In vivo effects of metal ions on conformation and mechanical performance of silkworm silks

Overexpression of bond-forming active protein for efficient production of silk with structural changes and properties enhanced in silkworm

Silkworm silk exhibits excellent mechanical properties, biocompatibility, and has potential applications in the biomedical sector. This study focused on enhancing the mechanical properties of Bombyx mori silk by overexpressing three bond-forming active proteins (BFAPs): AFP, HSP, and CRP in the silk glands of silkworms. Rheological tests confirmed increased viscoelasticity in the liquid fibroin stock solution of transgenic silkworms, and dynamic mechanical thermal analysis (DMTA) indicated that all three BFAPs participated in the interactions between fibroin molecular networks in transgenic silk. The mechanical property assay indicated that all three BFAPs improved the mechanical characteristics of transgenic silk, with AFP and HSP having the most significant effects. A synchrotron radiation Fourier transform infrared spectroscopy assay showed that all three BFAPs increased the β-sheet content of transgenic silk. Synchrotron radiation wide-angle X-ray diffraction assay showed that all three BFAPs changed the crystallinity, crystal size, and orientation factor of the silk. AFP and HSP significantly improved the mechanical attributes of transgenic silk through increased crystallinity, refined crystal size, and a slight decrease in orientation. This study opens new possibilities for modifying silk and other fiber materials.

A brief review on the mechanisms and approaches of silk spinning-inspired biofabrication

Silk spinning, observed in spiders and insects, exhibits a remarkable biological source of inspiration for advanced polymer fabrications. Because of the systems design, silk spinning represents a holistic and circular approach to sustainable polymer fabrication, characterized by renewable resources, ambient and aqueous processing conditions, and fully recyclable "wastes." Also, silk spinning results in structures that are characterized by the combination of monolithic proteinaceous composition and mechanical strength, as well as demonstrate tunable degradation profiles and minimal immunogenicity, thus making it a viable alternative to most synthetic polymers for the development of advanced biomedical devices. However, the fundamental mechanisms of silk spinning remain incompletely understood, thus impeding the efforts to harness the advantageous properties of silk spinning. Here, we present a concise and timely review of several essential features of silk spinning, including the molecular designs of silk proteins and the solvent cues along the spinning apparatus. The solvent cues, including salt ions, pH, and water content, are suggested to direct the hierarchical assembly of silk proteins and thus play a central role in silk spinning. We also discuss several hypotheses on the roles of solvent cues to provide a relatively comprehensive analysis and to identify the current knowledge gap. We then review the state-of-the-art bioinspired fabrications with silk proteins, including fiber spinning and additive approaches/three-dimensional (3D) printing. An emphasis throughout the article is placed on the universal characteristics of silk spinning developed through millions of years of individual evolution pathways in spiders and silkworms. This review serves as a stepping stone for future research endeavors, facilitating the in vitro recapitulation of silk spinning and advancing the field of bioinspired polymer fabrication.

Dynamic Changes and Characterization of the Metal Ions in the Silk Glands and Silk Fibers of Silkworm

Metal ions are involved in the conformational transition of silk fibroin and influence the structure and mechanical properties of silk fibers. However, the dynamic characteristics of metal ions during the formation of silk fibers remain unclear. In this study, we found that the silk glands of silkworms contain various metal elements, with varying levels of the metal elements in different zones of the glands and higher levels in the anterior silk glands. Additionally, the content of various metallic elements in the silk glands varied greatly before and after spinning, similar to their content in different cocoon layers, thus, indicating that the anterior silk glands maintain a certain metal ion environment for the transport and conformational transformation of the silk proteins. Most of the metallic elements located in fibroin were confirmed using degumming experiments. For the first time, a scanning electron microscope energy spectrometry system was used to characterize the metal elements in the cross-section of silk and cocoons. These findings have deepened our understanding of the relationship between the overall metal ion environment and silk fiber formation and help us further conceptualize the utilization of metal ions as targets to improve the mechanical properties of the silk fibers.

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.

