MEXi mammalian expression system

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 MEXi-293E cells or other mammalian cells, we provide a multitude of plasmids for stable or transient expression.

Acceptor vectors

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.