neuromuscular junction

Acetylcholinesterase (AChE) is an essential enzymatic component of the neuromuscular junction where it is responsible for terminating neurotransmission by the cholinergic motor neurons. The enzyme at the neuromuscular junction (NMJ) is contributed primarily by the skeletal muscle where it is produced at higher levels in the post-synaptic region of the fibers. The major form of AChE at the NMJ is a large asymmetric form consisting of three tetramers covalently attached to a three-stranded collagen-like tail which is responsible for anchoring it to the synaptic basal lamina. Its location and expression is regulated to a large extent by the motor neurons and occurs at the transcriptional, translational and post-translational levels. While its expression can be quite rapid in tissue cultured cells, its half-life in vivo appears to be quite long, about three weeks, although more rapidly turning over pools have been described. Finally the essential nature of this enzyme is underscored by the fact that no naturally occurring null mutations of the catalytic subunit have been described in higher organisms and the few dozen humans carrying mutations in the collagen tail responsible for anchoring the enzyme at the NMJ are severely affected.

Polyproline-rich peptides associated with Torpedo californica acetylcholinesterase tetramers

Acetylcholinesterase (AChE) terminates cholinergic neurotransmission by hydrolyzing acetylcholine. The collagen-tailed AChE tetramer is a product of 2 genes, ACHE and ColQ. The AChE tetramer consists of 4 identical AChE subunits and one polyproline-rich peptide, whose function is to hold the 4 AChE subunits together. Our goal was to determine the amino acid sequence of the polyproline-rich peptide(s) in Torpedo californica AChE (TcAChE) tetramers to aid in the analysis of images that will be acquired by cryo-EM. Collagen-tailed AChE was solubilized from Torpedo californica electric organ, converted to 300 kDa tetramers by digestion with trypsin, and purified by affinity chromatography. Polyproline-rich peptides were released by denaturing the TcAChE tetramers in a boiling water bath, and reducing disulfide bonds with dithiothreitol. Carbamidomethylated peptides were separated from TcAChE protein on a spin filter before they were analyzed by liquid chromatography tandem mass spectrometry on a high resolution Orbitrap Fusion Lumos mass spectrometer. Of the 64 identified collagen-tail (ColQ) peptides, 60 were from the polyproline-rich region near the N-terminus of ColQ. The most abundant proline-rich peptides were SVNKCCLLTPPPPPMFPPPFFTETNILQE, at 40% of total mass-spectral signal intensity, and SVNKCCLLTPPPPPMFPPPFFTETNILQEVDLNNLPLEIKPTEPSCK, at 27% of total intensity. The high abundance of these 2 peptides makes them candidates for the principal form of the polyproline-rich peptide in the trypsin-treated TcAChE tetramers.

Biochemical adaptation in brain Acetylcholinesterase during acclimation to sub-lethal temperatures in the eurythermal fish Tilapia mossambica

Tilapia mossambica is a eurythermal tropical fish. We studied the effect of temperature on the kinetics of brain Acetylcholinesterase (AChE) during adaptation to sublethal temperatures by acclimating the fish to 37 °C, and controls to 25 °C. Electrophoresis showed the presence of two AChE bands that did not change in position or intensity with acclimation period or temperature. The apparent K_m was 0.23 ± 0.01 mM ATChI and remained relatively constant over the in vitro assay temperature range 10 °C to 40 °C. Biochemical characterization suggested that the enzyme is a ‘eurytolerant protein’ in its stability of kinetic and thermal properties over a wide temperature range. Thermal stability and arrhenius plots suggested that the AChE was made up of two forms that differed in their thermal properties.The two molecular forms of acetylcholinesterase were purified from the brain of T. mossambica. Molecular weight studies revealed that the two forms were size isomers: a monomer of 59 KDa and a tetramer of 244 KDa. They differed in their K_ms, thermal stabilities and energies of activation. We suggest that biochemical adaptation to temperature in the brain acetylcholinerase system of the fish Tilapia mossambica is based on the aggregation-dissociation of these size isomers.

Activation of G protein-coupled receptor 30 by flavonoids leads to expression of acetylcholinesterase in cultured PC12 cells

Flavonoids, considered as phytoestrogen mainly deriving from fruit and vegetable, are known to have beneficial effects in brain functions. The role of flavonoids in induction of a cholinergic enzyme, acetylcholinesterase (AChE), was being explored here. In cultured PC12 cells, twenty-four commonly found flavonoids were tested for its induction on AChE activity. Fourteen flavonoids showed induction, and five of them had robust effect, i.e. daidzin, alpinetin, irisflorentin, cardamonin and lysionotin. The induction of AChE was fully blocked by pre-treatment of G15 (a selective G protein-coupled receptor 30 [GPR 30] antagonist), suggesting a direct involvement of a membrane-bound estrogen receptor, named as GPR 30, in the cultures. In addition, daidzin was further identified to induce expression of tetrameric globular form of proline-rich membrane anchor (PRiMA)-linked AChE. In parallel, application of daidzin in cultured PC12 cells significantly induced expression of neurofilaments, markers for neuronal differentiation. Taken together, flavonoids could induce the expression of AChE via GPR 30 in cultured PC12 cells, which could be a good candidate for possible treatment of the brain diseases.

Cryo-EM structure of the native butyrylcholinesterase tetramer reveals a dimer of dimers stabilized by a superhelical assembly

The quaternary structures of the cholinesterases, acetylcholinesterase (AChE) and butyrylcholinesterase (BChE), are essential for their localization and function. Of practical importance, BChE is a promising therapeutic candidate for intoxication by organophosphate nerve agents and insecticides, and for detoxification of addictive substances. Efficacy of the recombinant enzyme hinges on its having a long circulatory half-life; this, in turn, depends strongly on its ability to tetramerize. Here, we used cryoelectron microscopy (cryo-EM) to determine the structure of the highly glycosylated native BChE tetramer purified from human plasma at 5.7 Å. Our structure reveals that the BChE tetramer is organized as a staggered dimer of dimers. Tetramerization is mediated by assembly of the C-terminal tryptophan amphiphilic tetramerization (WAT) helices from each subunit as a superhelical assembly around a central lamellipodin-derived oligopeptide with a proline-rich attachment domain (PRAD) sequence that adopts a polyproline II helical conformation and runs antiparallel. The catalytic domains within a dimer are asymmetrically linked to the WAT/PRAD. In the resulting arrangement, the tetramerization domain is largely shielded by the catalytic domains, which may contribute to the stability of the human BChE (HuBChE) tetramer. Our cryo-EM structure reveals the basis for assembly of the native tetramers and has implications for the therapeutic applications of HuBChE. This mode of tetramerization is seen only in the cholinesterases but may provide a promising template for designing other proteins with improved circulatory residence times.

