Expression and Characterization of Human β-1, 4-Galactosyltransferase 1 (β4GalT1) Using Silkworm-Baculovirus Expression System

Baculovirus expression vector system (BEVS) is widely known as a mass-production tool to produce functional recombinant glycoproteins except that it may not be always suitable for medical practice due to the differences in the structure of N-linked glycans between insects and mammalian. Currently, various approaches have been reported to alter N-linked glycan structures of glycoproteins derived from insects into terminally sialylated complex-type N-glycans. In the light of those studies, we also proposed in vitro maturation of N-glycan with mass-produced and purified glycosyltransferases by silkworm-BEVS. β-1,4-Galactosyltransferase 1 (β4GalT1) is known as one of type II transmembrane enzymes that transfer galactose in a β-1, 4 linkage to accepter sugars, and a key enzyme for further sialylation of N-glycans. In this study, we developed a large-scale production of recombinant human β4GalT1 (rhβ4GalT1) with N- or C-terminal tags in silkworm-BEVS. We demonstrated that rhβ4GalT1 is N-glycosylated and without mucin-type glycosylation. Interestingly, we found that purified rhβ4GalT1 from silkworm serum presented higher galactosyltransferase activity than that expressed from cultured mammalian cells. We also validated the UDP-galactose transferase activity of produced rhβ4GalT1 proteins by using protein subtracts from silkworm silk gland. Taken together, rhβ4GalT1 from silkworms can become a valuable tool for producing high-quality recombinant glycoproteins with mammalian-like N-glycans.

CRISPR-Cas9 vectors for genome editing and host engineering in the baculovirus-insect cell system

The baculovirus-insect cell system (BICS) has been widely used to produce many different recombinant proteins for basic research and is being used to produce several biologics approved for use in human or veterinary medicine. Early BICS were technically complex and constrained by the relatively primordial nature of insect cell protein glycosylation pathways. Since then, recombination has been used to modify baculovirus vectors-which has simplified the system-and transform insect cells, which has enhanced its protein glycosylation capabilities. Now, CRISPR-Cas9 tools for site-specific genome editing are needed to facilitate further improvements in the BICS. Thus, in this study, we used various insect U6 promoters to construct CRISPR-Cas9 vectors and assessed their utility for site-specific genome editing in two insect cell lines commonly used as hosts in the BICS. We demonstrate the use of CRISPR-Cas9 to edit an endogenous insect cell gene and alter protein glycosylation in the BICS.

N-Glycan Modification of a Recombinant Protein via Coexpression of Human Glycosyltransferases in Silkworm Pupae

Recombinant proteins produced in insect cells and insects, unlike those produced in mammalian cells, have pauci-mannose-type N-glycans. In this study, we examined complex-type N-glycans on recombinant proteins via coexpression of human β-1,2-N-acetylglucosaminyltransferase II (hGnT II) and human β1,4-galactosyltransferase (hGalT I) in silkworm pupae, by using the Bombyx mori nucleopolyhedrovirus (BmNPV) bacmid system. The actin A3 promoter from B. mori and the polyhedrin promoter from Autographa californica multiple nucleopolyhedroviruses (AcMNPVs) were used to coexpress hGnT II and hGalT I. These recombinant BmNPVs were coexpressed with human IgG (hIgG), hGnT II and hGalT I in silkworm pupae. When hIgG was coexpressed with hGnT II, approximately 15% of all N-glycans were biantennary, with both arms terminally modified with N-acetylglucosamine (GlcNAc). In contrast, when hIgG was coexpressed with both hGnT II and hGalT I under the control of the polyhedrin promoter, 27% of all N-glycans were biantennary and terminally modified with GlcNAc, with up to 5% carrying one galactose and 11% carrying two. The obtained N-glycan structure was dependent on the promoters used for coexpression of hGnT II or hGalT I. This is the first report of silkworm pupae producing a biantennary, terminally galactosylated N-glycan in a recombinant protein. These results suggest that silkworms can be used as alternatives to insect and mammalian hosts to produce recombinant glycoproteins with complex N-glycans.

