Isolation and properties of a soluble sialidase from the culture fluid of Chinese hamster ovary cells

Cellular translocation and secretion of sialidases

Sialidases (or neuraminidases) catalyze the hydrolysis of sialic acid (Sia)-containing molecules, mostly the removal of the terminal Sia on glycans (desialylation) of either glycoproteins or glycolipids. Therefore, sialidases can modulate the functionality of the target glycoprotein or glycolipid and are involved in various biological pathways in health and disease. In mammalian cells, there are four kinds of sialidase, which are Neu1, Neu2, Neu3, and Neu4, based on their subcellular locations and substrate specificities. Neu1 is the lysosomal sialidase, Neu2 is the cytosolic sialidase, Neu3 is the plasma membrane-associated sialidase, and Neu4 is found in the lysosome, mitochondria, and endoplasmic reticulum. In addition to specific subcellular locations, sialidases can translocate to different subcellular localizations within particular cell conditions and stimuli, thereby participating in different cellular functions depending on their loci. Lysosomal sialidase Neu1 can translocate to the cell surface upon cell activation in several cell types, including immune cells, platelets, endothelial cells, and epithelial cells, where it desialylates receptors and thus impacts receptor activation and signaling. On the other hand, cells secrete sialidases upon activation. Secreted sialidases can serve as extracellular sialidases and cause the desialylation of both extracellular glycoproteins or glycolipids and cell surface glycoproteins or glycolipids on their own and other cells, thus playing roles in various biological pathways as well. This review discusses the recent advances and understanding of sialidase translocation in different cells and secretion from different cells under different conditions and their involvement in physiological and pathological pathways.

Production of α2,6-sialylated and non-fucosylated recombinant alpha-1-antitrypsin in CHO cells

Alpha-1-antitrypsin (A1AT) is an abundant serum inhibitor of serine proteases. A1AT deficiency is a common genetic disorder which is currently treated with augmentation therapies. These treatments involve weekly injections of patients with purified plasma-derived A1AT. Such therapies can be extremely expensive and rely on plasma donors. Hence, large-scale production of recombinant A1AT (rA1AT) could greatly benefit these patients, as it could decrease the cost of treatments, reduce biosafety concerns and ensure quantitative and qualitative controls of the protein. In this report, we sought to produce α2,6-sialylated rA1AT with our cumate-inducible stable CHO pool expression system. Our different CHO pools could reach volumetric productivities of 1,2 g/L. The human α2,6-sialyltransferase was stably expressed in these cells in order to mimic elevated α2,6-sialylation levels of native A1AT protein. Sialylation of the recombinant protein was stable over the duration of the fed-batch production phase and was higher in a pool where cells were sorted and enriched by FACS based on cell-surface α2,6-sialylation. Addition of ManNAc to the cell culture media during production enhanced both α2,3 and α2,6 A1AT sialylation levels whereas addition of 2F-peracetylfucose potently inhibited fucosylation of the protein. Finally, we demonstrated that rA1AT proteins exhibited human neutrophil elastase inhibitory activities similar to the commercial human plasma-derived A1AT.

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.

Sialidase specificity determined by chemoselective modification of complex sialylated glycans

Sialidases hydrolytically remove sialic acids from sialylated glycoproteins and glycolipids. Sialidases are widely distributed in nature and sialidase-mediated desialylation is implicated in normal and pathological processes. However, mechanisms by which sialidases exert their biological effects remain obscure, in part because sialidase substrate preferences are poorly defined. Here we report the design and implementation of a sialidase substrate specificity assay based on chemoselective labeling of sialosides. We show that this assay identifies components of glycosylated substrates that contribute to sialidase specificity. We demonstrate that specificity of sialidases can depend on structure of the underlying glycan, a characteristic difficult to discern using typical sialidase assays. Moreover, we discovered that Streptococcus pneumoniae sialidase NanC strongly prefers sialosides containing the Neu5Ac form of sialic acid versus those that contain Neu5Gc. We propose using this approach to evaluate sialidase preferences for diverse potential substrates.

Production of recombinant human antithrombin III on 20-L bioreactor scale: correlation of supernatant neuraminidase activity, desialylation, and decrease of biological activity of recombinant glycoprotein

Chinese hamster ovary (CHO) cells producing the recombinant glycoprotein human antithrombin III (rhAT III) were batch cultivated in a 20-L bioreactor for 13 days. Neuraminidase activity in cell-free supernatant was monitored during cultivation and free sialic acid was determined by HPLC. Neu5Acalpha(2-->3)Gal-specific Maackia amurensis and Galbeta(1-->4)GlcNAc-specific Datura stramonium agglutinin were used for determination of sialylated and desialylated rhAT III, respectively. A commercial test kit was used for evaluation of functional rhAT III activity. Supernatant neuraminidase as well as lactate dehydrogenase activity increased significantly during batch growth. The enhanced number of dead cells correlated with increased neuraminidase activity, which seemed to be principally due to cell lysis, resulting in release of cytosolic neuraminidase. Loss of terminally alpha(2-->3) linked sialic acids of the oligosaccharide portions of rhAT III, analyzed in lectin-based Western blot and lectin-adsorbent assays, correlated with a decrease of activity of rhAT III produced throughout long-term batch cultivation. Thus, structural oligosaccharide integrity as well as the functional activity of recombinant glycoprotein depend on the viability and mortality of the bioreactor culture, and batches with a high number of viable cells are required to guarantee production of glycoproteins with maximum biological activity. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 56: 441-448, 1997.

