Production of complex multiantennary N-glycans in Nicotiana benthamiana plants

In recent years, plants have been developed as an alternative expression system to mammalian hosts for the production of therapeutic proteins. Many modifications to the plant glycosylation machinery have been made to render it more human because of the importance of glycosylation for functionality, serum half-life, and the safety profile of the expressed proteins. These modifications include removal of plant-specific β1,2-xylose and core α1,3-fucose, and addition of bisecting N-acetylglucosamine, β1,4-galactoses, and sialic acid residues. Another glycosylation step that is essential for the production of complex human-type glycans is the synthesis of multiantennary structures, which are frequently found on human N-glycans but are not generated by wild-type plants. Here, we report both the magnICON-based transient as well as stable introduction of the α1,3-mannosyl-β1,4-N-acetylglucosaminyltransferase (GnT-IV isozymes a and b) and α1,6-mannosyl-β1,6-N-acetylglucosaminyltransferase (GnT-V) in Nicotiana benthamiana plants. The enzymes were targeted to the Golgi apparatus by fusing their catalytic domains to the plant-specific localization signals of xylosyltransferase and fucosyltransferase. The GnT-IV and -V modifications were tested in the wild-type background, but were also combined with the RNA interference-mediated knockdown of β1,2-xylosyltransferase and α1,3-fucosyltransferase. Results showed that triantennary Gn[GnGn] and [GnGn]Gn N-glycans could be produced according to the expected activities of the respective enzymes. Combination of the two enzymes by crossing stably transformed GnT-IV and GnT-V plants showed that up to 10% tetraantennary [GnGn][GnGn], 25% triantennary, and 35% biantennary N-glycans were synthesized. All transgenic plants were viable and showed no aberrant phenotype under standard growth conditions.     

Processing of complex N-glycans in IgG Fc-region is affected by core fucosylation

We investigated N-glycan processing of immunoglobulin G1 using the monoclonal antibody cetuximab (CxMab), which has a glycosite in the Fab domain in addition to the conserved Fc glycosylation, as a reporter. Three GlcNAc (Gn) terminating bi-antennary glycoforms of CxMab differing in core fucosylation (α1,3- and α1,6-linkage) were generated in a plant-based expression platform. These GnGn, GnGnF³, and GnGnF⁶ CxMab variants were subjected in vivo to further processing toward sialylation and GlcNAc diversification (bisected and branching structures). Mass spectrometry-based glycan analyses revealed efficient processing of Fab glycans toward envisaged structures. By contrast, Fc glycan processing largely depend on the presence of core fucose. A particularly strong support of glycan processing in the presence of plant-specific core α1,3-fucose was observed. Consistently, molecular modeling suggests changes in the interactions of the Fc carbohydrate chain depending on the presence of core fucose, possibly changing the accessibility. Here, we provide data that reveal molecular mechanisms of glycan processing of IgG antibodies, which may have implications for the generation of glycan-engineered therapeutic antibodies with improved efficacies.     

Engineered glycoforms of an antineuroblastoma IgG1 with optimized antibody-dependent cellular cytotoxic activity

The glycosylation pattern of chCE7, an antineuroblastoma chimeric IgG1, was engineered in Chinese hamster ovary cells with tetracycline-regulated expression of beta(1,4)-N-acetylglucosaminyltransferase III (GnTIII), a glycosyltransferase catalyzing formation of bisected oligosaccharides that have been implicated in antibody-dependent cellular cytotoxicity (ADCC). Measurement of the ADCC activity of chCE7 produced at different tetracycline levels showed an optimal range of GnTIII expression for maximal chCE7 in vitro ADCC activity, and this activity correlated with the level of constant region-associated, bisected complex oligosaccharides determined by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The new optimized variants of chCE7 exhibit substantial ADCC activity and, hence, may be useful for treatment of neuroblastoma. The strategy presented here should be applicable to optimize the ADCC activity of other therapeutic IgGs.     

