Production of human alpha-interferon in silkworm using a baculovirus vector

Complete genome determination of a European Bombyx mori nucleopolyhedrovirus (BmNPV) strain isolated in the French Cevennes

The Bombyx mori nucleopolyhedrovirus (BmNPV) is a critical pathogen in sericulture; yet, it also holds great promise for the bioproduction of recombinant proteins. Here, we report the characterization of a highly pathogenic strain from the French Cevennes region. Phylogenetic analyses reveal that this strain is related to a Brazilian BmNPV strain.

Specific nucleotide substitutions in the burst sequence enhance polyhedrin expression in alphabaculoviruses: improvement of baculovirus expression vectors

Alphabaculoviruses produce a large number of occlusion bodies (OBs) in host cells during the late stage of infection. OBs are mainly composed of the viral product polyhedrin (POLH), and the extremely high-level transcription of the polh gene has been exploited to express foreign proteins in insect cultured cells, larvae, and pupae. This polh hyper-transcription requires the "burst sequence" located between the transcriptional start site and the initiation codon. Here, we focused on the roles of the A-rich region within the burst sequence. We generated Bombyx mori nucleopolyhedrovirus mutants whose burst sequence contained "A-to-T" mutations in the A-rich region. Some mutants exhibited levels of polh promoter-driven reporter expression lower than or comparable to that of the wild type, whereas the mutants with TTT mutations at positions -16 to -14 in the polh upstream region showed a four- to fivefold increase in it. Most cases of single or double A-to-T mutations at -16 to -14 of the upstream region had small but significant effects on the expression level, while the triple mutation was the most effective. This enhancement was also observed in the Autographa californica multiple nucleopolyhedrovirus-based vector system, which is more commonly used for foreign protein expression. We also found that this triple mutation enhanced the accumulation of polh mRNA and POLH protein even in an OB-producing virus. These results indicate that specific mutations in the burst sequence have the potential to increase baculoviral protein expression at the transcriptional level and may improve foreign protein expression by baculoviruses.IMPORTANCEThe most notable characteristic of alphabaculoviruses is that they produce many proteinaceous occlusion bodies (OBs) during the very late stages of infection. The main component of these OBs is virus-encoded polyhedrin (POLH). The high expression of the polh gene led to the development of the baculovirus expression vector system (BEVS). Currently, this system is widely used for the production of vaccines, veterinary medicines, and reagents. Despite this background, the mechanisms by which baculoviruses ultimately produce large quantities of OBs remain largely unexplained, even after approximately 40 years since the BEVS development. Here, we discovered that three nucleotide substitutions in the polh burst sequence markedly increased the polh expression levels in both BmNPV- and AcMNPV-based BEVSs, regardless of the vector type. This discovery can be easily introduced into the currently used BEVS, possibly contributing to further improvements for achieving even higher expression of foreign proteins.

Baculoviruses remodel the cytoskeleton of insect hemocytes to breach the host basal lamina

Many pathogens and endosymbionts hijack the host's cytoskeleton for efficient propagation and transfer within or between host cells. Once released into the host's circulatory system, however, they have to confront structural barriers without utilizing host cell functions. Many insect viruses and insect-borne viruses can re-enter from the hemolymph into insect tissues despite the barrier of the basal lamina (BL), but the molecular mechanism remains unclear in many cases. Here, we demonstrate that Bombyx mori nucleopolyhedrovirus (BmNPV) remodels host hemocytes to breach the BL. We found that the viral membrane protein actin rearrangement-inducing factor 1 (ARIF-1) induces filopodia-like protrusions and invadosome-like structures in hemocytes, which play a critical role in attaching to the tissue surface, penetrating the tracheal BL and thus facilitating the transport of viral nucleocapsids into host tissues. Our findings clearly show the role of hemocyte infection in viral systemic spread and its molecular basis.

BV/ODV-E26 is a conserved baculoviral inhibitory factor for optimizing viral virulence in lepidopteran hosts

Alphabaculoviruses induce abnormal behavior in lepidopteran larval hosts. A baculoviral gene, bv/odv-e26, is crucial for behavioral manipulation in Bombyx mori larvae by Bombyx mori nucleopolyhedrovirus (BmNPV). However, how bv/odv-e26 fulfills its role in this phenotype remains largely unknown. In this study, we found that the overexpression of BmNPV bv/odv-e26 delayed viral infection in cultured cells and decreased pathogenicity in B. mori larvae. We also discovered that homologs of bv/odv-e26 are conserved more widely in alphabaculoviruses than previously thought. The inhibitory activity was demonstrated in bv/odv-e26 homologs of phylogenetically close and distant baculoviruses, indicating conserved inhibitory function among alphabaculoviruses. Furthermore, locomotory analyses revealed that bv/odv-e26 increased larval locomotory activity but had little effect on the timing of abnormal behavior initiation. Collectively, our findings demonstrate that bv/odv-e26 is a baculoviral inhibitory factor that is widely conserved in the genus alphabaculovirus and may reduce viral virulence for successful host behavioral manipulation.

CRISPR/Cas9- and Single-Stranded ODN-Mediated Knock-In in Silkworm Bombyx mori

Although genome editing techniques have made significant progress, introducing exogenous genes into the genome through knock-in remains a challenge in many organisms. In silkworm Bombyx mori, TALEN-mediated knock-in methods have been established. However, difficulties in construction and limitations of the target sequence have hindered the application of these methods. In the present study, we verified several CRISPR/Cas9-mediated knock-in methods to expand the application of gene knock-in techniques and found that the short single-stranded oligodeoxynucleotide (ssODN)-mediated method is the most effective in silkworms. Using ssODN-mediated methods, we established knock-in silkworm strains that harbor an attP sequence, a 50 bp phiC31 integrase recognition site, at either the BmHr38 (Hormone receptor 38) or Bmdsx (doublesex) locus. Additionally, we found that the long ssODN (lsODN)-mediated method successfully introduced the GAL4 gene at the doublesex locus in embryos. The present study provides valuable information on CRISPR/Cas9-mediated knock-in methods in silkworms, expanding the utility of genome editing techniques in insects and paving the way for analyzing gene and genome function in silkworms.

Differential expression of fibroin-related genes in middle silk glands is induced by dietary differences in a strain-dependent manner in Bombyx mori

The silkworm (Bombyx mori) is a model organism for lepidopteran insects. It is an oligophagous insect that primarily feeds on mulberry leaves and has industrial use for the production of raw silk. The development of artificial diets has provided an alternative nutrient source for silkworms; however, one significant issue is that the production of cocoons is lower in silkworms reared on artificial diets compared with those reared on mulberry leaves. The differences in the silk gland in the late-stage fifth instar silkworm larvae, when silk synthesis is most active, between those raised on artificial diets and mulberry leaves, are unknown. In this study, we identified differences in the transcriptomes of the middle and posterior silk glands of fifth instar day five silkworm larvae reared on artificial diets compared with those reared on mulberry leaves using three strains: Daizo, Nichi01, and J137 × C146. We found that the silk-related genes fibrohexamerin (fhx), fibroin-light-chain (fibL), and fibroin-heavy-chain (fibH) in the middle silk gland, and ser1 in the posterior silk gland, were differentially expressed in a strain-dependent manner. In silkworms reared on artificial diets, fhx, fibL, and fibH in the middle silk gland were upregulated in Nichi01 and downregulated in J137 × C146, whereas ser1 in the posterior silk gland was upregulated in J137 × C146 compared with silkworms reared on mulberry leaves. Our results demonstrate that the diet and strain of silkworm larvae affect the expression of genes related to silk production in their silk glands during the late fifth instar stage.

