Comparative Proteomics Reveal Diverse Functions and Dynamic Changes of Bombyx mori Silk Proteins Spun from Different Development Stages

A comprehensive gene expression analysis of the unique three-layered cocoon of the cecropia moth, Hyalophora cecropia

The larvae of the moth Hyalophora cecropia spin silk cocoons with morphologically distinct layers. We investigated the expression of the individual silk protein components of these cocoons in relation to the morphology of the silk gland and its affiliation to the different layers of the cocoon. The study used transcriptomic and proteomic analyses to identify 91 proteins associated with the silk cocoons, 63 of which have a signal peptide indicating their secretory nature. We checked the specificity of their expression in different parts of the SG and the presence of the corresponding protein products in each cocoon layer. Differences were observed among less abundant proteins with unclear functions. The representation of proteins in the inner envelope and intermediate space was similar, except for a higher proportion of probable contaminating proteins, mostly originating from the gut. On the other hand, the outer envelope contains a number of putative enzymes with unclear function. However, the protein most specific to the outer layer has sequence homology to putative serine/threonine kinase-like proteins and some adhesive proteins, and its closest homolog in Bombyx mori was found in the scaffold silk. This research provides valuable insights into the silk production of the cecropia moth, highlighting both similarities and differences to other moth species.

Mutation in the Bombyx mori BmGMC2 gene impacts silk production and silk protein synthesis

Silk is a natural protein fiber that is predominantly comprised of fibroin and sericin. In addition, it contains seroins, protease inhibitors, enzymes, and other proteins. We found an ecdysone oxidase BmGMC2, notably, which is specifically and highly expressed only in the silk glands of silkworms (Bombyx mori L.). It is also one of the main components of non-cocoon silk, however, its precise function remains unclear. In this study, we examined the spatiotemporal expression pattern of this protein and obtained a homozygous mutant strain (K-GMC2) using the CRISPR-Cas9 system. Compared to the wild-type strain (WT), the silk production and main silk proteins significantly decreased in the larval stage, and the adhesive strength of native silk proteins decreased in the final instar. Proteomic data indicated the abundance of ribosomal proteins decreased significantly in K-GMC2, differentially expressed proteins (DEPs) were enriched in pathways related to neurodegenerative diseases and genetic information processing, indicating that knockout may lead to a certain degree of cell stress, affecting the synthesis of silk proteins. This study investigated the expression pattern and gene function of ecdysone oxidase BmGMC2 in silk and silk glands, laying the groundwork for understanding the role of enzymes in the production of silk fibers.

A review on complete silk gene sequencing and de novo assembly of artificial silk

Silk, especially spider and insect silk, is a highly versatile biomaterial with potential applications in biomedicine, materials science, and biomimetic engineering. The primary structure of silk proteins is the basis for the mechanical properties of silk fibers. Biotechnologies such as single-molecule sequencing have facilitated an increasing number of reports on new silk genes and assembled silk proteins. Therefore, this review aims to provide a comprehensive overview of the recent advances in representative spider and insect silk proteins, focusing on identification methods, sequence characteristics, and de novo design and assembly. The review discusses three identification methods for silk genes: polymerase chain reaction (PCR)-based sequencing, PCR-free cloning and sequencing, and whole-genome sequencing. Moreover, it reveals the main spider and insect silk proteins and their sequences. Subsequent de novo assembly of artificial silk is covered and future research directions in the field of silk proteins, including new silk genes, customizable artificial silk, and the expansion of silk production and applications are discussed. This review provides a basis for the genetic aspects of silk production and the potential applications of artificial silk in material science and biomedical engineering.

A novel method for silkworm cocoons self-degumming and its effect on silk fibers

INTRODUCTION

Conventional hot-alkaline cocoon degumming techniques greatly weaken the physicochemical and mechanical properties of silk fibroin fiber, thus affecting the quality of silk fabric. Moreover, it causes massive energy waste and serious environmental pollution.

OBJECTIVE

This study aims to establish a novel cocoon self-degumming method by genetic modification of silkworm varieties and silk fibers.

METHODS

The self-degummed cocoon material was generated by specifically overexpressing trypsinogen protein in the sericin layer of silk thread; the effect of cocoon self-degumming method was evaluated by the degumming rate of sericin protein, the cleanliness and equivalent diameter of silk fibroin fiber; the basic characteristics of silk fibroin fiber degummed by cocoon self-degumming method and conventional hot-alkaline degumming technique were determined by electron microscopy, Fourier infrared spectroscopy, X-ray diffraction and tensile tests; the composition and biological activity of degummed sericin protein was respectively analyzed by liquid chromatograph-mass spectrometry and cytological experiments.

RESULTS

The genetically engineered self-degumming cocoon containing trypsinogen protein was successfully created, and the content of trypsinogen protein in silk was 47.14 ± 0.90 mg/g. The sericin protein in the self-degumming cocoon was removed out in water or 1 mM Tris-HCl buffer (pH = 8.0). Compared to alkaline-degummed silk fibroin, self-degummed silk fibroin had better cleanliness, thicker equivalent diameter, more complete silk structure and better mechanical property. In addition, sericin protein degummed from self-degumming cocoons significantly promoted cell proliferation and caused no obvious cytotoxicity.

CONCLUSION

Compared to conventional hot-alkaline degumming technique, the cocoon self-degumming method by genetically overexpressing trypsinogen protein in sericin layer of silk thread can self-degummed in a mild degumming condition, and gain silk fiber with better quality and more biologically active sericin protein products. This strategy can not only reduce the environmental impact, but also generate greater economic value, which will accelerate its application in the silk and pharmaceutical industries.

A pilot study of stable isotope fractionation in Bombyx mori rearing

Hydrogen, oxygen, carbon, and nitrogen isotopes derived from three different strains of silkworms at different life stages involved in silkworm rearing, were measured to understand the fractionation characteristics of stable isotopes at different stages of silkworm development, and to trace the movement of these isotopes from food to larva to excrement and finally to silk. We found that silkworm strain had little effect on δ²H, δ¹⁸O and δ¹³C values. However, a large difference was found in the δ¹⁵N levels of newly-hatched silkworms between Jingsong Haoyue and Hua Kang No. 3 orthogonal strains, suggesting that the mating and egg laying differences may result in an inconsistent kinetic nitrogen isotope fractionation. The δ¹³C values of silkworm pupae and silkworm cocoon also displayed significant differences, suggesting that heavy carbon isotopes are greatly fractionated from the larva to the silk during cocoon formation. Overall, these results may be used to clarify the relationship between isotope fractionation and the ecological process of the Bombyx mori and expand our ability to resolve stable isotope anomalies at a small regional-scale level.

