Isolation and Characterization of an Insect Cell Line Able to Perform Complex N-Linked Glycosylation on Recombinant Proteins

A database of crop pest cell lines

We have developed an online database describing the known cell lines from Coleoptera, Diptera, Hemiptera, Hymenoptera, and Lepidoptera that were developed from agricultural pests. Cell line information has been primarily obtained from previous compilations of insect cell lines. We conducted in-depth Internet literature searches and drew on Internet sources such as the Cellosaurus database (https://web.expasy.org/cellosaurus/), and inventories from cell line depositories. Here, we report on a new database of insect cell lines, which covers 719 cell lines from 86 species. We have not included cell lines developed from Drosophila because they are already known from published databases, such as https://dgrc.bio.indiana.edu/cells/Catalog. We provide the designation, tissue and species of origin, cell line developer, unique characteristics, its use in various applications, publications, and patents, and, when known, insect virus susceptibility. This information has been assembled and organized into a searchable database available at the link https://entomology.ca.uky.edu/aginsectcellsdatabase which will be updated on an ongoing basis.

Identification and Quantification of Anti-Gp.Mur Antibodies in Human Serum Using an Insect-Cell-Based System

Gp.Mur is a clinically relevant antigen of the MNS blood group system that is highly prevalent in several Asian populations. Its corresponding antibody, anti-Gp.Mur, has been implicated in hemolytic transfusion reactions and hemolytic disease of the fetus and newborn. Currently, identifying and confirming anti-Gp.Mur antibody presence in sera via agglutination of a panel of red blood cells (RBCs) is inefficient and difficult to quantify. Using a baculovirus expression system to express Gp.Mur antigen on insect cell surfaces, we have developed a quantitative cell-based system to confirm the presence of anti-Gp.Mur antibody in human serum. We obtained 10 serum samples preidentified as having anti-Gp.Mur antibody and another 4 samples containing noncorresponding antibodies from hospital patients. Insect cells displaying Gp.Mur antigen successfully adsorbed anti-Gp.Mur antibody in the sera and inhibited the RBC agglutination mediated by this antibody. By varying the concentration of Gp.Mur-displaying cells, we could grade levels of RBC agglutination by anti-Gp.Mur antibody. Densitometric analysis further enabled quantitative determinations of hemagglutination inhibition by Gp.Mur-displaying cells. We believe that this cell-based hemagglutination inhibition system greatly improves or supplements existing technology and is a convenient means for accurately identifying and quantifying anti-Gp.Mur antibody.

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.

'ZP domain' of human zona pellucida glycoprotein-1 binds to human spermatozoa and induces acrosomal exocytosis

BACKGROUND

The human egg coat, zona pellucida (ZP), is composed of four glycoproteins designated as zona pellucida glycoprotein-1 (ZP1), -2 (ZP2), -3 (ZP3) and -4 (ZP4) respectively. The zona proteins possess the archetypal 'ZP domain', a signature domain comprised of approximately 260 amino acid (aa) residues. In the present manuscript, attempts have been made to delineate the functional significance of the 'ZP domain' module of human ZP1, corresponding to 273-551 aa fragment of human ZP1.

METHODS

Baculovirus-expressed, nickel-nitrilotriacetic acid affinity chromatography purified 'ZP domain' of human ZP1 was employed to assess its capability to bind and subsequently induce acrosomal exocytosis in capacitated human spermatozoa using tetramethyl rhodamine isothiocyanate conjugated Pisum sativum Agglutinin in absence or presence of various pharmacological inhibitors. Binding characteristics of ZP1 'ZP domain' were assessed employing fluorescein isothiocyanate (FITC) labelled recombinant protein.

RESULTS

SDS-PAGE and immunoblot characterization of the purified recombinant protein (both from cell lysate as well as culture supernatant) revealed a doublet ranging from ~35-40 kDa. FITC- labelled 'ZP domain' of ZP1 binds primarily to the acrosomal cap of the capacitated human spermatozoa. A dose dependent increase in acrosomal exocytosis was observed when capacitated sperm were incubated with recombinant 'ZP domain' of human ZP1. The acrosome reaction mediated by recombinant protein was independent of Gi protein-coupled receptor pathway, required extra cellular calcium and involved both T- and L-type voltage operated calcium channels.

