Dimmed gene knockout shortens larval growth and reduces silk yield in the silkworm, Bombyx mori

Broad complex negatively regulates Fibrohexamerin/P25 by binding to the cis-element BMFA in the silkworm, Bombyx mori

Silk proteins, as natural macromolecular substances, hold significant potential for applications in biomaterials and biomedical fields. The expression of silk protein genes exhibits spatiotemporal specificity. Broad Complex (BrC), a key primary response factor to 20-hydroxyecdysone, plays a crucial role in metamorphosis. Our previous study showed that overexpression of BmBrC-Z2 significantly reduced fibroin gene Fibrohexamerin/P25 expression in the posterior silk gland. However, the underlying regulatory mechanism remains unclear. BMFA, a widely expressed factor that inhibits silk protein gene expression by recognizing BMFA elements, remains unidentified. Notably, the binding sequence of BmBrC-Z2 on the P25 promoter aligns with the BMFA element. Dual-Luciferase Reporter Assays, EMSA, and ChIP-PCR confirmed that BmBrC-Z2 directly binds to the BMFA element, thereby inhibiting P25 promoter activity. Furthermore, we demonstrated that BmBrC-Z2 and its isoform BmBrC-Z4 jointly bind to the BMFA element on the P25 promoter during the molting stage, whereas BmBrC-Z4 contributes a secondary role. Knocking out BmBrC-Z2 using the CRISPR/Cas9 system led to significant upregulation of silk protein genes during the molting stage in mutant larvae. These findings deepen our understanding of the complex regulatory mechanisms governing silk production and highlight the interplay between hormonal signaling and transcriptional regulation.

Dimm targets GDAP2 to regulate larval development in the silkworm, Bombyx mori

The basic helix-loop-helix (bHLH) domain transcription factors precisely regulate various developmental processes in insects. Dimm, a specific bHLH transcription factor, integrates the insulin/insulin-like growth factor signaling (IIS) and juvenile hormone signaling (JHS) pathways to modulate larval development in silkworms. However, the molecular mechanisms underlying this regulation are not yet fully understood. This study aimed to determine the targets of Dimm through which it regulates larval development. Chromatin immunoprecipitation followed by sequencing (ChIP-seq) revealed ganglioside-induced differentiation-associated protein 2 (GDAP2) as a direct downstream target gene of Dimm. Further study showed that Dimm directly binds to an enhancer element located in the second intron of the GDAP2 gene to promote its transcription. GDAP2 exhibited widespread expression across different stages and tissues of silkworms, regulated by both the IIS and the JHS pathways. The systemic knockout of GDAP2 leads to delayed larval development with a significant reduction in body weight; moreover, larval development was arrested at the 4th-instar stage. Further investigation unveiled that the inhibition of the ecdysone and innate immune signaling pathways in the mutant line led to abnormal larval development. A systematic investigation of the biological functions of GDAP2 offers valuable insights into the mechanism by which Dimm integrates IIS and JHS pathways to regulate the larval development of silkworms.

SGDAcn is a suppressor for silk gland endoreplication and development

Silkworm silk gland cells undergo multiple rounds of endoreplication, a process in which the genome is duplicated without cell division, leading to cellular polyploidization. This results in the accumulation of genomic DNA, serving as the foundation for rapid silk proteins synthesis. For the first time, we report a previously uncharacterized gene, SGDAcn, in the silkworm silk gland that clustered regularly interspaced short palindromic repeats (CRISPR) / CRISPR-associated nuclease 9-mediated SGDAcn knockout in the posterior silk gland increased cell size and enhanced silk production. SGDAcn knockout facilitated the progression of endoreplication by upregulating the expression of various cyclin genes and promoting energy metabolism, leading to a substantial increase in fibroin gene expression and its transcription factor Dimm, as well as the stimulation of ribosome biogenesis for messenger RNA translation and enhancement of eukaryotic translation initiation factors for protein synthesis. Our findings demonstrate that SGDAcn influences endoreplication, cell growth, and nucleolus size through SGDAcn-EGFR/PI3K/AKT and SGDAcn-NF-κB signaling pathways. Overall, SGDAcn acts as a negative regulator of silk gland development, affecting cell size and protein synthesis, thus modulating silk production. These mechanisms might be conserved in cell growth and progression, making an attractive target for genetic editing to improve silk yield in silkworms and potentially in mammalian cell growth regulation.

