Serpin-5 regulates prophenoloxidase activation and antimicrobial peptide pathways in the silkworm, Bombyx mori

Varroa destructor parasitism and Deformed wing virus infection in honey bees are linked to peroxisome-induced pathways

The ectoparasitic mite Varroa destructor transmits and triggers viral infections that have deleterious effects on honey bee colonies worldwide. We performed a manipulative experiment in which worker bees collected at emergence were exposed to Varroa for 72 h, and their proteomes were compared with those of untreated control bees. Label-free quantitative proteomics identified 77 differentially expressed A. mellifera proteins (DEPs). In addition, viral proteins were identified by orthogonal analysis, and most importantly, Deformed wing virus (DWV) was found at high levels/intensity in Varroa-exposed bees. Pathway enrichment analysis suggested that the main pathways affected included peroxisomal metabolism, cyto-/exoskeleton reorganization, and cuticular proteins. Detailed examination of individual DEPs revealed that additional changes in DEPs were associated with peroxisomal function. In addition, the proteome data support the importance of TGF-β signaling in Varroa-DWV interaction and the involvement of the mTORC1 and Hippo pathways. These results suggest that the effect of DWV on bees associated with Varroa feeding results in aberrant autophagy. In particular, autophagy is selectively modulated by peroxisomes, to which the observed proteome changes strongly corresponded. This study complements previous research with different study designs and suggests the importance of the peroxisome, which plays a key role in viral infections.

Localization of nitric oxide-producing hemocytes in Aedes and Culex mosquitoes infected with bacteria

Mosquitoes are significant vectors of various pathogens. Unlike vertebrates, insects rely solely on innate immunity. Hemocytes play a crucial role in the cellular part of the innate immune system. The gaseous radical nitric oxide (NO) produced by hemocytes acts against pathogens and also functions as a versatile transmitter in both the immune and nervous systems, utilizing cyclic guanosine monophosphate (cGMP) as a second messenger. This study conducted a parallel comparison of NO synthase (NOS) expression and NO production in hemocytes during Escherichia coli K12 infection in four vector species: Aedes aegypti, Aedes albopictus, Culex pipiens molestus, and Culex pipiens quinquefasciatus. Increased NOS expression by NADPH diaphorase (NADPHd) staining and NO production by immunofluorescence against the by-product L-citrulline were observed in infected mosquito hemocytes distributed throughout the abdomens. NADPHd activity and citrulline labeling were particularly found in periostial hemocytes near the heart, but also on the ventral nerve chord (VNC). Pericardial cells of Ae. aegypti and Cx. p. molestus showed increased citrulline immunofluorescence, suggesting their involvement in the immune response. Oenocytes displayed strong NADPHd and citrulline labeling independent of infection status. This comparative study, consistent with findings in other species, suggests a widespread phenomenon of NO's role in hemocyte responses during E. coli infection. Found differences within and between genera highlight the importance of species-specific investigations.

Serpin-4 Negatively Regulates Prophenoloxidase Activation and Antimicrobial Peptide Synthesis in the Silkworm, Bombyx mori

The prophenoloxidase (PPO) activation and Toll antimicrobial peptide synthesis pathways are two critical immune responses in the insect immune system. The activation of these pathways is mediated by the cascade of serine proteases, which is negatively regulated by serpins. In this study, we identified a typical serpin, BmSerpin-4, in silkworms, whose expression was dramatically up-regulated in the fat body and hemocytes after bacterial infections. The pre-injection of recombinant BmSerpin-4 remarkably decreased the antibacterial activity of the hemolymph and the expression of the antimicrobial peptides (AMPs) gloverin-3, cecropin-D, cecropin-E, and moricin in the fat body under Micrococcus luteus and Yersinia pseudotuberculosis serotype O: 3 (YP III) infection. Meanwhile, the inhibition of systemic melanization, PO activity, and PPO activation by BmSerpin-4 was also observed. Hemolymph proteinase 1 (HP1), serine protease 2 (SP2), HP6, and SP21 were predicted as the candidate target serine proteases for BmSerpin-4 through the analysis of residues adjacent to the scissile bond and comparisons of orthologous genes in Manduca sexta. This suggests that HP1, SP2, HP6, and SP21 might be essential in the activation of the serine protease cascade in both the Toll and PPO pathways in silkworms. Our study provided a comprehensive characterization of BmSerpin-4 and clues for the further dissection of silkworm PPO and Toll activation signaling.

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.

