Polyethyleneimine-based transient gene expression processes for suspension-adapted HEK-293E and CHO-DG44 cells

Production of cell surface and secreted glycoproteins in mammalian cells

Mammalian protein expression systems are becoming increasingly popular for the production of eukaryotic secreted and cell surface proteins. Here we describe methods to produce recombinant proteins in adherent or suspension human embryonic kidney cell cultures, using transient transfection or stable cell lines. The protocols are easy to scale up and cost-efficient, making them suitable for protein crystallization projects and other applications that require high protein yields.

The Strep-tag system for one-step affinity purification of proteins from mammalian cell culture

The Strep-tag-or its improved version Strep-tagII-is an eight amino acid sequence that can be easily fused or conjugated to any protein or peptide of interest and that was engineered for high affinity toward streptavidin, which otherwise is widely known as a tight biotin-binding reagent. Especially in combination with immobilized Strep-Tactin, a mutant streptavidin specifically optimized toward the Strep-tagII, this system enables the facile one-step affinity purification of various biomolecules, including oligomeric and even membrane proteins. The Strep-tagII/Strep-Tactin interaction shows exquisite specificity, thus allowing efficient separation from host cell proteins, and it can be reversed simply by addition of biotin (or a suitable derivative thereof, such as desthiobiotin). Therefore, this system has become very popular for the highly efficient affinity chromatography under biochemically mild conditions. Here, we describe the purification of Strep-tagged proteins from mammalian cell lysates and cell culture supernatants.

Uncovering methods for the prevention of protein aggregation and improvement of product quality in a transient expression system

Mammalian expression systems are used routinely for the production of recombinant proteins as therapeutic molecules as well as research tools. Transient expression has become increasingly popular in recent years due to its rapid timeline and improvements in expression level. While improvements to transient expression systems have focused mainly on the level of protein expression, the aspect of protein quality has received little attention. The removal of undesirable products, such as aggregation, depends primarily on purification, requiring additional cumbersome steps, which can lead to a lower product yield and longer timelines. In this study, we show that reducing the level of transcription by transfecting at a lower gene dose improves the quality of secreted molecules prone to aggregation. For gene dosing to have this effect, it is critical for the carrier DNA to be an empty vector containing the same elements as the gene containing plasmid. This approach can be used in combination with a temperature shift to hypothermic conditions during production to enhance the effect. The observed improvements not only minimized aggregation levels, but also generated products with overall superior quality, including more homogeneous signal peptide cleavage and N-linked glycosylation profiles. These techniques have produced a similar improvement in product quality with a variety of other molecules, suggesting that this may be a general approach to enhance product quality from transient expression systems.

Improved stability and efficacy of chitosan/pDNA complexes for gene delivery

Among polymeric polycations, chitosan has emerged as a powerful carrier for gene delivery. Only a few studies have focused on the stability of the chitosan/DNA complex under storage, although this is imperative for nanomedicinal applications. Here, we synthesized polyelectrolyte complexes at a charge ratio of 10 using 50 kDa chitosan and plasmid (p)DNA that encodes a GFP reporter. These preparations were stable up to 3 months at 4 °C and showed reproducible transfection efficiencies in vitro in HEK293 cells. In addition, we developed a methodology that increases the in vitro transfection efficiency of chitosan/pDNA complexes by 150% with respect to standard procedures. Notably, intracellular pDNA release and transfected cells peaked 5 days following transection of mitotically active cells. These new developments in formulation technology enhance the potential for polymeric nanoparticle-mediated gene therapy.

Production of functional human insulin-like growth factor binding proteins (IGFBPs) using recombinant expression in HEK293 cells

Insulin-like growth factor binding proteins (IGFBPs) display many functions in humans including regulation of the insulin-like growth factor (IGF) signaling pathway. The various roles of human IGFBPs make them attractive protein candidates in drug discovery. Structural and functional knowledge on human proteins with therapeutic relevance is needed to design and process the next generation of protein therapeutics. In order to conduct structural and functional investigations large quantities of recombinant proteins are needed. However, finding a suitable recombinant production system for proteins such as full-length human IGFBPs, still remains a challenge. Here we present a mammalian HEK293 expression method suitable for over-expression of secretory full-length human IGFBP-1 to -7. Protein purification of full-length human IGFBP-1, -2, -3 and -5 was conducted using a two-step chromatography procedure and the final protein yields were between 1 and 12mg protein per liter culture media. The recombinant IGFBPs contained PTMs and exhibited high-affinity interactions with their natural ligands IGF-1 and IGF-2.

Ecto-Fc MS identifies ligand-receptor interactions through extracellular domain Fc fusion protein baits and shotgun proteomic analysis

Ligand-receptor interactions represent essential biological triggers that regulate many diverse and important cellular processes. We have developed a discovery-based proteomic biochemical protocol that couples affinity purification with multidimensional liquid chromatographic tandem mass spectrometry (LCLC-MS/MS) and bioinformatic analysis. Compared with previous approaches, our analysis increases sensitivity, shortens analysis duration and boosts comprehensiveness. In this protocol, receptor extracellular domains are fused with the Fc region of IgG to generate fusion proteins that are purified from transfected HEK293T cells. These 'ecto-Fcs' are coupled to protein A beads and serve as baits for binding assays with prey proteins extracted from rodent brain. After capture, the affinity-purified proteins are digested into peptides and comprehensively analyzed by LCLC-MS/MS with ion-trap mass spectrometers. In 4 working days, this protocol can generate shortlists of candidate ligand-receptor protein-protein interactions. Our 'ecto-Fc MS' approach outperforms antibody-based approaches and provides a reproducible and robust framework for identifying extracellular ligand-receptor interactions.

Better and faster: improvements and optimization for mammalian recombinant protein production

Thanks to numerous technological advances, the production of recombinant proteins in mammalian cell lines has become an increasingly routine task that is no longer viewed as a heroic enterprise. While production in prokaryotic or lower eukaryotic systems may be more rapid and economical, the advantages of producing large amounts of protein that closely resembles the native form is often advantageous and may be essential for the realization of functionally active material for biological studies or biopharmaceuticals. The correct folding, processing and post-translational modifications conferred by expression in a mammalian cell is relevant to all classes of proteins, including cytoplasmic, secreted or integral membrane proteins. Therefore considerable efforts have focused on the development of growth media, cell lines, transformation methods and selection techniques that enable the production of grams of functional protein in weeks, rather than months. This review will focus on a plethora of methods that are broadly applicable to the high yield production of any class of protein (cytoplasmic, secreted or integral membrane) from mammalian cells.