Molecular cloning and functional characterization of a Lepidopteran insect beta4-N-acetylgalactosaminyltransferase with broad substrate specificity, a functional role in glycoprotein biosynthesis, and a potential functional role in glycolipid biosynthesis

Glycosylation in Drosophila S2 cells

In recent years, there has been a remarkable surge in the approval of therapeutic protein drugs, particularly recombinant glycoproteins. Drosophila melanogaster S2 cells have become an appealing platform for the production of recombinant proteins due to their simplicity and low cost in cell culture. However, a significant limitation associated with using the S2 cell expression system is its propensity to introduce simple paucimannosidic glycosylation structures, which differs from that in the mammalian expression system. It is well established that the glycosylation patterns of glycoproteins have a profound impact on the physicochemical properties, bioactivity, and immunogenicity. Therefore, understanding the mechanisms behind these glycosylation modifications and implementing measures to address it has become a subject of considerable interest. This review aims to comprehensively summarize recent advancements in glycosylation modification in S2 cells, with a particular focus on comparing the glycosylation patterns among S2, other insect cells, and mammalian cells, as well as developing strategies for altering the glycosylation patterns of recombinant glycoproteins.

Enzymatic glycan remodeling–metal free click (GlycoConnect™) provides homogenous antibody-drug conjugates with improved stability and therapeutic index without sequence engineering

Antibody-drug conjugates (ADCs) are increasingly powerful medicines for targeted cancer therapy. Inspired by the trend to further improve their therapeutic index by generation of homogenous ADCs, we report here how the clinical-stage GlycoConnect™ technology uses the globally conserved N-glycosylation site to generate stable and site-specific ADCs based on enzymatic remodeling and metal-free click chemistry. We demonstrate how an engineered endoglycosidase and a native glycosyl transferase enable highly efficient, one-pot glycan remodeling, incorporating a novel sugar substrate 6-azidoGalNAc. Metal-free click attachment of an array of cytotoxic payloads was highly optimized, in particular by inclusion of anionic surfactants. The therapeutic potential of GlycoConnect™, in combination with HydraSpace™ polar spacer technology, was compared to that of Kadcyla® (ado-trastuzumab emtansine), showing significantly improved efficacy and tolerability.

Glycosyltransferases: the multifaceted enzymatic regulator in insects

Glycosyltransferases (GTs) catalyse the reaction of glyco-conjugation of various biomolecules by transferring the saccharide moieties from an activated nucleotide sugar to nucleophilic glycosyl acceptor. In insects, GTs show diverse temporal and site-specific expression patterns and thus play significant roles in forming the complex biomolecular structures that are necessary for insect survival, growth and development. Several insects exhibit GT-mediated detoxification as a key defence strategy against plant allelochemicals and xenobiotic compounds, as well as a mechanism for pesticide cross-resistance. Also, these enzymes act as crucial effectors and modulators in various developmental processes of insects such as eye development, UV shielding, cuticle formation, epithelial development and other specialized functions. Furthermore, many of the known insect GTs have been shown to play a fundamental role in other physiological processes like body pigmentation, cuticular tanning, chemosensation and stress response. This review provides a detailed overview of the multifaceted functionality of insect GTs and summarizes numerous case studies associated with it.

Bombyx mori β1,4-N-acetylgalactosaminyltransferase possesses relaxed donor substrate specificity in N-glycan synthesis

N-Glycosylation is one of the most important post-translational protein modifications in eukaryotic cells. Although more than 200 N-glycogenes contributing to N-glycan biosynthesis have been identified and characterized, the information on insect N-glycosylation is still limited. Here, focusing on insect N-glycosylation, we characterized Bombyx mori N-acetylgalactosaminyltransferase (BmGalNAcT) participating in complex N-glycan biosynthesis in mammals. BmGalNAcT localized at the Golgi and was ubiquitously expressed in every organ and in the developmental stage of the middle silk gland of fifth instar larvae. Analysis of recombinant BmGalNAcT expressed in Sf9 cells showed that BmGalNAcT transferred GalNAc to non-reducing terminals of GlcNAcβ1,2-R with β1,4-linkage. In addition, BmGalNAcT mediated transfer of galactose and N-acetylglucosamine residues but not transfer of either glucose or glucuronic acid from the UDP-sugar donor substrate to the N-glycan. Despite this tri-functional sugar transfer activity, however, most of the endogenous glycoproteins of insect cells were present without GalNAc, Gal, or GlcNAc residues at the non-reducing terminal of β1,2-GlcNAc residue(s). Moreover, overexpression of BmGalNAcT in insect cells had no effect on N-acetylgalactosaminylation, galactosylation, or N-acetylglucosaminylation of the major N-glycan during biosynthesis. These results suggested that B. mori has a novel multifunctional glycosyltransferase, but the N-glycosylation is highly and strictly regulated by the endogenous N-glycosylation machineries.

