N-linked glycan structures of mouse interferon-β produced by Bombyx mori larvae

Low Surface Potential with Glycoconjugates Determines Insect Cell Adhesion at Room Temperature

Cell-coupled field-effect transistor (FET) biosensors have attracted considerable attention because of their high sensitivity to biomolecules. The use of insect cells (Sf21) as a core sensor element is advantageous due to their stable adhesion to sensors at room temperature. Although visualization of the insect cell-substrate interface leads to logical amplification of signals, the spatiotemporal processes at the interfaces have not yet been elucidated. We quantitatively monitored the adhesion dynamics of Sf21 using interference reflection microscopy (IRM). Specific adhesion signatures with ring-like patches along the cellular periphery were detected. A combination of zeta potential measurements and lectin staining identified specific glycoconjugates with low electrostatic potentials. The ring-like structures were disrupted after cholesterol depletion, suggesting a raft domain along the cell periphery. Our results indicate dynamic and asymmetric cell adhesion is due to low electrostatic repulsion with fluidic sugar rafts. We envision the logical design of cell-sensor interfaces with an electrical model that accounts for actual adhesion interfaces.

Human protein paucimannosylation: cues from the eukaryotic kingdoms

Paucimannosidic proteins (PMPs) are bioactive glycoproteins carrying truncated α- or β-mannosyl-terminating asparagine (N)-linked glycans widely reported across the eukaryotic domain. Our understanding of human PMPs remains limited, despite findings documenting their existence and association with human disease glycobiology. This review comprehensively surveys the structures, biosynthetic routes and functions of PMPs across the eukaryotic kingdoms with the aim of synthesising an improved understanding on the role of protein paucimannosylation in human health and diseases. Convincing biochemical, glycoanalytical and biological data detail a vast structural heterogeneity and fascinating tissue- and subcellular-specific expression of PMPs within invertebrates and plants, often comprising multi-α1,3/6-fucosylation and β1,2-xylosylation amongst other glycan modifications and non-glycan substitutions e.g. O-methylation. Vertebrates and protists express less-heterogeneous PMPs typically only comprising variable core fucosylation of bi- and trimannosylchitobiose core glycans. In particular, the Manα1,6Manβ1,4GlcNAc(α1,6Fuc)β1,4GlcNAcβAsn glycan (M2F) decorates various human neutrophil proteins reportedly displaying bioactivity and structural integrity demonstrating that they are not degradation products. Less-truncated paucimannosidic glycans (e.g. M3F) are characteristic glycosylation features of proteins expressed by human cancer and stem cells. Concertedly, these observations suggest the involvement of human PMPs in processes related to innate immunity, tumorigenesis and cellular differentiation. The absence of human PMPs in diverse bodily fluids studied under many (patho)physiological conditions suggests extravascular residence and points to localised functions of PMPs in peripheral tissues. Absence of PMPs in Fungi indicates that paucimannosylation is common, but not universally conserved, in eukaryotes. Relative to human PMPs, the expression of PMPs in plants, invertebrates and protists is more tissue-wide and constitutive yet, similar to their human counterparts, PMP expression remains regulated by the physiology of the producing organism and PMPs evidently serve essential functions in development, cell-cell communication and host-pathogen/symbiont interactions. In most PMP-producing organisms, including humans, the N-acetyl-β-hexosaminidase isoenzymes and linkage-specific α-mannosidases are glycoside hydrolases critical for generating PMPs via N-acetylglucosaminyltransferase I (GnT-I)-dependent and GnT-I-independent truncation pathways. However, the identity and structure of many species-specific PMPs in eukaryotes, their biosynthetic routes, strong tissue- and development-specific expression, and diverse functions are still elusive. Deep exploration of these PMP features involving, for example, the characterisation of endogenous PMP-recognising lectins across a variety of healthy and N-acetyl-β-hexosaminidase-deficient human tissue types and identification of microbial adhesins reactive to human PMPs, are amongst the many tasks required for enhanced insight into the glycobiology of human PMPs. In conclusion, the literature supports the notion that PMPs are significant, yet still heavily under-studied biomolecules in human glycobiology that serve essential functions and create structural heterogeneity not dissimilar to other human N-glycoprotein types. Human PMPs should therefore be recognised as bioactive glycoproteins that are distinctly different from the canonical N-glycoprotein classes and which warrant a more dedicated focus in glycobiological research.