Modifying an Insect Cell N-Glycan Processing Pathway Using CRISPR-Cas Technology

Fused lobes (FDL) is an enzyme that simultaneously catalyzes a key trimming reaction and antagonizes elongation reactions in the insect N-glycan processing pathway. Accordingly, FDL function accounts, at least in part, for major differences in the N-glycosylation patterns of glycoproteins produced by insect and mammalian cells. In this study, we used the CRISPR-Cas9 system to edit the fdl gene in Drosophila melanogaster S2 cells. CRISPR-Cas9 editing produced a high frequency of site-specific nucleotide insertions and deletions, reduced the production of insect-type, paucimannosidic products (Man3GlcNAc2), and led to the production of partially elongated, mammalian-type complex N-glycans (GlcNAc2Man3GlcNAc2) in S2 cells. As CRISPR-Cas9 has not been widely used to analyze or modify protein glycosylation pathways or edit insect cell genes, these results underscore its broad utility as a tool for these purposes. Our results also confirm the key role of FDL at the major branch point distinguishing insect and mammalian N-glycan processing pathways. Finally, the new FDL-deficient S2 cell derivative produced in this study will enable future bottom-up glycoengineering efforts designed to isolate insect cell lines that can efficiently produce recombinant glycoproteins with chemically predefined oligosaccharide side-chain structures.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2009-2010

This review is the sixth update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2010. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, arrays and fragmentation are covered in the first part of the review and applications to various structural typed constitutes the remainder. The main groups of compound that are discussed in this section are oligo and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals. Many of these applications are presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions and applications to chemical synthesis.

Engineering N-Glycosylation Pathway in Insect Cells: Suppression of β-N-Acetylglucosaminidase and Expression of β-1,4-Galactosyltransferase

Most insect cells have a simple N-glycosylation process and consequently paucimannosidic or simple core glycans predominate. It has been proposed that β-N-acetylglucosaminidase (GlcNAcase), a hexosaminidase in the Golgi membrane which removes a terminal N-acetylglucosamine (GlcNAc), might contribute to simple N-glycosylation profile in several insect cells including Drosophila S2. Here, we describe GlcNAcase suppression strategy using RNA interference (RNAi) to avoid the formation of paucimannosidic glycans in insect S2 cells. In addition, we describe coexpression of β(1,4)-galactosyltransferase (GalT) as a strategy to improve N-glycosylation pattern and enable recombinant therapeutic proteins to be produced in S2 cells with more complex N-glycans.

An Overview and History of Glyco-Engineering in Insect Expression Systems

Insect systems, including the baculovirus-insect cell and Drosophila S2 cell systems are widely used as recombinant protein production platforms. Historically, however, no insect-based system has been able to produce glycoproteins with human-type glycans, which often influence the clinical efficacy of therapeutic glycoproteins and the overall structures and functions of other recombinant glycoprotein products. In addition, some insect cell systems produce N-glycans with immunogenic epitopes. Over the past 20 years, these problems have been addressed by efforts to glyco-engineer insect-based expression systems. These efforts have focused on introducing the capacity to produce complex-type, terminally sialylated N-glycans and eliminating the capacity to produce immunogenic N-glycans. Various glyco-engineering approaches have included genetically engineering insect cells, baculoviral vectors, and/or insects with heterologous genes encoding the enzymes required to produce various glycosyltransferases, sugars, nucleotide sugars, and nucleotide sugar transporters, as well as an enzyme that can deplete GDP-fucose. In this chapter, we present an overview and history of glyco-engineering in insect expression systems as a prelude to subsequent chapters, which will highlight various methods used for this purpose.

