Ultrafine and High-Strength Silk Fibers Secreted by Bimolter Silkworms

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.

Tensile Strength Statistics of High-Performance Mono- and Multifilament Polymeric Materials: On the Validity of Normality

Recently, the statistical distributions of the mechanical properties, including tensile strength (σ), of several high-strength high-modulus oriented polymeric materials have been analyzed by employing the Weibull's and Gaussian statistical models. However, a more detailed comprehensive analysis of the distributions of the mechanical properties of these materials aimed to estimate the validity of normality by employing some other statistical approaches, is needed. In the present work, the σ statistical distributions of the seven high-strength oriented polymeric materials based on the polymers with three different chain architectures and conformations, ultra-high-molecular-weight polyethylene (UHMWPE), polyamide 6 (PA 6), and polypropylene (PP), each in the form of both single and multifilament fibers, have been investigated using graphical methods, such as the normal probability and quantile-quantile plots, and six selected formal normality tests, such as the Kolmogorov-Smirnov, Shapiro-Wilk, Lilliefors, Anderson-Darling, D'Agostino-K squared, and Chen-Shapiro tests. It has been found that the conformity of the σ distribution curves to the normal distribution, including the linearity of the normal probability plots, for the materials with lower strengths (σ < 1 GPa, quasi-ductile PA 6- and PP-based materials) is more correct as compared to those for the materials with markedly higher strengths (σ > 4 GPa, quasi-brittle UHMWPE-based materials). The impact of the sample type (single or multifilament fibers) on this behavior turned out to be negligible.

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.

Evolution of Statistical Strength during the Contact of Amorphous Polymer Specimens below the Glass Transition Temperature: Influence of Chain Length

A comprehensive study of the statistical distribution of the auto-adhesion lap-shear strength (σ) of amorphous polymer-polymer interfaces using various types of statistical tests and models is a useful approach aimed at a better understanding of the mechanisms of the self-healing interface. In the present work, this approach has been applied, for the first time, to a temperature (T) range below the bulk glass transition temperature (T_g^bulk). The interest of this T range consists in a very limited or even frozen translational segmental motion giving little or no chance for adhesion to occur. To clarify this issue, the two identical samples of entangled amorphous polystyrene (PS) with a molecular weight (M) of 10⁵ g/mol or 10⁶ g/mol were kept in contact at T = T_g^bulk - 33 °C for one day. The as-self-bonded PS-PS auto-adhesive joints (AJ) of PSs differing in M by an order of magnitude were fractured at ambient temperature, and their σ distributions were analyzed using the Weibull model, the quantile-quantile plots, the normality tests, and the Gaussian distribution. It has been shown that the Weibull model most correctly describes the σ statistical distributions of the two self-bonded PS-PS AJs with different M due to the joints' brittleness. The values of the Weibull modulus (a statistical parameter) m = 2.40 and 1.89 calculated for PSs with M = 10⁵ and 10⁶ g/mol, respectively, were rather close, indicating that the chain length has a minor effect on the σ data scatter. The Gaussian distribution has been found to be less appropriate for this purpose, though all the normality tests performed have predicted the correctness of the normal distribution for these PS-PS interfaces.

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.

Statistical Analysis of the Mechanical Behavior of High-Performance Polymers: Weibull’s or Gaussian Distributions?

This work addresses the following problem: which of the statistical approaches, Weibull’s or Gaussian, is more appropriate to correctly describe the statistical distributions of the mechanical properties of the high-performance polymer materials of different sample types (single or multifilament oriented fibers) and chain architectures (ultra-high-molecular-weight polyethylene, polyamide 6, or polypropylene)? Along with the routine mechanical properties such as strength, strain at break, and Young’s modulus, an apparent viscoelastic modulus and an apparent strain at break found when differentiating the stress–strain curves have been considered for the first time. For this purpose, a large sample number (50 in each series) has been tested. It has been shown that the values of the Weibull’s modulus (m) characterizing the data scatter were dependent both on the chain architecture and the sample type for the five elastic, viscoelastic and fracture characteristics analyzed. The Weibull’s model has been found to be more correct as compared to the Gaussian one. The different statistical approaches used for the analysis of the large arrays of the data are important for a better understanding of the deformation and fracture mechanisms of quasi-brittle and quasi-ductile high-performance polymer materials.

Fabrication of a Silk Sericin Hydrogel System Delivering Human Lactoferrin Using Genetically Engineered Silk with Improved Bioavailability to Alleviate Chemotherapy-Induced Immunosuppression

Chemotherapy is one of the main treatments for cancer; however, it usually causes severe atrophy of immune organs and self-immunity damage to patients. Human lactoferrin (hLF) is a multiple biofunctional protein in regulating the immune response and thus holds great promise to alleviate chemotherapy-caused immunosuppression. However, a sufficient hLF resource and efficient delivery of hLF remain a challenge. Here, we provide a useful strategy to simultaneously solve these two problems. A silk sericin hydrogel system delivering recombinant hLF (SSH-rhLF) was fabricated to alleviate the chemotherapeutic drug-caused side effects by rhLF-carrying silk cocoons, which were cost-effectively produced by a transgenic silkworm strain as the resource. SSH-rhLF with a uniform porous microstructural morphology, a dominant β-sheet internal structure, adjustable concentration and sustainable release of the rhLF, and non-cytotoxicity properties was demonstrated. Interestingly, the sericin hydrogel showed effective protection of the rhLF from degradation in the stomach and small intestine, thus prolonging the bioactivity and bioavailability of rhLF. As a result, the oral administration of SSH-rhLF with a low rhLF dose showed significant therapeutic effects on enhancing the immune organs of cyclophosphamide (CTX)-treated mice by protecting the splenic follicles, promoting the expression of immunoregulatory factors, and recovering the intestinal flora family from CTX-induced imbalance, which were similar to those achieved by oral administration of a high dose of free hLF in the solution form. The results suggest that the strategy of producing rhLF silk cocoons via feeding transgenic silkworms overcomes well the shortage of rhLF resources, improves the bioavailability of oral rhLF, and alleviates the side effects of chemotherapeutic drugs on immune organs. The oral SSH-rhLF will be promising for applications in cancer chemotherapy and immunity enhancement of patients.

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.

Antibacterial Mechanism of Silkworm Seroins

Seroin 1 and seroin 2 are abundant in silkworm cocoon silk and show strong antibacterial activities, and thus are thought to protect cocoon silk from damage by bacteria. In this study, we characterized the expression pattern of silkworm seroin 3, and found that seroin 3 is synthesized in the female ovary and secreted into egg to play its roles. After being infected, seroin 1, 2, and 3 were significantly up-regulated in the silkworm. We synthesized the full-length protein of seroin 1, 2, and 3 and their N/C-terminal domain (seroin-N/C), and compared the antimicrobial activities in vitro. All three seroins showed higher antibacterial activity against Gram-positive bacteria than against Gram-negative bacteria. Seroin 2 showed better antibacterial effect than seroin 1 and 3, whereas seroin 1/2/3-N was better than seroin 1/2/3-C. We found that seroin 2-C has stronger peptidoglycan binding ability than seroin 2-N per the ELISA test. The binding sites of seroin 2 with bacteria were blocked by peptidoglycan, which resulted in the loss of the antibacterial activity of seroin 2. Collectively, these findings suggest that seroin 1 and 2 play antibacterial roles in cocoon silk, whereas seroin 3 functions in the eggs. The three silkworm seroins have the same antibacterial mechanism, that is, binding to bacterial peptidoglycan by the C-terminal domain and inhibiting bacterial growth by the N-terminal domain.