Genetic engineering of recombinant glycoproteins and the glycosylation pathway in mammalian host cells

Glyco-engineered HEK 293-F cell lines for the production of therapeutic glycoproteins with human N-glycosylation and improved pharmacokinetics

N-glycosylated proteins produced in human embryonic kidney 293 (HEK 293) cells often carry terminal N-acetylgalactosamine (GalNAc) and only low levels of sialylation. On therapeutic proteins, such N-glycans often trigger rapid clearance from the patient bloodstream via efficient binding to asialoglycoprotein receptor (ASGP-R) and mannose receptor (MR). This currently limits the use of HEK 293 cells for therapeutic protein production. To eliminate terminal GalNAc, we knocked-out GalNAc transferases B4GALNT3 and B4GALNT4 by CRISPR/Cas9 in FreeStyle 293-F cells. The resulting cell line produced a coagulation factor VII-albumin fusion protein without GalNAc but with increased sialylation. This glyco-engineered protein bound less efficiently to both the ASGP-R and MR in vitro and it showed improved recovery, terminal half-life and area under the curve in pharmacokinetic rat experiments. By overexpressing sialyltransferases ST6GAL1 and ST3GAL6 in B4GALNT3 and B4GALNT4 knock-out cells, we further increased factor VII-albumin sialylation; for ST6GAL1 even to the level of human plasma-derived factor VII. Simultaneous knock-out of B4GALNT3 and B4GALNT4, and overexpression of ST6GAL1 further lowered factor VII-albumin binding to ASGP-R and MR. This novel glyco-engineered cell line is well-suited for the production of factor VII-albumin and presumably other therapeutic proteins with fully human N-glycosylation and superior pharmacokinetic properties.

Influence of protein/glycan interaction on site-specific glycan heterogeneity

To study how the interaction between N-linked glycans and the surrounding amino acids influences oligosaccharide processing, we used protein disulfide isomerase (PDI), a glycoprotein bearing 5 N-glycosylation sites, as a model system and expressed it transiently in a Chinese hamster ovary (CHO)-S cell line. PDI was produced as both secreted Sec-PDI and endoplasmic reticulum-retained glycoprotein (ER)-PDI, to study glycan processing by ER and Golgi resident enzymes. Quantitative site-specific glycosylation profiles were obtained, and flux analysis enabled modeling site-specific glycan processing. By altering the primary sequence of PDI, we changed the glycan/protein interaction and thus the site-specific glycoprofile because of the improved enzymatic fluxes at enzymatic bottlenecks. Our results highlight the importance of direct interactions between N-glycans and surface-exposed amino acids of glycoproteins on processing in the ER and the Golgi and the possibility of changing a site-specific N-glycan profile by modulating such interactions and thus the associated enzymatic fluxes. Altering the primary protein sequence can therefore be used to glycoengineer recombinant proteins.-Losfeld, M.-E., Scibona, E., Lin, C.-W., Villiger, T. K., Gauss, R., Morbidelli, M., Aebi, M. Influence of protein/glycan interaction on site-specific glycan heterogeneity.

Analysis of Three Epoetin Alpha Products by LC and LC-MS Indicates Differences in Glycosylation Critical Quality Attributes, Including Sialic Acid Content

Erythropoietin (EPO) is one of the main therapeutics used to treat anemic patients, greatly improving their quality of life. In this study, biosimilars Binocrit and a development product, called here CIGB-EPO, were compared to the originator product, Eprex. All three are epoetin alpha products, reputed to have similar glycosylation profiles. The quality, safety, and efficacy of this biotherapeutic depend on the following glycosylation critical quality attributes (GCQAs): sialylation, N-glycolyl-neuraminic acid (Neu5Gc) content, branching, N-acetyl-lactosamine (LacNAc) extensions, and O-acetylation pattern. Reverse-phase ultra-high-pressure liquid chromatography (RP-UHPLC) analysis of acid-released, 1,2-diamino-4,5-methylenedioxybenzene (DMB) labeled sialic acid derivatives and hydrophilic interaction liquid chromatography (HILIC) in combination with mass spectrometry (HILIC-UHPLC-MS) of procainamide (PROC) labeled N-glycans were the analytical tools used. An automated method for enzymatic release and PROC labeling was applied for the first time to the erythropoiesis stimulating agent (ESA) products, which facilitated novel, in-depth characterization, and allowed identification of precise structural features including the location of O-acetyl groups on sialic acid (SA) moieties. Samples were digested by a sialate-O-acetylesterase (NanS) to confirm the presence of O-acetyl groups. It was found that Eprex contained the greatest relative abundance of O-acetylated derivatives, Binocrit expressed the least Neu5Gc, and CIGB-EPO showed the greatest variety of high-mannose-phosphate structures. The sialylation and LacNAc extension patterns of the three ESAs were similar, with a maximum of four N-acetyl-neuraminic acid (Neu5Ac) moieties detected per glycan. Such differences in SA derivatization, particularly O-acetylation, could have consequences for the quality and safety of a biotherapeutic, as well as its efficacy.