A Genome-Wide Analysis of Serine Protease Inhibitors in Cydia pomonella Provides Insights into Their Evolution and Expression Pattern

Serine protease inhibitors (serpins) appear to be ubiquitous in almost all living organisms, with a conserved structure and varying functions. Serpins can modulate immune responses by negatively regulating serine protease activities strictly and precisely. The codling moth, Cydia pomonella (L.), a major invasive pest in China, can cause serious economic losses. However, knowledge of serpin genes in this insect remain largely unknown. In this study, we performed a systematic analysis of the serpin genes in C. pomonella, obtaining 26 serpins from the C. pomonella genome. Subsequently, their sequence features, evolutionary relationship, and expression pattern were characterized. Comparative analysis revealed the evolution of a number of serpin genes in Lepidoptera. Importantly, the evolutionary relationship and putative roles of serpin genes in C. pomonella were revealed. Additionally, selective pressure analysis found amino acid sites with strong evidence of positive selection. Interestingly, the serpin1 gene possessed at least six splicing isoforms with distinct reactive-center loops, and these isoforms were experimentally validated. Furthermore, we observed a subclade expansion of serpins, and these genes showed high expression in multiple tissues, suggesting their important roles in C. pomonella. Overall, this study will enrich our knowledge of the immunity of C. pomonella and help to elucidate the role of serpins in the immune response.

Serpin-1a and serpin-6 regulate the Toll pathway immune homeostasis by synergistically inhibiting the Spätzle-processing enzyme CLIP2 in silkworm, Bombyx mori

The Toll receptor signaling pathway is an important innate immune response of insects to pathogen infection; its extracellular signal transduction involves serine protease cascade activation. However, excessive or constitutive activation of the Toll pathway can be detrimental. Hence, the balance between activation and inhibition of the extracellular protease cascade must be tightly regulated to achieve favorable outcomes. Previous studies have shown that serpins-serine protease inhibitors-negatively regulate insect innate immunity by inhibiting extracellular protease cascade signaling. Although the roles of serpins in insect innate immunity are well described, the physiological mechanisms underlying their synergistic effects remain poorly understand. Here, we characterize the molecular mechanism by which serpin-1a and serpin-6 synergistically maintain immune homeostasis of the silkworm Toll pathway under physiological and pathological conditions. Through in vitro biochemical assays and in vivo bioassays, we demonstrate that clip-domain serine protease 2 (CLIP2), as the Toll cascade-activating terminal protease, is responsible for processing proSpätzle1 to induce the expression of antimicrobial peptides. Further biochemical and genetic analyses indicate that constitutively expressed serpin-1a and inducible serpin-6 synergistically target CLIP2 to maintain homeostasis of the silkworm Toll pathway under physiological and pathological conditions. Taken together, this study provides new insights into the precise regulation of Toll cascade activation signals in insect innate immune responses and highlights the importance and complexity of insect immune homeostasis regulation.

The Effects of Diet on the Immune Responses of the Oriental Armyworm Mythimna separata

Nutrients can greatly affect host immune defenses against infection. Possessing a simple immune system, insects have been widely used as models to address the relationships between nutrition and immunity. The effects of high versus low protein-to-carbohydrate ratio (P:C) diets on insect immune responses vary in different studies. To reveal the dietary manipulation of immune responses in the polyphagous agricultural pest oriental armyworm, we examined immune gene expression, phenoloxidase (PO) activity, and phagocytosis to investigate the immune traits of bacteria-challenged oriental armyworms, which were fed different P:C ratio diets. We found the oriental armyworms that were fed a 35:7 (P:C) diet showed higher phenoloxidase (PO) activity and stronger melanization, and those reared on a 28:14 (P:C) diet showed higher antimicrobial activity. However, different P:C diets had no apparent effect on the hemocyte number and phagocytosis. These results overall indicate that high P:C diets differently optimize humoral immune defense responses in oriental armyworms, i.e., PO-mediated melanization and antimicrobial peptide synthesis in response to bacteria challenge.

Tachinid parasitoid Exorista japonica affects the utilization of diet by changing gut microbial composition in the silkworm, Bombyx mori

Changes in both intake and digestion of feed have been demonstrated in the host following parasitization. However, its regulatory mechanism has not been clarified. In this study, silkworms and Exorista japonica were used as research objects to analyze the effect of parasitism on the midgut immune system of the silkworm. After being parasitized, the expressions of antimicrobial peptide (AMP) genes of silkworms showed a fluctuating trend of first upregulation and then downregulation, while phenoloxidase and lysozyme activities were inhibited. To study the possible impact of the downregulation of AMP genes on intestinal microorganisms, the characteristics of the intestinal microbial population of silkworms on the third day of parasitism were analyzed. The relative abundance of Firmicutes, Proteobacteria, and Bacteroidota decreased, while that of Actinobacteriota increased. The increased abundance of conditionally pathogenic bacteria Serratia and Staphylococcus might lead to a decrease in the amount of silkworm ingestion. Meanwhile, the abundance of Acinetobacter, Bacillus, Pseudomonas, and Enterobacter promotes an increase in the digestion of nutrients. This study indicated that the imbalance of intestinal microbial homeostasis caused by parasitism may affect the absorption and digestion of nutrients by the host. Collectively, our findings provided a new clue for further exploring the mechanism of nutrient transport among the host, parasitoid, and intestinal microorganisms.

