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MEXi mammalian expression system

Mammalian cells are often used for the expression of recombinant proteins when complex glycolysation and accurate protein folding are important. The MEXi system provides an affordable mammalian protein expression system along with all essential components. MEXi-293E cells are human embryonic kidney (HEK) cells derived from the 293 cell line, adapted to suspension growth in an affordable cell culturing medium with a low biotin content. This allows highly efficient and specific purification of recombinant proteins via Strep-Tactin®XT under physiological conditions. 

The MEXi system uses transient expression rather than stable transformation, to enable a quick and short-term production of the recombinant protein for several days after DNA transfection. This happens via episomal replication and expression of the GOI without the need for a prior integration by making use of the EBNA1 protein in combination with oriP-harboring plasmids. Further, the system deploys a polyethylenimine-based transfection, which is an affordable and quick method suitable even for hard-to-transfect cell lines, while simultaneously avoiding cytotoxic effects as in lipidbased techniques, or complicated viral packaging. Products beyond protein expression and purification are available from IBA Lifesciences, such as reagents for specific detection, using the Strep-tag® technology. This allows the entire research task to be easily performed using synchronized protocols within one technology platform.

Expression hosts

A particular focus should also be set on choosing the appropriate expression host for the protein of interest to ensure getting a biologically functional target. While E. coli are the most prominent host cells due to their easy handling and compatibility with a large portion of targets, eukaryotic cell lines, such as yeasts, insect cells, or mammalian cells are required to obtain complex proteins with proper folding and post-translational modifications. Mammalian cells have become the most popular host for recombinant protein expression, especially of antibodies, therapeutic proteins, or biopharmaceuticals. The Human Embryonic Kidney 293 (HEK) cells are commonly used in drug development and laboratory settings since they exhibit straightforward growth in culture, have a high transfection efficiency, and are amenable to a variety of transfection methods.

IBA’s MEXi-293E cells are such human embryonic kidney (HEK) cells derived from the 293 cell line, which are optimized for the expression of recombinant proteins in mammalian cells and specifically adapted to an efficient growth in the corresponding cell culture media MEXi TM (Transfection Medium) and MEXi CM (Culture Medium).

To allow easy access to the various expression hosts, we provide a multitude of plasmids targeting the different hosts, including E. coli and mammalian cells but also yeast and insect cells (Baculovirus vectors).

Acceptor vectors

Transfection/ Transformation

In the next step, the previously created expression vector carrying the gene of interest needs to be transferred into the chosen expression host for propagation and production of the target protein. This can be realized by various methods, whereof transformation and transfection are the most frequently applied. Transformation is the process of getting the recombinant vector from a reaction mixture or vector solution into a prokaryotic expression host, such as E. coli cells. It is a naturally occurring direct uptake of DNA fragments, which is triggered by different physicochemical stimuli (heat shock, calcium treatment, or electroporation) in the laboratory environment. Once arrived in the cell, the gene on the plasmid can simply be expressed by an appropriate induction method. In contrast, transfection is a more complicated technique to deliver the recombinant DNA construct into eukaryotic cells, or more precisely into their nucleus. In this method, transient pores have to be opened in the cell membrane (viral packaging with calcium phosphate treatment, electroporation, cationic polyamides, or lipid shuttling) to induce the uptake of the genetic construct into the cytosol, followed by the transport into the nucleus due to signal sequences provided by the vector. Through a repeated process of careful selection and amplification, stable clones with the foreign information integrated into their genome are fabricated.

Notwithstanding the advantage that all descendants of these cells will contain the gene of interest and attend the production of the target, stable transfection is a laborious process mainly recommended for the large-scale production of recombinant proteins. In contrast, to rapidly obtain research quantities of the target, transient expression is the method of choice. This technique results in a quick and short-term production of the recombinant protein for several days after DNA transfection. IBA’s MEXi mammalian expression system implements this transient approach to enable the simple and efficient episomal replication and expression of the GOI without the need for a prior integration by making use of the EBNA1 protein in combination with oriP-harboring plasmids. Further, the system deploys a polyethylenimine-based transfection, which is an affordable and quick method suitable even for hard-to-transfect cell lines, while simultaneously avoiding cytotoxic effects as in lipid-based techniques, or complicated viral packaging.