A Novel Terminator Primer and Enhancer Reagents for Direct Expression of PCR-Amplified Genes in Mammalian Cells

Escherichia coli plasmids are commonly used for gene expression experiments in mammalian cells, while PCR-amplified DNAs are rarely used even though PCR is a much faster and easier method to construct recombinant DNAs. One difficulty may be the limited amount of DNA produced by PCR. For direct utilization of PCR-amplified DNA in transfection experiments, efficient transfection with a smaller amount of DNA should be attained. For this purpose, we investigated two enhancer reagents, polyethylene glycol and tRNA, for a chemical transfection method. The addition of the enhancers to a commercial transfection reagent individually and synergistically exhibited higher transfection efficiency applicable for several mammalian cell culture lines in a 96-well plate. By taking advantage of a simple transfection procedure using PCR-amplified DNA, SV40 and rabbit β-globin terminator lengths were minimized. The terminator length is short enough to design in oligonucleotides; thus, terminator primers can be used for the construction and analysis of numerous mutations, deletions, insertions, and tag-fusions at the 3'-terminus of any gene. The PCR-mediated gene manipulation with the terminator primers will transform gene expression by allowing for extremely simple and high-throughput experiments with small-scale, multi-well, and mammalian cell cultures.

A Panel of Recombinant Mucins Carrying a Repertoire of Sialylated O-Glycans Based on Different Core Chains for Studies of Glycan Binding Proteins

Sialylated glycans serve as key elements of receptors for many viruses, bacteria, and bacterial toxins. The microbial recognition and their binding specificity can be affected by the linkage of the terminal sugar residue, types of underlying sugar chains, and the nature of the entire glycoconjugate. Owing to the pathobiological significance of sialylated glycans, we have engineered Chinese hamster ovary (CHO) cells to secrete mucin-type immunoglobulin-fused proteins carrying terminal α2,3- or α2,6-linked sialic acid on defined O-glycan core saccharide chains. Besides stably expressing P-selectin glycoprotein ligand-1/mouse immunoglobulin G2b cDNA (PSGL-1/mIgG2b), CHO cells were stably transfected with plasmids encoding glycosyltransferases to synthesize core 2 (GCNT1), core 3 (B3GNT6), core 4 (GCNT1 and B3GNT6), or extended core 1 (B3GNT3) chains with or without the type 1 chain-encoding enzyme B3GALT5 and ST6GAL1. Western blot and liquid chromatography-mass spectrometry analysis confirmed the presence of core 1, 2, 3, 4, and extended core 1 chains carrying either type 1 (Galb3GlcNAc) or type 2 (Galb4GlcNAc) outer chains with or without α2,6-linked sialic acids. This panel of recombinant mucins carrying a repertoire of sialylated O-glycans will be important tools in studies aiming at determining the fine O-glycan binding specificity of sialic acid-specific microbial adhesins and mammalian lectins.

PET of c-Met in Cancer with ⁶⁴Cu-Labeled Hepatocyte Growth Factor

The hepatocyte growth factor (HGF) and its receptor, c-Met, are actively involved in tumor progression and metastasis and are closely associated with a poor prognostic outcome for cancer patients. Thus, the development of PET agents that can assess c-Met expression would be extremely useful for diagnosing cancer and subsequently monitoring response to c-Met-targeted therapies. Here, we report the characterization of recombinant human HGF (rh-HGF) as a PET tracer for detection of c-Met expression in vivo.

METHODS

rh-HGF was expressed in human embryonic kidney 293 cells and purified by nickel-nitrilotriacetic acid affinity chromatography. The concentrated rh-HGF was conjugated to 2-S-(4-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid and labeled with (64)Cu. c-Met binding evaluation by flow cytometry was performed on both U87MG and MDA-MB-231 cell lines, which have a high level and a low level, respectively, of c-Met. PET imaging and biodistribution studies were performed on nude mice bearing U87MG and MDA-MB-231 xenografted tumors.

