Heterologous expression as a tool for gene identification and analysis

Advances in Mammalian cell line development technologies for recombinant protein production

From 2006 to 2011, an average of 15 novel recombinant protein therapeutics have been approved by US Food and Drug Administration (FDA) annually. In addition, the expiration of blockbuster biologics has also spurred the emergence of biosimilars. The increasing numbers of innovator biologic products and biosimilars have thus fuelled the demand of production cell lines with high productivity. Currently, mammalian cell line development technologies used by most biopharmaceutical companies are based on either the methotrexate (MTX) amplification technology or the glutamine synthetase (GS) system. With both systems, the cell clones obtained are highly heterogeneous, as a result of random genome integration by the gene of interest and the gene amplification process. Consequently, large numbers of cell clones have to be screened to identify rare stable high producer cell clones. As such, the cell line development process typically requires 6 to 12 months and is a time, capital and labour intensive process. This article reviews established advances in protein expression and clone screening which are the core technologies in mammalian cell line development. Advancements in these component technologies are vital to improve the speed and efficiency of generating robust and highly productive cell line for large scale production of protein therapeutics.

Characterization of gene localization and accessibility in DHFR-amplified CHO cells

Efficient transcription is critical for high yields of recombinant proteins by mammalian cells. We previously reported that dihydrofolate reductase (DHFR)-mediated gene amplification can augment transcriptional rates as well as increasing gene copy numbers in Chinese hamster ovary (CHO) cells.1 In an attempt to elucidate the mechanisms involved, we have employed several approaches to identify the epigenetic differences between cell clones with varying transcriptional rates. Transgene placement and accessibility varies between unrelated parental cell clones with differential transcriptional rates. However, we did not observe any apparent epigenetic differences between parental clones and their amplified progeny, indicating undiscovered regulatory mechanisms are responsible for the augmentation of transcriptional rates upon DHFR-mediated amplification.

Stability of protein production from recombinant mammalian cells

One of the most important criteria for successful generation of a therapeutic protein from a recombinant cell is to obtain a cell line that maintains stability of production. If this is not achieved it can generate problems for process yields, effective use of time and money, and for regulatory approval of products. However, selection of a cell line that sustains stability of production over the required time period may be difficult to achieve during development of a therapeutic protein. There are several studies in the literature that have reported on the instability of protein production from recombinant cell lines. The causes of instability of production are varied and, in many cases, the exact molecular mechanisms are unknown. The production of proteins by cells is modulated by molecular events at levels ranging from transcription, posttranscriptional processing, translation, posttranslational processing, to secretion. There is potential for regulation of stability of protein production at many or all of these stages. In this study we review published information on stability of protein production for three industrially important cell lines: hybridoma, Chinese hamster ovary (CHO), and nonsecreting (NS0) myeloma cell lines. We highlight the most likely molecular loci at which instability may be engendered and indicate other areas of protein production that may affect stability from mammalian cells. We also outline approaches that could help to overcome the problems associated with unpredictable expression levels and maximized production, and indicate the consequences these might have for stability of production.

The circumsporozoite protein of Plasmodium falciparum is expressed and localized to the cell surface in the free-living ciliate Tetrahymena thermophila

Heterologous expression is an important tool for characterization of protein function, structural studies, and production of antigen. While many different host systems have been utilized for the expression of Plasmodium falciparum proteins, the extreme AT-richness of its genome represents an obstacle to efficient expression. In addition, primary sequence motifs such as glycosyl phosphatidyl-inositol (GPI) cleavage/attachment sites of P. falciparum are not recognized in currently used expression hosts. Recently, DNA-mediated transformation has been used for expression of heterologous genes in the ciliated protozoan Tetrahymena thermophila. We report the stable expression of full-length P. falciparum circumsporozoite (CS) protein in T. thermophila. The expressed gene utilized the native CS protein N-terminal secretory signal sequence and the C-terminal GPI anchoring signal. Immunofluorescence imaging demonstrated that the CS protein was localized to the cell surface of Tetrahymena. Metabolic labeling with tritiated myristate resulted in incorporation of label into the recombinant CS protein, indicating that the protein was bound to the cell surface via a GPI anchor. This is the first report of the recognition of targeting and GPI anchoring signals of the P. falciparum CS protein in a heterologous expression host.