Enzymatic properties and subtle differences in the substrate specificity of phylogenetically distinct invertebrate N-glycan processing hexosaminidases

Fused lobes (FDL) hexosaminidases are the most recently genetically defined glycosidases involved in the biosynthesis of N-glycans in invertebrates, and their narrow specificity is essential for the generation of paucimannosidic N-glycans in insects. In this study, we explored the potential of FDL hexosaminidases in the utilization of different artificial and natural substrates, both as purified, native compounds or generated in vitro using various relevant glycosyltransferases. In addition to the already-known FDL enzyme from Drosophila melanogaster, we now have identified and characterized the Apis mellifera FDL homolog. The enzymatic properties of the soluble forms of the affinity-purified insect FDL enzymes, expressed in both yeast and insect cells, were compared with those of the phylogenetically distinct recombinant Caenorhabditis elegans FDL-like enzymes and the N-acetylgalactosamine (GalNAc)-specific Caenorhabditis hexosaminidase HEX-4. In tests with a range of substrates, including natural N-glycans, we show that the invertebrate FDL(-like) enzymes are highly specific for N-acetylglucosamine attached to the α1,3-mannose, but under extreme conditions also remove other terminal GalNAc and N-acetylglucosamine residues. Recombinant FDL also proved useful in the analysis of complex mixtures of N-glycans originating from wild-type and mutant Caenorhabditis strains, thereby aiding isomeric definition of paucimannosidic and hybrid N-glycans in this organism. Furthermore, differences in activity and specificity were shown for two site-directed mutants of Drosophila FDL, compatible with the high structural similarity of chitinolytic and N-glycan degrading exohexosaminidases in insects. Our studies are another indication for the variety of structural and function aspects in the GH20 hexosaminidase family important for both catabolism and biosynthesis of glycoconjugates in eukaryotes.

Biochemical characterization of a novel β-N-acetylhexosaminidase from the insect Ostrinia furnacalis

The β-N-acetylhexosaminidase FDL specifically removes the β-1,2-GlcNAc residue conjugated to the α-1,3-mannose residue of the core structure of insect N-glycans, playing significant physiological roles in post-translational modification in the Golgi apparatus. Little is known about its enzymatic properties. We obtained the OfFDL gene from the insect Ostrinia furnacalis by RT-PCR. The full length cDNA of FDL is 2241 bp carrying an opening reading frame of 1923 bp encoding 640 amino acids. The recombinant protein OfFDL in a soluble and active form was obtained with high purity through a two-step purification strategy. The recombinant OfFDL exclusively hydrolyzes the terminal β-1,2-GlcNAc residue from the α-1,3 branch instead of the α-1,6 branch of the substrate GnGn-PA. Several kinetic parameters including kcat/Km values toward four artificial substrates and Ki values of three representative hexosaminidase inhibitors were obtained.

Expression and N-glycan analysis of human 90K glycoprotein in Drosophila S2 cells

Human 90K (h90K; Mac-2-binding protein) glycoprotein is a potential pharmaceutical due to its inhibitory activity against cancer metastasis and expansion. Here, h90K glycoprotein was produced in insect Drosophila S2 cell system, and its N-glycan pattern was analyzed. A plasmid encoding h90K gene, fused with a hexahistidine tag under the control of Drosophila metallotionein promoter, was stably transfected into S2 cells. After copper sulfate induction, transfected S2 cells secreted recombinant h90K at a good expression level of 28mg/L in a 150-mL spinner flask culture. The purified recombinant h90K showed an apparent molecular weight of ∼78kDa which was much smaller than that (∼97kDa) of the natural h90K. Because de-N-glycosylated h90K appeared at ∼60kDa protein band, it was suggested that the recombinant h90K from S2 cells has small N-glycans with about half the molecular weight (∼18kDa) of N-glycans of the natural h90K. Through detail analyses using high-performance liquid chromatography and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, the S2-derived recombinant h90K was confirmed that it has simple paucimannosidic structures containing two or three mannose residues with core fucose as the major (∼79%) N-glycans.