RESULTS

The rh-HGF expression yield was 150-200 μg of protein per 5 × 10(6) cells after a 48-h transfection, with purity of approximately 85%-90%. Flow cytometry examination confirmed that rh-HGF had a strong and specific capacity to bind to c-Met. After (64)Cu labeling, PET imaging revealed specific and prominent uptake of (64)Cu-NOTA-rh-HGF in c-Met-positive U87MG tumors (percentage injected dose per gram, 6.8 ± 1.8 at 9 h after injection) and significantly lower uptake in c-Met-negative MDA-MB-231 tumors (percentage injected dose per gram, 1.8 ± 0.6 at 9 h after injection). The fact that sonication-denatured rh-HGF had significantly lower uptake in U87MG tumors, along with histology analysis, confirmed the c-Met specificity of (64)Cu-NOTA-rh-HGF.

CONCLUSION

This study provided initial evidence that (64)Cu-NOTA-rh-HGF visualizes c-Met expression in vivo, an application that may prove useful for c-Met-targeted cancer therapy.

Production of Highly Sialylated Recombinant Glycoproteins Using Ricinus communis Agglutinin-I-Resistant CHO Glycosylation Mutants

The degree of sialylation of therapeutic glycoproteins affects its circulatory half-life and efficacy because incompletely sialylated glycoproteins are cleared from circulation by asialoglycoprotein receptors present in the liver cells. Mammalian expression systems, often employed in the production of these glycoprotein drugs, produce heterogeneously sialylated products. Here, we describe how to produce highly sialylated glycoproteins using a Chinese hamster ovary (CHO) cell glycosylation mutant called CHO-gmt4 with human erythropoietin (EPO) as a model glycoprotein. The protocol describes how to isolate and characterize the CHO glycosylation mutants and how to assess the sialylation of the recombinant protein using isoelectric focusing (IEF). It further describes how to inactivate the dihydrofolate reductase (DHFR) gene in these cells using zinc finger nuclease (ZFN) technology to enable gene amplification and the generation of stable cell lines producing highly sialylated EPO.

Targeted biopharmaceuticals for cancer treatment

Cancer is a complex invasive genetic disease that causes significant mortality rate worldwide. Protein-based biopharmaceuticals have significantly extended the lives of millions of cancer patients. This article reviews the biological function and application of targeted anticancer biopharmaceuticals. We first discuss the specific antigens and core pathways that are used in the development of targeted cancer therapy. The innovative monoclonal antibodies, non-antibody proteins, and small molecules targeting these antigens or pathways are then reviewed. Finally, the current challenges in anticancer biopharmaceuticals development and the potential solutions to address these challenges are discussed.

Producing recombinant therapeutic glycoproteins with enhanced sialylation using CHO-gmt4 glycosylation mutant cells

Recombinant glycoprotein drugs require proper glycosylation for optimal therapeutic efficacy. Glycoprotein therapeutics are rapidly removed from circulation and have reduced efficacy if they are poorly sialylated. Ricinus communis agglutinin-I (RCA-I) was found highly toxic to wild-type CHO-K1 cells and all the mutants that survived RCA-I treatment contained a dysfunctional N-acetylglucosaminyltransferase I (GnT I) gene. These mutants are named CHO-gmt4 cells. Interestingly, upon restoration of GnT I, the sialylation of a model glycoprotein, erythropoietin, produced in CHO-gmt4 cells was shown to be superior to that produced in wild-type CHO-K1 cells. This addendum summarizes the applicability of this cell line, from transient to stable expression of the recombinant protein, and from a lab scale to an industrial scale perfusion bioreactor. In addition, CHO-gmt4 cells can be used to produce glycoproteins with mannose-terminated N-glycans. Recombinant glucocerebrosidase produced by CHO-gmt4 cells will not require glycan remodeling and may be directly used to treat patients with Gaucher disease. CHO-gmt4 cells can also be used to produce other glycoprotein therapeutics which target cells expressing mannose receptors.

