Engineering lepidopteran insect cells for sialoglycoprotein production by genetic transformation with mammalian beta 1,4-galactosyltransferase and alpha 2,6-sialyltransferase genes

Identification and validation of protective glycoproteins in Haemonchus contortus H11

Barbervax is the first and only available vaccine to protect animals against Haemonchus contortus - one of the most pathogenic parasites of small ruminants. This vaccine contains a kind of native antigen called H11, a glycoprotein complex derived from integral gut of this parasite. Native H11 has been shown to induce high levels (72-95%) of protection, but single or two recombinant molecules of H11 are consistently unsuccessful. An increasing number of aminopeptidases related to H11 have been characterized in the past three decades, but little is known about which ones are the key contributors to protective immunity. Our recent work has revealed that the immunoprotective effect of H11 is primarily associated with its N-glycan moieties. To identify key immunoprotective glycoproteins derived from H11 antigen, we employed glycan-related protective IgG antibodies combined with LC-MS/MS analysis and identified five glycosylated H11 proteins: H11, H11-1, H11-2, H11-4, and H11-5. Subsequently, we utilized the baculovirus-insect cell expression system and successfully expressed four H11 recombinant proteins including rH11, rH11-1, rH11-2 and rH11-4, which demonstrated similar aminopeptidase activity and comparable high-mannose and di-fucosylated N-glycan structures to those found on native H11. Immunization of goats with a cocktail of four rH11s induced a 66.29% reduction (p > 0.05) in total worm burden and cumulative fecal egg counts. High level of anti-rH11s IgG which could inhibit H. contortus intestinal aminopeptidase activity and larval development. Collectively, our study identified glycoprotein antigens from H11 and assessed their protective efficacy of a recombinant cocktail expressed in insect cells. This work will provide valuable insights into further development of recombinant vaccines against parasitic nematodes.

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.

Bioengineering of Antibody Fragments: Challenges and Opportunities

Antibody fragments are used in the clinic as important therapeutic proteins for treatment of indications where better tissue penetration and less immunogenic molecules are needed. Several expression platforms have been employed for the production of these recombinant proteins, from which E. coli and CHO cell-based systems have emerged as the most promising hosts for higher expression. Because antibody fragments such as Fabs and scFvs are smaller than traditional antibody structures and do not require specific patterns of glycosylation decoration for therapeutic efficacy, it is possible to express them in systems with reduced post-translational modification capacity and high expression yield, for example, in plant and insect cell-based systems. In this review, we describe different bioengineering technologies along with their opportunities and difficulties to manufacture antibody fragments with consideration of stability, efficacy and safety for humans. There is still potential for a new production technology with a view of being simple, fast and cost-effective while maintaining the stability and efficacy of biotherapeutic fragments.

Advances in CRISPR-Cas9 for the Baculovirus Vector System: A Systematic Review

The baculovirus expression vector systems (BEVS) have been widely used for the recombinant production of proteins in insect cells and with high insert capacity. However, baculovirus does not replicate in mammalian cells; thus, the BacMam system, a heterogenous expression system that can infect certain mammalian cells, was developed. Since then, the BacMam system has enabled transgene expression via mammalian-specific promoters in human cells, and later, the MultiBacMam system enabled multi-protein expression in mammalian cells. In this review, we will cover the continual development of the BEVS in combination with CRPISPR-Cas technologies to drive genome-editing in mammalian cells. Additionally, we highlight the use of CRISPR-Cas in glycoengineering to potentially produce a new class of glycoprotein medicines in insect cells. Moreover, we anticipate CRISPR-Cas9 to play a crucial role in the development of protein expression systems, gene therapy, and advancing genome engineering applications in the future.

A New Bacmid for Customized Protein Glycosylation Pathway Engineering in the Baculovirus-Insect Cell System

One attractive feature of the baculovirus-insect cell system (BICS) is the baculoviral genome has a large capacity for genetic cargo. This enables construction of viral vectors designed to accept multigene insertions, which has facilitated efforts to produce recombinant multisubunit protein complexes. However, the large genetic capacity of baculovirus vectors has not yet been exploited for multistep pathway engineering. Therefore, we created PolyBac, which is a novel baculovirus shuttle vector, or bacmid, that can be used for this purpose. PolyBac was designed to accept multiple transgene insertions by three different mechanisms at three different sites within the baculovirus genome. After constructing and characterizing PolyBac, we used it to isolate nine derivatives encoding various combinations of up to eight different protein N-glycosylation pathway functions, or glycogenes. We then used these derivatives, which were designed to progressively extend the endogenous insect cell pathway, to assess PolyBac's utility for protein glycosylation pathway engineering. This assessment was enabled by engineering each derivative to produce a recombinant influenza hemagglutinin (rH5), which was used to probe the impact of each glycoengineered PolyBac derivative on the endogenous insect cell pathway. Genetic analyses of these derivatives confirmed PolyBac can accept large DNA insertions. Biochemical analyses of the rH5 products showed each had distinct N-glycosylation profiles. Finally, the major N-glycan on each rH5 product was the predicted end product of the engineered N-glycosylation pathways encoded by each PolyBac derivative. These results generally indicate that PolyBac has utility for multistep metabolic pathway engineering and directly demonstrate that this new bacmid can be used for customized protein glycosylation pathway engineering in the BICS.