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.

Amino Acid Substitutions at P1 Position Change the Inhibitory Activity and Specificity of Protease Inhibitors BmSPI38 and BmSPI39 from Bombyx mori

It was found that silkworm serine protease inhibitors BmSPI38 and BmSPI39 were very different from typical TIL-type protease inhibitors in sequence, structure, and activity. BmSPI38 and BmSPI39 with unique structure and activity may be good models for studying the relationship between the structure and function of small-molecule TIL-type protease inhibitors. In this study, site-directed saturation mutagenesis at the P1 position was conducted to investigate the effect of P1 sites on the inhibitory activity and specificity of BmSPI38 and BmSPI39. In-gel activity staining and protease inhibition experiments confirmed that BmSPI38 and BmSPI39 could strongly inhibit elastase activity. Almost all mutant proteins of BmSPI38 and BmSPI39 retained the inhibitory activities against subtilisin and elastase, but the replacement of P1 residues greatly affected their intrinsic inhibitory activities. Overall, the substitution of Gly54 in BmSPI38 and Ala56 in BmSPI39 with Gln, Ser, or Thr was able to significantly enhance their inhibitory activities against subtilisin and elastase. However, replacing P1 residues in BmSPI38 and BmSPI39 with Ile, Trp, Pro, or Val could seriously weaken their inhibitory activity against subtilisin and elastase. The replacement of P1 residues with Arg or Lys not only reduced the intrinsic activities of BmSPI38 and BmSPI39, but also resulted in the acquisition of stronger trypsin inhibitory activities and weaker chymotrypsin inhibitory activities. The activity staining results showed that BmSPI38(G54K), BmSPI39(A56R), and BmSPI39(A56K) had extremely high acid-base and thermal stability. In conclusion, this study not only confirmed that BmSPI38 and BmSPI39 had strong elastase inhibitory activity, but also confirmed that P1 residue replacement could change their activity and inhibitory specificity. This not only provides a new perspective and idea for the exploitation and utilization of BmSPI38 and BmSPI39 in biomedicine and pest control, but also provides a basis or reference for the activity and specificity modification of TIL-type protease inhibitors.

POUM2 homeostasis regulates intimal remodeling and cells fate in the anterior silk gland of the silkworm

Tissue/organ remodeling and cells fate determination play key roles in the life cycle of animals. However, they are still poorly understood in insects, especially in the silkworm. The anterior silk gland (ASG) of the silkworm is essential for the formation and performance of silk fibers, but the regulatory mechanism of ASG remodeling and cells fate determination is less known. Here we found that silencing of POUM2 caused shorter ASG length, intimal structural defects, silkworm spinning failure, and the resultant naked pupae death, but cells number was not affected. Cells staining showed that DNA endoreduplication was not affected in the ASG. Transmission electron microscopy and chitin staining showed cuticle proteins and chitin were greatly reduced in the ASG during the molting period. Transcriptional analysis showed the expression profiles of cuticle proteins and chitin synthase were similar to that of POUM2 during the molting period, and POUM2 down-regulation reduced the expression of cuticle proteins, chitin synthase, autophagy and apoptosis-related genes. While the phenotype resulting from POUM2 over-expression was similar to that of POUM2 down-regulation. Cells staining revealed marked cells apoptosis with cells number reduction and inhibition of DNA endoreduplication in the ASG. Transcriptional analysis showed the expression of autophagy and apoptosis-related genes, and some cuticle proteins and chitin synthase were significantly up-regulated. The results suggest that POUM2 homeostasis regulates ASG intimal remodeling and cells fate, thus affecting ASG development, silkworm spinning and metamorphosis. Our studies not only offer potential molecular targets for genetic improvement of silk performance and molecular breeding of the silkworm, but also provide new insights into POU factor-mediated tissue remodeling and cells fate determination in insects.

Multi-Omics Integration to Reveal the Mechanism of Sericin Inhibiting LPS-Induced Inflammation

Sericin is a natural protein with high application potential, but the research on its efficacy is very limited. In this study, the anti-inflammatory mechanism of sericin protein was investigated. Firstly, the protein composition of sericin extracts was determined by Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS). This was then combined with Enzyme-linked Immunosorbent Assay (ELISA) and Quantitative Real-time PCR (qRT-PCR), and it was confirmed that the anti-inflammation ability of sericin was positively correlated with the purity of sericin 1 protein. Finally, RNA-seq was performed to quantify the inhibitory capacity of sericin sample SS2 in LPS-stimulated macrophages. The gene functional annotation showed that SS2 suppressed almost all PRRs signaling pathways activated by lipopolysaccharides (LPS), such as the Toll-like receptors (TLRs) and NOD-like receptors (NLRs) signaling pathways. The expression level of adaptor gene MyD88 and receptor gene NOD1 was significantly down-regulated after SS2 treatment. SS2 also reduced the phosphorylation levels of NF-κB P65, P38, and JNK, thereby reducing the expressions of IL-1β, IL-6, INOS, and other inflammatory cytokines. It was confirmed that sericin inhibited LPS-induced inflammation through MyD88/NF-κB pathway. This finding provides necessary theoretical support for sericin development and application.

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.

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.

