Continuous translation of circularized mRNA improves recombinant protein titer.

Recent success in demonstrating the translation of circular RNA open reading frames or circular mRNA, may offer a new avenue for improving recombinant protein production from cell and cell-free expression platforms. Initiation and termination are two rate limiting steps of translation. Circular RNA as a class of RNA is defined by covalent joining of terminal ends to give a closed loop structure. By encoding a gene lacking a stop codon on a circular RNA molecule an infinite open reading frame is generated permitting continuously translating circular mRNA (CTC mRNA). CTC mRNAs have shown promise in enhancing the production of multimeric polyproteins in bacterial cell-free expression systems. Problems arise when homogenous, functional post-translationally modified protein is required. To produce post-translationally modified, secreted protein from an CTC mRNA we investigated co-translational cleavage of nascent polypeptide chains by incorporating a 2 A "self-cleavage" peptide motif. Using a model recombinant human glycoprotein Erythropoietin (EPO) we demonstrate for the first time the ability to produce secreted protein from an infinite circular mRNA in live mammalian cells. Both cell-specific and volumetric productivity were improved by using circular mRNAs. This study pioneers the potential of recombinant protein production from CTC mRNA in mammalian cells.

Improved yield of rhEPO in CHO cells with synthetic 5' UTR.

The impact of local structure on mRNA translation is not well-defined pertaining to the 5' UTR. Reports suggest structural remodelling of the 5' UTR can significantly influence mRNA translation both in cis and trans however a new layer of complexity has been applied to this model with the now known reversible post-transcriptional chemical modification of RNA. N6-methyladenosine (m6A) is the most abundant internal base modification in mammalian mRNA. It has been reported that mRNAs harbouring m6A motifs in their 5' UTR have improved translation efficiency. The present study evaluated the addition of putative m6A motifs to the 5' UTR of a model recombinant human therapeutic glycoprotein, Erythropoietin (EPO), in a direct comparison with an A to T mutant and a no adenosine control. The m6A construct yielded significantly improved EPO titer in transient batch culture over no adenosine and m6T controls by 2.84 and 2.61-fold respectively. This study highlights that refinement of transgene RNA elements can yield significant improvements to protein titer.

CRISPR Deletion of miR-27 Impacts Recombinant Protein Production in CHO Cells.

MicroRNAs represent an interesting group of regulatory molecules with the unique ability of a single miRNA able to regulate the expression of potentially hundreds of target genes. In that regard, their utility has been demonstrated as a strategy to improve the cellular phenotypes important in the biomanufacturing of recombinant proteins. Common approaches to stably deplete miRNAs are the use of sponge decoy transcripts or shRNA inhibitors, both of which require the introduction and expression of extra genetic material in the cell. As an alternative, we implemented the CRISPR/Cas9 system in our laboratory to generate CHO cells which lack the expression of a specific miRNA for the purpose of functional studies. To implement the system, miR-27a/b was chosen as it has been shown to be upregulated during hypothermic conditions and therefore may be involved in influencing CHO cell growth and recombinant protein productivity. In this chapter, we present a protocol for targeting miRNAs in CHO cells using CRISPR/Cas9 and the analysis of the resulting phenotype, using miR-27 as an example. We show that it is possible to target miRNAs in CHO cells and achieved ≥80% targeting efficiency. Indel analysis and TOPO-TA cloning combined with Sanger sequencing showed a range of different indels. Furthermore, it was possible to identify clones with no detectable expression of mature miR-27b. Depletion of miR-27b led to improved viability in late stages of batch and fed-batch cultures, making it a potentially interesting target to improve bioprocess performance of CHO cells.

Regulation of multiple aquaporin genes in Arabidopsis by a pair of recently duplicated DREB transcription factors.

Identifying the transcription factors that mediate responses to abiotic stress is of fundamental importance in plant biology, not least because of their potential utility in crop improvement. The recently duplicated genes RAP2.4B and RAP2.4 encode transcription factors belonging to the abiotic stress-associated DREB A-6 clade in Arabidopsis thaliana. Both proteins localise exclusively to nuclei and show similar DRE-element-binding characteristics. Expression analysis of stressed and non-stressed plants revealed partially overlapping expression patterns. Both genes were highly expressed in stems and roots and were differentially induced in response to cold, dehydration and osmotic stress. RAP2.4B, however, was uniquely expressed at a high level in dry seeds and was induced by heat stress, while RAP2.4 was uniquely induced at a high level by salt stress. Microarray-based transcriptional profiling of double knockout and overexpression lines revealed altered expression of genes associated with adaptation to drought stress. Most strikingly, six aquaporin genes, five of which are members of a recently identified co-expression network, were downregulated in the double knockout line and correspondingly upregulated in the overexpression line, suggesting that these DREBs play a role in the regulation of water homeostasis.