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

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

Bmgsb is involved in the determination of cell fate by affecting the cell cycle genes in the silk gland of Bombyx mori

Silk gland is the only organ of silkworm that can produce silk protein, which is a natural macromolecular protein complex and widely utilized in various fields such as biomaterials and biomedicine. The development of silk gland and the expression patterns of silk protein crucial for the silk industry. In this study, the function of a transcription factor Bmgsb was investigated with CRISPR/Cas9 and transgenic system. It was found that the homozygous individuals in the Bmgsb KO line experienced spinning failure and pupae death, the AMSG exhibited defects, and the ASG displayed abnormal curvature. These phenotypes were accompanied by increased DNA endoreplication and significantly upregulated expression of fibroin genes in the ASG. RT-qPCR results confirmed significant upregulation of cell cycle-related genes, including cyclin G and cyclin T in the Bmgsb KO line. Furthermore, ectopic expression of Bmgsb in the PSG weakened PSG curvature, inhibited DNA endoreplication, and downregulated the expression of fibroin genes. These findings strongly suggest that Bmgsb plays a crucial role in determining cell fate in the silk gland and regulating the expression of silk protein through the cyclin pathway. Our research provides a theoretical foundation for further studies on organ differentiation and have implications for the silk industry.

A strategy for improving silk yield and organ size in silk-producing insects

Insect silks possess excellent biodegradability, biocompatibility and mechanical properties, and have numerous applications in biomedicine and tissue engineering. However, the application of silk fiber is hindered by its limited supply, especially from non-domesticated insects. In the present study, the silk yield and organ size of Bombyx mori were significantly improved through genetic manipulation of the target of rapamycin complex 1 (TORC1) pathway components. Silk protein synthesis and silk gland size were decreased following rapamycin treatment, inhibiting the TORC1 signaling pathway both in vivo and ex vivo. The overexpression of posterior silk gland-specific Rheb and BmSLC7A5 improved silk protein synthesis and silk gland size by activating the TORC1 signaling pathway. Silk yield in BmSLC7A5-overexpression silkworms was significantly increased by approximately 25%. We have demonstrated that the TORC1 signaling pathway is involved in the transcription and translation of silk genes and transcriptional activators via phosphorylation of p70 S6 kinase 1 and 4E-binding protein 1. Our findings present a strategy for increasing silk yield and organ size in silk-producing insects.

Broad Complex-Z2 directly activates BmMBF2 to inhibit the silk protein synthesis in the silkworm, Bombyx mori

Silk proteins, as natural macromolecules, have extensive applications in biomaterials and biomedicine. In the silkworm, the expression of silk protein genes is negatively associated with ecdysone during the molt stage, while it is positively correlated with juvenile hormone during the intermolt stage. In our previous study, overexpression of an isoform Z2 of Broad Complex (BmBrC-Z2), an ecdysone early response factor, significantly reduced the expression of silk protein genes. However, the underlying regulatory mechanism remains unclear. In this study, we conducted transcriptomic analysis and found that overexpressing BmBrC-Z2 significantly upregulated the expression level of multiprotein bridging factor 2 (BmMBF2), an inhibitor of fibroin heavy chain (FibH). Further investigations revealed that BmBrC-Z2 directly regulated BmMBF2 by binding to cis-regulatory elements, as demonstrated using Dual-Luciferase Reporter Gene Assay, EMSA, and ChIP-PCR assay. Additionally, when using the CRISPR/Cas9 system to knock out BmMBF2, silk protein genes were significantly upregulated during the molt stage of mutant larvae. These findings uncover the negative regulation of silk protein synthesis by the ecdysone signaling cascade, specifically through the manipulation of BmMBF2 expression during the molt stage. This study enhances to our understanding of the temporal regulatory mechanism governing silk protein synthesis and offers a potential strategy for improving silk yield.

PI3K-Akt-SGF1-Dimm pathway mediates the nutritional regulation of silk protein synthesis in Bombyx mori

The efficient synthesis of silk protein is heavily reliant on the ingestion of massive nutrients during the peak growth phase in the silkworm. However, the molecular mechanism of nutritional regulation of silk protein synthesis remains unknown. In this study, we investigated the impact of nutrient deficiency on the synthesis of silk protein. Nutritional deficiency led to a reduction in silk yield, accompanied by decreased levels of silk proteins and fibroin heavy chain (FibH)-activating transcription factors SGF1 and Dimm. Furthermore, insulin enhanced the protein levels of SGF1 and Dimm, which can be attenuated by specific inhibitors of PI3K. Co-immunoprecipitation analysis showed that the nutrient pathway factor protein kinase B (Akt) could interact with SGF1 protein. Knockdown of Akt reduced the phosphorylation level of SGF1 and impedes its nuclear translocation. Further studies revealed that SGF1 was directly bound to Fkh site in the 22-43 region upstream of ATG of Dimm gene to activate its transcription. In conclusion, during the peak growth phase, nutrition promotes the massive synthesis of silk protein through the PI3K-Akt-SGF1-Dimm pathway. This study offers valuable insights into the efficient synthesis of silk proteins and establishes a theoretical foundation for improving silk yield.