Site- and branch-specific sialylation of recombinant human interferon-gamma in Chinese hamster ovary cell culture

Since sialic acid content is known to be a critical determinant of the biological properties of glycoproteins, it is essential to characterize and monitor sialylation patterns of recombinant glycoproteins intended for therapeutic use. This study reports site- and branch-specific differences in sialylation of human interferon-gamma (IFN-gamma) derived from Chinese hamster ovary (CHO) cell culture. Sialylation profiles were quantitated by reversed-phase HPLC separations of the site-specific pools of tryptic glycopeptides representing IFN-gamma's two potential N-linked glycosylation sites (i.e., Asn(25) and Asn(97)). Although sialylation at each glycosylation site was found to be incomplete, glycans of Asn(25) were more heavily sialylated than those of Asn(97). Furthermore, Man(alpha1-3) arms of the predominant complex biantennary structures were more favorably sialylated than Man(alpha1-6) branches at each glycosylation site. When the sialylation profile was analyzed throughout a suspension batch culture, sialic acid content at each site and branch was found to be relatively constant until a steady decrease in sialylation was observed coincident with loss of cell viability. The introduction of a competitive inhibitor of sialidase into the culture supernatant prevented the loss of sialic acid after the onset of cell death but did not affect sialylation prior to cell death. This finding indicated that incomplete sialylation prior to loss of cell viability could be attributed to incomplete intracellular sialylation while the reduction in sialylation following loss of cell viability was due to extracellular sialidase activity resulting from cell lysis. Thus, both intracellular and extracellular processes defined the sialic acid content of the final product. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 55: 390-398, 1977.

Influence of Primatone RL supplementation on sialylation of recombinant human interferon-gamma produced by Chinese hamster ovary cell culture using serum-free media

Although serum-free media have been widely used in mammalian cell culture for therapeutic protein production, the effects of serum-substitutes on product quality have not been extensively examined. This study observed an adverse effect of Primatone RL, an animal tissue hydrolysate commonly used as a serum-substitute to promote cell growth, on sialylation of interferon-gamma (IFN-gamma) derived from Chinese hamster ovary (CHO) cell culture in both batch and fed-batch modes. In batch cultures, decreased sialylation was observed at each of the glycosylation sites (i.e., Asn(25) and Asn(97)) of IFN-gamma with the use of elevated concentrations of the peptone. Although poorest sialylation was obtained with the use of a growth-inhibiting concentration of Primatone RL, diminished sialylation was observed at the optimal peptone concentration for cell growth and product yield. Since incubation of the product in Primatone RL-supplemented acellular medium did not result in decreased sialylation, the negative effect of Primatone RL could not be attributed to extracellular desialylation of IFN-gamma by components of the peptone. In the fed-batch mode, a culture utilizing a serum-free feeding medium supplemented with Primatone RL demonstrated poorer sialylation than a similar culture not fed the peptone. The results of both the batch and fed-batch experiments indicate that the adverse effect of the peptone was not due solely to ammonia accumulation. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 56: 353-360, 1997.

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.

RNA interference of sialidase improves glycoprotein sialic acid content consistency

An important challenge facing therapeutic protein production in mammalian cell culture is the cleavage of terminal sialic acids on recombinant protein glycans by the glycosidase enzymes released by lysed cells into the supernatant. This undesired phenomenon results in a protein product which is rapidly cleared from the plasma by asialoglycoprotein receptors in the liver. In this study, RNA interference was utilized as a genetic approach to silence the activity of sialidase, a glycosidase responsible for cleaving terminal sialic acids on IFN-gamma produced by Chinese Hamster Ovary (CHO) cells. We first identified a 21-nt double stranded siRNA that reduced endogenous sialidase mRNA and protein activity levels. Potency of each siRNA sequences was compared using real time RT-PCR and a sialidase activity assay. We next integrated the siRNA sequence into CHO cells, allowing production and selection of stable cell lines. We isolated stable clones with sialidase activity reduced by over 60% as compared to the control cell line. Micellar electrokinetic chromatography (MEKC), thiobarbituric acid assay (TAA), and high performance anion exchange chromatography (HPAEC) coupled to amperometric detection were performed to analyze glycan site occupancy, sialic acid content, and distribution of asialo-/sialylated-glycan structures, respectively. Two of the stable clones successfully retained the full sialic acid content of the recombinant IFN-gamma, even upon cells' death. This was comparable to the case where a chemically synthesized sialidase inhibitor was used. These results demonstrated that RNA interference of sialidase can prevent the desialylation problem in glycoprotein production, resulting improved protein quality during the entire cell culture process.

Crystal Structure of the Human Cytosolic Sialidase Neu2

Gangliosides play key roles in cell differentiation, cell-cell interactions, and transmembrane signaling. Sialidases hydrolyze sialic acids to produce asialo compounds, which is the first step of degradation processes of glycoproteins and gangliosides. Sialidase involvement has been implicated in some lysosomal storage disorders such as sialidosis and galactosialidosis. Neu2 is a recently identified human cytosolic sialidase. Here we report the first high resolution x-ray structures of mammalian sialidase, human Neu2, in its apo form and in complex with an inhibitor, 2-deoxy-2,3-dehydro-N-acetylneuraminic acid (DANA). The structure shows the canonical six-blade beta-propeller observed in viral and bacterial sialidases with its active site in a shallow crevice. In the complex structure, the inhibitor lies in the catalytic crevice surrounded by ten amino acids. In particular, the arginine triad, conserved among sialidases, aids in the proper positioning of the carboxylate group of DANA within the active site region. The tyrosine residue, Tyr(334), conserved among mammalian and bacterial sialidases as well as in viral neuraminidases, facilitates the enzymatic reaction by stabilizing a putative carbonium ion in the transition state. The loops containing Glu(111) and the catalytic aspartate Asp(46) are disordered in the apo form but upon binding of DANA become ordered to adopt two short alpha-helices to cover the inhibitor, illustrating the dynamic nature of substrate recognition. The N-acetyl and glycerol moieties of DANA are recognized by Neu2 residues not shared by bacterial sialidases and viral neuraminidases, which can be regarded as a key structural difference for potential drug design against bacteria, influenza, and other viruses.

