Serine protease SP7 cleaves prophenoloxidase and is regulated by two serpins in Ostrinia furnacalis melanization

AMPK regulates HIF-1α to induce pupal diapause in the cotton bollworm, Helicoverpa armigera

Diapause is an adaptive strategy employed by insects to endure adverse environmental conditions and is characterized by reduced metabolic activity, primarily due to a decreased respiratory rate. AMP-activated protein kinase (AMPK) serves as an intracellular energy regulator, modulating energy metabolism in response to metabolic fluctuations. However, its role in pupal diapause of the cotton bollworm, Helicoverpa armigera, remains unclear. In this study, we found that AMPK and its active form, P-AMPK, are highly expressed in diapause-destined pupae. Furthermore, activation of AMPK delayed the development of nondiapause-destined pupae, suggesting a critical role for AMPK in the regulation of pupal diapause in H. armigera. Manipulating AMPK activity in H. armigera epidermal (HaEpi) cells and pupae significantly influenced the expression of hypoxia-inducible factor-1α (HIF-1α), which our laboratory previously reported as a key inducer of pupal diapause through the reduction of mitochondrial activity in H. armigera. Histone deacetylase 4 (HDAC4), a shuttle protein phosphorylated by AMPK which translocates between the cytoplasm and the nucleus, was found to exhibit significantly higher expression in diapause-destined pupal brains compared to their nondiapause counterparts. AMPK in both HaEpi cells and pupae positively regulated the protein levels of P-HDAC4 by binding to the HDAC4 promoter. Additionally, HDAC4 was shown to enhance HIF-1α expression in diapause-destined individuals. HDAC4 binds to and deacetylates heat shock protein 70 (HSP70), and reduced acetylation of HSP70 was found to significantly elevate HIF-1α protein levels. The AMPK-HIF-1α signaling pathway appears to play a pivotal role in reducing mitochondrial activity and facilitating diapause induction in H. armigera pupae.

Chemical Proteomics Reveals That Camptothecin Weakens Insect Immunity against Bacteria by Suppressing Antimicrobial Peptide Expression

Plant natural products are crucial in defending against herbivorous insects and are widely used in pest control, yet their mechanisms remain complex and insufficiently studied. This study employed a reverse strategy to investigate the mechanism of camptothecin (CPT), a botanical pesticide. By using a CPT-based chemical probe coupled with proteomic analysis, immune-related proteins, including those involved in prophenoloxidase (PPO) activation and antimicrobial peptide (AMP) synthesis, were identified in the Asian corn borer, Ostrinia furnacalis. The findings demonstrated that CPT weakens insect immunity, primarily by inhibiting AMP synthesis rather than affecting PPO activation. Specifically, CPT downregulated the expression of genes in the IMD pathway and those encoding AMPs (attacin and gloverin). Additionally, CPT-fed insects exhibited reduced antibacterial activity. This research uncovers a novel mechanism of CPT as an insect immunosuppressant, offering new insights that may enhance the application of CPT in pest control.

Bombyx mori serpin 3 is involved in innate immunity by interacting with serine protease 7 to regulate prophenoloxidase activation

A subfamily of conserved proteins called serpins plays crucial roles in various physiological functions, particularly in the activation pathway of the serine protease cascade, an essential component of insect innate immunity. Here, we found Bombyx mori serpin 3 (BmSerpin3) was most highly expressed in the fat body, and was up-regulated after exposure to bacteria, fungus and virus. Further, the expression of BmSerpin3 in the hemocytes, fat body, midgut of silkworm larvae, and BmN cells was up-regulated upon Bombyx mori nucleopolyhedrovirus (BmNPV) infection. Through Bac-to-Bac expression system, we obtained the active protein of BmSerpin3, and the enzyme activity assay showed that BmSerpin3 significantly inhibited the activity of both subtilisin and trypsin. In addition, BmSerpin3 could inhibit the activation of prophenoloxidase (PPO) in larvae. The knockdown of BmSerpin3 showed increased phenoloxidase (PO) activity compared to control after BmNPV infection. Ultimately, we confirmed that BmSerpin3 interacts with B. mori Serine Protease 7 (BmSP7). Hence, we hypothesize that BmSerpin3 is involved in innate immunity by interacting with BmSP7 to regulate the PPO activation cascade. Taken together, these results showed that BmSerpin3 play a role in silkworm innate immunity and lay a foundation for studying its functions.

