Silkworm expression system as a platform technology in life science

Alternative Methods of Vaccine Delivery: An Overview of Edible and Intradermal Vaccines

Vaccines are recognized worldwide as one of the most important tools for combating infectious diseases. Despite the tremendous value conferred by currently available vaccines toward public health, the implementation of additional vaccine platforms is also of key importance. In fact, currently available vaccines possess shortcomings, such as inefficient triggering of a cell-mediated immune response and the lack of protective mucosal immunity. In this regard, recent work has been focused on vaccine delivery systems, as an alternative to injectable vaccines, to increase antigen stability and improve overall immunogenicity. In particular, novel strategies based on edible or intradermal vaccine formulations have been demonstrated to trigger both a systemic and mucosal immune response. These novel vaccination delivery systems offer several advantages over the injectable preparations including self-administration, reduced cost, stability, and elimination of a cold chain. In this review, the latest findings and accomplishments regarding edible and intradermal vaccines are described in the context of the system used for immunogen expression, their molecular features and capacity to induce a protective systemic and mucosal response.

Expression, Purification, and Characterization of Recombinant Human α1-Antitrypsin Produced Using Silkworm-Baculovirus Expression System

Human α₁-antitrypsin (AAT) is the most abundant serine proteinase inhibitor (serpin) in the human plasma. Commercially available AAT for the medications of deficiency of α₁-antitrypsin is mainly purified from human plasma. There is a high demand for a stable and low-cost supply of recombinant AAT (rAAT). In this study, the baculovirus expression vector system using silkworm larvae as host was employed and a large amount of highly active AAT was recovered from the silkworm serum (~ 15 mg/10 ml) with high purity. Both the enzymatic activity and stability of purified rAAT were comparable with those of commercial product. Our results provide an alternative method for mass production of the active rAAT in pharmaceutical use.

Purification and functional characterization of tomato mosaic virus 130K protein expressed in silkworm pupae using a baculovirus vector

Tomato mosaic virus (ToMV; genus, Tobamovirus) is a member of the alpha-like virus superfamily of positive-strand RNA viruses, which includes many plant and animal viruses of agronomical and clinical importance. The genomes of alpha-like viruses encode replication-associated proteins that contain methyltransferase, helicase and/or polymerase domains. The three-dimensional structure of the helicase domain fragment of ToMV has been determined, but the structures of the other domains of alpha-like virus replication proteins are not available. In this study, we expressed full-length ToMV replication-associated protein 130 K, which contains the methyltransferase and helicase domains, using the baculovirus-silkworm expression system and purified the recombinant protein to near homogeneity. Purified 130 K, which was stable in phosphate buffer containing magnesium ions and ATP, formed a dimer in solution and hydrolyzed nucleoside 5'-triphosphates.

A functional polypeptide N-acetylgalactosaminyltransferase (PGANT) initiates O-glycosylation in cultured silkworm BmN4 cells

Mucin-type O-glycosylation is initiated by UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases (ppGalNAc-Ts or PGANTs), attaching GalNAc to serine or threonine residue of a protein substrate. In the insect model from Lepidoptera, silkworm (Bombyx mori), however, O-glycosylation pathway is totally unexplored and remains largely unknown. In this study, as the first report regarding protein O-glycosylation analysis in silkworms, we verified the O-glycan profile that a common core 1 Gal (β1-3) GalNAc disaccharide branch without terminally sialylated structure is mainly formed for a baculovirus-produced human proteoglycan 4 (PRG4) protein. Intriguingly, functional screenings in cultured silkworm BmN4 cells for nine Bmpgants reveal that Bmpgant2 is the solo functional BmPGANT for PRG4, implying that Bmpgants may have unique cell/tissue or protein substrate preferences. Furthermore, a recombinant BmPGANT2 protein was successfully purified from silkworm-BEVS and exhibited a high ability to transfer GalNAc for both peptide and protein substrates. Taken together, the present results clarified the functional BmPGANT2 in cultured silkworm cells, providing crucial fundamental insights for future studies dissecting the detailed silkworm O-glycosylation pathways and productions of glycoproteins with O-glycans.

Enhanced Production of Recombinant Protein by Fusion Expression with Ssp DnaB Mini-Intein in the Baculovirus Expression System

The baculovirus expression system (BES) is considered to be a very powerful tool for the expression of numerous difficult to express vertebrate proteins. Ssp DnaB mini-intein is a useful fusion partner for the production of recombinant proteins because it can be self-cleaved by controlling the pH and temperature, without additional treatment. To evaluate the utility of Ssp DnaB mini-intein in the BES, recombinant viruses were generated to express the enhanced green fluorescent protein, the VP2 protein of porcine parvovirus, and the E2 protein of classical swine fever virus fused to a mini-intein. As expected, intracellular self-cleavage of the mini-intein occurred during virus infection, but the cleavage initiation time varied depending on the target protein. Significantly enhanced protein production was observed for all of the target proteins that were fused to the mini-intein. This increase was enough to overcome the decrease in the fusion protein due to intracellular self-cleavage. The mini-intein in all of the recombinant fusion proteins was successfully cleaved by controlling the pH and temperature. These results suggest that the Ssp DnaB mini-intein is a useful fusion partner in the BES for easy purification and enhanced production of target proteins.

