Accurate measurement of global and specific protein synthesis rates is becoming increasingly important, especially in the context of biotechnological applications such as process modeling or selection of production cell clones. While quantification of total protein translation across whole cell populations is easily achieved, methods that are capable of tracking population dynamics at the single-cell level are still lacking. To address this need, we apply O-propargyl-puromycin (OPP) labeling to assess total protein synthesis in single recombinant Chinese hamster ovary (CHO) cells by flow cytometry. Thereby we demonstrate that global protein translation rates slightly increase with progression through the cell cycle during exponential growth. Stable CHO cell lines producing recombinant protein display similar levels of total protein synthesis as their parental CHO host cell line. Global protein translation does not correlate with intracellular product content of three model proteins, but the host cell line with high transient productivity has a higher OPP signal. This indicates that production cell lines with increased overall protein synthesis capacity can be identified by our method at the single-cell level. In conclusion, OPP-labeling allows rapid and reproducible assessment of global protein synthesis in single CHO cells, and can be multiplexed with DNA staining or any type of immunolabeling of specific proteins or markers for organelles.

Obtaining highly productive Chinese hamster ovary (CHO)-cell clones for the production of therapeutic proteins relies on multiple time-consuming selection steps. Several CHO-cell strains with high degrees of genomic and epigenetic variation are available. Each harbor potential advantages and disadvantages for any given product, particularly those considered difficult to express. A simple test system to quickly assess compatibility of cell line and product may therefore prove useful. Transient plasmid transfection falls short of the specific productivities of stable producer cells, making it unsuitable for the elucidation of high specific productivity bottlenecks. The aim of the study is to reach specific productivities approaching those of industrial production cell lines by transfection of in vitro transcribed mRNA. The system is characterized with respect to transfection efficacy (by quantitative PCR) and protein production (by flow cytometry and biolayer interferometry). Fluorescence of intracellular eGFP saturates at higher amounts of mRNA per cell, while the amount of secreted and intracellular EPO-Fc remain linearly correlated to the amount of mRNA taken up. Nevertheless, MS shows a severe reduction in N-glycosylation quality. This method allows for rapid elucidation of bottlenecks that would otherwise remain undetected until later during cell line development, giving insight into suitable strategies for preemptive targeted metabolic engineering and host cell line optimization.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.

N-glycosylation is defined as a key quality attribute for the majority of complex biological therapeutics. Despite many N-glycan engineering efforts, the demand to generate desired N-glycan profiles that may vary for different proteins in a reproducible manner is still difficult to fulfill in many cases. Stable production of homogenous structures with a more demanding level of processing, for instance high degrees of branching and terminal sialylation, is particularly challenging. Among many other influential factors, the level of productivity can steer N-glycosylation towards less mature N-glycan structures. Recently, we introduced an mRNA transfection system capable of elucidating bottlenecks in the secretory pathway by stepwise increase of intracellular model protein mRNA load. Here, this system was applied to evaluate engineering strategies for enhanced N-glycan processing. The tool proves to indeed be valuable for a quick assessment of engineering approaches on the cellular Nglycosylation capacity at high productivity. The gene editing approaches tested include overexpression of key Golgi-resident glycosyltransferases, partially coupled with multiple gene deletions. Changes in galactosylation, sialylation, and branching potential as well as N-acetyllactosamine formation were evaluated.

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