Gene targeting in the silkworm by use of a baculovirus

Identification and Characterization of Genes Related to Resistance of Autographa californica Nucleopolyhedrovirus Infection in Bombyx mori

Simple Summary

Autographa californica nucleopolyhedrovirus (AcMNPV) is a kind of baculovirus that was initially found and named for its host, but the previous study reveals several silkworm strains are preferentially susceptible to AcMNPV through intrahemocelical injection method. In the following study, genetics analysis showed that a set of potential genes which controlled resistance of AcMNPV was located on chromosome 3. In the present research, we performed Genome-Wide Association Studies to identify the gene that controls the resistance of AcMNPV, results show that the Niemann-Pick C1 (NPC-1) gene is strongly associated with this resistance. Then we found that there are several amino acid mutations in the protein sequence of BmNPC1 between two different resistance strains of Bombyx mori. RNAi results showed that BmNPC1 successfully suppressed virus infection ability and changed the expression pattern of viral genes.

Abstract

In Bombyx mori, as an important economic insect, it was first found that some strains were completely refractory to infection with Autographa californica nucleopolyhedrovirus (AcMNPV) through intrahemocelical injection; whereas almost all natural strains had difficulty resisting Bombyx mori nucleopolyhedrovirus (BmNPV), which is also a member of the family Baculoviridae. Previous genetics analysis research found that this trait was controlled by a potentially corresponding locus on chromosome 3, but the specific gene and mechanism was still unknown. With the help of the massive silkworm strain re-sequencing dataset, we performed the Genome-Wide Association Studies (GWAS) to identify the gene related to the resistance of AcMNPV in this study. The GWAS results showed that the Niemann-Pick type C1 (NPC-1) gene was the most associated with the trait. The knockdown experiments in BmN cells showed that BmNPC1 has a successful virus suppression infection ability. We found a small number of amino acid mutations among different resistant silkworms, which indicates that these mutations contributed to the resistance of AcMNPV. Furthermore, inhibition of the BmNPC1 gene also changed the viral gene expression of the AcMNPV, which is similar to the expression profile in the transcriptome data of p50 and C108 strains.

The Exact Timing of Microinjection of Parthenogenetic Silkworm Embryos Is Crucial for Their Successful Transgenesis

The use of parthenogenetic silkworm (Bombyx mori) strains, which eliminate the problem of recombination, is a useful tool for maintaining transgenic clonal lines. The generation of genetically identical individuals is becoming an important tool in genetic engineering, allowing replication of an existing advantageous trait combination without the mixing that occurs during sexual reproduction. Thus, an animal with a particular genetic modification, such as the ability to produce transgenic proteins, can reproduce more rapidly than by natural mating. One obstacle to the widespread use of parthenogenesis in silkworm genetic engineering is the relatively low efficiency of downstream transgenesis techniques. In this work, we seek to optimize the use of transgenesis in conjunction with the production of parthenogenetic individuals. We found that a very important parameter for the introduction of foreign genes into a parthenogenetic strain is the precise timing of embryo microinjection. Our modification of the original method increased the efficiency of transgene injection as well as the survival rate of injected embryos. We also provide a detailed description of the methodological procedure including a graphical overview of the entire protocol.

Thermochromic Silks for Temperature Management and Dynamic Textile Displays

HIGHLIGHTS

Wearable and smart textiles are constructed by integrating embroidery technology and 5G cloud communication, showing promising applications in temperature management and real-time dynamic textile displays. Thermochromism is introduced into the natural silk to produce high-performance thermochromic silks (TCSs) through a low cost, sustainable, efficient, and scalable strategy. The interfacial bonding of the continuously produced TCSs is in situ analyzed and improved through pre-solvent treatment and is confirmed using synchrotron Fourier transform infrared microspectroscopy.

ABSTRACT

Silks have various advantages compared with synthetic polymer fibers, such as sustainability, mechanical properties, luster, as well as air and humidity permeability. However, the functionalization of silks has not yet been fully developed. Functionalization techniques that retain or even improve the sustainability of silk production are required. To this end, a low-cost, effective, and scalable strategy to produce TCSs by integrating yarn-spinning and continuous dip coating technique is developed herein. TCSs with extremely long length (> 10 km), high mechanical performance (strength of 443.1 MPa, toughness of 56.0 MJ m⁻³, comparable with natural cocoon silk), and good interfacial bonding were developed. TCSs can be automatically woven into arbitrary fabrics, which feature super-hydrophobicity as well as rapid and programmable thermochromic responses with good cyclic performance: the response speed reached to one second and remained stable after hundreds of tests. Finally, applications of TCS fabrics in temperature management and dynamic textile displays are demonstrated, confirming their application potential in smart textiles, wearable devices, flexible displays, and human–machine interfaces. Moreover, combination of the fabrication and the demonstrated applications is expected to bridge the gap between lab research and industry and accelerate the commercialization of TCSs. [Image: see text]

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s40820-021-00591-w.