Aberration of Serum and Tissue N-Glycans in Mouse β1,4-GalT1 Y286L Mutant Variants

β1,4-GalT1 is a type II membrane glycosyltransferase. It catalyzes the production of lactose in the lactating mammary gland and is supposedly also involved in the galactosylation of terminal GlcNAc of complex-type N-glycans. In-vitro studies of the bovine β4Gal-T1 homolog showed that replacing a single residue of tyrosine with leucine at position 289 alters the donor substrate specificity from UDP-Gal to UDP-N-acetyl-galactosamine (UDP-GalNAc). The effect of this peculiar change in β1,4GalT1 specificity was investigated in-vivo, by generating biallelic Tyr286Leu β1,4GalT1 mice using CRISPR/Cas9 and crossbreeding. Mice bearing this mutation showed no appreciable defects when compared to wild-type mice, with the exception of biallelic female B4GALT1 mutant mice, which were unable to produce milk. The detailed comparison of wild-type and mutant mice derived from liver, kidney, spleen, and intestinal tissues showed only small differences in their N-glycan pattern. Comparable N-glycosylation was also observed in HEK 293 wild-type and knock-out B4GALT1 cells. Remarkably and in contrast to the other analyzed tissue samples, sialylation and galactosylation of serum N-glycans of biallelic Tyr286Leu GalT1 mice almost disappeared completely. These results suggest that β1,4GalT1 plays a special role in the synthesis of serum N-glycans. The herein described Tyr286Leu β1,4GalT1 mutant mouse model may, therefore, prove useful in the investigation of the mechanism which regulates tissue-dependent galactosylation.

α-L-Fucosidase from Bombyx mori has broad substrate specificity and hydrolyzes core fucosylated N-glycans

N-glycans play a role in physiological functions, including glycoprotein conformation, signal transduction, and antigenicity. Insects display both α-1,6- and α-1,3-linked fucose residues bound to the innermost N-acetylglucosamine of N-glycans whereas core α-1,3-fucosylated N-glycans are not found in mammals. Functions of insect core-fucosylated glycans are not clear, and no α-L-fucosidase related to the N-glycan degradation has been identified. In the genome of the domestic silkworm, Bombyx mori, a gene for a protein, BmFucA, belonging to the glycoside hydrolase family 29 is a candidate for an α-L-fucosidase gene. In this study, BmFucA was cloned and recombinantly expressed as a glutathione-S-transferase tagged protein (GST-BmFucA). Recombinant GST-BmFucA exhibited broad substrate specificity and hydrolyzed p-nitrophenyl α-L-fucopyranoside, 2'-fucosyllactose, 3-fucosyllactose, 3-fucosyl-N,N'-diacetylchitobiose, and 6-fucosyl-N,N'-diacetylchitobiose. Further, GST-BmFucA released fucose from both pyridylaminated complex-type and paucimannose-type glycans that were core-α-1,6-fucosylated. GST-BmFucA also shows hydrolysis activity for core-fucosylated glycans attached to phospholipase A₂ from bee venom. BmFucA may be involved in the catabolism of core-fucosylated N-glycans in B. mori.

Characterization of Bombyx mori N-acetylglucosaminyltransferase II splicing variants

N-Acetylglucosaminyltransferase II (GNTII), which catalyzes the transfer of N-acetylglucosamine to N-glycans, plays an essential role in the biosynthesis of branched and complex-type N-glycans. Some characteristics of the GNTIIs from various species have been identified, but not all features have been revealed because some insects have GNTII redundancies due to the possession of splicing variants. In this study, we focused on four splicing variants of silkworm Bombyx mori GNTII (BmGNTII) that differ only in the absence or presence of Exon 2, Exon 9 or both, and we characterized the spatiotemporal transcript levels and enzymatic properties of each. Two of the splicing variants, BmGNTII-B and BmGNTII-D, lack Exon 9, and were expressed more highly in silk glands than any other organs. With respect to the enzymatic properties, optimal temperature and pH were similar among the recombinant BmGNTIIs, but the specific activities and temperature stabilities differed according to the presence or absence of Exon 9 in the splicing variants. These results demonstrate that the B. mori genome encodes splicing variants of GNTII with different enzymatic properties.

Biochemical characterization and mutational analysis of silkworm Bombyx mori β-1,4-N-acetylgalactosaminyltransferase and insight into the substrate specificity of β-1,4-galactosyltransferase family enzymes

Silkworm Bombyx mori is one of the insect hosts for recombinant protein production at academic and industrial levels. B. mori and other insect cells can produce mammalian proteins with proper posttranslational modifications, such as N-glycosylation, but the structures of N-glycans in B. mori are mainly high mannose- and paucimannose-type, while mammals also produce hybrid- and complex-type glycans. Recently, complex-type N-glycans whose structures are different from mammalian ones have been identified in some insect cell N-glycomes at very low levels compared with levels of high mannose- and paucimannose-type glycans. However, their functions and the enzymes involved in the biosynthesis of insect complex-type N-glycans are not clear, and complex-type N-glycans, except for N-acetylglucosamine-terminated glycans, are still not identified in the B. mori N-glycome. Here, we focused on the β-1,4-galactosyltransferase family (also known as glycosyltransferase family 7, GT7) that contains mammalian β-1,4-galactosyltransferase and insect β-1,4-N-acetylgalactosaminyltransferase. A gene for a GT7 protein (BmGalNAcT) from B. mori was cloned, expressed in a soluble form using a silkworm expression system, and the gene product showed strict β-1,4-N-acetylgalactosaminyltransferase activity but not β-1,4-galactosyltransferase activity. A mutation in Ile298 or Ile310, which are predicted to be located in the active site, reduced its glycosyltransferase activity, suggesting that these residues and the corresponding residues are responsible for substrate specificity of GT7. These results suggested that BmGalNAcT may be involved in the complex-type N-glycans, and moreover, bioinformatics analysis revealed that B. mori might have an extra gene for a GT7 enzyme with different specificity in addition to the known insect GT7 glycosyltransferases.

