Enhancing recombinant glycoprotein sialylation through CMP-sialic acid transporter over expression in Chinese hamster ovary cells

Sialyllactose supplementation enhances sialylation of Fc-fusion glycoprotein in recombinant Chinese hamster ovary cell culture

Sialylation during N-glycosylation plays an important role in the half-life of therapeutic glycoproteins in vivo and has sparked interest in the production of therapeutic proteins using recombinant Chinese hamster ovary (rCHO) cells. To improve the sialylation of therapeutic proteins, we examined the effect of sialyllactose supplementation on sialylation of Fc-fusion glycoproteins produced in rCHO cells. Two enzymatically-synthesized sialyllactoses, 3'-sialyllactose (3'-SL) and 6'-sialyllactose (6'-SL), were administered separately to two rCHO cell lines producing the same Fc-fusion glycoprotein derived from DUKX-B11 and DG44, respectively. Two sialyllactoses successfully increased sialylation of Fc-fusion glycoprotein in both cell lines, as evidenced by isoform distribution, sialylated N-glycan formation, and sialic acid content. Increased sialylation by adding sialyllactose was likely the result of increased amount of intracellular CMP-sialic acid (CMP-SA), the direct nucleotide sugar for sialylation. Furthermore, the degree of sialylation enhanced by sialyllactoses was slightly effective or nearly similar compared with the addition of N-acetylmannosamine (ManNAc), a representative nucleotide sugar precursor, to increase sialylation of glycoproteins. The effectiveness of sialyllactose was also confirmed using three commercially available CHO cell culture media. Taken together, these results suggest that enzymatically-synthesized sialyllactose represents a promising candidate for culture media supplementation to increase sialylation of glycoproteins in rCHO cell culture.

From Efficiency to Yield: Exploring Recent Advances in CHO Cell Line Development for Monoclonal Antibodies

The increasing demand for biosimilar monoclonal antibodies (mAbs) has prompted the development of stable high-producing cell lines while simultaneously decreasing the time required for screening. Existing platforms have proven inefficient, resulting in inconsistencies in yields, growth characteristics, and quality features in the final mAb products. Selecting a suitable expression host, designing an effective gene expression system, developing a streamlined cell line generation approach, optimizing culture conditions, and defining scaling-up and purification strategies are all critical steps in the production of recombinant proteins, particularly monoclonal antibodies, in mammalian cells. As a result, an active area of study is dedicated to expression and optimizing recombinant protein production. This review explores recent breakthroughs and approaches targeted at accelerating cell line development to attain efficiency and consistency in the synthesis of therapeutic proteins, specifically monoclonal antibodies. The primary goal is to bridge the gap between rising demand and consistent, high-quality mAb production, thereby benefiting the healthcare and pharmaceutical industries.

Flow Cytometry-Based Detection of Siglec Ligands

Siglecs are a family of immunomodulatory cell surface receptors present on white blood cells. Binding to cell surface sialic acid-containing glycans modulates the proximity of Siglecs to other receptors that they regulate. This proximity is key to enabling signaling motifs on the cytosolic domain of Siglecs to modulate immune responses. Given the important roles that Siglecs play in helping to maintain immune homeostasis, a better understanding of their glycan ligands is needed to elucidate their roles in health and disease. A common approach for probing Siglec ligands on cells is to use soluble versions of the recombinant Siglecs in conjunction with flow cytometry. Flow cytometry has many advantages in enabling the relative levels of Siglec ligands between cell types to be rapidly quantified. Here, a step-by-step protocol is described on how to detect Siglec ligands most sensitively and accurately on cells by flow cytometry.

Sialylation as an Important Regulator of Antibody Function

Antibodies play a critical role in linking the adaptive immune response to the innate immune system. In humans, antibodies are categorized into five classes, IgG, IgM, IgA, IgE, and IgD, based on constant region sequence, structure, and tropism. In serum, IgG is the most abundant antibody, comprising 75% of antibodies in circulation, followed by IgA at 15%, IgM at 10%, and IgD and IgE are the least abundant. All human antibody classes are post-translationally modified by sugars. The resulting glycans take on many divergent structures and can be attached in an N-linked or O-linked manner, and are distinct by antibody class, and by position on each antibody. Many of these glycan structures on antibodies are capped by sialic acid. It is well established that the composition of the N-linked glycans on IgG exert a profound influence on its effector functions. However, recent studies have described the influence of glycans, particularly sialic acid for other antibody classes. Here, we discuss the role of glycosylation, with a focus on terminal sialylation, in the biology and function across all antibody classes. Sialylation has been shown to influence not only IgG, but IgE, IgM, and IgA biology, making it an important and unappreciated regulator of antibody function.