Diversity of Antimicrobial Peptides in Silkworm

Antimicrobial resistance is a phenomenon that the present-day world is witnessing that poses a serious threat to global health. The decline in the development of novel therapeutics over the last couple of decades has exacerbated the situation further. In this scenario, the pursuit of new alternative therapeutics to commonly used antibiotics has gained predominance amongst researchers across the world. Antimicrobial peptides (AMPs) from natural sources have drawn significant interest in the recent years as promising pharmacological substitutes over the conventional antibiotics. The most notable advantage of AMPs is that microorganisms cannot develop resistance to them. Insects represent one of the potential sources of AMPs, which are synthesized as part of an innate immune defence against invading pathogens. AMPs from different insects have been extensively studied, and silkworm is one of them. Diverse classes of AMPs (including attacins, cecropins, defensins, enbocins, gloverins, lebocins and moricins) were identified from silkworm that exhibit antimicrobial property against bacteria, fungi and viruses, indicating their potential therapeutic benefits. This review briefs about the immune responses of silkworm to invading pathogens, the isolation of AMPs from silkworms, AMPs reported in silkworms and their activity against various microorganisms.

Sericin Ser3 Ectopic Expressed in Posterior Silk Gland Affects Hemolymph Immune Melanization Response via Reducing Melanin Synthesis in Silkworm

The transgenesis of silkworms is an important way to innovate genetic resources and silk function. However, the silk-gland (SG) of transgenic silkworms, which is the most concerned target tissue of sericulture, often suffers from low vitality, stunting and other problems, and the reasons are still unknown. This study trans engineered recombinant Ser3, a middle silk gland (MSG) specific expression gene, in the posterior silk gland (PSG) of the silkworm, and studied hemolymph immune melanization response changes in mutant pure line SER (Ser3^+/+). The results showed that although the mutant had normal vitality, the melanin content and phenoloxidase (PO) activity in hemolymph related to humoral immunity were significantly reduced, and caused significantly slower blood melanization and weaker sterilization ability. The mechanism investigation showed that the mRNA levels and enzymatic activities of phenylalanine hydroxylase (PAH), tyrosine hydroxylase (TH) and dopamine decarboxylase (DDC) in the melanin synthesis pathway in mutant hemolymph, as well as the transcription levels of the PPAE, SP21 and serpins genes in the serine protease cascade were significantly affected. Moreover, the total antioxidant capacity, superoxide anion inhibition capacity and catalase (CAT) level related to the redox metabolic capacity of hemolymph were significantly increased, while the activities of superoxide dismutase (SOD) and glutathione reductase (GR), as well as the levels of hydrogen peroxide (H₂O₂) and glutathione (GSH), were significantly decreased. In conclusion, the anabolism of melanin in the hemolymph of PSG transgenic silkworm SER was inhibited, while the basic response level of oxidative stress was increased, and the hemolymph immune melanization response was decreased. The results will significantly improve the safe assessment and development of genetically modified organisms.

Insect phenoloxidase and its diverse roles: melanogenesis and beyond

Insect life on earth is greatly diversified despite being exposed to several infectious agents due to their diverse habitats and ecological niche. One of the major factors responsible for their successful establishment is having a powerful innate immune system. The most common and effective method used by insects in recognizing pathogen and non-self-substances is the melanization process among others. The key enzyme involved in melanin biosynthesis is the copper containing humoral defense enzyme, phenoloxidase (PO). This review focused on understanding about PO and that had been in research for nearly a century. The review elaborates about evolutionary significance of PO in arthropods, its relationship with mammalian tyrosinases, various substrates, activators and inhibitors involved in the activation of phenoloxidase cascade, as it requires an integrated system of activation that vary among insect species. The enzyme also plays a vital role in insect immunity by involving in several other immune functions like sclerotization, wound healing, opsonization, encapsulation and nodule formation. Further, gene knock down or knock out of PO genes and inhibition of PO-melanization cascade by several mechanisms can also be considered as promising future alternative to control serious pests by making them highly susceptible to any targeted attack.

Two venom serpins from the parasitoid wasp Microplitis mediator inhibit the host prophenoloxidase activation and antimicrobial peptide synthesis

Endoparasitoid wasps inject venom proteins into the hemocoel of host insects to ensure survival, growth, and development of their progenies by blocking host immunity. We previously identified ten serine protease inhibitors of the serpin superfamily in venom of the endoparasitoid wasp, Microplitis mediator, but it is unclear how these inhibitors may interact with host immune serine proteases. In this study, we investigated the functions of two serpins, MmvSPN-1 and MmvSPN-2, in the regulation of humoral immune responses in two hosts, the oriental armyworm Pseudaletia separate and the cotton bollworm Helicoverpa armigera, by dsRNA knockdown and biochemical assays using recombinant proteins. Knockdown of the two serpins resulted in increases in prophenoloxidase (PPO) activation and antimicrobial peptide (AMP) production in the hosts. After injection into the host hemocoel, the recombinant serpins inhibited PPO activation and AMP transcription. Mass spectrometry analysis of the pull-downs and in vitro reconstitution experiments revealed that HacSP29, a clip-domain serine protease in H. armigera, is the target of these two serpins. Therefore, these two inhibitors in the wasp venom may protect eggs from attacks by melanization and AMPs in the host 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.