Tissue-specific glycosylation in the honeybee: Analysis of the N-glycomes of Apis mellifera larvae and venom

BACKGROUND

Previous glycophylogenetic comparisons of dipteran and lepidopteran species revealed variations in the anionic and zwitterionic modifications of their N-glycans; therefore, we wished to explore whether species- and order-specific glycomic variations would extend to the hymenoptera, which include the honeybee Apis mellifera, an agriculturally- and allergologically-significant social species.

METHODS

In this study, we employed an off-line liquid chromatography/mass spectrometry approach, in combination with enzymatic and chemical treatments, to analyse the N-glycans of male honeybee larvae and honeybee venom in order to facilitate definition of isomeric structures.

RESULTS

The neutral larval N-glycome was dominated by oligomannosidic and paucimannosidic structures, while the neutral venom N-glycome displayed more processed hybrid and complex forms with antennal N-acetylgalactosamine, galactose and fucose residues including Lewis-like epitopes; the anionic pools from both larvae and venom contained a wide variety of glucuronylated, sulphated and phosphoethanolamine-modified N-glycans with up to three antennae. In comparison to honeybee royal jelly, there were more fucosylated and fewer Man_4/5-based hybrid glycans in the larvae and venom samples as well as contrasting antennal lengths.

CONCLUSIONS

Combining the current data on venom and larvae with that we previously published on royal jelly, a total honeybee N-glycomic repertoire of some 150 compositions can be proposed in addition to the 20 previously identified on specific venom glycoproteins.

SIGNIFICANCE

Our data are indicative of tissue-specific modification of the core and antennal regions of N-glycans in Apis mellifera and reinforce the concept that insects are capable of extensive processing to result in rather complex anionic oligosaccharide structures.

Comparisons of N-glycans across invertebrate phyla

Many invertebrates are either parasites themselves or vectors involved in parasite transmission; thereby, the interactions of parasites with final or intermediate hosts are often mediated by glycans. Therefore, it is of interest to compare the glycan structures or motifs present across invertebrate species. While a typical vertebrate modification such as sialic acid is rare in lower animals, antennal and core modifications of N-glycans are highly varied and range from core fucose, galactosylated fucose, fucosylated galactose, methyl groups, glucuronic acid and sulphate through to addition of zwitterionic moieties (phosphorylcholine, phosphoethanolamine and aminoethylphosphonate). Only in some cases are the enzymatic bases and the biological function of these modifications known. We are indeed still in the phase of discovering invertebrate glycomes primarily using mass spectrometry, but molecular biology and microarraying techniques are complementary to the determination of novel glycan structures and their functions.

Diversity and functions of protein glycosylation in insects

The majority of proteins is modified with carbohydrate structures. This modification, called glycosylation, was shown to be crucial for protein folding, stability and subcellular location, as well as protein-protein interactions, recognition and signaling. Protein glycosylation is involved in multiple physiological processes, including embryonic development, growth, circadian rhythms, cell attachment as well as maintenance of organ structure, immunity and fertility. Although the general principles of glycosylation are similar among eukaryotic organisms, insects synthesize a distinct repertoire of glycan structures compared to plants and vertebrates. Consequently, a number of unique insect glycans mediate functions specific to this class of invertebrates. For instance, the core α1,3-fucosylation of N-glycans is absent in vertebrates, while in insects this modification is crucial for the development of wings and the nervous system. At present, most of the data on insect glycobiology comes from research in Drosophila. Yet, progressively more information on the glycan structures and the importance of glycosylation in other insects like beetles, caterpillars, aphids and bees is becoming available. This review gives a summary of the current knowledge and recent progress related to glycan diversity and function(s) of protein glycosylation in insects. We focus on N- and O-glycosylation, their synthesis, physiological role(s), as well as the molecular and biochemical basis of these processes.

The underestimated N-glycomes of lepidopteran species

BACKGROUND

Insects are significant to the environment, agriculture, health and biotechnology. Many of these aspects display some relationship to glycosylation, e.g., in case of pathogen binding or production of humanised antibodies; for a long time, it has been considered that insect N-glycosylation potentials are rather similar and simple, but as more species are glycomically analysed in depth, it is becoming obvious that there is indeed a large structural diversity and interspecies variability.

METHODS

Using an off-line LC-MALDI-TOF MS approach, we have analysed the N-glycomes of two lepidopteran species (the cabbage looper Trichoplusia ni and the gypsy moth Lymantria dispar) as well as of the commonly-used T. ni High Five cell line.

RESULTS

We detected not only sulphated, glucuronylated, core difucosylated and Lewis-like antennal fucosylated structures, but also the zwitterion phosphorylcholine on antennal GlcNAc residues, a modification otherwise familiar from nematodes; in L. dispar, N-glycans with glycolipid-like antennae containing α-linked N-acetylgalactosamine were also revealed.

CONCLUSION

The lepidopteran glycomes analysed not only display core α1,3-fucosylation, which is foreign to mammals, but also up to 5% anionic and/or zwitterionic glycans previously not found in these species.

SIGNIFICANCE

The occurrence of anionic and zwitterionic glycans in the Lepidoptera data is not only of glycoanalytical and evolutionary interest, but is of biotechnological relevance as lepidopteran cell lines are potential factories for recombinant glycoprotein production.

