Reduction of CMP-N-acetylneuraminic acid hydroxylase activity in engineered Chinese hamster ovary cells using an antisense-RNA strategy

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.

Strategic feeding of NS0 and CHO cell cultures to control glycan profiles and immunogenic epitopes of monoclonal antibodies

The control of glycosylation profiles is essential to the consistent manufacture of therapeutic monoclonal antibodies that may be produced from a variety of cell lines including CHO and NS0. Of particular concern is the potential for generating non-human epitopes such as N-glycolylneuraminic acid (Neu5Gc) and Galα1-3 Gal that may be immunogenic. We have looked at the effects of a commonly used media supplements of manganese, galactose and uridine (MGU) on Mab production from CHO and NS0 cells in enhancing galactosylation and sialylation as well as the generation of these non-human glycan epitopes. In the absence of the MGU supplement, the humanized IgG1 antibody (Hu1D10) produced from NS0 cells showed a low level of mono- and di-sialylated structures (SI:0.09) of which 75 % of sialic acid was Neu5Gc. The chimeric human-llama Mab (EG2-hFc) produced from CHO cells showed an equally low level of sialylation (SI: 0.12) but the Neu5Gc content of sialic acid was negligible (<3%). Combinations of the MGU supplements added to the production cultures resulted in a substantial increase in the galactosylation of both Mabs (up to GI:0.78 in Hu1D10 and 0.81 in EG2-hFc). However, the effects on sialylation differed between the two Mabs. We observed a slight increase in sialylation of the EG2-hFc Mab by a combination of MG but it appeared that one of the components (uridine) was inhibitory to sialylation. On the other hand, MG or MGU increased sialylation of Hu1D10 substantially (SI:0.72) with an increase that could be attributed predominantly to the formation of Neu5Ac rather than Neu5Gc. The increased level of galactosylation observed with MG or MGU was attributed to an activation of the galactosyl transferase enzymes through enhanced intracellular levels of UDP-Gal and the availability of Mn²⁺ as an enzymic co-factor. However, this effect not only increased the desirable beta 1-4 Gal linkage to GlcNAc but unfortunately in NS0 cells increased the formation of Galα1-3 Gal which was shown to increase x3 in the presence of combinations of the MGU supplements. Supplementation of media with fetal bovine serum (FBS) increased the availability of free Neu5Ac which resulted in a significant increase in the sialylation of Hu1D10 from NS0 cells. This also resulted in a significant decrease in the proportion of Neu5Gc in the measured sialic acid from the Mab.

Osmolality as a lever to modulate the N-glycolylneuraminicacid (Neu5Gc) level of a recombinant glycoprotein produced in Chinese hamster ovary cells

Glycoproteins could be highly sialylated, and controlling the sialic acid levels for some therapeutic proteins is critical to ensure product consistency and efficacy. N-acetylneuraminic acid (Neu5Ac, or NANA) and N-glycolylneuraminic acid (Neu5Gc, or NGNA) are the two most common forms of sialic acids produced in mammalian cells. As Neu5Gc is not produced in humans and can elicit immune responses, minimizing Neu5Gc formation is important in controlling this quality attribute for complex glycoproteins. In this study, a sialylated glycoprotein was used as the model molecule to study the effect of culture osmolality on Neu5Gc. A 14-day fed-batch process with osmolality maintained at physiological levels produced high levels of Neu5Gc. Increase of culture osmolality reduced the Neu5Gc level up to 70-80%, and the effect was proportional to the osmolality level. Through evaluating different osmolality conditions (300-450 mOsm/kg) under low or high pCO₂ , we demonstrated that osmolality could be an effective process lever to modulate the Neu5Gc level. Potential mechanism of osmolality impact on Neu5Gc is discussed and is hypothesized to be cytosol NADH availability related. Compared with cell line engineering efforts, this simple process lever provides the opportunity to readily modulate the Neu5Gc level in a cell culture environment.