Biological Materials Processing: Time-Tested Tricks for Sustainable Fiber Fabrication

There is an urgent need to improve the sustainability of the materials we produce and use. Here, we explore what humans can learn from nature about how to sustainably fabricate polymeric fibers with excellent material properties by reviewing the physical and chemical aspects of materials processing distilled from diverse model systems, including spider silk, mussel byssus, velvet worm slime, hagfish slime, and mistletoe viscin. We identify common and divergent strategies, highlighting the potential for bioinspired design and technology transfer. Despite the diversity of the biopolymeric fibers surveyed, we identify several common strategies across multiple systems, including: (1) use of stimuli-responsive biomolecular building blocks, (2) use of concentrated fluid precursor phases (e.g., coacervates and liquid crystals) stored under controlled chemical conditions, and (3) use of chemical (pH, salt concentration, redox chemistry) and physical (mechanical shear, extensional flow) stimuli to trigger the transition from fluid precursor to solid material. Importantly, because these materials largely form and function outside of the body of the organisms, these principles can more easily be transferred for bioinspired design in synthetic systems. We end the review by discussing ongoing efforts and challenges to mimic biological model systems, with a particular focus on artificial spider silks and mussel-inspired materials.

Combined CRISPR toolkits reveal the domestication landscape and function of the ultra-long and highly repetitive silk genes

Highly repetitive sequences play a major structural and function role in the genome. In the present study, we developed Cas9-assisted cloning and SMRT sequencing of long repetitive sequences (CACS) to sequence and manipulate highly repetitive genes from eukaryotic genomes. CACS combined Cas9-mediated cleavage of a target segment from an intact genome, Gibson assembly cloning, and PacBio SMRT sequencing. Applying CACS, we directly cloned and sequenced the complete sequences of fibroin heavy chain (FibH) genes from 17 domesticated (Bombyx mori) and 7 wild (Bombyx mandarina) silkworms. Our analysis revealed the unique fine structure organization, genetic variations, and domestication dynamics of FibH. We also demonstrated that the length of the repetitive regions determined the mechanical properties of silk fiber, which was further confirmed by Cas9 editing of FibH. CACS is a simple, robust, and efficient approach, providing affordable accessibility to highly repetitive regions of a genome. STATEMENT OF SIGNIFICANCE: Silkworm silk is the earliest and most widely used animal fiber, and its excellent performance mainly depends on the fibroin heavy chain (FibH) protein. The FibH gene is the main breakthrough in understanding the formation mechanism and improvement of silk fiber. In the study, we developed a CACS method for characterizing the fine structure and domestication landscape of 24 silkworm FibH genes. We used CRISPR/Cas9 to edit the repetitive sequence of FibH genes, revealing the relationship between FibH genes and mechanical properties of silkworm silk. Our study is helpful in modifying silk genes to manipulate other valuable highly repetitive sequences, and provides insight for silkworm breeding.

Structural insight on the liquid silk from the middle silk gland of non-mulberry silkworm Antheraea assamensis

This study highlights the preliminary characterization of liquid silk from the middle silk gland (MSG) along with the in-silico analysis of the sericin protein of a less explored non mulberry silkworm Antheraea assamensis which is endemic to the North Eastern region of India. Various biophysical methods have been applied to elucidate the conformational patterns of the liquid silk present inside the MSG without removing the sericin layer. This will help us to know the actual features of the in vivo transitional status of the silk in the MSG which travel towards the anterior silk gland (ASG) prior to spinning. The SDS PAGE analysis represented the existence of the both fibroin and sericin bands in the sample. The structural pattern of the MSG liquid silk as revealed by various methods denoted the occurrence of β-sheet component along with some random coil and β-turn components which in turn suggests the transitional state of the liquid silk attributed to the existence of both the crystalline and amorphous contents. The thermo gravimetric study and the aggregation behavior analysis results proposed the occurrence of intermolecular hydrogen bonding between the sericin and fibroin in the MSG. This study will sensitize the better understanding of the behavior of the liquid silk in the MSG of non-mulberry silkworm A. assamensis and will open avenues for various application-based studies of this silk.Communicated by Ramaswamy H. Sarma.

