The Toll Signaling Pathway in the Chinese Oak Silkworm, Antheraea pernyi: Innate Immune Responses to Different Microorganisms

Identification and characterization of Spätzle in Myzus persicae and its role during microbial infection

As a Toll receptor ligand, Spätzle (Spz) plays a crucial role in activating the Toll pathway and participating in the innate immune response of insects. However, the immune function of Spz in Myzus persicae remains poorly understood. In this study, we identified and cloned 7 Spz genes from M. persicae, all containing a Spz domain (cystine-knot domain). Phylogenetic analysis revealed that the 7 different MpSpzs were divided into 6 groups within a single cluster with each Spz of Drosophila melanogaster and Acyrthosiphon pisum. These genes were mainly expressed in 1st-instar nymphs, hemolymph, and embryos and showed varying levels of positive response to infection with Escherichia coli, Staphylococcus aureus, and Beauveria bassiana. After gene-silencing of MpSpzs by RNA interference with injection of target gene-specific double-stranded RNA, microbial infection significantly increased the mortality of M. persicae compared to control groups. Further studies revealed that the suppression of MpSpz resulted in a significant reduction in lysozyme expression. The present study offers novel insights into the role of Spätzle in the innate immune response against microbial infection in M. persicae.

PxSpätzle3 Regulates the Toll Pathway To Affect Bacillus thuringiensis Susceptibility of Plutella xylostella

Spätzle is an important messenger in the Toll pathway of the insect innate immune system. However, the function of Spätzle in regulating the Toll pathway in Bacillus thuringiensis (Bt) susceptibility of Plutella xylostella is unclear. In this study, we cloned the Spätzle3 gene of P. xylostella. Molecular docking and yeast two-hybrid experiments indicated that PxSpätzle3 combined with PxToll6 to regulate the Toll pathway. After knocking out PxSpätzle3, the expression of downstream Toll pathway genes and antimicrobial peptides (AMPs) decreased. Antibacterial assays showed that PxGloverin2 could inhibit Bt8010. Further bioassays revealed that the susceptibility of the mutant to Bt8010 was significantly higher than that of the wild type. Intriguingly, the gut bacteria Enterobacter sp. EbPXG5 enhanced the lethality of Bt8010 to the mutant P. xylostella. Our findings clarify that PxSpätzle3 activates the Toll pathway by binding with PxToll6, regulates AMP production, and affects the susceptibility of P. xylostella to Bt8010. Additionally, our study uncovers the role of gut bacteria in this process. These insights provide new ideas for the sustainable control of pests.

Anti-oxidation enhancement, inflammation alleviation, and microbial composition optimization of using tussah (Antheraea pernyi) silk fibroin peptides for hyperglycaemia remission

OBJECTIVE

This study aimed to evaluate the positive effects on anti-oxidation, anti-inflammation, and microbial composition optimization of diabetic mice using tussah (Antheraea pernyi) silk fibroin peptides (TSFP), providing the theoretical foundation for making the use of silk resources of A. pernyi and incorporating as a supplement into the hypoglycemic foods.

METHOD

The animal model of diabetes was established successfully. Alloxan-induced diabetic mice were orally administered using TSFP, and the hypoglycaemic effects in vivo were systematically investigated.

RESULTS

The results indicated that TSFP could significantly reduce the fasting blood glucose (FBG) levels and suppress the mRNA expression of glycometabolism genes of diabetic mice. In addition, the TSFP could ameliorate the lipid dysbolism and contribute to a higher anti-oxidation capacity. Moreover, TSFP could alleviate pathological damages and hinder inflammatory processes of diabetic mice. Besides, the supplementation of TSFP presented a greater ability to shape and optimize the gut microbial composition by enriching the profitable bacteria and inhibiting the pathogenic microorganisms. Correlation analysis also revealed that the abundances of functional bacteria in the TSFP-treated groups exhibited better correlations with serum parameters, which would be of positive significance for blood glucose regulation and inflammation remission.

CONCLUSIONS

These results collectively corroborated the feasibility and superiority of using TSFP for hyperglycaemia remission via anti-oxidation enhancement, inflammation alleviation, and microbial composition optimization, contributing to a safely feasible and biologically efficient strategy for improving anti-diabetic effects.