Acute and long-term consequences of exposure to organophosphate nerve agents in humans

Nerve agents are organophosphate (OP) compounds and among the most powerful poisons known to man. A terrorist attack on civilian or military populations causing mass casualties is a real threat. The OP nerve agents include soman, sarin, cyclosarin, tabun, and VX. The major mechanism of acute toxicity is the irreversible inhibition of acetylcholinesterase. Acetylcholinesterase inhibition results in the accumulation of excessive acetylcholine levels in synapses, leading to progression of toxic signs including hypersecretions, tremors, status epilepticus, respiratory distress, and death. Miosis and rhinorrhea are the most common clinical findings in those individuals acutely exposed to OP nerve agents. Prolonged seizures are responsible for the neuropathology. The brain region that shows the most severe damage is the amygdala, followed by the piriform cortex, hippocampus, cortex, thalamus, and caudate/putamen. Current medical countermeasures are only modestly effective in attenuating the seizures and neuropathology. Anticonvulsants such as benzodiazepines decrease seizure activity and improve outcome, but their efficacy depends upon the administration time after exposure to the nerve agent. Administration of benzodiazepines may increase the risk for seizure recurrence. Recent studies document long-term neurologic and behavior deficits, and technological advances demonstrate structural brain changes on magnetic resonance imaging.

Genistein, a Phytoestrogen in Soybean, Induces the Expression of Acetylcholinesterase via G Protein-Coupled Receptor 30 in PC12 Cells

Genistein, 4′,5,7-trihydroxyisoflavone, is a major isoflavone in soybean, which is known as phytestrogen having known benefit to brain functions. Being a common phytestrogen, the possible role of genistein in the brain protection needs to be further explored. In cultured PC12 cells, application of genistein significantly induced the expression of neurofilaments (NFs), markers for neuronal differentiation. In parallel, the expression of tetrameric form of proline-rich membrane anchor (PRiMA)-linked acetyl-cholinesterase (G4 AChE), a key enzyme to hydrolyze acetylcholine in cholinergic synapses, was induced in a dose-dependent manner: this induction included the associated protein PRiMA. The genistein-induced AChE expression was fully blocked by the pre-treatment of H89 (an inhibitor of protein kinase A, PKA) and G15 (a selective G protein-coupled receptor 30 (GPR30) antagonist), which suggested a direct involvement of a membrane-bound estrogen receptor (ER), named as GPR30 in the cultures. In parallel, the estrogen-induced activation of GPR30 induced AChE expression in a dose-dependent manner. The genistein/estrogen-induced AChE expression was triggered by a cyclic AMP responding element (CRE) located on the ACHE gene promoter. The binding of this CRE site by cAMP response element-binding protein (CREB) induced ACHE gene transcription. In parallel, increased expression levels of miR132 and miR212 were found when cultured PC12 cells were treated with genistein or G1. Thus, a balance between production and destruction of AChE by the activation of GPR30 was reported here. We have shown for the first time that the activation of GPR30 could be one way for estrogen or flavonoids, possessing estrogenic properties, to enhance cholinergic functions in the brain, which could be a good candidate for possible treatment of neurodegenerative diseases.

Biogenesis, assembly and trafficking of acetylcholinesterase

Acetylcholinesterase (AChE) is expressed as several homomeric and heterooligomeric forms in a wide variety of tissues such as neurons in the central and peripheral nervous systems and their targets including skeletal muscle, endocrine and exocrine glands. In addition, glycolipid-anchored forms are expressed in erythropoietic and lymphopoietic cells. While transcriptional and post-transcriptional regulation is important for determining which AChE oligomeric forms are expressed in a given tissue, translational and post-translational regulatory mechanisms at the level of protein folding, assembly and sorting play equally important roles in assuring that the AChE molecules reach their intended sites on the cell surface in the appropriate numbers. This brief review will focus on the latter events in the cell with the goal of providing novel therapeutic interventional strategies for the treatment of organophosphate and carbamate pesticide and nerve agent exposure. This is an article for the special issue XVth International Symposium on Cholinergic Mechanisms.

Wnt3a induces the expression of acetylcholinesterase during osteoblast differentiation via the Runx2 transcription factor

Acetylcholinesterase (AChE) hydrolyzes acetylcholine to terminate cholinergic transmission in neurons. Apart from this AChE activity, emerging evidence suggests that AChE could also function in other, non-neuronal cells. For instance, in bone, AChE exists as a proline-rich membrane anchor (PRiMA)-linked globular form in osteoblasts, in which it is proposed to play a noncholinergic role in differentiation. However, this hypothesis is untested. Here, we found that in cultured rat osteoblasts, AChE expression was increased in parallel with osteoblastic differentiation. Because several lines of evidence indicate that AChE activity in osteoblast could be triggered by Wnt/β-catenin signaling, we added recombinant human Wnt3a to cultured osteoblasts and found that this addition induced expression of the ACHE gene and protein product. This Wnt3a-induced AChE expression was blocked by the Wnt-signaling inhibitor Dickkopf protein-1 (DKK-1). We hypothesized that the Runt-related transcription factor 2 (Runx2), a downstream transcription factor in Wnt/β-catenin signaling, is involved in AChE regulation in osteoblasts, confirmed by the identification of a Runx2-binding site in the ACHE gene promoter, further corroborated by ChIP. Of note, Runx2 overexpression in osteoblasts induced AChE expression and activity of the ACHE promoter tagged with the luciferase gene. Moreover, deletion of the Runx2-binding site in the ACHE promoter reduced its activity during osteoblastic differentiation, and addition of 5-azacytidine and trichostatin A to differentiating osteoblasts affected AChE expression, suggesting epigenetic regulation of the ACHE gene. We conclude that AChE plays a role in osteoblastic differentiation and is regulated by both Wnt3a and Runx2.

Transcriptional activity of acetylcholinesterase gene is regulated by DNA methylation during C2C12 myogenesis

The expression of acetylcholinesterase (AChE), an enzyme hydrolyzes neurotransmitter acetylcholine at vertebrate neuromuscular junction, is regulated during myogenesis, indicating the significance of muscle intrinsic factors in controlling the enzyme expression. DNA methylation is essential for temporal control of myogenic gene expression during myogenesis; however, its role in AChE regulation is not known. The promoter of vertebrate ACHE gene carries highly conserved CG-rich regions, implying its likeliness to be methylated for epigenetic regulation. A DNA methyltransferase inhibitor, 5-azacytidine (5-Aza), was applied onto C2C12 cells throughout the myotube formation. When DNA methylation was inhibited, the promoter activity, transcript expression and enzymatic activity of AChE were markedly increased after day 3 of differentiation, which indicated the putative role of DNA methylation. By bisulfite pyrosequencing, the overall methylation rate was found to peak at day 3 during C2C12 cell differentiation; a SP1 site located at -1826bp upstream of mouse ACHE gene was revealed to be heavily methylated. The involvement of transcriptional factor SP1 in epigenetic regulation of AChE was illustrated here: (i) the SP1-driven transcriptional activity was increased in 5-Aza-treated C2C12 culture; (ii) the binding of SP1 onto the SP1 site of ACHE gene was fully blocked by the DNA methylation; and (iii) the sequence flanking SP1 sites of ACHE gene was precipitated by chromatin immuno-precipitation assay. The findings suggested the role of DNA methylation on AChE transcriptional regulation and provided insight in elucidating the DNA methylation-mediated regulatory mechanism on AChE expression during muscle differentiation.

