Enhancing transient protein expression in HEK-293 cells by briefly exposing the culture to DMSO

Improvement strategies for transient gene expression in mammalian cells

Mammalian cells are suitable hosts for producing recombinant therapeutic proteins, with Chinese hamster ovary (CHO) and human embryonic kidney 293 (HEK293) cells being the most commonly used cell lines. Mammalian cell expression system includes stable and transient gene expression (TGE) system, with the TGE system having the advantages of short cycles and simple operation. By optimizing the TGE system, the expression of recombinant proteins has been significantly improved. Here, the TGE system and the detailed and up-to-date improvement strategies of mammalian cells, including cell line, expression vector, culture media, culture processes, transfection conditions, and co-expression of helper genes, are reviewed. KEY POINTS: • Detailed improvement strategies of transient gene expression system of mammalian cells are reviewed • The composition of transient expression system of mammalian cell are summarized • Proposed optimization prospects for transient gene expression systems.

Effects of an indole derivative on cell proliferation, transfection, and alternative splicing in production of lentiviral vectors by transient co-transfection

Lentiviral vectors derived from human immunodeficiency virus type I are widely used to deliver functional gene copies to mammalian cells for research and gene therapies. Post-transcriptional splicing of lentiviral vector transgene in transduced host and transfected producer cells presents barriers to widespread application of lentiviral vector-based therapies. The present study examined effects of indole derivative compound IDC16 on splicing of lentiviral vector transcripts in producer cells and corresponding yield of infectious lentiviral vectors. Indole IDC16 was shown previously to modify alternative splicing in human immunodeficiency virus type I. Human embryonic kidney 293T cells were transiently transfected by 3rd generation backbone and packaging plasmids using polyethyleneimine. Reverse transcription-quantitative polymerase chain reaction of the fraction of unspliced genomes in human embryonic kidney 293T cells increased up to 31% upon the indole's treatment at 2.5 uM. Corresponding yield of infectious lentiviral vectors decreased up to 4.5-fold in a cell transduction assay. Adjusting timing and duration of IDC16 treatment indicated that the indole's disruption of early stages of transfection and cell cycle had a greater effect on exponential time course of lentiviral vector production than its reduction of post-transcriptional splicing. Decrease in transfected human embryonic kidney 293T proliferation by IDC16 became significant at 10 uM. These findings indicated contributions by early-stage transfection, cell proliferation, and post-transcriptional splicing in transient transfection of human embryonic kidney 293T cells for lentiviral vector production.

Investigating dynamics of lentiviral vector secretion from HEK293T producer cells using a fractionated perfusion system

Mammalian cell culture is quickly becoming the go to engineering vehicle to mass produce viral vectors in a manner that is safe, convenient, reproducible, and cost and scale effective. Human embryonic kidney (HEK293) cells, in particular, have been utilized and customized (via differentiated transgene expression, modified culture parameters, addition of cytostatic culture agents) to increase vector yields. However, less attention has been made to understanding innate processes within the cells (such as, immune response, cell cycle, metabolism) themselves to better control or increase viral vector product yield. Accordingly, herein, the variation in viral production was studied from HEK cells over time using a one-way perfusion system and bioreactor to study the impact of external factors on secretion dynamics without retrotransduction. Specifically, the impact of cell density on viral titer, transduction efficiency, and LDH, was studied. Next, we look at the impact of using an inflammatory reporter cell line on viral output, and the secretion dynamics from HEK cells when we use sodium butyrate (cell cycle arrest agent). Lastly, we assess how downregulation of the PDK pathway increases viral titer. Altogether, we investigated the impact of various interventions to increase transient protein expression and viral output from HEK cells in a controlled and measurable environment to ultimately increase the efficiency of HEK cells for downstream clinical applications.