Genetic engineering approaches to improve posttranslational modification of biopharmaceuticals in different production platforms

The number of approved biopharmaceuticals, where product quality attributes remain of major importance, is increasing steadily. Within the available variety of expression hosts, the production of biopharmaceuticals faces diverse limitations with respect to posttranslational modifications (PTM), while different biopharmaceuticals demand different forms and specifications of PTMs for proper functionality. With the growing toolbox of genetic engineering technologies, it is now possible to address general as well as host- or biopharmaceutical-specific product quality obstacles. In this review, we present diverse expression systems derived from mammalians, bacteria, yeast, plants, and insects as well as available genetic engineering tools. We focus on genes for knockout/knockdown and overexpression for meaningful approaches to improve biopharmaceutical PTMs and discuss their applicability as well as future trends in the field.

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.

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.

A Novel Variant in CMAH Is Associated with Blood Type AB in Ragdoll Cats

The enzyme cytidine monophospho-N-acetylneuraminic acid hydroxylase is associated with the production of sialic acids on cat red blood cells. The cat has one major blood group with three serotypes; the most common blood type A being dominant to type B. A third rare blood type is known as AB and has an unclear mode of inheritance. Cat blood type antigens are defined, with N-glycolylneuraminic acid being associated with type A and N-acetylneuraminic acid with type B. Blood type AB is serologically characterized by agglutination using typing reagents directed against both A and B epitopes. While a genetic characterization of blood type B has been achieved, the rare type AB serotype remains genetically uncharacterized. A genome-wide association study in Ragdoll cats (22 cases and 15 controls) detected a significant association between blood type AB and SNPs on cat chromosome B2, with the most highly associated SNP being at position 4,487,432 near the candidate gene cytidine monophospho-N-acetylneuraminic acid hydroxylase. A novel variant, c.364C>T, was identified that is highly associated with blood type AB in Ragdoll cats and, to a lesser degree, with type AB in random bred cats. The newly identified variant is probably linked with blood type AB in Ragdoll cats, and is associated with the expression of both antigens (N-glycolylneuraminic acid and N-acetylneuraminic acid) on the red blood cell membrane. Other variants, not identified by this work, are likely to be associated with blood type AB in other breeds of cat.

At least two Fc Neu5Gc residues of monoclonal antibodies are required for binding to anti-Neu5Gc antibody

Two non-human glycan epitopes, galactose-α-1,3-galactose (α-gal) and Neu5Gc-α-2-6-galactose (Neu5Gc) have been shown to be antigenic when attached to Fab oligosaccharides of monoclonal antibodies (mAbs) , while α-gal attached to Fc glycans was not. However, the antigenicity of Neu5Gc on the Fc glycans remains unclear in the context that most mAbs carry only Fc glycans. After studying two clinical mAbs carrying significant amounts of Fc Neu5Gc, we show that their binding activity with anti-Neu5Gc antibody resided in a small subset of mAbs carrying two or more Fc Neu5Gc, while mAbs harboring only one Neu5Gc showed no reactivity. Since most Neu5Gc epitopes were distributed singly on the Fc of mAbs, our results suggest that the potential antigenicity of Fc Neu5Gc is low. Our study could be referenced in the process design and optimization of mAb production in murine myeloma cells and in the quality control of mAbs for industries and regulatory authorities.

Protein glycosylation in diverse cell systems: implications for modification and analysis of recombinant proteins

A major challenge for the biotechnology industry is to engineer the glycosylation pathways of expression systems to synthesize recombinant proteins with human glycosylation. Inappropriate glycosylation can result in reduced activity, limited half-life in circulation and unwanted immunogenicity. In this review, the complexities of glycosylation in human cells are explained and compared with glycosylation in bacteria, yeasts, fungi, insects, plants and nonhuman mammalian species. Key advances in the engineering of the glycosylation of expression systems are highlighted. Advances in the challenging and technically complex field of glycan analysis are also described. The emergence of a new generation of expression systems with sophisticated engineering for humanized glycosylation of glycoproteins appears to be on the horizon.