Novel recombinant feline interferon carrying N-glycans with reduced allergy risk produced by a transgenic silkworm system

Background

The generation of recombinant proteins for commercialisation must be cost-effective. Despite the cost-effective production of recombinant feline interferon (rFeIFN) by a baculovirus expression system, this rFeIFN carries insect-type N-glycans, with core α 1,3 fucosyl residues that act as potential allergens. An alternative method of production may yield recombinant glycoproteins with reduced antigenicity.

Results

A cDNA clone encoding the fifteenth subtype of FeIFN-α (FeIFN-α15) was isolated from a Japanese domestic cat. This clone encoded a protein of 189 amino acids with a molecular mass of 21.1 kDa. The rFeIFN-α15 was expressed using a transgenic silkworm system, which was expected to yield an N-glycan structure with reduced antigenicity compared with the protein produced by the baculovirus system. The resulting rFeIFN-α15 accumulated in the sericin layer of silk fibres and was easily extracted and purified by column chromatography. The N-terminal amino acid sequence of purified rFeIFN-α15 was identical to the mature form of natural sequence. Moreover, its N-glycans did not include detectable core α 1,3 fucosyl residues. Its anti-vesicular stomatitis virus activity (2.6 × 10⁸ units/mg protein) was comparable to that of the baculovirus-expressed rFeIFN.

Conclusions

The lower allergy risk of rFeIFN produced by the transgenic silkworm system than by the baculovirus expression system is due to the former lacking core α 1,3 fucosyl residues in its N-glycans. The rFeIFN-α15 produced by the transgenic silkworm system may be a prospective candidate for the next generation of rFeIFN in veterinary medicine.

Sialylation potentials of the silkworm, Bombyx mori; B. mori possesses an active α2,6-sialyltransferase

N-Glycosylation is an important post-translational modification in most secreted and membrane-bound proteins in eukaryotic cells. However, the insect N-glycosylation pathway and the potentials contributing to the N-glycan synthesis are still unclear because most of the studies on these subjects have focused on mammals and plants. Here, we identified Bombyx mori sialyltransferase (BmST), which is a Golgi-localized glycosyltransferase and which can modify N-glycans. BmST was ubiquitously expressed in different organs and in various stages of development and localized at the Golgi. Biochemical analysis using Sf9-expressed BmST revealed that BmST encoded α2,6-sialyltransferase and transferred N-acetylneuraminic acid (NeuAc) to the nonreducing terminus of Galβ1-R, but exhibited the highest activity toward GalNAcβ1,4-GlcNAc-R. Unlike human α2,6-sialyltransferase, BmST required the post-translational modification, especially N-glycosylation, for its full activity. N-Glycoprotein analysis of B. mori fifth instar larvae revealed that high-mannose-type structure was predominant and GlcNAc-linked and fucosylated structures were observed but endogenous galactosyl-, N-acetylgalactosaminyl- and sialyl-N-glycoproteins were undetectable under the standard analytical approach. These results indicate that B. mori genome encodes an α2,6-sialyltransferase, but further investigations of the sialylation potentials are necessary.

Improvement of glycosylation structure by suppression of β-N-acetylglucosaminidases in silkworm