CONCLUSIONS

Results described in the present study suggest that the 'ZP domain' module of human ZP1 has functional activity and may have a role during fertilization in humans.

Engineering and characterization of a baculovirus-expressed mouse/human chimeric antibody against transferrin receptor

Transferrin receptor (TfR) has been explored as a target for antibody-based therapy of cancer. In the previous study, we reported a murine anti-TfR monoclonal antibody (mAb) 7579 had good anti-tumor activities in vitro. In an attempt to reduce its immunogenicity and enhance its ability to recruit immune effector mechanism in vivo, we herein developed its chimera in the baculovirus/insect cell expression system based on the mating-assisted genetically integrated cloning (MAGIC) strategy. The chimeric light and heavy chains, containing human IgG1 constant regions, were correctly processed and assembled in insect cells, and then secreted into the mediums as heterodimeric H(2)L(2) immunoglobulins. Furthermore, analyses of antigen-binding assay and competitive binding assay indicated that the chimeric antibody possessed specificity and affinity similar to that of its parental murine antibody. Results of the antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC) assay verified that the chimeric antibody could efficiently mediate ADCC and CDC against TfR-overexpressing tumor cells. These results suggested that this baculovirus-expressed chimeric anti-TfR IgG1 might have the potential to be used for cancer immunotherapy. Meanwhile, the MAGIC strategy, facilitating the rapid generation of chimeric mAbs, could be one of the efficient strategies for antibody engineering.

Comparison of the N-linked glycosylation of human beta1,3-N-acetylglucosaminyltransferase 2 expressed in insect cells and silkworm larvae

N-Glycosylation of human beta1,3N-acetylglucosaminyltransferase 2 (beta3GnT2) is essential for its biological function. beta3GnT2 fused to GFP(uv) (GFP(uv)-beta3GnT2) was produced by non-virus expression systems in stably transformed insect cells and silkworm larvae using a recombinant BmNPV bacmid, and purified for analysis of N-glycosylation. The N-glycan structure of beta3GnT2 was identified by glycoamidase A digestion, labeling with 2-aminopyridine (PA), and HPLC mapping. The paucimannosidic N-glycan structure (73.2%) was predominant in stably transformed Trichoplusia ni cells. In contrast, N-glycan with Gal (21.3%) and GlcNAc (16.2%) terminal residues linked to Manalpha(1,3) branch were detected on beta3GnT2 expressed in silkworm larvae. The presence of terminal Gal and bisecting GlcNAc residues such as Galbeta1, 4GlcNAcbeta1, 2Manalpha1,3(GlcNAcbeta1,4)(Manalpha1,6)Manbeta1, 4GlcNAc is not typical structure for lepidopteran insect N-glycosylation. Although allergenic alpha1,3-fucose residues have been found in T. ni cells, only alpha1,6-fucose residues were attached to the beta3GnT2 glycan in silkworm larvae. Therefore, silkworm larvae might be a useful host for producing human glycoproteins.

A new Trichoplusia ni cell line for membrane protein expression using a baculovirus expression vector system

A new cell line, MSU-TnT4 (TnT4), was established from Trichoplusia ni embryos for use with baculovirus expression vectors and evaluated for its potential for membrane protein production. To evaluate membrane protein synthesis, recombinant baculoviruses were constructed to express the human neurotensin receptor 1 as an enhanced green fluorescent protein (GFP) fusion. TnT4 cells had a doubling time of 21 h and expressed the membrane-GFP fusion protein at approximately twice the level as Sf21 cells from the p10 promoter, as evaluated by GFP intensity. Expression of secreted alkaline phosphatase (SEAP) was similar to that of Sf21 cells. Expression of membrane-GFP fusion proteins in recombinant baculoviruses provides a rapid method for evaluating the potential of new cell lines for the production of membrane proteins using a baculovirus expression vector system (BEVS).

Choice of cellular protein expression system

Recombinant protein expression has become a standard laboratory tool, and a wide variety of systems and techniques are now in use. Because there are so many systems to choose from, the investigator has to be careful to use the combination that will give the best results for the protein being studied. This overview unit discusses expression and production choices, including post-translational modifications (e.g., glycosylation, acylation, sulfation, and removal of N-terminal methionine), in vivo and in vitro folding, and influence of downstream elements on expression.