Drosophila melanogaster resilin improves the mechanical properties of transgenic silk

Resilin is a natural protein with high extensibility and resilience that plays a key role in the biological processes of insects, such as flight, bouncing, and vocalization. This study used piggyBac-mediated transgenic technology to stably insert the Drosophila melanogaster resilin gene into the silkworm genome to investigate whether exogenous protein structures improve the mechanical properties of silkworm silk. Molecular detection showed that recombinant resilin was expressed and secreted into silk. Secondary structure and mechanical property analysis showed that the β-sheet content in silk from transgenic silkworms was higher than in wild-type silk. The fracture strength of silk fused with resilin protein was 7.2% higher than wild-type silk. The resilience of recombinant silk after one-time stretching and cyclic stretching was 20.5% and 18.7% higher than wild-type silk, respectively. In summary, Drosophila resilin can enhance the mechanical properties of silk, and this study is the first to improve the mechanical properties of silk using proteins other than spider silk, which broadens the possibilities for the design and application of biomimetic silk materials.

Stretching-Induced Conformational Transition of [3-13C]Ser- and [3-13C]Tyr-Antheraea yamamai Silk Fibroin before Spinning Investigated with 13C Solid-State NMR Spectroscopy

The conformational transition of [3-¹³C]Ser- and [3-¹³C]Tyr-Antheraea yamamai silk fibroin before spinning induced by stretching was investigated with ¹³C CP/MAS NMR spectroscopy. The α-helix content of the silk fibroin before stretching was found to be 31.6% based on the Ala and Ser peaks. With increasing stretching ratio, the α-helix and the random coil Ala Cβ peaks decreased gradually, while the β-sheet peak was observed at a stretching ratio of ×5 and increased rapidly upon further stretching. For Ser residue, the α-helix peak decreased monotonically with increasing stretching ratio, but the random coil peak increased slightly till the stretching ratio of ×5 and then decreased. A small β-sheet peak was observed before stretching and then increased rapidly starting from the stretching ratio of ×7. In contrast, a gradual decrease of random coil peak and an increase of β-sheet peak were observed for the Tyr residue. The results of this investigation may be helpful for further studies of fiber formation mechanism in A. yamamai and in the future design of artificial silk materials.

Azadirachtin inhibits the development and metabolism of the silk glands of Spodoptera frugiperda and affects spinning behavior

BACKGROUND

Spodoptera frugiperda is a major agricultural pest, and the dispersal of its larvae by spinning silk is one of the causes of crop damage. At present, there are relatively few reports of pest control that affect larvae spinning silk. In this study, the effect of spinning behavior of the S. frugiperda larvae was investigated through a series of experiments.

RESULTS

The 3rd instar larvae of S. frugiperda were exposed to azadirachtin, and the pathological changes in the silk glands of S. frugiperda and the differences in their metabolites were analyzed by scanning electron microscopy, histological sectioning, transmission electron microscopy and metabolomics. The results showed that azadirachtin could affect the silk gland of S. frugiperda. After 48 h of treatment with azadirachtin, the silk gland lumen of S. frugiperda appeared vacuolated. KEGG showed that 31 different metabolites were identified, of which 12 were upregulated and 19 were downregulated. These metabolites were enriched in 15 different metabolic pathways, which indicated that the silk gland of S. frugiperda was closely related to the formation of fatty acids and energy metabolism for the silk formation process.

CONCLUSIONS

This study provides a preliminary report of the effect of azadirachtin on the spinning behavior of the S. frugiperda larvae. Metabolomic results indicated that histidine, glycine and leucine, which are related to serine protein synthesis, were down-regulated. Azadirachtin can damage the silk glands of S. frugiperda and thus affect spinning behavior. This provides the basis for the control of S. frugiperda by spinning silk. © 2022 Society of Chemical Industry.