The identity of zona pellucida receptor on spermatozoa: an unresolved issue in developmental biology

Mammalian oocytes are surrounded by an acellular zona pellucida (ZP). Fertilization begins when a capacitated spermatozoon binds to the ZP. Defective sperm-ZP interaction is a cause of male infertility and reduced fertilization rates in clinical assisted reproduction treatment. Despite the importance of spermatozoa-ZP binding, the mechanisms and regulation of the interaction are unclear partly due to the failure in the identification of ZP receptor on spermatozoa. Most of the previous studies assumed that the sperm ZP receptor is a single molecular species, and a number of potential candidates had been suggested. Yet none of them can be considered as the sole sperm ZP receptor. Accumulated evidence suggested that the sperm ZP receptor is a dynamic multi-molecular structure requiring coordinated action of different proteins that are assembled into a functional complex during post-testicular maturation and capacitation. The complex components may include carbohydrate-binding, protein-binding and acrosomal matrix proteins which work as a suite to mediate spermatozoa-ZP interaction. This article aims to review the latest insights in the identification of the sperm ZP receptor. Continued investigation of the area will provide considerable understanding of the regulation of fertilization that will be useful for practical application in human contraception and reproductive medicine.

Thyrotropin isoforms: implications for thyrotropin analysis and clinical practice

Serum thyrotropin (TSH) is considered the single most sensitive and specific measure of thyroid function in the general population owing to its negative logarithmic association with free triiodothyronine and free thyroxine concentrations. It is therefore often the test of choice for screening, diagnosis, and monitoring of primary hypothyroidism. Serum TSH concentrations can be analyzed quantitatively using third-generation immunoassays, whereas its bioactivity can be measured by TSH activity assays in cell culture. Theoretically, if serum TSH concentrations are directly related to TSH activity, the two tests should yield comparable results. However, on occasion, the results are discordant, with serum concentrations being higher than TSH biological activity. This review focuses on the dissociation between the clinical state and serum TSH concentrations and addresses clinically important aspects of TSH analysis.

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.

Enhanced secretion of biologically active, non-glycosylated VEGF from Saccharomyces cerevisiae

Vascular endothelial growth factor (VEGF) mediates angiogenesis, which plays a critical role in the development and differentiation of the vascular system. VEGF is a homodimeric glycoprotein that contains one N-glycosylation site. In this study, we evaluated Saccharomyces cerevisiae expression systems producing glycosylated and non-glycosylated splice variants of human VEGF, VEGF121, and VEGF165. The pre region of the mating factor α1 (MFα1) signal sequence was found to perform better than the entire MFα1 prepro signal sequence in secreting glycosylated VEGF. Secretion of non-glycosylated VEGF165 was completely blocked, indicating the importance of glycosylation in VEGF165 secretion. Interestingly, non-glycosylated VEGF165 was secreted when guided by the MFα1 prepro signal sequence, albeit to a lesser degree, compared to glycosylated VEGF165. N-glycosylation in the pro region was required for the prepro sequence to promote VEGF secretion. Furthermore, substitution of asparagine at the VEGF glycosylation site with lysine or glutamic acid increased secretion of non-glycosylated VEGF, a finding not previously reported. Our findings suggest that S. cerevisiae could be a suitable host for secreting biologically active, non-glycosylated VEGF for clinical use.

Differential investigations from plasma-derived and recombinant Factor IX revealed major differences in post-translational modifications of activation peptides

Post-translational modifications (PTMs) located on the activation peptide (AP) of recombinant FIX (rFIX, BeneFIX(®) ) and plasma-derived FIX (pdFIX, Betafact(®) ) have been investigated by mass spectrometry to review the structural differences between these two products. Three major structural differences were pointed out. rFIX contains a low amount of phosphorylated and sulphated AP (4% for rFIX vs. 70% for pdFIX); rFIX N-glycans are only sialylated in the α2-3 linkage, whereas pdFIX N-glycans contain both type of α2-3 and α2-6 linkages, and rFIX does not contain any sialyl Lewis(X) glycoantigens contrary to pdFIX. These variations might participate in the in vivo potential different behaviours of the two molecules.