Development of serum-free media for lepidopteran insect cell lines

Lepidopteran insect cell culture technology has progressed to the point of becoming an essential part of one of the most successful eukaryotic expression systems and is increasingly used industrially on a large scale. Therefore, there is a constant need for convenient and low-cost culture media capable of supporting good insect cell growth and ensuring high yield of baculovirus as well as the strong expression of recombinant proteins. Vertebrate sera or invertebrate hemolymph were essential supplements in first-generation insect cell media. These supplements, however, are cumbersome and expensive for routine large-scale culture; thus, their use is now circumvented by substituting the essential growth factors present in these supplements with serum-free substances. Such non-serum supplements are typically of non-animal origin and include protein hydrolysates, lipid emulsions, and specialized substances (e.g., surfactants and shear damage protecting chemicals). These supplements need to complement the defined, synthetic basal medium to ensure that the fundamental nutritional needs of the cells are satisfied. Although there is a significant number of proprietary serum-free and low-protein or protein-free media on the market, the lack of information concerning their detailed composition is a drawback in their adoption for different applications, including their adaptation to the metabolic and kinetic analysis and monitoring of a given insect cell based bioprocess. Hence, there is wide appeal for formulating serum-free media based on a rational assessment of the metabolic requirements of the lepidopteran cells during both the growth and the production phases. Techniques such as statistical experimental design and genetic algorithms adapted to the cellular behavior and the bioreactor operation mode (batch, fed-batch, or perfusion) permit the formulation of versatile serum- and protein-free media. These techniques are illustrated with recent developments of serum-free media for the cultivation of commercially important Spodoptera frugiperda and Trichoplusia ni cell lines.

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.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update covering the period 1999-2000

This review describes the use of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry for the analysis of carbohydrates and glycoconjugates and continues coverage of the field from the previous review published in 1999 (D. J. Harvey, Matrix-assisted laser desorption/ionization mass spectrometry of carbohydrates, 1999, Mass Spectrom Rev, 18:349-451) for the period 1999-2000. As MALDI mass spectrometry is acquiring the status of a mature technique in this field, there has been a greater emphasis on applications rather than to method development as opposed to the previous review. The present review covers applications to plant-derived carbohydrates, N- and O-linked glycans from glycoproteins, glycated proteins, mucins, glycosaminoglycans, bacterial glycolipids, glycosphingolipids, glycoglycerolipids and related compounds, and glycosides. Applications of MALDI mass spectrometry to the study of enzymes acting on carbohydrates (glycosyltransferases and glycosidases) and to the synthesis of carbohydrates, are also covered.

N-glycan structures of human transferrin produced by Lymantria dispar (gypsy moth) cells using the LdMNPV expression system

N-glycan structures of recombinant human serum transferrin (hTf) expressed by Lymantria dispar (gypsy moth) 652Y cells were determined. The gene encoding hTf was incorporated into a Lymantria dispar nucleopolyhedrovirus (LdMNPV) under the control of the polyhedrin promoter. This virus was then used to infect Ld652Y cells, and the recombinant protein was harvested at 120 h postinfection. N-glycans were released from the purified recombinant human serum transferrin and derivatized with 2-aminopyridine; the glycan structures were analyzed by a two-dimensional HPLC and MALDI-TOF MS. Structures of 11 glycans (88.8% of total N-glycans) were elucidated. The glycan analysis revealed that the most abundant glycans were Man1-3(+/-Fucalpha6)GlcNAc2 (75.5%) and GlcNAcMan3(+/-Fucalpha6)GlcNAc2 (7.4%). There was only approximately 6% of high-mannose type glycans identified. Nearly half (49.8%) of the total N-glycans contained alpha(1,6)-fucosylation on the Asn-linked GlcNAc residue. However alpha(1,3)-fucosylation on the same GlcNAc, often found in N-glycans produced by other insects and insect cells, was not detected. Inclusion of fetal bovine serum in culture media had little effect on the N-glycan structures of the recombinant human serum transferrin obtained.