Rescue and Stabilization of Acetylcholinesterase in Skeletal Muscle by N-terminal Peptides Derived from the Noncatalytic Subunits

The vast majority of newly synthesized acetylcholinesterase (AChE) molecules do not assemble into catalytically active oligomeric forms and are rapidly degraded intracellularly by the endoplasmic reticulum-associated protein degradation pathway. We have previously shown that AChE in skeletal muscle is regulated in part post-translationally by the availability of the noncatalytic subunit collagen Q, and others have shown that expression of a 17-amino acid N-terminal proline-rich attachment domain of collagen Q is sufficient to promote AChE tetramerization in cells producing AChE. In this study we show that muscle cells, or cell lines expressing AChE catalytic subunits, incubated with synthetic proline-rich attachment domain peptides containing the endoplasmic reticulum retrieval sequence KDEL take up and retrogradely transport them to the endoplasmic reticulum network where they induce assembly of AChE tetramers. The peptides act to enhance AChE folding thereby rescuing them from reticulum degradation. This enhanced folding efficiency occurs in the presence of inhibitors of protein synthesis and in turn increases total cell-associated AChE activity and active tetramer secretion. Pulse-chase studies of isotopically labeled AChE molecules show that the enzyme is rescued from intracellular degradation. These studies provide a mechanistic explanation for the large scale intracellular degradation of AChE previously observed and indicate that simple peptides alone can increase the production and secretion of this critical synaptic enzyme in muscle tissue.

Polyproline tetramer organizing peptides in fetal bovine serum acetylcholinesterase

Acetylcholinesterase (AChE) in the serum of fetal cow is a tetramer. The related enzyme, butyrylcholinesterase (BChE), in the sera of humans and horse requires polyproline peptides for assembly into tetramers. Our goal was to determine whether soluble tetrameric AChE includes tetramer organizing peptides in its structure. Fetal bovine serum AChE was denatured by boiling to release non-covalently bound peptides. Bulk protein was separated from peptides by filtration and by high performance liquid chromatography. Peptide mass and amino acid sequence of the released peptides were determined by MALDI-TOF-TOF and LTQ-Orbitrap mass spectrometry. Twenty polyproline peptides, divided into 5 families, were identified. The longest peptide contained 25 consecutive prolines and no other amino acid. Other polyproline peptides included one non-proline amino acid, for example serine at the C-terminus of 20 prolines. A search of the mammalian proteome database suggested that this assortment of polyproline peptides originated from at least 5 different precursor proteins, none of which were the ColQ or PRiMA of membrane-anchored AChE. To date, AChE and BChE are the only proteins known that include polyproline tetramer organizing peptides in their tetrameric structure.

N-linked glycosylation of dimeric acetylcholinesterase in erythrocytes is essential for enzyme maturation and membrane targeting

Acetylcholinesterase (AChE) is well-known for its cholinergic functions in the nervous system; however, this enzyme is also found in other tissues where its function is still not understood. AChE is synthesized through alternative splicing as splicing variants, with isoforms including read-through (AChE(R)), tailed (AChE(T)) and hydrophobic (AChE(H)). In human erythrocytes, AChE(H) is a glycophosphatidylinositol-linked dimer on the plasma membrane. Three N-linked glycosylation sites have been identified in the catalytic domain of human AChE. Here, we investigate the roles of glycosylation in assembly and trafficking of human AChE(H). In transfected fibroblasts, expression of AChE(H) was able to mimic the function of the dimeric form of AChE on the erythrocyte membrane. A glycan-depleted form was constructed by site-directed mutagenesis. By comparison with the wild-type AChE(H), the mutant had a much lower enzymatic activity and a much higher K(m) value. In addition, the mutant was dimerized in the endoplasmic reticulum, but was not trafficked to the Golgi apparatus. The results suggest that the glycosylation may affect AChE(H) enzymatic activity and trafficking, but not dimer formation. The present findings indicate the significance of N-glycosylation in controlling the biosynthesis of the AChE(H) dimer form.

Molecular Assembly and Biosynthesis of Acetylcholinesterase in Brain and Muscle: the Roles of t-peptide, FHB Domain, and N-linked Glycosylation

Acetylcholinesterase (AChE) is responsible for the hydrolysis of the neurotransmitter, acetylcholine, in the nervous system. The functional localization and oligomerization of AChE T variant are depending primarily on the association of their anchoring partners, either collagen tail (ColQ) or proline-rich membrane anchor (PRiMA). Complexes with ColQ represent the asymmetric forms (A(12)) in muscle, while complexes with PRiMA represent tetrameric globular forms (G(4)) mainly found in brain and muscle. Apart from these traditional molecular forms, a ColQ-linked asymmetric form and a PRiMA-linked globular form of hybrid cholinesterases (ChEs), having both AChE and BChE catalytic subunits, were revealed in chicken brain and muscle. The similarity of various molecular forms of AChE and BChE raises interesting question regarding to their possible relationship in enzyme assembly and localization. The focus of this review is to provide current findings about the biosynthesis of different forms of ChEs together with their anchoring proteins.

The PRiMA-linked cholinesterase tetramers are assembled from homodimers: hybrid molecules composed of acetylcholinesterase and butyrylcholinesterase dimers are up-regulated during development of chicken brain

Acetylcholinesterase (AChE) is anchored onto cell membranes by the transmembrane protein PRiMA (proline-rich membrane anchor) as a tetrameric globular form that is prominently expressed in vertebrate brain. In parallel, the PRiMA-linked tetrameric butyrylcholinesterase (BChE) is also found in the brain. A single type of AChE-BChE hybrid tetramer was formed in cell cultures by co-transfection of cDNAs encoding AChE(T) and BChE(T) with proline-rich attachment domain-containing proteins, PRiMA I, PRiMA II, or a fragment of ColQ having a C-terminal GPI addition signal (Q(N-GPI)). Using AChE and BChE mutants, we showed that AChE-BChE hybrids linked with PRiMA or Q(N-GPI) always consist of AChE(T) and BChE(T) homodimers. The dimer formation of AChE(T) and BChE(T) depends on the catalytic domains, and the assembly of tetramers with a proline-rich attachment domain-containing protein requires the presence of C-terminal "t-peptides" in cholinesterase subunits. Our results indicate that PRiMA- or ColQ-linked cholinesterase tetramers are assembled from AChE(T) or BChE(T) homodimers. Moreover, the PRiMA-linked AChE-BChE hybrids occur naturally in chicken brain, and their expression increases during development, suggesting that they might play a role in cholinergic neurotransmission.

Conformational preferences of a 14-residue fibrillogenic peptide from acetylcholinesterase

A 14-residue fragment from near the C-terminus of the enzyme acetylcholinesterase (AChE) is believed to have a neurotoxic/neurotrophic effect acting via an unknown pathway. While the peptide is α-helical in the full-length enzyme, the structure and association mechanism of the fragment are unknown. Using multiple molecular dynamics simulations, starting from a tetrameric complex of the association domain of AChE and systematically disassembled subsets that include the peptide fragment, we show that the fragment is incapable of retaining its helicity in solution. Extensive replica exchange Monte Carlo folding and unfolding simulations in implicit solvent with capped and uncapped termini failed to converge to any consistent cluster of structures, suggesting that the fragment remains largely unstructured in solution under the conditions considered. Furthermore, extended molecular dynamics simulations of two steric zipper models show that the peptide is likely to form a zipper with antiparallel sheets and that peptides with mutations known to prevent fibril formation likely do so by interfering with this packing. The results demonstrate how the local environment of a peptide can stabilize a particular conformation.

