The digestive proteinase trypsin, alkaline A contributes to anti-BmNPV activity in silkworm (Bombyx mori)

Effects of baculovirus infection on intestinal microflora of BmNPV resistant and susceptible strain silkworm

Bombyx mori L. (Lepidoptera: Bombycidae) nucleopolyhedrovirus (BmNPV) is a serious pathogen causing huge economic losses to sericulture. There is growing evidence that the gut microbiota of silkworms plays a critical role in shaping host responses and interactions with viral infection. However, little is known about the differences in the composition and diversity of intestinal microflora, especially with respect to silkworm strain differences and BmNPV infection-induced changes. Here, we aim to explore the differences between BmNPV-resistant strain A35 and susceptible strain P50 silkworm and the impact of BmNPV infection on intestinal microflora in different strains. The 16S rDNA sequencing analysis revealed that the fecal microbial populations were distinct between A35 and P50 and were significantly changed post BmNPV infection in both strains. Further analysis showed that the BmNPV-resistant strain silkworm possessed higher bacterial diversity than the susceptible strain, and BmNPV infection reduced the diversity of intestinal flora assessed by feces in both silkworm strains. In response to BmNPV infection, the abundance of Muribaculaceae increased in P50 and decreased in A35, while the abundance of Enterobacteriaceae decreased in P50 and increased in A35. These results indicated that BmNPV infection had various effects on the abundance of fecal microflora in different silkworm strains. Our findings not only broadened the understanding of host-pathogen interactions but also provided theoretical help for the breeding of resistant strains and healthy rearing of silkworms based on symbiotic bacteria.

Insights into midgut cell types and their crucial role in antiviral immunity in the lepidopteran model Bombyx mori

The midgut, a vital component of the digestive system in arthropods, serves as an interface between ingested food and the insect's physiology, playing a pivotal role in nutrient absorption and immune defense mechanisms. Distinct cell types, including columnar, enteroendocrine, goblet and regenerative cells, comprise the midgut in insects and contribute to its robust immune response. Enterocytes/columnar cells, the primary absorptive cells, facilitate the immune response through enzyme secretions, while regenerative cells play a crucial role in maintaining midgut integrity by continuously replenishing damaged cells and maintaining the continuity of the immune defense. The peritrophic membrane is vital to the insect's innate immunity, shielding the midgut from pathogens and abrasive food particles. Midgut juice, a mixture of digestive enzymes and antimicrobial factors, further contributes to the insect's immune defense, helping the insect to combat invading pathogens and regulate the midgut microbial community. The cutting-edge single-cell transcriptomics also unveiled previously unrecognized subpopulations within the insect midgut cells and elucidated the striking similarities between the gastrointestinal tracts of insects and higher mammals. Understanding the intricate interplay between midgut cell types provides valuable insights into insect immunity. This review provides a solid foundation for unraveling the complex roles of the midgut, not only in digestion but also in immunity. Moreover, this review will discuss the novel immune strategies led by the midgut employed by insects to combat invading pathogens, ultimately contributing to the broader understanding of insect physiology and defense mechanisms.

Bmo-miR-3351 modulates glutathione content and inhibits BmNPV proliferation by targeting BmGSTe6 in Bombyx mori

