Analysis of the sericin1 promoter and assisted detection of exogenous gene expression efficiency in the silkworm Bombyx mori L

Identification of Helicoverpa armigera promoters for biotechnological applications

Helicoverpa armigera and Helicoverpa zea are highly polyphagous major agricultural pests with a global distribution. Their control is based on insecticides, however, new, effective, and environmentally friendly control tools are required to be developed and validated. In an effort to facilitate the development of advanced biotechnological tools in these species that will take advantage of new powerful molecular biology techniques like CRISPR/Cas9, we used available transcriptomic data and literature resources, in order to identify RNA polymerase II and III promoters active in RP-HzGUT-AW1(MG), a midgut derived cell line from Helicoverpa zea. Following functional analysis in insect cell lines, four RNA polymerase II promoters from the genes HaLabial, HaTsp-2A, HaPtx-I and HaCaudal were found to exhibit high transcriptional activity in vitro. The HaTsp-2A promoter did not exhibit any activity in the non-midgut derived cell lines Sf-9 and Hi-5 despite high sequence conservation among Lepidoptera, suggesting that it may function in a gut specific manner. Furthermore, considering the utility of RNA polymerase III U6 promoters in methodologies such as RNAi and CRISPR/Cas9, we identified and evaluated four different U6 promoters of H. armigera. In vitro experiments based on luciferase and GFP reporter assays, as well as in vivo experiments targeting an essential gene of Helicoverpa, indicate that these U6 promoters are functional and can be used to experimentally silence or knockout target genes through the expression of shRNAs and sgRNAs respectively. Taking our findings together, we provide a set of promoters useful for the genetic manipulation of Helicoverpa species, that can be used in various applications in the context of agricultural biotechnology.

Human epidermal growth factor-functionalized cocoon silk with improved cell proliferation activity for the fabrication of wound dressings

Human epidermal growth factor (hEGF) is a key factor involved in wound healing owing to its powerful ability to stimulate cell proliferation. In this study, we used piggyBac transposon technology to produce transgenic silkworms expressing the hEGF protein fused to truncated heavy chain (FibH-hEGF). The FibH-hEGF fusion protein was successfully expressed and secreted into silkworm cocoons. Compared to wild-type silk, the transgenic silkworm silk had the similar morphology about silks fiber surface and cocoon nets, while the secondary structure between the transgenic silk and wild-type silk was different. Most importantly, transgenic silkworm cocoon silk powder extract significantly increased human fibroblast FIB cell proliferation for a long duration with no apparent cytotoxicity. Our study provides a promising method for obtaining cost-effective and functional biomaterials for the fabrication of wound dressings.

Transgenic and Diet-Enhanced Silk Production for Reinforced Biomaterials: A Metamaterial Perspective

Silk fibers, which are protein-based biopolymers produced by spiders and silkworms, are fascinating biomaterials that have been extensively studied for numerous biomedical applications. Silk fibers often have remarkable physical and biological properties that typical synthetic materials do not exhibit. These attributes have prompted a wide variety of silk research, including genetic engineering, biotechnological synthesis, and bioinspired fiber spinning, to produce silk proteins on a large scale and to further enhance their properties. In this review, we describe the basic properties of spider silk and silkworm silk and the important production methods for silk proteins. We discuss recent advances in reinforced silk using silkworm transgenesis and functional additive diets with a focus on biomedical applications. We also explain that reinforced silk has an analogy with metamaterials such that user-designed atypical responses can be engineered beyond what naturally occurring materials offer. These insights into reinforced silk can guide better engineering of superior synthetic biomaterials and lead to discoveries of unexplored biological and medical applications of silk.

Functional characterization of the cis-regulatory region for the vitellogenin gene in Plutella xylostella

The vitellogenin gene promoter (VgP) is an essential cis-regulatory element that plays a significant role in transcription of the vitellogenin (Vg) gene, leading to the production of yolk protein in insects, including lepidopterans. However, the function of VgP is still not clear in Plutella xylostella. Here, we cloned a 5.1 kb DNA fragment of the cis-regulatory region adjacent to the 5' end of the Vg gene of P. xylostella (PxVg). We identified two promoter sites in that 5' upstream sequence of PxVg and performed in vitro analysis of two promoter sequences (PxVgP1, 4.9 kb, and PxVgP2, 2.9 kb) in the embryonic cell line of P. xylostella. PxVgP2 exhibited higher enhanced green fluorescent protein (EGFP) expression, so PxVgP2 was used for in vivo analysis. Strong EGFP fluorescence was observed in adult females and the fat body of females, with low expression in embryos. Our results suggest that PxVgP is an important stage-, tissue- and sex-specific endogenous cis-regulatory element in P. xylostella.

Extraordinary Mechanical Properties of Composite Silk Through Hereditable Transgenic Silkworm Expressing Recombinant Major Ampullate Spidroin

Spider dragline silk is a remarkable material that shows excellent mechanical properties, diverse applications, biocompatibility and biodegradability. Transgenic silkworm technology was used to obtain four types of chimeric silkworm/spider (termed composite) silk fibres, including different lengths of recombinant Major ampullate Spidroin1 (re-MaSp1) or recombinant Major ampullate Spidroin2 (re-MaSp2) from the black widow spider, Latrodectus hesperus. The results showed that the overall mechanical properties of composite silk fibres improved as the re-MaSp1 chain length increased, and there were significant linear relationships between the mechanical properties and the re-MaSp1 chain length (p < 0.01). Additionally, a stronger tensile strength was observed for the composite silk fibres that included re-MaSp1, which only contained one type of repetitive motif, (GA)n/An, to provide tensile strength, compared with the silk fibres that includedre-MaSp2, which has the same protein chain length as re-MaSp1 but contains multiple types of repetitive motifs, GPGXX and (GA)n/An. Therefore, the results indicated that the nature of various repetitive motifs in the primary structure played an important role in imparting excellent mechanical properties to the protein-based silk fibres. A silk protein with a single type of repetitive motif and sufficiently long chains was determined to be an additional indispensable factor. Thus, this study forms a foundation for designing and optimizing the structure of re-silk protein using a heterologous expression system.