Glycoengineering of Mammalian Expression Systems on a Cellular Level

Mammalian expression systems such as Chinese hamster ovary (CHO), mouse myeloma (NS0), and human embryonic kidney (HEK) cells serve a critical role in the biotechnology industry as the production host of choice for recombinant protein therapeutics. Most of the recombinant biologics are glycoproteins that contain complex oligosaccharide or glycan attachments representing a principal component of product quality. Both N-glycans and O-glycans are present in these mammalian cells, but the engineering of N-linked glycosylation is of critical interest in industry and many efforts have been directed to improve this pathway. This is because altering the N-glycan composition can change the product quality of recombinant biotherapeutics in mammalian hosts. In addition, sialylation and fucosylation represent components of the glycosylation pathway that affect circulatory half-life and antibody-dependent cellular cytotoxicity, respectively. In this chapter, we first offer an overview of the glycosylation, sialylation, and fucosylation networks in mammalian cells, specifically CHO cells, which are extensively used in antibody production. Next, genetic engineering technologies used in CHO cells to modulate glycosylation pathways are described. We provide examples of their use in CHO cell engineering approaches to highlight these technologies further. Specifically, we describe efforts to overexpress glycosyltransferases and sialyltransfereases, and efforts to decrease sialidase cleavage and fucosylation. Finally, this chapter covers new strategies and future directions of CHO cell glycoengineering, such as the application of glycoproteomics, glycomics, and the integration of 'omics' approaches to identify, quantify, and characterize the glycosylated proteins in CHO cells. Graphical Abstract.

Multiplexed engineering glycosyltransferase genes in CHO cells via targeted integration for producing antibodies with diverse complex-type N-glycans

Therapeutic antibodies are decorated with complex-type N-glycans that significantly affect their biodistribution and bioactivity. The N-glycan structures on antibodies are incompletely processed in wild-type CHO cells due to their limited glycosylation capacity. To improve N-glycan processing, glycosyltransferase genes have been traditionally overexpressed in CHO cells to engineer the cellular N-glycosylation pathway by using random integration, which is often associated with large clonal variations in gene expression levels. In order to minimize the clonal variations, we used recombinase-mediated-cassette-exchange (RMCE) technology to overexpress a panel of 42 human glycosyltransferase genes to screen their impact on antibody N-linked glycosylation. The bottlenecks in the N-glycosylation pathway were identified and then released by overexpressing single or multiple critical genes. Overexpressing B4GalT1 gene alone in the CHO cells produced antibodies with more than 80% galactosylated bi-antennary N-glycans. Combinatorial overexpression of B4GalT1 and ST6Gal1 produced antibodies containing more than 70% sialylated bi-antennary N-glycans. In addition, antibodies with various tri-antennary N-glycans were obtained for the first time by overexpressing MGAT5 alone or in combination with B4GalT1 and ST6Gal1. The various N-glycan structures and the method for producing them in this work provide opportunities to study the glycan structure-and-function and develop novel recombinant antibodies for addressing different therapeutic applications.

Glycoengineering Chinese hamster ovary cells: a short history

Biotherapeutic glycoproteins have revolutionised the field of pharmaceuticals, with new discoveries and continuous improvements underpinning the rapid growth of this industry. N-glycosylation is a critical quality attribute of biotherapeutic glycoproteins that influences the efficacy, half-life and immunogenicity of these drugs. This review will focus on the advances and future directions of remodelling N-glycosylation in Chinese hamster ovary (CHO) cells, which are the workhorse of recombinant biotherapeutic production, with particular emphasis on antibody products, using strategies such as cell line and protein backbone engineering.

N-Glycosylation of IgG and IgG-Like Recombinant Therapeutic Proteins: Why Is It Important and How Can We Control It?

Regulatory bodies worldwide consider N-glycosylation to be a critical quality attribute for immunoglobulin G (IgG) and IgG-like therapeutics. This consideration is due to the importance of posttranslational modifications in determining the efficacy, safety, and pharmacokinetic properties of biologics. Given its critical role in protein therapeutic production, we review N-glycosylation beginning with an overview of the myriad interactions of N-glycans with other biological factors. We examine the mechanism and drivers for N-glycosylation during biotherapeutic production and the several competing factors that impact glycan formation, including the abundance of precursor nucleotide sugars, transporters, glycosidases, glycosyltransferases, and process conditions. We explore the role of these factors with a focus on the analytical approaches used to characterize glycosylation and associated processes, followed by the current state of advanced glycosylation modeling techniques. This combination of disciplines allows for a deeper understanding of N-glycosylation and will lead to more rational glycan control.

Transfection of glycoprotein encoding mRNA for swift evaluation of N-glycan engineering strategies

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 N‐glycosylation 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.

Enhancement of sialylation in rIgG in glyco-engineered Chinese hamster ovary cells

Since about 70% of commercial biopharmaceutical products have been produced in Chinese hamster ovary (CHO) cells, this cell line is undeniably a workhorse for biopharmaceuticals production. Meanwhile, sialic acid terminals were reported to affect anti-inflammatory activity, antibody-dependent cellular cytotoxicity efficacy of IgG antibodies. Taking these findings together, we aimed to establish CHO cell lines that highly produce sialic acid terminals by overexpressing two N-acetylneuraminic acid-based key enzymes, α(2,6)-sialyltransferase and UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase using dihydrofolate reductase/methotrexate gene amplification method. Indeed, the number of total sialic acid terminal glycan structures increased tremendously, by 12-fold compared to the wild type in total protein extracts. With the methotrexate supplementation, a targeted cell line, CHOmt17-100, showed up to 1.4 times more sialylated structures of glycoforms in total proteins. Interestingly, immunoglobulin G, used as the model protein in CHOmt17-100, showed about 53% sialylated structures in its glycoforms. These resultant sialylated glycans exhibited more than approximately 14.5 times increase as compared to that of the wild type. Moreover, the resultant glycan structures mostly had N-acetylneuraminic acid terminals, while N-glycolylneuraminic acid terminal composition remained less than 5% as compared to the wild type. Engineered antibodies derived from CHO cell lines that produce high levels of sialic acid will contribute to the examination of glycoforms' efficacy and usefulness toward bio-better products.