MicroRNAs (miRNAs) are small non-coding RNAs that play pivotal roles in the host response to invading pathogens. Among these pathogens, Bombyx mori nucleopolyhedrovirus (BmNPV) is one of the main causes of substantial economic losses in sericulture, and there are relatively few studies on the specific functions of miRNAs in the B. mori-BmNPV interaction. Therefore, we conducted transcriptome sequencing to identify differentially expressed (DE) messenger RNAs (mRNAs) and miRNAs in the midgut of 2 B. mori strains (BmNPV-susceptible strain P50 and BmNPV-resistant strain A35) after BmNPV infection. Through correlation analysis of the miRNA and mRNA data, we identified a comprehensive set of 21 miRNAs and 37 predicted target mRNAs. Notably, miR-3351, which has high expression in A35, exhibited remarkable efficacy in suppressing BmNPV proliferation. Additionally, we confirmed that miR-3351 binds to the 3' untranslated region (3' UTR) of B. mori glutathione S-transferase epsilon 6 (BmGSTe6), resulting in its downregulation. Conversely, BmGSTe6 displayed an opposite expression pattern to miR-3351, effectively promoting BmNPV proliferation. Notably, BmGSTe6 levels were positively correlated with glutathione S-transferase activity, consequently influencing intracellular glutathione content in the infected samples. Furthermore, our investigation revealed the protective role of glutathione against BmNPV infection in BmN cells. In summary, miR-3351 modulates glutathione content by downregulating BmGSTe6 to inhibit BmNPV proliferation in B. mori. Our findings enriched the research on the role of B. mori miRNAs in the defense against BmNPV infection, and suggests that the antiviral molecule, glutathione, offers a novel perspective on preventing viral infection in sericulture.

Single-nucleus sequencing of silkworm larval midgut reveals the immune escape strategy of BmNPV in the midgut during the late stage of infection

The midgut is an important barrier against microorganism invasion and proliferation, yet is the first tissue encountered when a baculovirus naturally invades the host. However, only limited knowledge is available how different midgut cell types contribute to the immune response and the clearance or promotion of viral infection. Here, single-nucleus RNA sequencing (snRNA seq) was employed to analyze the responses of various cell subpopulations in the silkworm larval midgut to B. mori nucleopolyhedrovirus (BmNPV) infection. We identified 22 distinct clusters representing enteroendocrine cells (EEs), enterocytes (ECs), intestinal stem cells (ISCs), Goblet cell-like and muscle cell types in the BmNPV-infected and uninfected silkworm larvae midgut at 72 h post infection. Further, our results revealed that the strategies for immune escape of BmNPV in the midgut at the late stage of infection include (1) inhibiting the response of antiviral pathways; (2) inhibiting the expression of antiviral host factors; (3) stimulating expression levels of genes promoting BmNPV replication. These findings suggest that the midgut, as the first line of defense against the invasion of the baculovirus, has dual characteristics of "resistance" and "tolerance". Our single-cell dataset reveals the diversity of silkworm larval midgut cells, and the transcriptome analysis provides insights into the interaction between host and virus infection at the single-cell level.

The Bmtret1 Gene Family and Its Potential Role in Response to BmNPV Stress in Bombyx mori

Trehalose is a non-reducing disaccharide and participates in physiological activities such as organ formation, energy metabolism, and stress resistance in insects. The Bmtret1 gene family is mainly involved in in the sugar metabolism of silkworm. In the present study, phylogenetic analysis divided 21 Bmtret1 orthologs into three clades. These genes are equally distributed on the nine chromosomes. The cis-elements in the promoter regions of Bmtret1s indicated the possible function of Bmtret1s in response to hormones and environmental stimulus. The qPCR analysis showed the significantly different expression levels of Bmtret1s in different tissues and organs, indicating possible functional divergence. In addition, most Bmtret1s showed disturbed expression levels in response to silkworm nuclear polyhedrosis virus (BmNPV) stresses. Our results provide a clue for further functional dissection of the Tret1s in Bombyx mori and implicate them as potential regulators of antiviral responses.

Nuclear Factor Kappa B Promotes Ferritin Heavy Chain Expression in Bombyx mori in Response to B. mori Nucleopolyhedrovirus Infection