Improvement of glycosylation in insect cells with mammalian glycosyltransferases

The N-glycans of recombinant glycoproteins expressed in insect cells mainly contain high mannose or tri-mannose structures, which are truncated forms of the sialylated N-glycans found in mammalian cells. Because asialylated glycoproteins have a shorter half-life in blood circulation, we investigated if sialylated therapeutic glycoprotein can be produced from insect cells by enhancing the N-glycosylation machinery of the cells. We co-expressed in two insect cell lines, Sf9 and Ea4, the human alpha1-antitrypsin (halpha1AT) protein with a series of key glycosyltransferases, including GlcNAc transferase II (GnT2), beta1,4-galactosyltransferase (beta14GT), and alpha2,6-sialyltransferase (alpha26ST) by a single recombinant baculovirus. We demonstrated that the enhancement of N-glycosylation is cell type-dependent and is more efficient in Ea4 than Sf9 cells. Glycan analysis indicated that sialylated halpha1AT proteins were produced in Ea4 insect cells expressing the above-mentioned exogenous glycosyltransferases. Therefore, our expression strategy may simplify the production of humanized therapeutic glycoproteins by improving the N-glycosylation pathway in specific insect cells, with an ensemble of exogenous glycosyltransferases in a single recombinant baculovirus.

G protein-coupled receptor overexpression with the baculovirus-insect cell system: a tool for structural and functional studies

G protein-coupled receptors, whose topology shows seven transmembrane domains, form the largest known family of receptors involved in higher organism signal transduction. These receptors are generally of low natural abundance and overexpression is usually a prerequisite to their structural or functional characterisation. The baculovirus-insect cell system constitutes a versatile tool for the maximal production of receptors. This heterologous expression system also provides interesting alternatives for receptor functional studies in a well-controlled cellular context.

Beta-(1 --> 4)-galactosyltransferase activity in native and engineered insect cells measured with time-resolved europium fluorescence

To evaluate the ability of insect cells to produce complex-type N-glycans, beta-(1 --> 4)-galactosyltransferase (beta4GalT) activity in several insect cell lines was analyzed. For this purpose, we developed a simple and highly sensitive assay for beta-(1 --> 4)-galactosyltransferase (beta4GalT) activity, which is based on time-resolved fluorometry of europium. Bovine serum albumin (BSA) modified with GlcNAc (GlcNAc(44)-BSA) was used as the acceptor. GlcNAc(44)-BSA was coated on a 96-well microplate, and after incubation with the enzyme sample in the presence of UDP-Gal, Eu-labeled RCA(120) (Ricinus communis aggutin I), was added. RCA(120) binds to the Galbeta(1 --> 4)GlcNAc structure in the product, and the bound Eu-RCA(120) was measured by the fluorescence of europium. When bovine beta4Gal-T-I was used as a standard reference enzyme, a linear relationship between enzyme activity and fluorescent signal was obtained over the range of 0-1000 microUnits (IU). Using this system, we were able to measure a low but significant level of beta4GalT activity in Trichoplusia ni cells ('High Five'). In contrast, no endogenous beta4GalT activity was detected in a Spodoptera frugiperda (Sf-9) cell line. However, Sf-9 cells stably transfected with the bovine beta4GalT-I gene and 'High Five' cells infected with a baculovirus containing the same gene produced activity levels that were comparable to or greater than those found in Chinese hamster ovary cells. We also showed that the beta4GalT activity level observed in the baculovirus-infected T. ni cells under the control of immediate early promoter was highly dependent on the post-infection time, suggesting that galactosylation level may also be variable during the infection period.

Glycosylated and phosphorylated proteins--expression in yeast and oocytes of Xenopus: prospects and challenges--relevance to expression of thermostable proteins

Phosphorylation and glycosylation are important posttranslational events in the biosynthesis of proteins. The different degrees of phosphorylation and glycosylation of proteins have been an intriguing phenomenon. Advances in genetic engineering have made it possible to control the degree of glycosylation and phosphorylation of proteins. Structural biology of phosphorylated and glycosylated proteins has been advancing at a much slower pace due to difficulties in using high-resolution NMR studies in solution phase. Major difficulties have arisen from the inherent mobilities of phosphorylated and glycosylated side chains. This paper reviews molecular and structural biology of phosphorylated and glycosylated proteins expressed in eukaryotic expression systems which are especially suited for large-scale production of these proteins. In our laboratory, we have observed that eukaryotic expression systems are particularly suited for the expression of thermostable light-activated proteins, e.g., bacteriorhodopsins and plastocyanins.