Similarly to BmToll9-1, BmToll9-2 Is a Positive Regulator of the Humoral Immune Response in the Silkworm, Bombyx mori

Toll receptors play important roles in the development and innate immunity of insects. Previously, we reported the immunological function of BmToll9-2 in silkworm, Bombyx mori, larvae. In this study, we focused on the role of BmToll9-2 as a regulator in the Toll signaling pathway. The expressions of most signaling genes in the Toll pathway, as well as immune effectors, were reduced after the RNAi of BmToll9-2. Coincidentally, hemolymph from BmToll9-2-silenced larvae exhibited decreased antibacterial activity in the growth of Escherichia coli, demonstrated either by growth curve or inhibitory zone experiments. The oral administration of heat-inactivated E. coli and Staphylococcus aureus following the RNAi of BmToll9-2 up-regulated the expression of most signaling genes in the Toll pathway and downstream immune effectors. The above results indicate that BmToll9-2 is positively involved in the Toll signaling pathway. As a positive regulator, BmToll9-2 is shown to be activated preferentially against E. coli and, in turn, positively modulates the humoral immune response in antibacterial activity.

BmToll9-1 Is a Positive Regulator of the Immune Response in the Silkworm Bombyx mori

Toll receptors are involved in the development and innate immunity of insects. BmToll9-1 is an important immune receptor in the Toll pathway. Previous studies have focused on its role as a receptor in immune response. In this study, we aimed to investigate the role of BmToll9-1 as a regulator in the immune response. The expression profiles demonstrated that BmToll9-1 was predominantly expressed in the midgut. RNA interference (RNAi) of BmToll9-1 was found to be effective in the midgut via the injection of dsRNA, which resulted in smaller and lighter larvae and cocoons. Most signaling genes in the Toll pathway and downstream effector genes were downregulated after the RNAi of BmToll9-1. The hemolymph from BmToll9-1-silenced larvae showed decreased antibacterial activity against Escherichia coli, either in growth curve or inhibition zone experiments. The above results indicate that BmToll9-1 might be positively involved in the immune pathway of silkworm. As a positive regulator, BmToll9-1 might function mainly in the gut to maintain microbial homeostasis to regulate the growth of silkworms. Silencing of BmToll9-1 downregulates the signaling genes in the Toll pathway and antimicrobial peptide (AMP) production, resulting in decreased antibacterial activity in the hemolymph.

Immunological regulation by Toll-1 and Spätzle-4 in larval density-dependent prophylaxis of the oriental armyworm, Mythimna separata

High population density has been shown to alter insect prophylactic immunity. Toll-Spätzle pathway performs a key function in insect innate immune response. To determine the role of Toll and Spätzle, two main components of Toll-Spätzle pathway, in the density-dependent prophylaxis of Mythimna separata. We identified full-length cDNA encoding the Toll-1 and Spätzle-4 genes in M. separata (designed MsToll-1 and Ms Spätzle-4). Both MsToll-1 and MsSpätzle-4 were expressed throughout all developmental stages. MsToll-1 expression was highly in fat body and brain and MsSpätzle-4 was highly expressed in brain and Malpighian tubule. With increased larval density, MsToll-1 expression was markedly up-regulated. MsSpätzle-4 expression was found to be raised in larvae that were fed in high density (5 and 10 larvae per jar). Co-immunoprecipitation assays demonstrated that MsToll-1 interacted with MsSpätzle-4. Immune-related genes transcriptions were considerably reduced in high-density larvae MsToll-1 (or MsSpätzle-4) was silenced by dsRNA injection. Meanwhile, a discernible reduction in the survival rate of the larvae exposed to Bacillus thuringiensis infection with silence of MsToll-1 (or MsSpätzle-4) was observed. This study implies that prophylactic immunity was influenced by crowded larvae via modulating the Toll-Spätzle pathway in M. separata and allow for a new understanding of into density-dependent prophylaxis in insects.

