Getting the glycosylation right: implications for the biotechnology industry

Unraveling the Chinese hamster ovary cell line transcriptome by next-generation sequencing

The pyrosequencing technology from 454 Life Sciences and a novel assembly approach for cDNA sequences with the Newbler Assembler were used to achieve a major step forward to unravel the transcriptome of Chinese hamster ovary (CHO) cells. Normalized cDNA libraries originating from several cell lines and diverse culture conditions were sequenced and the resulting 1.84 million reads were assembled into 32,801 contiguous sequences, 29,184 isotigs, and 24,576 isogroups. A taxonomic classification of the isotigs showed that more than 70% of the assembled data is most similar to the transcriptome of Mus musculus, with most of the remaining isotigs being homologous to DNA sequences from Rattus norvegicus. Mapping of the CHO cell line contigs to the mouse transcriptome demonstrated that 9124 mouse transcripts, representing 6701 genes, are covered by more than 95% of their sequence length. Metabolic pathways of the central carbohydrate metabolism and biosynthesis routes of sugars used for protein N-glycosylation were reconstructed from the transcriptome data. All relevant genes representing major steps in the N-glycosylation pathway of CHO cells were detected. The present manuscript represents a data set of assembled and annotated genes for CHO cells that can now be used for a detailed analysis of the molecular functioning of CHO cell lines.

The genomic sequence of the Chinese hamster ovary (CHO)-K1 cell line

Since 1987, immortalized cells from the ovary of a Chinese hamster have been the workhorse for producing recombinant therapeutics, including monoclonal antibodies, blood factors, hormones, growth factors and enzymes. Xu et al. provide the genome sequence of the ancestral CHO-K1 cell line, which should aid in the optimization of current production cell lines.

Chinese hamster ovary (CHO)–derived cell lines are the preferred host cells for the production of therapeutic proteins. Here we present a draft genomic sequence of the CHO-K1 ancestral cell line. The assembly comprises 2.45 Gb of genomic sequence, with 24,383 predicted genes. We associate most of the assembled scaffolds with 21 chromosomes isolated by microfluidics to identify chromosomal locations of genes. Furthermore, we investigate genes involved in glycosylation, which affect therapeutic protein quality, and viral susceptibility genes, which are relevant to cell engineering and regulatory concerns. Homologs of most human glycosylation-associated genes are present in the CHO-K1 genome, although 141 of these homologs are not expressed under exponential growth conditions. Many important viral entry genes are also present in the genome but not expressed, which may explain the unusual viral resistance property of CHO cell lines. We discuss how the availability of this genome sequence may facilitate genome-scale science for the optimization of biopharmaceutical protein production.

Regulated multicistronic expression technology for mammalian metabolic engineering

Contemporary basic research is rapidly revealing increasingly complex molecular regulatory networks which are often interconnected via key signal integrators. These connections among regulatory and catalytic networks often frustrate bioengineers as promising metabolic engineering strategies are bypassed by compensatory metabolic responses or cause unexpected, undesired outcomes such as apoptosis, product protein degradation or inappropriate post- translational modification. Therefore, for metabolic engineering to achieve greater success in mammalian cell culture processes and to become important for future applications such as gene therapy and tissue engineering, this technology must be enhanced to allow simultaneous, in cases conditional, reshaping of metabolic pathways to access difficult-to-attain cell states. Recent advances in this new territory of multigene metabolic engineering are intimately linked to the development of multicistronic expression technology which allows the simultaneous, and in some cases, regulated expression of several genes in mammalian cells. Here we review recent achievements in multicistronic expression technology in view of multigene metabolic engineering.