A multi-omics analysis of recombinant protein production in Hek293 cells

Hek293 cells are the predominant hosts for transient expression of recombinant proteins and are used for stable expression of proteins where post-translational modifications performed by CHO cells are inadequate. Nevertheless, there is little information available on the key cellular features underpinning recombinant protein production in Hek293 cells. To improve our understanding of recombinant protein production in Hek293 cells and identify targets for the engineering of an improved host cell line, we have compared a stable, recombinant protein producing Hek293 cell line and its parental cell line using a combination of transcriptomics, metabolomics and fluxomics. Producer cultures consumed less glucose than non-producer cultures while achieving the same growth rate, despite the additional burden of recombinant protein production. Surprisingly, there was no indication that producer cultures compensated for the reduction in glycolytic energy by increasing the efficiency of glucose utilization or increasing glutamine consumption. In contrast, glutamine consumption was lower and the majority of genes involved in oxidative phosphorylation were downregulated in producer cultures. We observed an overall downregulation of a large number of genes associated with broad cellular functions (e.g., cell growth and proliferation) in producer cultures, and therefore speculate that a broad adaptation of the cellular network freed up resources for recombinant protein production while maintaining the same growth rate. Increased abundance of genes associated with endoplasmic reticulum stress indicated a possible bottleneck at the point of protein folding and assembly.

Isotope labeling in mammalian cells

Isotope labeling of proteins represents an important and often required tool for the application of nuclear magnetic resonance (NMR) spectroscopy to investigate the structure and dynamics of proteins. Mammalian expression systems have conventionally been considered to be too weak and inefficient for protein expression. However, recent advances have significantly improved the expression levels of these systems. Here, we provide an overview of some of the recent developments in expression strategies for mammalian expression systems in view of NMR investigations.

Huh-7 cell line as an alternative cultural model for the production of human like erythropoietin (EPO)

BACKGROUND AND AIMS

Erythropoietin (EPO) is a glycoprotein hormone which is required to regulate the production of red blood cells. Deficiency of EPO is known to cause anemia in chronically infected renal patients and they require regular blood transfusion. Availability of recombinant EPO has eliminated the need for blood transfusion and now it is extensively used for the treatment of anemia. Glycosylation of erythropoietin is essential for its secretion, stability, protein conformation and biological activity. However, maintenance of human like glycosylation pattern during manufacturing of EPO is a major challenge in biotechnology. Currently, Chinese hamster ovary (CHO) cell line is used for the commercial production of erythropoietin but this cell line does not maintain glycosylation resembling human system. With the trend to eliminate non-human constituent from biopharmaceutical products, as a preliminary approach, we have investigated the potential of human hepatoma cell line (Huh-7) to produce recombinant EPO.

MATERIALS AND METHODS

Initially, the secretory signal and Kozak sequences was added before the EPO mature protein sequence using overlap extension PCR technique. PCR-amplified cDNA fragments of EPO was inserted into mammalian expression vector under the control of the cytomegalovirus (CMV) promoter and transiently expressed in CHO and Huh-7 cell lines. After RT-PCR analysis, ELISA and Western blotting was performed to verify the immunochemical properties of secreted EPO.

RESULTS

Addition of secretory signal and Kozak sequence facilitated the extra-cellular secretion and enhanced the expression of EPO protein. Significant expression (P < 0.05) of EPO was observed in the medium from Huh-7 cell line.

CONCLUSION

Huh-7 cell line has a great potential to produce glycosylated EPO, suggesting the use of this cell line to produce glycoproteins of the therapeutic importance resembling to the natural human system.

Enhancing recombinant protein production in human cell lines with a constitutive transport element and mRNA export proteins

Recent research into mRNA maturation processes in the nucleus has identified a number of proteins involved in mRNA transcription, capping, splicing, end processing and export. Among them, the Tap-p15 heterodimer acts as an mRNA export receptor. Tap-p15 is recruited onto fully processed mRNA in the nucleus, which is ready for export to the cytoplasm, through associating with Aly or SR proteins on mRNA, or by directly associating with a constitutive transport element (CTE), an RNA element derived from type D retroviruses. mRNA containing a CTE is exported to the cytoplasm by directly associating with Tap-p15, even in the absence of Tap-recruiting proteins such as Aly or SR proteins on the mRNA. Here, we showed that the use of a CTE enhanced the expression of recombinant protein in human cell lines. The co-expression of reporter proteins and Tap-p15 also enhanced recombinant protein expression. Moreover, the use of a CTE and Tap-p15 synergistically further enhanced the recombinant protein expression. In addition to Tap-p15, several Tap-p15-recruiting proteins, including Aly and SR proteins, enhanced recombinant protein expression, albeit independently of the CTE. The incorporation of a CTE and Tap-p15-recruiting proteins into protein expression system is useful to increase recombinant protein yield in human cells.