Larval crowding effects during early development in the Chinese oak silkmoth Antheraea pernyi (Lepidoptera: Saturniidae)

Chinese sericulture relies in part on the rearing of the Chinese oak silkmoth Antheraea pernyi, an insect with key cultural and ecological roles. While feeding primarily on oak, Antheraea species are known to accept alternative hosts such as birch Betula sp with little to no apparent negative fitness consequences. This opens up the range of hostplants that could be used for large-scale rearing of A. pernyi for silk production and food, or used by this species in possible invasions. To date, however, the natural history and ecology of A. pernyi remain subject of investigation. For instance, we still do not know how individuals respond to crowding developmental environments, which is an important factor to consider for the ecology of the species as well as for commercial rearing. Here, I describe the implications of larval crowding to the survival and growth of A. pernyi larvae during early development. I show that higher crowding is associated with stronger negative effects on growth and survival, corroborating findings from other holometabolous insects. I then discuss the implications of this findings for our understanding of optimum larval crowding. Overall, the findings reveal important ecological information for an insect species key for provisioning and cultural ecosystem services.

High temperature exposure reduces the susceptibility of Helicoverpa armigera to its nucleopolyhedrovirus (HearNPV) by enhancing expression of heat shock proteins

BACKGROUND

High temperatures will occur more frequently with global warming, with potential impacts on the efficacy of biological control agents. Heat shock proteins (HSPs) are induced by high temperature, but their possible roles in pest resistance to entomopathogens remain unexplored. We investigated the effects of high temperature (35 °C) on Helicoverpa armigera resistance to H. armigera nucleopolyhedrovirus (HearNPV) and the putative roles of HSPs in this process.

RESULTS

Even short periods (24 h) of high temperature (35 °C) reduced mortality in HearNPV-infected H. armigera larvae. Sustained 35 °C exposure significantly shortened developmental time, and increased fresh weight and locomotor activity in infected larvae. Moreover, high temperature inhibited virus replication and thickened the epidermis of H. armigera, resulting in reduced spread of infection from cadavers. Real-time polymerase chain reaction (PCR) analysis showed that expression of 11 HSP genes was altered by the 35 °C treatment, and that mostly small heat shock protein (sHSP) genes were up-regulated, the same sHSPs were induced when larvae were infected with HearNPV. Finally, RNA interference (RNAi) suppression of these sHSPs showed that only Hsp24.91 and Hsp21.8 diminished H. armigera defensive responses to HearNPV infection.

CONCLUSION

Even short periods of exposure to high temperature can significantly reduce susceptibility of H. armigera larvae to HearNPV by stimulating the production of sHSPs which enhance immune responses, with important implications for the use of entomopathogens as biological control agents under global warming scenarios. © 2022 Society of Chemical Industry.

Tenebrio molitor Spätzle 1b Is Required to Confer Antibacterial Defense Against Gram-Negative Bacteria by Regulation of Antimicrobial Peptides

Innate immunity is the ultimate line of defense against invading pathogens in insects. Unlike in the mammalian model, in the insect model, invading pathogens are recognized by extracellular receptors, which activate the Toll signaling pathway through an extracellular serine protease cascade. In the Toll-NF-κB pathway, the extracellular spätzle protein acts as a downstream ligand for Toll receptors in insects. In this study, we identified a novel Spätzle isoform (TmSpz1b) from RNA sequencing database of Tenebrio molitor. TmSpz1b was bioinformatically analyzed, and functionally characterized for the antimicrobial function by RNA interference (RNAi). The 702 bp open reading frame of TmSpz1b encoded a putative protein of 233 amino acid residues. A conserved cystine-knot domain with seven cysteine residues in TmSpz1b was involved in three disulfide bridges and the formation of a spätzle dimer. TmSpz1b was mostly expressed in the hemocytes of T. molitor late instar larvae. The mRNA expression of TmSpz1b was highly induced in the hemocytes after Escherichia coli, Staphylococcus aureus, and Candida albicans stimulation of T. molitor larvae. TmSpz1b silenced larvae were significantly more susceptible to E. coli infection. In addition, RNAi-based functional assay characterized TmSpz1b to be involved in the positive regulation of antimicrobial peptide genes in hemocytes and fat bodies. Further, the TmDorX2 transcripts were downregulated in TmSpz1b silenced individuals upon E. coli challenge suggesting the relationship to Toll signaling pathway. These results indicate that TmSpz1b is involved in the T. molitor innate immunity, causes the sequestration of Gram-negative bacteria by the regulatory action of antimicrobial peptides, and enhances the survival of T. molitor larvae.