Gene-expression profiles for five key glycosylation genes for galactose-fed CHO cells expressing recombinant IL-4/13 cytokine trap

Recombinant protein glycosylation profiles have been shown to affect the in-vivo half-life, and therefore the efficacy and economics, for many therapeutics. While much research has been conducted correlating the effects of various stimuli on recombinant protein glycosylation characteristics, relatively little work has examined glycosylation-related gene-expression profiles. In this study, the effects of galactose feeding on the gene-expression profiles for five key glycosylation-related genes were determined for Chinese hamster ovary cells producing a recombinant IL-4/13 cytokine trap fusion. The genes investigated were sialidase, a putative alpha2,3-sialyltransferase, CMP-sialic acid transporter, beta1,4-galactosyltransferase, and UDP-galactosyltransferase. Additionally, the sialic acid content (sialylation) of the recombinant protein was examined. The peak sialic acid content of the IL-4/13 cytokine trap fusion protein was observed to be similar for the control and galactose-fed cultures. The gene-expression profiles for four of the glycosylation genes were observed to be sensitive to the glucose concentration and not significantly different for the control and galactose-fed cultures prior to glucose depletion. However, the sialidase gene-expression profiles were different for the control and galactose-fed cultures. The sialidase gene-expression profile increased significantly for the galactose-fed cultures prior to glucose depletion, whereas for the control cultures, the sialidase gene-expression profiles did not increase until the late stationary phase. The intracellular sialidase enzyme activity decreased exponentially with time for the control cultures; however, for the galactose-fed cultures, the intracellular sialidase enzyme activity decreased initially and then remained relatively high compared to the control cultures. These results indicate that the galactose feeding may increase the potential for desialylation, which offsets any improvements in the sialylation rate due to increased substrate levels. Thus, galactose feeding is an unnecessary expense for the production of the IL-4/13 cytokine trap fusion protein in a batch process.

Induction of lysosomal and plasma membrane-bound sialidases in human T-cells via T-cell receptor

Among the three isoenzymes of neuraminidase (Neu) or sialidase, Neu-1 has been suggested to be induced by cell activation and to be involved in IL (interleukin)-4 biosynthesis in murine T-cells. In the present study, we found that antigen-induced airway eosinophilia, a typical response dependent on Th2 (T-helper cell type 2) cytokines, as well as mRNA expression of Th2 cytokines, including IL-4, are suppressed in Neu-1-deficient mice, thereby demonstrating the in vivo role of murine Neu-1 in regulation of Th2 cytokines. To elucidate the roles of various sialidases in human T-cell activation, we investigated their tissue distribution, gene induction and function. Neu-1 is the predominant isoenzyme at the mRNA level in most tissues and cells in both mice and humans, including T-cells. T-cells also have significant levels of Neu-3 mRNAs, albeit much lower than those of Neu-1, whereas the levels of Neu-2 mRNAs are minimal. In human T-cells, both Neu-1 and Neu-3 mRNAs are significantly induced by T-cell-receptor stimulation, as is sialidase activity against 4-methylumbelliferyl- N -acetylneuramic acid (a substrate for both Neu-1 and Neu-3) and the ganglioside G(D1a) [NeuAcalpha2-3Galbeta1-3GalNAcbeta1-4(NeuAcalpha2-3)Galbeta1-4Glcbeta1-cer] (a substrate for Neu-3, but not for Neu-1). The expression of the two sialidase genes may be under differential regulation. Western blot analysis and enzymic comparison with recombinant sialidases have revealed that Neu-3 is induced as a major isoform in activated cells. The induction of Neu-1 and Neu-3 in T-cells is unique. In human monocytes and neutrophils stimulated with various agents, the only observation of sialidase induction has been by IL-1 in neutrophils. Functionally, a major difference has been observed in Jurkat T-cell lines over-expressing Neu-1- and Neu-3. Upon T-cell receptor stimulation, IL-2, interferon-gamma, IL-4 and IL-13 are induced in the Neu-1 line, whereas in the Neu-3 line the same cytokines are induced, with the exception of IL-4. Taken together, these results suggest an important immunoregulatory role for both Neu-1 and Neu-3 in humans.

Properties of Recombinant Human Cytosolic Sialidase HsNEU2

Recombinant human cytosolic sialidase (HsNEU2), expressed in Escherichia coli, was purified to homogeneity, and its substrate specificity was studied. HsNEU2 hydrolyzed 4-methylumbelliferyl alpha-NeuAc, alpha 2-->3 sialyllactose, glycoproteins (fetuin, alpha-acid glycoprotein, transferrin, and bovine submaxillary gland mucin), micellar gangliosides GD1a, GD1b, GT1b, and alpha 2-->3 paragloboside, and vesicular GM3. alpha 2-->6 sialyllactose, colominic acid, GM1 oligosaccharide, whereas micellar GM2 and GM1 were resistant. The optimal pH was 5.6, kinetics Michaelis-Menten type, V(max) varying from 250 IU/mg protein (GD1a) to 0.7 IU/mg protein (alpha(1)-acid glycoprotein), and K(m) in the millimolar range. HsNEU2 was activated by detergents (Triton X-100) only with gangliosidic substrates; the change of GM3 from vesicular to mixed micellar aggregation led to a 8.5-fold V(max) increase. HsNEU2 acted on gangliosides (GD1a, GM1, and GM2) at nanomolar concentrations. With these dispersions (studied in detailed on GM1), where monomers are bound to the tube wall or dilutedly associated (1:2000, mol/mol) to Triton X-100 micelles, the V(max) values were 25 and 72 microIU/mg protein, and K(m) was 10 and 15 x 10(-9) m, respectively. Remarkably, GM1 and GM2 were recognized only as monomers. HsNEU2 worked at pH 7.0 with an efficiency (compared with that at pH 5.6) ranging from 4% (on GD1a) to 64% (on alpha(1)-acid glycoprotein), from 7% (on GD1a) to 45% (on GM3) in the presence of Triton X-100, and from 30 to 40% on GM1 monomeric dispersion. These results show that HsNEU2 differentially recognizes the type of sialosyl linkage, the aglycone part of the substrate, and the supramolecular organization (monomer/micelle/vesicle) of gangliosides. The last ability might be relevant in sialidase interactions with gangliosides under physiological conditions.