Protein N-glycosylation and N-glycan trimming are required for postembryonic development of the pest beetle Tribolium castaneum

In holometabolous insects the transition from larva to adult requires a complete body reorganization and relies on N-glycosylated proteins. N-glycosylation is an important posttranslational modification that influences protein activity but its impact on the metamorphosis has not been studied yet. Here we used the red flour beetle, Tribolium castaneum, to perform a first comprehensive study on the involvement of the protein N-glycosylation pathway in metamorphosis. The transcript levels for genes encoding N-glycan processing enzymes increased during later developmental stages and, in turn, transition from larva to adult coincided with an enrichment of more extensively modified paucimannose glycans, including fucosylated ones. Blockage of N-glycan attachment resulted in larval mortality, while RNAi of α-glucosidases involved in early N-glycan trimming and quality control disrupted the larva to pupa transition. Additionally, simultaneous knockdown of multiple genes responsible for N-glycan processing towards paucimannose structures revealed their novel roles in pupal appendage formation and adult eclosion. Our findings revealed that, next to hormonal control, insect post-embryonic development and metamorphosis depend on protein N-glycan attachment and efficient N-glycan processing. Consequently, disruption of these processes could be an effective new approach for insect control.

Targeted release and fractionation reveal glucuronylated and sulphated N- and O-glycans in larvae of dipteran insects

Mosquitoes are important vectors of parasitic and viral diseases with Anopheles gambiae transmitting malaria and Aedes aegypti spreading yellow and Dengue fevers. Using two different approaches (solid-phase extraction and reversed-phase or hydrophilic interaction HPLC fractionation followed by MALDI-TOF MS or permethylation followed by NSI-MS), we examined the N-glycans of both A. gambiae and A. aegypti larvae and demonstrate the presence of a range of paucimannosidic glycans as well as bi- and tri-antennary glycans, some of which are modified with fucose or with sulphate or glucuronic acid residues; the latter anionic modifications were also found on N-glycans of larvae from another dipteran species (Drosophila melanogaster). The sulphate groups are attached primarily to core α-mannose residues (especially the α1,6-linked mannose), whereas the glucuronic acid residues are linked to non-reducing β1,3-galactose. Also, O-glycans were found to possess glucuronic acid and sulphate as well as phosphoethanolamine modifications. The presence of sulphated and glucuronylated N-glycans is a novel feature in dipteran glycomes; these structures have the potential to act as additional anionic glycan ligands involved in parasite interactions with the vector host.

The Drosophila neurally altered carbohydrate mutant has a defective Golgi GDP-fucose transporter

Studying genetic disorders in model organisms can provide insights into heritable human diseases. The Drosophila neurally altered carbohydrate (nac) mutant is deficient for neural expression of the HRP epitope, which consists of N-glycans with core α1,3-linked fucose residues. Here, we show that a conserved serine residue in the Golgi GDP-fucose transporter (GFR) is substituted by leucine in nac(1) flies, which abolishes GDP-fucose transport in vivo and in vitro. This loss of function is due to a biochemical defect, not to destabilization or mistargeting of the mutant GFR protein. Mass spectrometry and HPLC analysis showed that nac(1) mutants lack not only core α1,3-linked, but also core α1,6-linked fucose residues on their N-glycans. Thus, the nac(1) Gfr mutation produces a previously unrecognized general defect in N-glycan core fucosylation. Transgenic expression of a wild-type Gfr gene restored the HRP epitope in neural tissues, directly demonstrating that the Gfr mutation is solely responsible for the neural HRP epitope deficiency in the nac(1) mutant. These results validate the Drosophila nac(1) mutant as a model for the human congenital disorder of glycosylation, CDG-IIc (also known as LAD-II), which is also the result of a GFR deficiency.

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.

Structure-based evolutionary relationship of glycosyltransferases: a case study of vertebrate β1,4-galactosyltransferase, invertebrate β1,4-N-acetylgalactosaminyltransferase and α-polypeptidyl-N-acetylgalactosaminyltransferase

Cell surface glycans play important cellular functions and are synthesized by glycosyltransferases. Structure and function studies show that the donor sugar specificity of the invertebrate β1,4-N-acetyl-glactosaminyltransferase (β4GalNAc-T) and the vertebrate β1,4-galactosyltransferase I (β4Gal-T1) are related by a single amino acid residue change. Comparison of the catalytic domain crystal structures of the β4Gal-T1 and the α-polypeptidyl-GalNAc-T (αppGalNAc-T) shows that their protein structure and sequences are similar. Therefore, it seems that the invertebrate β4GalNAc-T and the catalytic domain of αppGalNAc-T might have emerged from a common primordial gene. When vertebrates emerged from invertebrates, the amino acid that determines the donor sugar specificity of the invertebrate β4GalNAc-T might have mutated, thus converting the enzyme to a β4Gal-T1 in vertebrates.