Unusual transfer of CutA into the secretory pathway, evidenced by fusion proteins with acetylcholinesterase

The mouse CutA protein exists as long and short components of 20 and 15 kDa, produced by the use of different in-frame ATGs initiation codons, and by proteolytic cleavage. We recently showed that, surprisingly, the longer, uncleaved component resides mostly in the secretory pathway and is secreted, whereas the shorter component resides mostly in the cytoplasm. To confirm these subcellular localizations, we constructed fusion proteins in which the catalytic domain of rat acetylcholinesterase was placed downstream of the CutA variants. The acquisition of an active conformation and N-glycosylation of the fusion proteins proved their transfer into the secretory pathway. We show that the CutA-AChE fusion proteins produced and secreted active, N-glycosylated molecules, while an AChE mutant lacking its secretory signal peptide did not produce any significant activity. Thus, an N-terminal CutA domain actually drives AChE into the endoplasmic reticulum and allows its secretion. This was observed with full length CutA, starting at Met1, and at a much lower level with the shorter mutants starting at Met24 and Met44, although the latter is not predicted to possess any signal peptide. These experiments illustrate the value of using AChE as a reporter and reveals an unusual protein trafficking and secretory process.

Targeting of acetylcholinesterase in neurons in vivo: a dual processing function for the proline-rich membrane anchor subunit and the attachment domain on the catalytic subunit

Acetylcholinesterase (AChE) accumulates on axonal varicosities and is primarily found as tetramers associated with a proline-rich membrane anchor (PRiMA). PRiMA is a small transmembrane protein that efficiently transforms secreted AChE to an enzyme anchored on the outer cell surface. Surprisingly, in the striatum of the PRiMA knock-out mouse, despite a normal level of AChE mRNA, we find only 2-3% of wild type AChE activity, with the residual AChE localized in the endoplasmic reticulum, demonstrating that PRiMA in vivo is necessary for intracellular processing of AChE in neurons. Moreover, deletion of the retention signal of the AChE catalytic subunit in mice, which is the domain of interaction with PRiMA, does not restore AChE activity in the striatum, establishing that PRiMA is necessary to target and/or to stabilize nascent AChE in neurons. These unexpected findings open new avenues to modulating AChE activity and its distribution in CNS disorders.

Respective roles of the catalytic domains and C-terminal tail peptides in the oligomerization and secretory trafficking of human acetylcholinesterase and butyrylcholinesterase

Butyrylcholinesterase (BChE) and the T splice variant of acetylcholinesterase that is predominant in mammalian brain and muscles (AChE(T)) possess a characteristic C-terminal tail (t) peptide. This t peptide allows their assembly into tetramers associated with the anchoring proteins ColQ and PRiMA. Although the t peptides of all vertebrate cholinesterases are remarkably similar and, in particular, contain seven strictly conserved aromatic residues, these enzymes differ in some of their oligomerization properties. To explore these differences, we studied human AChE (Aa) and BChE (Bb), and chimeras in which the t peptides (a and b) were exchanged (Ab and Ba). We found that secretion was increased by deletion of the t peptides, and that it was more efficient with a than with b. The patterns of oligomers were similar for Aa and Ab, as well as for Ba and Bb, indicating a predominant influence of the catalytic domains. However, addition of a cysteine within the aromatic-rich segment of the t peptides modified the oligomeric patterns: with a cysteine at position 19, the proportion of tetramers was markedly increased for Aa(S19C) and Ba(S19C), and to a lesser extent for Bb(N19C); the Ab(N19C) mutant produced all oligomeric forms, from monomers to hexamers. These results indicate that both the catalytic domains and the C-terminal t peptides contribute to the capacity of cholinesterases to form and secrete various oligomers. Sequence comparisons show that the differences between the t peptides of AChE and BChE are remarkably conserved among all vertebrates, suggesting that they reflect distinct functional adaptations.

Protein CutA undergoes an unusual transfer into the secretory pathway and affects the folding, oligomerization, and secretion of acetylcholinesterase

The mammalian protein CutA was first discovered in a search for the membrane anchor of mammalian brain acetylcholinesterase (AChE). It was co-purified with AChE, but it is distinct from the real transmembrane anchor protein, PRiMA. CutA is a ubiquitous trimeric protein, homologous to the bacterial CutA1 protein that belongs to an operon involved in resistance to divalent ions ("copper tolerance A"). The function of this protein in plants and animals is unknown, and several hypotheses concerning its subcellular localization have been proposed. We analyzed the expression and the subcellular localization of mouse CutA variants, starting at three in-frame ATG codons, in transfected COS cells. We show that CutA produces 20-kDa (H) and 15-kDa (L) components. The H component is transferred into the secretory pathway and secreted, without cleavage of a signal peptide, whereas the L component is mostly cytosolic. We show that expression of the longer CutA variant reduces the level of AChE, that this effect depends on the AChE C-terminal peptides, and probably results from misfolding. Surprisingly, CutA increased the secretion of a mutant possessing a KDEL motif at its C terminus; it also increased the formation of AChE homotetramers. We found no evidence for a direct interaction between CutA and AChE. The longer CutA variant seems to affect the processing and trafficking of secretory proteins, whereas the shorter one may have a distinct function in the cytoplasm.

Acetylcholinesterase associates differently with its anchoring proteins ColQ and PRiMA

Acetylcholinesterase tetramers are inserted in the basal lamina of neuromuscular junctions or anchored in cell membranes through the interaction of four C-terminal t peptides with proline-rich attachment domains (PRADs) of cholinesterase-associated collagen Q (ColQ) or of the transmembrane protein PRiMA (proline-rich membrane anchor). ColQ and PRiMA differ in the length of their proline-rich motifs (10 and 15 residues, respectively). ColQ has two cysteines upstream of the PRAD, which are disulfide-linked to two AChE(T) subunits ("heavy" dimer), and the other two subunits are disulfide-linked together ("light" dimer). In contrast, PRiMA has four cysteines upstream of the PRAD. We examined whether these cysteines could be linked to AChE(T) subunits in complexes formed with PRiMA in transfected COS cells and in the mammalian brain. For comparison, we studied complexes formed with N-terminal fragments of ColQ, N-terminal fragments of PRiMA, and chimeras in which the upstream regions containing the cysteines were exchanged. We also compared the effect of mutations in the t peptides on their association with the two PRADs. We report that the two PRADs differ in their interaction with AChE(T) subunits; in complexes formed with the PRAD of PRiMA, we observed light dimers, but very few heavy dimers, even though such dimers were formed with the PQ chimera in which the N-terminal region of PRiMA was associated with the PRAD of ColQ. Complexes with PQ or with PRiMA contained heavy components, which migrated abnormally in SDS-PAGE but probably resulted from disulfide bonding of four AChE(T) subunits with the four upstream cysteines of the associated protein.