Reinventing the Wheel: Synthetic Circular RNAs for Mammalian Cell Engineering.

The circular RNA renaissance is upon us. Recent reports demonstrate applications of synthetic circular RNA molecules as gene therapies and in the production of biologics from cell-based expression systems. Circular RNAs are covalently closed loop RNA species that are formed naturally through noncolinear splicing of pre-mRNA. Although once thought to be noncoding artefacts from splicing errors, it is now accepted that circular RNAs are abundant and have diverse functions in gene regulation and protein coding in eukaryotes. Numerous reports have investigated circular RNAs in various diseases, but the promise of synthetic circular RNAs in the production of recombinant proteins and as RNA-based therapies is only now coming into focus. This review highlights reported uses of synthetic circular RNAs and describes methods for generating these molecules.

Investigation and circumvention of transfection inhibition by ferric ammonium citrate in serum-free media for Chinese hamster ovary cells.

While reliable transfection methods are essential for Chinese hamster ovary (CHO) cell line engineering, reduced transfection efficiencies have been observed in several commercially prepared media. In this study, we aimed to assess common media additives that impede efficiency mediated by three chemical transfection agents: liposomal-based (Lipofectamine 2000), polymer-based (TransIT-X2), and lipopolyplex-based (TransIT-PRO). An in-house GFP-expressing vector and serum-free medium (BCR-F12: developed for the purposes of this study) were used to analyze transient transfection efficiencies of three CHO cell lines (CHO-K1, DG44, DP12). Compared to a selection of commercially available media, BCR-F12 displayed challenges associated with transfection in vendor-prepared formulations, with no detection when liposomal-based methods were used, reduced (<3%) efficiency observed when polymer-based methods were used and only limited efficiency (25%) with lipopolyplexes. Following a stepwise removal protocol, ferric ammonium citrate (FAC) was identified as the critical factor impeding transfection, with transfection enabled with the liposomal- and polymer-based methods and a 1.3- to 7-fold increased lipopolyplex efficiency observed in all cell lines in FAC-depleted media (-FAC), although lower viabilities were observed. Subsequent early addition of FAC (0.5-5 hr post-transfection) revealed 0.5 hr post-transfection as the optimal time to supplement in order to achieve transfection efficiencies similar to -FAC medium while retaining optimal cellular viabilities. In conclusion, FAC was observed to interfere with DNA transfection acting at early stages in all transfection agents and all cell lines studied, and a practical strategy to circumvent this problem is suggested.

Leaky Expression of the TET-On System Hinders Control of Endogenous miRNA Abundance.

With the ability to affect multiple genes and fundamental pathways simultaneously, miRNA engineering of Chinese Hamster Ovary (CHO) cells has significant advantages over single gene expression or repression. Tight control of these molecular triggers is desirable as it could in theory allow on/off or even tunable regulation of desirable cellular phenotypes. The present study investigated the potential of employing a tetracycline inducible (TET-On) system for conditional knockdown of specific miRNAs but encountered several challenges. The authors show a significant reduction in cell proliferation and culture viability when maintained in media supplemented with the TET-On induction agent Doxycycline at concentrations commonly reported. Calculation of a mature miRNA and miRNA sponge mRNA copy number demonstrates that leaky basal transgene expression in the un-induced state, is sufficient for significant miRNA knockdown. This work highlights challenges of the TET-On inducible expression system for controlled manipulation of endogenous miRNAs with two examples; miR-378 and miR-455. The authors suggest a solution involving isolation of highly inducible clones and use a single cell analysis platform to demonstrate the heterogeneity of basal expression and inducibility. Finally, the authors describe numerous strategies to minimize leaky transgene expression and alterations to current miRNA sponge design.