Isolation of ferritin and its interaction with BmNPV in the silkworm, Bombyx mori

Ferritin is a ubiquitous iron storage protein that plays an important role in host defence against pathogen infections. In the present study, native ferritin was isolated from the hemolymph of Bombyx mori using native-polyacrylamide gel electrophoresis (native-PAGE) and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The results revealed that ferritin consisted of two subunits, designated as BmFerHCH and BmFerLCH. Previously integrated previous transcriptome and iTRAQ data showed that the two subunits were down-regulated in resistant silkworm strain BC9 and there was no obvious change in the expression levels of the subunits in susceptible silkworm strain P50 after BmNPV infection. Virus overlay assays revealed that B. mori ferritin as the form of heteropolymer had an interaction with B. mori nucleopolyhedrovirus (BmNPV), but it can't interact with BmNPV after depolymerisation. What's more, reverse transcription quantitative PCR (RT-qPCR) analysis suggested that BmFerHCH and BmFerLCH could be induced by bacteria, virus and iron. This is the first study to extract B. mori ferritin successfully and confirms their roles in the process of BmNPV infection. All these results will lay a foundation for further research the function of B. mori ferritin.

Biodiversity, Evolution and Ecological Specialization of Baculoviruses: A Treasure Trove for Future Applied Research

The Baculoviridae, a family of insect-specific large DNA viruses, is widely used in both biotechnology and biological control. Its applied value stems from millions of years of evolution influenced by interactions with their hosts and the environment. To understand how ecological interactions have shaped baculovirus diversification, we reconstructed a robust molecular phylogeny using 217 complete genomes and ~580 isolates for which at least one of four lepidopteran core genes was available. We then used a phylogenetic-concept-based approach (mPTP) to delimit 165 baculovirus species, including 38 species derived from new genetic data. Phylogenetic optimization of ecological characters revealed a general pattern of host conservatism punctuated by occasional shifts between closely related hosts and major shifts between lepidopteran superfamilies. Moreover, we found significant phylogenetic conservatism between baculoviruses and the type of plant growth (woody or herbaceous) associated with their insect hosts. In addition, we found that colonization of new ecological niches sometimes led to viral radiation. These macroevolutionary patterns show that besides selection during the infection process, baculovirus diversification was influenced by tritrophic interactions, explained by their persistence on plants and interactions in the midgut during horizontal transmission. This complete eco-evolutionary framework highlights the potential innovations that could still be harnessed from the diversity of baculoviruses.

A Recombinant Baculovirus Efficiently Generates Recombinant Adeno-Associated Virus Vectors in Cultured Insect Cells and Larvae

Current large-scale recombinant adeno-associated virus (rAAV) production systems based on the baculovirus expression vector (BEV) remain complicated and cost-intensive, and they lack versatility and flexibility. Here we present a novel recombinant baculovirus integrated with all packaging elements for the production of rAAV. To optimize BEV construction, ribosome leaky-scanning mechanism was used to express AAV Rep and Cap proteins downstream of the PH and P10 promoters in the pFast.Bac.Dual vector, respectively, and the rAAV genome was inserted between the two promoters. The yields of rAAV2, rAAV8, and rAAV9 derived from the BEV-infected Sf9 cells exceeded 10⁵ vector genomes (VG) per cell. The BEV was shown to be stable and showed no apparent decrease of rAAV yield after at least four serial passages. The rAAVs derived from the new Bac system displayed high-quality and high-transduction activity. Additionally, rAAV2 could be efficiently generated from BEV-infected beet armyworm larvae at a per-larvae yield of 2.75 ± 1.66 × 10¹⁰ VG. The rAAV2 derived from larvae showed a structure similar to the rAAV2 derived from HEK293 cells, and it also displayed high-transduction activity. In summary, the novel BEV is ideally suitable for large-scale rAAV production. Further, this study exploits a potential cost-efficient platform for rAAV production in insect larvae.

A secretary bi-cistronic baculovirus expression system with improved production of the HA1 protein of H6 influenza virus in insect cells and Spodoptera litura larvae

A bi-cistronic baculovirus expression vector was constructed to facilitate the expression, detection, and isolation of the hemagglutinin (HA) fragment HA1 of H6N1 avian influenza virus (AIV) in an insect and a culture of its cells. In this construct, the GP67sp signal peptide promoted the secretion of the recombinant protein into the culture medium, and improved protein expression and purification. Enhanced green fluorescent protein, co-expressed through an internal ribosome entry site, served as a visible reporter for protein expression detection. The hemolymph of Spodoptera litura larvae infected with the bi-cistronic baculovirus was collected for the purification of the recombinant HA1, which was found to be glycosylated, and monomeric and trimeric forms of the recombinant HA1 were identified. Proteins expressed in both the cell culture and larvae served as effective subunit vaccines for the production of antiserum against HA. The antiserum recognized the H6 subtype of AIV but not the H5 subtype.

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HA1 of H6N1 influenza virus was expressed in insect and cell culture.

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The expressed HA1 was glycosylated, and estimated as monomeric and trimeric forms.

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The expressed HA1 served as an effective subunit vaccine for producing antisera.

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The antisera specifically recognized influenza H6 subtype but not the H5 subtype.