Time-course transcriptome data of silk glands in day 0-7 last-instar larvae of Bombyx mori (w1 pnd strain)

Time-course transcriptome expression data were constructed for four parts of the silk gland (anterior, middle, and posterior parts of the middle silk gland, along with the posterior silk gland) in the domestic silkworm, Bombyx mori, from days 0 to 7 of the last-instar larvae. For sample preparation, silk glands were extracted from one female and one male larva every 24 hours accurately after the fourth ecdysis. The reliability of these transcriptome data was confirmed by comparing the transcripts per million (TPM) values of the silk gene and quantitative reverse transcription PCR results. Hierarchical cluster analysis results supported the reliability of transcriptome data. These data are likely to contribute to the progress in molecular biology and genetic research using B. mori, such as elucidating the mechanism underlying the massive production of silk proteins, conducting entomological research using a meta-analysis as a model for lepidopteran insect species, and exploring medical research using B. mori as a model for disease species by utilising transcriptome data.

Codon Optimization-based Whole-gene Scanning Identifies Hidden Nucleotides Essential for Bombyx mori Nucleopolyhedrovirus polyhedrin Hyperexpression

During the late stage of infection, alphabaculoviruses produce many occlusion bodies (OBs) in the nuclei of the insect host's cells through the hyperexpression of polyhedrin (POLH), a major OB component encoded by polh. The strong polh promoter has been used to develop a baculovirus expression vector system for recombinant protein expression in cultured insect cells and larvae. However, the relationship between POLH accumulation and the polh coding sequence remains largely unelucidated. This study aimed to assess the importance of polh codon usage and/or nucleotide sequences in POLH accumulation by generating a baculovirus Bombyx mori nucleopolyhedrovirus (BmNPV) expressing mutant polh (co-polh) optimized according to the codon preference of its host insect. Although the deduced amino acid sequence of CO-POLH was the same as that of wild-type POLH, POLH accumulation was significantly lower in cells infected with the co-polh mutant. This reduction was due to decreased polh mRNA levels rather than translational repression. Analysis of mutant viruses with chimeric polh revealed that a 30 base-pair (bp) 5' proximal polh coding region was necessary for maintaining high polh mRNA levels. Sequence comparison of wild-type polh and co-polh identified five nucleotide differences in this region, indicating that these nucleotides were critical for polh hyperexpression. Furthermore, luciferase reporter assays showed that the 30 bp 5' coding region was sufficient for maintaining the polh promoter-driven high level of polh mRNA. Thus, our whole-gene scanning by codon optimization identified important hidden nucleotides for polh hyperexpression in alphabaculoviruses.

Complete genome sequence of a nucleopolyhedrovirus isolated from Bombyx mori in Hakozaki, Japan, obtained using Oxford Nanopore Technologies sequencing

I report the complete genome sequence of an isolate of nucleopolyhedrovirus from Bombyx mori, the domesticated silkworm, maintained for the long term at Kyushu University in Hakozaki, Fukuoka Prefecture, Japan. The genome is 127,783 bp long, with a G+C content of 40.3%, and contains 142 open reading frames.

The Development and Expansion of in vivo Germline Editing Technologies in Arthropods: Receptor-Mediated Ovary Transduction of Cargo (ReMOT Control) and Beyond

In the past 20 years, sequencing technologies have led to easy access to genomic data from nonmodel organisms in all biological realms. Insect genetic manipulation, however, continues to be a challenge due to various factors, including technical and cost-related issues. Traditional techniques such as microinjection of gene-editing vectors into early stage embryos have been used for arthropod transgenesis and the discovery of Clustered regularly interspaced short palindromic repeats and CRISPR-associated protein (CRISPR-Cas) technologies allowed for targeted mutagenesis and the creation of knockouts or knock-ins in arthropods. Receptor-Mediated Ovary Transduction of Cargo (ReMOT Control) acts as an alternative to embryonic microinjections, which require expensive equipment and extensive hands-on training. ReMOT Control's main advantage is its ease of use coupled with the ability to hypothetically target any vitellogenic species, as injections are administered to the egg-laying adult rather than embryos. After its initial application in the mosquito Aedes aegypti, ReMOT Control has successfully produced mutants not only for mosquitoes but for multiple arthropod species from diverse orders, such as ticks, mites, wasps, beetles, and true bugs, and is being extended to crustaceans, demonstrating the versatility of the technique. In this review, we discuss the current state of ReMOT Control from its proof-of-concept to the advances and challenges in the application across species after 5 years since its development, including novel extensions of the technique such as direct parental (DIPA)-CRISPR.

A seamless connection from the burst sequence to the start codon is essential for polyhedrin hyperexpression in alphabaculoviruses

Alphabaculoviruses produce a large number of occlusion bodies (OBs) in host cells during the late stage of infection. OBs are mainly composed of polyhedrin (POLH), and high-level transcription of the polh gene has been exploited to express foreign proteins in insect cells. While making Bombyx mori nucleopolyhedrovirus (BmNPV) polh mutants using a conventional transfer vector-based method, we noticed that a virus with a short sequence insertion just before the polh start codon produces fewer very small OBs. Detailed analysis of several BmNPV mutants revealed that insertions between the burst sequence and start codon markedly decrease POLH accumulation and polh transcription. We further confirmed this decrease using recombinant viruses expressing a reporter gene driven by the polh promoter. These findings underscore the critical importance of a seamless connection from the burst sequence to the start codon for baculovirus polh hyperexpression.

A method for screening the suppressor genes of siRNA and piRNA pathways using cultured silkworm cells

The BmN-4 cell line originates from the ovaries of silkworm, Bombyx mori , and possesses endogenous small interfering RNA (siRNA) and PIWI-interacting RNA (piRNA) pathways. BmN-4 cells are latently infected with Bombyx mori latent virus (BmLV), an RNA virus whose replication is strictly controlled by both siRNA and piRNA pathways. Knockdown or knockout of the core factors of these two small RNA pathways increases BmLV RNA amount, which in turn inhibits cell growth. Here, we used the known RNAi suppressor CrPV-1A to assess whether the BmN-4 cell line can be used for screening the suppressors of siRNA and piRNA pathways.