Glycoproteins from insect cells: sialylated or not?

Our growing comprehension of the biological roles of glycan moieties has created a clear need for expression systems that can produce mammalian-type glycoproteins. In turn, this has intensified interest in understanding the protein glycosylation pathways of the heterologous hosts that are commonly used for recombinant glycoprotein expression. Among these, insect cells are the most widely used and, particularly in their role as hosts for baculovirus expression vectors, provide a powerful tool for biotechnology. Various studies of the glycosylation patterns of endogenous and recombinant glycoproteins produced by insect cells have revealed a large variety of O- and N-linked glycan structures and have established that the major processed O- and N-glycan species found on these glycoproteins are (Gal beta1,3)GalNAc-O-Ser/Thr and Man3(Fuc)GlcNAc2-N-Asn, respectively. However, the ability or inability of insect cells to synthesize and compartmentalize sialic acids and to produce sialylated glycans remains controversial. This is an important issue because terminal sialic acid residues play diverse biological roles in many glycoconjugates. While most work indicates that insect cell-derived glycoproteins are not sialylated, some well-controlled studies suggest that sialylation can occur. In evaluating this work, it is important to recognize that oligosaccharide structural determination is tedious work, due to the infinite diversity of this class of compounds. Furthermore, there is no universal method of glycan analysis; rather, various strategies and techniques can be used, which provide glycobiologists with relatively more or less precise and reliable results. Therefore, it is important to consider the methodology used to assess glycan structures when evaluating these studies. The purpose of this review is to survey the studies that have contributed to our current view of glycoprotein sialylation in insect cell systems, according to the methods used. Possible reasons for the disagreement on this topic in the literature, which include the diverse origins of biological material and experimental artifacts, will be discussed. In the final analysis, it appears that if insect cells have the genetic potential to perform sialylation of glycoproteins, this is a highly specialized function that probably occurs rarely. Thus, the production of sialylated recombinant glycoproteins in the baculovirus-insect cell system will require metabolic engineering efforts to extend the native protein glycosylation pathways of insect cells.

N-glycan patterns of human transferrin produced in Trichoplusia ni insect cells: effects of mammalian galactosyltransferase

The N-glycans of human serum transferrin produced in Trichopulsia ni cells were analyzed to examine N-linked oligosaccharide processing in insect cells. Metabolic radiolabeling of the intra- and extracellular protein fractions revealed the presence of multiple transferrin glycoforms with molecular weights lower than that observed for native human transferrin. Consequently, the N-glycan structures of transferrin in the culture medium were determined using three-dimensional high performance liquid chromatography. The attached oligosaccharides included high mannose, paucimannosidic, and hybrid structures with over 50% of these structures containing one fucose, alpha(1,6)-, or two fucoses, alpha(1,6)- and alpha(1,3)-, linked to the Asn-linked N-acetylglucosamine. Neither sialic acid nor galactose was detected on any of the N-glycans. However, when transferrin was coexpressed with beta(1,4)-galactosyltransferase three additional galactose-containing hybrid oligosaccharides were obtained. The galactose attachments were exclusive to the alpha(1, 3)-mannose branch and the structures varied by the presence of zero, one, or two attached fucose residues. Furthermore, the presence of the galactosyltransferase appeared to reduce the number of paucimannosidic structures, which suggests that galactose attachment inhibits the ability of hexosaminidase activity to remove the terminal N-acetylglucosamine. The ability to promote galactosylation and reduce paucimannosidic N-glycans suggests that the oligosaccharide processing pathway in insect cells may be manipulated to mimic more closely that of mammalian cells.

Modifying secretion and post-translational processing in insect cells

The baculovirus-insect cell system is a valuable tool for the expression of heterologous proteins. Due to limitations in the intracellular processing environment, however, heterologous secreted and membrane proteins are often insoluble, poorly processed, or contain 'non-human' modifications. Recent attempts to modify the insect cell secretory pathway by overexpressing processing factors have demonstrated the potential to overcome these limitations.

