Production and secretion of high levels of recombinant human acetylcholinesterase in cultured cell lines: microheterogeneity of the catalytic subunit

Structure and function in rhodopsin: high level expression of a synthetic bovine opsin gene and its mutants in stable mammalian cell lines

Stable mammalian cell lines harboring a synthetic bovine opsin gene have been derived from the suspension-adapted HEK293 cell line. The opsin gene is under the control of the immediate-early cytomegalovirus promoter/enhancer in an expression vector that also contains a selectable marker (Neo) governed by a relatively weak promoter. The cell lines expressing the opsin gene at high levels are selected by growth in the presence of high concentrations of the antibiotic geneticin. Under the conditions used for cell growth in suspension, opsin is produced at saturated culture levels of more than 2 mg/liter. After reconstitution with 11-cis-retinal, rhodopsin is purified to homogeneity in a single step by immunoaffinity column chromatography. Rhodopsin thus prepared (> 90% recovery at concentrations of up to 15 microM) is indistinguishable from rhodopsin purified from bovine rod outer segments by the following criteria: (i) UV/Vis absorption spectra in the dark and after photobleaching and the rate of metarhodopsin II decay, (ii) initial rates of transducin activation, and (iii) the rate of phosphorylation by rhodopsin kinase. Although mammalian cell opsin migrates slower than rod outer segment opsin on SDS/polyacrylamide gels, presumably due to a different N-glycosylation pattern, their mobilities after deglycosylation are identical. This method has enabled the preparation of several site-specific mutants of bovine opsin in comparable amounts.

Structural modifications of the omega loop in human acetylcholinesterase

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

Aging of phosphylated human acetylcholinesterase: catalytic processes mediated by aromatic and polar residues of the active centre

We have examined the effects of 11 substitutions of active centre gorge residues of human acetylcholinesterase (HuAChE) on the rates of phosphonylation by 1,2,2-trimethylpropyl methyl-phosphonofluoridate (soman) and the aging of the resulting conjugates. The rates of phosphonylation were reduced to as little as one-seventieth, mainly in mutants of the hydrogen-bond network (Glu-202, Glu-450, Tyr-133). These recombinant enzymes as well as the F338A, W86A, W86F and D74N mutant HuAChEs varied in their resistance to aging (15-3300-fold relative to the wild type). The most dramatic resistance to aging was observed for the phosphonyl conjugate of the mutant W86A enzyme (1850-3300-fold relative to the wild type). It is proposed that Trp-86 contributes to the aging process by stabilizing the evolving carbonium ion on the 1,2,2-trimethylpropyl moiety, via charge-pi interaction. The rate-enhancing effect of Trp-86 provides a rationale for the unique facility of aging in soman-inhibited cholinesterases, compared with the corresponding conjugates in other serine hydrolases. Replacements of Glu-202 by aspartic acid, glutamine or alanine residues resulted in a similar (1/130-1/300) decrease of the rates of aging. A comparable decrease was also observed for the conjugate of the F338A mutant. These results, and the similar pH dependence of aging rates for the wild-type and E202Q and F338A mutant HuAChEs, indicate that Glu-202 is not involved in proton transfer to the phosphonyl moiety. On the basis of these findings and of molecular modelling we suggest that Glu-202 and Phe-338 contribute to the aging process by stabilizing the imidazolium of the catalytic triad His-447 via charge-charge and charge-pi interactions respectively, thereby facilitating an oxonium formation on the phosphonyl moiety.

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

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

Post-translational and activation-dependent modifications of the G protein-coupled thrombin receptor

The purpose of the present study was to analyze the post-translational and activation-dependent modifications of the G protein-coupled thrombin receptor. A human receptor cDNA was engineered to encode an epitope tag derived from the vesicular stomatitis virus glycoprotein at the COOH terminus of the receptor and expressed in human embryonic kidney 293 cells. We show here that the mature receptor is a glycosylated protein with an apparent molecular mass ranging from 68 to 80 kDa by SDS-polyacrylamide gel electrophoresis. Removal of asparagine-linked oligosaccharides with N-glycosidase F leads to the appearance of a 36-40-kDa receptor species. The current model for receptor activation by thrombin involves specific hydrolysis of the arginine-41/serine-42 (Arg-41/Ser-42) peptide bond. Cleavage of the receptor by thrombin was demonstrated directly by Western analyses performed on membranes and glycoprotein-enriched lysates from transfected cells. Whereas thrombin treatment of cells results in increased mobility of the receptor in SDS-polyacrylamide gel electrophoresis, we found that their treatment with the thrombin receptor agonist peptide leads to a decrease in thrombin receptor mobility due, in part, to phosphorylation. The serine proteases trypsin and plasmin also cleave and activate the receptor similar to thrombin, whereas chymotrypsin cleaves the receptor at a site distal to Arg-41, thus rendering it unresponsive to thrombin while still responsive to thrombin receptor agonist peptide.

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

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

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

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

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

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

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

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

Engineering resistance to 'aging' of phosphylated human acetylcholinesterase. Role of hydrogen bond network in the active center

Recombinant human acetylcholinesterase (HuAChE) and selected mutants (E202Q, Y337A, E450A) were studied with respect to catalytic activity towards charged and noncharged substrates, phosphylation with organophosphorus (OP) inhibitors and subsequent aging of the OP-conjugates. Amino acid E450, unlike residues E202 and Y337, is not within interaction distance from the active center. Yet, the bimolecular rates of catalysis and phosphylation are 30-100 fold lower for both E450A and E202Q compared to Y337A or the wild type and in both mutants the resulting OP-conjugates show striking resistance to aging. It is proposed that a hydrogen bond network, that maintains the functional architecture of the active center, involving water molecules and residues E202 and E450, is responsible for the observed behaviour.

Evaluation of anchorage-dependent cell propagation systems for production of human acetylcholinesterase by recombinant 293 cells

Production of recombinant human acetylcholinesterase (AChE) by a high producer human embryonic kidney cell line (293) was evaluated by three main cell propagation systems; surface propagator, fixed-bed reactor and stirred microcarrier cultures. The recombinant cell line expresses AChE levels as high as 10-20 mg/l/day. System productivities in either the surface propagator (multitray system), or in the fixed-bed reactor (polyurethane macroporous sponges) were 4-8 mg AChE/l/day during a production period of 8 days. Similar productive rates, yet longer production periods (up to 22 days), were obtained in microcarrier (MC) cultures using either polystyrene beads (Biosilon); collagen-coated dextran beads (Cytodex-3); or gelatin macroporous beads (Cultispher-G). Best results were obtained in an aggregate culture using cellulose beads charged with diethylaminoethyl (DEAE) groups, (Servacel), as carriers. In this culture, a system productivity of 6-10 mg/l/day was maintained for 28 days.