Depletion of endogenous miRNA-378-3p increases peak cell density of CHO DP12 cells and is correlated with elevated levels of ubiquitin carboxyl-terminal hydrolase 14.

miRNAs are potent molecular regulators of cellular behaviour. The manipulation of these small non-coding RNAs has been used to enhance industrially relevant phenotypes in Chinese Hamster Ovary (CHO) cells. We investigated the stable depletion of six miRNAs; miR-204-5p, 338-3p, 378-3p, 409-3p, 455-3p and 505-3p, robustly associated with cell growth rate from a previous profiling study. Inhibition of endogenous miR-378-3p function by miRNA-sponge-decoy improved peak cell density by 59%. Quantitative label free LC-MS/MS proteomic analysis of the fractionated cell cultures at day 4 and 8 of batch culture found 216 cytosolic and 114 membrane-associated proteins differentially expressed with stable miR-378-3p depletion. qRT-PCR of 8 genes; Clic4, Hnrnpa1, Prdx1, Actn4, Usp14, Srxn1, Canx and Gnb1, with unidirectional differential protein expression over the two time points of analysis was carried out. In-silico predictive algorithms; TargetScan and miRDB, were used to decipher possible direct targets of miR-378-3p. The Ubiquitin carboxyl-terminal hydrolase 14 (Usp14) protein was identified in the cytosolic fractions at both timepoints as differentially expressed with an increased abundance of 1.58-fold in the miR-378-3p depleted cells on day 8. Usp14 is a deubiquitinase (DUB) with previous reports of its up-regulation leading to increased proliferation of cancer cells. Overexpression of Usp14 in CHO cells had significant effects on cell growth supporting a role for Usp14 in the increased peak cell density seen with miR-378-3p depletion. This study highlights miR-378-3p as a novel engineering candidate for improving CHO cell growth. The use of subcellular fractionation also improved proteome coverage in the identification of novel miRNA targets.

miR-CATCH Identifies Biologically Active miRNA Regulators of the Pro-Survival Gene XIAP, in Chinese Hamster Ovary Cells.

Genetic engineering of mammalian cells is of interest as a means to boost bio-therapeutic protein yield. X-linked inhibitor of apoptosis (XIAP) overexpression has previously been shown to enhance CHO cell growth and prolong culture longevity while additionally boosting productivity. The authors confirmed this across a range of recombinant products (SEAP, EPO, and IgG). However, stable overexpression of an engineering transgene competes for the cells translational machinery potentially compromising product titre. MicroRNAs are attractive genetic engineering candidates given their non-coding nature and ability to regulate multiple genes simultaneously, thereby relieving the translational burden associated with stable overexpression of a protein-encoding gene. The large number of potential targets of a single miRNA, falsely predicted in silico, presents difficulties in identifying those that could be useful engineering tools. The authors explored the identification of direct miRNA regulators of the pro-survival endogenous XIAP gene in CHO-K1 cells by using a miR-CATCH protocol. A biotin-tagged antisense DNA oligonucleotide for XIAP mRNA is designed and used to pull down and capture bound miRNAs. Two miRNAs are chosen out of the 14 miRNAs identified for further validation, miR-124-3p and miR-19b-3p. Transient transfection of mimics for both results in the diminished translation of endogenous CHO XIAP protein whereas their inhibition increases XIAP protein levels. A 3'UTR reporter assay confirms miR-124-3p to be a bona fide regulator of XIAP in CHO-K1 cells. This method demonstrates a useful approach to finding miRNA candidates for CHO cell engineering without competing for the cellular translational machinery.

A proteomic profiling dataset of recombinant Chinese hamster ovary cells showing enhanced cellular growth following miR-378 depletion.

The proteomic data presented in this article provide supporting information to the related research article "Depletion of endogenous miRNA-378-3p increases peak cell density of CHO DP12 cells and is correlated with elevated levels of Ubiquitin Carboxyl-Terminal Hydrolase 14" (Costello et al., in press) [1]. Control and microRNA-378 depleted CHO DP12 cells were profiled using label-free quantitative proteomic profiling. CHO DP12 cells were collected on day 4 and 8 of batch culture, subcellular proteomic enrichment was performed, and subsequent fractions were analyzed by liquid chromatography tandem mass spectrometry (LC-MS/MS). Here we provide the complete proteomic dataset of proteins significantly differentially expressed by greater than 1.25-fold change in abundance between control and miR-378 depleted CHO DP12 cells, and the lists of all identified proteins for each condition.

Manipulating MiRNA Expression to Uncover Hidden Functions.

There are a numerous target prediction algorithms that allow users to identify putative targets of their microRNAs (miRNAs) of interest. Although these tools are useful to gain insight into the potential role of miRNAs in regulating cellular processes, physical manipulation of the expression of the miRNA is the most superior way to truly determine the function of the miRNA in the system of interest. This chapter outlines methods to reveal miRNA function by modulating miRNA expression by transient transfection of miRNA mimics and inhibitors, and stable overexpression and reduction of miRNA expression using plasmid overexpression and sponge vectors.