A Gut-Specific LITAF-Like Gene in Antheraea pernyi (Lepidoptera: Saturniidae) Involved in the Immune Response to Three Pathogens

Antheraea pernyi (Guérin-Méneville 1855) is an important resource for silk, food, and biohealth products; however, exogenous pathogens largely affect the commercial application potential of this species. Since the gut is a key organ for the digestion and absorption of nutrients as well as for immune defense, we used comparative transcriptome analysis to screen for a gut-specific molecular tool for further functional research in A. pernyi. In total, 3,331 differentially expressed genes (DEGs) were identified in the gut compared with all other pooled tissues of A. pernyi, including 1,463 upregulated genes in the gut. Among these, we further focused on a lipopolysaccharide-induced tumor necrosis factor-α factor (LITAF) gene because of its high gut-specific expression and the presence of a highly conserved SIMPLE-like domain, which is related to the immune response to pathogenic infections in many species. The cDNA sequence of ApLITAF was 447-bp long and contained a 243-bp open reading frame encoding an 80-amino acid protein. Immune challenge assays indicated that ApLITAF expression was significantly upregulated in the gut of A. pernyi naturally infected with nucleopolyhedrovirus (NPV) or fed leaves infected with the gram-negative bacterium Escherichia coli (Migula 1895) and the gram-positive bacterium Bacillus subtilis (Ehrenberg 1835). Cell transfection showed that ApLITAF localized to the lysosome. Collectively, these results suggested that ApLITAF played a role in the immune response of A. pernyi and could facilitate the future research and breeding application in this species.

TmSpz-like Plays a Fundamental Role in Response to E. coli but Not S. aureus or C. albican Infection in Tenebrio molitor via Regulation of Antimicrobial Peptide Production

The cystine knot protein Spätzle is a Toll receptor ligand that modulates the intracellular signaling cascade involved in the nuclear factor kappa B (NF-κB)-mediated regulation of antimicrobial peptide (AMP)-encoding genes. Spätzle-mediated activation of the Toll pathway is critical for the innate immune responses of insects against Gram-positive bacteria and fungi. In this study, the open reading frame (ORF) sequence of Spätzle-like from T. molitor (TmSpz-like) identified from the RNA sequencing dataset was cloned and sequenced. The 885-bp TmSpz-like ORF encoded a polypeptide of 294 amino acid residues. TmSpz-like comprised a cystine knot domain with six conserved cysteine residues that formed three disulfide bonds. Additionally, TmSpz-like exhibited the highest amino acid sequence similarity with T. castaneum Spätzle (TcSpz). In the phylogenetic tree, TmSpz-like and TcSpz were located within a single cluster. The expression of TmSpz-like was upregulated in the Malpighian tubules and gut tissues of T. molitor. Additionally, the expression of TmSpz-like in the whole body and gut of the larvae was upregulated at 24 h post-E. coli infection. The results of RNA interference experiments revealed that TmSpz-like is critical for the viability of E. coli-infected T. molitor larvae. Eleven AMP-encoding genes were downregulated in the E. coli-infected TmSpz-like knockdown larvae, which suggested that TmSpz-like positively regulated these genes. Additionally, the NF-κB-encoding genes (TmDorX1, TmDorX2, and TmRelish) were downregulated in the E. coli-infected TmSpz-like knockdown larvae. Thus, TmSpz-like plays a critical role in the regulation of AMP production in T. molitor in response to E. coli infection.

Characterization and expression analysis of genes encoding three small heat shock proteins in the oriental armyworm, Mythimna separata (Walker)