In situ preparation of molybdenum-dioxide-incorporated carbonized silk fiber and its application in supercapacitors

A previous study found that the capacitive behavior of nanoparticles fed to the silkworm can be delivered to carbonized silk fibers, which can be used to fabricate electrodes for the construction of flexible supercapacitors. However, the tendency of nanoparticles to aggregate decreases the quantity of nanoparticles that enter the silk and therefore reduces the capacitance performance of the prepared carbonized silk. Here, we sprayed ammonium molybdate tetrahydrate (AMT) on the surface of mulberry leaves used for feeding silkworms and investigated the effect of feeding AMT on the growth of silkworms and the properties of spun silk. The precursor incorporated into the silk was converted into scattered MoO2 NPs, which were embedded within the carbonized silk fiber (CSF) via carbothermal reduction. The specific capacitance of CSF obtained from silkworms fed with an aqueous solution of AMT-treated mulberry leaves reached up to 298 F/g at 0.2 g/A, which is much higher than that of the control group (102 F/g). Since AMT is highly water-soluble, and its concentration can be easily modulated, we believe that the proposed strategy is feasible for the large-scale fabrication of CSF with enhanced capacitive performance.

Insignificant Difference in Biocompatibility of Regenerated Silk Fibroin Prepared with Ternary Reagent Compared with Regenerated Silk Fibroin Prepared with Lithium Bromide

Bombyx mori silk fibroin (SF) is widely used in the field of biomaterials due to its excellent biocompatibility and mechanical properties. However, SF cannot be used directly in many applications and needs to be dissolved first. Lithium bromide (LiBr) is a traditional solvent which is usually used to dissolve SF. However, LiBr has several limitations, e.g., it is expensive, it is toxic to organisms, and it is environmentally unfriendly. Herein, we investigate the possibility of developing a ternary reagent system that is inexpensive, non-toxic to organisms, and environmentally friendly as an alternative for silk fibroin solubilization. The results confirm that regenerated silk fibroin (RSF) prepared using a ternary reagent has the same morphology and amino acid composition as that prepared using LiBr, but the RSF prepared using a ternary reagent still had a small amount of calcium residue even after long-term dialysis. Further research found that the residual calcium does not cause significant differences in the structure and biological performance of the RSF, such as its cytotoxicity, blood compatibility, and antibacterial properties. Therefore, we believe that ternary reagents are an ideal alternative solvent for dissolving SF.

Conservation of Three-Dimensional Structure of Lepidoptera and Trichoptera L-Fibroins for 290 Million Years

The divergence of sister orders Trichoptera (caddisflies) and Lepidoptera (moths and butterflies) from a silk-spinning ancestor occurred around 290 million years ago. Trichoptera larvae are mainly aquatic, and Lepidoptera larvae are almost entirely terrestrial—distinct habitats that required molecular adaptation of their silk for deployment in water and air, respectively. The major protein components of their silks are heavy chain and light chain fibroins. In an effort to identify molecular changes in L-fibroins that may have contributed to the divergent use of silk in water and air, we used the ColabFold implementation of AlphaFold2 to predict three-dimensional structures of L-fibroins from both orders. A comparison of the structures revealed that despite the ancient divergence, profoundly different habitats, and low sequence conservation, a novel 10-helix core structure was strongly conserved in L-fibroins from both orders. Previously known intra- and intermolecular disulfide linkages were accurately predicted. Structural variations outside of the core may represent molecular changes that contributed to the evolution of insect silks adapted to water or air. The distributions of electrostatic potential, for example, were not conserved and present distinct order-specific surfaces for potential interactions with or modulation by external factors. Additionally, the interactions of L-fibroins with the H-fibroin C-termini are different for these orders; lepidopteran L-fibroins have N-terminal insertions that are not present in trichopteran L-fibroins, which form an unstructured ribbon in isolation but become part of an intermolecular β-sheet when folded with their corresponding H-fibroin C-termini. The results are an example of protein structure prediction from deep sequence data of understudied proteins made possible by AlphaFold2.

