P25 Gene Knockout Contributes to Human Epidermal Growth Factor Production in Transgenic Silkworms

Complete BmFib-L knockout reveals its indispensable role in silk fiber formation

Silkworm (Bombyx mori), belonging to the order Lepidoptera, is an important model insect for economic and scientific research. The capacity of the silkworm to secrete robust silk renders it a valuable economic resource, while its biological characteristics offer insights into a number of scientific disciplines. Despite the extensive research conducted to elucidate the mechanisms of silk secretion, many aspects remain unclear. In this study, a mutant strain was generated by editing the BmFib-L gene for silk fibroin, BmFib-L was observed to terminate prematurely at 27 amino acids, using the CRISPR/Cas9 system. The BmFib-L was entirely absent in the mutant strain in comparison to the wild type (WT). On the third day of the fifth instar, the posterior silk glands of the mutant strain were dissected and found to have no significant morphological differences compared to the WT. However, the mutant cocoon phenotypes exhibited significant changes. The findings demonstrated that BmFib-L plays a pivotal role in silk formation, yet is not indispensable for silk secretion. Furthermore, BmFib-L exerts a discernible influence on the mechanical properties of silk. A comparative analysis of the transcriptome of the mutant strain and the WT revealed that the elevated expression of transporter and metabolism-related genes in the mutant strain preserved the equilibrium of the silk gland cells, thereby guaranteeing the typical development of the mutant silk gland cells. These findings offer a novel perspective for investigating the mechanism of silk secretion in silkworms.

Identification and functional study of Fib-L, a major silk fibroin gene component in rice leaf folders

The rice leaf folder, Cnaphalocrocis medinalis (Lepidoptera: Pyralidae), is a major migratory pest in rice agriculture. This pest is characterised by its larvae's ability to fold rice leaves using silk, a behaviour that culminates in the formation of a silken cocoon during the pupal stage. The fibroin light chain (CmFib-L) gene is crucial for silk production, yet its specific function in C. medinalis has reminded elusive. This study presents a comprehensive analysis of the CmFib-L gene, revealing its complete open reading frame (ORF) and expression patterns. Notably, the gene is highly expressed in the fifth-instar larvae and the silk gland, which are critical stages for silk production. Our experiments demonstrate that silencing the CmFib-L gene leads to a reduction in pupal weight, an extension of the pupal stage and a disorganised silk cocoon. Furthermore, the larval behaviour of leaf folding and spinning is significantly impaired when the expression of CmFib-L is downregulated. These findings not only show the importance of fibroin light chain in silk production but also reveal a new target gene to regulate and control the behaviour and development of C. medinalis.

Silk Protein Gene Engineering and Its Applications: Recent Advances in Biomedicine Driven by Molecular Biotechnology

Silk protein, as a natural polymer material with unique structures and properties, exhibits tremendous potential in the biomedical field. Given the limited production and restricted properties of natural silk proteins, molecular biotechnology has been extensively applied in silk protein genetic engineering to produce novel silk proteins with specific properties. This review outlines the roles of major model organisms, such as silkworms and spiders, in silk protein production, and provides a detailed introduction to the applications of gene editing technologies (eg, CRISPR-Cas9), transgenic expression technologies, and synthetic biology techniques in silk protein genetic engineering. By analyzing the genetic factors influencing silk protein expression, this review further elaborates on the innovative applications of silk proteins in drug delivery systems, tissue engineering and regenerative medicine (eg, skin, bone, cartilage, and vascular repair), as well as antibacterial immune strategies. Notably, modified silk proteins expressed by transgenic silkworms demonstrate significant advantages in enhancing drug bioavailability and promoting cell proliferation and differentiation. In conclusion, silk protein gene engineering, through continuous innovations in molecular biotechnology, has provided an effective pathway for the production of high-performance silk protein materials. The extensive applications of these modified silk proteins in the biomedical field have not only expanded the functionality of silk proteins but also offered new approaches to address medical challenges. In the future, the development of silk protein gene engineering will further rely on interdisciplinary integration to promote in-depth research and the expansion of industrial applications of silk proteins.