Effects of buffering conditions and culture pH on production rates and glycosylation of clinical phase I anti-melanoma mouse IgG3 monoclonal antibody R24

R24, a mouse IgG3 monoclonal antibody (MAb) against ganglioside GD3 (Neu5Acalpha8Neu5Acalpha3Gal beta4Glcbeta1Cer), can block tumor growth as reported in a series of clinical trials in patients with metastatic melanoma. The IgG molecule basically contains an asparagine-linked biantennary complex type oligosaccharide on the C(H)2 domain of each heavy chain, which is necessary for its in vivo effector function. The purpose of this study was to investigate the biotechnological production and particularly the glycosylation of this clinically important MAb in CO(2)/HCO(3) (-) (pH 7.4, 7.2, and 6.9) and HEPES buffered serum-free medium. Growth, metabolism, and IgG production of hybridoma cells (ATCC HB-8445) were analyzed on a 2-L bioreactor scale using fed-batch mode. Specific growth rates (mu) and MAb production rates (q(IgG)) varied significantly with maximum product yields at pH 6.9 (q(IgG) = 42.9 microg 10(-6) cells d(-1), mu = 0.30 d(-1)) and lowest yields in pH 7.4 adjusted batches (q(IgG) = 10.8 microg 10(-6) cells d(-1), mu = 0.40 d(-1)). N-glycans were structurally characterized by high pH anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD), matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF), and electrospray-ionization quadrupole time-of-flight (ESI-QTOF) mass spectrometry (MS). The highest relative amounts of agalacto and monogalacto biantennary complex type oligosaccharides were detected in the pH 7.2 (46% and 38%, respectively) and pH 6.9 (44% and 40%, respectively) cultivations and the uppermost quantities of digalacto (fully galactosylated) structures in the pH 7.4 (32%) and the HEPES (26%) buffered fermentation. In the experiments with HEPES buffering, antibodies with a molar Neu5Ac/Neu5Gc ratio of 3.067 were obtained. The fermentations at pH 7.2 and 6.9 resulted in almost equal molar Neu5Ac/Neu5Gc ratios of 1.008 and 0.985, respectively, while the alkaline shift caused a moderate overexpression of Neu5Ac deduced from the Neu5Ac/Neu5Gc quotient of 1.411. Different culture buffering gave rise to altered glycosylation pattern of the MAb R24. Consequently, a detailed molecular characterization of MAb glycosylation is generally recommended as a part of the development of MAbs for targeted in vivo immunotherapy to assure biochemical consistency of product lots and oligosaccharide-dependent biological activity.

Recombinant (2-->3)-alpha-sialyltransferase immobilized on nickel-agarose for preparative synthesis of sialyl Lewis(x) and Lewis(a) precursor oligosaccharides

The specificity of recombinant (2-->3)-alpha-sialyltransferase (ST3Gal-III), expressed in baculovirus-infected insect cells, has been determined with various oligosaccharide acceptors and sugar-nucleotide donors using a fluorescence based assay. Recombinant ST3Gal-III tagged with a polyhistidine tail was immobilized on Ni(2+)-NTA-Agarose as an active enzyme for use in the synthesis of three sialylated oligosaccharides: (i) the divalent molecule [alpha-Neu5Ac-(2-->3)-D-Galp-(1-->4)-beta-D-GlcpNAc-O-CH(2)](2)-C-(CH(2)OBn)(2) (12); (ii) the dansylated derivative, alpha-Neu5Ac-(2-->3)-D-Galp-(1-->3)-beta-D-GlcpNAc-O-(CH(2))(6)-NH-dansyl and; (iii) the tetrasacharide alpha-Neu5Ac-(2-->3)-beta-D-Galp-(1-->4)-beta-D-GlcpNAc-(1-->2)-alpha-D-Manp-O-CH(3). Compound 12 was itself prepared from the divalent N-acetyllactosamine molecule built on pentaerythritol by a chemo-enzymatic route.

Site-directed mutagenesis of human membrane-associated ganglioside sialidase: identification of amino-acid residues contributing to substrate specificity

Unlike microbial sialidases, mammalian sialidases possess strict substrate specificity, for example the human membrane-associated sialidase, which hydrolyzes only gangliosides. To cast light on the molecular basis of this narrow substrate preference, predicted active site amino-acid residues of the human membrane sialidase were altered by site-directed mutagenesis. When compared with the active site amino-acid residues proposed for Salmonella typhimurium sialidase, only five out of 13 residues were found to be different to the human enzyme, these being located upstream of the putative transmembrane region. Alteration of seven residues, including these five, was followed by transient expression of the mutant enzymes in COS-1 cells and characterization of their kinetic properties using various substrates. Substitution of glutamic acid (at position 51) by aspartic acid and of arginine (at position 114) by glutamine or alanine resulted in retention of good catalytic efficiency toward ganglioside substrates, whereas other substitutions caused a marked reduction. The mutant enzyme E51D exhibited an increase in hydrolytic activity towards GM2 as well as sialyllactose (which are poor substrates for the wild-type) with change to a lower Km and a higher Vmax. R114Q demonstrated a substrate specificity shift in the same direction as E51D, whereas R114A enhanced the preference for gangliosides GD3 and GD1a that are effectively hydrolyzed by the wild-type. The inhibition experiments using 2-deoxy-2,3-didehydro-N-acetylneuraminic acid were consistent with the results in the alteration of substrate specificity. The findings suggest that putative active-site residues of the human membrane sialidase contribute to its substrate specificity.