The Role of Filippi's Glands in the Silk Moths Cocoon Construction

Filippi’s glands (FGs), formerly also called Lyonet’s glands, are accessory secretory structures of the labial (silk) glands of lepidopteran caterpillars, which were implicated to play an important role in the maturation of the silk material and the construction of the cocoon. In our previous study, we have identified several species of giant silk moths that completely lack the FGs. Interestingly, the absence of FGs in these species correlates with the construction of a loose cocoon architecture. We investigated the functions of FGs by their surgical extirpation in the last instar larvae of the silkworm, Bombyx mori. We found that the absence of FGs altered the structure of the resulting cocoon, in which the different layers of silk were separated. In further experiments, we found no effects of the absence of FGs on larval cocoon formation behavior or on changes in cocoon mass or lipid content. Differential proteomic analysis revealed no significant contribution of structural proteins from FGs to silk cocoon material, but we identified several low abundance proteins that may play a role in posttranslational modifications of some silk proteins. Proteomic analysis also revealed a difference in phosphorylation of the N-terminal sequence of fibroin-heavy chain molecule. Thus, FGs appear to affect silk stickiness during spinning by regulating posttranslational modifications. This could also explain the link that exists between the absence of these glands and the formation of loose cocoons in some giant silk moth species.

Mutation in Bombyx mori fibrohexamerin (P25) gene causes reorganization of rough endoplasmic reticulum in posterior silk gland cells and alters morphology of fibroin secretory globules in the silk gland lumen

Larvae of many lepidopteran species produce a mixture of secretory proteins, known as silk, for building protective shelters and cocoons. Silk consists of a water-insoluble silk filament core produced in the posterior silk gland (PSG) and a sticky hydrophilic coating produced by the middle silk gland (MSG). In Bombyx mori, the fiber core comprises three proteins: heavy chain fibroin (Fib-H), light chain fibroin (Fib-L) and fibrohexamerin (Fhx, previously referred to as P25). To learn more about the role of Fhx, we used transcription activator-like effector nuclease (TALEN) mutagenesis and prepared a homozygous line with a null mutation in the Fhx gene. Our characterization of cocoon morphology and silk quality showed that the mutation had very little effect. However, a detailed inspection of the secretory cells in the posterior silk gland (PSG) of mid-last-instar mutant larvae revealed temporary changes in the morphology of the endoplasmic reticulum. We also observed a morphological difference in fibroin secretory globules stored in the PSG lumen of Fhx mutants, which suggests that their fibroin complexes have a slightly lower solubility. Finally, we performed an LC-MS-based quantitative proteomic analysis comparing mutant and wild-type (wt) cocoon proteins and found a high abundance of a 16 kDa secretory protein likely involved in fibroin solubility. Overall, our study shows that whilst Fhx is dispensable for silk formation, it contributes to the stability of fibroin complexes during intracellular transport and affects the morphology of fibroin secretory globules in the PSG lumen.

Role of sericin 1 in the immune system of silkworms revealed by transcriptomic and proteomic analyses after gene knockout

The domestic silkworm is a type of lepidopteran insect that feeds on mulberry leaves and has high economic value because of its ability to spin cocoons. Sericin 1 is an important component of silkworm cocoons, accounting for approximately 25% of the material. In this study, CRISPR/Cas9‐mediated gene editing was successfully used to destroy the sericin 1 gene, and homozygous mutants were obtained after continuous screening. Homozygous mutation resulted in premature termination of the translation of sericin 1 protein at 323 amino acids. Comparative transcriptomic and proteomic analyses of middle silk gland cells from wild‐type individuals and mutants were performed on the fourth day of the fifth instar, and the results suggest that sericin 1 plays an important role in the cellular immune system. In addition, the results suggest that sericin 1 has a synergistic effect with some protease inhibitors and that the secretion of these proteins is strictly regulated. These results will provide new insights into the function and expression pattern of sericin 1 and the mechanism of silk secretion.

Milligram scale expression, refolding, and purification of Bombyx mori cocoonase using a recombinant E. coli system

Silk is one of the most versatile biomaterials with signature properties of outstanding mechanical strength and flexibility. A potential avenue for developing more environmentally friendly silk production is to make use of the silk moth (Bombyx mori) cocoonase, this will at the same time increase the possibility for using the byproduct, sericin, as a raw material for other applications. Cocoonase is a serine protease utilized by the silk moth to soften the cocoon to enable its escape after completed metamorphosis. Cocoonase selectively degrades the glue protein of the cocoon, sericin, without affecting the silk-fiber made of the protein fibroin. Cocoonase can be recombinantly produced in E. coli, however, it is exclusively found as insoluble inclusion bodies. To solve this problem and to be able to utilize the benefits associated with an E. coli based expression system, we have developed a protocol that enables the production of soluble and functional protease in the milligram/liter scale. The core of the protocol is refolding of the protein in a buffer with a redox potential that is optimized for formation of native and intramolecular di-sulfide bridges. The redox potential was balanced with defined concentrations of reduced and oxidized glutathione. This E.coli based production protocol will, in addition to structure determination, also enable modification of cocoonase both in terms of catalytic function and stability. These factors will be valuable components in the development of alternate silk production methodology.

Silk of the common clothes moth, Tineola bisselliella, a cosmopolitan pest belonging to the basal ditrysian moth line

Many lepidopteran larvae produce silk secretions to build feeding tubes and cocoons that play important protective roles in their lives. Recent research on the silk of bombycoid and pyralid moths has shown that it contains several highly abundant silk components with remarkable mechanical properties. It was also found to contain a number of other proteins of which the functions have yet to be identified. To gain an overview of the silk composition in more primitive lepidopteran species and to identify the core silk components common to most species, we analyzed the cocoon proteins of Tineola bisselliella, which belongs to the basal ditrysian moth line. Using de novo transcriptome sequencing combined with mass spectrometry (MS)-based proteomics, we detected more than 100 secretory proteins in the silk cocoons. Fibroin, sericins, and protease inhibitors were found to be the most abundant proteins, along with several novel candidate silk components. We also verified the tissue and developmental stage specificity of the silk protein expression and characterized the morphology of both the silk glands and silk in T. bisselliella. Our study provides a detailed analysis of silk in the primitive moth, expands the known set of silk-specific genes in Lepidoptera, and helps to elucidate their evolutionary relationships.

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.

Sericin for Tissue Engineering

Sericin is a 10-to-400 kDa hydrophilic protein with high serine content and is a silk constituent together with fibroin. It is produced in the middle silk gland of the silkworm and encoded by four sericin genes. The molecular weight of sericin and its biological activity vary depending on the extraction method employed. Its chemical structure, in terms of random coil and β-sheet conformations, also differs with the extraction method, thereby extending its applications in various fields. Sericin, which was discarded in the textile industry in the past, is being applied and developed in the biomedical field, owing to its biological properties. In particular, many studies are underway in the field of tissue engineering, evaluating its applicability in burn dressing, drug delivery, bone regeneration, cartilage regeneration, and nerve regeneration.