The baculovirus-silkworm recombinant protein expression system is an excellent method for achieving high-level expression and post-translational modifications, especially glycosylation. However, the presence of paucimannosidic-type N-glycan in glycoproteins restricts their clinical use. Paucimannosidic-type N-glycan is produced by insect-specific membrane-binding-type β-N-acetylglucosaminidase (GlcNAcase). In the silkworm, BmGlcNAcase1, BmGlcNAcase2, and BmFDL are membrane-binding-type GlcNAcases. We investigated the localization of these GlcNAcases and found that BmFDL and BmGlcNAcase2 were mainly located in the fat body and hemolymph, respectively. The fat body is the main tissue of recombinant protein expression by baculovirus, and many glycoproteins are secreted into the hemolymph. These results suggest that inhibition of BmFDL and BmGlcNAcase2 could increase GlcNAc-type N-glycan levels. We therefore injected a GlcNAcase inhibitor into silkworms to investigate changes in the N-glycan structure of the glycoprotein expressed by baculovirus; modest levels of GlcNAc-type N-glycan were observed (0.8% of total N-glycan). Next, we generated a transgenic silkworm in which RNA interference (RNAi) reduced the BmFDL transcript level and enzyme activity to 25% and 50%, respectively, of that of the control silkworm. The proportion of GlcNAc-type N-glycan increased to 4.3% in the RNAi-transgenic silkworm. We conclude that the structure of N-glycan can be changed by inhibiting the GlcNAcases in silkworm.

Improved isolation and purification of functional human Fas receptor extracellular domain using baculovirus-silkworm expression system

To achieve an efficient isolation of human Fas receptor extracellular domain (hFasRECD), a fusion protein of hFasRECD with human IgG1 heavy chain Fc domain containing thrombin cleavage sequence at the junction site was overexpressed using baculovirus-silkworm larvae expression system. The hFasRECD part was separated from the fusion protein by the effective cleavage of the recognition site with bovine thrombin. Protein G column treatment of the reaction mixture and the subsequent cation-exchange chromatography provided purified hFasRECD with a final yield of 13.5mg from 25.0 ml silkworm hemolymph. The functional activity of the product was examined by size-exclusion chromatography analysis. The isolated hFasRECD less strongly interacted with human Fas ligand extracellular domain (hFasLECD) than the Fc domain-bridged counterpart, showing the contribution of antibody-like avidity in the latter case. The purified glycosylated hFasRECD presented several discrete bands in the disulphide-bridge non-reducing SDS-PAGE analysis, and virtually all of the components were considered to participate in the binding to hFasLECD. The attached glycans were susceptible to PNGase F digestion, but mostly resistant to Endo Hf digestion under denaturing conditions. One of the components exhibited a higher susceptibility to PNGase F digestion under non-denaturing conditions.

IgG2 dominancy and carbohydrate recognition specificity of C3H/He mouse antibodies directed to cross-reactive carbohydrate determinants (CCDs) bearing beta-(1,2)-xylose and alpha-(1,3)-fucose

Few common carbohydrate epitopes consisting of terminal beta-(1,2)-xylose and/or alpha-(1,3)-fucose residues are shared by a variety of glycoproteins from plants, insects and parasitic worms, termed cross-reactive carbohydrate determinant (CCD), and frequently recognized by IgE antibodies of patients with food and/or respiratory allergy, though clinical relevancy of such CCD-specific IgE is still controversial. Attention has also been focused on CCDs from the undesired post-translational modification of recombinant therapeutic proteins produced by transgenic plants and insects. In the present study, to clarify immunogenic potentials of CCD-bearing glycoproteins, the antibody response to a model plant glycoprotein, horseradish peroxidase (HRP) was investigated in a mouse model. C3H/He mice were immunized with HRP plus Al(OH)(3) or Freund's adjuvant, and IgG and IgE responses to CCDs in addition to HRP were analyzed by ELISA using some distinct glycoproteins with known N-glycan structures. IgE response to HRP was induced remarkably, whereas that to CCD was weaker and delayed. Moreover, apparent ratio of the CCD-specific antibodies to HRP-specific ones tended to be higher in IgG2a and IgG2b isotypes than IgG1, IgG3 and IgE. In contrast to rabbit antibodies, the CCD-specific antibodies from the mice gave poor reactivity with bromelain and honeybee phospholipase A2, suggesting the critical role of both beta-(1,2)-xylose and alpha-(1,3)-mannose in the CCD-recognition by the mouse antibodies. Moreover, the mouse antibodies showed weaker cross-reactivity to pollen- and insect-derived glycoproteins than the rabbit ones. Thus, in this mouse model, not only IgE but also IgG2 antibody responses to CCDs were induced by immunizing with a CCD-bearing glycoprotein, suggesting that CCDs affected not only Th2-type but also Th1-type antibody response at least in C3H/He mice.