Knockout of sialidase and pro-apoptotic genes in Chinese hamster ovary cells enables the production of recombinant human erythropoietin in fed-batch cultures

Sialic acid, a terminal monosaccharide present in N-glycans, plays an important role in determining both the in vivo half-life and the therapeutic efficacy of recombinant glycoproteins. Low sialylation levels of recombinant human erythropoietin (rhEPO) in recombinant Chinese hamster ovary (rCHO) cell cultures are considered a major obstacle to the production of rhEPO in fed-batch mode. This is mainly due to the accumulation of extracellular sialidases released from the cells. To overcome this hurdle, three sialidase genes (Neu1, 2, and 3) were initially knocked-out using the CRISPR/Cas9-mediated large deletion method in the rhEPO-producing rCHO cell line. Unlike wild type cells, sialidase knockout (KO) clones maintained the sialic acid content and proportion of tetra-sialylated rhEPO throughout fed-batch cultures without exhibiting a detrimental effect with respect to cell growth and rhEPO production. Additional KO of two pro-apoptotic genes, BAK and BAX, in sialidase KO clones (5X KO clones) further improved rhEPO production without any detrimental effect on sialylation. On day 10 in fed-batch cultures, the 5X KO clones had 1.4-times higher rhEPO concentration and 3.0-times higher sialic acid content than wild type cells. Furthermore, the proportion of tetra-sialylated rhEPO on day 10 in fed-batch cultures was 42.2-44.3% for 5X KO clones while it was only 2.2% for wild type cells. Taken together, KO of sialidase and pro-apoptotic genes in rCHO cells is a useful tool for producing heavily sialylated glycoproteins such as rhEPO in fed-batch mode.

Model-Driven Engineering of N-Linked Glycosylation in Chinese Hamster Ovary Cells

Chinese hamster ovary (CHO) cells are used for industrial production of protein-based therapeutics (i.e., "biologics"). Here we describe a method for combining systems-level kinetic models with a synthetic biology platform for multigene overexpression to rationally perturb N-linked glycosylation. Specifically, we sought to increase galactose incorporation on a secreted Immunoglobulin G (IgG) protein. We rationally design, build, and test a total of 23 transgenic cell pools that express single or three-gene glycoengineering cassettes comprising a total of 100 kilobases of engineered DNA sequence. Through iterative engineering and model refinement, we rationally increase the fraction of bigalactosylated glycans five-fold from 11.9% to 61.9% and simultaneously decrease the glycan heterogeneity on the secreted IgG. Our approach allows for rapid hypothesis testing and identification of synergistic behavior from genetic perturbations by bridging systems and synthetic biology.

Coexpression of CMP-sialic acid transporter reduces N-glycolylneuraminic acid levels of recombinant glycoproteins in Chinese hamster ovary cells

Recombinant glycoproteins expressed in Chinese hamster ovary (CHO) cells contain two forms of sialic acids; N-acetylneuraminic acid (Neu5Ac) as a major type and N-glycolylneuraminic acid (Neu5Gc) as a minor type. The Neu5Gc glycan moieties in therapeutic glycoproteins can elicit immune responses because they do not exist in human. In the present work, to reduce Neu5Gc levels of recombinant glycoproteins from CHO cell cultures, we coexpressed cytidine-5'-monophosphate-sialic acid transporter (CMP-SAT) that is an antiporter and transports cytosolic CMP-sialic acids (both forms) into Golgi lumen. When human erythropoietin was used as a target human glycoprotein, coexpression of CMP-SAT resulted in a significant decrease of Neu5Gc level by 41.4% and a notable increase of Neu5Ac level by 21.2%. This result could be reasonably explained by our hypothesis that the turnover rate of Neu5Ac to Neu5Gc catalyzed by CMP-Neu5Ac hydroxylase would be reduced through facilitated transportation of Neu5Ac into Golgi apparatus by coexpression of CMP-SAT. We confirmed the effects of CMP-SAT coexpression on the decrease of Neu5Gc level and the increase of Neu5Ac level using another glycoprotein human DNase I. Therefore, CMP-SAT coexpression might be an effective strategy to reduce the levels of undesired Neu5Gc in recombinant therapeutic glycoproteins from CHO cell cultures.

Improving Immunotherapy Through Glycodesign

Immunotherapy is revolutionizing health care, with the majority of high impact "drugs" approved in the past decade falling into this category of therapy. Despite considerable success, glycosylation-a key design parameter that ensures safety, optimizes biological response, and influences the pharmacokinetic properties of an immunotherapeutic-has slowed the development of this class of drugs in the past and remains challenging at present. This article describes how optimizing glycosylation through a variety of glycoengineering strategies provides enticing opportunities to not only avoid past pitfalls, but also to substantially improve immunotherapies including antibodies and recombinant proteins, and cell-based therapies. We cover design principles important for early stage pre-clinical development and also discuss how various glycoengineering strategies can augment the biomanufacturing process to ensure the overall effectiveness of immunotherapeutics.