A lef5-deficient baculovirus expression system with no virion contamination and promoting secretion

The baculovirus expression system (BEVS) is widely used in biomanufacturing. However, massive late and very late genes are expressed during baculovirus infection of the host cell, and the protein aggregation has a significant negative impact on the intercellular environment of cells and organelle function. Baculovirus particle contamination also hinders the purification of particulate products. In this study, a novel baculovirus vector of deficient-lef5 (Ac-Δlef5) was established to prevent the production of baculovirus particles. It could transduce cells and replicate viral DNA but the expression of the late and very late genes was reduced so that no virus was packed and budded. The Ac-Δlef5 expression system was successfully used to express a secreted protein, thyroid peroxidase (TPO). Compared to wild-type baculovirus (Ac-wt), Ac-Δlef5 delayed the decline in cell viability and prolonged the harvest period from 4 to 6 days after infection. When expressed in the Ac-Δlef5 system, 98% of TPO was secreted extracellularly, which was about 1.8 times that of the Ac-wt system. Meanwhile, the transcription levels of protein folding-related genes were significantly increased. The results show that the Ac-Δlef5 system is a potential novel viral-free baculovirus expression system, which omits the virion removal operation in biomanufacturing.

Improvement of Modular Protein Display Efficiency in SpyTag-Implemented Norovirus-like Particles

Genetic fusion and chemical conjugation are the most common approaches for displaying a foreign protein on the surface of virus-like particles (VLPs); however, these methods may negatively affect the formation and stability of VLPs. Here, we aimed to develop a modular display platform for protein decoration on norovirus-like particles (NoV-LPs) by combining the NoV-LP scaffold with the SpyTag/SpyCatcher bioconjugation system, as the NoV-LP is an attractive protein nanoparticle to carry foreign proteins for various applications. The SpyTagged-NoV-LPs were prepared by introducing SpyTag peptide into the C-terminus of the norovirus VP1 protein. To increase surface exposure of the SpyTag peptide on the NoV-LPs, two or three repeated extension linkers (EAAAK) were inserted between the SpyTag peptide and VP1 protein. Fluorescence proteins, EGFP and mCherry, were fused to SpyCatcher and employed as SpyTag conjugation partners. These VP1-SpyTag variants and SpyCatcher-fused EGFP and mCherry were separately expressed in silkworm fat bodies and purified. This study reveals that adding an extension linker did not disrupt the VLP formation; instead, it increased the particle size by 4-6 nm. The conjugation efficiency of the VP1-SpyTag variants with the extended linker improved from ∼15-35 to ∼50-63% based on the densitometric analysis, while it was up to 77% based on an optical quantification of EGFP and mCherry. Results indicate that the linker causes the SpyTag peptides to be positioned further away from the C-termini of VP1 and potentially increases the exposure of the SpyTag to the outer surface of the NoV-LPs, allowing more SpyTag/SpyCatcher complex formation on the VLP surface. Our study provides a strategy for enhancing the conjugation efficiency of NoV-LP and demonstrates the platform's utility for developing vaccines or functional nanoparticles.

Display of multiple proteins on engineered canine parvovirus-like particles expressed in cultured silkworm cells and silkworm larvae

Recent progress has been made dramatically in decorating virus-like particles (VLPs) on the surface or inside with functional molecules, such as antigens or nucleic acids. However, it is still challenging to display multiple antigens on the surface of VLP to meet the requirement as a practical vaccine candidate. Herein this study, we focus on the expression and engineering of the capsid protein VP2 of canine parvovirus for VLP display in the silkworm-expression system. The chemistry of the SpyTag/SpyCatcher (SpT/SpC) and SnoopTag/SnoopCatcher (SnT/SnC) are efficient protein covalent ligation systems to modify VP2 genetically, where SpyTag/SnoopTag are inserted into the N-terminus or two distinct loop regions (Lx and L2) of VP2. The SpC-EGFP and SnC-mCherry are employed as model proteins to evaluate their binding and display on six SnT/SnC-modified VP2 variants. From a series of protein binding assays between indicated protein partners, we showed that the VP2 variant with SpT inserted at the L2 region significantly enhanced VLP display to 80% compared to 5.4% from N-terminal SpT-fused VP2-derived VLPs. In contrast, the VP2 variant with SpT at the Lx region failed to form VLPs. Moreover, the SpT (Lx)/SnT (L2) double-engineered chimeric VP2 variants showed covalent conjugation capacity to both SpC/SnC protein partners. The orthogonal ligations between those binding partners were confirmed by both mixing purified proteins and co-infecting cultured silkworm cells or larvae with desired recombinant viruses. Our results indicate that a convenient VLP display platform was successfully developed for multiple antigen displays on demand. Further verifications can be performed to assess its capacity for displaying desirable antigens and inducing a robust immune response to targeted pathogens.

Delivering on the promise of recombinant silk-inspired proteins for drug delivery

Effective drug delivery is essential for the success of a medical treatment. Polymeric drug delivery systems (DDSs) are preferred over systemic administration of drugs due to their protection capacity, directed release, and reduced side effects. Among the numerous polymer sources, silks and recombinant silks have drawn significant attention over the past decade as DDSs. Native silk is produced from a variety of organisms, which are then used as sources or guides of genetic material for heterologous expression or engineered designs. Recombinant silks bear the outstanding properties of natural silk, such as processability in aqueous solution, self-assembly, drug loading capacity, drug stabilization/protection, and degradability, while incorporating specific properties beneficial for their success as DDS, such as monodispersity and tailored physicochemical properties. Moreover, the on-demand inclusion of sequences that customize the DDS for the specific application enhances efficiency. Often, inclusion of a drug into a DDS is achieved by simple mixing or diffusion and stabilized by non-specific molecular interactions; however, these interactions can be improved by the incorporation of drug-binding peptide sequences. In this review we provide an overview of native sources for silks and silk sequences, as well as the design and formulation of recombinant silk biomaterials as drug delivery systems in a variety of formats, such as films, hydrogels, porous sponges, or particles.

Overexpression Bombyx mori HEXIM1 Facilitates Immune Escape of Bombyx mori Nucleopolyhedrovirus by Suppressing BmRelish-Driven Immune Responses

Bombyx mori nucleopolyhedrovirus (BmNPV), a typical arthropod-specific enveloped DNA virus, is one of the most serious pathogens in silkworm farming, but the potential mechanisms of the evasion of innate immune responses from BmNPV infection are still poorly understood. HEXIM1 is an RNA-binding protein, best known as an inhibitor of positive transcription elongation factor b (P-TEFb), which controls transcription elongation by RNA polymerase II. In this study, Bombyx mori HEXIM1 (BmHEXIM1) was cloned and characterized, and its expression was found to be remarkably upregulated after BmNPV infection. Furthermore, BmHEXIM1 was detected to increase the proliferation of BmNPV, and its full length is essential for assisting BmNPV immune escape by suppressing BmRelish-driven immune responses. This study brought new insights into the mechanisms of immune escape of BmNPV and provided theoretical guidance for the breeding of BmNPV-resistant silkworm varieties.