Na-butyrate increases the production and alpha2,6-sialylation of recombinant interferon-gamma expressed by alpha2,6- sialyltransferase engineered CHO cells

A non-human like glycosylation pattern in human recombinant glycoproteins expressed by animal cells may compromise their use as therapeutic drugs. In order to correct the CHO glycosylation machinery, a CHO cell line producing recombinant human interferon- gamma (IFN) was transformed to replace the endogenous pseudogene with a functional copy of the enzyme alpha2,6-sialyltransferase (alpha2,6-ST). Both the parental and the modified CHO cell line were propagated in serum-free batch culture with or without 1 mM sodium butyrate. Although Na-butyrate inhibited cell growth, IFN concentration was increased twofold. The IFN sialylation status was determined using linkage specific sialidases and HPLC. Under non- induced conditions, IFN expressed by alpha2,6-engineered cells contained 68% of the total sialic acids in the alpha2,6- conformation and the overall molar ratio of sialic acids to IFN was 2.3. Sodium butyrate addition increased twofold the molar ratio of total sialic acids to IFN and 82% of total sialic acids on IFN were in the alpha2,6-conformation. In contrast, no effect of the sodium butyrate was noticed on the sialylation of the IFN secreted by the alpha2,6-ST deficient parental cell line. This study deals for the first time with the effect of Na-butyrate on CHO cells engineered to produce human like sialylation.

Recombinant glycoprotein E produced in mammalian cells in large-scale as an antigen for varicella-zoster-virus serology

A recombinant glycoprotein E (gE) from varicella-zoster virus (VZV) was generated and produced in Chinese Hamster Ovary (CHO) cells, in the development of a specific antigen for analysis of IgG antibodies to VZV. Several stable gE-secreting clones were established and one clone was adapted to growth in serum-free suspension culture. When the cells were cultured in a perfusion bioreactor, gE was secreted into the medium, from where it could be easily purified. The recombinant gE was then evaluated as a serological antigen in ELISA. When compared to a conventional whole virus antigen, the VZV gE showed similar results in ELISA-based seroprevalence studies of 854 samples derived from blood donors, students, ischemic stroke patients and their controls, including samples with border-line results in previous analyses. Eight samples (0.9%) were discordant, all being IgG-negative by the VZV gE ELISA and positive by the whole virus ELISA. The sensitivity and specificity of the VZV gE ELISA were 99.9% and 100%, respectively, compared to 100% and 88.9% for the VZV whole virus ELISA. The elderly subjects showed similar reactivities to both antigens, while VZV gE gave lower signals in the younger cohorts, suggesting that antibodies to gE may increase with age. It was concluded that the recombinant VZV gE from CHO cells was suitable as a serological antigen for the detection of IgG antibodies specific for VZV.

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.

The genotype of early-transmitting HIV gp120s promotes α (4) β(7)-reactivity, revealing α (4) β(7) +/CD4+ T cells as key targets in mucosal transmission

Mucosal transmission of HIV is inefficient. The virus must breach physical barriers before it infects mucosal CD4⁺ T cells. Low-level viral replication occurs initially in mucosal CD4⁺ T cells, but within days high-level replication occurs in Peyer's patches, the gut lamina propria and mesenteric lymph nodes. Understanding the early events in HIV transmission may provide valuable information relevant to the development of an HIV vaccine. The viral quasispecies in a donor contracts through a genetic bottleneck in the recipient, such that, in low-risk settings, infection is frequently established by a single founder virus. Early-transmitting viruses in subtypes A and C mucosal transmission tend to encode gp120s with reduced numbers of N-linked glycosylation sites at specific positions throughout the V1-V4 domains, relative to typical chronically replicating isolates in the donor quasispecies. The transmission advantage gained by the absence of these N-linked glycosylation sites is unknown. Using primary α₄β₇⁺/CD4⁺ T cells and a flow-cytometry based steady-state binding assay we show that the removal of transmission-associated N-linked glycosylation sites results in large increases in the specific reactivity of gp120 for integrin- α₄β₇. High-affinity for integrin α₄β₇, although not found in many gp120s, was observed in early-transmitting gp120s that we analyzed. Increased α₄β₇ affinity is mediated by sequences encoded in gp120 V1/V2. α₄β₇-reactivity was also influenced by N-linked glycosylation sites located in C3/V4. These results suggest that the genetic bottleneck that occurs after transmission may frequently involve a relative requirement for the productive infection of α₄β₇⁺/CD4⁺ T cells. Early-transmitting gp120s were further distinguished by their dependence on avidity-effects to interact with CD4, suggesting that these gp120s bear unusual structural features not present in many well-characterized gp120s derived from chronically replicating viruses. Understanding the structural features that characterize early-transmitting gp120s may aid in the design of an effective gp120-based subunit vaccine.