Small heat shock proteins (sHsps) function in the response of insects to abiotic stress; however, their role in response to biotic stress has been under-investigated. Mythimna separata, the oriental armyworm, is polyphenetic and exhibits gregarious and solitary phases in response to high and low population density, respectively. In this study, three genes were identified encoding sHsps, namely MsHsp19.7, MsHsp19.8 and MsHsp21.4, and expression levels in solitary and gregarious M. separata were compared. The deduced protein sequences of the three MsHsps had molecular weights of 19.7, 19.8 and 21.4 kDa, respectively, and contained a conserved α-crystalline domain. Real-time PCR analyses revealed that the three sHsps were transcribed in all developmental stages and were dramatically up-regulated at the 6^th larval stage in gregarious individuals. Expression of the three MsHsps was variable in different tissues of 6^th instar larvae, but exhibited consistent up- and down-regulation in the hindgut and Malpighian tubules of gregarious individuals, respectively. In addition, MsHsp19.7 and MsHsp19.8 were significantly induced when solitary forms were subjected to crowding for 36 h, but all three MsHsps were down-regulated when gregarious forms were isolated. Our findings suggest that population density functions as a stress factor and impacts MsHsps expression in M. separata.

Spätzle Homolog-Mediated Toll-Like Pathway Regulates Innate Immune Responses to Maintain the Homeostasis of Gut Microbiota in the Red Palm Weevil, Rhynchophorus ferrugineus Olivier (Coleoptera: Dryophthoridae)

Spätzle (Spz) is a dimeric ligand that responds to the Gram-positive bacterial or fungal infection by binding Toll receptors to induce the secretion of antimicrobial peptides. However, whether the Toll-like signaling pathway mediates the innate immunity of Rhynchophorus ferrugineus to modulate the homeostasis of gut microbiota has not been determined. In this study, we found that a Spz homolog, RfSpätzle, is a secretory protein comprising a signal peptide and a conservative Spz domain. RT-qPCR analysis revealed that RfSpätzle was significantly induced to be expressed in the fat body and gut by the systemic and oral infection with pathogenic microbes. The expression levels of two antimicrobial peptide genes, RfColeoptericin and RfCecropin, were downregulated significantly by RfSpätzle knockdown, indicating that their secretion is under the regulation of the RfSpätzle-mediated signaling pathway. After being challenged by pathogenic microbes, the cumulative mortality rate of RfSpätzle-silenced individuals was drastically increased as compared to that of the controls. Further analysis indicated that these larvae possessed the diminished antibacterial activity. Moreover, RfSpätzle knockdown altered the relative abundance of gut bacteria at the phylum and family levels. Taken together, these findings suggest that RfSpätzle is involved in RPW immunity to confer protection and maintain the homeostasis of gut microbiota by mediating the production of antimicrobial peptides.

Identification of Serratia marcescens isolated from Antheraea pernyi eggs and determination of bacterial pathogenicity and transmission pathway

The Chinese oak silkworm, Antheraea pernyi, is an important alternate host for mass production of Trichogramma parasitoids, which play a vital role in the biological control of lepidopterous pests in China. The quality of A. pernyi eggs is particularly important for mass production of these parasitoids. In this study, a pathogenic bacterium, isolated and purified from A. pernyi eggs that had turned gray in color, was identified as Serratia marcescens. We used morphology, biochemistry and 16S rDNA analysis to characterize the strain, which was named "APE strain". Serratia marcescens APE strain was determined to be the causal bacterium associated with the disease in the eggs, verified by a test based on Koch's Postulates. We tested the pathogenicity of S. marcescens APE strain on A. pernyi eggs; the percentage of diseased (gray) eggs reached 57.78% when uninfected eggs were exposed to a concentration of 1 × 10⁹ cfu/mL bacterial suspension for 7 h. S. marcescens was transmitted mechanically by Trichogramma parasitoids. The transmission rate was 25.56%. In a horizontal transmission test, the highest percentage of uninfected eggs that developed infections was 51.43% after being treated with contents of diseased eggs for 12 h. In a vertical transmission test, the number of infected eggs per treated adult female was 63.8-92.3 after treatment with different S. marcescens concentrations, significantly lower than the 304.3 eggs per female in the control group. Furthermore, the percentage of infected (gray) eggs produced by adult moths exposed to bacteria in the larval stage was 80.5-85.3%.