Protein glycosylation and its impact on biotechnology

Glycosylation is a post-translational modification that is of paramount importance in the production of recombinant pharmaceuticals as most recombinantly produced therapeutics are N- and/or O-glycosylated. Being a cell-system-dependent process, it also varies with expression systems and growth conditions, which result in glycan microheterogeneity and macroheterogeneity. Glycans have an effect on drug stability, serum half-life, and immunogenicity; it is therefore important to analyze and optimize the glycan decoration of pharmaceuticals. This review summarizes the aspects of protein glycosylation that are of interest to biotechnologists, namely, biosynthesis and biological relevance, as well as the tools to optimize and to analyze protein glycosylation.

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.

High throughput quantification of N-glycans using one-pot sialic acid modification and matrix assisted laser desorption ionization time-of-flight mass spectrometry

Appropriate glycosylation of recombinant therapeutic glycoproteins has been emphasized in biopharmaceutical industries because the carbohydrate component can affect safety, efficacy, and consistency of the glycoproteins. Reliable quantification methods are essential to ensure consistency of their products with respect to glycosylation, particularly sialylation. Mass spectrometry (MS) has become a popular tool to analyze glycan profiles and structures, showing high resolution and sensitivity with structure identification ability. However, quantification of sialylated glycans using MS is not as reliable because of the different ionization efficiency between neutral and acidic glycans. We report here that amidation in mild acidic conditions can be used to neutralize acidic N-glycans still attached to the protein. The resulting amidated N-glycans can then be released from the protein using PNGase F, and labeled with permanent charges on the reducing end to avoid any modification and the formation of metal adducts during MS analysis. The N-glycan modification, digestion, and desalting steps were performed using a single-pot method that can be done in microcentrifuge tubes or 96-well microfilter plates, enabling high throughput glycan analysis. Using this method we were able to perform quantitative MALDI-TOF MS of a recombinant human glycoprotein to determine changes in fucosylation and changes in sialylation that were in very good agreement with a normal phase HPLC oligosaccharide mapping method.

Glycosylation of therapeutic proteins: an effective strategy to optimize efficacy

During their development and administration, protein-based drugs routinely display suboptimal therapeutic efficacies due to their poor physicochemical and pharmacological properties. These innate liabilities have driven the development of molecular strategies to improve the therapeutic behavior of protein drugs. Among the currently developed approaches, glycoengineering is one of the most promising, because it has been shown to simultaneously afford improvements in most of the parameters necessary for optimization of in vivo efficacy while allowing for targeting to the desired site of action. These include increased in vitro and in vivo molecular stability (due to reduced oxidation, cross-linking, pH-, chemical-, heating-, and freezing-induced unfolding/denaturation, precipitation, kinetic inactivation, and aggregation), as well as modulated pharmacodynamic responses (due to altered potencies from diminished in vitro enzymatic activities and altered receptor binding affinities) and improved pharmacokinetic profiles (due to altered absorption and distribution behaviors, longer circulation lifetimes, and decreased clearance rates). This article provides an account of the effects that glycosylation has on the therapeutic efficacy of protein drugs and describes the current understanding of the mechanisms by which glycosylation leads to such effects.

Carbohydrate analysis throughout the development of a protein therapeutic

This review discusses the challenges involved in the characterization of the glycosylation of therapeutic glycoproteins. The focus is on methods that are most commonly used in regulatory filings and lot release testing of therapeutic glycoproteins. The different types of assays for carbohydrate analysis are reviewed, including the distinction between assays appropriate for lot release or better suited to testing during early drug development or in-depth characterization of the glycosylation. Characteristics of the glycoprotein and production process that should be considered when determining the amount of testing, the number of different methods to employ and when the testing should be performed during development of protein therapeutics is also discussed.

Overproduction of recombinant human hepatocyte growth factor in Chinese hamster ovary cells

Hepatocyte growth factor (HGF) is a potent multi-functional protein that affects morphogenesis, cell migration, organ regeneration, and tumor invasion in various tissues, and has thus been considered to have potential as a therapeutic target in various diseases. In our current study, we established Chinese hamster ovary (CHO) cells overexpressing recombinant human HGF (rhHGF) protein and in a 5 day batch culture process using a 7.5l bioreactor (5l working volume) and serum-free medium these cells could produce over 13 mg/l of rhHGF protein. The recombinant protein was then purified to homogeneity from the culture supernatant using a two-step chromatographic procedure that resulted in about a 35% recovery rate. This purified rhHGF was found to be a mixture of inactive pro-HGF and an active heterodimeric form of this protein with a higher molecular weight than its counterpart expressed from insect cells. This finding suggests that the glycosylation of rhHGF protein in CHO cells differs from that in insect cells. Inactive pro-HGF was found to rapidly convert to the active heterodimeric form of HGF in the presence of FBS (Fetal Bovine Serum), suggesting that this process would occur also when injected into human body. We further demonstrate in cell proliferation and scattering activity assays that our purified rhHGF protein preparation is functionally active with a half-maximal effective concentration of 36.3 ng/ml.