Characterization of cytosolic sialidase from Chinese hamster ovary cells: part I: cloning and expression of soluble sialidase in Escherichia coli

The cDNA of Chinese hamster ovary (CHO) cell cytosolic sialidase was amplified by RT-PCR and cloned into the pGEX-2T plasmid vector encoding for glutathione S-transferase (GST). Screening revealed transformed Escherichia coli clones with the constructed plasmid encoding the CHO cell sialidase sequence. After isopropyl-beta-D-thiogalactopyranoside (IPTG) induction, SDS-PAGE of the total protein extracts revealed a new protein of about 70 kDa, correlating with the molecular weight of a fusion protein composed of the GST (26 kDa) and the cloned cytosolic CHO cell sialidase (43 kDa). A soluble fusion protein was purified from sonified E. coli homogenates by one-step affinity chromatography on Glutathione Sepharose 4B, which showed sialidase activity towards 4-methyl-umbelliferyl-alpha-D-N-acetylneuraminic acid (MUF-Neu5Ac) substrate. Induction of cells with 0.1, 0.5, and 1.0 mM IPTG revealed highest total protein amounts after induction with 1.0 mM IPTG, but highest specific activity for affinity chromatography purified eluates from cultures induced with 0.1 mM IPTG. Therefore, large scale production was performed by inducing cells during exponential growth in a 25 L bioreactor for 3 h with 0.1 mM IPTG after chilling the cell suspension to 25 degrees C. The amount of 26.46 mg of 40-fold purified GST-sialidase with a specific activity of 0.999 U/mg protein was obtained from crude protein extracts by one-step affinity chromatography. 2-Deoxy-2,3-dehydro-N-acetylneuraminic acid (Neu5Ac2en) and Neu5Ac were competitive inhibitors for the sialidase, the former being the more effective one using MUF-Neu5Ac as the substrate. The cytosolic sialidase is capable of desialylating a wide spectrum of different types of gangliosides using a thin-layer chromatography overlay kinetic assay without detergents. This is the subject of the accompanying paper (Müthing, J.; Burg, M. Carbohydr. Res. 2001, 330, 347-356).

Characterization of cytosolic sialidase from Chinese hamster ovary cells: part II. Substrate specificity for gangliosides

Cytosolic Chinese hamster ovary (CHO) cell sialidase has been cloned as a soluble glutathione S-transferase (GST)-sialidase fusion protein with an apparent molecular weight of 69 kD in Escherichia coli. The enzyme has then been produced in mg quantities at 25-L bioreactor scale and purified by one-step affinity chromatography on glutathione sepharose (Burg, M.; Müthing, J. Carbohydr. Res. 2001, 330, 335-346). The cloned sialidase was probed for desialylation of a wide spectrum of different types of gangliosides using a thin-layer chromatography (TLC) overlay kinetic assay. Different gangliosides were separated on silica gel precoated TLC plates, incubated with increasing concentrations of sialidase (50 degreesU/mL up to 1.6 mU/mL) without detergents, and desialylated gangliosides were detected with specific anti-asialoganglioside antibodies. The enzyme exhibited almost identical hydrolysis activity in degradation of GM3(Neu5Ac) and GM3(Neu5Gc). A slightly enhanced activity, compared with reference Vibrio cholerae sialidase, was detected towards terminally alpha(2-3)-sialylated neolacto-series gangliosides IV3-alpha-Neu5Ac-nLc4Cer and VI3-alpha-Neu5Ac-nLc6Cer. The ganglio-series gangliosides G(D1a), G(D1b), and G(T1b), the preferential substrates of V. cholerae sialidase for generating cleavage-resistant G(M1), were less suitable targets for the CHO cell sialidase. The increasing evidence on colocalization of gangliosides and sialidase in the cytosol strongly suggests the involvement of the cytosolic sialidase in ganglioside metabolism on intracellular level by yet unknown mechanisms.

Ganglioside modulates ligand binding to the epidermal growth factor receptor

Whereas previous investigations have shown that pharmacologic addition of gangliosides inhibits keratinocyte proliferation by downregulating epidermal growth factor receptor phosphorylation, the underlying biochemical basis and physiologic relevance are unknown. Using Scatchard and displacement plots, we have shown that supplemental purified gangliosides decrease the binding of (125)I-labeled epidermal growth factor to keratinocyte-derived SCC12 cells. Conversely, SCC12 cells transfected with sialidase and thus depleted of gangliosides show increased ligand binding to the epidermal growth factor receptor, which is consistent with their increased proliferation in response to epidermal growth factor and transforming growth factor-alpha, and increased phosphorylation of the epidermal growth factor receptor, and downstream signal transduction pathway components. The mechanism of the altered binding appears to involve primarily decreased numbers of available receptors within the intact membrane, but not altered receptor protein expression. These studies provide evidence that the effect of gangliosides on keratinocyte proliferation results, at least in part, from the direct binding of ganglioside to the receptor and disruption of the receptor-ligand interaction. Manipulation of membrane ganglioside content may be a powerful new means to alter epidermal growth factor receptor-dependent cell proliferation.

Identification and expression of NEU3, a novel human sialidase associated to the plasma membrane

Several mammalian sialidases have been described so far, suggesting the existence of numerous polypeptides with different tissue distributions, subcellular localizations and substrate specificities. Among these enzymes, plasma-membrane-associated sialidase(s) have a pivotal role in modulating the ganglioside content of the lipid bilayer, suggesting their involvement in the complex mechanisms governing cell-surface biological functions. Here we describe the identification and expression of a human plasma-membrane-associated sialidase, NEU3, isolated starting from an expressed sequence tag (EST) clone. The cDNA for this sialidase encodes a 428-residue protein containing a putative transmembrane helix, a YRIP (single-letter amino acid codes) motif and three Asp boxes characteristic of sialidases. The polypeptide shows high sequence identity (78%) with the membrane-associated sialidase recently purified and cloned from Bos taurus. Northern blot analysis showed a wide pattern of expression of the gene, in both adult and fetal human tissues. Transient expression in COS7 cells permitted the detection of a sialidase activity with high activity towards ganglioside substrates at a pH optimum of 3.8. Immunofluorescence staining of the transfected COS7 cells demonstrated the protein's localization in the plasma membrane.

