Optimization of a suspension culture for a Theileria annulata-infected bovine cell line

Theileria annulata schizont transformed bovine lymphocytes show the feature of permanent proliferation in vitro culture. In this study, we optimized a suitable culture medium for transformed cells to ensure a high yield of quality cells in suspension culture. As the basis for the optimized medium, we combined 75% Gibco (GB) and 25% RPMI-1640 medium. Glucose, lactic acid, ammonia, growth factors and several kinds of amino acids at specific concentrations play important roles in maintaining the maximum growth rate and the quality of cells. The metabolic flow of 17 amino acids, glucose and nutrients was determined with high-performance liquid chromatography (HPLC) and cell viability analysis. The genetic stability of the TaSP and TaSE genes at different passages of the cell line in suspension culture was determined using PCR amplification. The optimal concentrations or tolerated levels of glucose, lactic acid and ammonia were 10-14, 2-5.5 and 3.5-5.5 mmol/L, respectively. Our data demonstrated that the potential utility of the medium optimized here to yield high quality cells compared with basal (normally used) medium. The medium also facilitated the easy maintenance of transformed cells with high yields and excellent quality for in vitro studies. This study also provides insight into the processes of optimization and vaccine development.

Expression of carbamoyl phosphate synthetase I and ornithine transcarbamoylase genes in Chinese hamster ovary dhfr-cells decreases accumulation of ammonium ion in culture media

Ammonium ion accumulation in mammalian cell culture media causes toxicity which inhibits cell growth and productivity. To reduce the level of the accumulated ammonium ion, carbamoyl phosphate synthetase I (CPS I) and ornithine transcarbamoylase (OTC) were used, which catalyze the first and second steps of the urea cycle in the liver. To examine the effects of overexpressed CPS I and OTC genes on the concentration of the ammonium ion in culture media, the two genes were introduced into Chinese hamster ovary (CHO) dhfr-cells. The CPS I expressing cell lines (CPS I-CHO) and both CPS I and OTC expressing cell lines (CPS I/OTC-CHO) were confirmed at the mRNA level and analyzed in terms of the cell growth and the accumulation of ammonium ion in culture media. The accumulation of ammonium ion was approximately 25-33% less in CPS I/OTC-CHO than in either CPS I-CHO or the vector-control cell lines. Interestingly however, the cell growth was approximately 15-30% faster in both CPS I-CHO and CPS I/OTC-CHO than in the control cell lines. Forced expression of urea cycle enzymes in the CHO cells revealed that both the expression of CPS I and OTC can reduce the accumulation of ammonium ion in the culture media.

Effects of ammonia on CHO cell growth, erythropoietin production, and glycosylation

The effect of ammonium chloride was determined on a culture of CHO cells transfected with the human erythropoietin (EPO) gene. Cell growth was inhibited above a culture concentration of 5 mM NH(4)Cl with an IC-50 determined to be 33 mM. The specific production of EPO increased with the addition of NH(4)Cl above 5 mM. At 10 mM NH(4)Cl, the final cell density after 4 days in culture was significantly lower but the final yield of EPO was significantly higher. This appeared to be due to continued protein production after cell growth had ceased. The metabolic effects of added NH(4)Cl included higher specific consumption rates of glucose and glutamine and an increased rate of production of alanine, glycine, and glutamate. The EPO analyzed from control cultures had a molecular weight range of 33-39 kDa and an isoelectric point range of 4.06-4.67. Seven distinct isoforms of the molecule were identified by two-dimensional electrophoresis. This molecular heterogeneity was ascribed to variable glycosylation. Complete enzymatic de-glycosylation resulted in a single molecular form with a molecular mass of 18 kDa. Addition of NH(4)Cl to the cultures caused a significant increase in the heterogeneity of the glycoforms as shown by an increased molecular weight and pI range. Enzymatic de-sialylation of the EPO from the ammonia-treated and control cultures resulted in identical electrophoretic patterns. This indicated that the effect of ammonia was in the reduction of terminal sialylation of the glycan structures which accounted for the increased pI. Selective removal of the N-glycan structures by PNGase F resulted in two bands identified as the O-glycan linked structure (19 kDa) and the completely de-glycosylated structure (18 kDa). The proportion of the O-linked glycan structure was reduced, and its pI increased in cultures to which ammonia was added. Thus, the glycosylation pattern altered by the presence of ammonia included a reduction in terminal sialylation of all the glycans and a reduction in the content of the O-linked glycan. The addition of a sialidase inhibitor to the cultures had no effect on the ammonia-induced increase in EPO heterogeneity. Also, the effect of ammonia on glycosylation could not be mimicked using the weak base chloroquine in our system.