Ectopic expression of BmeryCA in Bombyx mori increases silk yield and mechanical properties by altering the pH of posterior silk gland

The silk glands are the specialized tissue where silk protein synthesis, secretion, and conformational transitions take place, with pH playing a critical role in both silk protein synthesis and fiber formation. In the present study, we have identified erythrocyte carbonic anhydrase (BmeryCA) belonging to the α-CA class in the silk gland, which is a Zn2+ dependent metalloenzyme capable of efficiently and reversibly catalyzing the hydrated reaction of CO2 to HCO3-, thus participating in the regulation of acid-base balance. Multiple sequence alignments revealed that the active site of BmeryCA was highly conserved. Tissue expression profiling showed that BmeryCA had relatively high expression levels in hemolymph and epidermis but is barely expressed in the posterior silk gland (PSG). By specifically overexpressing BmeryCA in the PSG, we generated transgenic silkworms. Ion-selective microelectrode (ISM) measurements demonstrated that specifically overexpression of BmeryCA in the PSG led to a shift in pH from weakly alkaline to slightly neutral conditions. Moreover, the resultant PSG-specific BmeryCA overexpression mutant strain displayed a significant increase in both silk yield and silk fiber mechanical properties. Our research provided new insights into enhancing silk yield and improving the mechanical properties of silk fibers.

Silkworm Hemolymph and Cocoon Metabolomics Reveals Valine Improves Feed Efficiency of Silkworm Artificial Diet

Artificial silkworm diets significantly impact farm profitability. Sustainable cocoon production depends on the continuous improvement of feed efficiency to reduce costs and nutrient losses in the feed. This study used metabolomics to explore the differences in silkworm cocoons and hemolymph under two modes of rearing: an artificial diet and a mulberry-leaf diet. Nine metabolites of silkworm cocoons and hemolymph in the mulberry-leaf group were higher than those in the artificial-diet group. Enrichment analysis of the KEGG pathways for these metabolites revealed that they were mainly enriched in the valine, leucine, and isoleucine biosynthesis and degradation pathways. Hence, the artificial silkworm diet was supplemented various concentrations of valine were supplemented to with the aim of examining the impact of valine on their feeding and digestion of the artificial diet. The results indicated that valine addition had no significant effect on feed digestibility in the fifth-instar silkworm. Food intake in the 2% and 4% valine groups was significantly lower than that in the 0% valine group. However, the 2% and 4% valine groups showed significantly improved cocoon-production efficiency, at 11.3% and 25.1% higher, respectively. However, the cocoon-layer-production efficiencies of the 2% and 4% valine groups decreased by 7.7% and 13.9%, respectively. The research confirmed that valine is an effective substance for enhancing the feed efficiency of silkworms.

Juvenile hormone regulates silk gene expression by m6A RNA methylation

Juvenile hormone (JH) is an indispensable insect hormone that is critical in regulating insect development and physiology. N6-methyladenosine (m6A) is the most abundant modification of RNA that regulates RNA fate in eukaryotic organisms. However, the relationship between m6A and JH remains largely unknown. Here, we found that the application of a Juvenile hormone analog (JHA) extended the larval period of Bombyx mori and increased the weight and thickness of the cocoon. Interestingly, global transcriptional patterns revealed that m6A-related genes are specifically regulated by JHA in the posterior silk gland (PSG) that synthesizes the major component of cocoon silk. By transcriptome and m6A sequencing data conjointly, we discovered that JHA significantly regulated the m6A modification in the PSG of B. mori and many m6A-containing genes are related to nucleic acid binding, nucleus, and nucleobase-containing compound metabolism. Notably, 547 genes were significantly regulated by JHA at both the m6A modification and expression levels, especially 16 silk-associated genes, including sericin2, seroin1, Serine protease inhibitors 4 (BmSPI4), Serine protease inhibitors 5 (BmSPI5), and LIM domain-binding protein 2 (Ldb). Among them, 11 silk associated genes were significantly affected by METTL3 knockdown, validating that these genes are targets of m6A modification. Furthermore, we confirm that JHA directly regulates the expression of BmSPI4 and BmSPI5 through m6A modification of CDS regions. These results demonstrate the essential role of m6A methylation regulated by JH in PSG, and elucidate a novel mechanism by which JH affects silk gland development via m6A methylation. This study uncovers that m6A modification is a critical factor mediating the effect of JH in insects.