Improved strength of silk fibers in Bombyx mori trimolters induced by an anti-juvenile hormone compound

A comprehensive review of recent advances in silk sericin: Extraction approaches, structure, biochemical characterization, and biomedical applications

Silks are natural polymers that have been widely used for centuries. Silk consists of a filament core protein, termed fibroin, and a glue-like coating substance formed of sericin (SER) proteins. This protein is extracted from the silkworm cocoons (particularly Bombyx mori) and is mainly composed of amino acids like glycine, serine, aspartic acid, and threonine. Silk SER can be obtained using numerous methods, including enzymatic extraction, high-temperature, autoclaving, ethanol precipitation, cross-linking, and utilizing acidic, alkali, or neutral aqueous solutions. Given the versatility and outstanding properties of SER, it is widely fabricated to produce sponges, films, and hydrogels for further use in diverse biomedical applications. Hence, many authors reported that SER benefits cell proliferation, tissue engineering, and skin tissue restoration thanks to its moisturizing features, antioxidant and anti-inflammatory properties, and mitogenic effect on mammalian cells. Remarkably, SER is used in drug delivery depending on its chemical reactivity and pH-responsiveness. These unique features of SER enhance the bioactivity of drugs, facilitating the fabrication of biomedical materials at nano- and microscales, hydrogels, and conjugated molecules. This review thoroughly outlines the extraction techniques, biological properties, and respective biomedical applications of SER.

Identification and functional study of fhx-L1, a major silk component in Bombyx mori

The silkworm cocoon was composed of fibroins, sericins, protease inhibitors, and proteins of unknown function. In this study, we focused on fhx-L1 (fibrohexamerin-like1), which was the homolog of fibroin fhx (fibrohexamerin). We identified 154 fhx family genes in 44 Lepidoptera insects, and seven fhx-Ls were found in Bombyx mori. Fhx-L1 was the most abundant of these proteins in silk and was specifically expressed in the silk gland. Immunofluorescence analysis showed that fhx-L1 was secreted into the whole sericin layers, similar to sericin1 (ser1). Western blotting revealed that the fhx-L1 protein contains N-linked oligosaccharide chains. CRISPR/Cas9-mediated gene editing was used to generate a homozygous mutant of fhx-L1 (fhx-L1^KO). The cocoon of fhx-L1^KO was larger and fluffier than that of the wild-type (WT), which was attributed to the lower adhesion between silk fibers. We also found that the content of β-sheet in the mutant silk was lower than in the WT silk, which resulted in further deterioration of the mechanical properties of the fhx-L1^KO silk. Our study revealed the properties and function of fhx-L1 as a major structural component in silk. Then, our study provided a potential insight for in-depth study of silk protein function.

Analysis of histomorphometric and proteome dynamics inside the silk gland lumen of Bombyx mori revealed the dynamic change of silk protein during the molt stage

Silkworms spin different silks at different growth stages for specific purposes. The silk spun before the end of each instar is stronger than that at the beginning of each instar and cocoon silk. However, the compositional changes in silk proteins during this process are unknown. Consequently, we performed histomorphological and proteomic analyses of the silk gland to characterize changes from the instar end to the next instar beginning. The silk glands were collected on day 3 of third- and fourth-instar larvae (III-3 and IV-3) and the beginning of fourth-instar larvae (IV-0). Proteomic analysis identified 2961 proteins from all silk glands. Silk proteins P25 and Ser5 were significantly more abundant in III-3 and IV-3 than in IV-0, and many cuticular proteins and protease inhibitors increased significantly in IV-0 compared with III-3 and IV-3. This shift may cause mechanical property differences between the instar end and beginning silk. Using section staining, qPCR, and western blotting, we found for the first time that silk proteins were degraded first and then resynthesized during the molting stage. Furthermore, we revealed that fibroinase mediated the changes of silk proteins during molting. Our results provide insights into the molecular mechanisms of silk proteins dynamic regulation during molting.

Identification and characterization of sericin5 reveals non-cocoon silk sericin components with high β-sheet content and adhesive strength

Sericins are glue proteins on the surface of silk fibers. Four sericins have been characterized in silkworm, namely sericin1 (Ser1), sericin2 (Ser2), sericin3 (Ser3), and sericin4 (Ser4). In this study, we report a novel sericin, sericin5 (Ser5), which exists only in non-cocoon silk. We describe the sequence, exon-intron structure, and translation products of Ser5 in Bombyx mori. The Ser5 gene is approximately 22-kb long and comprises 16 exons. Ser5 protein has a size of 260 kDa, as determined by SDS-PAGE, western blot, and LC-MS/MS. Immunofluorescence analysis revealed that Ser5 co-localizes with Ser1 in the sericin layer. The expression pattern of Ser5 was detected at the transcriptional and translational levels. We systematically analyzed and compared the amino acid composition, repeat regions, and hydrophilicity of silkworm sericins. Morphological observations showed that non-cocoon silk had more sericin than cocoon silk. Circular dichroism spectra revealed that non-cocoon silk sericin contained more β-sheet structures than cocoon silk sericin. In addition, we found that the hydrophilicity and adhesive strength of native sericin increases gradually from the inner layer to the outer layer. This research enhances our understanding of various sericins from cocoon silk and non-cocoon silk with regard to their expression patterns, hydrophilicity, secondary structure and adhesive performances. STATEMENT OF SIGNIFICANCE: : Sericin is a natural biomaterial with diverse biological properties, which has long been used as tissue engineering and biomedical applications. However, the composition and distribution of sericins in different kinds of silk are still uncertain, and the properties difference between sericins have not yet been reported. Our study makes a significant contribution to the literature as it identifies the sequence, composition, hydrophilicity and adhesive property of sericins. Moreover, it provides key insights into the structure-function and function-distribution relationships associated with sericins. We believe that this study will arouse the interest to the readership of your journal as it identifies the new complete sequence of sericin and revealed the composition and properties of sericin, thus highlighting their future potentials applications in both the biomaterial and technical fields.