Production of recombinant human erythropoietin/Fc fusion protein by genetically manipulated chickens

We previously reported the production of human erythropoietin (hEpo) using genetically manipulated (GM) chickens. The recombinant hEpo was produced in the serum and egg white of the GM chickens, and the oligosaccharide chain structures of the serum-derived hEpo were more favorable than those of the egg white-derived hEpo. In the present study, a retroviral vector encoding an expression cassette for a fusion protein of hEpo and the Fc region of human immunoglobulin G (hEpo/Fc) was injected into developing chicken embryos, with the aim of recovering the serum-derived hEpo from egg yolk through the yolk accumulation mechanism of maternal antibodies. The GM chickens that hatched stably produced the hEpo/Fc fusion protein not only in their serum and egg white, but also in the egg yolk as expected. Lectin blot analyses revealed that significant amounts of the oligosaccharide chains of hEpo/Fc produced in the serum and eggs of GM chickens terminated with galactose, and that the oligosaccharide chains of the serum- and yolk-derived hEpo/Fc incorporated sialic acid residues. Moreover, biological activity assessment using Epo-dependent cells revealed that the yolk-derived hEpo/Fc exhibited a comparable performance to the serum- and CHO-derived hEpo/Fc. These results indicate that transport of Fc fusion proteins from the blood circulation to the yolk in chickens represents an effective strategy for the production of pharmaceutical glycoproteins using transgenic chicken bioreactors.

Myocilin promotes substrate adhesion, spreading and formation of focal contacts in podocytes and mesangial cells

Myocilin, a secreted glycoprotein of the olfactomedin family, is constitutively expressed in podocytes of the rat kidney and induced in mesangial cells during mesangioproliferative glomerulonephritis. As myocilin has been found to be associated with fibrillar components of the extracellular matrix, and adhesive properties have been shown for other members of the olfactomedin family, we hypothesized that myocilin might play a role in cell-matrix interactions in the glomerulus. To elucidate functional properties of myocilin, recombinant myocilin was expressed in 293 EBNA cells and purified by Ni-chelate and heparin chromatography. Culture plates were coated with myocilin, and primary rat mesangial cells and cells from an immortal murine podocyte cell line were seeded onto the plates in serum free conditions. Both cell types showed concentration-dependant attachment to myocilin, an effect that was statistically significant and could be blocked with specific antibodies. When compared to equal amounts of fibronectin or collagen 1, myocilin was less effective in promoting substrate adhesion. Synergistic effects in substrate adhesion were observed when myocilin was added to low concentrations of fibronectin. Twenty-five percent of cells that had attached to myocilin substrates showed spreading and expressed focal contacts which were labeled by vinculin/phalloidin staining. Comparable findings were observed when human or murine trabecular meshwork cells were seeded on myocilin substrates. Adhesive properties of myocilin required multimer formation, and were not observed when culture plates were coated with a C-terminal fragment of myocilin, containing the olfactomedin domain. We conclude that myocilin promotes substrate adhesion of podocytes and mesangial cells, and might contribute to cell-matrix adhesion of both cell types in vivo.

Structural analysis of the glycosylation of gene-activated erythropoietin (epoetin delta, Dynepo)

Recently, a novel recombinant human erythropoietin (epoetin delta, Dynepo) has been marketed in the European Union for the treatment of chronic kidney disease, cancer patients receiving chemotherapy, and so forth. Epoetin delta is engineered in cultures of the human fibrosarcoma cell line HT-1080 by homologous recombination and "gene activation." Unlike recombinant erythropoietins produced in other mammalian cells, epoetin delta is supposed to have a human-type glycosylation profile. However, the isoelectric focusing profile of epoetin delta differs from that of endogenous erythropoietin (both urinary and plasmatic). In this work, structural and quantitative analysis of the O- and N-glycans of epoetin delta was performed and compared with glycosylation from recombinant erythropoietin produced in Chinese hamster ovary (CHO) cells. From the comparison, significant differences in the sialylation of O-glycans were found. Furthermore, the N-glycan analysis indicated a lower heterogeneity from epoetin delta when compared with its CHO homologue, being predominantly tetraantennary without N-acetyllactosamine repeats in the former. The sialic acid characterization revealed the absence of N-glycolylneuraminic acid. The overall sugar profiles of both glycoproteins appeared to be significantly different and could be useful for maintaining pharmaceutical quality control, detecting the misuse of erythropoietin in sports, and establishing new avenues to link glycosylation with biological activity of glycoproteins.