Hemolymph protease-17b activates proHP6 to stimulate melanization and Toll signaling in Manduca sexta

Manduca sexta hemolymph protease-6 (HP6) plays a central role in coordinating antimicrobial responses, such as prophenoloxidase (PPO) activation and Toll signaling. Our previous studies indicated that HP5 and GP6 activate proHP6 in larval hemolymph and extraembryonic tissues, respectively. Here, we report the characterization of HP17b as another HP6 activating enzyme and its regulation by multiple serpins in hemolymph. The precursor of HP17b expressed in baculovirus infected Sf9 cells became spontaneously cleaved at two sites, and these products were purified together in one preparation named HP17b', a mixture of proHP17b, a 35 kDa intermediate, and HP17b. HP17b' converted proHP6 to HP6. As reported before, HP6 converted precursors of PPO activating protease-1 (PAP1) and HP8 to their active forms. HP8 activates proSpӓtzle-1 to turn on Toll signaling. We found HP17b' directly activated proSPHI and II to form a cofactor for PPO activation by PAP1. Supplementation of larval hemolymph with HP17b', HP17b, or proHP17b significantly increased PPO activation. Adding Micrococcus luteus to the reactions did not enhance PPO activation in the reactions containing HP17b', HP17b, or proHP17b. Using HP17b antibodies, we isolated from induced plasma HP17b fragments and associated proteins (e.g., serpin-4). Serpin-1A, 1J, 1J', 4, 5, or 6 reduced the activation of proHP6 by HP17b' through formation of covalent complexes with active HP17b. We detected an activity for proHP17b cleavage in hemolymph from bar-stage pharate pupae but failed to purify the protease due to its high instability. Other known HPs did not activate proHP17b in vitro. Together, these results suggest that HP17b is a clip-domain protease activated by an unknown endopeptidase in response to a danger signal and regulated by multiple serpins.

Parasitoid Serpins Evolve Novel Functions to Manipulate Host Homeostasis

Parasitoids introduce various virulence factors when parasitism occurs, and some taxa generate teratocytes to manipulate the host immune system and metabolic homeostasis for the survival and development of their progeny. Host-parasitoid interactions are extremely diverse and complex, yet the evolutionary dynamics are still poorly understood. A category of serpin genes, named CvT-serpins, was discovered to be specifically expressed and secreted by the teratocytes of Cotesia vestalis, an endoparasitoid of the diamondback moth Plutella xylostella. Genomic and phylogenetic analysis indicated that the C. vestalis serpin genes are duplicated and most of them are clustered into 1 monophyletic clade. Intense positive selection was detected at the residues around the P1-P1' cleavage sites of the Cv-serpin reactive center loop domain. Functional analyses revealed that, in addition to the conserved function of melanization inhibition (CvT-serpins 1, 16, 18, and 21), CvT-serpins exhibited novel functions, i.e. bacteriostasis (CvT-serpins 3 and 5) and nutrient metabolism regulation (CvT-serpins 8 and 10). When the host-parasitoid system is challenged with foreign bacteria, CvT-serpins act as an immune regulator to reprogram the host immune system through sustained inhibition of host melanization while simultaneously functioning as immune effectors to compensate for this suppression. In addition, we provided evidence that CvT-serpin8 and 10 participate in the regulation of host trehalose and lipid levels by affecting genes involved in these metabolic pathways. These findings illustrate an exquisite tactic by which parasitoids win out in the parasite-host evolutionary arms race by manipulating host immune and nutrition homeostasis via adaptive gene evolution and neofunctionalization.

