Comparative proteomic analysis of Bombyx mori hemocytes treated with destruxin A

Destruxin A inhibits the hemocytin-mediated hemolymph immunity of host insects to facilitate Metarhizium infection

Insects have an effective innate immune system to protect themselves against fungal invasion. Metarhizium employs a toxin-based strategy using a nonribosomal peptide called destruxin A (DA) to counteract the host immune response. However, the mechanism by which DA inhibits insect immunity is still unclear. Here, we identified 48 DA-binding proteins in silkworm hemolymph, with the binding affinity (KD) ranging from 2 to 420 μM. Among these proteins, hemocytin, an important immune factor, was determined to be the strongest DA-binding protein. DA binds to hemocytin and regulates its conformation in a multisite manner. Furthermore, DA exerts a significant inhibitory effect on hemocytin-mediated hemocyte aggregation. By disrupting the interaction between hemocytin, actin A3, and gelsolin, DA prevents the transformation of granules into vesicles in hemocytes. These vesicles are responsible for storing, maturing, and exocytosing hemocytin. Therefore, hemocytin secretion is reduced, and the formation of structures that promote aggregation in outer hemocytes is inhibited.

Identification of Host Molecules Involved in the Proliferation of Nucleopolyhedrovirus in Bombyx mori

The Bombyx mori nucleopolyhedrovirus (BmNPV), a foodborne infectious virus, is the pathogen causing nuclear polyhedrosis and high lethality in the silkworm. In this study, we characterized the molecules involved in BmNPV-silkworm interaction by RNA sequencing of the fat body isolated from the virus-susceptible strain P50. Kyoto Encyclopedia of Genes and Genomes (KEGG) annotation showed that the upregulated differentially expressed genes (DEGs) were mainly involved in translation, signal transduction, folding, sorting, and degradation, as well as transport and catabolism, while the downregulated DEGs were predominantly enriched in the metabolism of carbohydrates, amino acids, and lipids at 72 h post BmNPV infection. Knockout of the upregulated somatomedin-B and thrombospondin type-1 domain-containing protein, probable allantoicase, trifunctional purine biosynthetic protein adenosine-3, and Psl and pyoverdine operon regulator inhibited the proliferation of BmNPV, while knockout of the downregulated clip domain serine protease 3 and carboxylesterase clade H, member 1 promoted it. The molecules herein identified provide a foundation for developing strategies and designing drugs against BmNPV.

Effects of Destruxin A on Silkworm's Immunophilins

Destruxin A (DA), a major secondary metabolite of Metarhizium anisopliae, has anti-immunity to insects. However, the detailed mechanism and its interactions with target proteins are elusive. Previously, three immunophilins, peptidyl–prolyl cis–trans isomerase (BmPPI), FK506 binding-protein 45 (BmFKBP45) and BmFKBP59 homologue, were isolated from the silkworm, Bombyx mori Bm12 cell line following treatment with DA, which suggested that these proteins were possible DA-binding proteins. To validate the interaction between DA and the three immunophilins, we performed bio-layer interferometry (BLI) assay, and the results showed that DA has interaction with BmPPI, whose affinity constant value is 1.98 × 10⁻³ M and which has no affinity with FKBP45 and FKBP59 homologue in vitro. Furthermore, we investigated the affinity between DA and human PPI protein (HsPPIA) and the affinity constant (K_D) value is 2.22 × 10⁻³ M. Additionally, we compared the effects of silkworm and human PPI proteins produced by DA and immunosuppressants, cyclosporine A (CsA), and tacrolimus (FK506), by employing I2H (insect two-hybrid) in the SF-9 cell line. The results indicated that in silkworm, the effects created by DA and CsA were stronger than FK506. Furthermore, the effects created by DA in silkworm were stronger than those in humans. This study will offer new thinking to elucidate the molecular mechanism of DA in the immunity system of insects.

BmTudor-sn Is a Binding Protein of Destruxin A in Silkworm Bm12 Cells

Destruxin A (DA), a hexa-cyclodepsipeptidic mycotoxin secreted by the entomopathogenic fungus Metarhizium anisopliae, was reported to have an insecticidal effect and anti-immunity activity. However, its molecular mechanism of action remains unclear. Previously, we isolated several potential DA-affinity (binding) proteins in the Bombyx mori Bm12 cell line. By docking score using MOE2015, we selected three proteins—BmTudor-sn, BmPiwi, and BmAGO2—for further validation. First, using Bio-Layer Interferometry in vitro, we found that BmTudor-sn had an affinity interaction with DA at 125, 250, and 500 µM, while BmPiwi and BmAGO2 had no interaction signal with DA. Second, we employed standard immunoblotting to verify that BmTudor-sn is susceptible to DA, but BmPiwi and BmAGO2 are not. Third, to verify these findings in vivo, we used a target engagement strategy based on shifts in protein thermal stability following ligand binding termed the cellular thermal shift assay and found no thermal stability shift in BmPiwi and BmAGO2, whereas a shift was found for BmTudor-sn. In addition, in BmTudor-sn knockdown Bm12 cells, we observed that cell viability increased under DA treatment. Furthermore, insect two-hybrid system results indicated that the key site involved in DA binding to BmTudor-sn was Leu704. In conclusion, in vivo and in vitro experimental evidence indicated that BmTudor-sn is a binding protein of DA in silkworm Bm12 cells at the 100 µM level, and the key site of this interaction is Leu704. Our results provide new perspectives to aid in elucidating the molecular mechanism of action of DA in insects and developing new biopesticide.