Ferritin heavy chain (FerHCH) is a major component of ferritin and plays an important role in maintaining iron homeostasis and redox equilibrium. Our previous studies have demonstrated that the Bombyx mori ferritin heavy chain homolog (BmFerHCH) could respond to B. mori nucleopolyhedrovirus (BmNPV) infection. However, the mechanism by which BmNPV regulates the expression of BmFerHCH remains unclear. In this study, BmFerHCH increased after BmNPV infection and BmNPV infection enhanced nuclear factor kappa B (NF-κB) activity in BmN cells. An NF-κB inhibitor (PDTC) reduced the expression of the virus-induced BmFerHCH in BmN cells, and overexpression of BmRelish (NF-κB) increased the expression of virus-induced BmFerHCH in BmN cells. Furthermore, BmNPV infection enhanced BmFerHCH promoter activity. The potential NF-κB cis-regulatory elements (CREs) in the BmFerHCH promoter were screened by using the JASPAR CORE database, and two effective NF-κB CREs were identified using a dual luciferase reporting system and electrophoretic mobility shift assay (EMSA). BmRelish (NF-κB) bound to NF-κB CREs and promoted the transcription of BmFerHCH. Taken together, BmNPV promotes activation of BmRelish (NF-κB), and activated BmRelish (NF-κB) binds to NF-κB CREs of BmFerHCH promoter to enhance BmFerHCH expression. Our study provides a foundation for future research on the function of BmFerHCH in BmNPV infection.

Alteration of Manure Antibiotic Resistance Genes via Soil Fauna Is Associated with the Intestinal Microbiome

Livestock wastes contain high levels of antibiotic resistance genes (ARGs) and a variety of human-related pathogens. Bioconversion of livestock manure using larvae of the beetle Protaetia brevitarsis is an effective technique for waste reduction and value creation; however, the fate of manure ARGs during gut passage and interaction with the gut microbiome of P. brevitarsis remains unclear. To investigate this, we fed P. brevitarsis with dry chicken manure for 6 days and measured bacterial community dynamics and ARG abundance and diversity along the P. brevitarsis gut tract using high-throughput quantitative PCR and metagenomics approaches. The diversity of ARGs was significantly lower in larval midgut, hindgut, and frass than in raw chicken manure, and around 80% of pathogenicity-related genes (PRGs) exhibited reduced abundance. Network analysis demonstrated that Bacteroidetes and Firmicutes were the key bacterial phyla associated with ARG reduction. Metagenomic analysis further indicated that ARGs, mobile genetic elements (MGEs), and PRGs were simultaneously attenuated in the hindgut, implicating a decreased likelihood for horizontal gene transfer (HGT) of ARGs among bacteria and pathogens during manure bioconversion. Our findings demonstrated that the attenuation of ARGs is strongly associated with the variation of the gut microbiome of P. brevitarsis, providing insights into mechanisms of risk mitigation of ARG dissemination during manure bioconversion. IMPORTANCE Saprophagous fauna like the oriental edible beetle (P. brevitarsis) plays a fundamental role in converting organic wastes into biofertilizer. Accumulating evidence has shown that soil fauna can reduce the abundance of ARGs, although the underlying mechanism of ARG reduction is still unclear. In our previous research, we found a large reduction of ARGs in vegetable roots and leaves from frass compared with raw manure, providing a promising biofertilizer for soil-vegetable systems. Therefore, in this study, temporal dynamic changes in the microbiomes of the donor (chicken manure) and host (P. brevitarsis) were investigated, and we found a close association between the gut microbiome and the alteration of ARGs. These results shed new light on how the insect gut microbiome can mitigate manure-borne ARGs and provide insights into the bioconversion process via a typical member of the saprophagous fauna, P. brevitarsis.

Bombyx mori ferritin heavy-chain homolog facilitates BmNPV proliferation by inhibiting reactive oxygen species-mediated apoptosis