Differentiation of MSC and annulus fibrosus cells on genetically engineered silk fleece-membrane-composites enriched for GDF-6 or TGF-β3

Intervertebral disc (IVD) repair is a high-priority topic in our active and increasingly ageing society. Since a high number of people are affected by low back pain treatment options that are able to restore the biological function of the IVD are highly warranted. Here, we investigated whether the feasibility of genetically engineered (GE)-silk from Bombyx mori containing specific growth factors to precondition human bone-marrow derived mesenchymal stem cells (hMSC) or to activate differentiated human annulus fibrosus cells (hAFC) prior transplantation or for direct repair on the IVD. Here, we tested the hypothesis that GE-silk fleece can thrive human hMSC towards an IVD-like phenotype. We aimed to demonstrate a possible translational application of good manufacturing practice (GMP)-compliant GE-silk scaffolds in IVD repair and regeneration. GE-silk with growth and differentiation factor 6 (GDF-6-silk) or transforming growth factor β3 (TGF-β3, TGF-β3-silk) and untreated silk (cSilk) were investigated by DNA content, cell activity assay and glycosaminoglycan (GAG) content and their differentiation potential by qPCR analysis. We found that all silk types demonstrated a very high biocompatibility for both cell types, that is, hMSC and hAFC, as revealed by cell activity, and DNA proliferation assay. Further, analyzing qPCR of marker genes revealed a trend to differentiation toward an NP-like phenotype looking at the Aggrecan/Collagen 2 ratio which was around 10:1. Our results support the conclusion that our GE-silk scaffold treatment approach can thrive hMSC towards a more IVD-like phenotype or can maintain the phenotype of native hAFC. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1324-1333, 2018.

Biomedical Applications of Recombinant Silk-Based Materials

Silk is mostly known as a luxurious textile, which originates from silkworms first cultivated in China. A deeper look into the variety of silk reveals that it can be used for much more, in nature and by humanity. For medical purposes, natural silks were recognized early as a potential biomaterial for surgical threads or wound dressings; however, as biomedical engineering advances, the demand for high-performance, naturally derived biomaterials becomes more pressing and stringent. A common problem of natural materials is their large batch-to-batch variation, the quantity available, their potentially high immunogenicity, and their fast biodegradation. Some of these common problems also apply to silk; therefore, recombinant approaches for producing silk proteins have been developed. There are several research groups which study and utilize various recombinantly produced silk proteins, and many of these have also investigated their products for biomedical applications. This review gives a critical overview over of the results for applications of recombinant silk proteins in biomedical engineering.

Expression and Activation of Horseradish Peroxidase-Protein A/G Fusion Protein in Silkworm Larvae for Diagnostic Purposes

Recombinant protein production can create artificial proteins with desired functions by introducing genetic modifications to the target proteins. Horseradish peroxidase (HRP) has been used extensively as a reporter enzyme in biotechnological applications; however, recombinant production of HRP has not been very successful, hampering the utilization of HRP with genetic modifications. A fusion protein comprising an antibody binding protein and HRP will be an ideal bio-probe for high-quality HRP-based diagnostic systems. A HRP-protein A/G fusion protein (HRP-pAG) is designed and its production in silkworm (Bombyx mori) is evaluated for the first time. HRP-pAG is expressed in a soluble apo form, and is activated successfully by incubating with hemin. The activated HRP-pAG is used directly for ELISA experiments and retains its activity over 20 days at 4 °C. Moreover, HRP-pAG is modified with biotin by the microbial transglutaminase (MTG) reaction. The biotinylated HRP-pAG is conjugated with streptavidin to form a HRP-pAG multimer and the multimeric HRP-pAG produced higher signals in the ELISA system than monomeric HRP-pAG. The successful production of recombinant HRP in silkworm will contribute to creating novel HRP-based bioconjugates as well as further functionalization of HRP by applying enzymatic post-translational modifications.

Characterization of Recombinant Thermococcus kodakaraensis (KOD) DNA Polymerases Produced Using Silkworm-Baculovirus Expression Vector System

The KOD DNA polymerase from Thermococcus kodakarensis (Tkod-Pol) has been preferred for PCR due to its rapid elongation rate, extreme thermostability and outstanding fidelity. Here in this study, we utilized silkworm-baculovirus expression vector system (silkworm-BEVS) to express the recombinant Tkod-Pol (rKOD) with N-terminal (rKOD-N) or C-terminal (rKOD-C) tandem fusion tags. By using BEVS, we produced functional rKODs with satisfactory yields, about 1.1 mg/larva for rKOD-N and 0.25 mg/larva for rKOD-C, respectively. Interestingly, we found that rKOD-C shows higher thermostability at 95 °C than that of rKOD-N, while that rKOD-N is significantly unstable after exposing to long period of heat-shock. We also assessed the polymerase activity as well as the fidelity of purified rKODs under various conditions. Compared with commercially available rKOD, which is expressed in E. coli expression system, rKOD-C exhibited almost the same PCR performance as the commercial rKOD did, while rKOD-N did lower performance. Taken together, our results suggested that silkworm-BEVS can be used to express and purify efficient rKOD in a commercial way.

Purification of virus-like particles (VLPs) expressed in the silkworm Bombyx mori

Virus-like particles (VLPs) are a promising and developing option for vaccination and gene therapy. They are also interesting as shuttles for drug targeting. Currently, several different gene expression systems are available, among which the silkworm expression system is known for its mass production capacity. However, cost-effective purification with high purity of the target protein is a particular bottleneck for this system. The present review evaluates the advances in the purification of VLPs, especially from silkworm larval hemolymph. Beginning with applicable pre-treatments for VLPs over to chromatography methods and quality control of the purified VLPs. Whereupon the main focus is on the different chromatography approaches for the purification, but the structure of the VLPs and their intended use for humans make also the quality control important. Within this, the stability of the VLPs which has to be considered for the purification is as well discussed.