Expression of a novel human sialidase encoded by the NEU2 gene

Sialidases (E.C.3.2.1.18) belong to a group of glycohydrolytic enzymes, widely distributed in nature, which remove sialic acid residues from glycoproteins and glycolipids. All of the sialidase so far characterized at the molecular level share an Asp block, repeated three to five times in the primary structure, and an F/YRIP sequence motif which is part of the active site. Using a sequence homology-based approach, we previously identified a human gene, named NEU2, mapping to chromosome 2q37. NEU2 encoded protein is a polypeptide of 380 amino acids with two Asp block consensuses and the YRIP sequence in the amino terminal part of the primary structure. Here we demonstrate that NEU2 encodes a functional sialidase. NEU2 was expressed in COS7 cells, giving rise to a dramatic increase in the sialidase activity measured in cell extracts with the artificial substrate 4-MU-NANA. Using a rabbit polyclonal antiserum, on Western blots a protein band with a molecular weight of about 42 kDa was detectable, and its cytosolic localization was demonstrated with cell fractionation experiments. These results were confirmed using immunohistochemical techniques. NEU2 expression in E.coli cells allowed purification of the recombinant protein. As already observed in the enzyme expressed in COS7 cells, NEU2 pH optimum corresponds to 5.6 and the polypeptide showed a K(m)for 4-MU-NANA of 0.07 mM. In addition, based on the detectable similarities between the NEU2 amino acid sequence and bacterial sialidases, a prediction of the three-dimensional structure of the enzyme was carried out using a protein homology modeling approach.

Cryptic sialic acid binding lectins on human blood leukocytes can be unmasked by sialidase treatment or cellular activation

We recently reported that the sialic acid-specific binding sites of CD22 molecules on B cells are masked by endogenous ligands, and can be unmasked by sialidase treatment or cellular activation. Here, we show that many other human blood leukocyte types have endogenous sialic acid binding sites that can be unmasked by sialidase treatment. Truncation of sialic acid side chains on the soluble probes used for detection abolishes all binding, indicating the specificity of the interaction for the details of sialic acid structure. There is limited overlap between alpha2-6- and alpha2-3-sialic acid-specific binding sites, which are unmasked on monocytes, natural killer cells, a minority of mature T cells, neutrophils, and some cultured human leukemic cell lines. Activation with phorbol ester and calcium ionophore causes spontaneous exposure of some of the binding sites, occurring over a period of minutes on neutrophils and several hours on monocytes and U937 leukemia cells. Activation is accompanied by some evidence for desialylation of cell surface molecules. Thus, many human blood cells have specific binding sites for sialic acids, masked by endogenous sialylated ligands. Cellular activation can unmask these sites, possibly by the action of an endogenous sialidase. The nearly universal masking of such sites in unactivated blood cells could explain why many of these sialic acid-binding lectins have not been previously discovered. Similar considerations may apply to sialic acid binding lectins of other cell types and tissues.

Enhancing therapeutic glycoprotein production in Chinese hamster ovary cells by metabolic engineering endogenous gene control with antisense DNA and gene targeting

Recombinant glycoprotein therapeutics have proven to be invaluable pharmaceuticals for the treatment of chronic and life-threatening diseases. Although these molecules are extraordinarily efficacious, many diseases have high dosage requirements of several hundred milligrams of protein for each administration. Multiple doses at this level are often required for treatment. One of the major challenges currently facing the biotechnology industry is the development of large-scale, cost-effective production and manufacturing processes of these biologically synthesized molecules. Metabolic engineering of animal cell expression hosts promises to address this challenge by substantially enhancing recombinant protein quality, productivity, and biological activity. In this report, we describe a novel approach to metabolic engineering in Chinese hamster ovary cells by control of endogenous gene expression. Analysis of the advantages and limitations of using antisense DNA and gene targeting as a means of control are discussed and several gene candidates for regulation with these techniques are identified. Practical considerations for using these technologies to reduce the levels of the CHO cell sialidase (Warner et al., Glycobiology, 3, 455-463, 1993) as a model gene system for regulation are also presented.

Cloning, expression, and chromosomal mapping of a human ganglioside sialidase

Here we report the cDNA sequence of a human ganglioside sialidase. The cDNA was isolated from a human brain cDNA library by screening with a 240 bp probe generated by polymerase chain reaction using primers based on the sequences of rat cytosolic and bovine membrane sialidases which we previously cloned. The 3.0 kb cDNA encodes an open reading frame of 436 amino acids containing a putative transmenbrane domain and an Arg-Ile-Pro and three Asp-box sequences characteristic of sialidases and showing overall 83% and 39% identities to the bovine and rat enzymes, respectively. Northern blot analysis revealed high expression in skeletal muscle and testis, but low level in kidney, placenta, lung, and digestive organs. Transient expression of the cDNA in COS-1 cells resulted in a 130-fold increase in sialidase activity compared to the control level, and the activity was found to be almost specific for gangliosides. Fluorescent in situ hybridization allowed the human sialidase gene localized to chromosome 11 at q 13.5.

Cloning and characterization of NEU2, a human gene homologous to rodent soluble sialidases

Sialidases (EC 3.2.1.18) are a group of glycohydrolytic enzymes, widely distributed in nature, that cleave sialic acid residues from the oligosaccharide components of glycoconjugates. All of the sialidase enzymes thus far characterized share an Asp block, repeated three to five times in the primary structure, and an F/YRIP sequence motif that is part of the active site. Using a sequence homology-based approach, we have identified a novel human gene, named NEU2, mapping to chromosome 2q37. The nucleotide sequence analysis of the gene has shown that it contains only one intron of about 1.25 kb, and the longest open reading frame encodes a protein of 380 amino acids, with a two-Asp block consensus, and the YRIP sequence. In the putative promoter sequence there are a classical TATAA box and four E boxes, which are consensus binding sites for muscle-specific transcription factors. Northern blot analysis revealed expression of the NEU2 transcript only in skeletal muscle.

Sialylation of the Sialic Acid Binding Lectin Sialoadhesin Regulates Its Ability to Mediate Cell Adhesion

The macrophage-specific cell surface receptor sialoadhesin, which is a member of the newly recognized family of sialic acid binding lectins called siglecs, binds glycoprotein and glycolipid ligands containing a2-3–linked sialic acid on the surface of several leukocyte subsets. Recently, the sialic acid binding activity of the siglec CD22 has been demonstrated to be regulated by sialylation of the CD22 receptor molecule. In the present work, we show that desialylation of in vivo macrophage sialylconjugates enhances sialoadhesin-mediated lectin activity. Herein, we show that receptor sialylation of soluble sialoadhesin inhibits its binding to Jurkat cell ligands, and that charge-dependent repulsion alone cannot explain this inhibition. Furthermore, we show that the inhibitory effect of sialic acid is partially dependent on the presence of an intact exocyclic side chain. These results, in conjunction with previous findings, suggest that sialylation of siglecs by specific glycosyltransferases may be a common mechanism by which siglec-mediated adhesion is regulated.