Genetic engineered color silk: fabrication of a photonics material through a bioassisted technology

Silk produced by the silkworm Bombyx mori is an attractive material because of its luster, smooth and soft texture, conspicuous mechanical strength, good biocompatibility, slow biodegradation, and carbon neutral synthesis. Silkworms have been domesticated and bred for production of better quality and quantity of silk, resulting in the development of sericulture and the textile industry. Silk is generally white, so dyeing is required to obtain colored fiber. However, the dyeing process involves harsh conditions and generates a large volume of waste water, which have environmentally and economically negative impacts. Although some strains produce cocoons that contain pigments derived from the mulberry leaves that they eat, the pigments are distributed in the sericin layer and are lost during gumming. In trials for production of colored silk by feeding silkworms on diets containing dyes, only limited species of dye molecules were incorporated into the silk threads. A method for the generation of transgenic silkworm was established in conjunction with the discovery of green fluorescent protein (GFP), and silkworms carrying the GFP gene spun silk threads that formed cocoons that glowed bright green and still retained the original properties of silk. A wide range of color variation of silk threads has been obtained by replacing the GFP gene with the genes of other fluorescent proteins chosen from the fluorescent protein palette. The genetically modified silk with photonic properties can be processed to form various products including linear threads, 2D fabrics, and 3D materials. The transgenic colored silk could be economically advantageous due to addition of a new value to silk and reduction of cost for water waste, and environmentally preferable for saving water. Here, I review the literature regarding the production methods of fluorescent silk from transgenic silkworms and present examples of genetically modified color silk.

Bioengineered silkworms with butterfly cytotoxin-modified silk glands produce sericin cocoons with a utility for a new biomaterial

Genetically manipulated organisms with dysfunction of specific tissues are crucial for the study of various biological applications and mechanisms. However, the bioengineering of model organisms with tissue-specific dysfunction has not progressed because the challenges of expression of proteins, such as cytotoxins, in living cells of individual organisms need to be overcome first. Here, we report the establishment of a transgenic silkworm (Bombyx mori) with posterior silk glands (PSGs) that was designed to express the cabbage butterfly (Pieris rapae) cytotoxin pierisin-1A (P1A). P1A, a homolog of the apoptosis inducer pierisin-1, had relatively lower DNA ADP ribosyltransferase activity than pierisin-1; it also induced the repression of certain protein synthesis when expressed in B. mori-derived cultured cells. The transgene-derived P1A domain harboring enzymatic activity was successfully expressed in the transgenic silkworm PSGs. The glands showed no apoptosis-related morphological changes; however, an abnormal appearance was evident. The introduced truncated P1A resulted in the dysfunction of PSGs in that they failed to produce the silk protein fibroin. Cocoons generated by the silkworms solely consisted of the glue-like glycoprotein sericin, from which soluble sericin could be prepared to form hydrogels. Embryonic stem cells could be maintained on the hydrogels in an undifferentiated state and proliferated through stimulation by the cytokines introduced into the hydrogels. Thus, bioengineering with targeted P1A expression successfully produced silkworms with a biologically useful trait that has significant application potential.

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.

Cell proliferation by silk gut incorporating FGF-2 protein microcrystals

Silk gut processed from the silk glands of the silkworm could be an ideal biodegradable carrier for cell growth factors. We previously demonstrated that polyhedra, microcrystals of Cypovirus 1 polyhedrin, can serve as versatile carrier proteins. Here, we report the generation of a transgenic silkworm that expresses polyhedrin together with human basic fibroblast growth factor (FGF-2) in its posterior silk glands to utilize silk gut as a proteinaceous carrier to protect and slowly release active cell growth factors. In the posterior silk glands, polyhedrin formed polyhedral microcrystals, and FGF-2 became encapsulated within the polyhedra due to a polyhedron-immobilization signal. Silk gut powder prepared from posterior silk glands containing polyhedron-encapsulated FGF-2 stimulated the phosphorylation of p44/p42 MAP kinase and induced the proliferation of serum-starved NIH3T3 cells by releasing bioactive FGF-2. Even after a one-week incubation at 25 °C, significantly higher biological activity of FGF-2 was observed for silk gut powder incorporating polyhedron-encapsulated FGF-2 relative to silk gut powder with non-encapsulated FGF-2. Our results demonstrate that posterior silk glands incorporating polyhedron-encapsulated FGF-2 are applicable to the preparation of biodegradable silk gut, which can protect and release FGF-2 that is produced in a virus- and serum-free expression system with significant application potential.

Heritable genome editing with CRISPR/Cas9 in the silkworm, Bombyx mori

We report the establishment of an efficient and heritable gene mutagenesis method in the silkworm Bombyx mori using modified type II clustered regularly interspaced short palindromic repeats (CRISPR) with an associated protein (Cas9) system. Using four loci Bm-ok, BmKMO, BmTH, and Bmtan as candidates, we proved that genome alterations at specific sites could be induced by direct microinjection of specific guide RNA and Cas9-mRNA into silkworm embryos. Mutation frequencies of 16.7-35.0% were observed in the injected generation, and DNA fragments deletions were also noted. Bm-ok mosaic mutants were used to test for mutant heritability due to the easily determined translucent epidermal phenotype of Bm-ok-disrupted cells. Two crossing strategies were used. In the first, injected Bm-ok moths were crossed with wild-type moths, and a 28.6% frequency of germline mutation transmission was observed. In the second strategy, two Bm-ok mosaic mutant moths were crossed with each other, and 93.6% of the offsprings appeared mutations in both alleles of Bm-ok gene (compound heterozygous). In summary, the CRISPR/Cas9 system can act as a highly specific and heritable gene-editing tool in Bombyx mori.