Adaptation of the "in-gel release method" to N-glycome analysis of low-milligram amounts of material

Protein N-glycosylation is a post-translational modification which plays numerous crucial physiological roles. The N-glycan pattern varies depending on the species organs, tissues and even cell types and their respective physiological states. Obtaining enough starting material from a particular cell type or tissue for N-glycan purification by conventional methods can, in certain cases, be very difficult. Previously, a sensitive technique, the "in-gel release method" that allows the determination of N-glycans attached to a protein isolated by SDS-PAGE, has been developed in this and other laboratories. Here, we describe the adaptation of this method to obtain information on the N-glycome from minute amounts of tissue. The starting material, ranging from less than a milligram to a few milligrams of fresh tissue, is directly ground in Laemmli sample buffer and subject briefly to discontinuous Tris-glycine-SDS-PAGE. The Coomassie-stained band containing the majority of the proteins is subject to the "in-gel release method". The developed technique was used to analyze N-glycan patterns of different samples from Caenorhabditis elegans, Drosophila melanogaster, Spodoptera frugiperda, Trichoplusia ni, Nicotiana benthamiana, Arabidopsis thaliana, and Mus musculus. Furthermore, the technique was used to determine the effects of transient small-scale RNAi-mediated knock-down of a glycosylation-related gene in Drosophila Schneider 2 cell line.

Glycomic studies of Drosophila melanogaster embryos

With the complete genome sequence of Drosophila melanogaster defined a systematic approach towards understanding the function of glycosylation has become possible. Structural assignment of the entire Drosophila glycome during specific developmental stages could provide information that would shed further light on the specific roles of different glycans during development and pinpoint the activity of certain glycosyltransferases and other glycan biosynthetic genes that otherwise might be missed through genetic analyses. In this paper the major glycoprotein N- and O-glycans of Drosophila embryos are described as part of our initial undertaking to characterize the glycome of Drosophila melanogaster. The N-glycans are dominated by high mannose and paucimannose structures. Minor amounts of mono-, bi- and tri-antennary complex glycans were observed with GlcNAc and Galbeta1-4GlcNAc non-reducing end termini. O-glycans were restricted to the mucin-type core 1 Galbeta1-3GalNAc sequence.

Protein N-glycosylation in the baculovirus-insect cell system

One of the major advantages of the baculovirus-insect cell system is that it is a eukaryotic system that can provide posttranslational modifications, such as protein N-glycosylation. However, this is a vastly oversimplified view, which reflects a poor understanding of insect glycobiology. In general, insect protein glycosylation pathways are far simpler than the corresponding pathways of higher eukaryotes. Paradoxically, it is increasingly clear that various insects encode and can express more elaborate protein glycosylation functions in restricted fashion. Thus, the information gathered in a wide variety of studies on insect protein N-glycosylation during the past 25 years has provided what now appears to be a reasonably detailed, comprehensive, and accurate understanding of the protein N-glycosylation capabilities of the baculovirus-insect cell system. In this chapter, we discuss the models of insect protein N-glycosylation that have emerged from these studies and how this impacts the use of baculovirus-insect cell systems for recombinant glycoprotein production. We also discuss the use of these models as baselines for metabolic engineering efforts leading to the development of new baculovirus-insect cell systems with humanized protein N-glycosylation pathways, which can be used to produce more authentic recombinant N-glycoproteins for drug development and other biomedical applications.

Distinct contributions of β4GalNAcTA and β4GalNAcTB to Drosophila glycosphingolipid biosynthesis

Drosophila melanogaster has two beta4-N-acetylgalactosaminyltransferases, beta4GalNAcTA and beta4GalNAcTB, that are able to catalyse the formation of lacdiNAc (GalNAcbeta,4GlcNAc). LacdiNAc is found as a structural element of Drosophila glycosphingolipids (GSLs) suggesting that beta4GalNAcTs contribute to the generation of GSL structures in vivo. Mutations in Egghead and Brainaic, enzymes that generate the beta4GalNAcT trisaccharide acceptor structure GlcNAcbeta,3Manbeta,4GlcbetaCer, are lethal. In contrast, flies doubly mutant for the beta4GalNAcTs are viable and fertile. Here, we describe the structural analysis of the GSLs in beta4GalNAcT mutants and find that in double mutant flies no lacdiNAc structure is generated and the trisaccharide GlcNAcbeta,3Manbeta,4GlcbetaCer accumulates. We also find that phosphoethanolamine transfer to GlcNAc in the trisaccharide does not occur, demonstrating that this step is dependent on prior or simultaneous transfer of GalNAc. By comparing GSL structures generated in the beta4GalNAcT single mutants we show that beta4GalNAcTB is the major enzyme for the overall GSL biosynthesis in adult flies. In beta4GalNAcTA mutants, composition of GSL structures is indistinguishable from wild-type animals. However, in beta4GalNAcTB mutants precursor structures are accumulating in different steps of GSL biosynthesis, without the complete loss of lacdiNAc, indicating that beta4GalNAcTA plays a minor role in generating GSL structures. Together our results demonstrate that both beta4GalNAcTs are able to generate lacdiNAc structures in Drosophila GSL, although with different contributions in vivo, and that the trisaccharide GlcNAcbeta,3Manbeta,4GlcbetaCer is sufficient to avoid the major phenotypic consequences associated with the GSL biosynthetic defects in Brainiac or Egghead.