Correlation in Expression between LTR Retrotransposons and Potential Host Cis-Targets during Infection of Antherea pernyi with ApNPV Baculovirus

The published genome sequence of Antheraeayamamai (Saturnnidae) was used to construct a library of long terminal repeat (LTR)-retrotransposons that is representative of the wild silkmoth (Antherea) genus, and that includes 22,666 solo LTRs and 541 full-length LTRs. The LTR retrotransposons of Antheraeayamamai (AyLTRs) could be classified into the three canonical groups of Gypsy, Copia and Belpao. Eleven AyLTRs contained the env gene element, but the relationship with the env element of baculovirus, particularly A. yamamai and pernyi nucleopolyhedrovirus (AyNPV and ApNPV), was distant. A total of 251 “independent” full-length AyLTRs were identified that were located within 100 kb distance (downstream or upstream) of 406 neighboring genes in A. yamamai. Regulation of these genes might occur in cis by the AyLTRs, and the neighboring genes were found to be enriched in GO terms such as “response to stimulus”, and KEGG terms such as “mTOR signaling pathway” among others. Furthermore, the library of LTR-retrotransposons and the A. yamamai genome were used to identify and analyze the expression of LTR-retrotransposons and genes in ApNPV-infected and non-infected A. pernyi larval midguts, using raw data of a published transcriptome study. Our analysis demonstrates that 93 full-length LTR-retrotransposons are transcribed in the midgut of A. pernyi of which 12 significantly change their expression after ApNPV infection (differentially expressed LTR-retrotransposons or DELs). In addition, the expression of differentially expressed genes (DEGs) and neighboring DELs on the chromosome following ApNPV infection suggests the possibility of regulation of expression of DEGs by DELs through a cis mechanism, which will require experimental verification. When examined in more detail, it was found that genes involved in Notch signaling and stress granule (SG) formation were significantly up-regulated in ApNPV-infected A. pernyi larval midgut. Moreover, several DEGs in the Notch and SG pathways were found to be located in the neighborhood of particular DELs, indicating the possibility of DEG-DEL cross-regulation in cis for these two pathways.

Upregulation of a Trypsin-Like Serine Protease Gene in Antheraea pernyi (Lepidoptera: Saturniidae) Strains Exposed to Different Pathogens

Antheraea pernyi Guérin-Méneville is used for silk production and as a food resource. Its infection by exogenous pathogens, including microsporidia, fungi, bacteria, and virus, can lead to silkworm diseases, causing major economic losses. A trypsin-like serine protease gene (TLS) was found in A. pernyi transcriptome data resulting from two different infection experiments. The cDNA sequence of ApTLS was 1,020 bp in length and contained an open reading frame of 774 bp encoding a 257-amino acid protein (GenBank KF779933). The present study investigated the expression patterns of ApTLS after exposure to different pathogens, and in four different A. pernyi strains. Semiquantitative RT-PCR indicated that ApTLS was expressed in all developmental stages and was most expressed in the midgut. Quantitative real-time PCR indicated ApTLS was upregulated in the midgut of A. pernyi exposed to nucleopolyhedrovirus (ApNPV), Nosema pernyi, Enterococcus pernyi, and Beauveria bassiana infections, and the highest gene expression level was found under ApNPV infection. The strain Shenhuang No. 2 presented the lowest infection rate and the highest ApTLS gene expression level when exposed to ApNPV. Thus, ApTLS seems to be involved in innate defense reactions in A. pernyi.

Characterization of PmSpӓtzle 1 from the black tiger shrimp Peneaus monodon

Spätzle is a signaling ligand in innate immune response that signals pathogenic infection via Toll receptor and Toll pathway into the cells for the synthesis of antimicrobial proteins. Herein, three PmSpӓtzle isoforms were identified in Penaeus monodon, namely PmSpz1, 2 and 3. The PmSpz1 was chosen for detailed study. The PmSpz1 gene was expressed in all nine tissues tested including the hemocytes, stomach, hepatopancreas, gill, lymphoid tissue, eyestalk, muscle, intestine and heart. Its expression was up-regulated upon white spot syndrome virus (WSSV) infection. Western blot analysis of hemolymph showed that the PmSpz1 mostly existed as a cleaved active form awaiting to activate the Toll pathway. Injection of a recombinant PmSpz1 rendered the shrimp less susceptible to the WSSV infection. Injection of a recombinant active form of PmSpz1 into a normal shrimp activated the synthesis of crustinPm1, crustinPm7, ALFPm3, penaeidin3 but not penaeidin5 indicating that the expression of all antimicrobial proteins but not penaeidin5 was under the regulation of Toll pathway.