Baculoviruses in Gene Therapy and Personalized Medicine

This review will outline the role of baculoviruses in gene therapy and future potential in personalized medicine. Baculoviruses are a safe, non-toxic, non-integrative vector with a large cloning capacity. Baculoviruses are also a highly adaptable, low-cost vector with a broad tissue and host tropism due to their ability to infect both quiescent and proliferating cells. Moreover, they only replicate in insect cells, not mammalian cells, improving their biosafety. The beneficial properties of baculoviruses make it an attractive option for gene delivery. The use of baculoviruses in gene therapy has advanced significantly, contributing to vaccine production, anti-cancer therapies and regenerative medicine. Currently, baculoviruses are primarily used for recombinant protein production and vaccines.  This review will also discuss methods to optimize baculoviruses protein production and mammalian cell entry, limitations and potential for gene therapy and personalized medicine. Limitations such as transient gene expression, complement activation and virus fragility are discussed in details as they can be overcome through further genetic modifications and other methods. This review concludes that baculoviruses are an excllent candidate for gene therapy, personalized medicine and other biotherapeutic applications.

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.

The Coming Age of Insect Cells for Manufacturing and Development of Protein Therapeutics

Protein therapeutics is a rapidly growing segment of the pharmaceutical market. Currently, the majority of protein therapeutics are manufactured in mammalian cells for their ability to generate safe and efficacious human-like glycoproteins. The high cost of using mammalian cells for manufacturing has motivated a constant search for alternative host platforms. Insect cells have begun to emerge as a promising candidate, largely due to the development of the baculovirus expression vector system. While there are continuing efforts to improve insect-baculovirus expression for producing protein therapeutics, key limitations including cell lysis and the lack of homogeneous humanized glycosylation still remain. The field has started to see a movement toward virus-less gene expression approaches, notably the use of clustered regularly interspaced short palindromic repeats to address these shortcomings. This review highlights recent technological advances that are realizing the transformative potential of insect cells for the manufacturing and development of protein therapeutics.

Transforming Lepidopteran Insect Cells for Continuous Recombinant Protein Expression

The baculovirus expression vector system (BEVS) is widely used to produce large quantities of recombinant proteins. However, the yields of extracellular and membrane-bound proteins obtained with this system are often very low, possibly due to the adverse effects of baculovirus infection on the host insect cell secretory pathway. An alternative approach to producing poorly expressed proteins is to transform lepidopteran insect cells with the gene of interest under the control of promoters that are constitutively active in uninfected cells, thereby making cell lines that continuously express recombinant protein. This chapter provides an overview of the methods and considerations for making stably transformed lepidopteran insect cells. Techniques for the insertion of genes into continuous expression vectors, transfection of cells, and the selection and isolation of stably transformed Sf-9 clones by either colony formation or end-point dilution are described in detail.

Transcriptome analyses of insect cells to facilitate baculovirus-insect expression

The High Five cell line (BTI-TN-5B1-4) isolated from the cabbage looper, Trichoplusia ni is an insect cell line widely used for baculovirus-mediated recombinant protein expression. Despite its widespread application in industry and academic laboratories, the genomic background of this cell line remains unclear. Here we sequenced the transcriptome of High Five cells and assembled 25,234 transcripts. Codon usage analysis showed that High Five cells have a robust codon usage capacity and therefore suit for expressing proteins of both eukaryotic- and prokaryotic-origin. Genes involved in glycosylation were profiled in our study, providing guidance for engineering glycosylated proteins in the insect cells. We also predicted signal peptides for transcripts with high expression abundance in both High Five and Sf21 cell lines, and these results have important implications for optimizing the expression level of some secretory and membrane proteins.

Transforming Lepidopteran Insect Cells for Improved Protein Processing and Expression

The lepidopteran insect cells used with the baculovirus expression vector system (BEVS) are capable of synthesizing and accurately processing foreign proteins. However, proteins expressed in baculovirus-infected cells often fail to be completely processed, or are not processed in a manner that meets a researcher's needs. This chapter discusses a metabolic engineering approach that addresses this problem. Basically, this approach involves the addition of new or enhancement of existing protein processing functions in established lepidopteran insect cell lines. In addition to improvements in protein processing, this approach has also been used to improve protein expression levels obtained with the BEVS. Methods for engineering cell lines and assessing their properties as improved hosts for the BEVS are detailed. Examples of lepidopteran insect cell lines engineered for improved protein N-glycosylation, folding/trafficking, and expression are described in detail.

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.

Stable isotope labeling of glycoprotein expressed in silkworms using immunoglobulin G as a test molecule

Silkworms serve as promising bioreactors for the production of recombinant proteins, including glycoproteins and membrane proteins, for structural and functional protein analyses. However, lack of methodology for stable isotope labeling has been a major deterrent to using this expression system for nuclear magnetic resonance (NMR) structural biology. Here we developed a metabolic isotope labeling technique using commercially available silkworm larvae. The fifth instar larvae were infected with baculoviruses for co-expression of recombinant human immunoglobulin G (IgG) as a test molecule, with calnexin as a chaperone. They were subsequently reared on an artificial diet containing (15)N-labeled yeast crude protein extract. We harvested 0.1 mg of IgG from larva with a (15)N-enrichment ratio of approximately 80%. This allowed us to compare NMR spectral data of the Fc fragment cleaved from the silkworm-produced IgG with those of an authentic Fc glycoprotein derived from mammalian cells. Therefore, we successfully demonstrated that our method enables production of isotopically labeled glycoproteins for NMR studies.