O-glycosylation potential of lepidopteran insect cell lines

The enzyme activities involved in O-glycosylation have been studied in three insect cell lines, Spodoptera frugiperda (Sf-9), Mamestra brassicae (Mb) and Trichoplusia ni (Tn) cultured in two different serum-free media. The structural features of O-glycoproteins in these insect cells were investigated using a panel of lectins and the glycosyltransferase activities involved in O-glycan biosynthesis of insect cells were measured (i.e., UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase, UDP-Gal:core-1 beta1, 3-galactosyltransferase, CMP-NeuAc:Galbeta1-3GalNAc alpha2, 3-sialyltransferase, and UDP-Gal:Galbeta1-3GalNAc alpha1, 4-galactosyltransferase activities). First, we show that O-glycosylation potential depends on cell type. All three lepidopteran cell lines express GalNAcalpha-O-Ser/Thr antigen, which is recognized by soy bean agglutinin and reflects high UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase activity. Capillary electrophoresis and mass spectrometry studies revealed the presence of at least two different UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases in these insect cells. Only some O-linked GalNAc residues are further processed by the addition of beta1,3-linked Gal residues to form T-antigen, as shown by the binding of peanut agglutinin. This reflects relative low levels of UDP-Gal:core-1 beta1,3-galactosyltransferase in insect cells, as compared to those observed in mammalian control cells. In addition, we detected strong binding of Bandeiraea simplicifolia lectin-I isolectin B4 to Mamestra brassicae endogenous glycoproteins, which suggests a high activity of a UDP-Gal:Galbeta1-3GalNAc alpha1, 4-galactosyltransferase. This explains the absence of PNA binding to Mamestra brassicae glycoproteins. Furthermore, our results substantiated that there is no sialyltransferase activity and, therefore, no terminal sialic acid production by these cell lines. Finally, we found that the culture medium influences the O-glycosylation potential of each cell line.

Protein glycosylation: implications for in vivo functions and therapeutic applications

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

Efficient human IFN-gamma expression in the mammary gland of transgenic mice

Two hybrid genes (BLG-HuIFN-gamma2 and BLG-HuIFN-gamma3) were constructed on the basis of sheep beta-lactoglobulin (BLG) and human interferon-gamma (HuIFN-gamma) gene sequences. They were used to direct HuIFN-gamma synthesis in the mammary gland of transgenic mice. HuIFN-gamma was efficiently produced in the mammary gland of transgenic mice. BLG-HuIFN-gamma2 transgenic females expressed HuIFN-gamma in the milk at concentrations up to 570 mg/ml, and BLG-HuIFN-gamma3 transgenic females expressed up to 350 mg/ml. All females carrying the BLG-HuIFN-gamma3 gene expressed HuIFN-gamma in their milk. No significant changes were observed in the HuIFN-gamma expression level during the lactation period. Using RT-PCR analysis, ectopic expression for both hybrid genes was found in transgenic mice. Despite ectopic expression of HuIFN-gamma in transgenic mice, their development and pregnancy were normal. The heritability of the HuIFN-gamma expression level in milk was demonstrated up to the F2 generation. This work demonstrates that hybrid genes have the potential to develop in transgenic domestic animals producing HuIFN-gamma in milk.

Engineering N-glycosylation pathways in the baculovirus-insect cell system

The inability to produce eukaryotic glycoproteins with complex N-linked glycans is a major limitation of the baculovirus-insect cell expression system. Recent studies have demonstrated that metabolic engineering can be used to extend the glycoprotein processing capabilities of lepidopteran insect cells. This approach is being used to develop new baculovirus-insect cell expression systems that can produce more authentic recombinant glycoproteins and obtain new information on insect N-glycosylation pathways.

Expression of heterologous proteins in stable insect cell culture

Stable transformed insect cell lines have been used for producing many highly processed heterologous proteins. Current research has focused on development of new expression and selection systems, and enhancement of vector stability. Defining the variation of modification and processing capabilities between cell lines will further enhance complex protein production from insect cells.