Proteomics as a tool for tapping potential of entomopathogens as microbial insecticides

Biopesticides are collective pest control harnessing the knowledge of the target pest and its natural enemies that minimize the risks of synthetic pesticides. A subset of biopesticides; bioinsecticides, are specifically used in controlling insect pests. Entomopathogens (EPMs) are micro-organisms sought after as subject for bioinsecticide development. However, lack of understanding of EPM mechanism of toxicity and pathogenicity slowed the progress of bioinsecticide development. Proteomics is a useful tool in elucidating the interaction of entomopathogenic fungi, entomopathogenic bacteria, and entomopathogenic virus with their target host. Collectively, proteomics shed light onto insect host response to EPM infection, mechanism of action of EPM's toxic proteins and secondary metabolites besides characterizing secreted and membrane-bound proteins of EPM that more precisely describe relevant proteins for host recognition and mediating pathogenesis. However, proteomics requires optimized protein extraction methods to maximize the number of proteins for analysis and availability of organism's genome for a more precise protein identification.

Altered membrane rigidity via enhanced endogenous cholesterol synthesis drives cancer cell resistance to destruxins

Destruxins, secondary metabolites of entomopathogenic fungi, exert a wide variety of interesting characteristics ranging from antiviral to anticancer effects. Although their mode of action was evaluated previously, the molecular mechanisms of resistance development are unknown. Hence, we have established destruxin-resistant sublines of HCT116 colon carcinoma cells by selection with the most prevalent derivatives, destruxin (dtx)A, dtxB and dtxE. Various cell biological and molecular techniques were applied to elucidate the regulatory mechanisms underlying these acquired and highly stable destruxin resistance phenotypes. Interestingly, well-known chemoresistance-mediating ABC efflux transporters were not the major players. Instead, in dtxA- and dtxB-resistant cells a hyper-activated mevalonate pathway was uncovered resulting in increased de-novo cholesterol synthesis rates and elevated levels of lanosterol, cholesterol as well as several oxysterol metabolites. Accordingly, inhibition of the mevalonate pathway at two different steps, using either statins or zoledronic acid, significantly reduced acquired but also intrinsic destruxin resistance. Vice versa, cholesterol supplementation protected destruxin-sensitive cells against their cytotoxic activity. Additionally, an increased cell membrane adhesiveness of dtxA-resistant as compared to parental cells was detected by atomic force microscopy. This was paralleled by a dramatically reduced ionophoric capacity of dtxA in resistant cells when cultured in absence but not in presence of statins. Summarizing, our results suggest a reduced ionophoric activity of destruxins due to cholesterol-mediated plasma membrane re-organization as molecular mechanism underlying acquired destruxin resistance in human colon cancer cells. Whether this mechanism might be valid also in other cell types and organisms exposed to destruxins e.g. as bio-insecticides needs to be evaluated.

Destruxin A Induces and Binds HSPs in Bombyx mori Bm12 Cells

Destruxin A (DA) is a cyclodepsipeptidic mycotoxin isolated from the entomopathogenic fungus, Metarhizium anisopliae. It has insecticidal activity against host insect's innate immunity system, but the molecular mechanism is not yet elucidated. In our previous experiment, four HSPs (heat shock proteins, BmHSP70-3, BmHSP75, BmHSP83, and BmHSCP) were characterized from the specific protein electrophoretic bands of Bombyx mori Bm12 cell line treated with DA in the test of drug affinity responsive target stability (DARTS), which implied that these HSPs might be kinds of DA-affinity proteins, or DA induces them up-regulated expression. Therefore, in current research, the interactions of DA and HSPs were explored through analysis of bio-layer interferometry (BLI) employing FortBio OcteteQK. The expression levels of HSPs genes were surveyed by quantitative real-time polymerase chain reaction (qPCR). The results indicated that DA had no interactions with BmHSP70-3, BmHSP75, and BmHSP83, but had affinity to BmHSCP with a K_D value of 88.1 μM, in BLI analysis. However, the expression levels of all HSPs genes were significantly up-regulated after the Bm12 cells were treated by DA. In conclusion, DA can induce the four HSPs expression in Bm12 cells, but DA only binds to BmHSCP. Our research provides new insights on understanding of the action mechanisms of destruxins.