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

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

Insulin-like peptide 3 expressed in the silkworm possesses intrinsic disulfide bonds and full biological activity

Insulin-like peptide 3 (INSL3) is a member of the relaxin/insulin superfamily and is expressed in testicular Leydig cells. Essential for fetal testis descent, INSL3 has been implicated in testicular and sperm function in adult males via interaction with relaxin/insulin-like family peptide receptor 2 (RXFP2). The INSL3 is typically prepared using chemical synthesis or overexpression in Escherichia coli followed by oxidative refolding and proteolysis. Here, we expressed and purified full-length porcine INSL3 (pINSL3) using a silkworm-based Bombyx mori nucleopolyhedrovirus bacmid expression system. Biophysical measurements and proteomic analysis revealed that this recombinant pINSL3 exhibited the correct conformation, with the three critical disulfide bonds observed in native pINSL3, although partial cleavage occurred. In cAMP stimulation assays using RXFP2-expressing HEK293 cells, the recombinant pINSL3 possessed full biological activity. This is the first report concerning the production of fully active pINSL3 without post-expression treatments and provides an efficient production platform for expressing relaxin/insulin superfamily peptides.

In vivo production of recombinant proteins using occluded recombinant AcMNPV-derived baculovirus vectors

Trichoplusia ni insect larvae infected with vectors derived from the Autographa californica multiple nucleopolyhedrovirus (AcMNPV), are an excellent alternative to insect cells cultured in conventional bioreactors to produce recombinant proteins because productivity and cost-efficiency reasons. However, there is still a lot of work to do to reduce the manual procedures commonly required in this production platform that limit its scalability. To increase the scalability of this platform technology, a current bottleneck to be circumvented in the future is the need of injection for the inoculation of larvae with polyhedrin negative baculovirus vectors (Polh-) because of the lack of oral infectivity of these viruses, which are commonly used for production in insect cell cultures. In this work we have developed a straightforward alternative to obtain orally infective vectors derived from AcMNPV and expressing recombinant proteins that can be administered to the insect larvae (Trichoplusia ni) by feeding, formulated in the insect diet. The approach developed was based on the use of a recombinant polyhedrin protein expressed by a recombinant vector (Polh+), able to co-occlude any recombinant Polh- baculovirus vector expressing a recombinant protein. A second alternative was developed by the generation of a dual vector co-expressing the recombinant polyhedrin protein and the foreign gene of interest to obtain the occluded viruses. Additionally, by the incorporation of a reporter gene into the helper Polh+ vector, it was possible the follow-up visualization of the co-occluded viruses infection in insect larvae and will help to homogenize infection conditions. By using these methodologies, the production of recombinant proteins in per os infected larvae, without manual infection procedures, was very similar in yield to that obtained by manual injection of recombinant Polh- AcMNPV-based vectors expressing the same proteins. However, further analyses will be required for a detailed comparison of production yields reached by injection vs oral infections for different recombinant proteins. In conclusion, these results open the possibility of future industrial scaling-up production of recombinant proteins in insect larvae by reducing manual operations.

Increasing the yield of middle silk gland expression system through transgenic knock-down of endogenous sericin-1

Various genetically modified bioreactor systems have been developed to meet the increasing demands of recombinant proteins. Silk gland of Bombyx mori holds great potential to be a cost-effective bioreactor for commercial-scale production of recombinant proteins. However, the actual yields of proteins obtained from the current silk gland expression systems are too low for the proteins to be dissolved and purified in a large scale. Here, we proposed a strategy that reducing endogenous sericin proteins would increase the expression yield of foreign proteins. Using transgenic RNA interference, we successfully reduced the expression of BmSer1 to 50%. A total 26 transgenic lines expressing Discosoma sp. red fluorescent protein (DsRed) in the middle silk gland (MSG) under the control of BmSer1 promoter were established to analyze the expression of recombinant. qRT-PCR and western blotting showed that in BmSer1 knock-down lines, the expression of DsRed had significantly increased both at mRNA and protein levels. We did an additional analysis of DsRed/BmSer1 distribution in cocoon and effect of DsRed protein accumulation on the silk fiber formation process. This study describes not only a novel method to enhance recombinant protein expression in MSG bioreactor, but also a strategy to optimize other bioreactor systems.

iTRAQ-based quantitative proteomics analysis of molecular mechanisms associated with Bombyx mori (Lepidoptera) larval midgut response to BmNPV in susceptible and near-isogenic strains

Bombyx mori nucleopolyhedrovirus (BmNPV) has been identified as a major pathogen responsible for severe economic loss. Most silkworm strains are susceptible to BmNPV, with only a few highly resistant strains thus far identified. Here we investigated the molecular basis of silkworm resistance to BmNPV using susceptible (the recurrent parent P50) and resistant (near-isogenic line BC9) strains and a combination of iTRAQ-based quantitative proteomics, reverse-transcription quantitative PCR and Western blotting. By comparing the proteomes of infected and non-infected P50 and BC9 silkworms, we identified 793 differentially expressed proteins (DEPs). By gene ontology and KEGG enrichment analyses, we found that these DEPs are preferentially involved in metabolism, catalytic activity, amino sugar and nucleotide sugar metabolism and carbon metabolism. 114 (14.38%) DEPs were associated with the cytoskeleton, immune response, apoptosis, ubiquitination, translation, ion transport, endocytosis and endopeptidase activity. After removing the genetic background and individual immune stress response proteins, we identified 84 DEPs were found that are potentially involved in resistance to BmNPV. Further studies showed that a serine protease was down-regulated in P50 and up-regulated in BC9 after BmNPV infection. Taken together, these results provide insights into the molecular mechanism of silkworm response to BmNPV.