Metabolic engineering of CHO cells to prepare glycoproteins

As a complex and common post-translational modification, N-linked glycosylation affects a recombinant glycoprotein's biological activity and efficacy. For example, the α1,6-fucosylation significantly affects antibody-dependent cellular cytotoxicity and α2,6-sialylation is critical for antibody anti-inflammatory activity. Terminal sialylation is important for a glycoprotein's circulatory half-life. Chinese hamster ovary (CHO) cells are currently the predominant recombinant protein production platform, and, in this review, the characteristics of CHO glycosylation are summarized. Moreover, recent and current metabolic engineering strategies for tailoring glycoprotein fucosylation and sialylation in CHO cells, intensely investigated in the past decades, are described. One approach for reducing α1,6-fucosylation is through inhibiting fucosyltransferase (FUT8) expression by knockdown and knockout methods. Another approach to modulate fucosylation is through inhibition of multiple genes in the fucosylation biosynthesis pathway or through chemical inhibitors. To modulate antibody sialylation of the fragment crystallizable region, expressions of sialyltransferase and galactotransferase individually or together with amino acid mutations can affect antibody glycoforms and further influence antibody effector functions. The inhibition of sialidase expression and chemical supplementations are also effective and complementary approaches to improve the sialylation levels on recombinant glycoproteins. The engineering of CHO cells or protein sequence to control glycoforms to produce more homogenous glycans is an emerging topic. For modulating the glycosylation metabolic pathways, the interplay of multiple glyco-gene knockouts and knockins and the combination of multiple approaches, including genetic manipulation, protein engineering and chemical supplementation, are detailed in order to achieve specific glycan profiles on recombinant glycoproteins for superior biological function and effectiveness.

The contributions of individual galactosyltransferases to protein specific N-glycan processing in Chinese Hamster Ovary cells

Galactosylation as part of N-glycan processing is conducted by a set of beta-1,4-galactosyltransferases (B4GALTs), with B4GALT1 as the dominant isoenzyme for this reaction. Nevertheless, the exact contributions of this key-player as well as of the other isoenzymes involved in N-glycosylation, B4GALT2, B4GALT3 and B4GALT4, have not been studied in-depth. To increase the understanding of the protein- and site-specific activities of individual galactosyltransferases in Chinese Hamster Ovary cells, a panel of triple deletion cell lines was generated that expressed only one isoform of B4GALT each. Two model proteins were selected for this study to cover a large spectrum of possible N-glycan structures: erythropoietin and deamine-oxidase. They were expressed as Fc-fusion constructs (EPO-Fc and Fc-DAO) and their N-glycan processing status was analyzed by site-specific mass spectrometry. The sole activity of B4GALT1 resulted in a decrease of 15-21 % of fully galactosylated structures for erythropoietin, emphasizing the involvement of other isoenzymes. Interestingly, the contributions of B4GALT2 and B4GALT3 differed for the two model proteins. Unexpectedly, removal of galactosyltransferases influenced the overall process of N-glycan maturation, with the result of a higher occurrence of poorly processed oligosaccharides. In the context of high productivity cell lines, which can push N-glycan maturation towards incomplete galactosylation, galactosyltransferases are potential targets to ensure stable product quality. In view of our results, specifically engineered "designer" cell lines may be required for different proteins.

Inhibition of poly-LacNAc biosynthesis with release of CMP-Neu5Ac feedback inhibition increases the sialylation of recombinant EPO produced in CHO cells

Sialylation of recombinant therapeutic glycoproteins modulates their pharmacokinetic properties by affecting their in vivo half-life. N-glycan branching on glycoproteins increases the number of potential attachment sites for sialic acid. Here, we introduce a new approach for increasing the sialylation of recombinant human erythropoietin (rhEPO) produced in CHO cells by modulating poly-N-acetyllactosamine (poly-LacNAc) biosynthesis. We did not observe an increase in rhEPO sialylation, however, until the feedback inhibition by intracellular cytidine monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac), which is a limiting factor for sialylation, was released. Thus, we found that a combined approach inhibiting poly-LacNAc biosynthesis and releasing CMP-Neu5Ac feedback inhibition produces the most significant increase in rhEPO sialylation in metabolically engineered CHO cells. Furthermore, a detailed analysis of the resulting N-glycan structures using LC/MS revealed increased tri- and tetra- sialylated N-glycan structures accompanied by a reduction of di-sialylated N-glycan structures. These results validate our new approach for glycosylation engineering, and we expect this approach will be useful in future efforts to enhance the efficacy of other therapeutic glycoproteins.