Biosynthetic sericin 1-like protein skews dendritic cells to tolerogenic-like phenotype

Silkworm sericin has been widely exploited in biomaterials due to its favorable biological activities. However, the extraction processes of sericin from silkworm cocoons can alter the biological and biophysical properties, including a structural diversity of natural sericin. In addition, extracted natural sericin is often contaminated with fibroin that may be harmful to human cells. Induction of tolerogenic dendritic cell (DC) has become a strategy in biomaterial fields because this cell type plays a key role in immune modulation and wound healing. To overcome undesired effects of extracted natural sericin and to improve its biological properties, we biosynthesized sericin 1-like protein that contained only functional motifs and tested its biological activity and immunomodulatory properties in fibroblasts and DCs, respectively. In comparison to natural sericin, biosynthetic sericin 1 promoted collagen production in fibroblasts at a late time point. Furthermore, DCs treated with biosynthetic sericin 1 exhibited a tolerogenic-like phenotype with semimaturation and low production of proinflammatory cytokines, but high production of anti-inflammatory cytokine, IL-10. Biosynthetic sericin 1 might be developed as immunomodulator or immunosuppressant.

Proteome analysis of male accessory gland secretions in Leucinodes orbonalis Guenee (Lepidoptera: Crambidae), a Solanum melongena L. pest

Male accessory gland (MAG) proteins are transferred along with the sperm to females at the time of mating and have diverse effects on female reproductive physiology in a wide range of insects. In this study, we sought to identify the MAG proteins in Leucinodes orbonalis Guenee, a Solanum melongena L. pest, by analyzing the MAG proteins of virgin and mated male moths by nano-LC-ESI-MS/MS techniques. A total of 142 and 131 proteins in virgin and mated males were identified, respectively, among which 17 (12.0%) and 10 (7.6%) proteins were found to show secretory signals in virgin and mated males, respectively. These secretory proteins were shown to be involved in several biological processes in insects, including egg development, sperm-related functions/capacitation, defense, metabolism, and protein chaperoning. To the best of our knowledge, this is the first study to perform a proteome analysis of the MAG proteins of L. orbonalis, and offers an opportunity for further investigation of the functions of these proteins. In insects, certain MAG proteins are known to inhibit mating whereas others accelerate egg-laying. Therefore, the identification of these proteins in L. orbonalis may be useful for pest control.

Synthesis, secretion, and antifungal mechanism of a phosphatidylethanolamine-binding protein from the silk gland of the silkworm Bombyx mori

A silkworm cocoon contains several antimicrobial proteins such as protease inhibitors and seroins to provide protection for the enclosed pupa. In this study, we identified a new Bombyx mori phosphatidylethanolamine-binding protein (BmPEBP) with antimicrobial activity in the cocoon silk using semi-quantitative and quantitative RT-PCR, western blotting, and immunofluorescence. The results indicated that BmPEBP was synthesized in the middle silk gland and secreted into the sericin layer of the cocoon silk. Functional analysis showed that BmPEBP could inhibit the spore growth of four types of fungi, Candida albicans, Saccharomyces cerevisiae, Beauveriabassiana, and Aspergillus fumigates, by binding to the fungal cell membrane. Investigation of the interaction of BmPEBP with membrane phospholipids revealed that the protein showed a strong binding affinity to phosphatidylethanolamine, weak affinity to phosphatidylinositol, and no affinity to phosphatidylserine or phosphatidylcholine. Circular dichroism spectroscopy showed that binding to phosphatidylethanolamine caused conformational changes in the BmPEBP molecule by reducing β-sheet formation and inducing the appearance of an α-helix motif. We speculate that BmPEBP performs antifungal function in the cocoon silk through interaction with phosphatidylethanolamine in the fungal membrane.

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.

Insights into regulatory characteristics of the promoters of Sericin 1 and Sericin 3 in transgenic silkworms

Sericin, produced in the middle silk gland (MSG) of silkworms, is a group of glue proteins that coat and cement silk fibers. Several genes are known to encode sericin, but their spatiotemporal regulation has yet to be fully elucidated. Here, we report in detail the expression profiles of the promoters of two major sericin-coding genes, Sericin 1 (Ser1)and Sericin 3 (Ser3), by analyzing Gal4/UAS transgenic silkworms. We found that UAS-linked EGFP fluorescence in transgenic silkworms driven by Ser1-Gal4was detected in only the R3, R4 and R5 regions of MSG starting inday-3 fifth-instar larvae and was continuously expressed until silk gland degradation. In transgenic silkworms driven by Ser3-Gal4, EGFP fluorescence was detected at a low level in the R2 region of MSG since the last day of fifth-instar larvae, and the expression increased during the wandering stages and was continuously detected until silk gland degradation. The molecular detection of EGFP expression in each of the Gal4/UAS transgenic silkworms was consistent with fluorescence observations. These findings reveal clear differences in the regulatory characteristics of the promoters of Ser1and Ser3 and provide new insights into the regulatory mechanism of the expression of sericin-coding genes.

Kunitz-type protease inhibitor BmSPI51 plays an antifungal role in the silkworm cocoon

The protease inhibitors found in silkworm cocoons can be divided into several families, a majority of which contain serpin, TIL, or Kunitz domains. Previously, it has been reported that TIL-type protease inhibitors have antimicrobial activity. To date, however, it has not been determined whether the Kunitz-type protease inhibitor BmSPI51, the most abundant of cocoon protease inhibitors, plays an antimicrobial role. Thus, in this study, we sought to determine the biological role of BmSPI51 in silkworm cocoons. Our results obtained from real-time quantitative reverse transcription PCR and immunofluorescence analyses indicate that BmSPI51 is expressed exclusively in the silk glands during the larval fifth instar stage and is subsequently secreted into cocoon silk. Moreover, at a molar ratio of 1:1, BmSPI51 produced via prokaryotic expression exhibited inhibitory activity against trypsin and also proved to be highly stable over wide ranges of temperature and pH values. The expression of BmSPI51 was also found to be significantly upregulated in the larval fat body after infection with three species of fungi, namely, Candida albicans, Beauveria bassiana, and Saccharomyces cerevisiae. In vitro inhibition tests revealed that BmSPI51 significantly inhibited the sporular growth of all three of these fungal species. Further, results obtained from a binding assay showed that BmSPI51 binds to β-d-glucan and mannan on the surface of fungal cells. In this study, we, thus, revealed the antimicrobial activity of BmSPI51 and its underlying mechanism in silkworm, thereby contributing to our present understanding of defense mechanisms in silkworm cocoons.