BIOLOGICAL SIGNIFICANCE

Bombyx mori nucleopolyhedrovirus (BmNPV) is highly pathogenic, causing serious losses in sericulture every year. However, the molecular mechanisms of BmNPV infection and host defence remain unclear. Here we combined quantitative proteomic, bioinformatics, RT-qPCR and Western blotting analyses and found that BmNPV invasion causes complex protein alterations in the larval midgut, and that these changes are related to cytoskeleton, immune response, apoptosis, ubiquitination, translation, ion transport, endocytosis and endopeptidase activity. Five important differentially expression proteins were validation by independent approaches. These finding will help address the molecular mechanisms of silkworm resistance to BmNPV and provide a molecular target for resisting BmNPV.

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

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

Advantages of the Silkworm As an Animal Model for Developing Novel Antimicrobial Agents

The demand for novel antibiotics to combat the global spread of multi drug-resistant pathogens continues to grow. Pathogenic bacteria and fungi that cause fatal human infections can also kill silkworms and the infected silkworms can be cured by the same antibiotics used to treat infections in the clinic. As an invertebrate model, silkworm model is characterized by its convenience, low cost, no ethical issues. The presence of conserved immune response and similar pharmacokinetics compared to mammals make silkworm infection model suitable to examine the therapeutic effectiveness of antimicrobial agents. Based on this, we utilized silkworm bacterial infection model to screen the therapeutic effectiveness of various microbial culture broths and successfully identified a therapeutically effective novel antibiotic, lysocin E, which has a novel mode of action of binding to menaquinone, thus leading to membrane damage and bactericidal activity. The similar approach to screen potential antibiotics resulted in the identification of other therapeutically effective novel antibiotics, such as nosokomycin and ASP2397 (VL-2397). In this regard, we propose that the silkworm antibiotic screening model is very effective for identifying novel antibiotics. In this review, we summarize the advantages of the silkworm model and propose that the utilization of silkworm infection model will facilitate the discovery of novel therapeutically effective antimicrobial agents.

Non-conventional expression systems for the production of vaccine proteins and immunotherapeutic molecules

The increasing demand for recombinant vaccine antigens or immunotherapeutic molecules calls into question the universality of current protein expression systems. Vaccine production can require relatively low amounts of expressed materials, but represents an extremely diverse category consisting of different target antigens with marked structural differences. In contrast, monoclonal antibodies, by definition share key molecular characteristics and require a production system capable of very large outputs, which drives the quest for highly efficient and cost-effective systems. In discussing expression systems, the primary assumption is that a universal production platform for vaccines and immunotherapeutics will unlikely exist. This review provides an overview of the evolution of traditional expression systems, including mammalian cells, yeast and E.coli, but also alternative systems such as other bacteria than E. coli, transgenic animals, insect cells, plants and microalgae, Tetrahymena thermophila, Leishmania tarentolae, filamentous fungi, cell free systems, and the incorporation of non-natural amino acids.

Establishment of the BacMam system using silkworm baculovirus

The BacMam system uses modified insect viruses (baculoviruses) as vehicles to efficiently deliver genes for expression in mammalian cells. The technique can be widely applied to large-scale recombinant protein production with appropriate modifications, high-throughput screening platforms for cell-based assays, and the delivery of large genes. The silkworm system is often employed as a rapid and cost-effective approach for recombinant baculovirus generation. Here we have developed the novel BacMam system using silkworm baculovirus, and shown the successful expression of EGFP in mammalian cells. The transduction to mammalian cells via the BacMam system was improved by adding phosphate-buffered saline and sodium butyrate to the culture medium and lowering the temperature after viral infection. This study provides an alternative gene delivery system for mammalian cells, which has various potential applications, including efficient native protein production and gene therapy.

Towards next generation maggot debridement therapy: transgenic Lucilia sericata larvae that produce and secrete a human growth factor

BACKGROUND

Diabetes and its concurrent complications impact a significant proportion of the population of the US and create a large financial burden on the American health care system. FDA-approved maggot debridement therapy (MDT), the application of sterile laboratory-reared Lucilia sericata (green bottle fly) larvae to wounds, is a cost-effective and successful treatment for diabetic foot ulcers and other medical conditions. Human platelet derived growth factor-BB (PDGF-BB) is a secreted dimeric peptide growth factor that binds the PDGF receptor. PDGF-BB stimulates cell proliferation and survival, promotes wound healing, and has been investigated as a possible topical treatment for non-healing wounds. Genetic engineering has allowed for expression and secretion of human growth factors and other proteins in transgenic insects. Here, we present a novel concept in MDT technology that combines the established benefits of MDT with the power of genetic engineering to promote healing. The focus of this study is to create and characterize strains of transgenic L. sericata that express and secrete PDGF-BB at detectable levels in adult hemolymph, whole larval lysate, and maggot excretions/ secretions (ES), with potential for clinical utility in wound healing.