Assembly and regulation of acetylcholinesterase at the vertebrate neuromuscular junction

The collagen-tailed form of acetylcholinesterase (ColQ-AChE) is the major if not unique form of the enzyme associated with the neuromuscular junction (NMJ). This enzyme form consists of catalytic and non-catalytic subunits encoded by separate genes, assembled as three enzymatic tetramers attached to the three-stranded collagen-like tail (ColQ). This synaptic form of the enzyme is tightly attached to the basal lamina associated with the glycosaminoglycan perlecan. Fasciculin-2 is a snake toxin that binds tightly to AChE. Localization of junctional AChE on frozen sections of muscle with fluorescent Fasciculin-2 shows that the labeled toxin dissociates with a half-life of about 36 h. The fluorescent toxin can subsequently be taken up by the muscle fibers by endocytosis giving the appearance of enzyme recycling. Newly synthesized AChE molecules undergo a lengthy series of processing events before final transport to the cell surface and association with the synaptic basal lamina. Following co-translational glycosylation the catalytic subunit polypeptide chain interacts with several molecular chaperones, glycosidases and glycosyltransferases to produce a catalytically active enzyme that can subsequently bind to one of two non-catalytic subunits. These molecular chaperones can be rate limiting steps in the assembly process. Treatment of muscle cells with a synthetic peptide containing the PRAD attachment sequence and a KDEL retention signal results in a large increase in assembled and exportable AChE, providing an additional level of post-translational control. Finally, we have found that Pumilio2, a member of the PUF family of RNA-binding proteins, is highly concentrated at the vertebrate neuromuscular junction where it plays an important role in regulating AChE translation through binding to a highly conserved NANOS response element in the 3'-UTR. Together, these studies define several new levels of AChE regulation in electrically excitable cells.

Two new mutations of the human BCHE gene (IVS3-14T>C and L574fsX576)

The genetic variation of human butyrylcholinesterase is associated with the majority of prolonged cases of apnea in patients submitted to the muscle relaxant succinylcholine. The present study reports two new mutations of the BCHE gene in 346 Euro-Brazilians: IVS3-14T>C found in five heterozygotes (allele frequency: 0.72+/-0.32%) and L574fsX576 found in one heterozygote (allele frequency: 0.14+/-0.14%). These two variants were not found in 85 Guarani Amerindians. It is not expected that the IVS3-14T>C mutation may interfere in the splicing process and that the mutation found in exon 4 (L574fsX576) may disturb BChE tetramerization and activity.

Identification of cis-acting elements involved in acetylcholinesterase RNA alternative splicing

The 3' end of Acetylcholinesterase (AChE) pre-mRNA is processed by a complex mechanism of alternative splicing. Three different transcripts are generated and called R, H and T according respectively to the intron (intron 4') or exons (5 or 6) retained in the mature RNA. The relative expression of the specific transcripts depends on cell type, developmental stage or pathophysiological conditions. The aim of our study was to identify sequences involved in AChE pre-mRNA splicing choices. For this purpose, we constructed a minigene in which the constitutive exons were fused and followed by the entire alternative domain without 3' UTR. We transfected the wild-type or minigene mutated in the alternative domain in muscle or COS-7 cells and identified the splicing products by RPA, RT-PCR and sedimentation coefficients of the enzymatic molecular forms. We find that the alternative splicing domain contains most of the necessary signals to control splicing choices in skeletal muscle cells with the coding sequences of the domain having little effect on the splicing outcome. A branch point at an unusual location 278 nt from the 3' acceptor site of exon 6 is characterized. We further identify several regulatory sequences in the non-coding sequence of exon 5 that regulate the splicing pattern. Sequences that control the splice to exon 5 and those which influence intron 4' retention or splicing to exon 6 appear to be distinct.

The C-terminal T peptide of cholinesterases: structure, interactions, and influence on protein folding and secretion

Mammalian cholinergic tissues mostly express the T splice variant of acetylcholinesterase, in which the catalytic domain is associated with a C-terminal peptide of 40 residues, called the t peptide (Massoulié, 2002). Homologous t peptides exist in all vertebrate cholinesterases, acetylcholinesterases (AChEs), and butyrylcholinesterases (BChEs): they contain a series of seven conserved aromatic residues, including three tryptophans, and a cysteine at position-4 of their C-terminus. The major AChE isozyme of the nematode Caenorhabditis elegans also contains a similar peptide. Although the C-terminal t peptides do not seem to affect the catalytic activity of cholinesterases, they determine their physiological function, because they allow cholinesterase subunits of type T to form oligomers and to associate with structural anchoring proteins. When reduced to their catalytic domain, AChE subunits without a t peptide are active but remain monomeric and soluble.

Assembly of acetylcholinesterase tetramers by peptidic motifs from the proline-rich membrane anchor, PRiMA: competition between degradation and secretion pathways of heteromeric complexes

The membrane-bound form of acetylcholinesterase (AChE) constitutes the major component of this enzyme in the mammalian brain. These molecules are hetero-oligomers, composed of four AChE catalytic subunits of type T (AChE(T)), associated with a transmembrane protein of type 1, called PRiMA (proline-rich membrane anchor). PRiMA consists of a signal peptide, an extracellular domain that contains a proline-rich motif (14 prolines with an intervening leucine, P4LP10), a transmembrane domain, and a cytoplasmic domain. Expression of AChE(T) subunits in transfected COS cells with a truncated PRiMA, without its transmembrane and cytoplasmic domains (P(stp54) mutant), produced secreted heteromeric complexes (T4-P(stp54)), instead of membrane-bound tetramers. In this study, we used a series of deletions and point mutations to analyze the interaction between the extracellular domain of PRiMA and AChE(T) subunits. We confirmed the importance of the polyproline stretches and defined a peptidic motif (RP4LP10RL), which induces the assembly and secretion of a heteromeric complex with four AChE(T) subunits, nearly as efficiently as the entire extracellular domain of PRiMA. It is noteworthy that deletion of the N-terminal segment preceding the prolines had little effect. Interestingly, short PRiMA mutants, truncated within the proline-rich motif, reduced both cellular and secreted AChE activity, suggesting that their interaction with AChE(T) subunits induces their intracellular degradation.