Study on the Effect of Stretching on the Strength of Natural Silk Based on Different Feeding Methods

Silk is an important biological protein fiber, which has been widely developed and used in textile and biomedical fields due to its excellent mechanical properties and good biocompatibility. Strength is an important indicator that determines the value and use of silk. Although investigations have been made on the mechanical properties of silkworm silks and their dependence relationship with the microstructures, the variation of silk strength formed in the process of silkworm spinning has not been reported. By feeding the same strain of silkworms with mulberry leaves, mulberry leaves + artificial feed, and artificial feed, silks with three filament sizes were obtained, respectively. The tensile test results showed that the strength and filament size of silk are inversely proportional. The structure and fibrosis process of different-strength silks were analyzed. The results showed that, compared with ordinary silk, the β-sheet and crystallinity content of high-strength silk is higher, indicating that its fibrosis process is more sufficient. We proposed that the stretched degree of silk protein determines its structure and properties. During the spinning process of individual silkworms, the secretion of silk protein is not stable, which will cause changes in the stretched degree. The measurement results of the intraindividual stretched degree and strength verified that the degree of stretch determines the strength of the silk. This study not only provides a deeper understanding of the properties of silk protein but also is of interest for the design and development of advanced biomimetic silk materials.

Transgenic genome editing-derived antiviral therapy to nucleopolyhedrovirus infection in the industrial strain of the silkworm

The silkworm (Bombyx mori) is a domesticated and economically important insect. During the whole growth period, silkworm suffers various pathogen infection. Nearly 80% of silk cocoon crop losses are attributed to viral diseases. The circular double-stranded DNA virus Bombyx mori nuclepolyhedrovirus (BmNPV) is the major viral pathogen responsible for massive silkworm death and huge economic losses in the sericulture industry. Up to now, almost all the new strategies for developing immunity against BmNPV are in laboratory strains because of the lack of transgenic technology in industrial silkworm strains. We previously demonstrated that modification of BmNPV genome DNA with the antivirus transgenic CRISPR/Cas9 system effectively improved the resistance of laboratory silkworm strains to viral pathogens. The industrial strains are monovoltine or bivoltine. It is very difficult to break the diapause before microinjection for genetic transformation. Here, we show that the anti-BmNPV transgenic CRISPR/Cas9 system also works in the industrial silkworm strain Jingsong. In this study, we successfully broke diapause and obtained generation-skipping embryos and constructed two TG Jingsong lines. Both lines exhibited significantly enhanced immunity to BmNPV without significant changes in most of the commercially important traits. These results demonstrate that the construction of TG silkworm lines carrying a heritable immune defense system against BmNPV could be an effective strategy to enhance the resistance of industrial silkworm strains to the most devastating DNA virus. The research opened a window for genetic modification of the important strains from laboratory strains to industrial strains.

Horizontal Gene Transfer and Gene Duplication of β-Fructofuranosidase Confer Lepidopteran Insects Metabolic Benefits

Horizontal gene transfer (HGT) is a potentially critical source of material for ecological adaptation and the evolution of novel genetic traits. However, reports on posttransfer duplication in organism genomes are lacking, and the evolutionary advantages conferred on the recipient are generally poorly understood. Sucrase plays an important role in insect physiological growth and development. Here, we performed a comprehensive analysis of the evolution of insect β-fructofuranosidase transferred from bacteria via HGT. We found that posttransfer duplications of β-fructofuranosidase were widespread in Lepidoptera and sporadic occurrences of β-fructofuranosidase were found in Coleoptera and Hymenoptera. β-fructofuranosidase genes often undergo modifications, such as gene duplication, differential gene loss, and changes in mutation rates. Lepidopteran β-fructofuranosidase gene (SUC) clusters showed marked divergence in gene expression patterns and enzymatic properties in Bombyx mori (moth) and Papilio xuthus (butterfly). We generated SUC1 mutations in B. mori using CRISPR/Cas9 to thoroughly examine the physiological function of SUC. BmSUC1 mutant larvae were viable but displayed delayed growth and reduced sucrase activities that included susceptibility to the sugar mimic alkaloid found in high concentrations in mulberry. BmSUC1 served as a critical sucrase and supported metabolic homeostasis in the larval midgut and silk gland, suggesting that gene transfer of β-fructofuranosidase enhanced the digestive and metabolic adaptation of lepidopteran insects. These findings highlight not only the universal function of β-fructofuranosidase with a link to the maintenance of carbohydrate metabolism but also an underexplored function in the silk gland. This study expands our knowledge of posttransfer duplication and subsequent functional diversification in the adaptive evolution and lineage-specific adaptation of organisms.