A strategy for improving the mechanical properties of silk fibers through the combination of genetic manipulation and zinc ion crosslinking

Silk fiber is generally considered an excellent biological material due to its good biocompatibility, morphological plasticity and biodegradability. Previously, the construction of silkworm silk gland bioreactors based on the piggyBac transposon has been optimized. However, the inserted exogenous genes have problems such as position uncertainty, and expression is not strictly controlled. Here, we applied transcription activator-like effector nuclease (TALEN)-mediated homology-directed repair (HDR) to precisely insert histidine-rich cuticular protein (CP) into silkworm Sericin1 (Ser1) gene. The Ser1-CP fusion protein was successfully secreted into cocoon shell. Subsequently, based on the metal coordination ability of the histidine imidazole group, we crosslinked cocoon with metal ions in vitro. In this strategy, the mechanical properties of the fused silk fibers with crosslinked Zn2+ improved, and the maximum breaking stress of the crosslinked Zn2+-fused silk fibers was 23.5 % greater than that of the wild-type fibers. Analysis of the secondary structure of the silk protein showed that the fused silk fibers crosslinked with Zn2+ had more β-sheet structures. This study pioneered a method of improving the mechanical properties of silk fibers by crosslinking metal ions with fused exogenous proteins and expanded the application value of silk gland bioreactors in the development of novel biomaterials.

TALEN-mediated homologous-recombination-based fibroin light chain in-fusion expression system in Bombyx mori

Silkworm was the first domesticated insect and has important economic value. It has also become an ideal model organism with applications in genetic and expression studies. In recent years, the use of transgenic strategies has made the silkworm silk gland an attractive bioreactor for the production of recombinant proteins, in particular, piggyBac-mediated transgenes. However, owing to differences in regulatory elements such as promoters, the expression levels of exogenous proteins have not reached expectations. Here, we used targeted gene editing to achieve site-specific integration of exogenous genes on genomic DNA and established the fibroin light chain (FibL) in-fusion expression system by TALEN-mediated homology-directed recombination. First, the histidine-rich cuticular protein (CP) was successfully site-directed inserted into the native FibL, and the FibL-CP fusion gene was correctly transcribed and expressed in the posterior silk gland under the control of the endogenous FibL promoter, with a protein expression level comparable with that of the native FibL protein. Moreover, we showed based on molecular docking that the fusion of FibL with cuticular protein may have a negative effect on disulfide bond formation between the C-terminal domain of fibroin heavy chain (FibH) and FibL-CP, resulting in abnormal spinning and cocoon in homozygotes, indicating a significant role of FibL in silk protein formation and secretion. Our results demonstrate the feasibility of using the FibL fusion system to express exogenous proteins in silkworm. We expect that this bioreactor system will be used to produce more proteins of interest, expanding the application value of the silk gland bioreactor.

Transgenic modifications of silkworms as a means to obtain therapeutic biomolecules and protein fibers with exceptional properties

Transgenic modification of Bombyx mori silkworms is a benign approach for the production of silk fibers with extraordinary properties and also to generate therapeutic proteins and other biomolecules for various applications. Silk fibers with fluorescence lasting more than a year, natural protein fibers with strength and toughness exceeding that of spider silk, proteins and therapeutic biomolecules with exceptional properties have been developed using transgenic technology. The transgenic modifications have been done primarily by modifying the silk sericin and fibroin genes and also the silk producing glands. Although the genetic modifications were typically performed using the sericin 1 and other genes, newer techniques such as CRISPR/Cas9 have enabled successful modifications of both the fibroin H-chain and L-chain. Such modifications have led to the production of therapeutic proteins and other biomolecules in reasonable quantities at affordable costs for tissue engineering and other medical applications. Transgenically modified silkworms also have distinct and long-lasting fluorescence useful for bioimaging applications. This review presents an overview of the transgenic techniques for modifications of B. mori silkworms and the properties obtained due to such modifications with particular focus on production of growth factors, fluorescent proteins, and high performance protein fibers.

Ectopic expression of sericin enables efficient production of ancient silk with structural changes in silkworm

Bombyx mori silk is a super-long natural protein fiber with a unique structure and excellent performance. Innovative silk structures with high performance are in great demand, thus resulting in an industrial bottleneck. Herein, the outer layer sericin SER3 is ectopically expressed in the posterior silk gland (PSG) in silkworms via a piggyBac-mediated transgenic approach, then secreted into the inner fibroin layer, thus generating a fiber with sericin microsomes dispersed in fibroin fibrils. The water-soluble SER3 protein secreted by PSG causes P25's detachment from the fibroin unit of the Fib-H/Fib-L/P25 polymer, and accumulation between the fibroin layer and the sericin layer. Consequently, the water solubility and stability of the fibroin-colloid in the silk glandular cavity, and the crystallinity increase, and the mechanical properties of cocoon fibers, moisture absorption and moisture liberation of the silk also improve. Meanwhile, the mutant overcomes the problems of low survival and abnormal silk gland development, thus enabling higher production efficiency of cocoon silk. In summary, we describe a silk gland transgenic target protein selection strategy to alter the silk fiber structure and to innovate its properties. This work provides an efficient and green method to produce silk fibers with new functions.