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.

Engineering β1,4-galactosyltransferase I to reduce secretion and enhance N-glycan elongation in insect cells

β1,4-galactosyltransferase I (B4GALT1) is a Golgi-resident enzyme that elongates glycoprotein glycans, but a subpopulation of this enzyme is secreted following proteolytic cleavage in its stem domain. We hypothesized that engineering B4GALT1 to block cleavage and secretion would enhance its retention and, therefore, its function. To test this hypothesis, we replaced the cytoplasmic/transmembrane/stem (CTS) domains of B4GALT1 with those from human α1,3-fucosyltransferase 7 (FUT7), which is not cleaved and secreted. Expression of FUT7-CTS-B4GALT1 in insect cells produced lower levels of secreted and higher levels of intracellular B4GALT1 activity than the native enzyme. We also noted that the B4GALT1 used in our study had a leucine at position 282, whereas all other animal B4GALT1 sequences have an aromatic amino acid at this position. Thus, we examined the combined impact of changing the CTS domains and the amino acid at position 282 on intracellular B4GALT1 activity levels and N-glycan processing in insect cells. The results demonstrated a correlation between the levels of intracellular B4GALT1 activity and terminally galactosylated N-glycans, N-glycan branching, the appearance of hybrid structures, and reduced core fucosylation. Thus, engineering B4GALT1 to reduce its cleavage and secretion is an approach that can be used to enhance N-glycan elongation in insect cells.

Protein production using the baculovirus-insect cell expression system

The baculovirus-insect cell expression system is widely used in producing recombinant proteins. This review is focused on the use of this expression system in developing bioprocesses for producing proteins of interest. The issues addressed include: the baculovirus biology and genetic manipulation to improve protein expression and quality; the suppression of proteolysis associated with the viral enzymes; the engineering of the insect cell lines for improved capability in glycosylation and folding of the expressed proteins; the impact of baculovirus on the host cell and its implications for protein production; the effects of the growth medium on metabolism of the host cell; the bioreactors and the associated operational aspects; and downstream processing of the product. All these factors strongly affect the production of recombinant proteins. The current state of knowledge is reviewed.

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

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

A new insect cell glycoengineering approach provides baculovirus-inducible glycogene expression and increases human-type glycosylation efficiency

Insect cells are often glycoengineered using DNA constructs encoding foreign glyocoenzymes under the transcriptional control of the baculovirus immediate early promoter, ie1. However, we recently found that the delayed early baculovirus promoter, 39K, provides inducible and higher levels of transgene expression than ie1 after baculovirus infection (Lin and Jarvis, 2013). Thus, the purpose of this study was to assess the utility of the 39K promoter for insect cell glycoengineering. We produced two polyclonal transgenic insect cell populations in parallel using DNA constructs encoding foreign glycoenzymes under either ie1 (Sfie1SWT) or 39K (Sf39KSWT) promoter control. The surface of Sfie1SWT cells was constitutively sialylated, whereas the Sf39KSWT cell surface was only strongly sialylated after baculovirus infection, indicating Sf39KSWT cells were inducibly-glycoengineered. All nine glycogene-related transcript levels were induced by baculovirus infection of Sf39KSWT cells and most reached higher levels in Sf39KSWT than in Sfie1SWT cells at early times after infection. Similarly, galactosyltransferase activity, sialyltransferase activity, and sialic acid levels were induced and reached higher levels in baculovirus-infected Sf39KSWT cells. Finally, two different recombinant glycoproteins produced by baculovirus-infected Sf39KSWT cells had lower proportions of paucimannose-type and higher proportions of sialylated, complex-type N-glycans than those produced by baculovirus-infected Sfie1SWT cells. Thus, the 39K promoter provides baculovirus-inducible expression of foreign glycogenes, higher glycoenzyme activity levels, and higher human-type N-glycan processing efficiencies than the ie1 promoter, indicating that this delayed early baculovirus promoter has great utility for insect cell glycoengineering.

A novel baculovirus vector for the production of nonfucosylated recombinant glycoproteins in insect cells