Photoactivatable Agonist-Antagonist Pair as a Tool for Precise Spatiotemporal Control of Serotonin Receptor 2C Signaling

Orthogonal recreation of the signaling profile of a chemical synapse is a current challenge in neuroscience. This is due in part to the kinetics of synaptic signaling, where neurotransmitters are rapidly released and quickly cleared by active reuptake machinery. One strategy to produce a rapid rise in an orthogonally controlled signal is via photocaged compounds. In this work, photocaged compounds are employed to recreate both the rapid rise and equally rapid fall in activation at a chemical synapse. Specifically, a complementary pair of photocages based on BODIPY were conjugated to a 5-HT2C subtype-selective agonist, WAY-161503, and antagonist, N-desmethylclozapine, to generate "caged" versions of these drugs. These conjugates release the bioactive drug upon illumination with green light (agonist) or red light (antagonist). We report on the synthesis, characterization, and bioactivity testing of the conjugates against the 5-HT2C receptor. We then characterize the kinetics of photolysis quantitatively using HPLC and qualitatively in cell culture conditions stimulating live cells. The compounds are shown to be stable in the dark for 48 h at room temperature, yet photolyze rapidly when irradiated with visible light. In live cells expressing the 5-HT2C receptor, precise spatiotemporal control of the degree and length of calcium signaling is demonstrated. By loading both compounds in tandem and leveraging spectral multiplexing as a noninvasive method to control local small-molecule drug availability, we can reproducibly initiate and suppress intracellular calcium flux on a timescale not possible by traditional methods of drug dosing. These tools enable a greater spatiotemporal control of 5-HT2C modulation and will allow for more detailed studies of the receptors' signaling, interactions with other proteins, and native physiology.

Cell-cycle arrest at the G1/S boundary enhances transient voltage-gated ion channel expression in human and insect cells

Heterologous expression of recombinant ion channel subunits in cell lines is often limited by the presence of a low number of channels at the cell surface level. Here, we introduce a combination of two techniques: viral expression using the baculovirus system plus cell-cycle arrest at the G1/S boundary using either thymidine or hydroxyurea. This method achieved a manifold increase in the peak current density of expressed ion channels compared with the classical liposome-mediated transfection methods. The enhanced ionic current was accompanied by an increase in the density of gating charges, confirming that the increased yield of protein and ionic current reflects the functional localization of channels in the plasma membrane. This modified method of viral expression coordinated with the cell cycle arrest will pave the way to better decipher the structure and function of ion channels and their association with ion channelopathies.

Membrane Protein Binding Interactions Studied in Live Cells via Diethylpyrocarbonate Covalent Labeling Mass Spectrometry

Membrane proteins are vital in the human proteome for their cellular functions and make up a majority of drug targets in the U.S. However, characterizing their higher-order structures and interactions remains challenging. Most often membrane proteins are studied in artificial membranes, but such artificial systems do not fully account for the diversity of components present in cell membranes. In this study, we demonstrate that diethylpyrocarbonate (DEPC) covalent labeling mass spectrometry can provide binding site information for membrane proteins in living cells using membrane-bound tumor necrosis factor α (mTNFα) as a model system. Using three therapeutic monoclonal antibodies that bind TNFα, our results show that residues that are buried in the epitope upon antibody binding generally decrease in DEPC labeling extent. Additionally, serine, threonine, and tyrosine residues on the periphery of the epitope increase in labeling upon antibody binding because of a more hydrophobic microenvironment that is created. We also observe changes in labeling away from the epitope, indicating changes to the packing of the mTNFα homotrimer, compaction of the mTNFα trimer against the cell membrane, and/or previously uncharacterized allosteric changes upon antibody binding. Overall, DEPC-based covalent labeling mass spectrometry offers an effective means of characterizing structure and interactions of membrane proteins in living cells.

Impact of lipopolysaccharides on cultivation and recombinant protein expression in human embryonal kidney (HEK-293) cells

The human embryonal kidney 293 cell (HEK-293) is a widely used expression host for transient gene expression. The genes or plasmids used for the transient transfections are usually propagated and extracted from the gram-negative bacterium Escherichia coli, the workhorse for molecular biologists. As a gram-negative bacterium E. coli has an outer membrane (OM) containing lipopolysaccharides (LPS) or endotoxins. LPS are very potent inducers of inflammatory cytokines in the body. In early research phases DNA intended for transient transfections is not routinely checked for LPS-levels. In this study we addressed the question whether LPS has an impact on the cultivation and production of a recombinant antibody. At high concentrations the presence of LPS has a detrimental impact on cell viability and recombinant protein expression. But low LPS concentrations are tolerated and might even enhance protein expression levels.