A serotonin-induced N-glycan switch regulates platelet aggregation

Serotonin (5-HT) is a multifunctional signaling molecule that plays different roles in a concentration-dependent manner. We demonstrated that elevated levels of plasma 5-HT accelerate platelet aggregation resulting in a hypercoagulable state in which the platelet surface becomes occupied by several glycoproteins. Here we study the novel hypothesis that an elevated level of plasma 5-HT results in modification of the content of N-glycans on the platelet surface and this abnormality is associated with platelet aggregation. Mass spectrometry of total surface glycoproteins on platelets isolated from wild-type mice infused for 24 hours with saline or 5-HT revealed that the content of glycoproteins on platelets from 5-HT-infused mice switched from predominantly N-acetyl-neuraminic acid (Neu5Ac) to N-glycolyl-neuraminic acid (Neu5Gc). Cytidine monophosphate-N-acetylneuraminate hydroxylase (CMAH) synthesizes Neu5Gc from Neu5Ac. Up-regulation of Neu5Gc content on the platelet surface resulted from an increase in the catalytic function, not expression, of CMAH in platelets of 5-HT-infused mice. The highest level of Neu5Gc was observed in platelets of 5-HT-infused, 5-HT transporter-knock out mice, suggesting that the surface delineated 5-HT receptor on platelets may promote CMAH catalytic activity. These new findings link elevated levels of plasma 5-HT to altered platelet N-glycan content, a previously unrecognized abnormality that may favor platelet aggregation.

Glycosylation: impact, control and improvement during therapeutic protein production

The emergence of the biopharmaceutical industry represented a major revolution for modern medicine, through the development of recombinant therapeutic proteins that brought new hope for many patients with previously untreatable diseases. There is a ever-growing demand for these therapeutics that forces a constant technological evolution to increase product yields while simultaneously reducing costs. However, the process changes made for this purpose may also affect the quality of the product, a factor that was initially overlooked but which is now a major focus of concern. Of the many properties determining product quality, glycosylation is regarded as one of the most important, influencing, for example, the biological activity, serum half-life and immunogenicity of the protein. Consequently, monitoring and control of glycosylation is now critical in biopharmaceutical manufacturing and a requirement of regulatory agencies. A rapid evolution is being observed in this context, concerning the influence of glycosylation in the efficacy of different therapeutic proteins, the impact on glycosylation of a diversity of parameters/processes involved in therapeutic protein production, the analytical methodologies employed for glycosylation monitoring and control, as well as strategies that are being explored to use this property to improve therapeutic protein efficacy (glycoengineering). This work reviews the main findings on these subjects, providing an up-to-date source of information to support further studies.

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.

A new generation of carbohydrate-based therapeutics: recombinant mucin-type fusion proteins as versatile inhibitors of protein-carbohydrate interactions

Cell surface carbohydrates are essential for a multitude of biomedically important interactions that take place at the cell surface. Carbohydrate-binding proteins are, therefore, significant targets for the development of carbohydrate-based inhibitors. Due to their multivalent character, monovalent low-molecular-weight sugar homologues or analogues are usually poor inhibitors of these interactions. Recent advances in organic and chemoenzymatic synthesis of carbohydrates will undoubtedly increase the pace by which new multivalent carbohydrate-based drugs are developed. Knowledge gained on the glycosyltransferases that are involved in glycan biosynthesis can be used to engineer host cells for recombinant production of proteins with tailored glycan substitution. In particular, recombinant mucin-type proteins can serve as natural scaffolds for multivalent presentation of therapeutic carbohydrate determinants.

Cross validation of liquid chromatography-mass spectrometry and lectin array for monitoring glycosylation in fed-batch glycoprotein production

Glycosylation analysis of recombinant glycoproteins is of importance for the biopharmaceutical industry and the production of glycoprotein pharmaceuticals. A commercially available lectin array technology was evaluated for its ability to present a reproducible fingerprint of a recombinant CTLY4-IgG fusion glycoprotein expressed in large scale CHO-cell fermentation. The glycosylation prediction from the array was compared to traditional negative mode capillary LC-MS of released oligosaccharides. It was shown that both methods provide data that allow samples to be distinguished by their glycosylation pattern. This included information about sialylation, the presence of reducing terminal galactose β1-, terminal N-acetylglucosamine β1-, and antennary distribution. With both methods it was found that a general trend of increased sialylation was associated with an increase of the antenna and reduced amount of terminal galactose β1-, while N-acetylglucosamine β1- was less affected. LC-MS, but not the lectin array, provided valuable information about the sialic acid isoforms present, including N-acetylneuraminic acid, N-glycolylneuraminic acid and their O-acetylated versions. Detected small amounts of high-mannose structures by LC-MS correlated with the detection of the same epitope by the lectin array.