Insights into the quality of recombinant proteins produced by two different Bombyx mori expression systems

The silkworm, Bombyx mori, is an attractive host for recombinant protein production due to its high expression efficiency, quality, and quantity. Two expression systems have been widely used for recombinant protein production in B. mori: baculovirus/silkworm expression system and transgenic silkworm expression system. Both expression systems enable high protein production, but the qualities of the resulting recombinant proteins have not been well evaluated. In this study, we expressed bovine interferon γ (IFN-γ) using the two systems and examined the quality of the resulting proteins in terms of N-glycosylation and protein cleavage. Both expression systems successfully produced IFN-γ as an N-glycoprotein. Although the production in the baculovirus/silkworm expression system was much more efficient than that in the transgenic silkworm expression system, unexpected variants of IFN-γ were also produced in the former system due to the different N-glycosylation and C-terminal truncations. These results indicate that while high protein production could be achieved in the baculovirus/silkworm expression system, unintentional protein modification might occur, and therefore protein expression in the transgenic silkworm expression system is preferable from the point-of-view of N-glycosylation of the recombinant protein and evasion of unexpected attack by a protease in B. mori.

A database of crop pest cell lines

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

Mutations in the polyhedrin NLS affect the assembly and polyhedral shape of alphabaculovirus occlusion bodies

Alphabaculoviruses produce occlusion bodies (OBs) in the nucleus of the infected cells at the late stage of infection. OBs are mainly composed of a single viral protein called polyhedrin (POLH). Autographa californica multiple nucleopolyhedrovirus (AcMNPV) POLH possesses a monopartite nuclear localization signal sequence (NLS), KRKK, from 32nd to 35th residues. However, the functions of POLH NLS of other alphabaculoviruses remain unknown. Here, POLH NLS mutants of Bombyx mori nucleopolyhedrovirus (BmNPV) were generated and NLS function as well as the relationship between NLS and OB localization or morphology was investigated. Deletion or mutation of BmNPV POLH NLS severely affected POLH and OB intracellular localization. Additionally, viruses in which the arginine residue at the 33rd position of POLH was mutated produced a lower number of OBs, which was presumably due to decreased POLH accumulation in the infected cells. Furthermore, cytoplasmic OBs were morphologically aberrant, even though nuclear OB morphology was normal in the same cell. These results indicate that NLS is required for nuclear localization and efficient accumulation of BmNPV POLH, which heavily affect the number and morphology of OBs.

In vivo enzymatic digestion of HRV 3C protease cleavage sites-containing proteins produced in a silkworm-baculovirus expression system

Baculovirus expression vector system (BEVS) has been recognized as a potent protein expression system in engineering valuable enzymes and vaccines. Various fusion tags facilitate protein purification, leaving the potential risk to influence the target protein's biological activity negatively. It is of great interest to consider removing the additional tags using site-specific proteases, such as human rhinoviruses (HRV) 3C protease. The current study validated the cleavage activity of 3C protease in Escherichia coli and silkworm-BEVS systems by mixing the cell or fat body lysates of 3C protein and 3C site containing target protein in vitro. Further verification has been performed in the fat body lysate from co-expression of both constructs, showing remarkable cleavage efficiency in vivo silkworm larvae. We also achieved the glutathione-S-transferase (GST) tag-cleaved product of the VP15 protein from the White spot syndrome virus after purification, suggesting that we successfully established a coinfection-based recognition-and-reaction BEVS platform for the tag-free protein engineering.

Bibliometric Analysis of Trends in Mulberry and Silkworm Research on the Production of Silk and Its By-Products

Simple Summary

Over the past two decades scientific research on sericulture, the agricultural activity of silk production, generated a great number of outputs in the form of articles reported and classified by one of the most well-known and used database dealing with scientific literature. This occurrence demonstrates an increasing interest in this sector especially starting from 2000s; results presented in relevant papers showed their applicability even in fields apparently not related to silk production as commonly meant, like medicine, cosmetics, and engineering. To understand how sericulture has been transcending its usual boundaries, which are its current “hotspots”, and links with other fields of study, the authors propose a text-mining based analysis of the outputs of scientific research on sericulture and silk; the final goal is to establish “quantitative” indicators for researchers, entrepreneurs, and scholars.

Abstract

Traditionally, sericulture is meant as the agricultural activity of silk production, from mulberry (Morus sp.pl.) cultivation to silkworm (Bombyx mori L.) rearing. The aim of the present work is to analyze the trends and outputs of scientific research on sericulture-related topics during the last two decades, from 2000 to 2020. In this work the authors propose a text-mining analysis of the titles, abstracts and keywords of scientific articles focused on sericulture and available in the SCOPUS database considering the above-mentioned period of time; from this article collection, the 100 most recurrent terms were extracted and studied in detail. The number of publications per year in sericulture-related topics increased from 87 in 2000 to 363 in 2020 (+317%). The 100 most recurrent terms were then aggregated in clusters. The analysis shows how in the last period scientific research, besides the traditional themes of sericulture, also focused on alternative products obtainable from the sericultural practice, as fruits of mulberry trees (increment of +134% of the occurrences in the last five years) and chemical compounds as antioxidants (+233% of occurrences), phenolics (+330% of occurrences) and flavonoids (+274% of occurrences). From these considerations, the authors can state how sericulture is an active and multidisciplinary research field.

SilkBase: an integrated transcriptomic and genomic database for Bombyx mori and related species

We introduce SilkBase as an integrated database for transcriptomic and genomic resources of the domesticated silkworm Bombyx mori and related species. SilkBase is the oldest B. mori database that was originally established as the expressed sequence tag database since 1999. Here, we upgraded the database by including the datasets of the newly assembled B. mori complete genome sequence, predicted gene models, bacterial artificial chromosome (BAC)-end and fosmid-end sequences, complementary DNA (cDNA) reads from 69 libraries, RNA-seq data from 10 libraries, PIWI-interacting RNAs (piRNAs) from 13 libraries, ChIP-seq data of 9 histone modifications and HP1 proteins and transcriptome and/or genome data of four B. mori-related species, i.e. Bombyx mandarina, Trilocha varians, Ernolatia moorei and Samia ricini. Our new integrated genome browser easily provides a snapshot of tissue- and stage-specific gene expression, alternative splicing, production of piRNAs and histone modifications at the gene locus of interest. Moreover, SilkBase is useful for performing comparative studies among five closely related lepidopteran insects.

Database URL: https://silkbase.ab.a.u-tokyo.ac.jp

Sirt5 Inhibits BmNPV Replication by Promoting a Relish-Mediated Antiviral Pathway in Bombyx mori

Silent information regulators (Sirtuins) belong to the family of nicotinamide adenine dinucleotide (NAD⁺)-dependent histone deacetylases (HDACs) that have diverse functions in cells. Mammalian Sirtuins have seven isoforms (Sirt1–7) which have been found to play a role in viral replication. However, Sirtuin members of insects are very different from mammals, and the function of insect Sirtuins in regulating virus replication is unclear. The silkworm, Bombyx mori, as a model species of Lepidoptera, is also an important economical insect. B. mori nucleopolyhedrovirus (BmNPV) is a major pathogen that specifically infects silkworms and causes serious losses in the sericulture industry. Here, we used the infection of the silkworm by BmNPV as a model to explore the effect of Sirtuins on virus replication. We initially knocked down all silkworm Sirtuins, and then infected with BmNPV to analyze its replication. Sirt2 and Sirt5 were found to have potential antiviral functions in the silkworm. We further confirmed the antiviral function of silkworm Sirt5 through its effects on viral titers during both knockdown and overexpression experiments. Additionally, Suramin, a Sirt5 inhibitor, was found to promote BmNPV replication. In terms of molecular mechanism, it was found that silkworm Sirt5 might promote the immune pathway mediated by Relish, thereby enhancing the host antiviral response. This study is the first to explore the role of Sirtuins in insect-virus interactions, providing new insights into the functional role of members of the insect Sirtuin family.