Genome-wide open chromatin regions and their effects on the regulation of silk protein genes in Bombyx mori

Nucleosome-depleted open chromatin regions (OCRs) often harbor transcription factor (TF) binding sites that are associated with active DNA regulatory elements. To investigate the regulation of silk-protein genes, DNA molecules isolated from the silk glands of third-day fifth-instar silkworm larvae and embryo-derived (BmE) cells were subjected to formal dehyde-assisted isolation of regulatory elements (FAIRE) and high-throughput sequencing. In total, 68,000 OCRs were identified, and a number of TF-binding motifs were predicted. In particular, OCRs located near silk-protein genes contained potential binding sites for functional TFs. Moreover, many TFs were found to bind to clusters of OCRs upstream of silk-protein genes, and to regulate the expression of these genes. The expression of silk protein genes may be related not only to regulating TFs (such as fkh, Bmdimm, and Bmsage), but also to developmental and hormone-induced TFs (such as zen, eve, Br, and eip74ef). Elucidation of genome-wide OCRs and their regulatory motifs in silk protein genes will provide valuable data and clues for characterizing the mechanisms of transcriptional control of silk protein genes.

Transgenic silkworms secrete the recombinant glycosylated MRJP1 protein of Chinese honeybee, Apis cerana cerana

Major royal jelly protein-1 (MRJP1) is the most abundant glycoprotein of royal jelly (RJ) and is considered a potential component of functional foods. In this study, we used silkworm transgenic technology to obtain five transgenic silkworm lineages expressing the exogenous recombinant Chinese honeybee, Apis cerana cerana, protein-1 (rAccMRJP1) under the control of a fibroin light chain (Fib-L) promoter in the posterior silk glands. The protein was successfully secreted into cocoons; specifically, the highest rAccMRJP1 protein content was 0.78% of the dried cocoons. Our results confirmed that the protein band of the exogenous rAccMRJP1 protein expressed in the transgenic silkworm lineages was a glycosylated protein. Therefore, this rAccMRJP1 protein could be used as an alternative standard protein sample to measure the freshness of RJ. Moreover, we also found that the overall trend between the expression of the endogenous and exogenous genes was that the expression level of the endogenous Fib-L gene declined as the expression of the exogenous rAccMRJP1 gene increased in the transgenic silkworm lineages. Thus, by employing genome editing technology to reduce silk protein expression levels, a silkworm bioreactor expression system could be developed as a highly successful system for producing various valuable heterologous proteins, potentially broadening the applications of the silkworm.

Protein kinase A-mediated phosphorylation of the Broad-Complex transcription factor in silkworm suppresses its transcriptional activity

The insect-specific transcription factor Broad-Complex (BR-C) is transcriptionally activated by the steroid 20-hydroxyecdysone (20E) and regulates the expression of many target genes involved in insect growth and development. However, although the transcriptional regulation of BR-C proteins has been well studied, how BR-C is regulated at post-transcription and -translation levels is poorly understood. To this end, using liquid chromatography-tandem mass spectrometry analysis, we identified residue Ser-186 as a phosphorylation site of BR-C in silkworm. Site-directed mutagenesis and treatment with specific kinase activators and inhibitors indicated that the Ser-186 residue in silkworm BR-C is phosphorylated by protein kinase A (PKA). Immunostaining assays disclosed that PKA-mediated phosphorylation of silkworm BR-C has no effect on its nuclear import. However, luciferase reporter analysis, electrophoretic mobility shift assays, and chromatin immunoprecipitation revealed that the PKA phosphorylation event suppresses the transcriptional activation of silkworm BR-C target genes and that this inhibition was caused by repression of BR-C binding to its DNA targets. Of note, both in vitro and ex vivo experiments disclosed that a continuous 20E signal inhibits the PKA-mediated BR-C phosphorylation and also the cAMP/PKA pathway, indicating that 20E's inhibitory effect on PKA-mediated phosphorylation of silkworm BR-C contributes to maintaining BR-C transcriptional activity. In conclusion, our findings indicate that PKA-mediated phosphorylation inhibits silkworm BR-C activity by interfering with its binding to DNA and that 20E signaling relieves PKA-mediated phosphorylation of BR-C, thereby maintaining its transcriptional activity.

Characterization of Transgenic Silkworm Yielded Biomaterials with Calcium-Binding Activity

Silk fibers have many inherent properties that are suitable for their use in biomaterials. In this study, the silk fibroin was genetically modified by including a Ca-binding sequence, [(AGSGAG)₆ASEYDYDDDSDDDDEWD]₂ from shell nacreous matrix protein. It can be produced as fibers by transgenic silkworm. The Ca-binding activity and mineralization of the transgenic silk fibroin were examined in vitro. The results showed that this transgenic silk fibroin had relatively higher Ca-binding activity than unmodified silk fibroin. The increased Ca-binding activity could promote the usage of silk fibroin as a biomaterial in the pharmaceutical industry. This study shows the possibility of using silk fibroin as a mineralization accelerating medical material by generating genetically modified transgenic silkworm.