Glycosylation is an important attribute of baculovirus-insect cell expression systems, but some insect cell lines produce core α1,3-fucosylated N-glycans, which are highly immunogenic and render recombinant glycoproteins unsuitable for human use. To address this problem, we exploited a bacterial enzyme, guanosine-5'-diphospho (GDP)-4-dehydro-6-deoxy-d-mannose reductase (Rmd), which consumes the GDP-l-fucose precursor. We expected this enzyme to block glycoprotein fucosylation by blocking the production of GDP-l-fucose, the donor substrate required for this process. Initially, we engineered two different insect cell lines to constitutively express Rmd and isolated subclones with fucosylation-negative phenotypes. However, we found the fucosylation-negative phenotypes induced by Rmd expression were unstable, indicating that this host cell engineering approach is ineffective in insect systems. Thus, we constructed a baculovirus vector designed to express Rmd immediately after infection and facilitate the insertion of genes encoding any glycoprotein of interest for expression later after infection. We used this vector to produce a daughter encoding rituximab and found, in contrast to an Rmd-negative control, that insect cells infected with this virus produced a nonfucosylated form of this therapeutic antibody. These results indicate that our Rmd(+) baculoviral vector can be used to solve the immunogenic core α1,3-fucosylation problem associated with the baculovirus-insect cell system. In conjunction with existing glycoengineered insect cell lines, this vector extends the utility of the baculovirus-insect cell system to include therapeutic glycoprotein production. This new vector also extends the utility of the baculovirus-insect cell system to include the production of recombinant antibodies with enhanced effector functions, due to its ability to block core α1,6-fucosylation.

Utility of temporally distinct baculovirus promoters for constitutive and baculovirus-inducible transgene expression in transformed insect cells

Genetically transformed lepidopteran insect cell lines have biotechnological applications as constitutive recombinant protein production platforms and improved hosts for baculovirus-mediated recombinant protein production. Insect cell transformation is often accomplished with a DNA construct(s) encoding a foreign protein(s) under the transcriptional control of a baculovirus immediate early promoter, such as the ie1 promoter. However, the potential utility of increasingly stronger promoters from later baculovirus gene classes, such as delayed early (39K), late (p6.9), and very late (polh), has not been systematically assessed. Hence, we produced DNA constructs encoding secreted alkaline phosphatase (SEAP) under the transcriptional control of each of the four temporally distinct classes of baculovirus promoters, used them to transform insect cells, and compared the levels of SEAP RNA and protein production obtained before and after baculovirus infection. The ie1 construct was the only one that supported SEAP protein production by transformed insect cells prior to baculovirus infection, confirming that only immediate early promoters can be used to isolate transformed insect cells for constitutive recombinant protein production. However, baculovirus infection activated transgene expression by all four classes of baculovirus promoters. After infection, cells transformed with the very late (polh) and late (p6.9) promoter constructs produced the highest levels of SEAP RNA, but only low levels of SEAP protein. Conversely, cells transformed with the immediate early (ie1) and delayed early (39K) promoter constructs produced lower levels of RNA, but equal or higher levels of SEAP protein. Unexpectedly, the 39K promoter construct provided tightly regulated, baculovirus-inducible protein production at higher levels than the later promoter constructs. Thus, this study demonstrated the utility of the 39K promoter for insect cell engineering, particularly when one requires higher levels of effector protein production than obtained with ie1 and/or when constitutive transgene expression adversely impacts host cell fitness and/or genetic stability.

MultiBac complexomics

Recombinant production of multiprotein complexes is an emerging focus in academic and pharmaceutical research and is expected to play a key role in addressing complex biological questions in health and disease. Here we describe MultiBac, a state-of-the-art eukaryotic expression technology utilizing an engineered baculovirus to infect insect cells. The robust and flexible concept of MultiBac allows for simultaneous expression of multiple proteins in a single cell, which can be used to produce protein complexes and to recapitulate metabolic pathways. The MultiBac system has been set up as an open-access platform technology at the European Molecular Biology Laboratory (EMBL) in Grenoble, France. The performance of this platform and its access modalities to the scientific community are detailed in this article. The MultiBac system has been instrumental for unlocking the function of a number of essential multiprotein complexes and recent examples are discussed. This article presents a novel concept for the customized production of glycosylated protein targets using SweetBac, a modified MultiBac vector system. Finally, this article outlines how MultiBac may further develop in the future to serve applications in both academic and industrial research and development.

Impact of a human CMP-sialic acid transporter on recombinant glycoprotein sialylation in glycoengineered insect cells

Insect cells are widely used for recombinant glycoprotein production, but they cannot provide the glycosylation patterns required for some biotechnological applications. This problem has been addressed by genetically engineering insect cells to express mammalian genes encoding various glycoprotein glycan processing functions. However, for various reasons, the impact of a mammalian cytosine-5'-monophospho (CMP)-sialic acid transporter has not yet been examined. Thus, we transformed Spodoptera frugiperda (Sf9) cells with six mammalian genes to generate a new cell line, SfSWT-4, that can produce sialylated glycoproteins when cultured with the sialic acid precursor, N-acetylmannosamine. We then super-transformed SfSWT-4 with a human CMP-sialic acid transporter (hCSAT) gene to isolate a daughter cell line, SfSWT-6, which expressed the hCSAT gene in addition to the other mammalian glycogenes. SfSWT-6 cells had higher levels of cell surface sialylation and also supported higher levels of recombinant glycoprotein sialylation, particularly when cultured with low concentrations of N-acetylmannosamine. Thus, hCSAT expression has an impact on glycoprotein sialylation, can reduce the cost of recombinant glycoprotein production and therefore should be included in ongoing efforts to glycoengineer the baculovirus-insect cell system. The results of this study also contributed new insights into the endogenous mechanism and potential mechanisms of CMP-sialic acid accumulation in the Golgi apparatus of lepidopteran insect cells.