In the first days following sexual transmission, HIV replication occurs initially at relatively low levels in mucosal tissues because of a paucity of CD4⁺ T cell targets for the virus to infect. After a period of days, virus accesses specific gut tissues that are enriched in activated CD4⁺ T cells, where near-exponential replication ensues. The period of time before HIV accesses gut tissues represents a window of opportunity where a microbicide, pre-exposure and/or post-exposure antiretroviral prophylaxis or a vaccine-induced immune response could block infection. We previously reported that the HIV envelope protein gp120 binds to integrin α₄β₇ on the surface of CD4⁺ T cells. α₄β₇ mediates the homing of CD4⁺ T cells into the gut tissues where HIV can replicate exponentially. Here we report that the genotypic features that distinguish viruses isolated within the first month after infection, termed early-transmitting isolates, promote increased steady-state reactivity with α₄β₇. This property likely provides these viruses with enhanced transmission-fitness. These results suggest that the infection of α₄β₇⁺/CD4⁺ T cells can play an important role early in HIV transmission. These findings have potentially important implications in the design of interventions to block the mucosal transmission of HIV.

Rapid characterization of N-linked glycans from secreted and gel-purified monoclonal antibodies using MALDI-ToF mass spectrometry

Recombinant monoclonal antibodies (MAbs) are increasingly being used for therapeutic use and correct glycosylation of these MAbs is essential for their correct function. Glycosylation profiles are host cell- and antibody class-dependent and can change over culture time and environmental conditions. Therefore, rapid monitoring of glycan addition/status is of great importance for process validity. We describe two workflows of generally applicability for glycan profiling of purified and gel-purified MAbs produced in NS0 and CHO cells, in which small-scale antibody purification and buffer exchange is combined with PNGase F glycan cleavage and graphite HyperCarb desalting. MALDI-ToF mass spectrometry is used for sensitive detection of glycan forms, with the ability to confirm glycan structures by selective ion fragmentation. Both workflows are rapid, technically simple and amenable to automation, and use in multi-well formats.

Glycobiomimics and glycobiosensors

Following steady advances in analytical technologies, our knowledge in glycomics is now increasing rapidly. Over the last decade, specific glycans have been described that are associated with a range of diseases, such as cancer and inflammation, with host-pathogen interactions and with various stages during stem cell development and differentiation. Simultaneously, deeper structural insight has been gained on glycosylated biopharmaceutical protein therapeutics manufactured in CHO (Chinese-hamster ovary) and other cell systems. This glycomic information is highly relevant for clinicians and biomanufacturing industries as a new class of glycobiomarkers emerges. However, current methods of glycoanalysis are primarily research tools and are not suitable for point-of-care on-site detection and analysis, or sensor devices. Lectin-based glycan detection provides the most promising approach to fill these gaps. However, the limited availability of lectins with high specificity and sensitivity for specific glycan motifs presents one of the main challenges in building reliable glycobiosensors. Recent reports have demonstrated the use of recombinant protein engineering, phage display and aptamer technologies in the production of lectin mimics, as well as the construction of biosensors that are capable of rapidly detecting glycan motifs at low levels in both a labelled and label-free manner. These are primarily proof-of-principle reports at this stage, but some of the approaches, either alone or in combination, will lead to functional glycobiosensors in the coming years which will be valuable tools for the clinical, biopharmaceutical and life science research communities.

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.

Recent Advances in Electrochemical Glycobiosensing

Biosensors based on electrochemical transduction mechanisms have recently made advances into the field of glycan analysis. These glyco-biosensors offer simple, rapid, sensitive, and economical approaches to the measurement need for rapid glycan analysis for biomarker detection, cancer and disease diagnostics, and bioprocess monitoring of therapeutic glycoproteins. Although the prevalent methods of glycan analysis (high-performance liquid chromatography, mass spectrometry, and nuclear magnetic resonance spectroscopy) provide detailed identification and structural analysis of glycan species, there are significantly few low-cost, rapid glycan assays available for diagnostic and screening applications. Here we review instances in which glyco-biosensors have been used for glycan analysis using a variety of electrochemical transduction mechanisms (e.g., amperometric, potentiometric, impedimetric, and voltammetric), selective binding agents (e.g., lectins and antibodies), and redox species (e.g., enzyme substrates, inorganic, and nanomaterial).