Cell lines with reduced UDP-N-acetylhexosamine pool in the presence of ammonium

The glycosylation of pharmaglycoproteins from recombinant cell lines can be affected by an uncontrolled accumulation of ammonium in the medium. Glucosamine-6-phosphate isomerase (GPI) has been proposed as the key enzyme responsible for elevating the intracellular UDP-N-acetylhexosamine pool (UDPGNAc) by accepting ammonium from the medium of cultured mammalian cells. As previously reported, the increased UDPGNAc pool then affects the N-glycan complexity in glycoproteins. To understand the entry of extracellular ammonium into the cellular metabolism, GPI has been isolated to homogeneity from BHK-21 cells and characterized. Thus, the complete pathway by which ammonium enters the cellular metabolism was elucidated. To reduce the negative effects of ammonium, GPI was inhibited using two different strategies. First, the addition of mannose to the culture media and, second, antisense RNA expression. In both cases, the cellular UDPGNAc pool was suppressed in the presence of high ammonium concentrations in the medium. However, constant suppression of the UDPGNAc pool could not be achieved by antisense RNA expression because antisense clones were apparently unstable. Further studies showed that the main reason for instability was the inducibility of GPI by its substrate ammonium. GPI was induced to a factor of two under ammonium-containing medium conditions. We propose gene knockout technology for GPI repression to obtain cell lines consisting of an UDPGNAc pool unaffected by the presence of ammonium.

Intracellular pH responses of hybridoma and yeast to substrate addition and acid challenge

The pHi responses of hybridoma and yeast cells to substrate and external acid additions were measured using the fluorescent pHi indicator, 9-aminoacridine. The pHi change, following CCCP addition, indicated by 9AA, compared very well with that indicated by BCECF. No change in pHi was observed following glucose or glutamine additions to hybridoma cells under glucose- and glutamine-absent conditions. Also, no change in pHi was observed when glucose was added in the presence of low glutamine and when glutamine was added in the presence of low glucose. However, in the presence of amiloride, the pHi of hybridoma cells decreased following glucose addition. Intracellular pH responses of hybridoma and yeast cells to decreases in external pH, effected by acid addition, were dependent on the cellular energy state and acid type. Cells controlled their pHi more tightly under energy-poor conditions compared to energy-rich conditions, and sulfuric acid (strong acid) caused larger changes in pHi compared to pyruvic acid (weak acid). In response to acid addition under energy-rich conditions, the magnitudes of pHi increases in hybridoma were smaller in the presence of amiloride compared to that in the absence of amiloride. Further, pHi responses to a decrease in external pH were slower at submetabolic temperatures.

Mechanisms of ammonia and ammonium ion toxicity in animal cells: transport across cell membranes

A model for transport of ammonia and ammonium ions across cell membranes is presented. The model suggests that ammonium ions compete with potassium ions for inward transport, over the cytoplasmic membrane, via potassium transport proteins like the Na+/K(+)-ATPase and the Na+K+2Cl(-)-cotransporter. It also explains the difference between the ammonia/ammonium that is added to the cells and which is formed by the cells during metabolism of amino acids, especially glutamine and glutamate. The ammonium transport and subsequent events lead to predictable intracellular and extracellular pH (pHe) changes. Experiments which verified the model and the predicted consequences were performed by measurements of the pHe in concentrated cell suspensions. Addition of ammonium ions caused a time-dependent pHe increase which was inhibited by potassium ions. The test system is not per se specific for transport measurements but the effect of potassium ions on the pHe strongly favors our suggested model. Simple diffusion of ammonium ions would not be counteracted by potassium ions. The results show that ammonium ion transport in the murine myeloma cell line (Sp2/0-Ag14) used is inhibited by an excess of potassium ions. Results from experiments with specific inhibitors of suggested transport proteins were not conclusive. It is postulated that one important toxic effect of ammonia/ammonium is an increased demand for maintenance energy, caused by the need to maintain ion gradients over the cytoplasmic membrane. The results also suggest that potassium ions can be used to detoxify ammonia/ammonium in animal cell cultivations.

Evidence for posttranscriptional stimulation of monoclonal antibody secretion by l-glutamine during slow hybridoma growth

The addition of 5-40 mM L-glutamine to batch cultures of a murine hybridoma following the cessation of rapid growth significantly stimulated monoclonal antibody (mAb) synthesis and secretion per cell. Stimulation of mAb secretion following the cessation of rapid growth was also observed in response to addition of mitochondrial intermediates of glutamate oxidation and was not found to be the result of release of transiently stored mAb. Less than 1% of the secreted mAb was detected by ELISA in isolated hybridoma lysosomes. This stimulation was posttranscriptional and not the result of enhancement of levels of mAb mRNAs or stabilization of heavy (H) or light (L) chain encoding message. Sub-inhibitory levels of lysosomotrophic weak bases stimulated release of lysosomal contents but did not result in release of intact or partially degraded mAb. Inhibition of aspartic proteinase activity secreted by the hybridoma did not enhance mAb secretion even though a high level of mAb degrading proteinase activity was continuously secreted during both rapid and slow growth. These responses indicate that during slow growth, the addition of L-glutamine increases the availability of cellular ATP generated by mitochondrial respiration which stimulates some posttranscriptional step in the pathway of mAb secretion such as the rate of H or L chain translation, chain assembly, interorganelle transport or vesicular transport from the Golgi to the cell membrane.