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.

Impact of product-related factors on immunogenicity of biotherapeutics

All protein therapeutics have the potential to be immunogenic. Several factors, including patient characteristics, disease state, and the therapy itself, influence the generation of an immune response. Product-related factors such as the molecule design, the expression system, post-translational modifications, impurities, contaminants, formulation and excipients, container, closure, as well as degradation products are all implicated. However, a critical examination of the available data shows that clear unequivocal evidence for the impact of these latter factors on clinical immunogenicity is lacking. No report could be found that clearly deconvolutes the clinical impact of the product attributes on patient susceptibility. Aggregation carries the greatest concern as a risk factor for immunogenicity, but the impact of aggregates is likely to depend on their structure as well as on the functionality (e.g., immunostimulatory or immunomodulatory) of the therapeutic. Preclinical studies are not yet capable of assessing the clinically relevant immunogenicity potential of these product-related factors. Simply addressing these risk factors as part of product development will not eliminate immunogenicity. Minimization of immunogenicity has to begin at the molecule design stage by reducing or eliminating antigenic epitopes and building in favorable physical and chemical properties.

Engineering mammalian cells in bioprocessing - current achievements and future perspectives

Over the past 20 years, we have seen significant improvements in product titres from 50 mg/l to 5-10 g/l, a more than 100-fold increase. The main methods that have been employed to achieve this increase in product titre have been through the manipulation of culture media and process control strategies, such as the optimization of fed-batch processes. An alternative means to increase productivity has been through the engineering of host cells by altering cellular processes. Recombinant DNA technology has been used to over-express or suppress specific genes to endow particular phenotypes. Cellular processes that have been altered in host cells include metabolism, cell cycle, protein secretion and apoptosis. Cell engineering has also been employed to improve post-translational modifications such as glycosylation. In this article, an overview of the main cell engineering strategies previously employed and the impact of these strategies are presented. Many of these strategies focus on engineering cell lines with more efficient carbon metabolism towards reducing waste metabolites, achieving a biphasic production system by engineering cell cycle control, increasing protein secretion by targeting specific endoplasmic reticulum stress chaperones, delaying cell death by targeting anti-apoptosis genes, and engineering glycosylation by enhancing recombinant protein sialylation and antibody glycosylation. Future perspectives for host cell engineering, and possible areas of research, are also discussed in this review.

Glycosylation pattern of humanized IgG-like bispecific antibody produced by recombinant CHO cells

The glycosylation pattern of a humanized anti-EGFRxanti-CD3 bispecific single-chain diabody with an Fc portion (hEx3-scDb-Fc) produced by recombinant Chinese hamster ovary cells was evaluated and compared with those of a recombinant humanized anti-IL-8 antibody (IgG1) and human serum IgG. N-Linked oligosaccharide structures were estimated by two-dimensional high-performance liquid chromatography and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. No sialylation was observed with purified hEx3-scDb-Fc and the anti-IL-8 antibody. From the analysis of neutral oligosaccharides, approximately more than 90% of the N-linked oligosaccharides of hEx3-scDb-Fc and the anti-IL-8 antibody were alpha-1,6-fucosylated. The galactosylated biantennary oligosaccharides comprise over 40% of the total N-linked oligosaccharides in both hEx3-scDb-Fc and the anti-IL-8 antibody. The fully galactosylated biantennary oligosaccharides from hEx3-scDb-Fc and the anti-IL-8 antibody accounted for only 10% of the N-linked; however, more than 20% of the N-linked oligosaccharides were fully galactosylated biantennary oligosaccharides in human serum IgG. The glycosylation pattern of hEx3-scDb-Fc was quite similar to that of the anti-IL-8 antibody.