High yield expression of leptospirosis vaccine candidates LigA and LipL32 in the methylotrophic yeast Pichia pastoris

Background

Leptospirosis, a zoonosis caused by Leptospira spp., is recognized as an emergent infectious disease. Due to the lack of adequate diagnostic tools, vaccines are an attractive intervention strategy. Recombinant proteins produced in Escherichia coli have demonstrated promising results, albeit with variable efficacy. Pichia pastoris is an alternative host with several advantages for the production of recombinant proteins.

Results

The vaccine candidates LigANI and LipL32 were cloned and expressed in P. pastoris as secreted proteins. Large-scale expression resulted in a yield of 276 mg/L for LigANI and 285 mg/L for LipL32. The recombinant proteins were glycosylated and were recognized by antibodies present in the sera of patients with severe leptospirosis.

Conclusions

The expression of LigANI and LipL32 in P. pastoris resulted in a significant increase in yield compared to expression in E. coli. In addition, the proteins were secreted, allowing for easy purification, and retained the antigenic characteristics of the native proteins, demonstrating their potential application as subunit vaccine candidates.

Rapid whole monoclonal antibody analysis by mass spectrometry: An ultra scale-down study of the effect of harvesting by centrifugation on the post-translational modification profile

With the trend towards the generation and production of increasing numbers of complex biopharmaceutical (protein based) products, there is an increased need and requirement to characterize both the product and production process in terms of robustness and reproducibility. This is of particular importance for products from mammalian cell culture which have large molecular structures and more often than not complex post-translational modifications (PTMs) that can impact the efficacy, stability and ultimately the safety of the final product. It is therefore vital to understand how the operating conditions of a bioprocess affect the distribution and make up of these PTMs to ensure a consistent quality and activity in the final product. Here we have characterized a typical bioprocess and determined (a) how the time of harvest from a mammalian cell culture and, (b) through the use of an ultra scale-down mimic how the nature of the primary recovery stages, affect the distribution and make up of the PTMs observed on a recombinant IgG(4) monoclonal antibody. In particular we describe the use of rapid whole antibody analysis by mass spectrometry to analyze simultaneously the changes that occur to the cleavage of heavy chain C-terminal lysine residues and the glycosylation pattern, as well as the presence of HL dimers. The time of harvest was found to have a large impact upon the range of glycosylation patterns observed, but not upon C-terminal lysine cleavage. The culture age had a profound impact on the ratio of different glycan moieties found on antibody molecules. The proportion of short glycans increased (e.g., (G0F)(2) 20-35%), with an associated decrease in the proportion of long glycans with culture age (e.g., (G2F)(2) 7-4%, and G1F/G2F from 15.2% to 7.8%). Ultra scale-down mimics showed that subsequent processing of these cultures did not change the post-translational modifications investigated, but did increase the proportion of half antibodies present in the process stream. The combination of ultra scale-down methodology and whole antibody analysis by mass spectrometry has demonstrated that the effects of processing on the detailed molecular structure of a monoclonal antibody can be rapidly determined early in the development process. In this study we have demonstrated this analysis to be applicable to critical process design decisions (e.g., time of harvest) in terms of achieving a desired molecular structure, but this approach could also be applied as a selection criterion as to the suitability of a platform process for the preparation of a new drug candidate. Also the methodology provides means for bioprocess engineers to predict at the discovery phase how a bioprocess will impact upon the quality of the final product.

Characterization of site-specific O-glycan structures within the mucin-like domain of alpha-dystroglycan from human skeletal muscle