Innovative use of a bacterial enzyme involved in sialic acid degradation to initiate sialic acid biosynthesis in glycoengineered insect cells

The baculovirus/insect cell system is widely used for recombinant protein production, but it is suboptimal for recombinant glycoprotein production because it does not provide sialylation, which is an essential feature of many glycoprotein biologics. This problem has been addressed by metabolic engineering, which has extended endogenous insect cell N-glycosylation pathways and enabled glycoprotein sialylation by baculovirus/insect cell systems. However, further improvement is needed because even the most extensively engineered baculovirus/insect cell systems require media supplementation with N-acetylmannosamine, an expensive sialic acid precursor, for efficient recombinant glycoprotein sialylation. Our solution to this problem focused on E. coli N-acetylglucosamine-6-phosphate 2'-epimerase (GNPE), which normally functions in bacterial sialic acid degradation. Considering that insect cells have the product, but not the substrate for this enzyme, we hypothesized that GNPE might drive the reverse reaction in these cells, thereby initiating sialic acid biosynthesis in the absence of media supplementation. We tested this hypothesis by isolating transgenic insect cells expressing E. coli GNPE together with a suite of mammalian genes needed for N-glycoprotein sialylation. Various assays showed that these cells efficiently produced sialic acid, CMP-sialic acid, and sialylated recombinant N-glycoproteins even in growth media without N-acetylmannosamine. Thus, this study demonstrated that a eukaryotic recombinant protein production platform can be glycoengineered with a bacterial gene, that a bacterial enzyme which normally functions in sialic acid degradation can be used to initiate sialic acid biosynthesis, and that insect cells expressing this enzyme can produce sialylated N-glycoproteins without N-acetylmannosamine supplementation, which will reduce production costs in glycoengineered baculovirus/insect cell systems.

SweetBac: a new approach for the production of mammalianised glycoproteins in insect cells

Recombinant production of therapeutically active proteins has become a central focus of contemporary life science research. These proteins are often produced in mammalian cells, in order to obtain products with post-translational modifications similar to their natural counterparts. However, in cases where a fast and flexible system for recombinant production of proteins is needed, the use of mammalian cells is limited. The baculoviral insect cell system has proven to be a powerful alternative for the expression of a wide range of recombinant proteins in short time frames. The major drawback of baculoviral systems lies in the inability to perform mammalian-like glycosylation required for the production of therapeutic glycoproteins. In this study we integrated sequences encoding Caenorhabditis elegans N-acetylglucosaminyltransferase II and bovine β1,4-galactosyltransferase I into the backbone of a baculovirus genome. The thereby generated SweetBac virus was subsequently used for the production of the human HIV anti-gp41 antibody 3D6 by integrating heavy and light chain open reading frames into the SweetBac genome. The parallel expression of target genes and glycosyltransferases reduced the yield of secreted antibody. However, the overall expression rate, especially in the recently established Tnao38 cell line, was comparable to that of transient expression in mammalian cells. In order to evaluate the ability of SweetBac to generate mammalian-like N-glycan structures on 3D6 antibody, we performed SDS-PAGE and tested for the presence of terminal galactose using Riccinus communis agglutinin I. The mammalianised variants of 3D6 showed highly specific binding to the lectin, indicating proper functionality. To confirm these results, PNGase A released N-glycans were analyzed by MALDI-TOF-MS and shown to contain structures with mainly one or two terminal galactose residues. Since the presence of specific N-glycans has an impact on antibodies ability to exert different effector functions, we tested the binding to human Fc gamma receptor I present on U937 cells.

A new glycoengineered insect cell line with an inducibly mammalianized protein N-glycosylation pathway

The inability to produce recombinant glycoproteins with authentic N-glycans is a limitation of many heterologous protein expression systems. In the baculovirus-insect cell system, this limitation has been addressed by glycoengineering insect cell lines with mammalian genes encoding protein N-glycosylation functions ("glycogenes") under the transcriptional control of constitutive promoters. However, a potential problem with this approach is that the metabolic load imposed by the expression of multiple transgenes could adversely impact the growth and/or stability of glycoengineered insect cell lines. Thus, we created a new transgenic insect cell line (SfSWT-5) with an inducibly mammalianized protein N-glycosylation pathway. Expression of all six glycogenes was induced when uninfected SfSWT-5 cells were cultured in growth medium containing doxycycline. Higher levels of expression and induction were observed when SfSWT-5 cells were cultured with doxycycline and infected with a baculovirus. Interestingly, there were no major differences in the short-term growth properties of SfSWT-5 cells cultured with or without doxycycline. Furthermore, there were no major differences in the phenotypic stability of these cells after continuous culture for over 300 passages with or without doxycycline. Baculovirus-infected Sf9 and SfSWT-5 cells produced about the same amounts of a model recombinant glycoprotein, but only the latter sialylated this product and sialylation was more pronounced when the cells were treated with doxycycline. In summary, this is the first report of a lower eukaryotic system with an inducibly mammalianized protein N-glycosylation pathway and the first to examine how the presumed metabolic load imposed by multiple transgene expression impacts insect cell growth and stability.