Sialylation of the Sialic Acid Binding Lectin Sialoadhesin Regulates Its Ability to Mediate Cell Adhesion

The macrophage-specific cell surface receptor sialoadhesin, which is a member of the newly recognized family of sialic acid binding lectins called siglecs, binds glycoprotein and glycolipid ligands containing a2-3–linked sialic acid on the surface of several leukocyte subsets. Recently, the sialic acid binding activity of the siglec CD22 has been demonstrated to be regulated by sialylation of the CD22 receptor molecule. In the present work, we show that desialylation of in vivo macrophage sialylconjugates enhances sialoadhesin-mediated lectin activity. Herein, we show that receptor sialylation of soluble sialoadhesin inhibits its binding to Jurkat cell ligands, and that charge-dependent repulsion alone cannot explain this inhibition. Furthermore, we show that the inhibitory effect of sialic acid is partially dependent on the presence of an intact exocyclic side chain. These results, in conjunction with previous findings, suggest that sialylation of siglecs by specific glycosyltransferases may be a common mechanism by which siglec-mediated adhesion is regulated.

Protein glycosylation: implications for in vivo functions and therapeutic applications

The glycosylation machinery in eukaryotic cells is available to all proteins that enter the secretory pathway. There is a growing interest in diseases caused by defective glycosylation, and in therapeutic glycoproteins produced through recombinant DNA technology route. The choice of a bioprocess for commercial production of recombinant glycoprotein is determined by a variety of factors, such as intrinsic biological properties of the protein being expressed and the purpose for which it is intended, and also the economic target. This review summarizes recent development and understanding related to synthesis of glycans, their functions, diseases, and various expression systems and characterization of glycans. The second section covers processing of N- and O-glycans and the factors that regulate protein glycosylation. The third section deals with in vivo functions of protein glycosylation, which includes protein folding and stability, receptor functioning, cell adhesion and signal transduction. Malfunctioning of glycosylation machinery and the resultant diseases are the subject of the fourth section. The next section covers the various expression systems exploited for the glycoproteins: it includes yeasts, mammalian cells, insect cells, plants and an amoeboid organism. Biopharmaceutical properties of therapeutic proteins are discussed in the sixth section. In vitro protein glycosylation and the characterization of glycan structures are the subject matters for the last two sections, respectively.

Electrophoretic methods for process monitoring and the quality assessment of recombinant glycoproteins

In many ways electrophoretic techniques appear ideal for quality monitoring of proteins and are thus well suited for the analysis of recombinant glycoproteins. The requirements of high throughput, comparative analysis and resolution of many variants are met by several electrophoretic techniques. A wide variety of such techniques are available to biotechnologists in the rapidly developing area of recombinant glycoproteins. It is the aim of this review to specifically cover recent work which has been applied to the analysis of DNA-derived glycoproteins, both from a process control standpoint and final product validation. All major areas of electrophoresis including sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), isoelectric focusing and techniques utilizing capillary electrophoresis are covered, with emphasis on analysis of glycoforms and oligosaccharide profiles of recombinant glycoproteins. As illustration, actual examples rather than standard glycoproteins are given to indicate the potential and limitations which may be encountered. It is anticipated that this review will prove a useful and practical guide to the latest developments by indicating the relevant merits of different methods.

Glycosylation provides both stimulatory and inhibitory effects on cell surface and soluble CD44 binding to hyaluronan

Glycosylation has been implicated in the regulation of CD44-mediated cell binding of hyaluronan (HA). However, neither the relative contribution of N- and O-linked glycans nor the oligosaccharide structures that alter CD44 affinity for HA have been elucidated. To determine the effect of selective alteration of CD44 oligosaccharide composition on the affinity of CD44 for HA, we developed a novel strategy based on the use of affinity capillary electrophoresis (ACE). Soluble recombinant CD44-immunoglobulin fusion proteins were overproduced in the mutant CHO cell line ldl-D, which has reversible defects in both N- and O-linked oligosaccharide synthesis. Using this cell line, a panel of recombinant glycosidases, and metabolic glycosidase inhibitors, CD44 glycoforms with defined oligosaccharide structures were generated and tested for HA affinity by ACE. Because ldl-D cells express endogenous cell surface CD44, the effect of any given glycosylation change on the ability of cell surface and soluble CD44 to bind HA could be compared. Four distinct oligosaccharide structures were found to effect CD44-mediated HA binding: (a) the terminal alpha2,3-linked sialic acid on N-linked oligosaccharides inhibited binding; (b) the first N-linked N-acetylglucosamine residue enhanced binding; (c) O-linked glycans on N-deglycosylated CD44 enhanced binding; and (d) N-acetylgalactosamine incorporation into non-N-linked glycans augmented HA binding by cell surface CD44. The first three structures induced up to a 30-fold alteration in the intrinsic CD44 affinity for HA (Kd = 5 to >150 microM). The fourth augmented CD44-mediated cellular HA avidity without changing the intrinsic HA affinity of soluble CD44.