BdcSP10 is a prophenoloxidase-activating protease in Bactrocera dorsalis

Insects rely on a powerful and efficient innate immune system against microbial invaders. One of the most important immune processes is the melanization reaction, in which eumelanin is synthesized and deposited on the physically injured site or the surface of invading pathogens. The melanization reaction is mediated by prophenoloxidase (PPO), which is synthesized as an inactive zymogen and requires proteolytic activation through a clip serine protease cascade. This cascade has been characterized in several Lepidoptera insect species, but it is less understood in most Diptera insects. Here, with the means of reverse genetics and biochemistry, we characterized the function of a clip serine protease BdcSP10 from the oriental fruit fly Bactrocera dorsalis (Hendel), a significant agriculture pest to a broad variety of fruit and vegetable crops. BdcSP10 knockdown inhibited the melanization reaction and rendered adult flies more vulnerable to pathogenic infections. In addition, purified and activated BdcSP10 proteases promoted the melanization reaction in larval hemolymph and directly cleaved and activated purified PPO1 and PPO2 in vitro. Taken together, we identified BdcSP10 as a PPO-activating protease and validated its important role in the defense against microbial infection in B. dorsalis. This work broadens the understanding of the activation mechanism of the melanization reaction in Diptera insects.

Serpin-4 Facilitates Baculovirus Infection by Inhibiting Melanization in Asian Corn Borer, Ostrinia furnacalis (Guenée)

Phenoloxidase (PO)-catalyzed melanization is a vital immune response in insects for defense against pathogen infection. This process is mediated by clip domain serine proteases and regulated by members of the serpin superfamily. We here revealed that the infection of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) significantly inhibited the PO activity in Ostrinia furnacalis hemolymph and induced the expression of O. furnacalis serpin-4. Addition of recombinant serpin-4 protein to O. furnacalis hemolymph resulted in a great increase of AcMNPV copies. Serpin-4 significantly suppressed the PO activity and the amidase activity in cleaving colorimetric substrate IEARpNA (IEARase activity) of hemolymph. Further experiments indicated it formed covalent complexes with three serine proteases (SP1, SP13 and SP105) and prevented them from cleaving their cognate downstream proteases in vitro. Altogether, O. furnacalis melanization restricted AcMNPV replication and serpin-4 facilitated AcMNPV infection by inhibiting serine proteases, SP1, SP13, and SP105 which were all involved in the melanization response.

A TIL-Type Serine Protease Inhibitor Involved in Humoral Immune Response of Asian Corn Borer Ostrinia furnaculis

To elucidate the application value of insect endogenous protease and its inhibitor genes in pest control, we analyzed in detail the transcriptome sequence of the Asian corn borer, Ostrinia furnacalis. We obtained 12 protease genes and 11 protease inhibitor genes, and comprehensively analyzed of their spatiotemporal expression by qRT-PCR. In which, a previous unstudied serine protease inhibitor gene attracted our attention. It belongs to the canonical serine proteinase inhibitor family, a trypsin inhibitor-like cysteine-rich domain (TIL)-type protease inhibitor, but its TIL domain lacks two cysteine residues, and it was named as ACB-TIL. Its expression level is relatively very low in the absence of pathogen stimulation, and can be up-regulated expression induced by Gram-negative bacteria (Escherichia coli), virus (BmNPV), and dsRNA (dsEGFP), but cannot be induced by fungus spores (Metarrhizium anisopliae). Prokaryotic expressed ACB-TIL protein can significantly inhibit the melanization in vitro. Injecting this protein into insect body can inhibit the production of antimicrobial peptides of attacin, lebocin and gloverin. Inhibition of ACB-TIL by RNAi can cause the responses of other immune-, protease- and inhibitor-related genes. ACB-TIL is primarily involved in Asian corn borer humoral immunity in responses to Gram-negative bacteria and viruses. This gene can be a potential target for pest control since this will mainly affect insect immune response.