Glycoengineering in CHO Cells: Advances in Systems Biology

For several decades, glycoprotein biologics have been successfully produced from Chinese hamster ovary (CHO) cells. The therapeutic efficacy and potency of glycoprotein biologics are often dictated by their post-translational modifications, particularly glycosylation, which unlike protein synthesis, is a non-templated process. Consequently, both native and recombinant glycoprotein production generate heterogeneous mixtures containing variable amounts of different glycoforms. Stability, potency, plasma half-life, and immunogenicity of the glycoprotein biologic are directly influenced by the glycoforms. Recently, CHO cells have also been explored for production of therapeutic glycosaminoglycans (e.g., heparin), which presents similar challenges as producing glycoproteins biologics. Approaches to controlling heterogeneity in CHO cells and directing the biosynthetic process toward desired glycoforms are not well understood. A systems biology approach combining different technologies is needed for complete understanding of the molecular processes accounting for this variability and to open up new venues in cell line development. In this review, we describe several advances in genetic manipulation, modeling, and glycan and glycoprotein analysis that together will provide new strategies for glycoengineering of CHO cells with desired or enhanced glycosylation capabilities.

Enhancement of glycosylation by stable co-expression of two sialylation-related enzymes on Chinese hamster ovary cells

Sialic acid plays important roles in stabilization and modulation of the interaction of molecules and membranes in organisms. Due to its high electronegativity, sialic acid can promote binding effects of molecules and support the transportation of drugs and ions in cells. This also strengthens cells against degradation from glycosidases and proteases. Hence sialic acid helps glycoproteins extend their half-lives and bioactivity. On the other hand, Chinese hamster ovary (CHO) cells have been widely used as a workhorse in biopharmaceutical fields in part due to the similarity between their glycan properties and those in humans. Thus, a high sialylation produced by CHO host cell line is strongly desired. In this study, we simultaneously overexpressed two key sialylated-based enzymes human β-galactoside α(2,6) sialyltransferase I and UDP-GlcNAc 2-epimerase/ManNAc kinase to achieve greater sialylation pattern produced host cells. The single-cell line thus-generated produced an approximately 41.6% higher level of total free sialic acid, and the glycan profiles showed a significant increase of more than 7-fold in the relative amount of total sialylated N-glycan as compared to the wild-type. These results demonstrated that co-expression of these two sialylated-based key enzymes yielded a cell line that effectively produced glycoproteins with superior sialylation and achievable human-like glycoforms.

Enhancing the sialylation of recombinant EPO produced in CHO cells via the inhibition of glycosphingolipid biosynthesis

Sialylation regulates the in vivo half-life of recombinant therapeutic glycoproteins, affecting their therapeutic efficacy. Levels of the precursor molecule cytidine monophospho-N-acetylneuraminic acid (CMP-Neu5Ac) are considered a limiting factor in the sialylation of glycoproteins. Here, we show that by reducing the amount of intracellular CMP-Neu5Ac consumed for glycosphingolipid (GSL) biosynthesis, we can increase the sialylation of recombinant human erythropoietin (rhEPO) produced in CHO cells. Initially, we found that treating CHO cells with a potent inhibitor of GSL biosynthesis increases the sialylation of the rhEPO they produce. Then, we established a stable CHO cell line that produces rhEPO in the context of repression of the key GSL biosynthetic enzyme UDP-glucose ceramide glucosyltransferase (UGCG). These UGCG-depleted cells show reduced levels of gangliosides and significantly elevated levels of rhEPO sialylation. Upon further analysis of the resulting N-glycosylation pattern, we discovered that the enhanced rhEPO sialylation could be attributed to a decrease in neutral and mono-sialylated N-glycans and an increase in di-sialylated N-glycans. Our results suggest that the therapeutic efficacy of rhEPO produced in CHO cells can be improved by shunting intracellular CMP-Neu5Ac away from GSL biosynthesis and toward glycoprotein sialylation.

Combining metabolic and process engineering strategies to improve recombinant glycoprotein production and quality

Increasing recombinant protein production while ensuring a high and consistent protein quality remains a challenge in mammalian cell culture process development. In this work, we combined a nutrient substitution approach with a metabolic engineering strategy that improves glucose utilization efficiency. This combination allowed us to tackle both lactate and ammonia accumulation and investigate on potential synergistic effects on protein production and quality. To this end, HEK293 cells overexpressing the pyruvate yeast carboxylase (PYC2) and their parental cells, both stably producing the therapeutic glycoprotein interferon α2b (IFNα2b), were cultured in media deprived of glutamine but containing chosen substitutes. Among the tested substitutes, pyruvate led to the best improvement in growth (integral of viable cell density) for both cell lines in batch cultures, whereas the culture of PYC2 cells without neither glutamine nor any substitute displayed surprisingly enhanced IFNα2b production. The drastic reduction in both lactate and ammonia in the cultures translated into extended high viability conditions and an increase in recombinant protein titer by up to 47% for the parental cells and the PYC2 cells. Product characterization performed by surface plasmon resonance biosensing using Sambucus nigra (SNA) lectin revealed that the increase in yield was however accompanied by a reduction in the degree of sialylation of the product. Supplementing cultures with glycosylation precursors and a cofactor were effective at counterbalancing the lack of glutamine and allowed improvement in IFNα2b quality as evaluated by lectin affinity. Our study provides a strategy to reconcile protein productivity and quality and highlights the advantages of PYC2-overexpressing cells in glutamine-free conditions.