KPI5 Is Involved in the Regulation of the Expression of Antibacterial Peptide Genes and Hemolymph Melanization in the Silkworm, Bombyx mori

Kunitz-type protease inhibitors (KPIs) are ubiquitously found in many organisms, and participate in various physiological processes. However, their function in insects remains to be elucidated. In the present study, we characterized and functionally analyzed silkworm KPI5. Sequence analysis showed that KPI5 contains 85 amino acids with six conserved cysteine residues, and the P1 site is a phenylalanine residue. Inhibitory activity and stability analyses indicated that recombinant KPI5 protein significantly inhibited the activity of chymotrypsin and was highly tolerant to temperature and pH. The spatio-temporal expression profile analysis showed that KPI5 was synthesized in the fat body and secreted into the hemolymph. In vivo induction analysis showed that the expression of KPI5 in the fat body was significantly upregulated by pathogen-associated molecular patterns (PAMPs). Binding assays suggested that KPI5 can bind to pathogens and PAMPs. In vitro pathogen growth inhibition assay and encapsulation analysis indicated that KPI5 can neither kill pathogenic bacteria directly nor promote the encapsulation of agarose beads by silkworm hemocytes. Recombinant protein injection test and CRISPR/Cas9-mediated knockdown showed that KPI5 promotes the expression of antimicrobial peptides (AMPs) in the fat body. Moreover, the survival rate of individuals in the KPI5 knockdown group was significantly lower than that of the control group after pathogen infection. Phenoloxidase (PO) activity assays showed that KPI5 significantly inhibited the hemolymph PO activity and melanization induced by PAMPs. These findings suggested that KPI5 plays a dual regulatory role in innate immunity by promoting the expression of antimicrobial peptides in the fat body and inhibiting hemolymph melanization. Our study furthers the understanding of the function of insect KPIs and provides new insights into the regulatory mechanism of insect immune homeostasis.

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

Melanization is an innate immune response in insects to defend against the invading pathogens and parasites. During melanization, prophenoloxidase (PPO) requires proteolytic activation by its upstream prophenoloxidase-activating protease (PAP). We here cloned a full-length cDNA for a serine protease, named as SP7, from Ostrinia furnacalis. The open reading frame of SP7 encodes 421-amino acid residue protein with a 19-residue signal peptide. qRT-PCR analysis showed that SP7 mRNA levels were significantly upregulated upon exposure to microbial infection. Recombinant SP7 zymogen was activated by serine protease SP2. The active SP7 could cleave O. furnacalis PPOs including PPO2, PPO1b and PPO3. Additionally, active SP7 could form covalent complexes with serine protease inhibitor serpin-3 and serpin-4. The activity of SP7 in cleaving a colorimetric substrate IEARpNA or O. furnacalis PPOs was efficiently blocked by either serpin-3 or serpin-4. Our work thus revealed that SP7 and SP2 partially constituted a PPO activation cascade in which SP7 was activated by SP2 and then likely worked as a PAP. SP7 was effectively regulated by serpin-3 and serpin-4. The results would allow further advances in the understanding of melanization mechanisms in O. furnacalis.

Social immunity in honey bees: royal jelly as a vehicle in transferring bacterial pathogen fragments between nestmates

Social immunity is a suite of behavioral and physiological traits that allow colony members to protect one another from pathogens, and includes the oral transfer of immunological compounds between nestmates. In honey bees, royal jelly is a glandular secretion produced by a subset of workers that is fed to the queen and young larvae, and which contains many antimicrobial compounds. A related form of social immunity, transgenerational immune priming (TGIP), allows queens to transfer pathogen fragments into their developing eggs, where they are recognized by the embryo's immune system and induce higher pathogen resistance in the new offspring. These pathogen fragments are transported by vitellogenin (Vg), an egg-yolk precursor protein that is also used by nurses to synthesize royal jelly. Therefore, royal jelly may serve as a vehicle to transport pathogen fragments from workers to other nestmates. To investigate this, we recently showed that ingested bacteria are transported to nurses' jelly-producing glands, and here, we show that pathogen fragments are incorporated into the royal jelly. Moreover, we show that consuming pathogen cells induces higher levels of an antimicrobial peptide found in royal jelly, defensin-1.