Core promoter regulates the expression of cathepsin B gene in the fat body of Bombyx mori

Bombyx mori cathepsin B (BmCatB) is involved in the programmed cell death of the fat body during B. mori metamorphosis. For a better understanding of the functional regulatory mechanism, the promoter region of BmCatB in the transcriptional regulation has been identified and analyzed in the present study. BmCatB promoter region performed by the 5' truncation or mutagenesis of EcREs was inserted in the pFA3Luc-A3RL double fluorescence expression vector to activate the fireflies luciferase (FLuc) gene. The results indicated that the dual-luciferase activity of BmCatB gene in the silkworm larval fat body is regulated by the length of promoter. Site-directed mutagenesis of EcRE experiment has shown that the EcREs are up-regulated significantly in the regulation of the BmCatB promoter. A 142bp region (-1165 to -1023) and EcREs are the mainly fat-body tissue-specificity related region and could function as a core promoter element.

Recent progress in genome engineering techniques in the silkworm, Bombyx mori

Rapid advances in genome engineering tools, such as zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and the clustered regularly interspaced palindromic repeats/CRISPR-associated (CRISPR/Cas) system, have enabled efficient gene knockout experiments in a wide variety of organisms. Here, we review the recent progress in targeted gene disruption techniques in the silkworm, Bombyx mori. Although efficiency of targeted mutagenesis was very low in an early experiment using ZFNs, recent studies have shown that TALENs can induce highly efficient mutagenesis of desired target genes in Bombyx. Notably, mutation frequencies induced by TALENs can reach more than 50% of G0 gametes. Thus, TALENs can now be used as a standard tool for gene targeting studies, even when mutant phenotypes are unknown. We also propose guidelines for experimental design and strategy for knockout experiments in Bombyx. Genome editing technologies will greatly increase the usefulness of Bombyx as a model for lepidopteran insects, the major agricultural pests, and lead to sophisticated breeding of Bombyx for use in sericulture and biotechnology.

Baculovirus-mediated gene transfer in butterfly wings in vivo: an efficient expression system with an anti-gp64 antibody

BACKGROUND

Candidate genes for color pattern formation in butterfly wings have been known based on gene expression patterns since the 1990s, but their functions remain elusive due to a lack of a functional assay. Several methods of transferring and expressing a foreign gene in butterfly wings have been reported, but they have suffered from low success rates or low expression levels. Here, we developed a simple, practical method to efficiently deliver and express a foreign gene using baculovirus-mediated gene transfer in butterfly wings in vivo.

RESULTS

A recombinant baculovirus containing a gene for green fluorescent protein (GFP) was injected into pupae of the blue pansy butterfly Junonia orithya (Nymphalidae). GFP fluorescence was detected in the pupal wings and other body parts of the injected individuals three to five days post-injection at various degrees of fluorescence. We obtained a high GFP expression rate at relatively high virus titers, but it was associated with pupal death before color pattern formation in wings. To reduce the high mortality rate caused by the baculovirus treatment, we administered an anti-gp64 antibody, which was raised against baculovirus coat protein gp64, to infected pupae after the baculovirus injection. This treatment greatly reduced the mortality rate of the infected pupae. GFP fluorescence was observed in pupal and adult wings and other body parts of the antibody-treated individuals at various degrees of fluorescence. Importantly, we obtained completely developed wings with a normal color pattern, in which fluorescent signals originated directly from scales or the basal membrane after the removal of scales. GFP fluorescence in wing tissues spatially coincided with anti-GFP antibody staining, confirming that the fluorescent signals originated from the expressed GFP molecules.

CONCLUSIONS

Our baculovirus-mediated gene transfer system with an anti-gp64 antibody is reasonably efficient, and it can be an invaluable tool to transfer, express, and functionally examine foreign genes in butterfly wings and also in other non-model insect systems.

Functional silk: colored and luminescent

Silkworm silk is among the most widely used natural fibers for textile and biomedical applications due to its extraordinary mechanical properties and superior biocompatibility. A number of physical and chemical processes have also been developed to reconstruct silk into various forms or to artificially produce silk-like materials. In addition to the direct use and the delicate replication of silk's natural structure and properties, there is a growing interest to introduce more new functionalities into silk while maintaining its advantageous intrinsic properties. In this review we assess various methods and their merits to produce functional silk, specifically those with color and luminescence, through post-processing steps as well as biological approaches. There is a highlight on intrinsically colored and luminescent silk produced directly from silkworms for a wide range of applications, and a discussion on the suitable molecular properties for being incorporated effectively into silk while it is being produced in the silk gland. With these understanding, a new generation of silk containing various functional materials (e.g., drugs, antibiotics and stimuli-sensitive dyes) would be produced for novel applications such as cancer therapy with controlled release feature, wound dressing with monitoring/sensing feature, tissue engineering scaffolds with antibacterial, anticoagulant or anti-inflammatory feature, and many others.

Expression of hGM-CSF in silk glands of transgenic silkworms using gene targeting vector

The silk gland of the silkworm is a highly specialized organ that has the wonderful ability to synthesize and secrete silk protein. To express human granucyto-macrophage colony-stimulating factor (hGM-CSF) in the posterior silk glands of gene-targeted silkworms, a targeting vector pSK-FibL-L-A3GFP-PH-GMCSF-LPA-FibL-R was constructed, harboring a 1.2 kb portion of the left homogenous arm (FibL-L), a 0.5 kb portion of the right homogenous arm (FibL-R), fibroin H-chain-promoter-driven hGM-CSF and silkworm actin 3-promoter-driven gfp. The targeting vector was then introduced into the eggs of silkworm, and the transgenic silkworms were verified by PCR and DNA hybridization after being screened for the gfp gene. Western blotting analysis using an antibody against hGM-CSF demonstrated a specific band with a molecular weight of 22 kD in the silk glands of the G3 generation transgenic silkworms. The level of expression of hGM-CSF in the posterior silk glands of the G3 generation transgenic silkworms was approximately 2.70 ng/g of freeze-dried powdered posterior silk gland. These results showed that the heterologous gene could be introduced into the silkworm genome and expressed successfully. Further more, the exogenous genes existing in the G5 transgenic silkworm identified by PCR confirmed its integration stability. In addition, the silk glands containing expressed hGM-CSF performed the function of significantly increasing leukocyte count of CY-treated mice in a time-and-dose-dependent manner.