Production of recombinant Japanese eel gonadotropins by baculovirus in silkworm larvae

Recombinant follicle-stimulating hormone (reFSH) and luteinizing hormone (reLH) of the Japanese eel Anguilla japonica were produced by baculovirus in silkworm Bombyx mori larvae. cDNAs encoding Japanese eel gonadotropin subunits (i.e., FSH beta, LH beta, and common alpha) were introduced into the baculovirus, which was infected into silkworm larvae after propagation of the recombinant virus in B. mori culture cells. A 100ml solution of pooled hemolymph from silkworm larvae containing reFSH or reLH were obtained from approximately 250 infected larvae. Ten milliliters of hemolymph were applied to Ni-affinity choromatography, and 5.6 and 3.5mg of partially purified reFSH and reLH were obtained, respectively. Using Western blot analysis concentrations of reFSH and reLH in the original hemolymph was estimated to be 2.2 and 1.1mg/ml, respectively. Biological activities of reFSH and reLH were assessed in vitro and in vivo. Purified reFSH and reLH induced eel oocyte maturation in vitro, and administration of hemolymph containing reFSH or reLH induced spermatogenesis in vivo in sexually immature Japanese eel. The present study indicates that a baculovirus-silkworm system could produce large amounts of biologically active recombinant fish gonadotropins for use in investigations in reproductive endocrinology and/or aquaculture of fish.

Cloning and characterization of a β-N-acetylglucosaminidase (BmFDL) from silkworm Bombyx mori

In insects, β-N-acetylglucosaminidase (GlcNAcase) participates in critical physiological processes such as fertilization, metamorphosis, and glycoconjugate degradation. Insects produce glycoproteins carrying paucimannosidic-type N-glycans, the terminal GlcNAc residue of which is cleaved by a GlcNAc-linkage specific GlcNAcase, also known as the fused lobes (FDL) protein. To obtain information on the structure of GlcNAcases and insight into their contribution to physiological processes, we cloned Bombyx mori FDL (BmFDL) from silkworm larvae. The full-length cDNA (1.9 kb) encoded a protein of 633 amino acids with 42% amino acid sequence identity to Drosophila melanogaster FDL (DmFDL). Recombinant BmFDL cleaved only β-1,2-linked GlcNAc residues from the α-1,3 branch of biantennary N-glycan. This substrate specificity was similar to that of DmFDL. Microsomal FDL activity was inhibited by anti-BmFDL antibodies. Taken together, our results suggest that BmFDL is a N-glycan-processing GlcNAcase in B. mori.

Identification of genes encoding N-glycan processing beta-N-acetylglucosaminidases in Trichoplusia ni and Bombyx mori: Implications for glycoengineering of baculovirus expression systems

Glycoproteins produced by non-engineered insects or insect cell lines characteristically bear truncated, paucimannose N-glycans in place of the complex N-glycans produced by mammalian cells. A key reason for this difference is the presence of a highly specific N-glycan processing beta-N-acetylglucosaminidase in insect, but not in mammalian systems. Thus, reducing or abolishing this enzyme could enhance the ability of glycoengineered insects or insect cell lines to produce complex N-glycans. Of the three insect species routinely used for recombinant glycoprotein production, the processing beta-N-acetylglucosaminidase gene has been isolated only from Spodoptera frugiperda. Thus, the purpose of this study was to isolate and characterize the genes encoding this important processing enzyme from the other two species, Bombyx mori and Trichoplusia ni. Bioinformatic analyses of putative processing beta-N-acetylglucosaminidase genes isolated from these two species indicated that each encoded a product that was, indeed, more similar to processing beta-N-acetylglucosaminidases than degradative or chitinolytic beta-N-acetylglucosaminidases. In addition, over-expression of each of these genes induced an enzyme activity with the substrate specificity characteristic of processing, but not degradative or chitinolytic enzymes. Together, these results demonstrated that the processing beta-N-acetylglucosaminidase genes had been successfully isolated from Trichoplusia ni and Bombyx mori. The identification of these genes has the potential to facilitate further glycoengineering of baculovirus-insect cell expression systems for the production of glycosylated proteins.