The mutation of SPI51, a protease inhibitor of silkworm, resulted in the change of antifungal activity during domestication

Domestication of silkworm has led to alterations in various gene expression patterns. For instance, many protease inhibitors were significantly downregulated in the domestic silkworm cocoon compared to its wild progenitor. Considering that SPI51 is the most abundant protease inhibitor in silkworm cocoons, herein, we compared the gene structures and sequences of SPI51 from B. mori (BmoSPI51) and B. mandarina (BmaSPI51). Comparing to the "RGGFR" active site in BmaSPI51, that of BmoPI51 is "KGSFP" and the C-terminal "YNTCECSCP" tail sequence is lost in the latter. To investigate the effect elicited by the active site and tail sequences on the function of SPI51, we expressed two mutated forms of BmoSPI51, namely, BmoSPI51 + tail and BmoSPI51M. BmoSPI51, BmoSPI51 + tail and BmoSPI51M were compared and found to have similar levels of inhibitory activity against trypsin. However, the BmoSPI51 + tail and BmoSPI51M proteins exhibited significantly stronger capacities to inhibit fungi growth, compared to BmoSPI51. We concluded that the specific amino acid sequence of the active site, as well as its the disulfide bond formed by C-terminal sequence in the BmaSPI51, represent the key factors responsible for its higher antifungal activity. This study provided new insights into the antifungal mechanisms elicited by protease inhibitors in the cocoons of silkworms.

Ultrafine and High-Strength Silk Fibers Secreted by Bimolter Silkworms

Ultrafine fibers are widely employed because of their lightness, softness, and warmth retention. Although silkworm silk is one of the most applied natural silks, it is coarse and difficult to transform into ultrafine fibers. Thus, to obtain ultrafine high-performance silk fibers, we employed anti-juvenile hormones in this study to induce bimolter silkworms. We found that the bimolter cocoons were composed of densely packed thin fibers and small apertures, wherein the silk diameter was 54.9% less than that of trimolter silk. Further analysis revealed that the bimolter silk was cleaner and lighter than the control silk. In addition, it was stronger (739 MPa versus 497 MPa) and more stiffness (i.e., a higher Young's modulus) than the trimolter silk. FTIR and X-ray diffraction results revealed that the excellent mechanical properties of bimolter silk can be attributed to the higher β-sheet content and crystallinity. Chitin staining of the anterior silk gland suggested that the lumen is narrower in bimolters, which may lead to the formation of greater numbers of β-sheet structures in the silk. Therefore, this study reveals the relationship between the structures and mechanical properties of bimolter silk and provides a valuable reference for producing high-strength and ultrafine silk fibers.

Study on the effect of graphene oxide (GO) feeding on silk properties based on segmented precise measurement

Silk is widely used in the biomedical field (e.g., surgical sutures) for its excellent mechanical properties and biocompatibility. The properties of silk can be further enhanced by a multitude of methods, including nano particle feeding, which is convenient and green. Generally, the filament length of a silkworm cocoon ranges from 1300 to 1700 m. Despite the fact that the filament size, a key factor affecting the mechanical properties of silk, varies along the length, evaluation of strengthened silk by segment and the specific distribution along the length has not been reported. Therefore, in the present study, we fed silkworms with graphene oxide-sprayed mulberry leaves and evaluated the silk properties segment by segment. The silk's strength and elongation were significantly enhanced, with more α-helical/random coils and thicker mesophase regions. Specifically, the silk from 2‰ GO-treated group had higher strength in the first 60% of the length, whereas the silk from 1‰ GO-treated group was stronger in the last 40% of the length. Notably, the silk from 1‰ GO-treated group had the highest strength and Young's modulus in the last 20% of the length, indicating that this segment is more suitable for use as a surgical suture. Our findings demonstrate that different silk segments offer a great range of desirable assets, and the feasibility to select a specific segment with the desired properties for a specific application.

Structural and Mechanical Properties of Silk from Different Instars of Bombyx mori

Silkworm silk has excellent mechanical properties, biocompatibility, and promising applications in the biomedical sector. Silkworms spin silk at the beginning and end of each of their five instar stages, as well as spinning mature silk after the fifth instar. We evaluated the mechanical properties and structure of 10 kinds of silk fibers from different stages. A tensile test showed that instar beginning silk, instar end silk, and mature silk possess distinct properties. Attenuated total reflectance Fourier-transform infrared spectroscopy and X-ray diffraction results showed that the excellent mechanical properties of instar end silk are attributed to higher β-sheet content and suitable crystallinity. Liquid chromatography-tandem mass spectrometry showed that P25 protein content in IV-E silk is 2.9× higher than that of cocoon silk. This study can offer guidelines for further biomimetic investigations into the design and manufacture of artificial silk protein fibers with novel function.