The widespread effect of beta 1,4-galactosyltransferase on N-glycan processing

We investigated beta 1,4-GalT (UDP-galactose: beta-d-N-acetylglucosaminide beta 1,4-galactosyltransferase) in terms of intracellular competition with GnT-IV (UDP-N-acetylglucosamine: alpha1,3-d-mannoside beta1,4-N-acetylglucosaminyltransferase) and GnT-V (UDP-N-acetylglucosamine: alpha1,6-d-mannoside beta 1,6-N-acetylglucosaminyltransferase). The beta 1,4-GalT-I gene was introduced into Chinese hamster ovary (CHO) cells producing human interferon (hIFN)-gamma (IM4/V/IV cells) and five clones expressing various levels of beta 1,4-GalT were isolated. As we previously reported, parental IM4/V/IV cells express high levels of GnT-IVa and -V and produce hIFN-gamma having primarily tetraantennary sugar chains. The branching of sugar chains on hIFN-gamma was suppressed in the beta 1,4-GalT-enhanced clones to a level corresponding to the intracellular activity of beta 1,4-GalT relative to GnTs. Moreover, the contents of hybrid-type and high-mannose-type sugar chains increased in these clones. The results showed that beta 1,4-GalT widely affects N-glycan processing by competing with GnT-IV, GnT-V, and alpha-mannosidase II in cells and also by some other mechanisms that suppress the conversion of high-mannose-type sugar chains to the hybrid type.     

Glycoengineering in plants for the development of N-glycan structures compatible with biopharmaceuticals

Plants and plant cells are emerging as promising alternatives for biopharmaceutical production with improved safety and efficiency. Plant cells are capable of performing post-translational modifications (PTMs) similar to those of mammalian cells and are safer than mammalian cells with regard to contamination by infectious pathogens, including animal viruses. However, a major obstacle to producing biopharmaceuticals in plants lies in the fact that plant-derived N-glycans include plant-specific sugar residues such as β1,2-xylose and α1,3-fucose attached to a pentasaccharide core (Man₃GlcNAc₂) as well as β1,3-galactose and α1,4-fucose involved in Lewis a (Le^a) epitope formation that can evoke allergic responses in the human body. In addition, sugar residues such as α1,6-fucose, β1,4-galactose and α2,6-sialic acid, which are thought to play important roles in the activity, transport, delivery and half-life of biopharmaceuticals are absent among the N-glycans naturally found in plants. In order to take advantage of plant cells as a system in which to produce biopharmaceuticals development of plants producing N-glycan structures compatible with biopharmaceuticals is necessary. In this article we summarize the current state of biopharmaceutical production using plants as well as what is known about N-glycosylation processes occurring in the endoplasmic reticulum and Golgi apparatus in plants. Finally, we propose and discuss a strategy for and the associated technical barriers of producing customized N-glycans via removal of enzyme genes that add plant-specific sugar residues and introducing enzyme genes that add sugar residues absent in plants.     

Reduced immunogenicity of Arabidopsis hgl1 mutant N-glycans caused by altered accessibility of xylose and core fucose epitopes