Influence of Hyperproteinemia on Insect Innate Immune Function of the Circulatory System in Bombyx mori

Simple Summary

Hyperproteinemia, a condition of elevated protein levels in the blood, is associated with a diverse range of human and animal diseases. However, there is no reliable hyperproteinemia disease models or modeling methods in mammal or other organisms, and the effect of hyperproteinemia on immunity is still unknown. Our work succeeded in constructing an animal model of hyperproteinemia with no primary disease effects and a controllable plasma protein concentration (PPC) in an invertebrate model organism, Bombyx mori. Our work confirmed that high PPC enhances hemolymph phagocytosis via a rapid increase in granulocytes and inhibited hemolymph melanization due to inhibition of the prophenoloxidase (PPO) signaling pathway, and also upregulated the gene expression of antimicrobial peptides via activating the Toll and Imd pathways in NF-κB signaling, and showed an inconsistent antibacterial activity for Gram-positive and Gram-negative bacteria. Our results show that high PPC had multiple significant effects on the innate immune function of the silkworm circulatory system and is expected to be improved by endocrine hormones. Our work explores the pathogenesis of hyperproteinemia in an invertebrate model, and expands the scope for silkworm biomedical applications, even use for a potential drug development platform.

Abstract

Metabolic disorders of the circulatory system of animals (e.g., hyperglycemia and hyperlipidemia) can significantly affect immune function; however, since there is currently no reliable animal model for hyperproteinemia, its effects on immunity remain unclear. In this study, we established an animal model for hyperproteinemia in an invertebrate silkworm model, with a controllable plasma protein concentration (PPC) and no primary disease effects. We evaluated the influence of hyperproteinemia on innate immunity. The results showed that high PPC enhanced hemolymph phagocytosis via inducing a rapid increase in granulocytes. Moreover, while oenocytoids increased, the plasmacytes quickly dwindled. High PPC inhibited hemolymph melanization due to decreased phenoloxidase (PO) activity in the hemolymph via inhibiting the expression of the prophenoloxidase-encoding genes, PPO1 and PPO2. High PPC upregulated the gene expression of antimicrobial peptides via differential activation of the Toll and Imd signaling pathways associated with NF-κB signaling, followed by an induction of inconsistent antibacterial activity towards Gram-positive and Gram-negative bacteria in an animal model of high PPC. Therefore, high PPC has multiple significant effects on the innate immune function of the silkworm circulatory system.

Truncated reactive center loop decrease the inhibitory activity of Antheraea pernyi serine protease inhibitor 6

Here, we assessed the effect of a systematic change in reactive center loop (RCL) length, N-terminal to the reactive center, on the inhibitory activity of the recombinant Apserpin-6. The domain prediction results indicated that the RCL is located between the amino acid numbered 359-379 at the C-terminal of Apserpin-6. The N-terminal variable region for amino acid positions P7-P1 of the RCL of Apserpin-6 was truncated or extended by residue deletion or insertion using site-directed mutagenesis. The recombinant Apserpin-6 with one or two residues insertion in RCL had no effect on prophenoloxidase (proPO) activity, whereas deletion of one or two residues in RCL lowered the efficiency of inhibition of Apserpin-6. The results of this study will facilitate the understanding of inhibition mechanism of RCL on proPO activity.

Reactive oxygen species and antimicrobial peptides are sequentially produced in silkworm midgut in response to bacterial infection

The silkworm, Bombyx mori, is utilized as a research model in many aspects of biological studies, including genetics, development and immunology. Previous biochemical and genomic studies have elucidated the silkworm immunity in response to infections elicited by bacteria, fungi, microsporidia, and viruses. The intestine serves as the front line in the battle between insects and ingested harmful microorganisms. In this study, we performed RNA sequencing (RNA-seq) of the larval silkworm midgut after oral infection with the Gram-positive bacterium Bacillus bombysepticus and the Gram-negative bacterium Yersinia pseudotuberculosis. This enables us to get a comprehensive understanding of the midgut responses to bacterial infection. We found that B. bombysepticus induced much stronger immune responses than Y. pseudotuberculosis did. Bacterial infection resulted in more energy consumption including carbohydrates and fatty acids. The midgut immune system was characterized by the generation of reactive oxygen species and antimicrobial peptides. The former played a critical role in eliminating invading bacteria during early stage, while the latter executed during late stage. Our results provide an integrated insight into the midgut systematic responses to bacterial infection.