MEXi mammalian expression workflow

MEXi workflow and components

The MEXi system was developed to offer an affordable mammalian protein expression system coming along with all essential components:

  • MEXi-293E cell line
  • A variety of pDSG expression vectors 
  • MEXi-CM culture medium
  • MEXi-TM transfection medium

Further advantages: 

  • Transient expression for time- and cost-efficient protein production 
  • Multiple expression vectors with different features 
  • Optimized for Twin-Strep-tag®:Strep-Tactin®XT system for highly pure proteins 
  • IP-free system

Application examples

The MEXi system consisting of MEXi 293E cells, pDSG-IBA expression vectors, MEXi-TM transfection medium, and MEXi-CM culture medium was applied for expression of different proteins. Subsequent purification occurred either with Strep-Tactin® or Strep-Tactin®XT. Yield and purity were analyzed by SDS-PAGE.

Metalloprotein

Type: metalloprotein, hydrolase     

Yield : 143 mg/l.  

Secreted Alkaline Phosphatase (SEAP) was fused with a C-terminal Twin-Strep-tag® and the BM40 secretion signal via cloning into pDSG-IBA102. MEXi 293E cells  (1.5 x 106 cells/ml)  were transfected in  MEXi-TM transfection medium (17 ml) with polyethylenimine (PEI, 25 kDa). Afterwards, the cell culture was incubated for 4 hours (37 °C, 5% CO2, 125 rpm) and then diluted with MEXi-CM culture medium to reach a cell density of 0.75 x 106 cells/ml. The cells were kept at 37 °C, 5% CO2, and 125 rpm for 7 days in order to obtain high protein yields. For purification, the cells were pelleted and the supernatant, containing the SEAP protein, was harvested. Free Biotin was blocked by BioLock containing avidin. The SEAP protein was finally purified using a gravity flow Strep-Tactin® column.

Antibody

Type: antibody

 Yield: 96 mg/l (represented by heavy (HC) and light chain (LC)) 

The monoclonal rat antibody (mAb) was cloned into the pDSG-IBA102 vector in order to fuse the heavy chain (HC) C-terminally with the Twin-Strep-tag® and the BM40 secretion signal. The transfection of MEXi 293E cells was performed in MEXi-TM transfection medium (250 ml) with polyethylenimine (PEI, 25 kDa). The cells were incubated for 4 hours (37 °C, 5% CO2, 125 rpm) and when a cell density of 3 x 106 cells/ml was reached, 250ml MEXi-CM culture medium was added. Afterwards, the culture was shifted to 32 °C and incubated until day 10. In order to divide the cells from the supernatant, the cell suspension was centrifuged according to the MEXi manual. Free biotin was blocked by BioLock containing avidin. The supernatant was used for protein purification via a gravity flow Strep-Tactin® column. WET FRED was used to facilitate loading of the large supernatant volume onto the column.

Customer protein

Yield: 318 mg/l  

 The coding sequence of the customer protein was cloned into pDSG-IBA102 leading to a recombinant protein with C-terminal Twin-Strep-tag® and BM40 secretion signal. 1050 ml of MEXi-TM was inoculated with MEXi 293E cells. Subsequently, plasmid DNA was added followed by addition of 25 kDa linear PEI. The cells were incubated for 4 hours in MEXi-TM medium at 37 °C, 5% CO2, and 125 rpm in an orbital shaking incubator. Cells were diluted to 7.5 x 105 cells/ml by addition of one volume MEXi-CM and kept at 37 °C for 7 days. Afterwards, cells were pelleted and the supernatant containing the customer protein was harvested. The customer protein was finally purified using Strep-Tactin®XT. The figure shows the elution fractions (E1-E3). A dilution of 1:10 was prepared for E1 and E2 for SDS-PAGE analysis.