Recombinant protein-based viral disease diagnostics in veterinary medicine

Identification of pathogens or antibody response to pathogens in human and animals modulates the treatment strategies for naive population and subsequent infections. Diseases can be controlled and even eradicated based on the epidemiology and effective prophylaxis, which often depends on development of efficient diagnostics. In addition, combating newly emerging diseases in human as well as animal healthcare is challenging and is dependent on developing safe and efficient diagnostics. Detection of antibodies directed against specific antigens has been the method of choice for documenting prior infection. Other than zoonosis, development of inexpensive vaccines and diagnostics is a unique problem in animal healthcare. The advent of recombinant DNA technology and its application in the biotechnology industry has revolutionized animal healthcare. The use of recombinant DNA technology in animal disease diagnosis has improved the rapidity, specificity and sensitivity of various diagnostic assays. This is because of the absence of host cellular proteins in the recombinant derived antigen preparations that dramatically decrease the rate of false-positive reactions. Various recombinant products are used for disease diagnosis in veterinary medicine and this article discusses recombinant-based viral disease diagnostics currently used for detection of pathogens in livestock and poultry.

Identification of genes encoding N-glycan processing beta-N-acetylglucosaminidases in Trichoplusia ni and Bombyx mori: Implications for glycoengineering of baculovirus expression systems

Glycoproteins produced by non-engineered insects or insect cell lines characteristically bear truncated, paucimannose N-glycans in place of the complex N-glycans produced by mammalian cells. A key reason for this difference is the presence of a highly specific N-glycan processing beta-N-acetylglucosaminidase in insect, but not in mammalian systems. Thus, reducing or abolishing this enzyme could enhance the ability of glycoengineered insects or insect cell lines to produce complex N-glycans. Of the three insect species routinely used for recombinant glycoprotein production, the processing beta-N-acetylglucosaminidase gene has been isolated only from Spodoptera frugiperda. Thus, the purpose of this study was to isolate and characterize the genes encoding this important processing enzyme from the other two species, Bombyx mori and Trichoplusia ni. Bioinformatic analyses of putative processing beta-N-acetylglucosaminidase genes isolated from these two species indicated that each encoded a product that was, indeed, more similar to processing beta-N-acetylglucosaminidases than degradative or chitinolytic beta-N-acetylglucosaminidases. In addition, over-expression of each of these genes induced an enzyme activity with the substrate specificity characteristic of processing, but not degradative or chitinolytic enzymes. Together, these results demonstrated that the processing beta-N-acetylglucosaminidase genes had been successfully isolated from Trichoplusia ni and Bombyx mori. The identification of these genes has the potential to facilitate further glycoengineering of baculovirus-insect cell expression systems for the production of glycosylated proteins.

Silkworm expression system as a platform technology in life science

Many recombinant proteins have been successfully produced in silkworm larvae or pupae and used for academic and industrial purposes. Several recombinant proteins produced by silkworms have already been commercialized. However, construction of a recombinant baculovirus containing a gene of interest requires tedious and troublesome steps and takes a long time (3-6 months). The recent development of a bacmid, Escherichia coli and Bombyx mori shuttle vector, has eliminated the conventional tedious procedures required to identify and isolate recombinant viruses. Several technical improvements, including a cysteine protease or chitinase deletion bacmid and chaperone-assisted expression and coexpression, have led to significantly increased protein yields and reduced costs for large-scale production. Terminal N-acetyl glucosamine and galactose residues were found in the N-glycan structures produced by silkworms, which are different from those generated by insect cells. Genomic elucidation of silkworm has opened a new chapter in utilization of silkworm. Transgenic silkworm technology provides a stable production of recombinant protein. Baculovirus surface display expression is one of the low-cost approaches toward silkworm larvae-derived recombinant subunit vaccines. The expression of pharmaceutically relevant proteins, including cell/viral surface proteins, membrane proteins, and guanine nucleotide-binding protein (G protein) coupled receptors, using silkworm larvae or cocoons has become very attractive. Silkworm biotechnology is an innovative and easy approach to achieve high protein expression levels and is a very promising platform technology in the field of life science. Like the "Silkroad," we expect that the "Bioroad" from Asia to Europe will be established by the silkworm expression system.