Chitin and cuticle proteins form the cuticular layer in the spinning duct of silkworm

Chitin is found in the exoskeleton and peritrophic matrix of arthropods, but recent studies have also identified chitin in the spinning duct of silk-spinning arthropods. Here, we report the presence and function of chitin and cuticle proteins ASSCP1 and ASSCP2 in the spinning duct of silkworm. We show that chitin and these proteins are co-located in the cuticular layer of the spinning duct. Ultrastructural analysis indicates that the cuticular layer has a multilayer structure by layered stacking of the chitin laminae. After knocking down ASSCP1 and ASSCP2, the fine structure of this layer was disrupted, which had negative impacts on the mechanical properties of silk. This work clarifies the function of chitin in the spinning duct of silkworm. Chitin and cuticle proteins are the main components of the cuticular layer, providing the shearing stress during silk fibrillogenesis and regulating the final mechanical properties of silk. STATEMENT OF SIGNIFICANCE: Recent studies have identified chitin in the spinning duct of silk-spinning arthropods. However, the role of chitin in this specific organ remains unclear. This study reports that chitin and cuticle proteins form the cuticular layer, a unique structure of the spinning duct of silkworm. This layer with a precise laminate structure gives the spinning duct flexible properties, provides shearing forces for silk fibrillogenesis, and contributes to silk final mechanical properties. Our work clarifies the component, ultrastructure, and biological significance of the silkworm cuticular layer, describes the specific process of silk fiber formation, and proposes new molecular targets (chitin and cuticle proteins) for the improvement of animal silks.

Fiber Formation and Mechanical Properties of Bombyx mori Silk Are Regulated by Vacuolar-Type ATPase

The mechanism of silk fiber formation in silkworms, Bombyx mori, is of particular scientific interest because it is closely related to the mechanical properties of silk fibers. However, there are still substantial knowledge gaps in understanding the details of this mechanism. Studies have found a pH gradient in the silk gland of silkworms. A vacuolar-type ATPase (V-ATPase) is thought to be involved in establishing this pH gradient. Although it is reported that the pH gradient plays a role in silk fibrillogenesis, the direct relationship between V-ATPase and silk mechanical properties is unclear. Thus, this study aims to clarify this relationship. We found that V-ATPase is highly and stably expressed in the anterior silk gland (ASG) and maintains the pH gradient and the fine structure of ASG. Inhibition of V-ATPase activity increased the β-sheet content and crystallinity of silk fibers. Tensile testing showed that the mechanical properties of silk fibers improved after inhibiting V-ATPase activity. All the data suggest that V-ATPase is a key factor in regulating silk fibrillogenesis and is related to the final mechanical properties of the silk fibers. V-ATPase is a potential target for silk mechanical property improvement.

Recombinant Silk Proteins with Additional Polyalanine Have Excellent Mechanical Properties

This paper explores the structures of exogenous protein molecules that can effectively improve the mechanical properties of silkworm silk. Several transgenic vectors fused with the silkworm fibroin light chain and type 3 repeats in different multiples of the ampullate dragline silk protein 1 (MaSp1) from black widow spider with different lengths of the polyalanine motifs were constructed for this study. Transgenic silkworms were successfully obtained by piggyBac-mediated microinjection. Molecular detection showed that foreign proteins were successfully secreted and contained within the cocoon shells. According to the prediction of PONDR^® VSL2 and PONDR^® VL-XT, the type 3 repeats and the polyalanine motif of the MaSp1 protein were amorphous. The results of FTIR analysis showed that the content of β-sheets in the silk of transgenic silkworms engineered with transgenic vectors with additional polyalanine was significantly higher than that of wild-type silkworm silk. Additionally, silk with a higher β-sheet content had better fracture strength and Young’s modulus. The mechanical properties of silk with longer chains of exogenous proteins were improved. In general, our results provide theoretical guidance and technical support for the large-scale production of excellent bionic silk.