Arabidopsis N-glycosylation mutants with enhanced salt sensitivity show reduced immunoreactivity of complex N-glycans. Among them, hybrid glycosylation 1 (hgl1) alleles lacking Golgi α-mannosidase II are unique, because their glycoprotein N-glycans are hardly labeled by anti-complex glycan antibodies, even though they carry β1,2-xylose and α1,3-fucose epitopes. To dissect the contribution of xylose and core fucose residues to plant stress responses and immunogenic potential, we prepared Arabidopsis hgl1 xylT double and hgl1 fucTa fucTb triple mutants by crossing previously established T-DNA insertion lines and verified them by mass spectrometry analyses. Root growth assays revealed that hgl1 fucTa fucTb but not hgl1 xylT plants are more salt-sensitive than hgl1, hinting at the importance of core fucose modification and masking of xylose residues. Detailed immunoblot analyses with anti-β1,2-xylose and anti-α1,3-fucose rabbit immunoglobulin G antibodies as well as cross-reactive carbohydrate determinant-specific human immunoglobulin E antibodies (present in sera of allergy patients) showed that xylose-specific reactivity of hgl1 N-glycans is indeed reduced. Based on three-dimensional modeling of plant N-glycans, we propose that xylose residues are tilted by 30° because of untrimmed mannoses in hgl1 mutants. Glycosidase treatments of protein extracts restored immunoreactivity of hgl1 N-glycans supporting these models. Furthermore, among allergy patient sera, untrimmed mannoses persisting on the α1,6-arm of hgl1 N-glycans were inhibitory to immunoreaction with core fucoses to various degrees. In summary, incompletely trimmed glycoprotein N-glycans conformationally prevent xylose and, to lesser extent, core fucose accessibility. Thus, in addition to N-acetylglucosaminyltransferase I, Golgi α-mannosidase II emerges as a so far unrecognized target for lowering the immunogenic potential of plant-derived glycoproteins.     

Jack bean α-mannosidase digestion profile of hybrid-type N-glycans: effect of reaction pH on substrate preference

Jack bean α-mannosidase (JBM) is a well-studied plant vacuolar α-mannosidase, and is widely used as a tool for the enzymatic analysis of sugar chains of glycoproteins. In this study, the JBM digestion profile of hybrid-type N-glycans was examined using pyridylamino (PA-) sugar chains. The digestion efficiencies of the PA-labeled hybrid-type N-glycans Manα1,6(Manα1,3)Manα1,6(GlcNAcβ1,2Manα1,3)Manβ1,4GlcNAcβ1,4GlcNAc-PA (GNM5-PA) and Manα1,6(Manα1,3)Manα1,6(Galβ1,4GlcNAcβ1,2Manα1,3)Manβ1,4GlcNAcβ1,4GlcNAc-PA (GalGNM5-PA) were significantly lower than that of the oligomannose-type N-glycan Manα1,6(Manα1,3)Manα1,6Manβ1,4GlcNAcβ1,4GlcNAc-PA (M4-PA), and the trimming pathways of GNM5-PA and GalGNM5-PA were different from that of M4-PA, suggesting a steric hindrance to the JBM activity caused by GlcNAcβ1-2Man(α) residues of the hybrid-type N-glycans. We also found that the substrate preference of JBM for the terminal Manα1-6Man(α) and Manα1-3Man(α) linkages in the hybrid-type N-glycans was altered by the change in reaction pH, suggesting a pH-dependent change in the enzyme-substrate interaction.     

Oligosaccharide preferences of beta1,4-galactosyltransferase-I: crystal structures of Met340His mutant of human beta1,4-galactosyltransferase-I with a pentasaccharide and trisaccharides of the N-glycan moiety