The glycosylation of the extracellular protein alpha-dystroglycan is important for its ligand-binding activity, and altered or blocked glycosylation is associated with several forms of congenital muscular dystrophies. By immunoprecipitation and sialic acid capture-and-release enrichment strategies, we isolated tryptic glycopeptides of alpha-dystroglycan from human skeletal muscle. Nano-liquid chromatography tandem mass spectrometry was used to identify both glycopeptides and peptides corresponding to the mucin-like and C-terminal domain of alpha-dystroglycan. The O-glycans found had either Hex-O-Thr or HexNAc-O-Ser/Thr anchored structures, which were often elongated and frequently, but not always, terminated with sialic acid. The HexNAc-O-Ser/Thr, but not Hex-O-Thr glycopeptides, displayed heterogeneity regarding glycan core structures and level of glycosylation site occupancy. We demonstrate for the first time glycan attachment sites of the NeuAcHexHexNAcHex-O structure corresponding to the anticipated Neu5Acalpha3Galbeta4GlcNAcbeta2Man-O-glycan (sLacNAc-Man), within the mucin-like domain of human alpha-dystroglycan from human skeletal muscle. Twenty-five glycopeptides were characterized from human alpha-dystroglycan, which provide insight to the complex in vivo O-glycosylation of alpha-dystroglycan.

Reduction of N-linked xylose and fucose by expression of rat beta1,4-N-acetylglucosaminyltransferase III in tobacco BY-2 cells depends on Golgi enzyme localization domain and genetic elements used for expression

Plant-specific N-glycosylation, such as the introduction of core alpha1,3-fucose and beta1,2-xylose residues, is a major obstacle to the utilization of plant cell- or plant-derived recombinant therapeutic proteins. The beta1,4-N-acetylglucosaminyltransferase III (GnTIII) introduces a bisecting GlcNAc residue into N-glycans, which exerts a high level of substrate mediated control over subsequent modifications, for example inhibiting mammalian core fucosylation. Based on similar findings in plants, we used Nicotianatabacum BY-2 cells to study the effects of localization and expression levels of GnTIII in the remodeling of the plant N-glycosylation pathway. The N-glycans produced by the cells expressing GnTIII were partially bisected and practically devoid of the paucimannosidic type which is typical for N-glycans produced by wildtype BY-2 suspension cultured cells. The proportion of human-compatible N-glycans devoid of fucose and xylose could be increased from an average of 4% on secreted protein from wildtype cells to as high as 59% in cells expressing chimeric GnTIII, named GnTIII(A.th.) replacing its native localization domain with the cytoplasmic tail, transmembrane, and stem region of Arabidopsis thaliana mannosidase II. The changes in N-glycosylation observed were dependent on the catalytic activity of GnTIII, as the expression of catalytically inactive GnTIII mutants did not show a significant effect on N-glycosylation.

Regulation of intracellular signaling by extracellular glycan remodeling

The plasma membrane of eukaryotic cells is coated with carbohydrates. By virtue of their extracellular position and recognizable chemical features, cell surface glycans mediate many receptor-ligand interactions. Recently, mammalian extracellular hydrolytic enzymes have been shown to modify the structure of cell surface glycans and consequently alter their binding properties. These cell surface glycan remodeling events can cause rapid changes in critical signal transduction phenomena. This Review highlights recent studies on the roles of eukaryotic extracellular sialidases, sulfatases, and a deacetylase in regulation of intracellular signaling. We also describe possible therapies that target extracellular glycan remodeling processes and discuss the potential for new discoveries in this area.

Generating NK cell receptor-Fc chimera proteins from 293T cells and considerations of appropriate glycosylation

The use of recombinant receptors as a scientific tool has become widespread in many research fields. Of particular interest are the natural killer (NK) receptors that play a major role in the immune response against tumors and virus-infected cells. We present here (i) a detailed protocol for the production and purification of soluble recombinant NK cell receptors tagged with human IgG1-Fc (thus termed receptor-Fc chimera or receptor-Ig fusion protein) and (ii) a protocol for cell staining with these recombinant receptor-Fc chimeras. As these recombinant proteins are produced in eukaryotic cells, we further discuss the glycosylation pattern of these receptors that might interfere with their ligand-binding phenotype.

Purification of human antibodies from transgenic corn using aqueous two-phase systems

A recombinant human antibody expressed in corn was purified using aqueous two-phase extraction. The antibody was an immunoglobulin G fully unglycosylated. Using systems of different compositions and/or pHs in each of one or two partitioning stages followed by one more stage in which the antibody was precipitated at the liquid/liquid interface facilitated the removal of different impurities in each stage. The best system yields a product 72% pure (22-fold purification) with a yield of 49%. The optimum extraction was done in two partitioning stages followed by an interfacial precipitation stage using poly(ethylene)glycol/potassium phosphate systems. NaCl was added to the first stage to eliminate large molecular weight impurities. The pH in the first stage was kept at 6 but a pH of 8 was used in the second stage and in the precipitation stage.