The influence of metal ions on native silk rheology

Whilst flow is the basis for silk fibre formation, subtle changes in a silk feedstocks' chemical environment may serve to increase both energetic efficiency and control hierarchical structure development during spinning. Despite the role of pH being largely understood, the influence of metal ions is not, only being inferred by correlative work and observations. Through a combination of rheology and microscopy, we provide a causative study of how the most abundant metal ions in the silk feedstock, Ca²⁺ and K⁺, affect its flow properties and structure. Our results show that Ca²⁺ ions increase viscosity and prevent molecular alignment and aggregation, providing ideal storage conditions for unspun silk. In contrast, the addition of K⁺ ions promotes molecular alignment and aggregation and therefore seems to transfer the silk feedstock into a spinning state which confirms recent 'sticky reptation' modelling hypotheses. Additionally, we characterised the influence of the ubiquitous kosmotropic agent Li⁺, used to prepare regenerated silk solutions, and find that it promotes molecular alignment and prevents aggregation which may permit a range of interesting artificial silk processing techniques to be developed. In summary, our results provide a clearer picture of how metal ions co-ordinate, control and thus contribute towards silk protein self-assembly which in turn can inspire structuring approaches in other biopolymer systems.

Composition and in silico structural analysis of fibroin from liquid silk of non-mulberry silkworm Antheraea assamensis

Silk is spun from the liquid precursor known as liquid silk secreted from the posterior part and stored in the silk gland lumen with occurrence of many momentary events. The liquid silk in the silk gland is transformed to the spun silk fibre. In this study the elucidation of the protein components of liquid silk from the posterior part of the silk gland (PSG) of saturniid silkworm Antheraea assamensis along with its structural characterization has been reported. The 3D model of the N-terminal amorphous portion with some repeat crystalline motifs (19-255) of core protein fibroin has also been constructed. 1D and 2D electrophoresis revealed the homo-dimeric structure of the silk protein. Secondary structure analysis by Circular dichroism, FTIR spectroscopy showed α helical structural component as predominant conformation in the liquid silk. The crystalline structure investigated through X ray diffraction (XRD) analysis also revealed the presence of less ordered amorphous α helical conformation in the liquid silk. The 3D structural model proposed of the residues from 19 to 255 has revealed structural stability throughout the molecular dynamics simulation process. This study will provide the detailed structural information and in silico analysis of the core protein present in the liquid silk of PSG.

Variations in the Metallic Ion Concentration in the Silk Gland and Cocoon of Silkworm Antheraea assamensis helfer

The natural spinning process in silkworms involves the conformation transition of the liquid silk protein present in the silk gland to make fiber. This conformation transition is influenced by different factors, and some studies suggested that changes in the metallic ion concentrations is one of them (Zhou et al. 2005). This study investigated the changes in the metallic ion compositions in the silk glands (before and during spinning) and cocoons of non-mulberry silkworm Antheraea assamensis helfer. Intact silk glands were dissected from mature 5th instar A. assamensis larvae. The glands were rinsed with deionized water and divided into five divisions: posterior silk gland (PSG), middle silk gland (MSG), anterior silk gland (ASG), posterior middle (PM) and anterior middle (AM) division of silk gland. Cocoon pieces and the gland parts were dried and digested in acid mixture to quantify the metallic contents in an atomic absorption spectrophotometer (Shimadzu, AA7000). We determined seven metals (Na, K, Mg, Ca, Cu, Zn, Fe) present in the different parts of the secretory pathway as well as in the fibers of A.assamensis. Our results suggested that the concentrations of Mg, Ca, Na, and K were more abundant in the gland than the Cu and Zn. Fe concentration was found comparatively less in amount in the gland. Amount of Ca found to be higher in the cocoons. The differences in the metallic ion concentrations in the gland parts before and during spinning suggested the possibility of their role in the formation of silk thread from luminal silk.