A nonlethal sampling method to obtain, generate and assemble whole blood transcriptomes from small, wild mammals.

The acquisition of tissue samples from wild populations is a constant challenge in conservation biology, especially for endangered species and protected species where nonlethal sampling is the only option. Whole blood has been suggested as a nonlethal sample type that contains a high percentage of bodywide and genomewide transcripts and therefore can be used to assess the transcriptional status of an individual, and to infer a high percentage of the genome. However, only limited quantities of blood can be nonlethally sampled from small species and it is not known if enough genetic material is contained in only a few drops of blood, which represents the upper limit of sample collection for some small species. In this study, we developed a nonlethal sampling method, the laboratory protocols and a bioinformatic pipeline to sequence and assemble the whole blood transcriptome, using Illumina RNA-Seq, from wild greater mouse-eared bats (Myotis myotis). For optimal results, both ribosomal and globin RNAs must be removed before library construction. Treatment of DNase is recommended but not required enabling the use of smaller amounts of starting RNA. A large proportion of protein-coding genes (61%) in the genome were expressed in the blood transcriptome, comparable to brain (65%), kidney (63%) and liver (58%) transcriptomes, and up to 99% of the mitogenome (excluding D-loop) was recovered in the RNA-Seq data. In conclusion, this nonlethal blood sampling method provides an opportunity for a genomewide transcriptomic study of small, endangered or critically protected species, without sacrificing any individuals.

Re-programming CHO cell metabolism using miR-23 tips the balance towards a highly productive phenotype.

microRNA engineering of CHO cells has already proved successful in enhancing various industrially relevant phenotypes and producing various recombinant products. A single miRNA's ability to interact with multiple mRNA targets allows their regulatory capacity to extend to processes such as cellular metabolism. Various metabolic states have previously been associated with particular CHO cell phenotypes such as glycolytic or oxidative metabolism accommodating growth and productivity, respectively. miR-23 has previously been demonstrated to play a role in glutamate metabolism resulting in enhanced oxidative phosphorylation through the TCA cycle. Re-programming cellular bioenergetics through miR-23 could tip the balance, forcing mammalian production cells to be more productive by favoring metabolic channelling into oxidative metabolism. CHO clones depleted of miR-23 using a miR-sponge decoy demonstrated an average ∼three-fold enhanced specific productivity with no impact on cell growth. Using a cell respirometer, mitochondrial activity was found to be enhanced by ∼30% at Complex I and II of the electron transport system. Additionally, label-free proteomic analysis uncovered various potential novel targets of miR-23 including LE1 and IDH1, both implicated in oxidative metabolism and mitochondrial activity. These results demonstrate miRNA-based engineering as a route to re-programming cellular metabolism resulting in increased productivity, without affecting growth.

CHO cell culture longevity and recombinant protein yield are enhanced by depletion of miR-7 activity via sponge decoy vectors.

Improving the efficiency of recombinant protein production by CHO cells is highly desirable as more complex proteins (MAbs, fusion proteins, blood/clotting factors, etc.) go into development and come onto the market. Previous reports have shown that microRNA (miRNA)-7 overexpression arrests the growth of CHO cells and that its depletion increases the proliferation of various cell types. In this study we generated stable CHO clones that overexpressed a miR-7-specific decoy transcript (sponge) downstream of a green fluorescent protein reporter gene. The miR-7 sponge efficiently diverted miR-7 away from its endogenous targets as exemplified by the increased expression of CDC7. Although the sponge effectively sequestered miR-7, it also appeared to protect the bound miRNA sequence from degradation in the cell, as exemplified by the apparent increase in mature miR-7 levels without any change in primary transcription. Phenotypically, CHO clones with sequestered miR-7 displayed improved maximum cell density (40%), significantly improved viability and an almost two-fold increase in yield of secreted protein in a fed-batch culture. These findings demonstrate that miRNA sponge transcripts could potentially be used in cell line development projects to generate producer clones that grow to higher densities and last longer in the bioreactor - thereby improving product yield.

Versatile dual reporter gene systems for investigating stop codon readthrough in plants.