Amplified fragment length polymorphism mapping of quantitative trait loci for economically important traits in the silkworm, Bombyx mori

Cocoon related characteristics are economically important traits in the silkworm, Bombyx mori L. (Lepidoptera: Bombycidae). In this study a genetic linkage map was developed that identified QTL controlling the cocoon weight, cocoon shell weight, and cocoon shell percentage using 161 amplified fragment length polymorphism (AFLP) markers. Twenty PstI/TaqI primer combinations were employed to genotype 78 F(2) progenies derived from a cross between P107 Japanese inbred line and Khorasan Lemon Iranian native strain. Among polymorphic markers, 159 AFLP markers were assigned to 24 linkage groups at the LOD threshold of 2.5 that varied in length from 4 to 299 cM. The total length of the linkage map was 2747 cM, giving an average marker resolution of 19.31 cM. A total of 21 AFLP markers were identified that were distributed over the ten linkage groups linked to the three studied traits using the composite interval mapping method. The explained variation rate by QTL controlling cocoon weight, cocoon shell weight, and cocoon shell percentage ranged from 0.02% to 64.85%, 0.2% to 49.11%, and 0.04% to 84.20%, respectively. These QTL controlled by different actions as well as under dominance, additive, partial dominance, dominance, and over dominance.

From genome to proteome: great progress in the domesticated silkworm (Bombyx mori L.)

As the only truly domesticated insect, the silkworm not only has great economic value, but it also has value as a model for genetics and molecular biology research. Genomics and proteomics have recently shown vast potential to be essential tools in domesticated silkworm research, especially after the completion of the Bombyx mori genome sequence. This paper reviews the progress of the domesticated silkworm genome, particularly focusing on its genetic map, physical map and functional genome. This review also presents proteomics, the proteomic technique and its application in silkworm research.

Molecular switches for pheromone release from a moth pheromone-binding protein

Pheromone-binding proteins (PBPs) are involved in the uptake of pheromones from pores on the antennae, transport through an aqueous environment surrounding the olfactory receptor neurons, and fast delivery to pheromone receptors. We tested the hypothesis that a C-terminal segment and a flexible loop are involved in the release of pheromones to membrane-bound receptors. We expressed in Escherichia coli 11 mutants of the PBP from the silkworm moth, BmorPBP, taking into consideration structural differences between the forms with high and low binding affinity. The N-terminus was truncated and His-69, His-70 and His-95 at the base of a flexible loop, and a cluster of acidic residues at the C-terminus were mutated. Binding assays and circular dichroism analyses support a mechanism involving protonation of acidic residues Asp-132 and Glu-141 at the C-terminus and histidines, His-70 and His-95, in the base of a loop covering the binding pocket. The former leads to the formation of a new alpha-helix, which competes with pheromone for the binding pocket, whereas positive charge repulsion of the histidines opens the opposite side of the binding pocket.

Spread of recombinant Autographa californica nucleopolyhedrovirus in various tissues of silkworm Bombyx mori determined by real-time PCR

A cassette harboring luciferase reporter driven by Bombyx mori A3 promoter was transferred to the bacmid AcDeltaEGT to generate the recombinant virus AcNPVA3Luc (where Ac represents Autographa californica, NPV represents nucleopolyhedrovirus, and A3Luc represents the firefly luciferase reporter cassette driven by the A3 promoter). Recombinant baculovirus was injected into the hemocoele of newly ecdysed fifth instar larvae of the silkworm. The infection of virus in various silkworm tissues was determined by real-time PCR. The profile of viral infection showed that the copy number of recombinant AcNPV (rAcNPV) increased the fastest in the hemocyte, followed by the fat body, Malpighian tubule, middle gut, and silk gland. Detecting in nonpermissive strain silkworm showed that there was no significant difference in the entry of rAcNPV into all tested tissues. The difference in viral infection reflected mainly the big difference in replication of rAcNPV in various tissues of silkworm larvae. Real-time quantitative RT-PCR showed that it was due to the different expression of genes involved in viral DNA replication.

Construction of gene expression systems in insect cell lines using promoters from the silkworm, Bombyx mori

The promoter regions of the Bombyx mori HSC70-4 and B. mori TCTP genes characterized previously were used for the construction of a series of constitutive gene expression systems active in cultured cells. The relative abilities of these promoters were evaluated by comparing those of a silkworm actin A3 (BmActin3) promoter, which is used widely as the first choice. A series of constitutive expression systems constructed were assayed for the transcription efficiency by connecting four reporter cDNAs, firefly luciferase, 3GFP, Ds-Red, and beta-galactosidase gene using the Gateway LR reaction. The insertion of an intron enhancer into the site between the TCTP promoter and gene increased the transcription of the BmTCTP promoter by 10-fold. The insertion of the IE-1 gene and HR3 enhancer to the all three promoters were found to increase the transcription up to 560 times.

A novel target-specific gene delivery system combining baculovirus and sequence-specific long interspersed nuclear elements

Transposable elements are valuable for somatic and germ-line transformation. However, long interspersed nuclear elements (LINEs) have not been used because of poor information on the transposition mechanism. We have developed a novel gene delivery system combining baculovirus AcNPV and two silkworm LINEs, SART1 and R1, which integrate into specific sequences of telomeric repeats and 28S ribosomal DNA, respectively. When two LINEs containing the enhanced green fluorescent protein gene recombined into AcNPV were infected into fifth instar larvae of the silkworm, we observed target-specific retrotransposition of LINEs at 72h post-infection, using polymerase chain reaction amplification and sequencing. Telomere- and 28S rDNA-specific transposition occurred in all nine tissues tested, including the ovary and testis. This is the first demonstration of site-specific gene delivery in living larvae. Insertion efficiencies were dependent on the virus titer for injection and the host strains of Bombyx mori. Using this system, we successfully detected the intergeneration transmission of retrotransposed sequences. In addition, AcNPV-mediated SART1 also transposed into telomere of another lepidopteran, Orgyia recens, suggesting that this system is useful for a wide variety of AcNPV-infectious insects. Site-specific gene delivery by virus-mediated LINE will be a potential gene therapy tool to avoid harmful unexpected insertions.