A novel sugar analog enhances sialic acid production and biotherapeutic sialylation in CHO cells

A desirable feature of many therapeutic glycoprotein production processes is to maximize the final sialic acid content. In this study, the effect of applying a novel chemical analog of the sialic acid precursor N-acetylmannosamine (ManNAc) on the sialic acid content of cellular proteins and a model recombinant glycoprotein, erythropoietin (EPO), was investigated in CHO-K1 cells. By introducing the 1,3,4-O-Bu₃ ManNAc analog at 200-300 µM into cell culture media, the intracellular sialic acid content of EPO-expressing cells increased ∼8-fold over untreated controls while the level of cellular sialylated glycoconjugates increased significantly as well. For example, addition of 200-300 µM 1,3,4-O-Bu₃ ManNAc resulted in >40% increase in final sialic acid content of recombinant EPO, while natural ManNAc at ∼100 times higher concentration of 20 mM produced a less profound change in EPO sialylation. Collectively, these results indicate that butyrate-derivatization of ManNAc improves the capacity of cells to incorporate exogenous ManNAc into the sialic acid biosynthetic pathway and thereby increase sialylation of recombinant EPO and other glycoproteins. This study establishes 1,3,4-O-Bu₃ ManNAc as a novel chemical supplement to improve glycoprotein quality and sialylation levels at concentrations orders of magnitude lower than alternative approaches. Biotechnol. Bioeng. 2017;114: 1899-1902. © 2017 Wiley Periodicals, Inc.

Glycoengineering of CHO Cells to Improve Product Quality

Chinese hamster ovary (CHO) cells represent the predominant platform in biopharmaceutical industry for the production of recombinant biotherapeutic proteins, especially glycoproteins. These glycoproteins include oligosaccharide or glycan attachments that represent one of the principal components dictating product quality. Especially important are the N-glycan attachments present on many recombinant glycoproteins of commercial interest. Furthermore, altering the glycan composition can be used to modulate the production quality of a recombinant biotherapeutic from CHO and other mammalian hosts. This review first describes the glycosylation network in mammalian cells and compares the glycosylation patterns between CHO and human cells. Next genetic strategies used in CHO cells to modulate the sialylation patterns through overexpression of sialyltransfereases and other glycosyltransferases are summarized. In addition, other approaches to alter sialylation including manipulation of sialic acid biosynthetic pathways and inhibition of sialidases are described. Finally, this review also covers other strategies such as the glycosylation site insertion and manipulation of glycan heterogeneity to produce desired glycoforms for diverse biotechnology applications.

Engineer Medium and Feed for Modulating N-Glycosylation of Recombinant Protein Production in CHO Cell Culture

Chinese hamster ovary (CHO) cells have become the primary expression system for the production of complex recombinant proteins due to their long-term success in industrial scale production and generating appropriate protein N-glycans similar to that of humans. Control and optimization of protein N-glycosylation is crucial, as the structure of N-glycans can largely influence both biological and physicochemical properties of recombinant proteins. Protein N-glycosylation in CHO cell culture can be controlled and tuned by engineering medium, feed, culture process, as well as genetic elements of the cell. In this chapter, we will focus on how to carry out experiments for N-glycosylation modulation through medium and feed optimization. The workflow and typical methods involved in the experiment process will be presented.

N-Glycosylation Design and Control of Therapeutic Monoclonal Antibodies

The N-linked glycan profiles on recombinant monoclonal antibody therapeutics significantly affect antibody biological functions and are largely determined by host cell genotypes and culture conditions. A key step in bioprocess development for monoclonal antibodies (mAbs) involves optimization and control of N-glycan profiles. With pressure from pricing and biosimilars looming, more efficient and effective approaches are sought in the field of glycoengineering. Metabolic studies and mathematical modeling are two such approaches that optimize bioprocesses by better understanding and predicting glycosylation. In this review, we summarize a group of strategies currently used for glycan profile modulation and control. Metabolic analysis and mathematical modeling are then explored with an emphasis on how these two techniques can be utilized to advance glycoengineering.

Cell line development for biomanufacturing processes: recent advances and an outlook

At the core of a biomanufacturing process for recombinant proteins is the production cell line. It influences the productivity and product quality. Its characteristics also dictate process development, as the process is optimized to complement the producing cell to achieve the target productivity and quality. Advances in the past decade, from vector design to cell line screening, have greatly expanded our capability to attain producing cell lines with certain desired traits. Increasing availability of genomic and transcriptomic resources for industrially important cell lines coupled with advances in genome editing technology have opened new avenues for cell line development. These developments are poised to help biosimilar manufacturing, which requires targeting pre-defined product quality attributes, e.g., glycoform, to match the innovator's range. This review summarizes recent advances and discusses future possibilities in this area.

Chinese hamster ovary (CHO) host cell engineering to increase sialylation of recombinant therapeutic proteins by modulating sialyltransferase expression