Current strategies for subunit and genetic viral veterinary vaccine development

Developing vaccines for livestock provides researchers with the opportunity to perform efficacy testing in the natural hosts. This enables the evaluation of different strategies, including definition of effective antigens or antigen combinations, and improvement in delivery systems for target antigens so that protective immune responses can be modulated or potentiated. An impressive amount of knowledge has been generated in recent years on vaccine strategies and consequently a wide variety of antigen delivery systems is now available for vaccine research. This paper reviews several antigen production and delivery strategies other than those based on the use of live viral vectors. Genetic and protein subunit vaccines as well as alternative production systems are considered in this review.

Baculovirus expression systems for production of recombinant proteins in insect and mammalian cells

Baculovirus vector systems are extensively used for the expression of foreign gene products in insect and mammalian cells. New advances increase the possibilities and applications of the baculovirus expression system, which makes it possible to express multiple genes simultaneously within a single infected insect cell and to obtain multimeric proteins functionally similar to their natural analogs. Recombinant viruses with expression cassettes active in mammalian cells are used to deliver and express genes in mammalian cells in vitro and in vivo. Further improvement of the baculovirus expression system and its adaptation to specific target cells can open up a wide variety of applications. The review considers recent achievements in the use of modified baculoviruses to express recombinant proteins in eukaryotic cells, advantages and drawbacks of the baculovirus expression system, and ways to optimize the expression of recombinant proteins in both insect and mammalian cell lines.

Semi-synthesis of glycoproteins from E. coli through native chemical ligation

Sufficient quantities of homogeneous samples of post-translationally modified proteins are often not readily available from biological sources to facilitate structure-function investigations. Native chemical ligation (NCL) is a convenient method for the production of biologically active proteins from smaller fragments. Such an approach allows protein modifications to be introduced in a controlled fashion into smaller peptide fragments which are amenable to total chemical synthesis. These fragments of defined sequence and structure can be elaborated to full-length proteins through NCL reactions with suitable components derived from bacterial origin. This report describes methods for the bacterial production of components for NCL and their use in typical reactions.

Re-visiting the endogenous capacity for recombinant glycoprotein sialylation by baculovirus-infected Tn-4h and DpN1 cells

It was previously reported that Tn-4h and DpN1 cells have the endogenous capacity to efficiently sialylate secreted alkaline phosphatase (SEAP) when infected with a baculovirus expression vector. In contrast, it has been found that lepidopteran insect cell lines that are more widely used as hosts for baculovirus vectors typically fail to sialylate SEAP and other recombinant glycoproteins. Thus, the N-glycan processing capabilities of Tn-4h and DpN1 cells are of potential interest to investigators using the baculovirus expression system for recombinant glycoprotein production. In this study, we experimentally re-assessed the ability of Tn-4h and DpN1 cells to sialylate SEAP with Sf9 and glyco-engineered Sf9 cells (SfSWT-1) as negative and positive controls, respectively. Our results showed that the SEAP purified from SfSWT-1 cells was strongly sialylated and initially indicated that the SEAP purified from Tn-4h cells was weakly sialylated. However, further analyses suggested that the SEAP produced by Tn-4h cells only appeared to be sialylated because it was contaminated with an electrophoretically indistinguishable sialoglycoprotein derived from fetal bovine serum. We subsequently expressed, purified, and analyzed a second recombinant glycoprotein (GST-SfManI) from all four cell lines and found that only the SfSWT-1 cells were able to detectably sialylate this product. Together, these results showed that neither Tn-4h nor DpN1 cells efficiently sialylated SEAP or GST-SfManI when infected by baculovirus expression vectors. Furthermore, they suggested that previous reports of efficient SEAP sialylation by Tn-4h and DpN1 cells probably reflect contamination with a sialylated, co-migrating glycoprotein, perhaps bovine fetuin, derived from the serum used in the insect cell growth medium.

Production of a recombinant mouse monoclonal antibody in transgenic silkworm cocoons

In the present study, we describe the production of transgenic silkworms expressing a recombinant mouse mAb in their cocoons. Two transgenic lines, L- and H-, were generated that carried cDNAs encoding the L- and H-chains of a mouse IgG mAb, respectively, under the control of the enhancer-linked sericin-1 promoter. Cocoon protein analysis indicated that the IgG L- or H-chain was secreted into the cocoons of each line. We also produced a transgenic line designated L/H, which carried both cDNAs, by crossing the L- and H-lines. This line efficiently produced the recombinant mAb as a fully assembled H(2)L(2) tetramer in its cocoons, with negligible L- or H-chain monomer and H-chain dimer production. Thus, the H(2)L(2) tetramer was synthesized in, and secreted from, the middle silk gland cells. Crossing of the L/H-line with a transgenic line expressing a baculovirus-derived trans-activator produced a 2.4-fold increase in mAb expression. The recombinant mAb was extracted from the cocoons with a buffer containing 3 m urea and purified by protein G affinity column chromatography. The antigen-binding affinity of the purified recombinant mAb was identical to that of the native mAb produced by a hybridoma. Analysis of the structure of the N-glycans attached to the recombinant mAb revealed that the mAb contained high mannose-, hybrid- and complex-type N-glycans. By contrast, insect-specific paucimannose-type glycans were not detected. Fucose residues alpha-1,3- and alpha-1,6-linked to the core N-acetylglucosamine residue, both of which are found in insect N-glycans, were not observed in the N-glycans of the mAb.