Chemoenzymatic elaboration of monosaccharides using engineered cytochrome P450BM3 demethylases

Polysaccharides comprise an extremely important class of biopolymers that play critical roles in a wide range of biological processes, but the synthesis of these compounds is challenging because of their complex structures. We have developed a chemoenzymatic method for regioselective deprotection of monosaccharide substrates using engineered Bacillus megaterium cytochrome P450 (P450(BM3)) demethylases that provides a highly efficient means to access valuable intermediates, which can be converted to a wide range of substituted monosaccharides and polysaccharides. Demethylases displaying high levels of regioselectivity toward a number of protected monosaccharides were identified using a combination of protein and substrate engineering, suggesting that this approach ultimately could be used in the synthesis of a wide range of substituted mono- and polysaccharides for studies in chemistry, biology, and medicine.

Identification of cell culture conditions to control N-glycosylation site-occupancy of recombinant glycoproteins expressed in CHO cells

The effect of different cell culture conditions on N-glycosylation site-occupancy has been elucidated for two different recombinant glycoproteins expressed in Chinese hamster ovary (CHO) cells, recombinant human tissue plasminogen activator (t-PA) and a recombinant enzyme (glycoprotein 2-GP2). Both molecules contain a N-glycosylation site that is variably occupied. Different environmental factors that affect the site-occupancy (the degree of occupied sites) of these molecules were identified. Supplementing the culture medium with additional manganese or iron increased the fraction of fully occupied t-PA (type I t-PA) by approximately 2.5-4%. Decreasing the cultivation temperature from 37 to 33 degrees C or 31 degrees C gradually increased site-occupancy of t-PA up to 4%. The addition of a specific productivity enhancer, butyrate, further increased site-occupancy by an additional 1% under each cultivation temperature tested. In addition, the thyroid hormones triiodothyronine and thyroxine increased site-occupancy of t-PA compared to control conditions by about 2%. In contrast, the addition of relevant nucleoside precursor molecules involved in N-glycan biosynthesis (e.g., uridine, guanosine, mannose) either had no effect or slightly reduced site-occupancy. For the recombinant enzyme (GP2), it was discovered that culture pH and the timing of butyrate addition can be used to control N-glycan site-occupancy within a specific range. An increase in culture pH correlated with a decrease in site-occupancy. Similarly, delaying the timing for butyrate addition also decreased site-occupancy of this molecule. These results highlight the importance of understanding how cell culture conditions and media components can affect the product quality of recombinant glycoproteins expressed in mammalian cell cultures. Furthermore, the identification of relevant factors will enable one to control product quality attributes, specifically N-glycan site-occupancy, within a specific range when applied appropriately.

The production of biopharmaceuticals in plant systems

Biopharmaceuticals present the fastest growing segment in the pharmaceutical industry, with an ever widening scope of applications. Whole plants as well as contained plant cell culture systems are being explored for their potential as cheap, safe, and scalable production hosts. The first plant-derived biopharmaceuticals have now reached the clinic. Many biopharmaceuticals are glycoproteins; as the Golgi N-glycosylation machinery of plants differs from the mammalian machinery, the N-glycoforms introduced on plant-produced proteins need to be taken into consideration. Potent systems have been developed to change the plant N-glycoforms to a desired or even superior form compared to the native mammalian N-glycoforms. This review describes the current status of biopharmaceutical production in plants for industrial applications. The recent advances and tools which have been utilized to generate glycoengineered plants are also summarized and compared with the relevant mammalian systems whenever applicable.