Transgenic approaches to study wing color pattern development in Lepidoptera

The extremely diverse lepidopteran wing patterns make useful models to study the evolution of development and the molecular changes that enable it. Until now, the implication of candidate genes in the differentiation of color patterns has relied primarily on correlational evidence, i.e., gene expression patterns in a developing wing mapping closely to the adult color pattern. The use of transgenic techniques in the Lepidoptera, including the manipulation of gene expression, will finally allow researchers to test hypotheses of gene function at various levels of the patterning hierarchy, from signaling ligands and transcription factors to pigment enzymes. Here we present an overview of transgenic techniques employed in lepidopteran systems and highlight areas where current and future research will provide exciting opportunities to deepen our understanding of the mechanisms of morphological evolution.

Generation of hybrid transgenic silkworms that express Bombyx mori prolyl-hydroxylase alpha-subunits and human collagens in posterior silk glands: Production of cocoons that contained collagens with hydroxylated proline residues

Prolyl 4-hydroxylase (P4H) is a heterotetramer enzyme consisting of alpha-subunits (P4Halpha) and beta-subunits (P4Hbeta), and is required for collagen biosynthesis. Previously, we generated transgenic silkworms that produced human type III collagen fragments (mini-collagens) in the posterior silk gland (PSG). However, prolyl 4-hydroxylation did not occur on the mini-collagens, because in spite of an abundant expression of P4Hbeta in PSGs, P4Halpha expression was quite low there, thus resulting in an insufficient activity of P4H. In this study we aimed at generating hybrid transgenic silkworms whose PSGs are capable of producing mini-collagens and enough P4H for their prolyl 4-hydroxylation. Isolated PSGs were bombarded with fibroin L-chain gene promoter-driven vectors containing Bombyx mori P4Halpha (BmP4Halpha) cDNAs and were transplanted into the hemolymphatic cavity. The P4H activity in the PSG cells significantly increased, indicating that the expressed BmP4Halpha formed active tetramers with endogenous BmP4Hbeta. Using germ-line transgenesis technology, silkworms were generated that synthesized BmP4Halpha in PSG cells. The P4H activity in the transgenic silkworms was 130-fold higher than that of wild-type counterparts. Finally, we generated hybrid transgenic silkworms that expressed cDNAs of both BmP4Halpha and mini-collagen in PSG cells. They spun cocoons that contained mini-collagens whose appropriate proline residues had been adequately hydroxylated.

Milestones leading to the genetic engineering of baculoviruses as expression vector systems and viral pesticides

The baculovirus expression vector system (BEVS) is widely established as a highly useful and effective eukaryotic expression system. Thousands of soluble and membrane proteins that, in general, are correctly folded, modified, sorted and assembled to produce highly authentic recombinant proteins have been cloned and expressed. This historical chronology and perspective will focus on the original, peer-reviewed discoveries that were pioneering and seminal to the development of the BEVS and that provided the basis for subsequent and more recent developments and applications.

Construction of a single nucleotide polymorphism linkage map for the silkworm, Bombyx mori, based on bacterial artificial chromosome end sequences

We have developed a linkage map for the silkworm Bombyx mori based on single nucleotide polymorphisms (SNPs) between strains p50T and C108T initially found on regions corresponding to the end sequences of bacterial artificial chromosome (BAC) clones. Using 190 segregants from a backcross of a p50T female x an F1 (p50T x C108T) male, we analyzed segregation patterns of 534 SNPs between p50T and C108T, detected among 3840 PCR amplicons, each associated with a p50T BAC end sequence. This enabled us to construct a linkage map composed of 534 SNP markers spanning 1305 cM in total length distributed over the expected 28 linkage groups. Of the 534 BACs whose ends harbored the SNPs used to construct the linkage map, 89 were associated with 107 different ESTs. Since each of the SNP markers is directly linked to a specific genomic BAC clone and to whole-genome sequence data, and some of them are also linked to EST data, the SNP linkage map will be a powerful tool for investigating silkworm genome properties, mutation mapping, and map-based cloning of genes of industrial and agricultural interest.

Transformed lepidopteran cells expressing a protein of the silkmoth fat body display enhanced susceptibility to baculovirus infection and produce high titers of budded virus in serum-free media

Baculovirus vectors constitute important tools for therapeutic protein production and mammalian cell transduction for gene therapy applications. A prerequisite for such applications is that the cell lines in which baculoviruses are propagated be maintained in serum-free media that are devoid of potential human pathogens. However, in serum-free media, the performance of baculovirus-based systems can be significantly reduced. In this report, we show that silkmoth-derived host cell lines for the Bombyx mori-nuclear polyhedrosis virus (BmNPV) that are transformed with the gene for the promoting protein (PP), a silkmoth-derived secreted factor containing a lipid-binding domain, display enhanced susceptibility to BmNPV infection and enhanced budded virus productivity in serum-free media. For transformed silkmoth cells maintained in serum-free media, the rate of BmNPV entry is enhanced by two orders of magnitude relative to the untransformed cells, while the rate of budded virus production is increased five-fold. The infectivity-enhancing effect can be also conferred to normal cells grown in serum-free media by addition of conditioned media from the transformed cells, which contain the secreted recombinant PP. Thus, PP substitutes for serum factors whose presence facilitates baculovirus entry into the cells. However, the effects of silkmoth-derived PP may be specific to the BmNPV-silkmoth system since little or no changes in viral infectivity are obtained by PP expression in Trichoplusia ni-derived High-Fivetrade mark cells grown in serum-free media and infected with a different baculovirus (AcNPV).