beta-1,4-Galactosyltransferase-I (beta4Gal-T1) transfers galactose from UDP-galactose to N-acetylglucosamine (GlcNAc) residues of the branched N-linked oligosaccharide chains of glycoproteins. In an N-linked biantennary oligosaccharide chain, one antenna is attached to the 3-hydroxyl-(1,3-arm), and the other to the 6-hydroxyl-(1,6-arm) group of mannose, which is beta-1,4-linked to an N-linked chitobiose, attached to the aspargine residue of a protein. For a better understanding of the branch specificity of beta4Gal-T1 towards the GlcNAc residues of N-glycans, we have carried out kinetic and crystallographic studies with the wild-type human beta4Gal-T1 (h-beta4Gal-T1) and the mutant Met340His-beta4Gal-T1 (h-M340H-beta4Gal-T1) in complex with a GlcNAc-containing pentasaccharide and several GlcNAc-containing trisaccharides present in N-glycans. The oligosaccharides used were: pentasaccharide GlcNAcbeta1,2-Manalpha1,6 (GlcNAcbeta1,2-Manalpha1,3)Man; the 1,6-arm trisaccharide, GlcNAcbeta1,2-Manalpha1,6-Manbeta-OR (1,2-1,6-arm); the 1,3-arm trisaccharides, GlcNAcbeta1,2-Manalpha1,3-Manbeta-OR (1,2-1,3-arm) and GlcNAcbeta1,4-Manalpha1,3-Manbeta-OR (1,4-1,3-arm); and the trisaccharide GlcNAcbeta1,4-GlcNAcbeta1,4-GlcNAc (chitotriose). With the wild-type h-beta4Gal-T1, the K(m) of 1,2-1,6-arm is approximately tenfold lower than for 1,2-1,3-arm and 1,4-1,3-arm, and 22-fold lower than for chitotriose. Crystal structures of h-M340H-beta4Gal-T1 in complex with the pentasaccharide and various trisaccharides at 1.9-2.0A resolution showed that beta4Gal-T1 is in a closed conformation with the oligosaccharide bound to the enzyme, and the 1,2-1,6-arm trisaccharide makes the maximum number of interactions with the enzyme, which is in concurrence with the lowest K(m) for the trisaccharide. Present studies suggest that beta4Gal-T1 interacts preferentially with the 1,2-1,6-arm trisaccharide rather than with the 1,2-1,3-arm or 1,4-1,3-arm of a bi- or tri-antennary oligosaccharide chain of N-glycan.     

Molecular cloning and characterization of human GnT-IX,a novel beta1,6-N-acetylglucosaminyltransferase that is specifically expressed in the brain

A novel beta1,6-N-acetylglucosaminyltransferase (beta1, 6GnT) cDNA was identified by a BLAST search using the amino acid sequence of human GnT-V as a query. The full-length sequence was determined by a combination of 5'-rapid amplification of cDNA end analysis and a further data base search. The open reading frame encodes a 792 amino acid protein with a type II membrane protein structure typical of glycosyltransferases. The entire sequence identity to human GnT-V is 42%. When pyridylaminated (PA) agalacto biantennary N-linked oligosaccharide was used as an acceptor substrate, the recombinant enzyme generated a novel product other than the expected GnT-V product, (GlcNAcbeta1,2-Manalpha1,3-)[GlcNAcbeta1,2-(GlcNAcbeta1,6-)Manalpha1,6-]Manbeta1,4-GlcNAcbeta1,4-GlcNAc-PA. This new product was identified as [GlcNAcbeta1,2-(GlcNAcbeta1,6-)Manalpha1,3-][Glc-NAcbeta1,2-(GlcNAcbeta1,6-)Manalpha1,6-]Manbeta1,4-GlcNAcbeta1,4-GlcNAc-PA by mass spectrometry and 1H NMR. Namely, the new GnT (designated as GnT-IX) has beta1,6GnT activity not only to the alpha1,6-linked mannose arm but also to the alpha1,3-linked mannose arm of N-glycan, forming a unique structure that has not been reported to date. Northern blot analysis showed that the GnT-IX gene is exclusively expressed in the brain, whereas the GnT-V gene is expressed ubiquitously. These results suggest that GnT-IX is responsible for the synthesis of a unique oligosaccharide structure in the brain.     