Towards abolition of immunogenic structures in insect cells: characterization of a honey-bee (Apis mellifera) multi-gene family reveals both an allergy-related core alpha1,3-fucosyltransferase and the first insect Lewis-histo-blood-group-related antigen-synthesizing enzyme

Glycoproteins from honey-bee (Apis mellifera), such as phospholipase A2 and hyaluronidase, are well-known major bee-venom allergens. They carry N-linked oligosaccharide structures with two types of alpha1,3-fucosylation: the modification by alpha1,3-fucose of the innermost core GlcNAc, which constitutes an epitope recognized by IgE from some bee-venom-allergic patients, and an antennal Lewis-like GalNAcbeta1,4(Fucalpha1,3)GlcNAc moiety. We now report the cloning and expression of two cDNAs encoding the relevant active alpha1,3-FucTs (alpha1,3-fucosyltransferases). The first sequence, closest to that of fruitfly (Drosophila melanogaster) FucTA, was found to be a core alpha1,3-FucT (EC 2.4.1.214), as judged by several enzyme and biochemical assays. The second cDNA encoded an enzyme, most related to Drosophila FucTC, that was shown to be capable of generating the Le(x) [Galbeta1-4(Fucalpha1-3)GlcNAc] epitope in vitro and is the first Lewis-type alpha1,3-FucT (EC 2.4.1.152) to be described in insects. The transcription levels of these two genes in various tissues were examined: FucTA was found to be predominantly expressed in the brain tissue and venom glands, whereas FucTC transcripts were detected at highest levels in venom and hypopharyngeal glands. Very low expression of a third homologue of unknown function, FucTB, was also observed in various tissues. The characterization of these honey-bee gene products not only accounts for the observed alpha1,3-fucosylation of bee-venom glycoproteins, but is expected to aid the identification and subsequent down-regulation of the FucTs in insect cell lines of biotechnological importance.

Functional roles for beta1,4-N-acetlygalactosaminyltransferase-A in Drosophila larval neurons and muscles

Adult Drosophila mutant for the glycosyltransferase beta1,4-N-acetlygalactosaminyltransferase-A (beta4GalNAcTA) display an abnormal locomotion phenotype, indicating a role for this enzyme, and the glycan structures that it generates, in the neuromuscular system. To investigate the functional role of this enzyme in more detail, we turned to the accessible larval neuromuscular system and report here that larvae mutant for beta4GalNAcTA display distinct nerve and muscle phenotypes. Mutant larvae exhibit abnormal backward crawling, reductions in nerve terminal bouton number, decreased spontaneous transmitter-release frequency, and short, wide muscles. This muscle shape change appears to result from hypercontraction since the individual sarcomeres are shorter in mutant muscles. Analysis of muscle calcium signals showed altered calcium handling in the mutant, suggesting a mechanism by which hypercontraction could occur. All of these phenotypes can be rescued by a transgene carrying the beta4GalNAcTA genomic region. Tissue-specific expression, using the Gal4-UAS system, reveals that neural expression rescues the mutant crawling phenotype, while muscle expression rescues the muscle defect. Tissue-specific expression did not appear to rescue the decrease in neuromuscular junction bouton number, suggesting that this defect arises from cooperation between nerve and muscle. Altogether, these results suggest that beta4GalNAcTA has at least three distinct functional roles.

Dynamic developmental elaboration of N-linked glycan complexity in the Drosophila melanogaster embryo

The structural diversity of glycoprotein N-linked oligosaccharides is determined by the expression and regulation of glycosyltransferase activities and by the availability of the appropriate acceptor/donor substrates. Cells in different tissues and in different developmental stages utilize these control points to manifest unique glycan expression patterns in response to their surroundings. The activity of a Toll-like receptor, called Tollo/Toll-8, induces a pattern of incompletely defined, but neural specific, glycan expression in the Drosophila embryo. Understanding the full extent of the changes in glycan expression that result from altered Tollo/Toll-8 signaling requires characterization of the complete N-linked glycan profile of both wild-type and mutant embryos. N-Linked glycans harvested from wild-type or mutant embryos were subjected to direct structural analysis by analytic and preparative high pressure liquid chromatography, by multidimensional mass spectrometry, and by exoglycosidase digestion, revealing a predominance of high mannose and paucimannose glycans. Di-, mono-, and nonfucosylated forms of hybrid, complex biantennary, and triantennary glycans account for 12% of the total wild-type glycan profile. Two sialylated glycans bearing N-acetylneuraminic acid were detected, the first direct demonstration of this modification in Drosophila. Glycan profiles change during normal development consistent with increasing alpha-mannosidase II and core fucosyl-transferase enzyme activities, and with decreasing activity of the Fused lobes processing hexosaminidase. In tollo/toll-8 mutants, a dramatic, expected loss of difucosylated glycans is accompanied by unexpected decreases in monofucosylated and nonfucosylated hybrid glycans and increases in some nonfucosylated paucimannose and biantennary glycans. Therefore, tollo/toll-8 signaling influences flux through several processing steps that affect the maturation of N-linked glycans.