N-Glycans of human proteins possess both α2,6- and α2,3-linked terminal sialic acid (SA). Recombinant glycoproteins produced in Chinese hamster overy (CHO) only have α2,3-linkage due to the absence of α2,6-sialyltransferase (St6gal1) expression. The Chinese hamster ST6GAL1 was successfully overexpressed using a plasmid expression vector in three recombinant immunoglobulin G (IgG)-producing CHO cell lines. The stably transfected cell lines were enriched for ST6GAL1 overexpression using FITC-Sambucus nigra (SNA) lectin that preferentially binds α2,6-linked SA. The presence of α2,6-linked SA was confirmed using a novel LTQ Linear Ion Trap Mass Spectrometry (LTQ MS) method including MSn fragmentation in the enriched ST6GAL1 Clone 27. Furthermore, the total SA (mol/mol) in IgG produced by the enriched ST6GAL1 Clone 27 increased by 2-fold compared to the control. For host cell engineering, the CHOZN(®) GS host cell line was transfected and enriched for ST6GAL1 overexpression. Single-cell clones were derived from the enriched population and selected based on FITC-SNA staining and St6gal1 expression. Two clones ("ST6GAL1 OE Clone 31 and 32") were confirmed for the presence of α2,6-linked SA in total host cell protein extracts. ST6GAL1 OE Clone 32 was subsequently used to express SAFC human IgG1. The recombinant IgG expressed in this host cell line was confirmed to have α2,6-linked SA and increased total SA content. In conclusion, overexpression of St6gal1 is sufficient to produce recombinant proteins with increased sialylation and more human-like glycoprofiles without combinatorial engineering of other sialylation pathway genes. This work represents our ongoing effort of glycoengineering in CHO host cell lines for the development of "bio-better" protein therapeutics and cell culture vaccine production.

Strategies for Engineering Protein N-Glycosylation Pathways in Mammalian Cells

Complexity and heterogeneity of oligosaccharides present a considerable challenge to the biopharmaceutical industry to manufacture biotherapeutics with reproducible and consistent glycoform profiles. Mammalian cells, especially Chinese hamster ovary cells, are the most widely used platform for the production of biotherapeutics. The glycans produced are predominantly of the complex type, with some differences between human and nonhuman mammalian glycosylation existing. This review briefly summarizes metabolic glyco-engineering strategies used in mammalian cells in order to alter the glycosylation patterns attached to proteins applied for diverse biotechnology applications.

Stabilization of cellular mitochondrial enzyme complex and sialyltransferase activity through supplementation of 30Kc19 protein

In previous studies, 30Kc19, a lipoprotein in silkworm hemolymph, enhanced productivity and glycosylation by expression of a 30Kc19 gene or supplementation with a recombinant 30Kc19 protein. Additionally, 30Kc19 exhibited enzyme-stabilizing and cell-penetrating abilities in vitro. In this study, we hypothesized that supplemented 30Kc19 penetrated into the cell and enhanced the stability of the cellular enzyme. We investigated this using in vitro and cellular assessments. The activity of sialyltransferase (ST) and isolated mitochondrial complex I/III was enhanced with 30Kc19 in dose-dependent manner while initial reaction rate was unchanged, suggesting that 30Kc19 enhanced enzyme stability rather than specific activity. For intracellular enzyme activity assessment, ST activity inside erythropoietin (EPO)-producing Chinese hamster ovary (CHO) cells increased more than 25 % and mitochondrial complex II activity in HeLa cells increased more than 50 % with 30Kc19. The increase in intracellular ST activity resulted in an increase in sialic acid content of glycoproteins produced in CHO cells supplemented with 30Kc19. Similarly, enhanced mitochondrial complex activity increased mitochondrial membrane potential and ATP production in HeLa cells with 30Kc19 by over 50 %. Because 30Kc19 stabilized intracellular enzymes for glycosylation and enhanced protein productivity with supplementation in the culture medium, we expect that 30Kc19 can be a valuable tool for effective industrial recombinant protein production.

Biological Insights into Therapeutic Protein Modifications throughout Trafficking and Their Biopharmaceutical Applications

Over the lifespan of therapeutic proteins, from the point of biosynthesis to the complete clearance from tested subjects, they undergo various biological modifications. Therapeutic influences and molecular mechanisms of these modifications have been well appreciated for some while remained less understood for many. This paper has classified these modifications into multiple categories, according to their processing locations and enzymatic involvement during the trafficking events. It also focuses on the underlying mechanisms and structural-functional relationship between modifications and therapeutic properties. In addition, recent advances in protein engineering, cell line engineering, and process engineering, by exploring these complex cellular processes, are discussed and summarized, for improving functional characteristics and attributes of protein-based biopharmaceutical products.

CHO glycosylation mutants as potential host cells to produce therapeutic proteins with enhanced efficacy

CHO glycosylation mutants, pioneered by Stanley and co-workers, have proven to be valuable tools in glycobiology and biopharmaceutical research. Here we aim to provide a summary of our efforts to isolate industrially applicable CHO glycosylation mutants, termed CHO-gmt cells, using cytotoxic lectins and zinc-finger nuclease technology. The genetic defects in the glycosylation machinery in these cells lead to the production of recombinant glycoproteins with consistent and unique glycan structures. In addition, these mutant cells can be easily adapted to serum-free medium in suspension cultures, the condition used by the biotech industry for large-scale production of recombinant therapeutics. In light of the critical impact of glycosylation on biopharmaceutical performances, namely, safety and efficacy, the CHO-gmt lines have enormous potential in producing glycoprotein therapeutics with optimal glycosylation profiles, thus, representing a panel of ideal host cell lines for producing recombinant biopharmaceuticals with improved safety profiles and enhanced efficacy.