Solid-phase differential display and bacterial expression systems in selection and functional analysis of cDNAs

Differential gene expression can be expected during activation and differentiation of cells as well as during pathological conditions, such as cancer. A number of strategies have been described to identify and understand isolated differentially expressed genes. The differential display methodology has rapidly become a widely used technique to identify differentially expressed mRNAs. In this chapter we described a variant of the differential display method based on solid-phase technology. The solid-phase procedure offers an attractive alternative to solution-based differential display because minute amounts of sample can be analyzed in considerably less time than previously. The employed solid support, monodisperse super paramagnetic beads, which circumvents precipitation and centrifugations steps, has also allowed for optimization of the critical enzymatic and preparative steps in the differential display methodology. We also described how bacterial expression can be used as a means to elucidate gene function. An efficient dual-expression system was presented, together with a basic concept describing how parallel expression of selected portions of cDNAs can be used for production of cDNA-encoded proteins as parts of affinity-tagged fusion proteins. The fusion proteins are suitable both for the generation of antibodies reactive to the target cDNA-encoded protein and for the subsequent affinity enrichment of such antibodies. Affinity-enriched antibodies have proved to be valuable tools in various assays, including immunoblotting and immunocytochemical staining, and can thus be used to localize the target cDNA-encoded protein to certain cells in a tissue section or even to a specific cell compartment or organelle within a cell. High-resolution localization of a cDNA-encoded protein would provide valuable information toward the understanding of protein function.

Mutant analysis of Prevotella sp. plaA-lacZ fusion protein expression in Escherichia coli: support for an essential role of the stem-loop

This study investigated the involvement of RNA folding in the synthesis of a fusion protein with beta-galactosidase activity. The coding gap region of the Prevotella loescheii adhesin gene plaA was fused in-frame with the Escherichia coli lacZ gene on plasmid pSK105. N-Terminal sequencing of the expressed plaA-lacZ protein indicated that it resulted from translational initiation at a fortuitous ribosomal-binding site within the plaA sequence at nt 570. Specific mutations were introduced in the stem-loop region that precedes the gap sequence. Analysis of stem-loop mutants, together with the introduction of compensatory mutations that restored activity, supports a requirement for stem-loop formation within the plaA sequence preceding the translational initiation site. A mutation reducing the predicted size of the loop, but preserving the stem structure, inactivated fusion protein synthesis. A suppressor mutation predicted to restore the size of the loop restored efficient fusion protein synthesis. In addition, the sequence preceding the translational start site of the plaA-lacZ fusion has several similarities to sequences that function as translational enhancers in prokaryotes. These include a stem-loop structure, an A-U rich region preceding the initiation codon, and a region of homology to 16S rRNA.Key words: site-directed mutagenesis, stem-loop formation, fusion protein, translational initiation, translational enhancer.

Codon optimization for high-level expression of human erythropoietin (EPO) in mammalian cells

Codon bias has been observed in many species. The usage of selective codons in a given gene is positively correlated with its expression efficiency. As an experimental approach to study codon-usage effects on heterologous gene expression in mammalian cells, we designed two human erythropoietin (EPO) genes, one in which native codons were systematically substituted with codons frequently found in highly expressed human genes and the other with codons prevalent in yeast genes. Relative performances of the re-engineered EPO genes were evaluated with various combinations of promoters and signal leader sequences. Under the comparable set of combinations, mature EPO gene with human high-frequency codons gave a considerably higher level of expression than that with yeast high-frequency codons. However, the levels of EPO expression varied, depending on the alternate combinations. Since the promoters and the signal leader sequences that we used are known to be equally efficient in gene expression, we hypothesized that the varied expression levels were due to the linear sequence between the promoter and the coding gene sequence. To test this possibility, we designed the EPO gene with hybrid codon usage in which the 5'-proximal region of the EPO gene was synthesized with yeast-biased codons and the rest with human-biased codons. This codon-usage hybrid EPO gene substantially enhanced the level of EPO transcripts and proteins up to 2.9-fold and 13.8-fold, respectively, when compared to the level reached by the original counterpart. Our results suggest that the linear sequence between the promoter and the 5'-proximal region of a gene plays an important role in achieving high-level expression in mammalian cells.