From planta to pharma with glycosylation in the toolbox

Plant-specific glycosylation has long been a major limitation to the extensive use of plant-made pharmaceuticals in human therapy. Our goal here is to highlight the progress recently made towards humanization of N-glycosylation in plants and to illustrate that plant-typical N- and O-glycosylation progressively emerge as additional advantages for using this promising expression system.

N-glycan structures and N-glycosylation sites of mouse soluble intercellular adhesion molecule-1 revealed by MALDI-TOF and FTICR mass spectrometry

Intercellular adhesion molecule-1 (ICAM-1) is a heavily N-glycosylated transmembrane protein comprising five extracellular Ig-like domains. The soluble isoform of ICAM-1 (sICAM-1), consisting of its extracellular part, is elevated in the cerebrospinal fluid of patients with severe brain trauma. In mouse astrocytes, recombinant mouse sICAM-1 induces the production of the CXC chemokine macrophage inflammatory protein-2 (MIP-2). MIP-2 induction is glycosylation dependent, as it is strongly enhanced when sICAM-1 carries sialylated, complex-type N-glycans as synthesized by wild-type Chinese hamster ovary (CHO) cells. The present study was aimed at elucidating the N-glycosylation of mouse sICAM-1 expressed in wild-type CHO cells with regard to sialylation, N-glycan profile, and N-glycosylation sites. Ion-exchange chromatography and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) of the released N-glycans showed that sICAM-1 mostly carried di- and trisialylated complex-type N-glycans with or without one fucose. In some sialylated N-glycans, one N-acetylneuraminic acid was replaced by N-glycolylneuraminic acid, and approximately 4% carried a higher number of sialic acid residues than of antennae. The N-glycosylation sites of mouse sICAM-1 were analyzed by MALDI-Fourier transform ion cyclotron resonance (FTICR)-MS and nanoLC-ESI-FTICR-MS of tryptic digests of mouse sICAM-1 expressed in the Lec1 mutant of CHO cells. All nine consensus sequences for N-glycosylation were found to be glycosylated. These results show that the N-glycans that enhance the MIP-2-inducing activity of mouse sICAM-1 are mostly di- and trisialylated complex-type N-glycans including a small fraction carrying more sialic acid residues than antennae and that the nine N-glycosylation sites of mouse sICAM-1 are all glycosylated.

Engineering cells for cell culture bioprocessing--physiological fundamentals

In the past decade, we have witnessed a tremendous increase in the number of mammalian cell-derived therapeutic proteins with clinical applications. The success of making these life-saving biologics available to the public is partly due to engineering efforts to enhance process efficiency. To further improve productivity, much effort has been devoted to developing metabolically engineered producing cells, which possess characteristics favorable for large-scale bioprocessing. In this article we discuss the fundamental physiological basis for cell engineering. Different facets of cellular mechanisms, including metabolism, protein processing, and the balancing pathways of cell growth and apoptosis, contribute to the complex traits of favorable growth and production characteristics. We present our assessment of the current state of the art by surveying efforts that have already been undertaken in engineering cells for a more robust process. The concept of physiological homeostasis as a key determinant and its implications on cell engineering is emphasized. Integrating the physiological perspective with cell culture engineering will facilitate attainment of dream cells with superlative characteristics.

GlycoVis: Visualizing glycan distribution in the proteinN-glycosylation pathway in mammalian cells

Glycosylation has profound effects on the quality of recombinant proteins produced in mammalian cells. The biosynthetic pathways of N-linked glycans on glycoproteins involves a relatively small number of enzymes and nucleotide sugars. Many of these glycoconjugate enzymes can utilize multiple N-glycans as substrates, thus generating a large number of glycan intermediates, and making the biosynthetic pathway resemble a network with diverging and converging paths. The N-glycans on secreted glycoprotein molecules include not only terminal glycans, but also pathway intermediates. To better assess the glycan distribution and the potential route of their synthesis, we created GlycoVis, a visualization program that displays the distribution and the potential reaction paths leading to each N-glycan on the reaction network. The substrate specificities of the enzymes involved were organized into a relationship matrix. With the input of glycan distribution data, the program outputs a reaction pathway map which labels the relative abundance levels of different glycans with different colors. The program also traces all possible reaction paths leading to each glycan and identifies each pathway on the map. Glycoform distribution of Chinese Hamster Ovary cell-derived tissue plasminogen activator (TPA), and human and mouse IgG were used as illustrations for the application of GlycoVis. In addition, the intracellular and secreted IgG from an NS0 producer cell line were isolated, and their glycoform profiles were displayed using GlycoVis for comparison. This visualization tool facilitates the analysis of potential reaction paths utilized under different physiological or culture conditions, and may provide insight on the potential targets for metabolic engineering.