RESULTS

We have engineered and confirmed transgene insertion in several strains of L. sericata that express human PDGF-BB. Using a heat-inducible promoter to control the pdgf-b gene, pdgf-b mRNA was detected via semi-quantitative PCR upon heat shock. PDGF-BB protein was also detectable in larval lysates and adult hemolymph but not larval ES. An alternative, tetracycline-repressible pdgf-b system mediated expression of pdgf-b mRNA when maggots were raised on diet that lacked tetracycline. Further, PDGF-BB protein was readily detected in whole larval lysate as well as larval ES.

CONCLUSIONS

Here we show robust, inducible expression and production of human PDGF-BB protein from two conditional expression systems in transgenic L. sericata larvae. The tetracycline-repressible system appears to be the most promising as PDGF-BB protein was detectable in larval ES following induction. Our system could potentially be used to deliver a variety of growth factors and anti-microbial peptides to the wound environment with the aim of enhancing wound healing, thereby improving patient outcome in a cost-effective manner.

Efficient expression of single chain variable fragment antibody against paclitaxel using the Bombyx mori nucleopolyhedrovirus bacmid DNA system and its characterizations

A single chain variable fragment (scFv), the smallest unit of functional recombinant antibody, is an attractive format of recombinant antibodies for various applications due to its small fragment and possibility of genetic engineering. Hybridoma clone 3A3 secreting anti-paclitaxel monoclonal antibody was used to construct genes encoding its variable domains of heavy (VH) and light (VL) chains. The VH and VL domains were linked to be the PT-scFv3A3 using flexible peptide linker in a format of VH-(GGGGS)5-VL. The PT-scFv3A3 was primarily expressed using the pET28a(+) vector in the Escherichia coli system, and was then further expressed by using the Bombyx mori nucleopolyhedrovirus (BmNPV) bacmid DNA system. Interestingly, the reactivity of PT-scFv3A3 expressed in the hemolymph of B. mori using the BmNPV bacmid DNA system was much higher than that expressed in the E. coli system. Using indirect competitive enzyme-linked immunosorbent assay (icELISA), the PT-scFv3A3 (B. mori) reacted not only with immobilized paclitaxel, but also with free paclitaxel in a concentration-dependent manner, with the linear range of free paclitaxel between 0.156 and 5.00 µg/ml. The PT-scFv3A3 (B. mori) exhibited less cross-reactivity (%) than its parental MAb clone 3A3 against paclitaxel-related compounds, including docetaxel (31.1 %), 7-xylosyltaxol (22.1 %), baccatin III (<0.68 %), 10-deacetylbaccatin III (<0.68 %), 1-hydroxybaccatin I (<0.68 %), and 1-acetoxy-5-deacetylbaccatin I (<0.68 %). With the exception of cephalomannine, the cross-reactivity was slightly increased to 8.50 %. The BmNPV bacmid DNA system was a highly efficient expression system of active PT-scFv3A3, which is applicable for PT-scFv3A3-based immunoassay of paclitaxel. In addition, the PT-scFv3A3 can be applied to evaluate its neutralizing property of paclitaxel or docetaxel toxicity.

Eri silkworm (Samia ricini), a non-mulberry host system for AcMNPV mediated expression of recombinant proteins

The baculovirus expression system (BVES) based on Autographa californica nucleopolyhedrovirus (AcMNPV) is widely used for the expression of eukaryotic proteins. Several insect cells/larvae that are permissive to AcMNPV have been routinely used as hosts to express heterologous proteins. Domesticated Eri silkworm (Samia ricini), reared in many parts of India, Japan and China, is a non-mulberry silkworm. The present study shows that the Eri silkworm larvae are susceptible to intra-haemocoelical inoculation of AcMNPV. The virus replicates in the larva, as indicated by an increased viral loads in the haemolymph upon injection of a recombinant AcMNPV carrying green fluorescent protein gene. The virus showed localized replication in different tissues including the fat body, haemocytes, tracheal matrix and in the Malphigian tubules. The larval system was successfully used to express heterologous protein, by infecting with a recombinant AcMNPV carrying the 3ABC coding sequence of foot-and-mouth disease virus (FMDV). The study shows that the Eri silkworm larva can be a potential alternative bioreactor, for scaling up of the recombinant proteins employing the baculovirus system.

The Influence of SV40 polyA on Gene Expression of Baculovirus Expression Vector Systems

The simian virus 40 polyadenylation signal (SV40 polyA) has been routinely inserted downstream of the polyhedrin promoter in many baculovirus expression vector systems (BEVS). In the baculovirus prototype Autographa californica multiple nucleopolyhedrovirus (AcMNPV), the polyhedrin promoter (very late promoter) transcribes its gene by a viral RNA polymerase therefore there is no supporting evidence that SV40 polyA is required for the proper gene expression under the polyhedrin promoter. Moreover, the effect of the SV40 polyA sequence on the polyhedrin promoter activity has not been tested either at its natural polyhedrin locus or in other loci in the viral genome. In order to test the significance of adding the SV40 polyA sequence on gene expression, the expression of the enhanced green fluorescent protein (egfp) was evaluated with and without the presence of SV40 polyA under the control of the polyhedrin promoter at different genomic loci (polyherin, ecdysteroid UDP-glucosyltransferase (egt), and gp37). In this study, spectrofluorometry and western blot showed reduction of EGFP protein for all recombinant viruses with SV40 polyA, whereas qPCR showed an increase in the egfp mRNA levels. Therefore, we conclude that SV40 polyA increases mRNA levels but decreases protein production in the BEVS when the polyhedrin promoter is used at different loci. This work suggests that SV40 polyA in BEVSs should be replaced by an AcMNPV late gene polyA for optimal protein production or left untouched for optimal RNA production (RNA interference applications).