Establishment of a Novel Baculovirus–Silkworm Expression System

The baculovirus vector expression system is a well-established tool for foreign protein production and gene delivery. In this study, we constructed a recombinant baculovirus vector system. The UAS promotor region and Bombyx mori nucleopolyhedrovirus (BmNPV) polyhedrin coding region were ligated into a pFastBac Dual vector to obtain a BmBac-UPS recombinant bacmid. The recombinant bacmid BmBac-Gal4 was generated by the same strategy which has a Gal4 coding region controlled by the IE2 promoter. BmBac-UPS and BmBac-IGal4 were co-infected into silkworm BmN cells to confirm the ability of the UAS/Gal4 system to form polyhedrons in B. mori cells. Furthermore, the recombinant viruses were tested for infection efficiency and the ability to generate polyhedra in transgenic B. mori cell line BmE. The results showed that recombinant viruses have the ability to form polyhedrons and gain raised pathogenicity when orally infected B. mori larvae and are applied as the preferred tool for foreign gene delivery and expression

Current status, and the developments of hosts and expression systems for the production of recombinant human cytokines

Cytokines consist of peptides, proteins and glycoproteins, which are biological signaling molecules, and boost cell-cell communication in immune reactions to stimulate cellular movements in the place of trauma, inflammation and infection. Recombinant cytokines are designed in such a way that they have generalized immunostimulation action or stimulate specific immune cells when the body encounters immunosuppressive signals from exogenous pathogens or other tumor microenvironments. Recombinant cytokines have improved the treatment processes for numerous diseases. They are also beneficial against novel toxicities that arise due to pharmacologic immunostimulators that lead to an imbalance in the regulation of cytokine. So, the production and use of recombinant human cytokines as therapeutic proteins are significant for medical treatment purposes. For the improved production of recombinant human cytokines, the development of host cells such as bacteria, yeast, fungi, insect, mammal and transgenic plants, and the specific expression systems for individual hosts is necessary. The recent advancements in the field of genetic engineering are beneficial for easy and efficient genetic manipulations for hosts as well as expression cassettes. The use of metabolic engineering and systems biology approaches have tremendous applications in recombinant protein production by generating mathematical models, and analyzing complex biological networks and metabolic pathways via simulations to understand the interconnections between metabolites and genetic behaviors. Further, the bioprocess developments and the optimization of cell culture conditions would enhance recombinant cytokines productivity on large scales.

Host-Dependent Producibility of Recombinant Cypridina noctiluca Luciferase With Glycosylation Defects

Cypridina noctiluca luciferase (CLuc) is a secreted luminescent protein that reacts with its substrate (Cypridina luciferin) to emit light. CLuc is known to be a thermostable protein and has been used for various research applications, including in vivo imaging and high-throughput reporter assays. Previously, we produced a large amount of recombinant CLuc for crystallographic analysis. However, this recombinant protein did not crystallize, probably due to heterogeneous N-glycan modifications. In this study, we produced recombinant CLuc without glycan modifications by introducing mutations at the N-glycan modification residues using mammalian Expi293F cells, silkworms, and tobacco Bright Yellow-2 cells. Interestingly, recombinant CLuc production depended heavily on the expression hosts. Among these selected hosts, we found that Expi293F cells efficiently produced the recombinant mutant CLuc without significant effects on its luciferase activity. We confirmed the lack of N-glycan modifications for this mutant protein by mass spectrometry analysis but found slight O-glycan modifications that we estimated were about 2% of the ion chromatogram peak area for the detected peptide fragments. Moreover, by using CLuc deletion mutants during the investigation of O-glycan modifications, we identified amino acid residues important to the luciferase activity of CLuc. Our results provide invaluable information related to CLuc function and pave the way for its crystallographic analysis.

Bioremediation of Industrial Pollutants by Insects Expressing a Fungal Laccase

Inadequate management of household and industrial wastes poses major challenges to human and environmental health. Advances in synthetic biology may help address these challenges by engineering biological systems to perform new functions such as biomanufacturing of high-value compounds from low-value waste streams and bioremediation of industrial pollutants. The current emphasis on microbial systems for biomanufacturing, which often requires highly preprocessed inputs and sophisticated infrastructure, is not feasible for many waste streams. Furthermore, concerns about transgene biocontainment have limited the release of engineered microbes or plants for bioremediation. Engineering of animals may provide opportunities for utilizing various waste streams that are not suitable for microbial biomanufacturing while effective transgene biocontainment options should enable in situ bioremediation. Here, we engineer the model insect Drosophila melanogaster to express a functional laccase from the fungus Trametes trogii. Laccase-expressing flies reduced concentrations of the endocrine disruptor bisphenol A by more than 50% when present in their growth media. A lyophilized powder prepared from engineered adult flies retained substantial enzymatic activity, degrading more than 90% of bisphenol A and the textile dye indigo carmine in aqueous solutions. Our results demonstrate that transgenic animals may be used to bioremediate environmental contaminants in vivo and serve as novel production platforms for industrial enzymes. These results support further development of insects, and possibly other animals, as bioproduction platforms and their potential use in bioremediation.

Molecular mechanism responsible for the hyperexpression of baculovirus polyhedrin

One of the amazing phenomena in the baculovirus life cycle is the hyperexpression of the very late gene, polyhedrin (polh), causing the production of the occlusion bodies where progeny virions are embedded. However, to date, the molecular mechanism underlying its hyperexpression is not completely elucidated. Considering that, in this review, the mechanism responsible for its hyperexpression from the previous studies up to now was comprehensively summarized from three aspects, namely, the structure characteristics of the polh promoter and transcription regulation, the structure and translation regulation of the polh mRNA, and especially the regulators that influence the expression of polh gene. Moreover, this review will help us obtain a better understanding about the hyperexpression of polh, and also provide guidance for improving the expression efficiency of the foreign proteins by adopting the baculovirus expression vector system.

Platforms for Production of Protein-Based Vaccines: From Classical to Next-Generation Strategies

To date, vaccination has become one of the most effective strategies to control and reduce infectious diseases, preventing millions of deaths worldwide. The earliest vaccines were developed as live-attenuated or inactivated pathogens, and, although they still represent the most extended human vaccine types, they also face some issues, such as the potential to revert to a pathogenic form of live-attenuated formulations or the weaker immune response associated with inactivated vaccines. Advances in genetic engineering have enabled improvements in vaccine design and strategies, such as recombinant subunit vaccines, have emerged, expanding the number of diseases that can be prevented. Moreover, antigen display systems such as VLPs or those designed by nanotechnology have improved the efficacy of subunit vaccines. Platforms for the production of recombinant vaccines have also evolved from the first hosts, Escherichia coli and Saccharomyces cerevisiae, to insect or mammalian cells. Traditional bacterial and yeast systems have been improved by engineering and new systems based on plants or insect larvae have emerged as alternative, low-cost platforms. Vaccine development is still time-consuming and costly, and alternative systems that can offer cost-effective and faster processes are demanding to address infectious diseases that still do not have a treatment and to face possible future pandemics.