Glyco-engineering of moss lacking plant-specific sugar residues

The commercial production of complex pharmaceutical proteins from human origin in plants is currently limited through differences in protein N-glycosylation pattern between plants and humans. On the one hand, plant-specific alpha(1,3)-fucose and beta(1,2)-xylose residues were shown to bear strong immunogenic potential. On the other hand, terminal beta(1,4)-galactose, a sugar common on N-glycans of pharmaceutically relevant proteins, e.g., antibodies, is missing in plant N-glycan structures. For safe and flexible production of pharmaceutical proteins, the humanisation of plant protein N-glycosylation is essential. Here, we present an approach that combines avoidance of plant-specific and introduction of human glycan structures. Transgenic strains of the moss Physcomitrella patens were created in which the alpha(1,3)-fucosyltransferase and beta(1,2)-xylosyltransferase genes were knocked out by targeted insertion of the human beta(1,4)-galactosyltransferase coding sequence in both of the plant genes (knockin). The transgenics lacked alpha(1,3)-fucose and beta(1,2)-xylose residues, whereas beta(1,4)-galactose residues appeared on protein N-glycans. Despite these significant biochemical changes, the plants did not differ from wild type with regard to overall morphology under standard cultivation conditions. Furthermore, the glyco-engineered plants secreted a transiently expressed recombinant human protein, the vascular endothelial growth factor, in the same concentration as unmodified moss, indicating that the performed changes in glycosylation did not impair the secretory pathway of the moss. The combined knockout/knockin approach presented here, leads to a new generation of engineered moss and towards the safe and flexible production of correctly processed pharmaceutical proteins with humanised N-glycosylation profiles.     

Rapid high yield production of different glycoforms of Ebola virus monoclonal antibody

Background

Fc-glycosylation of monoclonal antibodies (mAbs) has profound implications on the Fc-mediated effector functions. Alteration of this glycosylation may affect the efficiency of an antibody. However, difficulties in the production of mAbs with homogeneous N-glycosylation profiles in sufficient amounts hamper investigations of the potential biological impact of different glycan residues.

Methodology/Principal Findings

Here we set out to evaluate a transient plant viral based production system for the rapid generation of different glycoforms of a monoclonal antibody. Ebola virus mAb h-13F6 was generated using magnICON expression system in Nicotiana benthamiana, a plant species developed for commercial scale production of therapeutic proteins. h-13F6 was co-expressed with a series of modified mammalian enzymes involved in the processing of complex N-glycans. Using wild type (WT) plants and the glycosylation mutant ΔXTFT that synthesizes human like biantennary N-glycans with terminal N-acetylglucosamine on each branch (GnGn structures) as expression hosts we demonstrate the generation of h-13F6 complex N-glycans with (i) bisected structures, (ii) core α1,6 fucosylation and (iii) β1,4 galactosylated oligosaccharides. In addition we emphasize the significance of precise sub Golgi localization of enzymes for engineering of IgG Fc-glycosylation.

Conclusion

The method described here allows the efficient generation of a series of different human-like glycoforms at large homogeneity of virtually any antibody within one week after cDNA delivery to plants. This accelerates follow up functional studies and thus may contribute to study the biological role of N-glycan residues on Fcs and maximizing the clinical efficacy of therapeutic antibodies.     

Relations of the type and branch of surface N-glycans to cell adhesion, migration and integrin expressions

The relations between the structure of cell surface N-glycans to cell behaviors were studied in H7721 human hepatocarcinoma cell line, which predominantly expressed complex-type N-glycans on the surface. 1-Deoxymannojirimycin (DMJ) and swaisonine (SW), the specific inhibitor of Golgi alpha-mannosidase II or I, were selected to block the processing of N-glycans at the steps of high mannose and hybrid type respectively. All-trans retinoic acid (ATRA) and antisense cDNA of N-acetylglucosaminyltransferase-V (GnT-V) were used to suppress the expression of GnT-V and decreased the GlcNAc beta1,6-branching or tri-/tetra-antennary structure of surface N-glycans. The structural alterations of N-glycans were verified by sequential lectin affinity chromatography of [3H] mannose-labeled glycans isolated from the cell surface. The cell adhesions to fibronectin (Fn) and human umbilical vein epithelial cell (HUVEC), as well as cell migration (including chemotaxis and invasion) were selected as the parameters of cell behaviors. It was found that cell adhesion and migration were significantly decreased in SW and DMJ treated cells, suggesting that complex type N-glycan is critical for the above cell behaviors. ATRA and antisense GnTV enhanced cell adhesion to Fn but reduce cell adhesion to HUVEC and cell migration. These results reveal that cell surface complex-type N-glycans with GlcNAc beta1,6 branch are more effective than those without this branch in the cell adhesion to HUVEC and cell migration, but N-glycan without GlcNAc beta1,6-branch is the better one in mediating the cell adhesion to Fn. The integrin alpha5beta1 (receptor of Fn) on cell surface was unchanged by DMJ and SW. In contrast, ATRA up regulated alpha5, but not beta1, and antisense GnT-V decreased both alpha5 and beta1. This findings suggest that both the structure of N-glycan and the expression of integrin on cell surface are two of the important factors in the determination of cell adhesion to Fn, a complex biological process.     