Polyethylene glycosylation prolongs the circulatory stability of recombinant human butyrylcholinesterase

Previous studies in rodents and non-human primates have demonstrated that pretreatment of animals with cholinesterases could provide significant protection against organophosphate (OP) nerve agent toxicity. Gene delivery/therapy is emerging as an approach to achieve high-level expression of proteins in vivo that are very similar to their native counterparts. Recently, adenoviral (Ad) vectors have proven to be excellent vehicles for delivering genes to cells in vitro and in vivo. In this study, we explored the use of the newly designed AdenoVATOR system for the expression of recombinant human butyrylcholinesterase (rHu BChE) in human embryonic kidney 293A (HEK-293A) cells. In these cells, rHu BChE was expressed as mostly tetrameric form by the simultaneous expression of proline-rich attachment domain. By optimizing the culture conditions, 1.5-2.0 U/ml of rHu BChE could be expressed in HEK-293A cells. Recombinant Hu BChE was purified to homogeneity by ammonium sulfate fractionation followed by affinity column chromatography using procainamide Sepharose and cobalt Sepharose gels. The enzymatic and physico-chemical properties of purified rHu BChE were similar to those of native serum-derived Hu BChE. To determine the suitability of this preparation for use as an antidote against highly toxic nerve agents, its pharmacokinetics were evaluated in mice. Recombinant Hu BChE exhibited a mean residence time of 18.3 h which was 2.5-fold shorter than that observed for native Hu BChE in mice. However, rHu BChE chemically modified with polyethyleneglycol (PEG) displayed a mean residence time of 36.2 h suggesting that PEG-modification can prolong the circulatory stability of rHu BChE. The efficacy of Ad-Hu BChE to induce the production of therapeutic levels of bioscavenger in vivo is under evaluation.

Determinants of the t Peptide Involved in Folding, Degradation, and Secretion of Acetylcholinesterase

The C-terminal 40-residue t peptide of acetylcholinesterase (AChE) forms an amphiphilic alpha helix with a cluster of seven aromatic residues. It allows oligomerization and induces a partial degradation of AChE subunits through the endoplasmic reticulum-associated degradation pathway. We show that the t peptide induces the misfolding of a fraction of AChE subunits, even when mutations disorganized the cluster of aromatic residues or when these residues were replaced by leucines, indicating that this effect is due to hydrophobic residues. Mutations in the aromatic-rich region affected the cellular fate of AChE in a similar manner, with or without mutations that prevented dimerization. Degradation was decreased and secretion was increased when aromatic residues were replaced by leucines, and the opposite occurred when the amphiphilic alpha helix was disorganized. The last two residues (Asp-Leu) somewhat resembled an endoplasmic reticulum retention signal and caused a partial retention but only in mutants possessing aromatic residues in their t peptide. Our results suggested that several "signals" in the catalytic domain and in the t peptide act cooperatively for AChE quality control.

Structural and functional organization of synaptic acetylcholinesterase

The expression of the synaptic asymmetric form of the enzyme acetylcholinesterase (AChE) depends of two different genes: the gene that encodes for the catalytic subunit and the gene that encodes for the collagenic tail, ColQ. Asymmetric AChE is specifically localized to the basal lamina at the neuromuscular junction (NMJ). This highly organized distribution pattern suggests the existence of one or more specific binding sites in ColQ required for its anchorage to the synaptic basal lamina. Recent evidence support this notion: first, the presence of two heparin-binding domains in ColQ that interact with heparan sulfate proteoglycans (HSPGs) at the synaptic basal lamina; and second, a knockout mouse for perlecan, a HSPG concentrated in nerve-muscle contact, in which absence of asymmetric AChE at the NMJ is observed. The physiological importance of collagen-tailed AChE form in skeletal muscle has been illustrated by the identification of several mutations in the ColQ gene. These mutations determine end-plate acetylcholinesterase deficiency and induce one type of synaptic functional disorders observed in Congenital Myasthenic Syndromes (CMSs).

The synaptic acetylcholinesterase tetramer assembles around a polyproline II helix

Functional localization of acetylcholinesterase (AChE) in vertebrate muscle and brain depends on interaction of the tryptophan amphiphilic tetramerization (WAT) sequence, at the C-terminus of its major splice variant (T), with a proline-rich attachment domain (PRAD), of the anchoring proteins, collagenous (ColQ) and proline-rich membrane anchor. The crystal structure of the WAT/PRAD complex reveals a novel supercoil structure in which four parallel WAT chains form a left-handed superhelix around an antiparallel left-handed PRAD helix resembling polyproline II. The WAT coiled coils possess a WWW motif making repetitive hydrophobic stacking and hydrogen-bond interactions with the PRAD. The WAT chains are related by an approximately 4-fold screw axis around the PRAD. Each WAT makes similar but unique interactions, consistent with an asymmetric pattern of disulfide linkages between the AChE tetramer subunits and ColQ. The P59Q mutation in ColQ, which causes congenital endplate AChE deficiency, and is located within the PRAD, disrupts crucial WAT-WAT and WAT-PRAD interactions. A model is proposed for the synaptic AChE(T) tetramer.

Screening assays for cholinesterases resistant to inhibition by organophosphorus toxicants

Methods to measure resistance to inhibition by organophosphorus toxicants (OP) for mutants of butyrylcholinesterase (EC 3.1.1.8; BChE) and acetylcholinesterase (EC 3.1.1.7; AChE) enzymes were devised. Wild-type cholinesterases were completely inhibited by 0.1 mM echothiophate or 0.001 mM diisopropylfluorophosphate, but human BChE mutants G117H, G117D, L286H, and W231H and snake AChE mutant HFQT retained activity. Tissues containing a mixture of cholinesterases could be assayed for amount of G117H BChE. For example, the serum of transgenic mice expressing human G117H BChE contained 0.5 microg/ml human G117H BChE, 2 microg/ml wild-type mouse BChE, and 0.06 microg/ml wild-type mouse AChE. The oligomeric structure of G117H BChE in the serum of transgenic mice was determined by nondenaturing gel electrophoresis followed by staining for butyrylthiocholine hydrolysis activity in the presence of 0.1 mM echothiophate. Greater than 95% of the human G117H BChE in transgenic mouse serum was a tetramer. To visualize the distribution of G117H BChE in tissues of transgenic mice, sections of small intestine were treated with echothiophate and then stained for BChE activity. Both wild-type and G117H BChE were in the epithelial cells of the villi. These assays can be used to identify OP-resistant cholinesterases in culture medium and in animal tissues.

Elements of the C-terminal t peptide of acetylcholinesterase that determine amphiphilicity, homomeric and heteromeric associations, secretion and degradation

The C-terminal t peptide (40 residues) of vertebrate acetylcholinesterase (AChE) T subunits possesses a series of seven conserved aromatic residues and forms an amphiphilic alpha-helix; it allows the formation of homo-oligomers (monomers, dimers and tetramers) and heteromeric associations with the anchoring proteins, ColQ and PRiMA, which contain a proline-rich motif (PRAD). We analyzed the influence of mutations in the t peptide of Torpedo AChE(T) on oligomerization and secretion. Charged residues influenced the distribution of homo-oligomers but had little effect on the heteromeric association with Q(N), a PRAD-containing N-terminal fragment of ColQ. The formation of homo-tetramers and Q(N)-linked tetramers required a central core of four aromatic residues and a peptide segment extending to residue 31; the last nine residues (32-40) were not necessary, although the formation of disulfide bonds by cysteine C37 stabilized T(4) and T(4)-Q(N) tetramers. The last two residues of the t peptide (EL) induced a partial intracellular retention; replacement of the C-terminal CAEL tetrapeptide by KDEL did not prevent tetramerization and heteromeric association with Q(N), indicating that these associations take place in the endoplasmic reticulum. Mutations that disorganize the alpha-helical structure of the t peptide were found to enhance degradation. Co-expression with Q(N) generally increased secretion, mostly as T(4)-Q(N) complexes, but reduced it for some mutants. Thus, mutations in this small, autonomous interaction domain bring information on the features that determine oligomeric associations of AChE(T) subunits and the choice between secretion and degradation.