Identification of a Gene Cluster for the Biosynthesis of a Long, Galactose-Rich Exopolysaccharide in Lactobacillus rhamnosus GG and Functional Analysis of the Priming Glycosyltransferase

Cell surface polysaccharides have an established role as virulence factors in human bacterial pathogens. Less documented are the biosynthesis and biological functions of surface polysaccharides in beneficial bacteria. We identified a gene cluster that encodes the enzymes and regulatory and transporter proteins for the different steps in the biosynthesis of extracellular polysaccharides (EPS) of the well-documented probiotic strain Lactobacillus rhamnosus GG. Subsequent mutation of the welE gene, encoding the priming glycosyltransferase within this cluster, and comparative phenotypic analyses of wild-type versus mutant strains confirmed the specific function of this gene cluster in the biosynthesis of high-molecular-weight, galactose-rich heteropolymeric EPS molecules. The phenotypic analyses included monomer composition determination, estimation of the polymer length of the isolated EPS molecules, and single-molecule force spectroscopy of the surface polysaccharides. Further characterization of the welE mutant also showed that deprivation of these long, galactose-rich EPS molecules results in an increased adherence and biofilm formation capacity of L. rhamnosus GG, possibly because of less shielding of adhesins such as fimbria-like structures.

Microbial factories for recombinant pharmaceuticals

Most of the hosts used to produce the 151 recombinant pharmaceuticals so far approved for human use by the Food and Drug Administration (FDA) and/or by the European Medicines Agency (EMEA) are microbial cells, either bacteria or yeast. This fact indicates that despite the diverse bottlenecks and obstacles that microbial systems pose to the efficient production of functional mammalian proteins, namely lack or unconventional post-translational modifications, proteolytic instability, poor solubility and activation of cell stress responses, among others, they represent convenient and powerful tools for recombinant protein production. The entering into the market of a progressively increasing number of protein drugs produced in non-microbial systems has not impaired the development of products obtained in microbial cells, proving the robustness of the microbial set of cellular systems (so far Escherichia coli and Saccharomyces cerevisae) developed for protein drug production. We summarize here the nature, properties and applications of all those pharmaceuticals and the relevant features of the current and potential producing hosts, in a comparative way.

Baculovirus-insect cell expression systems

In the early 1980s, the first-published reports of baculovirus-mediated foreign gene expression stimulated great interest in the use of baculovirus-insect cell systems for recombinant protein production. Initially, this system appeared to be the first that would be able to provide the high production levels associated with bacterial systems and the eukaryotic protein processing capabilities associated with mammalian systems. Experience and an increased understanding of basic insect cell biology have shown that these early expectations were not completely realistic. Nevertheless, baculovirus-insect cell expression systems have the capacity to produce many recombinant proteins at high levels and they also provide significant eukaryotic protein processing capabilities. Furthermore, important technological advances over the past 20 years have improved upon the original methods developed for the isolation of baculovirus expression vectors, which were inefficient, required at least some specialized expertise and, therefore, induced some frustration among those who used the original baculovirus-insect cell expression system. Today, virtually any investigator with basic molecular biology training can relatively quickly and efficiently isolate a recombinant baculovirus vector and use it to produce their favorite protein in an insect cell culture. This chapter will begin with background information on the basic baculovirus-insect cell expression system and will then focus on recent developments that have greatly facilitated the ability of an average investigator to take advantage of its attributes.

Sialylation in protostomes: a perspective from Drosophila genetics and biochemistry

Numerous studies have revealed important functions for sialylation in both prokaryotes and higher animals. However, the genetic and biochemical potential for sialylation in Drosophila has only been confirmed recently. Recent studies suggest significant similarities between the sialylation pathways of vertebrates and insects and provide evidence for their common evolutionary origin. These new data support the hypothesis that sialylation in insects is a specialized and developmentally regulated process which likely plays a prominent role in the nervous system. Yet several key issues remain to be addressed in Drosophila, including the initiation of sialic acid de novo biosynthesis and understanding the structure and function of sialylated glycoconjugates. This review discusses our current knowledge of the Drosophila sialylation pathway, as compared to the pathway in bacteria and vertebrates. We arrive at the conclusion that Drosophila is emerging as a useful model organism that is poised to shed new light on the function of sialylation not only in protostomes, but also in a larger evolutionary context.

Initiation of protein O glycosylation by the polypeptide GalNAcT-1 in vascular biology and humoral immunity

Core-type protein O glycosylation is initiated by polypeptide N-acetylgalactosamine (GalNAc) transferase (ppGalNAcT) activity and produces the covalent linkage of serine and threonine residues of proteins. More than a dozen ppGalNAcTs operate within multicellular organisms, and they differ with respect to expression patterns and substrate selectivity. These distinctive features imply that each ppGalNAcT may differentially modulate regulatory processes in animal development, physiology, and perhaps disease. We found that ppGalNAcT-1 plays key roles in cell and glycoprotein selective functions that modulate the hematopoietic system. Loss of ppGalNAcT-1 activity in the mouse results in a bleeding disorder which tracks with reduced plasma levels of blood coagulation factors V, VII, VIII, IX, X, and XII. ppGalNAcT-1 further supports leukocyte trafficking and residency in normal homeostatic physiology as well as during inflammatory responses, in part by providing a scaffold for the synthesis of selectin ligands expressed by neutrophils and endothelial cells of peripheral lymph nodes. Animals lacking ppGalNAcT-1 are also markedly impaired in immunoglobulin G production, coincident with increased germinal center B-cell apoptosis and reduced levels of plasma B cells. These findings reveal that the initiation of protein O glycosylation by ppGalNAcT-1 provides a distinctive repertoire of advantageous functions that support vascular responses and humoral immunity.

