A minimal cytomegalovirus intron A variant can improve transgene expression in different mammalian cell lines

Optimization of the intron sequences combined with the CMV promoter increases recombinant protein expression in CHO cells

To meet the requirements of the biopharmaceutical industry, improving the yield of recombination therapeutic protein (RTP) from Chinese hamster ovary (CHO) cells is necessary. The human cytomegalovirus (CMV) promoter is widely used for RTP expression in CHO cells. To further improve RTP production, we truncated the human CMV intron and further evaluated the effect of four synthetic introns, including ctEF-1α first, EF-1α first, chimeric, and β-globin introns combined with the CMV promoter on recombinant expression levels in transient and stably recombinant CHO cells. The results showed that the truncated, EF-1α first, chimeric, and β-globin introns can significantly improve stable transgene expression in CHO cells. The qPCR results indicated that the mRNA level of transgene increased through optimizing intron sequences combined with the CMV promoter. Transcriptomics analysis was performed and found that differential expression of genes involved in mRNA processing, RNA export from nucleus, cytoplasmic translation, transcriptional activation and cell cycle regulation. In conclusion, optimization of the intron sequences combined with the CMV promoter can achieve a higher yield of recombinant proteins in CHO cells. This will be valuable for generating CHO cell lines with high productivity for industrial applications.

Current Practice in Bicistronic IRES Reporter Use: A Systematic Review

Bicistronic reporter assays have been instrumental for transgene expression, understanding of internal ribosomal entry site (IRES) translation, and identification of novel cap-independent translational elements (CITE). We observed a large methodological variability in the use of bicistronic reporter assays and data presentation or normalization procedures. Therefore, we systematically searched the literature for bicistronic IRES reporter studies and analyzed methodological details, data visualization, and normalization procedures. Two hundred fifty-seven publications were identified using our search strategy (published 1994-2020). Experimental studies on eukaryotic adherent cell systems and the cell-free translation assay were included for further analysis. We evaluated the following methodological details for 176 full text articles: the bicistronic reporter design, the cell line or type, transfection methods, and time point of analyses post-transfection. For the cell-free translation assay, we focused on methods of in vitro transcription, type of translation lysate, and incubation times and assay temperature. Data can be presented in multiple ways: raw data from individual cistrons, a ratio of the two, or fold changes thereof. In addition, many different control experiments have been suggested when studying IRES-mediated translation. In addition, many different normalization and control experiments have been suggested when studying IRES-mediated translation. Therefore, we also categorized and summarized their use. Our unbiased analyses provide a representative overview of bicistronic IRES reporter use. We identified parameters that were reported inconsistently or incompletely, which could hamper data reproduction and interpretation. On the basis of our analyses, we encourage adhering to a number of practices that should improve transparency of bicistronic reporter data presentation and improve methodological descriptions to facilitate data replication.

SV40 intron, a potent strong intron element that effectively increases transgene expression in transfected Chinese hamster ovary cells

Chinese hamster ovary (CHO) cells have become the most widely utilized mammalian cell line for the production of recombinant proteins. However, the product yield and transgene instability need to be further increased and solved. In this study, we investigated the effect of five different introns on transgene expression in CHO cells. hCMV intron A, adenovirus tripartite leader sequence intron, SV40 intron, Chinese hamster EF-1alpha gene intron 1 and intervening sequence intron were cloned downstream of the eGFP expression cassette in a eukaryotic vector, which was then transfected into CHO cells. qRT-PCR and flow cytometry were used to explore eGFP expression levels. And gene copy number was also detected by qPCR, respectively. Furthermore, the erythropoietin (EPO) protein was used to test the selected more strong intron. The results showed that SV40 intron exhibited the highest transgene expression level among the five compared intron elements under transient and stable transfections. In addition, the SV40 intron element can increase the ratio of positive colonies and decrease the coefficient of variation in transgene expression level. Moreover, the transgene expression level was not related to the gene copy number in stable transfected CHO cells. Also, the SV40 intron induced higher level of EPO expression than IVS intron in transfected CHO cell. In conclusion, SV40 intron is a potent strong intron element that increases transgene expression, which can readily be used to more efficient transgenic protein production in CHO cells.

Characterization of an In Vivo Z-DNA Detection Probe Based on a Cell Nucleus Accumulating Intrabody

Left-handed Z-DNA is a physiologically unstable DNA conformation, and its existence in vivo can be attributed to localized torsional distress. Despite evidence for the existence of Z-DNA in vivo, its precise role in the control of gene expression is not fully understood. Here, an in vivo probe based on an anti-Z-DNA intrabody is proposed for native Z-DNA detection. The probe was used for chromatin immunoprecipitation of potential Z-DNA-forming sequences in the human genome. One of the isolated putative Z-DNA-forming sequences was cloned upstream of a reporter gene expression cassette under control of the CMV promoter. The reporter gene encoded an antibody fragment fused to GFP. Transient co-transfection of this vector along with the Z-probe coding vector improved reporter gene expression. This improvement was demonstrated by measuring reporter gene mRNA and protein levels and the amount of fluorescence in co-transfected CHO-K1 cells. These results suggest that the presence of the anti-Z-DNA intrabody can interfere with a Z-DNA-containing reporter gene expression. Therefore, this in vivo probe for the detection of Z-DNA could be used for global correlation of Z-DNA-forming sequences and gene expression regulation.

[Intron 2 of human beta-globin in 3'-untranslated region enhances expression of chimeric genes]

Possibility to enhance heterologous gene expression in mammalian cells by introduction of an intron in 3' untranslated region (UTR) was investigated. To this end, a fragment of human beta-globin gene with intron 2 and flanked exon regions was introduced into vector encoding green fluorescent protein TagGFP2 after the TagGFP2 stop-codon (Int+). The distance between the stop-codon and the exonjunction was 35 nucleotides. It ensured that Int+ mRNA was resistant to degradation by nonsense mediated decay (NMD) machinery. A control vector Int- contained corresponding intronless sequence of the beta-globin mRNA. On the same plasmid, the second gene encoded far-red fluorescent protein Katushka was used to normalize fluorescence for transfection efficiency and expression level in individual cells. Transiently transfected HEK293T cells were analysed by flow cytometry. It was shown that cells transfected with plasmid carrying the Int+ gene possess 1.8 ± 0.2 fold higher green fluorescence compared to Int- cells. The observed effect was used to enhance expression of destabilized variants of yellow fluorescent protein TurboYFP-dest with high degradation rate in mammalian cells. We believe that introduction of beta-globin intron in the 3'-UTR of the chimeric gene can be used to enhance its expression and may be advantageous in some cases when usage of 5'-UTR intron is inappropriate.

Engineering cells to improve protein expression

Cellular engineering of bacteria, fungi, insect cells and mammalian cells is a promising methodology to improve recombinant protein production for structural, biochemical, and commercial applications. Increased understanding of the host organism biology has suggested engineering strategies targeting bottlenecks in transcription, translation, protein processing and secretory pathways, as well as cell growth and survival. A combination of metabolic engineering and synthetic biology has been used to improve the properties of cells for protein production, which has resulted in enhanced yields of multiple protein classes.