Glycoproteome in silkworm Bombyx mori and alteration by BmCPV infection

The biological functions of protein glycosylation have been increasingly recognized but not yet been very well understood, especially in lower organisms. Silkworm as a model lepidopteran insect and important economic insect, has been widely studied in life science, however, the current knowledge on the glycosylation status of its proteome is not satisfactory, and little is known about how pathogenic infections could affect the glycosylation status. This study performed large scale glycosite mapping for the silkworm Bombyx mori P50 strain, and quantitatively compared with that infected with the Bombyx mori cytoplasmic polyhedrosis virus (BmCPV). Some 400 glycoproteins were mapped in the silkworm, including N- and O-glycoproteins. Upon virus infection, the glycosylation levels of 41 N-glycopeptides were significantly changed, some of them belonging to transmembrane glycoproteins. The O-glycosylation profiles were also affected. In addition, 4 BmCPV-encoded viral proteins were found to be glycosylated for the first time, including polyhedrin, P101, VP3, and the NS protein. This study drafted a silkworm protein glycosylation map and underlined the potential impact of virus infection on glycosylation. SIGNIFICANCE: This study reveals the characteristics of the glycoproteome in the silkworm strain P50, and quantitatively compared to that infected by the virus BmCPV, which underlines the impact of virus infection on the alteration of protein glycosylation in invertebrate species. Our findings add to the knowledge of the post translational modifications of this model organism, and also uncovered for the first time the glycosylation status of the viral proteins expressed by BmCPV.

Antimicrobial peptides fromBombyx mori: a splendid immune defense response in silkworms

Bombyx mori L., a primary producer of silk, is the main tool in the sericulture industry and provides the means of livelihood to a large number of people. Silk cocoon crop losses due to bacterial infection pose a major threat to the sericulture industry. Bombyx mori L., a silkworm of the mulberry type, has a sophisticated inherent innate immune mechanism to combat such invasive pathogens. Among all the components in this defense system, antimicrobial peptides (AMPs) are notable due to their specificity towards the invading pathogens without harming the normal host cells. Bombyx mori L. so far has had AMPs identified that belong to six different families, namely cecropin, defensin, moricin, gloverin, attacin and lebocin, which are produced by the Toll and immune deficiency (IMD) pathways. Their diverse modes of action depend on microbial pathogens and are still under investigation. This review examines the recent progress in understanding the immune defense mechanism of Bombyx mori based on AMPs.

AMPs produced by B. mori induced by microbial challenge in the fat body.

Bmserpin2 Is Involved in BmNPV Infection by Suppressing Melanization in Bombyx mori

Melanization, an important defense response, plays a vital role in arthropod immunity. It is mediated by serine proteases (SPs) that convert the inactive prophenoloxidase (PPO) to active phenoloxidase (PO) and is tightly regulated by serine protease inhibitors (serpins) which belong to a well distributed superfamily in invertebrates, participating in immune mechanisms and other important physiological processes. Here, we investigated the Bmserpin2 gene which was identified from a transcriptome database in response to Bombyx mori nucleopolyhedrovirus (BmNPV) infection. Quantitative real-time polymerase chain reaction (qRT-PCR) results showed that Bmserpin2 was expressed in all tissues, with maximum expression in fat body. Upon BmNPV infection, the expression of Bmserpin2 was up-regulated in P50 (susceptible strain) and BC9 (resistant strain) in haemocytes, fat body and the midgut. However, up-regulation was delayed in BC9 (48 or 72 h), in contrast to P50 (24 h), after BmNPV infection. Meanwhile, Bmserpin2 could delay or inhibit melanization in silkworm haemolymph. Significant increased PO activity can be observed in Bmserpin2-depleted haemolymph under NPV infection. Furthermore, the viral genomic DNA copy number was decreased in Bmserpin2-depleted haemolymph. We conclude that Bmserpin2 is an inducible gene which might be involved in the regulation of PPO activation and suppressed melanization, and have a potential role in the innate immune system of B. mori.

Role of serine protease inhibitors in insect-host-pathogen interactions

Serine protease inhibitors (serpins), evolutionary old, structurally conserved molecules, are a superfamily of proteins found in almost all living organisms. Serpins are relatively large, typically 350-500 amino acids in length, with three β-sheets and seven to nine α-helices folding into a conserved tertiary structure with a reactive center loop. Serpins perform various physiological functions in insects, including development, digestion, host-pathogen interactions, and innate immune response. In insects, the innate immune system is characterized as the first and major defense system against the invasion of microorganisms. Serine protease cascades play a critical role in the initiation of innate immune responses, such as melanization and the production of antimicrobial peptides, and are strictly and precisely regulated by serpins. Herein, we provide a microreview on the role of serpins in the insect-host-pathogen interactions, emphasizing their role in immune responses, particularly in diamondback moth (Plutella xylostella), highlighting the important discoveries and also the gaps that remain to be explored in future studies.