Inactivation of Bombyx mori macula-like virus under physical conditions

The Bombyx mori macula-like virus (BmMLV) is a member of the genus Maculavirus, family Tymoviridae, and contains a positive-sense single-stranded RNA genome. Previously, we reported that almost all B. mori-derived cell lines have already been contaminated with BmMLV via an unknown infection route. Since B. mori-derived cell lines are used for the baculovirus expression vector system, the invasion of BmMLV will cause a serious safety risk in the production of recombinant proteins. In this study, to determine the inactivation effectiveness of BmMLV, viruses were treated with various temperatures as well as gamma and ultraviolet (UV) light radiation. After these treatments, the virus solutions were inoculated into BmMLV-free BmVF cells. At 7 days postinoculation, the amount of virus in cells was evaluated by real-time reverse transcription PCR. Regarding heat treatment, conditions under 56°C for 3 h were tolerated, whereas infectivity disappeared after treatment at 75°C for 1 h. Regarding gamma radiation treatment, viruses were relatively stable at 1 kGy; however, their infectivity was entirely eliminated at a dose of 10 kGy. With 254 nm UV-C treatment, viruses were still active at less than 120 mJ/cm(2); however, their infectivity was completely lost at greater than 140 mJ/cm(2) UV-C radiation. These results provide quantitative evidence of the potential for BmMLV inactivation under a variety of physical conditions.

Effective isotope labeling of proteins in a mammalian expression system

Isotope labeling of biologically interesting proteins is a prerequisite for structural and dynamics studies by NMR spectroscopy. Many of these proteins require mammalian cofactors, chaperons, or posttranslational modifications such as myristoylation, glypiation, disulfide bond formation, or N- or O-linked glycosylation; and mammalian cells have the necessary machinery to produce them in their functional forms. Here, we describe recent advances in mammalian expression, including an efficient adenoviral vector-based system, for the production of isotopically labeled proteins. This system enables expression of mammalian proteins and their complexes, including proteins that require posttranslational modifications. We describe a roadmap to produce isotopically labeled (15)N and (13)C posttranslationally modified proteins, such as the outer domain of HIV-1 gp120, which has four disulfide bonds and 15 potential sites of N-linked glycosylation. These methods should allow NMR spectroscopic analysis of the structure and function of posttranslationally modified and secreted, cytoplasmic, or membrane-bound proteins.

Targeted glycoengineering extends the protein N-glycosylation pathway in the silkworm silk gland

The silkworm silk glands are powerful secretory organs that can produce and secrete proteins at high levels. As such, it has been suggested that the biosynthetic and secretory power of the silk gland can be harnessed to produce and secrete recombinant proteins in tight or loose association with silk fibers. However, the utility of the silkworm platform is constrained by the fact that it has a relatively primitive protein N-glycosylation pathway, which produces relatively simple insect-type, rather than mammalian-type N-glycans. In this study, we demonstrate for the first time that the silk gland protein N-glycosylation pathway can be glycoengineered. We accomplished this by using a dual piggyBac vector encoding two distinct mammalian glycosyltransferases under the transcriptional control of a posterior silk gland (PSG)-specific promoter. Both mammalian transgenes were expressed and each mammalian N-glycan processing activity was induced in transformed silkworm PSGs. In addition, the transgenic animals produced endogenous glycoproteins containing significant proportions of mammalian-type, terminally galactosylated N-glycans, while the parental animals produced none. This demonstration of the ability to glycoengineer the silkworm extends its potential utility as a recombinant protein production platform.

High yield exogenous protein HPL production in the Bombyx mori silk gland provides novel insight into recombinant expression systems

The silk gland of Bombyx mori (BmSG) has gained significant attention by dint of superior synthesis and secretion of proteins. However, the application of BmSG bioreactor is still a controversial issue because of low yields of recombinant proteins. Here, a 3057 bp full-length coding sequence of Hpl was designed and transformed into the silkworm genome, and then the mutant (Hpl/Hpl) with specific expression of Hpl in posterior BmSG (BmPSG) was obtained. In the mutants, the transcription level of Fib-L and P25, and corresponding encoding proteins, did not decrease. However, the mRNA level of Fib-H was reduced by 71.1%, and Fib-H protein in the secreted fibroin was decreased from 91.86% to 71.01%. The mRNA level of Hpl was 0.73% and 0.74% of Fib-H and Fib-L, respectively, while HPL protein accounted for 18.85% of fibroin and 15.46% of the total amount of secreted silk protein. The exogenous protein was therefore very efficiently translated and secreted. Further analysis of differentially expressed gene (DEG) was carried out in the BmPSG cells and 891 DEGs were detected, of which 208 genes were related to protein metabolism. Reduced expression of endogenous silk proteins in the BmPSG could effectively improve the production efficiency of recombinant exogenous proteins.