Rice defense responses are induced upon leaf rolling by an insect herbivore

BACKGROUND

Plant defense against herbivores begins with perception. The earlier plant detects the harm, the greater plant will benefit in its arm race with the herbivore. Before feeding, the larvae of the rice pest Cnaphalocrocis medinalis, initially spin silk and fold up a leaf. Rice can detect and protect itself against C. medinalis feeding. However, whether rice could perceive C. medinalis leaf rolling behavior is currently unknown. Here, we evaluated the role of leaf rolling by C. medinalis and artificial leaf rolling in rice plant defense and its indirect effect on two important C. medinalis parasitoids (Itoplectis naranyae and Apanteles sp.) through a combination of volatile profiling, gene-transcriptional and phytohormonal profiling.

RESULTS

Natural leaf rolling by C. medinalis resulted in an increased attraction of I. naranyae when compared to the undamaged plant after 12 h. Volatile analysis revealed that six out of a total 22 components significantly increased in the headspace of C. medinalis rolled plant when compared to undamaged plant. Principal component analysis of these components revealed similarities in the headspace of undamaged plant and artificially rolled plant while the headspace volatiles of C. medinalis rolled plant deferred significantly. Leaf rolling and feeding by C. medinalis up-regulated the plant transcriptome and a series of jasmonic acid (JA) and salicylic acid (SA) related genes. While feeding significantly increased JA level after 12 to 36 h, rolling significantly increased SA level after 2 to 12 h. Compared to artificial rolling, natural rolling significantly increased JA level after 36 h and SA level after 2 and 12 h.

CONCLUSIONS

Our findings suggest that natural leaf rolling by C. medinalis can be perceived by rice plant. The detection of this behavior may serve as an early warning signal in favor of the rice plant defenses against C. medinalis.

Overexpression of Gloverin2 in the Bombyx mori silk gland enhances cocoon/silk antimicrobial activity

The Bombyx mori cocoon/silk possesses many immune-related components, including protease inhibitors, seroins, and antimicrobial peptides, which likely help to protect the pupating larva from infection. However, the natural antimicrobial activity of the B. mori cocoon/silk is still too weak for biomedical applications. With the goal of enhancing this natural activity, we constructed a transgenic vector to overexpress the B. mori antimicrobial peptide Gloverin2 (BmGlv2) under control of the silk gland-specific Serion1 promoter. Transgenic silkworms were generated via embryo microinjection. A low level of BmGlv2 was expressed in the non-transgenic silk gland, but BmGlv2 was efficiently overexpressed and proteolytically activated in the transgenic line. Overexpressed BmGlv2 was secreted and incorporated into the silk during spanning without affecting cocoon/silk formation. Moreover, the transgenic cocoon/silk had significantly greater inhibitory activity against bacteria and fungi than the non-transgenic cocoon/silk. This strategy could help enhance the antimicrobial performance and biomedical application of silk.

Comparative analysis of proteins from Bombyx mori and Antheraea pernyi cocoons for the purpose of silk identification

Achieving efficient identification of silk protein requires highly sensitive analytical techniques and favorable extraction methods, which is of great significance to the research of ancient silk, especially for the controversial issue of the silk origin. In this paper, proteomics and western blot were proposed to analyze the silk proteins of Bombyx mori (B. mori) and Antheraea pernyi (A. pernyi) dissolved by different methods. First, the differences in secondary structure were detected via spectroscopy. LC-MS/MS was then employed to characterize the peptides of silk proteins precisely. LiBr solution exhibited outstanding dissolution effect on B. mori cocoon, with 87 proteins detected; while copper-ethylenediamine solution (CED) was more appropriate for A. pernyi cocoon, and 16 proteins were identified in A. pernyi-CED. In addition to fibroin and sericin, abundant seroins, enzymes, protease inhibitors, other functional proteins and uncharacterized proteins were detected. Based on the LC-MS/MS data, diagnostic antibodies for the two species were prepared, and fibroin was successfully identified by western blot assay because both dissolution methods were gentle and did not destroy the antigenic epitopes in the protein molecule. Owing to their good specificity and high sensitivity, these diagnostic antibodies have good application prospects in immunoassays of different silk species. SIGNIFICANCE: This study presents the comprehensive analysis on silk identification of proteins from B. mori and A. pernyi extracted by different methods via the proteomic and immunology as well as the conventional approaches. Great coverage of two cocoon proteomes was accomplished, which demonstrated the outstanding difference in components and abundance. Based on the proteomics analysis, the diagnostic antibodies against two species were prepared and identified the corresponding fibroin successfully in the completed protein mixtures. To our knowledge, the proteomic and immunology procedures with high efficiency, sensitivity and specificity are novel analysis on the silk identification and has great potential in the field of ancient silk detection.