Silkworm expression and sugar profiling of human immune cell surface receptor, KIR2DL1

Immune cell surface receptors are directly involved in human diseases, and thus represent major drug targets. However, it is generally difficult to obtain sufficient amounts of these receptors for biochemical and structural studies because they often require posttranslational modifications, especially sugar modification. Recently, we have established a bacmid expression system for the baculovirus BmNPV, which directly infects silkworms, an attractive host for the large-scale production of recombinant sugar-modified proteins. Here we produced the human immune cell surface receptor, killer cell Ig-like receptor 2DL1 (KIR2DL1), by using the BmNPV bacmid expression system, in silkworms. By the direct injection of the bacmid DNA, the recombinant KIR2DL1 protein was efficiently expressed, secreted into body fluids, and purified by Ni(2+) affinity column chromatography. We further optimized the expression conditions, and the final yield was 0.2mg/larva. The sugar profiling revealed that the N-linked sugars of the purified protein comprised very few components, two paucimannose-type oligosaccharides, Manalpha1-6Manbeta1-4GlcNAcbeta1-4GlcNAc and Manalpha1-6Manbeta1-4GlcNAcbeta1-4(Fucalpha1-6)GlcNAc. This revealed that the protein product was much more homogeneous than the complex-sugar type product obtained by mammalian cell expression. The surface plasmon resonance analysis demonstrated that the purified KIR2DL1 protein exhibited specific binding to the HLA-Cw4 ligand. Moreover, the CD spectrum showed the proper secondary structure. These results clearly suggested that the silkworm expression system is quite useful for the expression of cell surface receptors that require posttranslational modifications, as well as for their structural and binding studies, due to the relatively homogeneous N-linked sugar modifications.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2003-2004

This review is the third update of the original review, published in 1999, on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings the topic to the end of 2004. Both fundamental studies and applications are covered. The main topics include methodological developments, matrices, fragmentation of carbohydrates and applications to large polymeric carbohydrates from plants, glycans from glycoproteins and those from various glycolipids. Other topics include the use of MALDI MS to study enzymes related to carbohydrate biosynthesis and degradation, its use in industrial processes, particularly biopharmaceuticals and its use to monitor products of chemical synthesis where glycodendrimers and carbohydrate-protein complexes are highlighted.

Recombinant protein production by a Kaiko-baculovirus system

Nucleopolyhedrovirus, a baculovirus, generates many intranuclear polyhedra in lepidopterous insects. The replacement of the polyhedra gene with a target gene, under a potent polyhedrin promoter, is widely used to express recombinant proteins. In this chapter, we describe the application of a highly efficient and reproducible baculovirus expression system with high throughput using Kaiko (silkworm).

Human IgG1 expression in silkworm larval hemolymph using BmNPV bacmids and its N-linked glycan structure

A Bombyx mori nucleopolyhedrovirus (BmNPV) bacmid expressing heavy and light chains of human 29IJ6 IgG was constructed and used to secrete recombinant antibody into silkworm larval hemolymph. Fifth instar silkworm larvae were reared and injected into the dorsum of the larvae with recombinant cysteine protease- and chitinase-deficient BmNPV (BmNPV-CP(-)-Chi(-)) bacmid/29IJ6 IgG and harvested after approximately 6 days. The total yield of recombinant 29IJ6 IgG was 36 microg/larvae, which is equivalent to 8 mg/kg of larvae. The recombinant antibody was purified to homogeneity using a HiTrap rProtein A FF column with a purification yield of 83.1%. The purified protein was identified by Western blot and ELISA experiments. The N-linked glycan structure of the purified protein was determined by the HPLC mapping method. The N-glycans of the 29IJ6 IgG glycoprotein produced in, and secreted by the silkworm larvae were composed exclusively of two kinds of paucimannose-type oligosaccharides, Manalpha1-6Manbeta1-4GlcNAcbeta1-4(Fucalpha1-6)GlcNAc and Manalpha1-6(Manalpha1-3)Manbeta1-4GlcNAcbeta1-4(Fucalpha1-6)GlcNAc.