Effect of genetic background on glycosylation heterogeneity in human antithrombin produced in the mammary gland of transgenic goats

Glycosylation is involved in the correct folding, targeting, bioactivity and clearance of therapeutic glycoproteins. With the development of transgenic animals as expression systems it is important to understand the impact of different genetic backgrounds and lactations on glycosylation. We have evaluated the glycosylation of recombinant antithrombin produced in several transgenic goat lines, from cloned animals and from different types of lactation including induced lactations. Our results show glycosylation patterns from the protein expressed in animals, derived from the same founder goat, are mostly comparable. Furthermore, the protein expressed in two cloned goats had highly consistent oligosaccharide profiles and similar carbohydrate composition. However, there were significantly different oligosaccharide profiles from the proteins derived from different founder goats. Artificial induction of lactation did not have significant effects on overall carbohydrate structures when compared to natural lactation. The only major difference was that recombinant antithrombin from induced lactations contained a slightly higher ratio of N-acetylneuraminic acid to N-glycolylneuraminic acid and less amount of oligosaccharides containing N-glycolylneuraminic acid. The oligosaccharides from all animals were a mixture of high mannose-, hybrid- and complex-type oligosaccharides. Sialic acid was present as alpha-2,6-linkage and no alpha-1,3-linked galactose was observed. These results indicate that transgenic animals with closely related genetic backgrounds express recombinant protein with comparable glycosylation.

Characterization of biologically active bovine pituitary FSH purified by immunoaffinity chromatography using a monoclonal antibody

A substantial amount of highly purified, biologically active bovine FSH was isolated from pituitary extracts by immunoaffinity chromatography based on a novel anti-bovine FSH beta-subunit monoclonal antibody. The biological activity was assessed in vitro using a steroidogenic granulosa cell line constitutively expressing the FSH receptor. Amino acid analysis, N-terminal amino acid sequencing, and peptide mass mapping demonstrated that primary structure modifications do not contribute to the heterogeneity of bovine FSH. The monosaccharide composition of the N-linked oligosaccharides was quantified and remarkably two distinct forms of sialic acids, N-acetyl- and N-glycolyl-neuraminic acids were found. In conclusion, we showed that isoform differences in bovine FSH is likely due only to sugar chain heterogeneity, and we give the first evidence that two substituted sialic acids contribute to the diversity of mammalian glycoprotein hormone isoforms.

Posttranslational modification of therapeutic proteins in plants

Plants have emerged as an alternative to current systems for the production of therapeutic proteins. The advantages of plants for the low-cost and large-scale production of safe and biologically active mammalian proteins have been documented recently. A major advantage of transgenic plants over production systems that are based on yeast or Escherichia coli is their ability to perform most of the posttranslational modifications (PTMs) that are required for the bioactivity and pharmacokinetics of recombinant therapeutic proteins. Furthermore, recent advances in the control of PTMs in transgenic plants have made it possible for plants to perform, at least to some extent, human-like modifications of recombinant proteins. Hence, plants have become a suitable alternative to animal cell factories for the production of therapeutic proteins.

Production and glycosylation of plant-made pharmaceuticals: the antibodies as a challenge

Antibodies have long been recognized for their diagnostic and therapeutic potential. The rapidly increasing number of monoclonal antibodies approved for immunotherapy has paved the way to an even greater demand for these molecules. In order to satisfy this growing demand and to increase the production capacity, alternative systems based on antibody production in transgenic organisms are being actively explored. In this paper, we focus on transgenic plants as a promising system for the scale-up and processing of plant-made pharmaceuticals. In particular, we point out the advantages and limitations induced by glycosylation of plant-made antibodies for human therapy.