Characterization of anti-CD20 monoclonal antibody produced by transgenic silkworms (Bombyx mori)

In response to the successful use of monoclonal antibodies (mAbs) in the treatment of various diseases, systems for expressing recombinant mAbs using transgenic animals or plants have been widely developed. The silkworm (Bombyx mori) is a highly domesticated insect that has recently been used for the production of recombinant proteins. Because of their cost-effective breeding and relatively easy production scale-up, transgenic silkworms show great promise as a novel production system for mAbs. In this study, we established a transgenic silkworm stably expressing a human-mouse chimeric anti-CD20 mAb having the same amino acid sequence as rituximab, and compared its characteristics with rituximab produced by Chinese hamster ovary (CHO) cells (MabThera®). The anti-CD20 mAb produced in the transgenic silkworm showed a similar antigen-binding property, but stronger antibody-dependent cell-mediated cytotoxicity (ADCC) and weaker complement-dependent cytotoxicity (CDC) compared to MabThera. Post-translational modification analysis was performed by peptide mapping using liquid chromatography/mass spectrometry. There was a significant difference in the N-glycosylation profile between the CHO- and the silkworm-derived mAbs, but not in other post-translational modifications including oxidation and deamidation. The mass spectra of the N-glycosylated peptide revealed that the observed biological properties were attributable to the characteristic N-glycan structures of the anti-CD20 mAbs produced in the transgenic silkworms, i.e., the lack of the core-fucose and galactose at the non-reducing terminal. These results suggest that the transgenic silkworm may be a promising expression system for the tumor-targeting mAbs with higher ADCC activity.

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.

Phosphorylation of Ser-204 and Tyr-405 in human malonyl-CoA decarboxylase expressed in silkworm Bombyx mori regulates catalytic decarboxylase activity

Decarboxylation of malonyl-CoA to acetyl-CoA by malonyl-CoA decarboxylase (MCD; EC 4.1.1.9) is a vital catalytic reaction of lipid metabolism. While it is established that phosphorylation of MCD modulates the enzymatic activity, the specific phosphorylation sites associated with the catalytic function have not been documented due to lack of sufficient production of MCD with proper post-translational modifications. Here, we used the silkworm-based Bombyx mori nucleopolyhedrovirus (BmNPV) bacmid system to express human MCD (hMCD) and mapped phosphorylation effects on enzymatic function. Purified MCD from silkworm displayed post-translational phosphorylation and demonstrated coherent enzymatic activity with high yield (-200 μg/silkworm). Point mutations in putative phosphorylation sites, Ser-204 or Tyr-405 of hMCD, identified by bioinformatics and proteomics analyses reduced the catalytic activity, underscoring the functional significance of phosphorylation in modulating decarboxylase-based catalysis. Identified phosphorylated residues are distinct from the decarboxylation catalytic site, implicating a phosphorylation-induced global conformational change of MCD as responsible in altering catalytic function. We conclude that phosphorylation of Ser-204 and Tyr-405 regulates the decarboxylase function of hMCD leveraging the silkworm-based BmNPV bacmid expression system that offers a fail-safe eukaryotic production platform implementing proper post-translational modification such as phosphorylation.

Heme precursor injection is effective for Arthromyces ramosus peroxidase fusion protein production by a silkworm expression system

Recombinant peroxidase from Arthromyces ramosus, fused with domains of antibody-binding proteins, was successfully obtained by a silkworm larvae expression system. The catalytic activity of the fusion peroxidase was increased 6-fold with the injection of 5-aminolevulinic acid into silkworm larvae as a heme precursor.

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.

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.

Efficient and cost effective production of active-form human PKB using silkworm larvae

Protein kinase B (PKB) also known as Akt is involved in many signal transduction pathways. As alterations of the PKB pathway are found in a number of human malignancies, PKB is considered an important drug target for cancer therapy. However, production of sufficient amounts of active PKB for biochemical and structural studies is very costly because of the necessity of using a higher organism expression system to obtain phosphorylated PKB. Here, we report efficient production of active PKBα using the BmNPV bacmid expression system with silkworm larvae. Following direct injection of bacmid DNA, recombinant PKBα protein was highly expressed in the fat bodies of larvae, and could be purified using a GST-tag and then cleaved. A final yield of approximately 1 mg PKBα/20 larvae was recorded. Kinase assays showed that the recombinant PKBα possessed high phosphorylation activity. We further confirmed phosphorylation on the activation loop by mass spectrometric analysis. Our results indicate that the silkworm expression system is of value for preparation of active-form PKBα with phosphorylation on the activation loop. This efficient production of the active protein will facilitate further biochemical and structural studies and stimulate subsequent drug development.

The advances and perspectives of recombinant protein production in the silk gland of silkworm Bombyx mori

The silk gland of silkworm Bombyx mori, is one of the most important organs that has been fully studied and utilized so far. It contributes finest silk fibers to humankind. The silk gland has excellent ability of synthesizing silk proteins and is a kind tool to produce some useful recombinant proteins, which can be widely used in the biological, biotechnical and pharmaceutical application fields. It's a very active area to express recombinant proteins using the silk gland as a bioreactor, and great progress has been achieved recently. This review recapitulates the progress of producing recombinant proteins and silk-based biomaterials in the silk gland of silkworm in addition to the construction of expression systems. Current challenges and future trends in the production of valuable recombinant proteins using transgenic silkworms are also discussed.