Effects of Osiris9a on Silk Properties in Bombyx mori Determined by Transgenic Overexpression

Osiris is an insect-specific gene family with multiple biological roles in development, phenotypic polymorphism, and protection. In the silkworm, we have previously identified twenty-five Osiris genes with high evolutionary conservation and remarkable synteny among several insects. Bombxy moriOsiris9a (BmOsi9a) is expressed only in the silk gland, particularly in the middle silk gland (MSG). However, the biological function of BmOsi9a is still unknown. In this study, we overexpressed BmOsi9a in the silk gland by germline transgene expression. BmOsi9a was overexpressed not only in the MSG but also in the posterior silk gland (PSG). Interestingly, BmOsi9a could be secreted into the lumen in the MSG but not in the PSG. In the silk fiber, overexpressed BmOsi9a interacted with Sericin1 in the MSG, as confirmed by a co-immunoprecipitation assay. The overexpression of BmOsi9a altered the secondary structure and crystallinity of the silk fiber, thereby changing the mechanical properties. These results provide insight into the mechanisms underlying silk proteins secretion and silk fiber formation.

Disruption of the Metal Ion Environment by EDTA for Silk Formation Affects the Mechanical Properties of Silkworm Silk

Silk fiber has become a research focus because of its comprehensive mechanical properties. Metal ions can influence the conformational transition of silk fibroin. Current research is mainly focused on the role of a single ion, rather than the whole metal ion environment. Here, we report the effects of the overall metal ion environment on the secondary structure and mechanical properties of silk fibers after direct injection and feeding of silkworms with EDTA. The metal composition of the hemolymph, silk gland, and silk fiber changed significantly post EDTA treatment. Synchrotron FTIR analysis indicated that the secondary structure of silk fiber after EDTA treatment changed dramatically; particularly, the β-sheets decreased and the β-turns increased. Post EDTA treatment, the silk fiber had significantly decreased strength, Young’s modulus, and toughness as compared with the control groups, while the strain exhibited no obvious change. These changes can be attributed to the change in the metal ion environment in the silk fibroin and sericin in the silk gland. Our investigation provides a new theoretical basis for the natural silk spinning process, and our findings could help develop a method to modify the mechanical properties of silk fiber using metal ions.

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.

GC/MS-based metabolomics analysis reveals active fatty acids biosynthesis in the Filippi's gland of the silkworm, Bombyx mori, during silk spinning

The Filippi's gland, also called the Lyonet's gland, is in truth a pair of tiny glands that are unique to lepidopteran insects. Although the ultrastructure of the Filippi's gland has been well-understood, the specific biological function of this gland in silk spinning is still unclear. Previous studies proposed a hypothesis that this gland might synthesize and secrete some substances into the anterior silk gland (ASG) to help silk spinning. In order to identify these metabolites, a GC/MS-based metabolomics technique was introduced. A total of 59 metabolites, including fatty acids, amino acids, and sugars, were identified in glands from silkworm larvae in the feeding and silk spinning stages. Abundance and pathway analyses revealed that these metabolites had different abundances during gland development and silk spinning, which may facilitate the transport of small molecules and ions. The most interesting result is that the Filippi's gland has a very active fatty acid biosynthesis process during spinning, suggesting that it may synthesize lipids or waxes and secrete them into the ASG to promote silk spinning. This data provides instructive insight into the biological functions of Filippi's gland from both silkworms and other lepidoptera.

Effect of Different Additives in Diets on Secondary Structure, Thermal and Mechanical Properties of Silkworm Silk

In this study, eight types of materials including nanoparticles (Cu and CaCO₃), metallic ions (Ca2+ and Cu2+), and amino acid substances (serine, tyrosine, sericin amino acid, and fibroin amino acid) were used as additives in silkworm diets to obtain in-situ modified silk fiber composites. The results indicate that tyrosine and fibroin amino acids significantly increase potassium content in silk fibers and induce the transformation of α-helices and random coils to β-sheet structures, resulting in higher crystallinities and better mechanical properties. However, the other additives-modified silk fibers show a decrease in β-sheet contents and a slight increase or even decrease in tensile strengths. This finding provides a green and effective approach to produce mechanically enhanced silk fibers with high crystallinity on a large scale. Moreover, the modification mechanisms of these additives were discussed in this study, which could offer new insights into the design and regulation of modified fibers or composites with desirable properties and functions.