Transcription Analysis of the Stress and Immune Response Genes to Temperature Stress in Ostrinia furnacalis

Ostrinia furnacalis is one of the most important pests on maize. O. furnacalis larvae are frequently exposed to the temperature challenges such as high temperature in summer and cold temperature in winter in the natural environment. High and low temperature stress, like any abiotic stress, impairs the physiology and development of insects. Up to now, there is limited information about gene regulation and signaling pathways related to the high and cold stress response in O. furnacalis. High-throughput sequencing of transcriptome provides a new approach for detecting stress and immune response genes under high and low temperature stresses in O. furnacalis. In the present study, O. furnacalis larvae were treated with the temperature at 8 and 40°C, and the responses of O. furnacalis larvae to the temperature stress were investigated through RNA-sequencing and further confirmation. The results showed that immune responses were up-regulated in larvae by the cold stress at 8°C while some stress response genes, such as HSP family, GST-2, Bax inhibitor and P450, were significantly increased at 40°C. Furthermore, quantitative real time polymerase chain reaction were performed to quantify the expression levels of immune related genes, such as PGRP-LB, antimicrobial peptides, lysozyme, serine protease and stress response genes such as small HSPs and HSP90, and the expression levels of these genes were similar to the RNA-seq results. In addition, the iron storage protein Ferritin was found to be involved in the response to temperature stress, and the changes of total iron concentration in the hemolymph were, in general, consistent with the expression levels of Ferritin. Taken together, our results suggested that the stress response genes were involved in the defense against the heat stress at 40°C, and the immune responses triggered by cold stress might provide protection for larvae from cold stress at 8°C. More interestingly, our results showed that during the responses to temperature stress, the total iron concentration in hemolymph regulated by Ferritin increased, which might help O. furnacalis in surviving the low and high temperature stress.

Label-free LC-MS/MS proteomic analysis of the hemolymph of silkworm larvae infected with Beauveria bassiana

Beauveria bassiana, a pathogen of the economically important silkworm (Bombyx mori), causes serious losses in the sericulture industry; however, the mechanisms underlying B. bassiana infection and the silkworm response are not fully understood. To obtain new insights into the interaction between B. bassiana and its host, hemolymph samples from fifth instar silkworm larvae infected with B. bassiana were analyzed at 36-h post-inoculation using a label-free LC-MS/MS proteomic technique. In total, 671 proteins were identified in the hemolymph, including 87 differentially expressed proteins, 42 up-regulated and 45 down-regulated in infected larvae. Six were detected only in infected larvae, and five were detected only in uninfected larvae. Based on GO annotations, 48 of the differentially expressed proteins were involved in molecular functions, 42 were involved in biological processes, and 39 were involved in cell components. A KEGG pathway analysis indicated that these differentially expressed proteins participate in 85 signal transduction pathways, including the amoebiasis, MAPK signaling, Hippo signaling, Toll and Imd signaling, and lysosome pathways. The silkworm hemolymph is the main site for B. bassiana replication. We identified differentially expressed proteins involved in the regulation of the host response to B. bassiana infection, providing important experimental data for the identification of key factors contributing to the interaction between the pathogenic fungus and its host.

A QM protein from Bombyx mori negatively regulates prophenoloxidase activation and melanization by interacting with Jun protein

The QM gene that encodes for the ribosomal protein L10 was firstly identified from human tumour cells as a tumour suppressor. In this study, a QM gene was identified in silkworm Bombyx mori (BmQM) and its immunomodulatory function was explored. BmQM messenger RNA (mRNA) and protein were highly expressed in the silk gland and fat body, and expressed in all stages of silkworm growth. After challenged with four different microorganisms, the expression levels of BmQM mRNA in fat body or haemocytes were significantly upregulated compared with the control. After knock-down of BmQM gene, the expressions of some immune genes (PGRPS6, Gloverin0, Lysozyme and Moricin) were affected, and the transcripts of prophenoloxidase1 and prophenoloxidase2 have different degrees of change. The phenoloxidase activity was significantly reduced when the purified recombinant BmQM protein was injected. Recombinant BmQM protein inhibited systemic melanization and suppressed prophenoloxidase activation stimulated by Micrococcus luteus, but it did not affect phenoloxidase activity. Far-western blotting assays showed that the BmQM protein interacted with silkworm BmJun protein, which negatively regulates AP-1 expression. Our results indicated that BmQM protein could affect some immune gene expression and negatively regulate the prophenoloxidase-activating system, and it may play an important role in regulation of the innate immunity in insects.

RNA-seq profiles of putative genes involved in specific immune priming in Bombyx mori haemocytes

BACKGROUND

The immune system of many invertebrates, including insects, has been shown to comprise memory, or specific immune priming. However, knowledge of the molecular mechanisms especially the candidate immune-related genes mediated the specificity of the immune priming are still very scarce and fragmentary. We therefore used two closely related Gram-negative pathogenic bacteria (Photorhabdus luminescens TT01 and P. luminescens H06) as the priming agents and employed Illumina/Solexa platform to investigate the transcriptional changes of the haemocytes of Bombyx mori larvae after priming.