Ferritin heavy-chain homolog (FerHCH), an iron-binding protein, plays an important role in the host defense against oxidative stress and pathogen infections. In our previous research, Bombyx mori native ferritin had an interaction with B. mori nucleopolyhedrovirus (BmNPV). However, the underlying molecular mechanism of single ferritin homolog responses to BmNPV infection remains unclear. In this study, we found that BmNPV titer and B. mori FerHCH (BmFerHCH) expression were positively correlated with the ferric iron concentration. We performed RNA interference (RNAi) and overexpression experiments to investigate the effects of BmFerHCH on BmNPV proliferation. BmFerHCH knockdown suppressed BmNPV proliferation in vivo and in vitro, whereas BmFerHCH overexpression facilitated BmNPV proliferation. In addition, the oxidative stress level was increased significantly in BmN cells after budded virus infection, while BmFerHCH could neutralize the increased ROS production induced by BmNPV. Of note, we found that ROS was involved in BmNPV-induced apoptosis. Through inhibiting ROS, apoptosis was suppressed by BmFerHCH, whereas BmFerHCH knockdown facilitated apoptosis. Therefore, we hypothesize that BmFerHCH-mediated inhibition of virus-induced apoptosis depends on suppressing ROS accumulation and, thereby, facilitates virus replication. These results suggest that BmFerHCH plays an important role in facilitating BmNPV proliferation and modulating BmFerHCH is potential strategy for studying host-pathogen interactions.

Genome-wide identification of lipases in silkworm (Bombyx mori) and their spatio-temporal expression in larval midgut

Lipases play crucial roles in food digestion by degrading dietary lipids into free fatty acids and glycerols. The domesticated silkworm (Bombyx mori) has been widely used as an important Lepidopteran model for decades. However, little is known about the lipase gene family in the silkworm, especially their hydrolytic activities as digestive enzymes. In this study, a total of 38 lipase genes were identified in the silkworm genome. Phylogenetic analysis indicated that they were divided into three major groups. Twelve lipases were confirmed to be expressed in the midgut at both transcriptional and translational levels. They were grouped into the same gene cluster, suggesting that they could have similar physiological functions. Quantitative real-time PCR (qRT-PCR) analyses indicated that lipases were mainly expressed in anterior and middle midgut regions, and their expression levels varied greatly along the length of midgut. A majority of lipases were down-regulated in the midgut when larvae stopped feeding. However, a unique lipase gene (Bmlip10583) showed low expression level during feeding stage, but it was significantly up-regulated during the larvae-pupae transition. These results demonstrated that expression of silkworm lipases was spatially and temporally regulated in the midgut during larval development. Taken together, our results provide a fundamental research of the lipase gene family in the silkworm.

Bombyx mori β-1,3-Glucan Recognition Protein 4 (BmβGRP4) Could Inhibit the Proliferation of B. mori Nucleopolyhedrovirus through Promoting Apoptosis

β-1,3-glucan recognition proteins (βGRPs) as pattern recognition receptors (PRRs) play an important role in recognizing various pathogens and trigger complicated signaling pathways in insects. In this study, we identified a Bombyx mori β-1,3-glucan recognition protein gene named BmβGRP4, which showed differential expression, from a previous transcriptome database. The full-length cDNA sequence was 1244 bp, containing an open reading frame (ORF) of 1128 bp encoding 375 amino acids. BmβGRP4 was strongly expressed in the larval stages and highly expressed in the midgut of B. mori larvae in particular. After BmNPV infection, the expression of BmβGRP4 was reduced significantly in the midgut. Furthermore, a significant increase in the copy number of BmNPV was observed after the knockdown of BmβGRP4 in 5th instar larvae, while the overexpression of BmβGRP4 suppressed the proliferation of BmNPV in BmN cells. Subsequently, the expression analysis of several apoptosis-related genes and observation of the apoptosis morphology demonstrated that overexpression of BmβGRP4 facilitated apoptosis induced by BmNPV in BmN cells. Moreover, BmβGRP4 positively regulated the phosphatase and tensin homolog gene (BmPTEN), while expression of the inhibitor of apoptosis gene (BmIAP) was negatively regulated by BmβGRP4. Hence, we hypothesize that BmNPV infection might suppress BmPTEN and facilitate BmIAP to inhibit cell apoptosis by downregulating the expression of BmβGRP4 to escape host antiviral defense. Taken together, these results show that BmβGRP4 may play a role in B. mori response to BmNPV infection and lay a foundation for studying its functions.