BACKGROUND: Translation is most often terminated when a ribosome encounters the first in-frame stop codon (UAA, UAG or UGA) in an mRNA. However, many viruses (and some cellular mRNAs) contain "stop" codons that cause a proportion of ribosomes to terminate and others to incorporate an amino acid and continue to synthesize a "readthrough", or C-terminally extended, protein. This dynamic redefinition of codon meaning is dependent on specific sequence context. METHODOLOGY: We describe two versatile dual reporter systems which facilitate investigation of stop codon readthrough in vivo in intact plants, and identification of the amino acid incorporated at the decoded stop codon. The first is based on the reporter enzymes NAN and GUS for which sensitive fluorogenic and histochemical substrates are available; the second on GST and GFP. CONCLUSIONS: We show that the NAN-GUS system can be used for direct in planta measurements of readthrough efficiency following transient expression of reporter constructs in leaves, and moreover, that the system is sufficiently sensitive to permit measurement of readthrough in stably transformed plants. We further show that the GST-GFP system can be used to affinity purify readthrough products for mass spectrometric analysis and provide the first definitive evidence that tyrosine alone is specified in vivo by a 'leaky' UAG codon, and tyrosine and tryptophan, respectively, at decoded UAA, and UGA codons in the Tobacco mosaic virus (TMV) readthrough context.

Intron-regulated expression of SUVH3, an Arabidopsis Su(var)3-9 homologue.

SU(VAR)3-9 proteins are key regulators of heterochromatin structure and function in plants, mammals, Drosophila, and yeast. In contrast to animals and fungi, plants contain numerous Su(var)3-9 homologues (SUVH), the members of which form a discrete subfamily. The SU(VAR)3-9 and SUVH proteins associate with heterochromatin and possess histone methyltransferase activity, indicating that they participate in the organization of transcriptionally repressive chromatin. The Arabidopsis thaliana genome contains 10 SUVH genes, belonging to four phylogenetically distinct subgroups: SUVH1, SUVH2, SUVH4, and SUVH5. The structure and expression of SUVH3, a member of the SUVH1 subgroup was investigated. SUVH3 was shown to be broadly expressed during plant development with the highest levels found in proliferating cells. The encoded protein localized in subnuclear foci and remained associated with condensed chromosomes throughout mitosis. A deletion analysis of the SUVH3 upstream region further revealed that an intron located in the 5' UTR is a key regulator of strong and constitutive SUVH3 expression.

Gene duplication, exon gain and neofunctionalization of OEP16-related genes in land plants.

OEP16, a channel protein of the outer membrane of chloroplasts, has been implicated in amino acid transport and in the substrate-dependent import of protochlorophyllide oxidoreductase A. Two major clades of OEP16-related sequences were identified in land plants (OEP16-L and OEP16-S), which arose by a gene duplication event predating the divergence of seed plants and bryophytes. Remarkably, in angiosperms, OEP16-S genes evolved by gaining an additional exon that extends an interhelical loop domain in the pore-forming region of the protein. We analysed the sequence, structure and expression of the corresponding Arabidopsis genes (atOEP16-S and atOEP16-L) and demonstrated that following duplication, both genes diverged in terms of expression patterns and coding sequence. AtOEP16-S, which contains multiple G-box ABA-responsive elements (ABREs) in the promoter region, is regulated by ABI3 and ABI5 and is strongly expressed during the maturation phase in seeds and pollen grains, both desiccation-tolerant tissues. In contrast, atOEP-L, which lacks promoter ABREs, is expressed predominantly in leaves, is induced strongly by low-temperature stress and shows weak induction in response to osmotic stress, salicylic acid and exogenous ABA. Our results indicate that gene duplication, exon gain and regulatory sequence evolution each played a role in the divergence of OEP16 homologues in plants.

Mutation of a family 8 glycosyltransferase gene alters cell wall carbohydrate composition and causes a humidity-sensitive semi-sterile dwarf phenotype in Arabidopsis.

The genome of Arabidopsis thaliana contains about 400 genes coding for glycosyltransferases, many of which are predicted to be involved in the synthesis and remodelling of cell wall components. We describe the isolation of a transposon-tagged mutant, parvus, which under low humidity conditions exhibits a severely dwarfed growth phenotype and failure of anther dehiscence resulting in semi-sterility. All aspects of the mutant phenotype were partially rescued by growth under high-humidity conditions, but not by the application of growth hormones or jasmonic acid. The mutation is caused by insertion of a maize Dissociation (Ds) element in a gene coding for a putative Golgi-localized glycosyltransferase belonging to family 8. Members of this family, originally identified on the basis of similarity to bacterial lipooligosaccharide glycosyltransferases, include enzymes known to be involved in the synthesis of bacterial and plant cell walls. Cell-wall carbohydrate analyses of the parvus mutant indicated reduced levels of rhamnogalacturonan I branching and alterations in the abundance of some xyloglucan linkages that may, however, be indirect consequences of the mutation.