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

Optimizing the transient transfection process of HEK-293 suspension cells for protein production by nucleotide ratio monitoring

Large scale, transient gene expression (TGE) is highly dependent of the physiological status of a cell line. Therefore, intracellular nucleotide pools and ratios were used for identifying and monitoring the optimal status of a suspension cell line used for TGE. The transfection efficiency upon polyethyleneimine (PEI)-mediated transient gene delivery into HEK-293 cells cultured in suspension was investigated to understand the effect of different culture and transfection conditions as well as the significance of the culture age and the quality of the cell line used. Based on two different bicistronic model plasmids expressing the human erythropoietin gene (rHuEPO) in the first position and green fluorescent protein as reporter gene in the second position and vice versa, a completely serum-free transient transfection process was established. The process makes use of a 1:1 mixture of a special calcium-free DMEM and the FreeStyle™ 293 Expression Medium. Maximum transfectability was achieved by adjusting the ratio for complex formation to one mass part of DNA and three parts of PEI corresponding to an N/P (nitrogen residues/DNA phosphates) ratio of 23 representing a minimum amount of DNA for the polycation-mediated gene delivery. Applying this method, maximum transfectabilities between 70 and 96 % and a rHuEPO concentration of 1.6 μg mL(-1) 72 h post transfection were reached, when rHuEPO gene was expressed from the first position of the bicistronic mRNA. This corresponded to 10 % of the total protein concentration in the cell-free supernatant of the cultures in protein-free medium. Up to 30 % higher transfectabilities were found for cells of early passages compared to those from late passages under protein-free culture conditions. In contrast, when the same cells were propagated in serum-containing medium, higher transfectabilities were found for late-passage cells, while up to 40 % lower transfectabilities were observed for early-passage cells. Nucleotide pools were measured during all cell cultivations and the nucleoside triphosphate/uridine ratios were calculated. These 'nucleotide ratios' changed in an age-dependent manner and could be used to distinguish early- from late-passage cells. The observed effects were also dependent on the presence of serum in the culture. Nucleotide ratios were shown being applied to investigate the optimal passage number of cultured cell lines for achieving a maximum productivity in cultures used for transient gene expression. Furthermore, these nucleotide ratios proved to be different for transfected and untransfected cells, providing a high potential tool to monitor the status of transfection under various culture conditions.

High cell density and productivity culture of Chinese hamster ovary cells in a fluidized bed bioreactor

A recombinant Chinese hamster ovary clone was cultivated in a 2L Cytopilot Mini fluidized bed bioreactor using Cytoline 1 microcarriers and a 10L B. Braun stirred tank bioreactor with Cytodex 1 microcarriers. Cytoline 1 is a macroporous polyethylene microcarrier and Cytodex 1 is a solid DEAE-dextran microcarrier. Cytoline 1 microcarriers in the fluidized bed bioreactor were gently mixed by an uplifting flow. Circulation and sparging in Cytopilot Mini were separated from the fluidized microcarrier bed. Cytopilot Mini bioreactor with Cytoline 1 microcarriers offered 2.3 times more surface area than the stirred tank bioreactor. The 2L fluidized bed bioreactor accommodated approximately half the cells in the 10L stirred tank bioreactor. Moreover, Cytopilot Mini had approximately three times more product output rate and 5.5 times higher specific productivity than the stirred tank bioreactor.

Packed-bed bioreactors for mammalian cell culture: bioprocess and biomedical applications

This article describes the development history of packed-bed bioreactors (PBRs) used for the culture of mammalian cells. It further reviews the current applications of PBRs and discusses the steps forward in the development of these systems for bioprocess and biomedical applications. The latest generation of PBRs used in bioprocess applications achieve very high cell densities (>10(8) cells ml(-1)) leading to outstandingly high volumetric productivity. However, a major bottleneck of such PBRs is their relatively small volume. The current maximal volume appears to be in the range of 10 to 30 l. A scale-up of more than 10-fold would be necessary for these PBRs to be used in production processes. In biomedical applications, PBRs have proved themselves as compact bioartificial organs, but their metabolic activity declines frequently within 1 to 2 weeks of operation. A main challenge in this field is to develop cell lines that grow consistently to high cell density in vitro and maintain a stable phenotype for a minimum of 1 to 2 months. Achieving this will greatly enhance the usefulness of PBR technology in clinical practice.