Over-expression of human clusterin increases stress resistance and extends lifespan in Drosophila melanogaster

Clusterin is a disulfide-linked heterodimeric glycoprotein that has been implicated in a variety of biological processes. Its expression has been shown to be elevated during cellular senescence and normal aging, but it is uncertain whether clusterin protects against aging or whether its expression is a consequence of aging. To investigate the functions of clusterin during organismal aging, we established transgenic Drosophila alleles to induce the expression of the secretory form of human clusterin (hClu(S)) using the Gal4/UAS system. hClu(S) protein (~60 kDa) was detected in both adult homogenates and larval hemolymphs of flies ubiquitously overexpressing hClu(S) (da-Gal4>UAS-hClu(S)) and in motoneurons (D42-Gal4>UAS-hClu(S)). Interestingly, the mean lifespans of these hClu(S)-overexpressing flies were significantly greater than those of control flies that exhibited no hClu(S) induction. hClu(S)-overexpressing flies also showed significantly greater tolerance to heat shock, wet starvation, and oxidative stress. Furthermore, amounts of reactive oxygen species (ROS) in whole bodies were significantly lower in hClu(S)-overexpressing flies. In addition, clusterin was found to prevent the inactivation of glutamine synthetase (GS) by metal-catalyzed oxidation (MCO) in vitro, and this protection was only supported by thiol-reducing equivalents, such as, DTT or GSH, and not by ascorbate (a non-thiol MCO system). Furthermore, this protection against GS inactivation by clusterin was abolished by reacting clusterin with N-ethylmaleimide, a sulfhydryl group-modifying agent. Taken together, these results suggest that a disulfide-linked form of clusterin functions as an antioxidant protein via its cysteine sulfhydryl groups to reduce ROS levels and delay the organismal aging in fruit flies.

Substrate specificities and intracellular distributions of three N-glycan processing enzymes functioning at a key branch point in the insect N-glycosylation pathway

Man(α1-6)[GlcNAc(β1-2)Man(α1-3)]ManGlcNAc(2) is a key branch point intermediate in the insect N-glycosylation pathway because it can be either trimmed by a processing β-N-acetylglucosaminidase (FDL) to produce paucimannosidic N-glycans or elongated by N-acetylglucosaminyltransferase II (GNT-II) to produce complex N-glycans. N-acetylglucosaminyltransferase I (GNT-I) contributes to branch point intermediate production and can potentially reverse the FDL trimming reaction. However, there has been no concerted effort to evaluate the relationships among these three enzymes in any single insect system. Hence, we extended our previous studies on Spodoptera frugiperda (Sf) FDL to include GNT-I and -II. Sf-GNT-I and -II cDNAs were isolated, the predicted protein sequences were analyzed, and both gene products were expressed and their acceptor substrate specificities and intracellular localizations were determined. Sf-GNT-I transferred N-acetylglucosamine to Man(5)GlcNAc(2), Man(3)GlcNAc(2), and GlcNAc(β1-2)Man(α1-6)[Man(α1-3)]ManGlcNAc(2), demonstrating its role in branch point intermediate production and its ability to reverse FDL trimming. Sf-GNT-II only transferred N-acetylglucosamine to Man(α1-6)[GlcNAc(β1-2)Man(α1-3)]ManGlcNAc(2), demonstrating that it initiates complex N-glycan production, but cannot use Man(3)GlcNAc(2) to produce hybrid or complex structures. Fluorescently tagged Sf-GNT-I and -II co-localized with an endogenous Sf Golgi marker and Sf-FDL co-localized with Sf-GNT-I and -II, indicating that all three enzymes are Golgi resident proteins. Unexpectedly, fluorescently tagged Drosophila melanogaster FDL also co-localized with Sf-GNT-I and an endogenous Drosophila Golgi marker, indicating that it is a Golgi resident enzyme in insect cells. Thus, the substrate specificities and physical juxtapositioning of GNT-I, GNT-II, and FDL support the idea that these enzymes function at the N-glycan processing branch point and are major factors determining the net outcome of the insect cell N-glycosylation pathway.