Productive infection of Autographa californica nucleopolyhedrovirus in silkworm Bombyx mori strain Haoyue due to the absence of a host antiviral factor

We have reported that several silkworm strains are permissive to intrahemocoelical infection of Autographa californica nucleopolyhedrovirus (AcNPV), contrary to the general belief that AcNPV cannot infect silkworm. In the present study, we address whether the intrahemocoelical infection of AcNPV to the silkworm was an exceptional phenomenon, and the possible genetic basis underlying it. Wilder range test of 31 strains of silkworm Bombyx mori for intrahemocoelical AcNPV infection led to the identification of 14 permissive strains and 17 nonpermissive strains, indicating that the intrahemocoelical infection of AcNPV to the silkworm was not a rare and isolated phenomenon. Productive infection was shown in permissive silkworms, by EGFP fluorescence in various tissues when expression of reporter gene controlled by a very late viral promoter polh. The viral titer in larval hemolymph of permissive silkworms increased and maintained at a higher level hundredfold more than the initial amount of virus, indicating viral replication. A series of genetic cross experiments suggested the existence of only one dominant host anti-AcNPV gene or a set of genetically linked genes, which prevent AcNPV infection in nonpermissive silkworm strain Qingsong and are absent in permissive silkworm strain Haoyue.

In vivo DNA double-strand breaks enhance gene targeting in cultured silkworm cells

Alteration of genomic information through homologous recombination (HR) is a powerful tool for reverse genetics in bacteria, yeast, and mice. The low frequency of HR is, however, a major obstacle to achieve efficient gene targeting. In this study, we have developed an assay system for investigating the frequency of gene targeting in cultured silkworm cells using a firefly luciferase gene as a reporter. The introduction of a DNA double-strand break (DSB) either in the chromosomal target locus or in the targeting construct drastically increased the frequency of gene targeting. Interestingly, the inhibition of poly(ADP-ribose) polymerase (PARP), a protein known to play an important role in overall suppression of the HR pathway, stimulated the targeting efficiency, whereas the overexpression of two silkworm RecA homologs, BmRad51 and BmDmc1, had no effect. The presently devised assay system may serve as a useful tool to improve the gene targeting efficiency in the silkworm (Bombyx mori).

Charge-selective recognition at fibroin-modified electrodes for analytical application

A novel fibroin-modified electrode with charge recognition is reported. The characteristics of silk fibroin membranes have been exploited for analytical applications. The membrane, with an isoelectric point of pH 4.5, was applied to graphite and carbon-fiber electrodes. The modified electrode was negatively charged in solutions of pH > 4.5, and so rejected anions and attracted cations. In solutions of pH < 4.5 the electrode was positively charged, and so rejected cations and attracted anions. The pH-responsive charge recognition of the modified electrode was investigated for some neurocompounds. A fibroin carbon-fiber electrode was used for in-vivo determination of the concentration of the cationic neurotransmitter dopamine (DA).

The genetics and genomics of the silkworm, Bombyx mori

We review progress in applying molecular genetic and genomic technologies to studies in the domesticated silkworm, Bombyx mori, highlighting its use as a model for Lepidoptera, and in sericulture and biotechnology. Dense molecular linkage maps are being integrated with classical linkage maps for positional cloning and marker-assisted selection. Classical mutations have been identified by a candidate gene approach. Cytogenetic and sequence analyses show that the W chromosome is composed largely of nested full-length long terminal repeat retrotransposons. Z-chromosome-linked sequences show a lack of dosage compensation. The downstream sex differentiation mechanism has been studied via the silkworm homolog of doublesex. Expressed sequence tagged databases have been used to discover Lepidoptera-specific genes, provide evidence for horizontal gene transfer, and construct microarrays. Physical maps using large-fragment bacterial artificial chromosome libraries have been constructed, and whole-genome shotgun sequencing is underway. Germline transformation and transient expression systems are well established and available for functional studies, high-level protein expression, and gene silencing via RNA interference.

Recent advances in transgenic arthropod technology

The ability to insert foreign genes into arthropod genomes has led to a diverse set of potential applications for transgenic arthropods, many of which are designed to advance public health or improve agricultural production. New techniques for expressing foreign genes in arthropods have now been successfully used in at least 18 different genera. However, advances in field biology are lagging far behind those in the laboratory, and considerable work is needed before deployment in nature can be a reality. A mechanism to drive the gene of interest though a natural population must be developed and thoroughly evaluated before any field release, but progress in this area has been limited. Likewise, serious consideration of potential risks associated with deployment in nature has been lacking. This review gives an overview of the most promising techniques for expressing foreign genes in arthropods, considers the potential risks associated with their deployment, and highlights the areas of research that are most urgently needed for the field to advance out of the laboratory and into practice.