Expression of PRiMA in the mouse brain: membrane anchoring and accumulation of 'tailed' acetylcholinesterase

We analysed the expression of PRiMA (proline-rich membrane anchor), the membrane anchor of acetylcholinesterase (AChE), by in situ hybridization in the mouse brain. We compared the pattern of PRiMA transcripts with that of AChE transcripts, as well as those of choline acetyltransferase and M1 muscarinic receptors which are considered pre- and postsynaptic cholinergic markers. We also analysed cholinesterase activity and its molecular forms in several brain structures. The results suggest that PRiMA expression is predominantly or exclusively related to the cholinergic system and that anchoring of cholinesterases to cell membranes by PRiMA represents a limiting factor for production of the AChE tailed splice variant (AChET)-PRiMA complex, which represents the major AChE component in the brain. This enzyme species is mostly associated with cholinergic neurons because the pattern of PRiMA mRNA expression largely coincides with that of ChAT. We also show that, in both mouse and human, PRiMA proteins exist as two alternative splice variants which differ in their cytoplasmic regions.

The C-terminal t peptide of acetylcholinesterase forms an α helix that supports homomeric and heteromeric interactions

Acetylcholinesterase subunits of type T (AChET) possess an alternatively spliced C-terminal peptide (t peptide) which endows them with amphiphilic properties, the capacity to form various homo-oligomers and to associate, as a tetramer, with anchoring proteins containing a proline rich attachment domain (PRAD). The t peptide contains seven conserved aromatic residues. By spectroscopic analyses of the synthetic peptides covering part or all of the t peptide of Torpedo AChET, we show that the region containing the aromatic residues adopts an alpha helical structure, which is favored in the presence of lipids and detergent micelles: these residues therefore form a hydrophobic cluster in a sector of the helix. We also analyzed the formation of disulfide bonds between two different AChET subunits, and between AChET subunits and a PRAD-containing protein [the N-terminal fragment of the ColQ protein (QN)] possessing two cysteines upstream or downstream of the PRAD. This shows that, in the complex formed by four T subunits with QN (T4-QN), the t peptides are not folded on themselves as hairpins but instead are all oriented in the same direction, antiparallel to that of the PRAD. The formation of disulfide bonds between various pairs of cysteines, introduced by mutagenesis at various positions in the t peptides, indicates that this complex possesses a surprising flexibility.

Trimerization domain of the collagen tail of acetylcholinesterase

In the collagen-tailed forms of cholinesterases, each subunit of a specific triple helical collagen, ColQ, may be attached through a proline-rich domain (PRAD) situated in its N-terminal noncollagenous region, to tetramers of acetylcholinesterase (AChE) or butyrylcholinesterase (BChE). This heteromeric assembly ensures the functional anchoring of AChE in extracellulare matrices, for example, at the neuromuscular junction. In this study, we analyzed the influence of deletions in the noncollagenous C-terminal region of ColQ on its capacity to form a triple helix. We show that an 80-residue segment located downstream of the collagenous regions contains the trimerization domain, that it can form trimers without the collagenous regions, and that a pair of cysteines located at the N-boundary of this domain facilitates oligomerization, although it is not absolutely required. We further show that AChE subunits can associate with nonhelical collagen ColQ monomers, forming ColQ-associated tetramers (G4-Q), which are secreted or are anchored at the cell surface when the C-terminal domain of ColQ is replaced by a GPI-addition signal.

Transcriptional regulation of gene expression by the coding sequence: An attempt to enhance expression of human AChE

In a previous report, Morel and Massoulié showed that Bungarus AChE (bBAChE) is produced more efficiently than rat AChE in various expression systems, mainly because the Bungarus coding sequence exerts a stimulatory effect on transcription (Morel and Massoulié, 2000). They reported that a 5' Bungarus fragment could partially transfer this property to a CAT expression vector. This appeared to offer the possibility of increasing the production of recombinant proteins. In the present paper, we show that insertion of this fragment in the transcribed region, before the polyadenylation site, may have either stimulatory or inhibitory effects, depending on the vector and on the reporter gene. Since the stimulatory effect of Bungarus coding region could not be attached to a small number of discrete motifs, we reasoned that it might result from a general feature of the sequence. Therefore it might be possible to partially transfer this property to the very homologous human AChE (hHAChE) coding sequence by modifications based on synonymous codons, which increased nucleotide identity between the 5' fragment (721 nucleotides) of bBAChE and hHAChE from 71% to 85%. The production of human AChE in transfected COS cells was increased nearly 2-fold with this modified construct, but still remained about 4-fold smaller than that of Bungarus AChE. There was no change in expression level in transformed Pichia pastoris. We thus confirm that coding sequences can strongly influence gene expression, but in a manner that depends on the context and cannot yet be predicted.

Overloading and removal of N-glycosylation targets on human acetylcholinesterase: effects on glycan composition and circulatory residence time

Optimization of post-translational modifications was shown to affect the ability of recombinant human acetylcholinesterase (rHuAChE) produced in HEK-293 cells to be retained in the circulation for prolonged periods of time [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]. To evaluate the possible contribution of the number of appended N-glycans in determining the pharmacokinetic behaviour of AChE, a series of sixteen recombinant human AChE glycoforms, differing in their number of appended N-glycans (2, 3, 4 or 5 glycans), state of assembly (dimeric or tetrameric) and terminal glycan sialylation (partially or fully sialylated) were generated. Extensive structural analysis of N-glycans demonstrated that the various glycan types associated with all the different rHuAChE glycoforms are essentially similar both in structure and abundance, and that production of the various glycoforms in the sialyltransferase-overexpressing 293ST-2D6 cell line resulted in the generation of enzyme species that carry glycans sialylated to the same extent. Pharmacokinetic profiling of the rHuAChE glycoforms in their fully tetramerized and sialylated state clearly demonstrated that circulatory longevity correlated directly with the number of attached N-glycans (mean residence times for rHuAChE glycoforms harbouring 2, 3, and 4 glycans=200, 740, and 1055 min respectively). This study provides evidence that glycan loading, together with N-glycan terminal processing and enzyme subunit oligomerization, operate in a hierarchical and concerted manner in determining the pharmacokinetic characteristics of AChE.

PRiMA: the membrane anchor of acetylcholinesterase in the brain

As a tetramer, acetylcholinesterase (AChE) is anchored to the basal lamina of the neuromuscular junction and to the membrane of neuronal synapses. We have previously shown that collagen Q (ColQ) anchors AChE at the neuromuscular junction. We have now cloned the gene PRiMA (proline-rich membrane anchor) encoding the AChE anchor in mammalian brain. We show that PRiMA is able to organize AChE into tetramers and to anchor them at the surface of transfected cells. Furthermore, we demonstrate that AChE is actually anchored in neural cell membranes through its interaction with PRiMA. Finally, we propose that only PRiMA anchors AChE in mammalian brain and muscle cell membranes.