Functional analysis and characterization of antimicrobial phosphatidylethanolamine-binding protein BmPEBP in the silkworm Bombyx mori

Phosphatidylethanolamine-binding proteins (PEBPs) are a class of highly conserved, biologically diverse proteins, which are widely distributed in plants, insects, and mammals. In this study, a Bombyx mori PEBP (BmPEBP) gene was reported, which encodes a protein composed of 209 amino acid residues. BmPEBP includes a predicted signal peptide, indicating that it is an extracellular protein, which differs from the cytoplasmic PEBPs of plants and mammals. Recombinant soluble BmPEBP was successfully synthesized using a prokaryotic expression system and was then purified effectively by Ni²⁺-NTA affinity chromatography and gel filtration. Far-ultraviolet circular dichroism spectra indicated that BmPEBP had a well-defined β-sheet structure, with the β-sheet content accounting for about 41% of the protein. BmPEBP had a relatively stable structure at temperatures ranging from 15 °C to 57.5 °C. The Tm, ΔH, and ΔS of BmPEBP were 62.27 °C ± 0.14 °C, 570.10 ± 0.17 kJ/mol, and 1.70 ± 0.03 KJ/(mol·K), respectively. Homology modeling analysis suggested that the active sites of BmPEBP were conserved, comprising Pro96, His111, and His143. Quantitative real-time PCR showed that BmPEBP was highly expressed in the silk gland and had very low expression in other tissues. However, BmPEBP expression was significantly upregulated in the larval fat body after infection with two kinds of fungi, Beauveria bassiana and Candida albicans. Moreover, in vitro fungal inhibition tests showed that BmPEBP could significantly inhibit the sporular growth of Saccharomyces cerevisiae, C. albicans, B. bassiana, and Aspergillus fumigatus. To our knowledge, this is the first report to reveal the antifungal role of a PEBP in insects.

Species identification of Bombyx mori and Antheraea pernyi silk via immunology and proteomics

In recent years, increasing attention has been paid to the origin, transmission and communication of silk. However, this is still an unsolved mystery in archaeology. The identification of silk-producing species, especially silk produced by Bombyx mori (B. mori) and Antheraea pernyi (A. pernyi), is of key significance to address this challenge. In this study, two innovative methods, i.e. immunology and proteomics, were proposed and successfully established for the species identification of silks. ELISAs result demonstrated that the two prepared antibodies exhibited high sensitivity and specificity in distinguishing B. mori and A. pernyi silk. No cross-reactivity with each other was observed. Moreover, biomarkers were obtained for Bombyx and Antheraea through proteomic analysis. It was also confirmed that the biomarkers were suitable for identifying the species that produced a given silk sample. Compared with conventional methods for distinguishing silk species, immunological and proteomics techniques used in tandem can provide intact information and have the potential to provide accurate and reliable information for species identification.

Deep Insight into the Transcriptome of the Single Silk Gland of Bombyx mori

The silk gland synthesizes and secretes a large amount of protein and stores liquid silk protein at an extremely high concentration. Interestingly, silk proteins and serine protease inhibitors are orderly arranged in the silk gland lumen and cocoon shells. Silk fiber formation and the spinning mechanism have not been fully elucidated. Therefore, we conducted a comparative transcriptome analysis of seven segments of the single silk gland to characterize internal changes in the silk gland during the 5th instar of mature larvae. In total, 3121 differentially expressed genes were identified in the seven segments. Genes highly expressed in the middle silk gland (MSG) were mainly involved in unsaturated fatty acid biosynthesis, fatty acid metabolism, apoptosis—fly, and lysosome pathways, whereas genes highly expressed in the posterior silk gland (PSG) were mainly involved in ribosome, proteasome, citrate cycle, and glycolysis/gluconeogenesis pathways. Thus, the MSG and PSG differ greatly in energy source use and function. Further, 773 gradually upregulated genes (from PSG to MSG) were involved in energy metabolism, silk protein synthesis, and secretion, suggesting that these genes play an important role in silk fiber formation. Our findings provide insights into the mechanism of silk protein synthesis and transport and silk fiber formation.

The expansion of genes encoding soluble silk components in the greater wax moth, Galleria mellonella

Lepidopteran silk is a complex assembly of proteins produced by a pair of highly specialized labial glands called silk glands. Silk composition has been examined only in a handful of species. Here we report on the analysis of silk gland-specific transcriptomes from three developmental stages of the greater wax moth, Galleria mellonella, combined with proteomics, Edman microsequencing and northern blot analysis. In addition to the genes known earlier, we identified twenty seven candidate cDNAs predicted to encode secretory proteins, which may represent novel silk components. Eight were verified by proteomic analysis or microsequencing, and several others were confirmed by similarity with known silk genes and their expression patterns. Our results revealed that most candidates encode abundant secreted proteins produced by middle silk glands including ten sericins, two seroins, one or more mucins, and several sequences without apparent similarity to known proteins. We did not detect any novel PSG-specific protein, confirming that there are only three fibroin subunits. Our data not only show that the number of sericin genes in the greater wax moth is higher than in other species thus far examined, but also the total content of soluble proteins in silk is twice as high in G. mellonella than in B. mori or A. yamamai. Our data will serve as a foundation for future identification and evolutionary analysis of silk proteins in the Lepidoptera.

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.

A proteotranscriptomic study of silk-producing glands from the orb-weaving spiders

A proteotranscriptomic approach provides a biochemical basis for understanding the intricate spinning process and complex structural features of spider silk proteins.

The fungal-resistance factors BmSPI38 and BmSPI39 predominantly exist as tetramers, not monomers, in Bombyx mori

Previous studies have indicated that trypsin inhibitor-like cysteine-rich domain (TIL)-type protease inhibitors, BmSPI38 and BmSPI39, suppress conidial germination and integument penetration of entomopathogenic fungi by inhibiting their cuticle-degrading proteases and might functions as fungal-resistance factors in the silkworm. To date, the physiological forms and functional significance of multimerization of BmSPI38 and BmSPI39 remain unknown. In this study, we investigated the physiological forms of BmSPI38 and BmSPI39 in Bombyx mori silkworms using multiple complementary methods, including activity staining, reducing and nonreducing sodium dodecyl sulfate polyacrylamide gel electrophoresis, matrix-assisted laser desorption ionization time-of-flight mass spectrometry, western blotting and immunofluorescence. We found that recombinant BmSPI38 and BmSPI39 tend to form homologous multimers, and their dimers, trimers and tetramers possessed intense inhibitory activity against subtilisin A from Bacillus licheniformis. In contrast, their monomers showed no detectable inhibitory activity. Both BmSPI38 and BmSPI39 also exist mainly as stable tetramers in silkworm tissues, and they also predominantly function as a tetramer in these tissues. This study is the first to demonstrate this preferred quaternary form of a TIL-type protease inhibitor and will likely help to elucidate the mechanisms of BmSPI38 and BmSPI39 in the innate immune response of the silkworm.