Integration of the 5' end of the retrotransposon, R2Bm, can be complemented by homologous recombination

R2Bm is a non-long-terminal-repeat (non-LTR) retrotransposon that was identified at a specific target site in the 28S rRNA genes of the silkworm, Bombyx mori. Although in vitro analysis has revealed that the 3' end of R2Bm is integrated into the target site by means of target-primed reverse transcription (TPRT), the mechanism of the 5' end integration is not well understood. We established a novel in vivo system to assay the insertion mechanism of R2Bm using a cultured cell line, C65, and a baculovirus, AcNPV, as host and vector, respectively. The 3' end of R2Bm integrated at the target site in the rRNA genes of C65 cells when an AcNPV containing both the full-length 3' UTR and the entire open reading frame (ORF) of R2Bm was introduced while the 5' end integration was incorrect. The 5' end of R2Bm was integrated, however, when the 28S gene sequence upstream of the R2Bm target site was added to the R2Bm sequence. Thus, in our assay, homologous sequences were likely essential for the successful integration of the entire R2Bm into the host cell genome. We also demonstrated that the failure to integrate caused by a frame-shifted ORF was rescued by co-infection with a helper virus that contained only the R2Bm ORF. This indicates that R2 retrotransposition can be complemented in trans. These findings suggest that the host's mechanism for DNA repair may be necessary for the integration of the 5' end of R2Bm and that R2Bm protein may only have the ability to integrate the 3' end of the element by TPRT.

New and highly efficient method for silkworm transgenesis usingAutographa californica nucleopolyhedrovirus andpiggyBac transposable elements

We have developed a new method for the transgenesis of the silkworm, Bombyx mori. This method couples the use of recombinant baculoviruses with the use of the piggyBac transposable element. One recombinant AcNPV, designated the helper virus, is designed to express the piggyBac transposase under the control of the Drosophila hsp70 promoter. Another recombinant AcNPV encoded the gene to be incorporated into the silkworm genome, in this case a green fluorescent protein (GFP) gene, under the control of B. mori actin A3 promoter and franked by the piggyBac inverted terminal repeats. Preblastoderm eggs were inoculated with a fine needle coated with a mixture of these two recombinant baculoviruses. Most of the inoculated larvae hatched and a high proportion of the newly hatched G0 larvae expressed the GFP marker. Transgenesis was confirmed by Southern blot analysis of G1 insects, sequencing the insertion site junctions isolated by inverse PCR, and the marker segregated in Mendelian fashion, as evidenced by the appearance of green fluorescence in G2 insects. Thus, transgenic silkworms were easily and efficiently obtained using this new method.

The expression of GFP under the control of fibroin promotor in primary ovarian cells of Antheraea pernyi

The fibroin promoter can stably express foreign gene in lepidopteran cells. Total RNA was extracted from the gland of silkworm, Antheraea pernyi and the transcription initiation site of fibroin gene of A. pernyi was identified by RNA ligase mediated rapid amplification of cDNA ends (RLM-RACE). The expression vector (pGFP-N2/Fib) was constructed by use of replacing the CMV promoter with the fibroin promoter. The results of visual screening under a fluorescent inverted microscope and Western blot analysis indicated that the GFP gene was expressed in the primary cells of ovary origins from A. pernyi.

The ecdysone regulatory cascade and ovarian development in lepidopteran insects: insights from the silkmoth paradigm

The developing ovariole of the silkmoth Bombyx mori represents an excellent model for studies on the changes that occur in gene expression during the execution of long-term developmental programs. All stages of follicle development, which differ from each other by 2-2.5 h of developmental time, are organized in a single array and can be isolated simultaneously for physiological, biochemical and gene expression studies from a single animal. Recently, significant progress has been made toward the understanding of the hormonal regulation and autonomous implementation of the developmental program that governs follicular cell differentiation during oogenesis in Bombyx. In this review, the developmental career of the ovarian follicle during pharate adult development is discussed in view of the new physiological, biochemical and gene expression data that have recently accumulated. An overview is presented of the changes in expression of structural and regulatory genes and their hormonal regulation in the developing follicle during the transitions among the broad developmental periods of previtellogenesis, vitellogenesis and choriogenesis. Ovarian development in silkmoth pharate adults is induced by 20-hydroxy-ecdysone (20E) through the activation of a regulatory cascade similar to the one observed during Drosophila development. The transition from previtellogenesis to vitellogenesis corresponds to a late response to the hormone and is characterized by the induction of the expression of a unique isoform of the nuclear receptor BmHR3 and the follicular cell-specific yolk protein ESP. The transition from early to middle and late vitellogenesis and choriogenesis, on the other hand, is regulated by positively and negatively acting intra- and extra-ovarian factors. In vitro cultures of developing ovarioles reveal the requirement for the presence of an as yet unidentified growth factor(s) in the hemolymph, while the follicle developmental arrest that is observed after treatment with the ecdysone agonist tebufenozide indicates the requirement for a decline in the level of 20E. The initiation of choriogenesis is characterized by the transcriptional activation of the gene BmGATAbeta that encodes GATA transcription factors, and the chorion genes in the follicular cells. Furthermore, modulation of the activity of BmGATAbeta at the posttranscriptional level is crucial for the stage-specific activation of chorion genes during late choriogenesis. The developing ovariole of the silkmoth is, therefore, emerging as an important model for the study of insect oogenesis and the action of the steroid hormone 20E at the molecular level.

Construction of targeting vector and expression of green fluorescent protein in the silkworm, Antheraea pernyi

The transcription initiation site of Anrheraea pernyi was identified, and a gene targeting vector was constructed. The green fluorescent protein (GFP) gene was inserted into this vector under the control of a fibroin promoter. This recombinant vector was used to target the GFP gene to the fibroin region of the silkworm genome. The DNA was injected into the testes during the pupae, and was also transferred into the egg via the sperm during fertilization. The analysis showed that the GFP gene was integrated into the fibroin gene on the genome by homologous recombination, and was expressed in the silk gland.