Safety evaluation of the double mutant 5-enol pyruvylshikimate-3-phosphate synthase (2mEPSPS) from maize that confers tolerance to glyphosate herbicide in transgenic plants

Glyphosate tolerance can be conferred by decreasing the herbicide's ability to inhibit the enzyme 5-enol pyruvylshikimate-3-phosphate synthase, which is essential for the biosynthesis of aromatic amino acids in all plants, fungi, and bacteria. Glyphosate tolerance is based upon the expression of the double mutant 5-enol pyruvylshikimate-3-phosphate synthase (2mEPSPS) protein. The 2mEPSPS protein, with a lower binding affinity for glyphosate, is highly resistant to the inhibition by glyphosate and thus allows sufficient enzyme activity for the plants to grow in the presence of herbicides that contain glyphosate. Based on both a review of published literature and experimental studies, the potential safety concerns related to the transgenic 2mEPSPS protein were assessed. The safety evaluation supports that the expressed protein is innocuous. The 2mEPSPS enzyme does not possess any of the properties associated with known toxins or allergens, including a lack of amino acid sequence similarity to known toxins and allergens, a rapid degradation in simulated gastric and intestinal fluids, and no adverse effects in mice after intravenous or oral administration (at 10 or 2000 mg/kg body weight, respectively). In conclusion, there is a reasonable certainty of no harm resulting from the inclusion of the 2mEPSPS protein in human food or in animal feed.

An efficient platform for screening expression and crystallization of glycoproteins produced in human cells

Glycoproteins are involved in diverse biological processes ranging from extracellular contact and recognition to intracellular signaling. Crystal structures of glycoproteins would yield tremendous insight into these processes. But glycoprotein structural analysis has been hindered by difficulties in expressing milligram quantities of stable, homogeneous protein and determining which modifications will yield samples amenable to crystallization. We describe a platform, which we have proven to be effective for rapidly screening expression and crystallization of a challenging glycoprotein target. In this protocol, multiple glycoprotein ectodomain constructs are produced in parallel by transient expression of adherent human embryonic kidney (HEK) 293T cells and are subsequently screened for crystals in microscale quantities by free interface diffusion. As a result, recombinant proteins are produced and processed in a native, mammalian environment, and crystallization screening can be accomplished with as little as 65 microg of protein. Moreover, large numbers of constructs can be generated, screened and scaled up for expression and crystallization, with results obtained in 4 weeks.

Comparison of the N-linked glycosylation of human beta1,3-N-acetylglucosaminyltransferase 2 expressed in insect cells and silkworm larvae

N-Glycosylation of human beta1,3N-acetylglucosaminyltransferase 2 (beta3GnT2) is essential for its biological function. beta3GnT2 fused to GFP(uv) (GFP(uv)-beta3GnT2) was produced by non-virus expression systems in stably transformed insect cells and silkworm larvae using a recombinant BmNPV bacmid, and purified for analysis of N-glycosylation. The N-glycan structure of beta3GnT2 was identified by glycoamidase A digestion, labeling with 2-aminopyridine (PA), and HPLC mapping. The paucimannosidic N-glycan structure (73.2%) was predominant in stably transformed Trichoplusia ni cells. In contrast, N-glycan with Gal (21.3%) and GlcNAc (16.2%) terminal residues linked to Manalpha(1,3) branch were detected on beta3GnT2 expressed in silkworm larvae. The presence of terminal Gal and bisecting GlcNAc residues such as Galbeta1, 4GlcNAcbeta1, 2Manalpha1,3(GlcNAcbeta1,4)(Manalpha1,6)Manbeta1, 4GlcNAc is not typical structure for lepidopteran insect N-glycosylation. Although allergenic alpha1,3-fucose residues have been found in T. ni cells, only alpha1,6-fucose residues were attached to the beta3GnT2 glycan in silkworm larvae. Therefore, silkworm larvae might be a useful host for producing human glycoproteins.

Improved protein sequence coverage by on resin deglycosylation and cysteine modification for biomarker discovery

Membrane proteins and secreted factors (soluble proteins or extracellular matrix components) are the targets of most monoclonal antibodies, which are currently in clinical development. These proteins are frequently post-translationally modified, e.g. by the formation of disulfide bonds or by glycosylation, which complicates their identification using proteomics technologies. Here, we describe a novel methodology for the on resin deglycosylation and cysteine modification of proteins after in vitro, in vivo or ex vivo biotinylation. Biotinylated proteins are captured on streptavidin resin and all subsequent modifications, as well as the proteolytic digestion, which yields peptides for MS analysis, are performed on resin. Using biotinylated bovine fetuin-A as a test protein, an improvement in sequence coverage from 7.9 to 58.7% could be shown, including the identification of all three glycosylation sites. Furthermore, a complex mixture derived from the ex vivo biotinylation of vascular structures in human kidney with cancer obtained by perfusion after surgical resection revealed almost a doubling of sequence coverage for all checked proteins when analyzed by LC-MALDI TOF/TOF.