Comparative Proteome Analysis Reveals that Cuticular Proteins Analogous to Peritrophin-Motif Proteins are Involved in the Regeneration of Chitin Layer in the Silk Gland of Bombyx mori at the Molting Stage

The silk gland of silkworm produces silk proteins during larval development. Many studies have long focused on the silk gland of the fifth instar larvae, but few have investigated this gland at other larval stages. In the present study, the silk gland proteomes of the fourth instar and fourth molt are analyzed using liquid chromatography-tandem mass spectrometry. In total, 2654 proteins are identified from the silk gland. A high abundance of ribosomal proteins and RR-motif chitin-binding proteins is identified during day 2 of the fourth instar (IV-2) larval developmental stage, and the expression of cuticular proteins analogous to peritrophin (CPAP)-motif chitin-binding proteins is higher during the fourth molt (IV-M). In all, nine enzymes are found to be involved in the chitin regeneration pathway in the silk gland. Among them, two chitinase and two chitin deacetylases are identified as CPAP-motif proteins. Furthermore, the expression of CPAP3-G, the most abundant CPAP-motif cuticular protein in the silk gland during the IV-M stage, is investigated using western blot and immunofluorescence analyses; CPAP3-G shows a reverse changing trend with chitin in the silk gland. The findings of this study suggest that CPAP-motif chitin-binding proteins are involved in the degradation of the chitin layer in the silk gland. The data have been deposited to the ProteomeXchange with identifier PXD008677.

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.

Osiris9a is a major component of silk fiber in lepidopteran insects

In a previous high-throughput proteomics study, it was found that the silkworm cocoon contains hundreds of complex proteins, many of which have unknown functions, in addition to fibroins, sericins, and some protease inhibitors. Osiris was one of the proteins with no known function. In this study, we identified the Osiris gene family members and constructed a phylogenetic tree based on the sequences from different species. Our results indicate that the Osiris9 gene subfamily contains six members; it is specifically expressed in lepidopteran insects and has evolved by gene duplication. An Osiris gene family member from Bombyx mori was designated as BmOsiris9a (BmOsi9a) on the basis of its homology to Drosophila melanogaster Osiris9. The expression pattern of BmOsi9a showed that it was highly expressed only in the middle silk gland of silkworm larvae, similar to Sericin1 (Ser1). BmOsi9a was visualized as two bands in western blot analysis, implying that it probably undergoes post-translational modifications. Immunohistochemistry analysis revealed that BmOsi9a was synthesized and secreted into the lumen of the middle silk gland, and was localized in the sericin layer in the silk fiber. BmOsi9a was found in the silk fibers of not only three Bombycidae species, viz. B. mori, B. mandarina, and B. huttoni, but also in the fibers collected from Saturniidae species, including Antheraea assama, Antheraea mylitta, and Samia cynthia. Although the exact biological function of Osi9a in the silk fibers is unknown, our results are important because they demonstrate that Osi9a is a common structural component of silk fiber and is expressed widely among the silk-producing Bombycidae and Saturniidae insects. Our results should help in understanding the role of Osi9a in silk fibers.

Wild Silkworm Cocoon Contains More Metabolites Than Domestic Silkworm Cocoon to Improve Its Protection

The silk of silkworm consists of fibroin fiber coated by sericins. In addition, some nonprotein components were also identified in the sericin fraction. The presence of nonprotein components in the silk has not been well explained. In the present study, methods based on gas chromatography-mass spectrometry were used to identify the metabolites in the cocoon silk from a wild silkworm and two domestic silkworm strains. In total, 45 metabolites were in the cocoon silk, including organic acids, fatty acids, carbohydrates, amino acids, and hydrocarbons. Comparative analyses revealed that 17 metabolites were significant more in the wild silkworm cocoon than in the domestic silkworm cocoon, including three organic acids, three fatty acids, three aldoses, four sugar alcohols, three hydrocarbons, and pyridine. Of them, citric acid in the wild silkworm cocoon is more than 40 times that in the domestic silkworm cocoon, which may have protective value against microbes. The carbohydrate, lipid, and the long-chain hydrocarbons may act as water repellent to make the pupa survive longer in the dry environment. Many metabolites in the cocoon silk may play roles to improve the silk resistance. Lots of nonprotein components were identified in the silk for the first time, providing useful data for understanding the biological function of the cocoon silk.

Quantitative proteomic analysis of the fall armyworm saliva

Lepidopteran larvae secrete saliva on plant tissues during feeding. Components in the saliva may aid in food digestion, whereas other components are recognized by plants as cues to elicit defense responses. Despite the ecological and economical importance of these plant-feeding insects, knowledge of their saliva composition is limited to a few species. In this study, we identified the salivary proteins of larvae of the fall armyworm (FAW), Spodoptera frugiperda; determined qualitative and quantitative differences in the salivary proteome of the two host races-corn and rice strains-of this insect; and identified changes in total protein concentration and relative protein abundance in the saliva of FAW larvae associated with different host plants. Quantitative proteomic analyses were performed using labeling with isobaric tags for relative and absolute quantification followed by liquid chromatography-tandem mass spectrometry. In total, 98 proteins were identified (>99% confidence) in the FAW saliva. These proteins were further categorized into five functional groups: proteins potentially involved in (1) plant defense regulation, (2) herbivore offense, (3) insect immunity, (4) detoxification, (5) digestion, and (6) other functions. Moreover, there were differences in the salivary proteome between the FAW strains that were identified by label-free proteomic analyses. Thirteen differentially identified proteins were present in each strain. There were also differences in the relative abundance of eleven salivary proteins between the two FAW host strains as well as differences within each strain associated with different diets. The total salivary protein concentration was also different for the two strains reared on different host plants. Based on these results, we conclude that the FAW saliva contains a complex mixture of proteins involved in different functions that are specific for each strain and its composition can change plastically in response to diet type.