Enhanced sialylation of recombinant human erythropoietin in Chinese hamster ovary cells by combinatorial engineering of selected genes

Therapeutic glycoproteins with exposed galactose (Gal) residues are cleared rapidly from the bloodstream by asialoglycoprotein receptors in hepatocytes. Various approaches have been used to increase the content of sialic acid, which occupies terminal sites of N- or O-linked glycans and thereby increases the half-life of therapeutic glycoproteins. We enhanced sialylation of human erythropoietin (EPO) by genetic engineering of the sialylation pathway in Chinese hamster ovary (CHO) cells. The enzyme GNE (uridine diphosphate-N-acetyl glucosamine 2-epimerase)/MNK (N-acetyl mannosamine kinase), which plays a key role in the initial two steps of sialic acid biosynthesis, is regulated by cytidine monophosphate (CMP)-sialic acid through a feedback mechanism. Since sialuria patient cells fail in regulating sialic acid biosynthesis by feedback mechanism, various sialuria-like mutated rat GNEs were established and subjected to in vitro activity assay. GNE/MNK-R263L-R266Q mutant showed 93.6% relative activity compared with wild type and did not display feedback inhibition. Genes for sialuria-mutated rat GNE/MNK, Chinese hamster CMP-sialic acid transporter and human α2,3-sialyltransferase (α2,3-ST) were transfected simultaneously into recombinant human (rh) EPO-producing CHO cells. CMP-sialic acid concentration of engineered cells was significantly (>10-fold) increased by sialuria-mutated GNE/MNK (R263L-R266Q) expression. The sialic acid content of rhEPO produced from engineered cells was 43% higher than that of control cells. Ratio of tetra-sialylated glycan of rhEPO produced from engineered cells was increased ∼32%, but ratios of asialo- and mono-sialylated glycans were decreased ∼50%, compared with control. These findings indicate that sialuria-mutated rat GNE/MNK effectively increases the intracellular CMP-sialic acid level. The newly constructed host CHO cell lines produced more highly sialylated therapeutic glycoproteins through overexpression of sialuria-mutated GNE/MNK, CMP-SAT and α2,3-ST.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for the period 2005-2006

This review is the fourth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2006. The review covers fundamental studies, fragmentation of carbohydrate ions, method developments, and applications of the technique to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, glycated proteins, glycolipids from bacteria, glycosides, and various other natural products. There is a short section on the use of MALDI-TOF mass spectrometry for the study of enzymes involved in glycan processing, a section on industrial processes, particularly the development of biopharmaceuticals and a section on the use of MALDI-MS to monitor products of chemical synthesis of carbohydrates. Large carbohydrate-protein complexes and glycodendrimers are highlighted in this final section.

Protein glycosylation control in mammalian cell culture: past precedents and contemporary prospects

Protein glycosylation is a post-translational modification of paramount importance for the function, immunogenicity, and efficacy of recombinant glycoprotein therapeutics. Within the repertoire of post-translational modifications, glycosylation stands out as having the most significant proven role towards affecting pharmacokinetics and protein physiochemical characteristics. In mammalian cell culture, the understanding and controllability of the glycosylation metabolic pathway has achieved numerous successes. However, there is still much that we do not know about the regulation of the pathway. One of the frequent conclusions regarding protein glycosylation control is that it needs to be studied on a case-by-case basis since there are often conflicting results with respect to a control variable and the resulting glycosylation. In attempts to obtain a more multivariate interpretation of these potentially controlling variables, gene expression analysis and systems biology have been used to study protein glycosylation in mammalian cell culture. Gene expression analysis has provided information on how glycosylation pathway genes both respond to culture environmental cues, and potentially facilitate changes in the final glycoform profile. Systems biology has allowed researchers to model the pathway as well-defined, inter-connected systems, allowing for the in silico testing of pathway parameters that would be difficult to test experimentally. Both approaches have facilitated a macroscopic and microscopic perspective on protein glycosylation control. These tools have and will continue to enhance our understanding and capability of producing optimal glycoform profiles on a consistent basis.

A flexible state-space approach for the modeling of metabolic networks II: advanced interrogation of hybridoma metabolism

Having previously introduced the mathematical framework of topological metabolic analysis (TMA) - a novel optimization-based technique for modeling metabolic networks of arbitrary size and complexity - we demonstrate how TMA facilitates unique methods of metabolic interrogation. With the aid of several hybridoma metabolic investigations as case-studies (Bonarius et al., 1995, 1996, 2001), we first establish that the TMA framework identifies biologically important aspects of the metabolic network under investigation. We also show that the use of a structured weighting approach within our objective provides a substantial modeling benefit over an unstructured, uniform, weighting approach. We then illustrate the strength of TAM as an advanced interrogation technique, first by using TMA to prove the existence of (and to quantitatively describe) multiple topologically distinct configurations of a metabolic network that each optimally model a given set of experimental observations. We further show that such alternate topologies are indistinguishable using existing stoichiometric modeling techniques, and we explain the biological significance of the topological variables appearing within our model. By leveraging the manner in which TMA implements metabolite inputs and outputs, we also show that metabolites whose possible metabolic fates are inadequately described by a given network reconstruction can be quickly identified. Lastly, we show how the use of the TMA aggregate objective function (AOF) permits the identification of modeling solutions that can simultaneously consider experimental observations, underlying biological motivations, or even purely engineering- or design-based goals.