Role of a single amino acid in the evolution of glycans of invertebrates and vertebrates

Structures of glycoconjugate N-glycans and glycolipids of invertebrates show significant differences from those of vertebrates. These differences are due largely to the vertebrate beta1,4-galactosyltransferase-1 (beta4Gal-T1), which is found as a beta1,4-N-acetylgalactosaminyltransferase (beta4GalNAc-T1) in invertebrates. Mutation of Tyr285 to Ile or Leu in human beta4Gal-T1 converts the enzyme into an equally efficient beta4GalNAc-T1. A comparison of all the human beta4Gal-T1 ortholog enzymes shows that this Tyr285 residue in human beta4Gal-T1 is conserved either as Tyr or Phe in all vertebrate enzymes, while in all invertebrate enzymes it is conserved as an Ile or Leu. We find that mutation of the corresponding Ile residue to Tyr in Drosophila beta4GalNAc-T1 converts the enzyme to a beta4Gal-T1 by reducing its N-acetylgalactosaminyltransferase activity by nearly 1000-fold, while enhancing its galactosyltransferase activity by 80-fold. Furthermore, we find that, similar to the vertebrate/mammalian beta4Gal-T1 enzymes, the wild-type Drosophila beta4GalNAc-T1 enzyme binds to a mammary gland-specific protein, alpha-lactalbumin (alpha-LA). Thus, it would seem that, during the evolution of vertebrates from invertebrates over 500 million years ago, beta4Gal-T1 appeared as a result of the single amino acid substitution of Tyr or Phe for Leu or Ile in the invertebrate beta4GalNAc-T1. Subsequently, the pre-existing alpha-LA-binding site was utilized during mammalian evolution to synthesize lactose in the mammary gland during lactation.

Drosophila beta 1,4-N-acetylgalactosaminyltransferase-A synthesizes the LacdiNAc structures on several glycoproteins and glycosphingolipids

The GalNAcbeta1,4GlcNAc (LacdiNAc or LDN) structure is a more common structural feature in invertebrate glycoconjugates when compared with the Galbeta1,4GlcNAc structure. Recently, beta1,4-N-acetylgalactosaminyltransferase (beta4GalNAcT) was identified in some invertebrates including Drosophila. However, the LDN structure has not been reported in Drosophila, and the biological function of LDN remains to be determined. In this study, we examined acceptor substrate specificity of Drosophila beta4GalNAcTA by using some N- and O-glycans on glycoproteins and neutral glycosphingolipids (GSLs). GalNAc was efficiently transferred toward N-glycans, O-glycans, and the arthro-series GSLs. Moreover, we showed that dbeta4GalNAcTA contributed to the synthesis of the LDN structure in vivo. The dbeta4GalNAcTA mRNA was highly expressed in the developmental and adult neuronal tissues. Thus, these results suggest that dbeta4GalNAcTA acts on the terminal GlcNAc residue of some glycans for the synthesis of LDN, and the LDN structure may play a role in the physiological or neuronal development of Drosophila.

Protein N-glycosylation in the baculovirus-insect cell expression system and engineering of insect cells to produce "mammalianized" recombinant glycoproteins

Baculovirus expression vectors are frequently used to express glycoproteins, a subclass of proteins that includes many products with therapeutic value. The insect cells that serve as hosts for baculovirus vector infection are capable of transferring oligosaccharide side chains (glycans) to the same sites in recombinant proteins as those that are used for native protein N-glycosylation in mammalian cells. However, while mammalian cells produce compositionally more complex N-glycans containing terminal sialic acids, insect cells mostly produce simpler N-glycans with terminal mannose residues. This structural difference between insect and mammalian N-glycans compromises the in vivo bioactivity of glycoproteins and can potentially induce allergenic reactions in humans. These features obviously compromise the biomedical value of recombinant glycoproteins produced in the baculovirus expression vector system. Thus, much effort has been expended to characterize the potential and limits of N-glycosylation in insect cell systems. Discoveries from this research have led to the engineering of insect N-glycosylation pathways for assembly of mammalian-style glycans on baculovirus-expressed glycoproteins. This chapter summarizes our knowledge of insect N-glycosylation pathways and describes efforts to engineer baculovirus vectors and insect cell lines to overcome the limits of insect cell glycosylation. In addition, we consider other possible strategies for improving glycosylation in insect cells.

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.

Molecular cloning and functional characterization of beta-N-acetylglucosaminidase genes from Sf9 cells

Sf9, a cell line derived from the lepidopteran insect, Spodoptera frugiperda, is widely used as a host for recombinant glycoprotein expression and purification by baculovirus vectors. Previous studies have shown that this cell line has one or more beta-N-acetylglucosaminidase activities that may be involved in the degradation and/or processing of N-glycoprotein glycans. However, these enzymes and their functions remain poorly characterized. Therefore, the goal of this study was to isolate beta-N-acetylglucosaminidase genes from Sf9 cells, over-express the gene products, and characterize their enzymatic activities. A degenerate PCR approach yielded three Sf9 cDNAs, which appeared to encode two distinct beta-N-acetylglucosaminidases, according to bioinformatic analyses. Baculovirus-mediated expression of these two cDNA products induced membrane-associated beta-N-acetylglucosaminidase activities in Sf9 cells, which cleaved terminal N-acetylglucosamine residues from the alpha-3 and -6 branches of a biantennary N-glycan substrate with acidic pH optima and completely hydrolyzed chitotriose to its constituent N-acetylglucosamine monomers. GFP-tagged forms of both enzymes exhibited punctate cytoplasmic fluorescence, which did not overlap with either lysosomal or Golgi-specific dyes. Together, these results indicated that the two new Sf9 genes identified in this study encode broad-spectrum beta-N-acetylglucosaminidases that appear to have unusual intracellular distributions. Their relative lack of substrate specificity and acidic pH optima are consistent with a functional role for these enzymes in glycoprotein glycan and chitin degradation, but not with a role in N-glycoprotein glycan processing.