RNA-Seq Analyses for Two Silkworm Strains Reveals Insight into Their Susceptibility and Resistance to Beauveria bassiana Infection

The silkworm Bombyx mori is an economically important species. White muscardine caused by Beauveria bassiana is the main fungal disease in sericulture, and understanding the silkworm responses to B. bassiana infection is of particular interest. Herein, we investigated the molecular mechanisms underlying these responses in two silkworm strains Haoyue (HY, sensitive to B. bassiana) and Kang 8 (K8, resistant to B. bassiana) using an RNA-seq approach. For each strain, three biological replicates for immersion treatment, two replicates for injection treatment and three untreated controls were collected to generate 16 libraries for sequencing. Differentially expressed genes (DEGs) between treated samples and untreated controls, and between the two silkworm strains, were identified. DEGs and the enriched Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways of the two strains exhibited an obvious difference. Several genes encoding cuticle proteins, serine proteinase inhibitors (SPI) and antimicrobial peptides (AMP) and the drug metabolism pathway involved in toxin detoxification were considered to be related to the resistance of K8 to B. bassiana. These results revealed insight into the resistance and susceptibility of two silkworm strains against B. bassiana infection and provided a roadmap for silkworm molecular breeding to enhance its resistance to B. bassiana.

Protease inhibitors in Bombyx mori silk might participate in protecting the pupating larva from microbial infection

Pupae inside cocoons rarely suffer from disease. It is apparent that some factors in the cocoon exert antimicrobial effects whereby the pupae inside can be protected from microbial infection. In the present study, we investigated the expression of cocoon protease inhibitors using immunoblotting and activity staining. Enzymatic hydrolysis of cocoon proteins in vitro was performed to characterize their roles in protecting the cocoon from microbial proteases. We found that some protease inhibitors, particularly trypsin inhibitor-like (TIL)-type protease inhibitors, can be secreted into the cocoon layer during the spinning process, thereby providing effective protection to the cocoon and pupa by inhibiting the extracellular proteases that can be secreted by pathogens.

Histomorphometric and transcriptomic features characterize silk glands' development during the molt to intermolt transition process in silkworm

The molt-intermolt cycle is an essential feature in holometabolous and hemimetabolous insects' development. In the silkworm, silk glands are under dramatic morphological and functional changes with fibroin genes' transcription being repeatedly turned off and on during the molt-intermolt cycles. However, the molecular mechanisms controlling it are still unknown. Here, silk gland's histomorphology and transcriptome analysis were used to characterize changes in its structure and gene expression patterns from molt to intermolt stages. By using section staining and transmission electron microscope, a renewable cell damage was detected in the silk gland at the molt stage, and an increased number of autophagosomes and lysosomes were found in silk gland cells' cytoplasm. Next, by using RNA sequencing, 54,578,413 reads were obtained, of which 85% were mapped to the silkworm reference genome. The expression level analysis of silk protein genes and silk gland transcription factors revealed that fibroin heavy chain, fibroin light chain, P25/fhx, sericin1, sericin3 and Dimm had consistent alteration trends in temporal expression. In addition, differentially expressed genes (DEGs) were identified, and most of the DEGs associated with ecdysone signal transduction, mRNA degradation, protein proteolysis, and autophagy were significantly down-regulated in the transition from molt to intermolt, suggesting that these pathways were activated for the silk gland renewal. These findings provide insights into the molecular mechanisms of silk gland development and silk protein genes transcriptional regulation during the molt to intermolt transition process.

DIFFERENTIAL PROTEOME ANALYSIS OF THE MALE AND FEMALE ANTENNAE FROM Holotrichia parallela

To understand the olfactory mechanisms of Holotrichia parallela antennae in detecting volatile compounds in the environment, protein profiles of H. parallela antennae were analyzed using two-dimensional electrophoresis followed by mass spectrometry and bioinformatics analyses. Approximately 1,100 protein spots in silver staining gel were detected. Quantitative image analysis revealed that in total 47 protein spots showed significant changes in different genders of adult antennae. Thirty-five differentially expressed proteins were identified by Matrix assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF/TOF) tandem mass spectrometer, among which 65.7% are involved in carbohydrate and energy metabolism, antioxidant system, transport, and amino acid/nucleotide metabolism. Some proteins identified here have not been reported previously in insect antennae. Identified male-biased proteins included odorant-binding protein 4, pheromone-binding protein-related protein 2, odorant-binding protein 14, prophenoloxidase-I, acyl-CoA dehydrogenase, aldo-keto reductase-like, carbamoyl phosphate synthetase, etc. whereas some proteins are female biased, such as antennae-rich cytochrome P450, aldehyde dehydrogenase, and putative glutamine synthetase. Alterations in the levels of some proteins were further confirmed by real time polymerase chain reaction (RT-PCR). The proteomic resources displayed here are valuable for the discovery of proteins from H. parallela antennae.

Structure, evolution, and expression of antimicrobial silk proteins, seroins in Lepidoptera

The silks of silkworm and waxworm contain abundant antimicrobial proteins, including protease inhibitors and seroins. Protease inhibitors have antifungal activities, whereas seroins have antiviral and antibacterial activities. In order to obtain insights into the structure, evolution, and expression of seroins, we performed an extensive survey based on the available genome, transcriptome, and expressed sequence tags datasets. Sixty-four seroins were identified in 32 lepidopteran species. The phylogenetic and structural analyses revealed that seroins can be classified into five subfamilies: seroin 1, seroin 2, seroin 3, seroin 2 + 1, and seroin 3 + 3. It is interesting that seroin 2 + 1 contains two tandem seroin domains, seroin 2 and seroin 1, whereas seroin 3 + 3 has two tandem seroin 3 domains. Each seroin domain contains a proline-rich N-terminal motif and a conserved C-terminal motif. The transcriptome and EST data indicated that seroin 1 and seroin 2 genes were expressed in the silk gland but seroin 3 genes were not. Semi-quantitative RT-PCR and western blot analyses suggested that seroin 1 and seroin 2 were constantly accumulated in the silk gland of silkworm during the fifth instar, and then secreted into cocoon silk during spinning. Immunofluorescence analyses indicated that seroin 1 was secreted into the fibroin and sericin layers, whereas seroin 2 protein was only secreted into the sericin layer. However, the antimicrobial activity of seroin 2 was more effective than that of seroin 1. The presence of seroin 1 in the fibroin layer suggested that this protein not only acts as an antimicrobial protein, but might also play a role in the assembly and secretion of fibroins. Seroin 3, which was first identified here, might be related to pheromone synthesis or recognition, as it was highly expressed in male antennae and in the pheromone gland.