Amino acid domains control the circulatory residence time of primate acetylcholinesterases in rhesus macaques (Macaca mulatta)

An array of 13 biochemically well defined molecular forms of bovine, human and newly cloned rhesus macaque (Macaca mulatta) AChEs (acetylcholinesterases) differing in glycosylation and subunit assembly status were subjected to comparative pharmacokinetic studies in mice and rhesus macaques. The circulatory lifetimes of recombinant bovine, macaque and human AChEs in mice were governed by previously determined hierarchical rules; the longest circulatory residence time was obtained when AChE was fully sialylated and tetramerized [Kronman, Chitlaru, Elhanany, Velan and Shafferman (2000) J. Biol. Chem. 275, 29488-29502; Chitlaru, Kronman, Velan and Shafferman (2001) Biochem. J. 354, 613-625]. In rhesus macaques, bovine molecular forms still obeyed the same hierarchical rules, whereas primate AChEs showed significant deviation from this behaviour. Residence times of human and rhesus AChEs were effectively extended by extensive sialylation, but subunit tetramerization and N-glycan addition had a marginal effect on their circulatory longevity in macaques. It appears that the major factor responsible for the differential pharmacokinetics of bovine and primate AChEs in macaques is related to differences in primary structure, suggesting the existence of a specific mechanism for the circulatory clearance of primate AChEs in rhesus macaques. The 35 amino acids that differ between bovine and primate AChEs are clustered within three defined domains, all located at the enzyme surface, and may therefore mediate the facilitated removal of primate cholinesterases specifically from the circulation of monkeys. These surface domains can be effectively masked by poly(ethylene glycol) appendage, resulting in the generation of chemically modified human and macaque AChEs that reside in the circulation for extraordinarily long periods of time (mean residence time of 10000 min). This extended residence time is similar to that displayed by native macaque butyrylcholinesterase (9950 min), which is the prevalent cholinesterase form in the circulation of adult macaques.

Scale-up of a myoblast culture process

The effects of different types of cell carriers, strategies for cell transfer on carriers, and of several fusion inhibitors on the growth kinetics of primary human myoblasts culture were studied in order to develop a bioprocess suitable for the treatment of Duchenne muscular dystrophy based on the transplantation of unfused cells. Our results indicate that myoblast production is larger on Cytodex 1 and 3 than on polypropylene or polyester fabrics and on a commercial porous macrocarrier. Myoblast growth conditions with Cytodex 1 were further investigated to establish the bioprocess operating conditions. It was found that microcarrier density of 3 g DW l(-1), inoculum density of 2x10(5) cells ml(-1), and continuous agitation speed of 30-rpm result in final myoblast production comparable to static cultures. However, for all the culture conditions used, myoblasts growth kinetics exhibited a lag phase that lasted a minimum of 1 week prior to growth, the end of the lag phase correlating with the appearance of microcarrier aggregates. Based on this observation, we propose that aggregation promotes cell growth by offering a network of very large inter-particular pores that protect cells from mechanical stress. We took advantage of the presence of these aggregates for the scale-up of the culture process. Indeed, using myoblast-loaded microcarrier-aggregates instead of myoblast suspension to inoculate a fresh suspension of microcarriers significantly reduced the duration of the lag phase and allowed the scale-up of the bioprocess at the 500-ml scale. In order to ensure the production of unfused myoblasts, the efficiency of five different fusion inhibitors was investigated. Only calpeptin (9.1 microg ml(-1)) significantly inhibited the fusion of the myoblasts, while TGFbeta (50 ng ml(-1)) and LPA (10 microg ml(-1)) increased myoblasts growth but did not affect fusion, sphingosine (30 microg ml(-1)) induced a 50% death and NMMA (25 microg ml(-1)) had no effect on either growth or fusion. Finally, transplantation trials on severe combined immunodeficient mice showed that microcarrier-cultured human myoblasts grown using the optimized bioprocess resulted in grafts as successful as myoblasts grown in static cultures. The bioprocess, therefore, prove to be suitable for the large-scale production of myoblasts required for muscular dystrophy treatment.

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

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

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.

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.

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.