N-Linked glycosylation of a baculovirus-expressed recombinant glycoprotein in insect larvae and tissue culture cells

The potential of insect cell cultures and larvae infected with recombinant baculoviruses to produce authentic recombinant glycoproteins cloned from mammalian sources was investigated. A comparison was made of the N-linked glycans attached to secreted alkaline phosphatase (SEAP) produced in four species of insect larvae and their derived cell lines plus one additional insect cell line and larvae of one additional species. These data survey N-linked oligosaccharides produced in four families and six genera of the order Lepidoptera. Recombinant SEAP expressed by recombinant isolates of Autographa californica and Bombyx mori nucleopolyhedroviruses was purified from cell culture medium, larval hemolymph or larval homogenates by phosphate affinity chromatography. The N-linked oligosaccharides were released with PNGase-F, labeled with 8-aminonaphthalene-1-3-6-trisulfonic acid, fractionated by polyacrylamide gel electrophoresis, and analyzed by fluorescence imaging. The oligosaccharide structures were confirmed with exoglycosidase digestions. Recombinant SEAP produced in cell lines of Lymantria dispar (IPLB-LdEIta), Heliothis virescens (IPLB-HvT1), and Bombyx mori (BmN) and larvae of Spodoptera frugiperda, Trichoplusia ni , H.virescens , B.mori , and Danaus plexippus contained oligosaccharides that were structurally identical to the 10 oligosaccharides attached to SEAP produced in T.ni cell lines. The oligosaccharide structures were all mannose-terminated. Structures containing two or three mannose residues, with and without core fucosylation, constituted more than 75% of the oligosaccharides from the cell culture and larval samples.

Heterologous expression of G-protein-coupled receptors: human opioid receptors under scrutiny

G-protein-coupled receptors whose topology shows seven transmembrane domains form the largest known family of receptors involved in higher organism signal transduction. Despite increasing knowledge on the functioning mechanisms of these receptors, almost no structural data are available but only a few models. Structural studies using a wide range of physical and biochemical techniques may require fairly large (up to several milligrams) amounts of purified protein. Since such quantities are not naturally available, overexpression is prerequisite. Heterologous expression systems are then assayed for maximal production of a protein facsimile. Heterologous systems may also provide interesting alternatives for receptor functional studies in a different cellular context. Opioid receptors will be used as an example to discuss aspects related to the choice and suitability of several different expression systems for the intended analysis of G-protein-coupled receptor properties. General implications will be outlined.

The glycosylation heterogeneity of recombinant human IFN-gamma

The cloning of the cDNA for human interferon-gamma (IFN-gamma) has resulted in its expression in Escherichia coli, baculovirus-infected insect cells, Chinese hamster ovary (CHO) cells, and the mammary gland of transgenic mice. Large quantities of highly purified recombinant IFN-gamma have been generated, aided by the use of highly specific neutralizing monoclonal antibodies, with a view to its production as a human therapeutic protein. The primary source of structural heterogeneity for IFN-gamma during its production in mammalian expression systems is glycosylation, which can profoundly affect the three-dimensional structure of a glycoprotein and its biological function. A number of analytical approaches have been developed recently to allow a detailed analysis of the carbohydrate structures associated with IFN-gamma, the principal advances being in the areas of capillary electrophoresis and mass spectrometry. The implementation of these high-resolution analytical tools to determine the glycosylation profile of IFN-gamma makes it one of the best characterized recombinant glycoproteins. Recombinant human IFN-gamma acts as a model secretory glycoprotein, typifying the intrinsic glycosylation processing events associated with production of a potential therapeutic glycoprotein.

Baculoviruses as expression vectors

Recent advances in baculovirus expression vector technology include improvements to methods for the selection of recombinant viruses and further developments in virion display vectors. It is now also possible to modify the host cell glycosylation pathway to alter the structure of glycans added to the recombinant polypeptide. Baculovirus vectors also continue to be modified to facilitate gene expression in mammalian cells.

Baculovirus vectors for expression in insect cells

Recombinant baculoviruses now represent a mature technology in which vector development, particularly for the control of expression level, has reached a plateau. However, other aspects of expression, such as the production of multiple proteins, improved product purification or maximizing protein processing, remain areas for novel vector and host cell development. This year has seen these topics come to the fore in descriptions of new expression systems.