Remodeling of the major mouse xenoantigen, Galalpha1-3Galbeta1-4GlcNAc-R, by N-acetylglucosaminyltransferase-III

beta-D-Mannoside beta-1,4-N-acetylglucosaminyltransferase III (GnT-III) catalyses the attachment of an N-acetylglucosamine (GlcNAc) residue to mannose in the beta(1-4) configuration in N-glycans, and forms a bisecting GlcNAc. We have generated transgenic mice that contain the human GnT-III gene under the control of the mouse albumin enhancer/promoter [Lee et al., (2003)]. Overexpression of this gene in mice reduced the antigenicity of N-glycans to human natural antibodies, especially in the case of the alpha-Gal epitope, Galalpha1-3Galbeta1-4GlcNAc-R. Study of endothelial cells from the GnT-III transgenic mice revealed a significant reduction in antigenicity, and a dramatic decrease in both complement- and natural killer cell-mediated mouse cell lysis. Changes in the enzymatic activities of other glycosyltransferases, such as alpha1,3-galactosyltransferase, and alpha-6-D-mannoside beta-1,6 N-acetylglucosaminyltransferase V, did not point to any interaction between GnT-III and these enzymes in the transgenic mice, suggesting that this approach may be useful in clinical xenotransplantation.     

Tetracycline-regulated overexpression of glycosyltransferases in Chinese hamster ovary cells.

The glycosylation patterns of recombinant therapeutic glycoproteins can be engineered by overexpression of glycosyltransferases in the host cells used for glycoprotein production. Most prior glycosylation engineering experiments have involved constitutive expression of cloned glycosyltransferases. Here we use tetracycline-regulated expression of two glycosyltransferases, N-acetylglucosaminlytransferases III and V (GnTIII and GnTV) to manipulate glycoform biosynthesis in Chinese hamster ovary (CHO) cells and to study the effect of glycosyltransferase overexpression on this host. The amount of GnTIII and GnTV in these cells, and the glycosylation patterns of several cellular glycoproteins, could be controlled simply by manipulating the concentration of tetracycline in the culture medium. Using this system, it was found that overexpression of either GnTIII or GnTV to high levels led to growth inhibition and was toxic to the cells, indicating that this may be a general feature of glycosyltransferase overexpression. This phenomenon has not been reported previously, probably due to the widespread use of constitutive promoters, and should be taken into account when designing vectors for glycosylation engineering. The growth inhibition effect sets an upper limit to the level of glycosyltransferase overexpression, and may thereby also limit the maximum extent of in vivo modification of poorly accessible glycosylation sites. Also, such inhibition implies a bound on constitutive glycosyltransferase expression which can be cloned.     

Comprehensive glyco-proteomic analysis of human alpha1-antitrypsin and its charge isoforms