H3K4me3 histone modification in baculovirus-infected silkworm cells

Baculovirus infection modulates the chromatin states and gene expression of host insect cells. Here we performed chromatin immunoprecipitation followed by deep sequencing (ChIP-seq) of H3 trimethylated at Lys4 (H3K4me3) histone modification in Bombyx mori nucleopolyhedrovirus-infected Bombyx mori cells. The ChIP-seq data revealed the changes of the genome-wide distribution and accumulation of euchromatic histone marks in host insect cells during the progression of baculovirus infection.

Active Human and Murine Tumor Necrosis Factor α Cytokines Produced from Silkworm Baculovirus Expression System

The tumor necrosis factor α (TNFα) has been employed as a promising reagent in treating autoimmunity and cancer diseases. To meet the substantial requirement of TNFα proteins, we report in this study that mature types of recombinant human and murine TNFα proteins are successfully expressed in the baculovirus expression system using silkworm larvae as hosts. The biological activities of purified products were verified in culture murine L929 cells, showing better performance over a commercial Escherichia coli-derived murine TNFα. By comparing the activity of purified TNFα with or without the tag removal, it is also concluded that the overall activity of purified TNFα cytokines could be further improved by the complete removal of C-terminal fusion tags. Collectively, our current attempt demonstrates an alternative platform for supplying high-quality TNFα products with excellent activities for further pharmaceutical and clinical trials.

Bombyx mori nucleopolyhedrovirus ptp and egt genes are dispensable for triggering enhanced locomotory activity and climbing behavior in Bombyx mandarina larvae

Baculoviruses are classic pathogens that alter host behavior to enhance their dispersal and transmission. While viral protein tyrosine phosphatase (ptp) has been considered as a critical factor for inducing enhanced locomotory activity, preceding investigations have reported that viral ecdysteroid UDP-glucosyltransferase (egt) contributes to triggering climbing behavior in some virus and host species. Here we found that both egt and ptp were dispensable for these abnormal behaviors in Bombyx mandarina larvae induced by Bombyx mori nucleopolyhedrovirus, thus implying that there is an unknown core mechanism of baculovirus-induced alteration of host behaviors.

Bombyx mori Pupae Efficiently Produce Recombinant AAV2/HBoV1 Vectors with a Bombyx mori Nuclear Polyhedrosis Virus Expression System

Recombinant adeno-associated virus (AAV) vectors have broad application prospects in the field of gene therapy. The establishment of low-cost and large-scale manufacturing is now the general agenda for industry. The baculovirus-insect cell/larva expression system has great potential for these applications due to its scalability and predictable biosafety. To establish a more efficient production system, Bombyx mori pupae were used as a new platform and infected with recombinant Bombyx mori nuclear polyhedrosis virus (BmNPV). The production of a chimeric recombinant adeno-associated virus (rAAV) serotype 2/human bocavirus type-1 (HBoV1) vector was used to evaluate the efficiency of this new baculovirus expression vector (BEV)–insect expression system. For this purpose, we constructed two recombinant BmNPVs, which were named rBmNPV/AAV2Rep-HBoV1Cap and rBmNPV/AAV2ITR-eGFP. The yields of rAAV2/HBoV1 derived from the rBmNPV/AAV2Rep-HBoV1Cap and rBmNPV/AAV2ITR-eGFP co-infected BmN cells exceeded 2 × 10⁴ vector genomes (VG) per cell. The rBmNPV/AAV2Rep-HBoV1Cap and rBmNPV/AAV2ITR-eGFP can express stably for at least five passages. Significantly, rAAV2/HBoV1 could be efficiently generated from BmNPV-infected silkworm larvae and pupae at average yields of 2.52 × 10¹² VG/larva and 4.6 × 10¹² VG/pupa, respectively. However, the vectors produced from the larvae and pupae had a high percentage of empty particles, which suggests that further optimization is required for this platform in the future. Our work shows that silkworm pupae, as an efficient bioreactor, have great potential for application in the production of gene therapy vectors.

Loss of p24 from the Bombyx mori nucleopolyhedrovirus genome results in the formation of cuboidal occlusion bodies

Some insect viruses produce the occlusion body (OB), a large crystalline particle comprising a viral protein that occludes virions to protect them from harsh environments. The shapes and sizes of OBs are diverse depending on baculovirus species, but the detailed molecular mechanism determining them has yet to be totally clarified yet. Here we generated Bombyx mori nucleopolyhedrovirus (BmNPV) mutants of the p24 gene that encodes a viral capsid protein and found that p24-mutated BmNPVs produced cuboidal OBs with a slightly larger size than typical truncated octahedral OBs produced by wild-type BmNPVs. Meanwhile, p24 disruption has no significant impact on progeny virus production and viral pathogenicity. In addition, we experimentally demonstrated that a single amino acid substitution found in the P24 protein of the BmNPV Cubic isolate caused cuboidal OB production. These results suggest that p24 has a crucial role in generating the typical shape of OBs.

CRISPR/Cas9-mediated mutagenesis of Ago2 and Siwi in silkworm cultured cells

The BmN-4 cell line, originated from the silkworm Bombyx mori ovary, possesses endogenous small interfering RNA (siRNA) and PIWI-interacting RNA (piRNA) pathways. We performed CRISPR/Cas9-mediated genome editing of Ago2 and Siwi, which are the core factors for siRNA and piRNA pathways, respectively, to understand the importance of the two distinct small RNA pathways in this cell line. We found that approximately half of the alleles contained loss-of-function mutations in both Ago2- and Siwi-mutated cells. The mutated cells grew at a slower rate compared to the control cells, strongly suggesting that the siRNA and piRNA pathways are both crucial for the normal growth of BmN-4 cells. The amounts of piRNAs decreased markedly in the Siwi-mutated cells, but global de-repression of transposable elements was not observed. Although the RNA amount of latently infected RNA virus, Bombyx mori macula-like virus (BmLV), increased in both Ago2- and Siwi-mutated cells, the siRNA and piRNA pathways showed a bias toward targeting BmLV genomic and subgenomic RNA, respectively. These results indicate the common, specific, and crucial roles of the two small RNA pathways in B. mori cultured cells.

Hsp90 function is required for stable transcription of the baculovirus transactivator ie-1 gene

A molecular chaperone heat shock protein 90 (Hsp90) is required for efficient infection by several viruses. Hsp90 has been recently implicated in baculovirus infection, but its exact role remains obscure. This study investigated the effect of 17-N-allylamino-17-demethoxygeldanamycin (17-AAG), an Hsp90-specific inhibitor, on Bombyx mori nucleopolyhedrovirus (BmNPV) infection. The 17-AAG treatment significantly decreased the production of budded viruses and occlusion bodies in BmNPV-infected Bombyx mori cultured cells. Immunoblot and SDS-PAGE analyses showed that the expression of early and delayed early gene products, DBP and BRO, was delayed and dysregulated, and the very late gene product POLH was almost completely diminished. RT-qPCR experiments revealed that 17-AAG treatment did not affect initiation of the immediate early gene ie-1 expression, but the expression decreased by ∼50 % during the late stage of infection. 17-AAG treatment also decreased ie-1 promoter activity by ∼50 %. In addition, the expression of delayed early and late genes was dysregulated and inhibited, respectively. These results indicated that Hsp90 function is required for stable ie-1 transcription. Inhibiting Hsp90 function negatively affects ie-1 expression, resulting in dysregulation of delayed early genes and a severe decrease in late and very late gene expression.