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.

Expression of recombinant cyclooxygenase 1 in Drosophila melanogaster S2 cells transformed with human beta1,4-galactosyltransferase and Galbeta1,4-GlcNAc alpha2,6-sialyltransferase

We examined the expression of human cyclooxygenase-1 (COX-1) in Drososphila melanogaster S2 (S2) cells transformed with cDNAs encoding beta1,4-galactosyltransferase (GalT) and Galbeta1,4-GlcNAc alpha2,6-sialyltransferase (ST). Southern blot analysis indicated that multiple copies of the glycosyltransferases genes were integrated into the S2 cell genome. A lectin blot analysis also indicated that recombinant COX-1 from S2COX-1/GalT-ST cells contained the glycan residues of beta1,4-linked galactose and alpha2,6-linked sialic acid. The specific peroxidase activity of recombinant sialylated COX-1 from S2COX-1/GalT-ST cells was 41,250 U mg(-1), indicating an increase of approximately 22% compared with a non-sialylated control (33,850 U mg(-1)) from S2COX-1 cells.

Identification of bacterial DNA markers for the detection of human fecal pollution in water

We used genome fragment enrichment and bioinformatics to identify several microbial DNA sequences with high potential for use as markers in PCR assays for detection of human fecal contamination in water. Following competitive solution-phase hybridization of total DNA from human and pig fecal samples, 351 plasmid clones were sequenced and were determined to define 289 different genomic DNA regions. These putative human-specific fecal bacterial DNA sequences were then analyzed by dot blot hybridization, which confirmed that 98% were present in the source human fecal microbial community and absent from the original pig fecal DNA extract. Comparative sequence analyses of these sequences suggested that a large number (43.5%) were predicted to encode bacterial secreted or surface-associated proteins. Deoxyoligonucleotide primers capable of annealing to a subset of 26 of the candidate sequences predicted to encode factors involved in interactions with host cells were then used in the PCR and did not amplify markers in DNA from any additional pig fecal specimens. These 26 PCR assays exhibited a range of specificity in tests with 11 other animal sources, with more than half amplifying markers only in specimens from dogs or cats. Four assays were more specific, detecting markers only in specimens from humans, including those from 18 different human populations examined. We then demonstrated the potential utility of these assays by using them to detect human fecal contamination in several impacted watersheds.

Construction and characterization of new piggyBac vectors for constitutive or inducible expression of heterologous gene pairs and the identification of a previously unrecognized activator sequence in piggyBac

BACKGROUND

We constructed and characterized several new piggyBac vectors to provide transposition of constitutively- or inducibly-expressible heterologous gene pairs. The dual constitutive control element consists of back-to-back copies of a baculovirus immediate early (ie1) promoter separated by a baculovirus enhancer (hr5). The dual inducible control element consists of back-to-back copies of a minimal cytomegalovirus (CMVmin) promoter separated by a synthetic operator (TetO7), which drives transcription in the presence of a mutant transcriptional repressor plus tetracycline.

RESULTS

Characterization of these vectors revealed an unexpected position effect, in which heterologous genes adjacent to the 3'- terminal region ("rightward" genes) were consistently expressed at higher levels than those adjacent to the 5'-terminal region ("leftward" genes) of the piggyBac element. This position effect was observed with all six heterologous genes examined and with both transcriptional control elements. Further analysis demonstrated that this position effect resulted from stimulation of rightward gene expression by the internal domain sequence of the 3'-terminal region of piggyBac. Inserting a copy of this sequence into the 5'- terminal repeat region of our new piggyBac vectors in either orientation stimulated leftward gene expression. Representative piggyBac vectors designed for constitutive or inducible expression of heterologous gene pairs were shown to be functional as insect transformation vectors.

CONCLUSION

This study is significant because (a) it demonstrates the utility of a strategy for the construction of piggyBac vectors that can provide constitutive or inducible heterologous gene pair expression and (b) it reveals the presence of a previously unrecognized transcriptional activator in piggyBac, which is an important and increasingly utilized transposable element.

Transforming lepidopteran insect cells for improved protein processing

The lepidopteran insect cells used with the baculovirus expression vector system (BEVS) are capable of synthesizing and accurately processing foreign proteins. However, proteins expressed in baculovirus-infected cells often fail to be completely processed, or are not processed in a manner that meets a researcher's needs. This chapter discusses a metabolic engineering approach that addresses this problem. Basically, this approach involves the addition of new or enhancement of existing protein processing functions in established lepidopteran insect cell lines. Methods for engineering these cell lines and assessing their properties as improved hosts for the BEVS are detailed. Examples of lepidopteran insect cell lines engineered for improved protein N-glycosylation and trafficking are described.