The effect of individual N-glycans on enzyme activity

In a series of investigations, N-glycosylation has proven to be a key determinant of enzyme secretion, activity, binding affinity and substrate specificity, enabling a protein to fine-tune its activity. In the majority of cases elimination of all putative N-glycosylation sites of an enzyme results in significantly reduced protein secretion levels, while removal of individual N-glycosylation sites often leads to the expression of active enzymes showing markedly reduced catalytic activity, with the decreased activity often commensurate with the number of glycosylation sites available, and the fully deglycosylated enzymes showing only minimal activity relative to their glycosylated counterparts. On the other hand, several cases have also recently emerged where deglycosylation of an enzyme results in significantly increased catalytic activity, binding affinity and altered substrate specificity, highlighting the very unique and diverse roles that individual N-glycans play in regulating enzyme function.

Enhancement of recombinant erythropoietin production in CHO cells in an incubator without CO(2) addition

The effect of low levels of carbon dioxide (CO(2)) in the gas phase on the production of recombinant human erythropoietin (EPO)in CHO cells was explored. A T-flask culture in an incubator without CO(2) addition showed a slow cell growth initially followed by the cessation of growth, while other cultures incubated under 0.5-5% CO(2) concentrations grew normally at the same rate during the entire period of cultivation. Interestingly, the production of EPO in the culture incubated under no CO(2) supply was highest among the tested cultures. The cell specific secretion rate of EPO (q(EPO)) of the culture under no CO(2) supply was about 3 times higher than that of the culture under 5% CO(2) supply. Western blot analysis and in vivo bioassay of EPO showed no apparent changes in EPO quality between the two cases of different CO(2) environments (air vs. 5% CO(2)), suggesting robust glycosylation of EPO by CHO cells even under very reduced CO(2) environment. Various combinations of the two extreme cases, with 5% CO(2) supply (suitable for cell growth) and no CO(2) addition (better for EPO production), were made in order to maximize the volumetric productivity of EPO secretion (P(V)) in CHO cells. The P(V) of the cultures programmed with initial incubation under 5% CO(2) followed by no CO(2) supply was about 2 times superior to that of the culture incubated only under no CO(2) supply. The P(V) of the culture under no CO(2) supply was slightly lower than that of culture grown under 5% CO(2). However, the q(EPO) of the no CO(2) supply case was more than 5 times higher than that of the culture under 5% CO(2) supply. In conclusion, we have demonstrated that a simple programming of CO(2) supply to an incubator can enhance the production of EPO in CHO cells remarkably, without any apparent change of the EPO quality.

Influence of intracellular nucleotide and nucleotide sugar contents on recombinant interferon-gamma glycosylation during batch and fed-batch cultures of CHO cells

Both the macroheterogeneity of recombinant human IFN-gamma produced by CHO cells and intracellular levels of nucleotides and sugar nucleotides, have been characterized during batch and fed-batch cultures carried out in different media. Whereas PF-BDM medium was capable to maintain a high percentage of the doubly- glycosylated glycoforms all over the process, mono-glycosylated and non-glycosylated forms increased during the batch culture using SF-RPMI medium. Intracellular level of UTP was higher in PF-BDM all over the batch culture compared to the SF-RPMI process. UDP-Gal accumulated only during the culture performed in PF-BDM medium, probably as a consequence of the reduced UDP-Glc synthesis flux in SF-RPMI medium. When the recombinant CHO cells were cultivated in fed-batch mode, the UTP level remained at a relatively high value in serum-containing RPMI and its titer increased during the fed-phase indicating an excess of biosynthesis. Besides, an accumulation of UDP-Gal occurred as well. Those results all together indicate that UTP and UDP-Glc syntheses in CHO cells cultivated in SF-RPMI medium in batch process, could be limiting during the glycosylation processes of the recombinant IFN-gamma. At last, the determination of the energetic status of the cells over the three studied processes suggested that a relationship between the adenylate energy charge and the glycosylation macroheterogeneity of the recombinant IFN-gamma may exist.