Comparing N-glycan processing in mammalian cell lines to native and engineered lepidopteran insect cell lines

In the past decades, a large number of studies in mammalian cells have revealed that processing of glycoproteins is compartmentalized into several subcellular organelles that process N-glycans to generate complex-type oligosaccharides with terminal N -acetlyneuraminic acid. Recent studies also suggested that processing of N-glycans in insect cells appear to follow a similar initial pathway but diverge at subsequent processing steps. N-glycans from insect cell lines are not usually processed to terminally sialylated complex-type structures but are instead modified to paucimannosidic or oligomannose structures. These differences in processing between insect cells and mammalian cells are due to insufficient expression of multiple processing enzymes including glycosyltransferases responsible for generating complex-type structures and metabolic enzymes involved in generating appropriate sugar nucleotides. Recent genomics studies suggest that insects themselves may include many of these complex transferases and metabolic enzymes at certain developmental stages but expression is lost or limited in most lines derived for cell culture. In addition, insect cells include an N -acetylglucosaminidase that removes a terminal N -acetylglucosamine from the N-glycan. The innermost N -acetylglucosamine residue attached to asparagine residue is also modified with alpha(1,3)-linked fucose, a potential allergenic epitope, in some insect cells. In spite of these limitations in N-glycosylation, insect cells have been widely used to express various recombinant proteins with the baculovirus expression vector system, taking advantage of their safety, ease of use, and high productivity. Recently, genetic engineering techniques have been applied successfully to insect cells in order to enable them to produce glycoproteins which include complex-type N-glycans. Modifications to insect N-glycan processing include the expression of missing glycosyltransferases and inclusion of the metabolic enzymes responsible for generating the essential donor sugar nucleotide, CMP- N -acetylneuraminic acid, required for sialylation. Inhibition of N -acetylglucosaminidase has also been applied to alter N-glycan processing in insect cells. This review summarizes current knowledge on N-glycan processing in lepidopteran insect cell lines, and recent progress in glycoengineering lepidopteran insect cells to produce glycoproteins containing complex N-glycans.

Biosynthesis of human-type N-glycans in heterologous systems

Insects, yeasts and plants generate widely different N-glycans, the structures of which differ significantly from those produced by mammals. The processing of the initial Glc2Man9GlcNAc2 oligosaccharide to Man8GlcNAc2 in the endoplasmic reticulum shows significant similarities among these species and with mammals, whereas very different processing events occur in the Golgi compartments. For example, yeasts can add 50 or even more Man residues to Man(8-9)GlcNAc2, whereas insect cells typically remove most or all Man residues to generate paucimannosidic Man(3-1)GlcNAc2N-glycans. Plant cells also remove Man residues to yield Man(4-5)GlcNAc2, with occasional complex GlcNAc or Gal modifications, but often add potentially allergenic beta(1,2)-linked Xyl and, together with insect cells, core alpha(1,3)-linked Fuc residues. However, genomic efforts, such as expression of exogenous glycosyltransferases, have revealed more complex processing capabilities in these hosts that are not usually observed in native cell lines. In addition, metabolic engineering efforts undertaken to modify insect, yeast and plant N-glycan processing pathways have yielded sialylated complex-type N-glycans in insect cells, and galactosylated N-glycans in yeasts and plants, indicating that cell lines can be engineered to produce mammalian-like glycoproteins of potential therapeutic value.

Functional analysis of Drosophila beta1,4-N-acetlygalactosaminyltransferases

Members of the mammalian beta1,4-galactosyltransferase family are among the best studied glycosyltransferases, but the requirements for all members of this family within an animal have not previously been determined. Here, we describe analysis of two Drosophila genes, beta4GalNAcTA (CG8536) and beta4GalNAcTB (CG14517), that are homologous to mammalian beta1,4-galactosyltransferases. Like their mammalian homologs, these glycosyltransferases use N-acetylglucosamine as an acceptor substrate. However, they transfer N-acetylgalactosamine rather than galactose. This activity, together with amino acid sequence similarity, places them among a group of recently identified invertebrate beta1,4-N-acetylgalactosaminyltransferases. To investigate the biological functions of these genes, null mutations were generated by imprecise excision of a transposable element (beta4GalNAcTA) or by gene-targeted homologous recombination (beta4GalNAcTB). Flies mutant for beta4GalNAcTA are viable and fertile but display behavioral phenotypes suggestive of essential roles for GalNAc-beta1,4-GlcNAc containing glycoconjugates in neuronal and/or muscular function. beta4GalNAcTB mutants are viable and display no evident morphological or behavioral phenotypes. Flies doubly mutant for both genes display only the behavioral phenotypes associated with mutation of beta4GalNAcTA. Thus Drosophila homologs of the mammalian beta4GalT family are essential for neuromuscular physiology or development but are not otherwise required for viability, fertility, or external morphology.

Structure and catalytic cycle of β-1,4-galactosyltransferase

Beta-1,4-galactosyltransferase-1, a housekeeping enzyme that functions in the synthesis of glycoconjugates, has two flexible loops, one short and one long. Upon binding a metal ion and UDP-galactose, the loops change from an open to a closed conformation, repositioning residues to lock the ligands in place. Residues at the N-terminal region of the long loop form the metal-binding site and those at the C-terminal region form a helix, which becomes part of the binding site for the oligosaccharide acceptor; the remaining residues cover the bound sugar-nucleotide. After binding of the oligosaccharide acceptor and transfer of the galactose moiety, the product disaccharide unit is ejected and the enzyme returns to the open conformation, repeating the catalytic cycle.