Human alpha1-antitrypsin (A1PI) is a well-known glycoprotein in human plasma important for the protection of tissues from proteolytic enzymes. The three N-glycosylation sites of A1PI contain diantennary N-glycans but also triantennary and even traces of tetraantennary structures leading to the typical IEF pattern observed for A1PI. Here we present an approach to characterize A1PI isoforms from human plasma and its PTMs by LC-ESI-MS and LC-ESI-MS/MS of peptides obtained by proteolytic digestion. The single cysteine residue of A1PI formed a disulfide bridge with free cysteine. The variability of the number of antennae and hence sialic acids on glycosylation site N107, which even contained minute amounts of tetraantennary structures, emerged as a major cause for the IEF pattern of A1PI. Only negligible amounts of triantennary structures were identified attached to N70, and exclusively diantennary structures were present on site N271 in each of the isoforms analyzed. Exoglycosidase digests revealed alpha2,6-linked neuraminic acids on diantennary N-glycans, and triantennary contained additionally one single alpha2,3-neuraminic acid per N-glycan, which, together with a fucose, formed a sialyl Lewis X determinant on the beta1,4-linked N-acetylglucosamine, as shown by 2-D-HPLC of pyridylaminated asialoglycans. Fucosylation of diantennary structures was marginal and of the core alpha1,6 type.     

Influence of an ER-retention signal on the N-glycosylation of recombinant human α-l-iduronidase generated in seeds of Arabidopsis

Processes associated with late events of N-glycosylation within the plant Golgi complex are a major limitation to the use of plant-based systems to produce recombinant pharmaceutical proteins for parenteral administration. Specifically, sugars added to the N-glycans of a recombinant protein during glycan maturation to complex forms (e.g. β1,2 xylose and α1,3 fucose) can render the product immunogenic. In order to avoid these sugars, the human enzyme α-L-iduronidase (IDUA, EC 3.2.1.76), with a C-terminal ER-retention sequence SEKDEL, was expressed in seeds of complex-glycan-deficient (cgl) mutant and wild-type (Col-0) Arabidopsis thaliana, under the control of regulatory (5'-, signal-peptide-encoding-, and 3'-) sequences from the arcelin 5-I gene of Phaseolus vulgaris (cgl-IDUA-SEKDEL and Col-IDUA-SEKDEL, respectively). The SEKDEL motif had no adverse effect on the specific activity of the purified enzyme. Surprisingly, the majority of the N-glycans of Col-IDUA-SEKDEL were complex N-glycans (i.e. contained xylose and/or fucose) (88 %), whereas complex N-glycans comprised a much lower proportion of the N-glycans of cgl-IDUA-SEKDEL (26 %), in which high-mannose forms were predominant. In contrast to the non-chimeric IDUA of cgl seeds, which is mainly secreted into the extracellular spaces, the addition of the SEKDEL sequence to human recombinant IDUA expressed in the same background led to retention of the protein in ER-derived vesicles/compartments and its partial localization in protein storage vacuoles. Our data support the contention that the use of a C-terminal ER retention motif as an effective strategy to prevent or reduce complex N-glycan formation, is protein specific.     

Expression of rat beta(1,4)-N-acetylglucosaminyltransferase III in Nicotiana tabacum remodels the plant-specific N-glycosylation

Plant N -linked glycans differ substantially from their mammalian counterparts, mainly with respect to modifications of the core glycan, which typically contains a β(1,2)-xylose and an α(1,3)-fucose. The addition of a bisecting N -acetylglucosamine residue by β(1,4)- N -acetylglucosaminyltransferase III (GnTIII) is known to control the processing of N -linked glycans in mammals, for example by preventing α(1,6)-fucosylation of the core glycan. In order to outcompete plant-specific β(1,2)-xylose and α(1,3)-fucose modifications, rat GnTIII was expressed either with its native localization domain (GnTIII) or with the cytoplasmic tail, transmembrane domain and stem region (CTS) of Arabidopsis thaliana mannosidase II (ManII) (GnTIII^A.th.). Both CTSs targeted enhanced yellow fluorescent protein (eYFP) to a brefeldin A-sensitive compartment, indicative of Golgi localization. GnTIII expression increased the fraction of N -glycans devoid of xylose and fucose from 13% ± 7% in wild-type plants to 60% ± 8% in plants expressing GnTIII^A.th.. N -Glycans of plants expressing rat GnTIII contained three major glycan structures of complex bisected, complex, or hybrid bisected type, accounting for 70%–85% of the total N -glycans. On expression of GnTIII^A.th., N -glycans displayed a higher heterogeneity and were of hybrid type. Co-expression of A. thaliana ManII significantly increased the amount of complex bisected structures relative to the plants expressing GnTIII or GnTIII^A.th., whereas co-expression of human ManII did not redirect the pool of hybrid structures towards complex-type structures. The method described offers the advantage that it can be implemented in any desired plant system for effective removal of β(1,2)-xylose and α(1,3)-fucose from the N -glycan.     