Identification of a novel group of putative Arabidopsis thaliana beta-(1,3)-galactosyltransferases

To begin biochemical and molecular studies on the biosynthesis of the type II arabinogalactan chains on arabinogalactan-proteins (AGPs), we adopted a bioinformatic approach to identify and systematically characterise the putative galactosyltransferases (GalTs) responsible for synthesizing the beta-(1,3)-Gal linkage from CAZy GT-family-31 from Arabidopsis thaliana. These analyses confirmed that 20 members of the GT-31 family contained domains/motifs typical of biochemically characterised beta-(1,3)-GTs from mammalian systems. Microarray data confirm that members of this family are expressed throughout all tissues making them likely candidates for the assembly of the ubiquitously found AGPs. One member, At1g77810, was selected for further analysis including location studies that confirmed its presence in the Golgi and preliminary enzyme substrate specificity studies that demonstrated beta-(1,3)-GalT activity. This bioinformatic/molecular study of CAZy GT-family-31 was validated by the recent report of Strasser et al. (Plant Cell 19:2278-2292, 2007) that another member of this family (At1g26810; GALT1) encodes a beta-(1,3)-GalT involved in the biosynthesis of the Lewis a epitope of N-glycans in Arabidopsis thaliana.

Antibody-based enzyme-linked lectin assay (ABELLA) for the sialylated recombinant human erythropoietin present in culture supernatant

The terminal sialic acid of human erythropoietin (hEPO) is essential for in vivo activity. The current resorcinol and HPLC methods for analyzing alpha2,3-linked sialic acid require more than a microgram of purified rhEPO, and purification takes a great deal of time and labor. In this study, we assessed the use of an antibody-based enzyme-linked lectin assay (ABELLA) for analyzing non-purified recombinant hEPO (rhEPO). The major problem of this method was the high background due to terminal sialylation of components of the assay (antibody and bovine serum albumin) other than rhEPO. To solve this problem, we used a monoclonal antibody (Mab 287) to capture the rhEPO, and oxidized the bovine serum albumin used for blocking with meta-periodate. The sialic acid content of non-purified rhEPO measured by ABELLA was similar to that obtained by the resorcinol method on purified rhEPO. ABELLA has advantages such as adaptability and need for minimal amounts of rhEPO (40 ng/ml). Our observations suggest that ABELLA should reduce the time and labor needed to improve culture conditions so as to increase protein sialylation, and also facilitate the study of sialylation mechanisms.

Functional characterization of the initiation enzyme of S-layer glycoprotein glycan biosynthesis in Geobacillus stearothermophilus NRS 2004/3a

The glycan chain of the S-layer glycoprotein of Geobacillus stearothermophilus NRS 2004/3a is composed of repeating units [-->2)-alpha-l-Rhap-(1-->3)-beta-l-Rhap-(1-->2)-alpha-l-Rhap-(1-->], with a 2-O-methyl modification of the terminal trisaccharide at the nonreducing end of the glycan chain, a core saccharide composed of two or three alpha-l-rhamnose residues, and a beta-d-galactose residue as a linker to the S-layer protein. In this study, we report the biochemical characterization of WsaP of the S-layer glycosylation gene cluster as a UDP-Gal:phosphoryl-polyprenol Gal-1-phosphate transferase that primes the S-layer glycoprotein glycan biosynthesis of Geobacillus stearothermophilus NRS 2004/3a. Our results demonstrate that the enzyme transfers in vitro a galactose-1-phosphate from UDP-galactose to endogenous phosphoryl-polyprenol and that the C-terminal half of WsaP carries the galactosyltransferase function, as already observed for the UDP-Gal:phosphoryl-polyprenol Gal-1-phosphate transferase WbaP from Salmonella enterica. To confirm the function of the enzyme, we show that WsaP is capable of reconstituting polysaccharide biosynthesis in WbaP-deficient strains of Escherichia coli and Salmonella enterica serovar Typhimurium.