Optimizing Expression Systems - Industry experts from Fujifilm Diosynth and Boehringer Ingelheim discuss methods for optimizing protein expression in bacterial and mammalian cell lines. - BioPharm

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Optimizing Expression Systems
Industry experts from Fujifilm Diosynth and Boehringer Ingelheim discuss methods for optimizing protein expression in bacterial and mammalian cell lines.


BioPharm International
Volume 25, Issue 7, pp. 24-27

Most biopharmaceutials are produced in either Chinese Hamster Ovary (CHO) cells or in Escherichia Coli. Here, biopharmaceutical manufacturers provide examples of how they have optimized protein expression in these commonly used cell types to increase product titers and effieciency.

FUJIFILM DIOSYNTH BIOTECHNOLOGIES' PAVEWAY EXPRESSION SYSTEM

Jonathan Pointon, PhD, principal scientist, at Fujifilm Diosynth Biotechnologies

One of the major bottlenecks in the production of biopharmaceuticals is the efficient expression of therapeutic proteins in microbial or mammalian cells. Fujifilm Diosynth Biotechnologies' Escherichia coli pAVEway expression system described here has been developed to ensure high product titers and efficient scale up to GMP manufacture while minimizing issues such as leaky expression (i.e., expression of recombinant protein in the absence of inducer), that can have detrimental effects on host and plasmid stability. Moreover, gene to fermentation can take as little as five weeks using the generic platform pAVEway fermentation processes outlined below.

How it works


Figure 1: Schematic representation of pAVEway repression and subsequent induction. The lac repressor tetramer binds to each pPOP, positioned either side of the promoter. This causes a DNA loop to form and in combination with the increased affinity of the lac repressor for pPOP compared to native lac operator sequences, extremely tight repression is observed. Addition of the inducer (IPTG, in yellow) displaces the lac repressor tetramer allowing transcription of gene of interest mRNA to begin. (FIGURES 1– 3 COURTESY OF FUJIFILM DIOSYNTH TECHNOLOGIES)
Several key components of the pAVEway expression vectors contribute to the functionality of the system. The use of a number of powerful E. coli RNA polymerase promoters, such as T7A3, λpL and tac, opens up a large host range in comparison with the popular T7 system that is limited to hosts carrying the λDE3 prophage. Although the λDE3 prophage lacks the elements required to excise the prophage, there is evidence suggesting some λDE3 strains release phage particles under certain conditions, which is highly undesirable in fermentation plants (1).


Figure 2: A generic pAVEway fermentation. Before induction, tight repression of basal expression ensures all cells are capable of protein expression. After induction, recombinant cells continue to grow (blue graph) and express protein (red graph) leading to high titers.
Control over basal expression from the strong pAVEway promoters is provided by the use of perfectly palindromic lac operator (pPOP) sequences. pPOP sequences facilitate much tighter binding of the lac repressor tetramer than native lac operator sequences. Moreover, the use of two optimally spaced pPOP sequences (91– 92 base pairs apart), one upstream and one downstream of the promoter, leads to DNA looping and, in combination with tighter lac repressor binding, ensures extremely tight control over basal expression (see Figure 1). This level of control is extremely important in large scale (i.e., ≥5 L) fermentations as it allows high biomass accumulation prior to induction.


Figure 3: Control of gene transcription with varying IPTG inducer concentration. The ability to tune protein expression allows protein translation to match the folding/secretion capacity of the cell.
In addition, such tight repression can be helpful when expressing proteins that are potentially toxic to E. coli, because induction is performed at a higher biomass than is achievable in a leaky expression system. Because leaky expression can have a seriously negative impact on cell growth and plasmid stability, the ability to effectively switch off protein expression until induction ensures that all cells are capable of protein production. For an expression system used in large-scale manufacture this is a highly desirable characteristic and demonstrates control over the process (see Figure 2). This feature also enables a generic high cell-density fermentation protocol to be used for any protein, with no specific optimization required (see Figure 2). Additional backbone components that confer further stability on pAVEway vectors led to the development of an antibiotic-free production system without compromising product yield. The presence of antibiotic in fermentation media is increasingly becoming a regulatory concern so the ability to run antibiotic-free pAVEway fermentations highlights the versatility of the system.


Table I: Examples of proteins produced using the pAVEway expression system. Titers shown are from the first fermentations using the generic fermentation process for each accumulation method (intracellular soluble/insoluble or secreted soluble). Subsequent fermentation optimization has lead to titers in excess of 20 g/L for erythropoietin, tumour necrosis factor-α and granulocyte colony stimulating factor (G-CSF), in addition to titers of 1.1 g/L for the secreted D1.3 Fab fragment.
The rate of target gene transcription in the pAVEway system can be controlled by varying the concentration of inducer (IPTG), because there is a linear relationship between IPTG concentration and gene expression from these vectors (see Figure 3). The ability to control the rate of expression can be useful in situations where the maximum rate of expression may not be needed for optimal accumulation of the appropriate form of the protein. For example, it may be beneficial to slowly express a recombinant protein targeted for secretion, so that the host secretion machinery is not overloaded as this can greatly reduce the growth and productivity of recombinant cells. Similarly, if producing a soluble protein intracellularly, for which E coli has a limited folding capacity, especially for complex mammalian proteins, a slower induction phase may aid in the folding process. In addition to the tuning effects exerted by varying IPTG concentration, different combinations of the two promoter region components lead to a range of pAVEway vectors with expression kinetics that can be tailored to the requirements of a specific protein and its production route, whether it be intracellular insoluble/soluble or secreted soluble. When combined with generic high cell-density fermentation protocols (possible due to the tight repression of pAVEway vectors), high titers of a diverse range of biopharmaceuticals, from viral and bacterial proteins through to complex mammalian proteins such as growth factors, cytokines, and antibody fragments have been produced. These titers can be boosted further with subsequent fermentation optimization (see Table I).

Reference:

1. S. Shuman, Proc. Natl. Acad. Sci. USA, 86, 3489–3493, (1989).


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