Modulation of therapeutic antibody effector functions by glycosylation engineering: influence of Golgi enzyme localization domain and co-expression of heterologous beta1, 4-N-acetylglucosaminyltransferase III and Golgi alpha-mannosidase II

The effector functions elicited by IgG antibodies strongly depend on the carbohydrate moiety linked to the Fc region of the protein. Therefore several approaches have been developed to rationally manipulate these glycans and improve the biological functions of the antibody. Overexpression of recombinant beta1,4-N-acetylglucosaminyltransferase III (GnT-III) in production cell lines leads to antibodies enriched in bisected oligosaccharides. Moreover, GnT-III overexpression leads to increases in non-fucosylated and hybrid oligosaccharides. Such antibody glycovariants have increased antibody-dependent cellular cytotoxicity (ADCC). To explore a further variable besides overexpression of GnT-III, we exchanged the localization domain of GnT-III with that of other Golgi-resident enzymes. Our results indicate that chimeric GnT-III can compete even more efficiently against the endogenous core alpha1,6-fucosyltransferase (alpha1,6-FucT) and Golgi alpha-mannosidase II (ManII) leading to higher proportions of bisected non-fucosylated hybrid glycans ("Glyco-1" antibody). The co-expression of GnT-III and ManII led to a similar degree of non-fucosylation as that obtained for Glyco-1, but the majority of the oligosaccharides linked to this antibody ("Glyco-2") are of the complex type. These glycovariants feature strongly increased ADCC activity compared to the unmodified antibody, while Glyco-1 (hybrid-rich) features reduced complement-dependent cytotoxicity (CDC) compared to Glyco-2 or unmodified antibody. We show that apart from GnT-III overexpression, engineering of GnT-III localization is a versatile tool to modulate the biological activities of antibodies relevant for their therapeutic application.     

Identification of core alpha 1,3-fucosylated glycans and cloning of the requisite fucosyltransferase cDNA from Drosophila melanogaster. Potential basis of the neural anti-horseadish peroxidase epitope

For many years, polyclonal antibodies raised against the plant glycoprotein horseradish peroxidase have been used to specifically stain the neural and male reproductive tissue of Drosophila melanogaster. This epitope is considered to be of carbohydrate origin, but no glycan structure from Drosophila has yet been isolated that could account for this cross-reactivity. Here we report that N-glycan core alpha1,3-linked fucose is, as judged by preabsorption experiments, indispensable for recognition of Drosophila embryonic nervous system by anti-horseradish peroxidase antibody. Further, we describe the identification by matrix-assisted laser desorption-ionization time-of-flight mass spectrometry and high performance liquid chromatography of two Drosophila N-glycans that, as already detected in other insects, carry both alpha1,3- and alpha1,6-linked fucose residues on the proximal core GlcNAc. Moreover, we have isolated three cDNAs encoding alpha1,3-fucosyltransferase homologues from Drosophila. One of the cDNAs, when transformed into Pichia pastoris, was found to direct expression of core alpha1,3-fucosyltransferase activity. This recombinant enzyme preferred as substrate a biantennary core alpha1,6-fucosylated N-glycan carrying two non-reducing N-acetylglucosamine residues (GnGnF6; Km 11 microm) over the same structure lacking a core fucose residue (GnGn; Km 46 microm). The Drosophila core alpha1,3-fucosyltransferase enzyme was also shown to be able to fucosylate N-glycan structures of human transferrin in vitro, this modification correlating with the acquisition of binding to anti-horseradish peroxidase antibody.