Enhanced Production of Monomeric Interferon-β by CHO Cells through the Control of Culture Conditions

The enhancement of recombinant protein expression of a transfected cell line is essential for the development of an efficient large-scale bioprocess. The effect of various media additives and temperature conditions were studied in an attempt to optimize protein production, stability, and protein glycosylation from a Chinese hamster ovary (CHO) cell line producing human beta-interferon (Hu-beta-IFN). We observed a decrease in the ELISA response of the glycoprotein in the later stages of batch cultures, which was attributed to molecular aggregation. Cells were subjected to various concentrations of glycerol, dimethyl sulfoxide (DMSO), and sodium butyrate (NaBu) in a variety of culture systems and conditions. The addition of both NaBu and DMSO resulted in higher specific productivities but reduced growth rates that resulted in a net reduction of interferon produced. Glycerol appeared to stabilize the secreted beta-IFN, resulting in reduced aggregation, despite a decrease in cell growth rate. Glycosylation analysis of isolated beta-IFN showed a time-dependent decrease in sialylation in batch culture that was ameliorated by the presence of glycerol. Low-temperature conditions (30 degrees C) had the greatest effect on productivity with a significant increase in beta-IFN titer as well as a reduction in the degree of molecular aggregation.

The Drosophila fused lobes Gene Encodes an N-Acetylglucosaminidase Involved in N-Glycan Processing

Most processed, e.g. fucosylated, N-glycans on insect glycoproteins terminate in mannose, yet the relevant modifying enzymes require the prior action of N-acetylglucosaminyltransferase I. This led to the hypothesis that a hexosaminidase acts during the course of N-glycan maturation. To determine whether the Drosophila melanogaster genome indeed encodes such an enzyme, a cDNA corresponding to fused lobes (fdl), a putative beta-N-acetylglucosaminidase with a potential transmembrane domain, was cloned. When expressed in Pichia pastoris, the enzyme exhibited a substrate specificity similar to that previously described for a hexosaminidase activity from Sf-9 cells, i.e. it hydrolyzed exclusively the GlcNAc residue attached to the alpha1,3-linked mannose of the core pentasaccharide of N-glycans. It also hydrolyzed p-nitrophenyl-N-acetyl-beta-glucosaminide, but not chitooligosaccharides; in contrast, Drosophila HEXO1 and HEXO2 expressed in Pichia cleaved both these substrates but not N-glycans. The localization of recombinant FDL tagged with green fluorescent protein in Drosophila S2 cells by immunoelectron microscopy showed that this enzyme transits through the Golgi, is present on the plasma membrane and in multivesicular bodies, and is secreted. Finally, the N-glycans of two lines of fdl mutant flies were analyzed by mass spectrometry and reversed-phase high-performance liquid chromatography. The ratio of structures with terminal GlcNAc over those without (i.e. paucimannosidic N-glycans) was drastically increased in the fdl-deficient flies. Therefore, we conclude that the fdl gene encodes a novel hexosaminidase responsible for the occurrence of paucimannosidic N-glycans in Drosophila.

The genetics and genomics of the silkworm, Bombyx mori

We review progress in applying molecular genetic and genomic technologies to studies in the domesticated silkworm, Bombyx mori, highlighting its use as a model for Lepidoptera, and in sericulture and biotechnology. Dense molecular linkage maps are being integrated with classical linkage maps for positional cloning and marker-assisted selection. Classical mutations have been identified by a candidate gene approach. Cytogenetic and sequence analyses show that the W chromosome is composed largely of nested full-length long terminal repeat retrotransposons. Z-chromosome-linked sequences show a lack of dosage compensation. The downstream sex differentiation mechanism has been studied via the silkworm homolog of doublesex. Expressed sequence tagged databases have been used to discover Lepidoptera-specific genes, provide evidence for horizontal gene transfer, and construct microarrays. Physical maps using large-fragment bacterial artificial chromosome libraries have been constructed, and whole-genome shotgun sequencing is underway. Germline transformation and transient expression systems are well established and available for functional studies, high-level protein expression, and gene silencing via RNA interference.