Drosophila melanogaster S2 cells for expression of heterologous genes: From gene cloning to bioprocess development

In the present review we discuss strategies that have been used for heterologous gene expression in Drosophila melanogaster Schneider 2 (S2) cells using plasmid vectors. Since the growth of S2 cells is not dependent on anchorage to solid substrates, these cells can be easily cultured in suspension in large volumes. The factors that most affect the growth and gene expression of S2 cells, namely cell line, cell passage, inoculum concentration, culture medium, temperature, dissolved oxygen concentration, pH, hydrodynamic forces and toxic metabolites, are discussed by comparison with other insect and mammalian cells. Gene expression, cell metabolism, culture medium formulation and parameters involved in cellular respiration are particularly emphasized. The experience of the authors with the successful expression of a biologically functional protein, the rabies virus glycoprotein (RVGP), by recombinant S2 cells is presented in the topics covered.

Diversity in protein glycosylation among insect species

BACKGROUND

A very common protein modification in multicellular organisms is protein glycosylation or the addition of carbohydrate structures to the peptide backbone. Although the Class of the Insecta is the largest animal taxon on Earth, almost all information concerning glycosylation in insects is derived from studies with only one species, namely the fruit fly Drosophila melanogaster.

METHODOLOGY/PRINCIPAL FINDINGS

In this report, the differences in glycoproteomes between insects belonging to several economically important insect orders were studied. Using GNA (Galanthus nivalis agglutinin) affinity chromatography, different sets of glycoproteins with mannosyl-containing glycan structures were purified from the flour beetle (Tribolium castaneum), the silkworm (Bombyx mori), the honeybee (Apis mellifera), the fruit fly (D. melanogaster) and the pea aphid (Acyrthosiphon pisum). To identify and characterize the purified glycoproteins, LC-MS/MS analysis was performed. For all insect species, it was demonstrated that glycoproteins were related to a broad range of biological processes and molecular functions. Moreover, the majority of glycoproteins retained on the GNA column were unique to one particular insect species and only a few glycoproteins were present in the five different glycoprotein sets. Furthermore, these data support the hypothesis that insect glycoproteins can be decorated with mannosylated O-glycans.

CONCLUSIONS/SIGNIFICANCE

The results presented here demonstrate that oligomannose N-glycosylation events are highly specific depending on the insect species. In addition, we also demonstrated that protein O-mannosylation in insect species may occur more frequently than currently believed.

A functional carbohydrate chip platform for analysis of carbohydrate-protein interaction

A carbohydrate chip based on glass or other transparent surfaces has been suggested as a potential tool for high-throughput analysis of carbohydrate-protein interactions. Here we proposed a facile, efficient, and cost-effective method whereby diverse carbohydrate types are modified in a single step and directly immobilized onto a glass surface, with retention of functional orientation. We modified various types of carbohydrates by reductive amination, in which reducing sugar groups were coupled with 4-(2-aminoethyl)aniline, which has di-amine groups at both ends. The modified carbohydrates were covalently attached to an amino-reactive NHS-activated glass surface by formation of stable amide bonds. This proposed method was applied for efficient construction of a carbohydrate microarray to analyze carbohydrate-protein interactions. The carbohydrate chip prepared using our method can be successfully used in diverse biomimetic studies of carbohydrates, including carbohydrate-biomolecule interactions, and carbohydrate sensor chip or microarray development for diagnosis and screening.

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.

The influenza A virus hemagglutinin glycosylation state affects receptor-binding specificity

In this study we evaluated the receptor-binding properties of recombinant soluble hemagglutinin (HA) trimers (subtype H2 and H7) produced in insect S2 cells, human HEK293T or HEK293S GnTI(-) cells, which produce proteins with paucimannose, complex or high-mannose N-linked glycans, respectively. The results show that HA proteins that only differ in their glycosylation status possess different receptor fine specificities. HEK293T cell-produced HA displayed a very narrow receptor specificity. However, when treated with neuraminidase this HA was able to bind more glycans with similar specificity as HEK293S GnTI(-) cell-produced HA. Insect cell-produced HA demonstrated decreased receptor specificity. As a consequence, differences in HA fine receptor specificities could not be observed with the insect cell-, but were readily detected with the HEK293S GnTI(-) cell-produced HAs.