Characterization of human lysosomal neuraminidase defines the molecular basis of the metabolic storage disorder sialidosis

Neuraminidases (sialidases) have an essential role in the removal of terminal sialic acid residues from sialoglycoconjugates and are distributed widely in nature. The human lysosomal enzyme occurs in complex with beta-galactosidase and protective protein/cathepsin A (PPCA), and is deficient in two genetic disorders: sialidosis, caused by a structural defect in the neuraminidase gene, and galactosialidosis, in which the loss of neuraminidase activity is secondary to a deficiency of PPCA. We identified a full-length cDNA clone in the dbEST data base, of which the predicted amino acid sequence has extensive homology to other mammalian and bacterial neuraminidases, including the F(Y)RIP domain and "Asp-boxes." In situ hybridization localized the human neuraminidase gene to chromosome band 6p21, a region known to contain the HLA locus. Transient expression of the cDNA in deficient human fibroblasts showed that the enzyme is compartmentalized in lysosomes and restored neuraminidase activity in a PPCA-dependent manner. The authenticity of the cDNA was verified by the identification of three independent mutations in the open reading frame of the mRNA from clinically distinct sialidosis patients. Coexpression of the mutant cDNAs with PPCA failed to generate neuraminidase activity, confirming the inactivating effect of the mutations. These results establish the molecular basis of sialidosis in these patients, and clearly identify the cDNA-encoded protein as lysosomal neuraminidase.

Getting the glycosylation right: implications for the biotechnology industry

Glycosylation is the most extensive of all the posttranslational modifications, and has important functions in the secretion, antigenicity and clearance of glycoproteins. In recent years major advances have been made in the cloning of glycosyltransferase enzymes, in understanding the varied biological functions of carbohydrates, and in the accurate analysis of glycoprotein heterogeneity. In this review we discuss the impact of these advances on the choice of a recombinant host cell line, in optimizing cell culture processes, and in choosing the appropriate level of glycosylation analysis for each stage of product development.

Removal of sialic acid from a glycoprotein in CHO cell culture supernatant by action of an extracellular CHO cell sialidase

We have directly tested the hypothesis that Chinese hamster ovary (CHO) cell-produced glycoproteins are subject to extracellular degradation by a sialidase endogenous to the CHO cell line. Factors important to understanding the potential for extracellular degradation are addressed including the glycoprotein specificity, subcellular source, mechanism of release, and stability of the sialidase activity. The extracellular CHO cell sialidase apparently originates from the cytosol of the cells, and is released to the cell culture supernatant as a result of damage to the cellular membrane. The extracellular sialidase is active toward a variety of CHO cell-produced glycoproteins, and can hydrolyze sialic acid from the recombinant glycoprotein gp120 in the culture supernatant. While measuring the actual degradation of a glycoprotein by extracellular CHO cell sialidase can be difficult, data presented here suggest that the level of degradation can be estimated indirectly by using a more convenient fluorescent substrate, 4-methylumbelliferyl-alpha-D-N-acetylneuraminic acid, to quantify sialidase activity. Degradation by sialidase is minimized through addition of the sialidase inhibitor 2,3-dehydro-2-deoxy-N-acetylneuraminic acid to the culture supernatant. The results in this study suggest additional potential approaches for minimizing degradation by sialidase, including isolation of a sialidase-deficient CHO cell line.

Processing of C-terminal lysine and arginine residues of proteins isolated from mammalian cell culture

C-terminal Lys or Arg residues whose presence was expected based on gene sequence information are often absent in proteins isolated from mammalian cell culture. This discrepancy is believed to be due to the activity of one or more basic carboxypeptidases. Internal Arg/Lys residues that become C-terminal upon proteolysis or zymogen activation, such as in the two-chain form of tissue plasminogen activator, may also be removed from the mature protein. Charge heterogeneity results when this type of processing is incomplete; such heterogeneity can be detected by isoelectric focusing or ion-exchange chromatography. The absence of C-terminal basic residues is not usually a regulatory concern, as plasma-derived proteins are often similarly processed.

Structural analysis of the sialylated N- and O-linked carbohydrate chains of recombinant human erythropoietin expressed in Chinese hamster ovary cells. Sialylation patterns and branch location of dimeric N-acetyllactosamine units

The N-linked carbohydrate chains of recombinant human erythropoietin expressed in CHO cells were quantitatively released with peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase F, separated from the remaining O-glycoprotein by gel-permeation chromatography, and subsequently fractionated via FPLC on Mono Q, HPLC on Lichrosorb-NH2 and high-pH anion-exchange chromatography on CarboPac PA1. The purified sialylated oligosaccharides were analyzed by one-dimensional and two-dimensional 500-MHz 1H-NMR spectroscopy. When necessary, oligosaccharides were treated with endo-beta-galactosidase (and N-acetyl-beta-glucosaminidase) followed by 1H-NMR analysis of the incubation products, to obtain additional structural information. Di-, tri-, tri'- and tetraantennary N-acetyllactosamine-type oligosaccharides occur which can be completely (major) or partially (minor) sialylated. Three different types of alpha 2-3-linked sialic acids are present, namely, N-acetylneuraminic acid (95%), N-glycolylneuraminic acid (2%) and N-acetyl-9-O-acetylneuraminic acid (3%). In the case of partial sialylation, a non-random distribution of the sialic acids over the branches is observed. One or two extra N-acetyllactosamine units, being exclusively located in the branches attached to the alpha 1-6-linked Man residue, can be present in completely or partially sialylated di-, tri'-, and tetraantennary oligosaccharides. Tetraantennary oligosaccharides with N-acetyllactosamine repeats could be digested quantitatively with endo-beta-galactosidase from Bacteroides fragilis, whereas under the same conditions tri' antennary oligosaccharides hardly reacted (< 15%). Using endo-beta-galactosidase from Escherichia freundii, these tri'antennary oligosaccharides could be digested more extensively (> 75%). The O-linked carbohydrate chains were released from the O-glycoprotein by alkaline borohydride treatment, and purified via FPLC on Mono Q and HPLC on Lichrosorb-NH2. Two O-glycans were found, namely, Neu5Ac alpha 2-3Gal beta 1-3GalNAc-ol and Neu5Ac alpha 2-3Gal beta 1-3(Neu5Ac alpha 2-6)GalNAc-ol.

The effect of cell-culture conditions on the oligosaccharide structures of secreted glycoproteins

Glycoprotein oligosaccharide structure influences numerous important protein properties. In recent years, a number of studies have demonstrated that cell-culture methodology can significantly affect the oligosaccharide structures of recombinant proteins and antibodies, and, in the past year in particular, several of the specific environmental variables responsible for these effects have been identified.