High-resolution analysis of baculovirus-induced host manipulation in the domestic silkworm, Bombyx mori

Many parasites manipulate host behaviour to enhance their transmission. Baculoviruses induce enhanced locomotory activity (ELA) combined with subsequent climbing behaviour in lepidopteran larvae, which facilitates viral dispersal. However, the mechanisms underlying host manipulation system are largely unknown. Previously, larval locomotion during ELA was summarized as the distance travelled for a few minutes at several time points, which are unlikely to characterize ELA precisely, as ELA typically persists for several hours. In this study, we modified a recently developed method using time-lapse recording to characterize locomotion of Bombyx mori larvae infected with B. mori nucleopolyhedrovirus (BmNPV) for 24 h at 3 s resolution. Our data showed that the locomotion of the mock-infected larvae was restricted to a small area, whereas the BmNPV-infected larvae exhibited a large locomotory area. These results indicate that BmNPV dysregulates the locomotory pattern of host larvae. Furthermore, both the mock- and BmNPV-infected larvae showed periodic cycles of movement and stationary behaviour with a similar frequency, suggesting the physiological mechanisms that induce locomotion are unaffected by BmNPV infection. In contrast, the BmNPV-infected larvae exhibited fast and long-lasting locomotion compared with mock-infected larvae, which indicates that locomotory speed and duration are manipulated by BmNPV.

Bombyx mori nucleopolyhedrovirus Bm46 is essential for efficient production of infectious BV and nucleocapsid morphogenesis

Bombyx mori nucleopolyhedrovirus (BmNPV) orf46 (Bm46), the orthologues of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ac57, is a highly conserved gene in group Ⅰ and group Ⅱ nucleopolyhedroviruses (NPVs). However, its function in viral life cycle is unclear. Our results indicated that Bm46 transcript was detected from infected cells at 12 h post infection, while Bm46 protein was detectable from 24 to 72 h post infection. Upon the deletion of Bm46, fewer infectious BVs were produced by titer assays, but neither viral DNA synthesis nor occlusion bodies (OBs) production was affected. Electron microscopy revealed that Bm46 knockout interrupted nucleocapsid assembly and occlusion-derived virus (ODV) embedding, resulting in aberrant capsid-like tubular structures accumulated in the RZ (ring zone). Interestingly, this abnormally elongated capsid structures were consistent with the immunofluorescence microscopy results showing that VP39 assembled into long filaments and cables in the RZ. Moreover, DNA copies decreased by 30 % in occlusion bodies (OBs) produced by Bm46-knockout virus. qRT-PCR and Western blot analysis showed that the expression of VP39 was affected by Bm46 disruption. Taken together, our findings clearly pointed out that Bm46 played an important role in BV production and the proper formation of nucleocapsid morphogenesis.

Construction of the Antheraea pernyi (Lepidoptera: Saturniidae) Multicapsid Nucleopolyhedrovirus Bacmid System

In this study, we established the Antheraea pernyi multicapsid nucleopolyhedrovirus (AnpeNPV) bacmid system for the construction of a Bac-to-Bac expression system and the generation of virus mutants. The CopyRight pSMART BAC cloning vector harboring the chloramphenicol resistance gene was introduced into the AnpeNPV genome to produce the AnpeNPV bacmid that could be propagated in Escherichia coli with stable replication. The enhanced green fluorescent protein (EGFP) was successfully expressed in both Tn-Hi5 cells and A. pernyi pupae using the AnpeNPV Bac-to-Bac expression system. To generate the AnpeNPV mutants, we developed the AnpeNPV bacmid/λ Red recombination system that facilitated the deletion of viral genes from the AnpeNPV genome. The genes cathepsin and chitinase were deleted and a derivative AnpeNPV Bac-to-Bac expression system was constructed. Furthermore, we demonstrated that the novel expression system could be used to express human epidermal growth factor in A. pernyi pupae. Taken together, the AnpeNPV bacmid system provides a powerful tool to create the AnpeNPV Bac-to-Bac expression system for protein expression in A. pernyi pupae. Further, it helps to knock-out genes from the AnpeNPV genome with λ Red recombination system for identification of the role of viral genes involved in regulating gene expression, DNA replication, virion structure, and infectivity during the AnpeNPV infection process.

Expression of recombinant apopholasin using a baculovirus-silkworm multigene expression system and activation via dehydrocoelenterazine

Pholasin is a photoprotein derived from the glowing bivalve mollusk, Pholas dactylus. Even though the chemical structure of the prosthetic group (chromophore) responsible for the light emission character of the mollusk remains unknown, research has shown that the presence of dehydrocoelenterazine (DCL) increased light emission and that the dithiothreitol adduct of DCL was isolated from Pholasin®. To date, our research has been focused on activating apopholasin, the naturally occurring apoprotein of Pholasin®, using DCL. In the current study, the expression of recombinant apopholasin via a baculovirus-silkworm multigene expression system is reported. Additionally, the purification of apopholasin using a Flag®-affinity column, the activation of apopholasin using DCL, and the initiation of its luminescent character through the addition of a peroxidase-hydrogen peroxide mixture are reported. The peroxidase-H₂O₂-dependent luminescence was observed from the recombinant apopholasin activated with DCL.

Bombyx mori nucleopolyhedrovirus Bm96 suppresses viral virulence in Bombyx mori larvae

Bombyx mori nucleopolyhedrovirus (BmNPV) is a severe pathogen for the domestic silkworm, Bombyx mori. BmNPV harbors over 140 protein-coding genes in its 128.4 kilobase pair-long double-stranded genome. However, many BmNPV genes are still uncharacterized. Here we investigated the role of BmNPV Bm96 in both B. mori cultured cells and larvae. We found that Bm96 is mainly expressed at the late stage of infection and accumulation of Bm96 protein peaks at 24 h post infection (hpi) and declines gradually at 48 hpi in B. mori cultured cells. Compared with the wild-type viruses, Bm96-deletion viruses exhibited higher viral propagation and fast-killing phenotype in B. mori larvae. These results strongly suggest that Bm96 negatively regulates the propagation of BmNPV in B. mori larvae. Furthermore, we observed that larvae infected with Bm96-deletion viruses showed lower locomotory activity at the late stage of infection compared with those infected with the wild-type viruses.