A Review of the Emerging Role of Silk for the Treatment of the Eye

Silk is a remarkable biopolymer with a long history of medical use. Silk fabrications have a robust track record for load-bearing applications, including surgical threads and meshes, which are clinically approved for use in humans. The progression of top-down and bottom-up engineering approaches using silk as the basis of a drug delivery or cell-loaded matrix helped to re-ignite interest in this ancient material. This review comprehensively summarises the current applications of silk for tissue engineering and drug delivery, with specific reference to the eye. Additionally, the review also covers emerging trends for the use of silk as a biologically active biopolymer for the treatment of eye disorders. The review concludes with future capabilities of silk to contribute to advanced, electronically-enhanced ocular drug delivery concepts.

Changes in Silk Feedstock Rheology during Cocoon Construction: The Role of Calcium and Potassium Ions

Variation in silk feedstocks is a barrier both to our understanding of natural spinning and biomimetic endeavors. To address this, compositional changes are investigated in feedstock specimens from the domesticated silkworm (Bombyx mori). It is found that the feedstock viscosity decreased systematically by over two orders of magnitude during cocoon construction. Potential factors such as protein concentration, molecular weight, pH, or the presence of trehalose are excluded, whereas a clear correlation appear between viscosity and the relative concentrations of Ca²⁺ and K⁺ ions. It is expected that Ca²⁺ ions would favor "salt bridges" between acidic (Asp and Glu) amino acids, leading to an increased viscosity, whereas K⁺ ions would compete for these sites, thereby reducing viscosity. Thus, these findings suggest a simple, systematic yet sophisticated control of feedstock viscosity in the silkworm, which in turn can be applied to future industrial silk production.

Proteome profile of spinneret from the silkworm, Bombyx mori

The silkworm spinneret is an important tissue for silk fibrillogenesis and spinning. All biochemical processes during silk fibrillogenesis are correlated with silk properties. Understanding the role of spinneret in silk fibrillogenesis may help to reveal the mechanism of silk fibrillogenesis as well as improve silk quality for commercial purposes. Thus, we profiled the proteome of silkworm spinneret. A total of 1572 proteins and 232 differential abundance proteins were identified. Silk fibrillogenesis-related proteins, such as cuticle proteins, ion-transporting proteins, muscular proteins, and energy metabolic proteins, were abundant in spinneret. Metabolic pathway and GO enrichment analyses revealed that the identified proteins were involved in energy metabolism, chitin binding, and cuticle construction. Active energy metabolism may provide abundant energy for the muscle contraction as well as ion and water exchange. The chitin binding and cuticle construction process may provide sufficient shear forces for silk formation. Our data suggest that silkworm spinneret provides a suitable physiological and biochemical environment for silk fibrillogenesis. These proteins are potential targets for improving silk quality in the silk industry. Data are available via ProteomeXchange with identifier PXD004455.

Accelerated biodegradation of silk sutures through matrix metalloproteinase activation by incorporating 4-hexylresorcinol

Silk suture material is primarily composed of silk fibroin and regarded as a non-resorbable material. It is slowly degraded by proteolysis when it is implanted into the body. 4-Hexylresorcinol (4HR) is a well-known antiseptic. In this study, the biodegradability of 4HR-incorporated silk sutures were compared to that of untreated silk sutures and polyglactin 910 sutures, a commercially available resorbable suture. 4HR-incorporated silk sutures exhibited anti-microbial properties. Matrix metalloproteinase (MMP) can digest a wide spectrum of proteins. 4HR increased MMP-2, -3, and -9 expression in RAW264.7 cells. MMP-2, -3, and -9 were able to digest not only silk fibroin but also silk sutures. Consequently, 59.5% of the 4HR-incorporated silk suture material remained at 11 weeks after grafting, which was similar to that of polyglactin 910 degradation (56.4% remained). The residual amount of bare silk suture material at 11 weeks after grafting was 91.5%. The expression levels of MMP-2, -3 and -9 were high in the 4HR-incorporated silk suture-implanted site 12 weeks after implantation. In conclusion, 4HR-treated silk sutures exhibited anti-microbial properties and a similar level of bio-degradation to polyglactin 910 sutures and induced higher expression of MMP-2, -3, and -9 in macrophages.