Acetylcholinesterase H and T dimers are associated through the same contact. Mutations at this interface interfere with the C-terminal T peptide, inducing degradation rather than secretion

Acetylcholinesterase (AChE) exists as AChE(H) and AChE(T) subunits, which differ by their C-terminal H or T peptides, generating glycophosphatidylinositol-anchored dimers and various oligomers, respectively. We introduced mutations in the four-helix bundle interface of glycophosphatidylinositol-anchored dimers, and analyzed their effect on the production and oligomerization of AChE(H), of AChE(T), and of truncated subunits, AChE(C) (without H or T peptide). Dimerization was reduced for all types of subunits, showing that they interact through the same contact zone; the formation of amphiphilic tetramers (Torpedo AChE(T)) and 13.5 S oligomers (rat AChE(T)) was also suppressed. Oligomerization appeared totally blocked by introduction of an N-linked glycan on the surface of helix alpha(7,8). Other point mutations did not affect the synthesis or the catalytic properties of AChE but reduced or blocked the secretion of AChE(T) subunits. Secretion of AChE(T) was partially restored by co-expression with Q(N), a secretable protein containing a proline-rich attachment domain (PRAD); Q(N) organized PRAD-linked tetramers, except for the N-glycosylated mutants. Thus, the simultaneous presence of an abnormal four-helix bundle zone and an exposed T peptide targeted the enzyme toward degradation, indicating a cross-talk between the catalytic and tetramerization domains.

Addition of a glycophosphatidylinositol to acetylcholinesterase. Processing, degradation, and secretion

We introduced various mutations and modifications in the GPI anchoring signal of rat acetylcholinesterase (AChE). 1) The resulting mutants, expressed in transiently transfected COS cells, were initially produced at the same rate, in an active form, but the fraction of GPI-anchored AChE and the steady state level of AChE activity varied over a wide range. 2) Productive interaction with the GPI addition machinery led to GPI anchoring, secretion of uncleaved protein, and secretion of a cleaved protein, in variable proportions. Unproductive interaction led to degradation; poorly processed molecules were degraded rather than retained intracellularly or secreted. 3) An efficient glypiation appeared necessary but not sufficient for a high level of secretion; the cleaved, secreted protein was possibly generated as a by-product of transamidation. 4) Glypiation was influenced by a wider context than the triplet omega/omega + 1/omega + 2, particularly omega - 1. 5) Glypiation was not affected by the closeness of the omega site to the alpha(10) helix of the catalytic domain. 6) A cysteine could simultaneously form a disulfide bond and serve as an omega site; however, there was a mutual interference between glypiation and the formation of an intercatenary disulfide bond, at a short distance upstream of omega. 7) Glypiation was not affected by the presence of an N-glycosylation site at omega or in its vicinity or by the addition of a short hydrophilic, highly charged peptide (FLAG; DYKDDDDK) at the C terminus of the hydrophobic region.

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.

Two distinct proteins are associated with tetrameric acetylcholinesterase on the cell surface

In mammalian brain, acetylcholinesterase (AChE) exists mostly as a tetramer of 70-kDa catalytic subunits that are linked through disulfide bonds to a hydrophobic subunit P of approximately 20 kDa. To characterize P, we reduced the disulfide bonds in purified bovine brain AChE and sequenced tryptic fragments from bands in the 20-kDa region. We obtained sequences belonging to at least two distinct proteins: the P protein and another protein that was not disulfide-linked to catalytic subunits. Both proteins were recognized in Western blots by antisera raised against specific peptides. We cloned cDNA encoding the second protein in a cDNA library from bovine substantia nigra and obtained rat and human homologs. We call this protein mCutA because of its homology to a bacterial protein (CutA). We could not demonstrate a direct interaction between mCutA and AChE in vitro in transfected cells. However, in a mouse neuroblastoma cell line that produced membrane-bound AChE as an amphiphilic tetramer, the expression of mCutA antisense mRNA eliminated cell surface AChE and decreased the level of amphiphilic tetramer in cell extracts. mCutA therefore appears necessary for the localization of AChE at the cell surface; it may be part of a multicomponent complex that anchors AChE in membranes, together with the hydrophobic P protein.

Hierarchy of post-translational modifications involved in the circulatory longevity of glycoproteins. Demonstration of concerted contributions of glycan sialylation and subunit assembly to the pharmacokinetic behavior of bovine acetylcholinesterase

The tetrameric form of native serum-derived bovine acetylcholinesterase is retained in the circulation for much longer periods (mean residence time, MRT = 1390 min) than recombinant bovine acetylcholinesterase (rBoAChE) produced in the HEK-293 cell system (MRT = 57 min). Extensive matrix-assisted laser desorption ionization-time of flight analyses established that the basic structures of the N-glycans associated with the native and recombinant enzymes are similar (the major species (50-60%) are of the biantennary fucosylated type and 20-30% are of the triantennary type), yet the glycan termini of the native enzyme are mostly capped with sialic acid (82%) and alpha-galactose (12%), whereas glycans of the recombinant enzyme exhibit a high level of exposed beta-galactose residues (50%) and a lack of alpha-galactose. Glycan termini of both fetal bovine serum and rBoAChE were altered in vitro using exoglycosidases and sialyltransferase or in vivo by a HEK-293 cell line developed specifically to allow efficient sialic acid capping of beta-galactose-exposed termini. In addition, the dimeric and monomeric forms of rBoAChE were quantitatively converted to tetramers by complexation with a synthetic peptide representing the human ColQ-derived proline-rich attachment domain. Thus by controlling both the level and nature of N-glycan capping and subunit assembly, we generated and characterized 9 distinct bovine AChE glycoforms displaying a 400-fold difference in their circulatory lifetimes (MRT = 3.5-1390 min). This revealed some general rules and a hierarchy of post-translation factors determining the circulatory profile of glycoproteins. Accordingly, an rBoAChE was generated that displayed a circulatory profile indistinguishable from the native form.

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.

Differences in expression of acetylcholinesterase and collagen Q control the distribution and oligomerization of the collagen-tailed forms in fast and slow muscles

The collagen-tailed forms of acetylcholinesterase (AChE) are accumulated at mammalian neuromuscular junctions. The A(4), A(8), and A(12) forms are expressed differently in the rat fast and slow muscles; the sternomastoid muscle contains essentially the A(12) form at end plates, whereas the soleus muscle also contains extrajunctional A(4) and A(8) forms. We show that collagen Q (ColQ) transcripts become exclusively junctional in the adult sternomastoid but remain uniformly expressed in the soleus. By coinjecting Xenopus oocytes with AChE(T) and ColQ mRNAs, we reproduced the muscle patterns of collagen-tailed forms. The soleus contains transcripts ColQ1 and ColQ1a, whereas the sternomastoid only contains ColQ1a. Collagen-tailed AChE represents the first evidence that synaptic components involved in cholinergic transmission may be differently regulated in fast and slow muscles.