Functional binding of E-selectin to its ligands is enhanced by structural features beyond its lectin domain

Selectins are key to mediating interactions involved in cellular adhesion and migration, underlying processes such as immune responses, metastasis, and transplantation. Selectins are composed of a lectin domain, an epidermal growth factor (EGF)-like domain, multiple short consensus repeats (SCRs), a transmembrane domain, and a cytoplasmic tail. It is well-established that the lectin and EGF domains are required to mediate interactions with ligands; however, the contributions of the other domains in mediating these interactions remain obscure. Using various E-selectin constructs produced in a newly developed silkworm-based expression system and several assays performed under both static and physiological flow conditions, including flow cytometry, glycan array analysis, surface plasmon resonance, and cell-rolling assays, we show here that a reduction in the number of SCR domains is correlated with a decline in functional E-selectin binding to hematopoietic cell E- and/or L-selectin ligand (HCELL) and P-selectin glycoprotein ligand-1 (PSGL-1). Moreover, the binding was significantly improved through E-selectin dimerization and by a substitution (A28H) that mimics an extended conformation of the lectin and EGF domains. Analyses of the association and dissociation rates indicated that the SCR domains, conformational extension, and dimerization collectively contribute to the association rate of E-selectin-ligand binding, whereas just the lectin and EGF domains contribute to the dissociation rate. These findings provide the first evidence of the critical role of the association rate in functional E-selectin-ligand interactions, and they highlight that the SCR domains have an important role that goes beyond the structural extension of the lectin and EGF domains.

Whole-genome sequencing and comparative transcriptome analysis of Bombyx mori nucleopolyhedrovirus La strain

The Bombyx mori nucleopolyhedrovirus (BmNPV) La is a variant BmNPV strain isolated in Laos. La has different features from BmNPV type strain T3 in virulence, production of the polyhedrin protein, and the formation of multicapsid occlusion-derived viruses. Here, the whole-genome sequence of La was compared to the sequences of nine BmNPV and two Bombyx mandarina nucleopolyhedrovirus strains. The complete La genome consisted of 127,618 base pairs with a G + C content of 40.3% and contained putative 136 open reading frames encoding more than 60 amino acids. The La genome lacked the bro-b gene and had the highest identity with that of the T3 strain. A comparison of the transcriptomes of La- and T3-infected cells showed that the expression levels of the polyhedrin and cathepsin genes were greater in cells infected with La as compared to those infected with T3. Interestingly, the virus genes with different RNA levels between the two BmNPV strains were assembled into five clusters in the genome of La. Also, the RNA levels of host ribosomal protein genes were significantly decreased in cells infected with La as compared to those infected with T3.

Construction of a Baculovirus Derivative to Produce Linearized Antheraea pernyi (Lepidoptera: Saturniidae) Multicapsid Nucleopolyhedrovirus Genomic DNA

In the Antheraea pernyi multicapsid nucleopolyhedrovirus (AnpeNPV)-based expression vector system, the frequency of homologous recombination events between wild-type AnpeNPV DNA and the transfer vector is low, resulting in a small amount of recombinant virus. Previous reports have indicated that linearized baculovirus DNA can increase the proportion of recombinant virus relative to the total progeny. To improve the recombination efficiency, we constructed a linearized derivative of AnpeNPV, referred to as AnpeNPVPhEGFP-AvrII, in which egfp flanked by AvrII restriction sites was located at the polyhedrin locus and driven by the polyhedrin promoter. Linear AnpeNPV DNA was obtained by the treatment of AnpeNPVPhEGFP-AvrII genomic DNA with AvrII endonuclease. The infectivity and recombinogenic activity between the linearized and circular viral DNA were evaluated by quantitative real-time polymerase chain reactions. We demonstrated that the linearized AnpeNPV DNA produced only small numbers of infectious budded viruses, accounting for approximately 4.5% of the budded virus production of wild-type AnpeNPV DNA in A. pernyi pupae. However, the linearized AnpeNPV DNA substantially increased recombinant virus production after cotransfection with an appropriate transfer vector; relative abundance of the recombinant virus was approximately 5.5-fold higher than that of the wild-type AnpeNPV DNA in A. pernyi pupae. The linearization of AnpeNPV DNA will facilitate the purification of recombinant viruses using the AnpeNPV-based expression vector system and the construction of an AnpeNPV-based bacmid system.

A construction strategy for a baculovirus-silkworm multigene expression system and its application for coexpression of type I and type II interferons

The Bombyx mori nucleopolyhedrovirus (BmNPV) baculovirus expression system (BES) is a eukaryotic expression system. It possesses great capability for post‐translation modification in expression of foreign proteins. With the counterselection cassette rpsL‐neo and phage λ‐Red recombinase, the defective‐rescue BmNPV BES reBmBac can be employed for efficient heterologous multigene coexpression at different gene sites in one baculovirus genome. In the present study, a recombinant baculovirus, reBm‐Cαγ, carrying two types of chicken interferon (IFN) genes (chIFN‐α and chIFN‐γ) was constructed using the reBmBac system. The chIFN‐α and chIFN‐γ genes were inserted into the same baculovirus genome at the polyhedron and p10 gene sites, respectively. The recombinant baculovirus was capable of coexpressing both chIFN‐α and chIFN‐γ. The expression levels of the two types of IFN in the coexpression product were exponentially high, at approximately 1.7 and 2.5 times higher, respectively, than those in the corresponding single‐expression products. The increase in expression level corresponds to replacement of the nonessential p10 gene in the reBm‐Cαγ recombinant baculovirus. This coexpression of recombinant chicken IFNs showed superior antiviral activity.

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.

Chichen type III interferon produced by silkworm bioreactor induces ISG expression and restricts ALV-J infection in vitro

Type III interferon (IFN-λ) has recently been shown to exert a significant antiviral impact against viruses in vertebrates. Avian leukosis virus subgroup J (ALV-J), which causes tumor disease and immunosuppression in infected chicken, is a retrovirus that is difficult to prevent and control because of a lack of vaccines and drugs. Here, we obtained chicken IFN-λ (chIFN-λ) using a silkworm bioreactor and demonstrated that chIFN-λ has antiviral activity against ALV-J infection of both chicken embryo fibroblast cell line (DF1) and epithelial cell line (LMH). We found that chIFN-λ triggered higher levels of particular type III interferon-stimulated genes (type III ISGs) including myxovirus resistance protein (Mx), viperin (RSAD2), and interferon-inducible transmembrane protein 3 (IFITM3) in DF1 and LMH cells. Furthermore, over-expression of Mx, viperin, and IFITM3 could inhibit ALV-J infection in DF1 and LMH cells. Therefore, these results suggested that the anti-ALV-J function of chIFN-λ was specifically implemented by induction of expression of type III ISGs. Our data identified chIFN-λ as a critical antiviral agent of ALV-J infection and provides a potentially and attractive platform for the production of commercial chIFN-λ.

Preincubation with a low concentration of methyl-β-cyclodextrin enhances baculovirus expression system productivity

OBJECTIVES

To enhance the productivity of foreign protein in culture cells using baculovirus expression system.

RESULTS

A low concentration of MβCD, with the optimal application concentration of 0.25 mM and the appropriate preincubation time range from 10 to 120 min, can efficiently enhance expression levels in both the AcMNPV and BmNPV expression systems.

CONCLUSIONS

Preincubation with a low concentration MβCD enhance baculovirus infection and foreign protein expression productivity.