RESULTS

In total, 23.0 Gbp of sequence data and 153,331,564 reads were generated, representing 10,496 genes. Approximately 89% of the genes or sequenced reads could be aligned to the silkworm reference genome. The differentially expressed genes (DEGs) of PBS-vs-TT01 (up-regulated expression of TT01 relative to PBS), PBS-vs-H06 (up-regulated expression of H06 relative to PBS) and TT01-vs-H06 (up-regulated expression of H06 relative to TT01) were 707, 159 and 461 respectively. In addition, expression patterns of 25 selected DEGs derived from quantitative real-time polymerase chain reaction (qRT-PCR) were consistent with their transcript abundance changes obtained by transcriptomic analyses. The DEGs are mainly related to pattern recognition receptors (PRRs), antimicrobial peptides (AMPs), signaling molecular, effector molecules, phagosome and spliceosome, indicating that they have participated in the regulation of the specific immune priming in the B. mori larvae.

CONCLUSIONS

The transcriptome profiling data sets from this study will provide valuable resources to better understand the molecular and biological mechanisms regulating the specificity of invertebrates' immune priming. All these will shed light on controlling insect pests or preventing epidemic of infectious diseases in economic invertebrates.

Effects of Bombyx mori nuclear polyhedrosis virus on serpin and antibacterial peptide expression in B. mori

The silkworm (Bombyx mori) is a typical and economically important lepidopteran species, and research has resulted in the development and accumulation of breeding lines. Studies of immune-related silkworm genes not only promote our understanding of silkworm immune response mechanisms, but they also inform insect immune molecular diversity research. Here, silkworm proteins were screened using proteomics after Bombyx mori nuclear polyhedrosis virus (BmNPV) infection, and 2368 silkworm proteins were identified, including six antimicrobial peptides and 12 serpins. The mRNA expression levels of these 18 proteins were examined at different times. The results indicated that attacin had the highest expression level, while serpin-5 and cecropin-D exhibited a negative regulatory correlation. These results provide a significant step toward a deeper understanding of B. mori immunoregulation.

Silkworm serpin32 functions as a negative-regulator in prophenoloxidase activation

The extracellular serine protease cascade is an essential component of insect humoral immunity. Serine protease inhibitors (serpins) play an important regulatory role in the process of insect immunity by regulating the serine protease cascade pathway. We aimed to clarify the function of Bmserpin32 in this study. First, we performed homologous sequence alignment and phylogenetic analysis of Bmserpin32. Bmserpin32 was found to share 64% amino acid sequence identity with Manduca sexta serpin7, an immunomodulatory protein. Bmserpin32 cDNA was cloned, and the recombinant Bmserpin32 protein was expressed in Escherichia coli and purified by nickel-nitrilotriacetic acid affinity and gel filtration chromatography. The activity assay showed that Bmserpin32 had significant inhibitory activity against trypsin. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry and site-directed mutagenesis combined with activity assays indicated that the cleavage site of Bmserpin32 is between Arg³⁵⁹ and Ile³⁶⁰. After infection with E. coli or Micrococcus luteus, the expression level of Bmserpin32 in immune-related tissues was significantly upregulated. In addition, Bmserpin32 could delay or inhibit the melanization of hemolymph by inhibiting the activation of prophenoloxidase in larval hemolymph. Furthermore, a physiological target of Bmserpin32 was identified as the clip protease, BmPAP3, an apparent ortholog of M. sexta propenoloxidase-activating protease-3. Our observations enable a better understanding of the physiological role of Bmserpin32 in regulating melanization in silkworm.

Insights into the venom protein components of Microplitis mediator, an endoparasitoid wasp

Endoparasitoid wasps deliver a variety of maternal factors, such as venom proteins, viruses, and virus-like particles, from their venom and calyx fluid into hosts and thereby regulate the hosts' immune response, metabolism and development. The endoparasitoid, Microplitis mediator, is used as an important biological agent for controlling the devastating pest Helicoverpa armigera. In this study, using an integrated transcriptomic and proteomic analysis approach, we identified 75 putative venom proteins in M. mediator. The identified venom components were consistent with other known parasitoid wasps' venom proteins, including metalloproteases, serine protease inhibitors, and glycoside hydrolase family 18 enzymes. The metalloprotease and serpin family showed extensive gene duplications in venom apparatus. Isobaric tags for relative and absolute quantitation (iTRAQ) based quantitative proteomics revealed 521 proteins that were differentially expressed at 6 h and 24 h post-parasitism, including 10 wasp venom proteins that were released into the host hemolymph. Further analysis indicated that 511 differentially expressed proteins (DEP) from the host are primarily involved in the immune response, material metabolism, and extracellular matrix receptor interaction. Taken together, our results on parasitoid wasp venoms have the potential to enhance the application of endoparasitoid wasps for controlling insect pest.

Manipulation of the silkworm immune system by a metalloprotease from the pathogenic bacterium Pseudomonas aeruginosa

Antimicrobial peptide (AMP) production and melanization are two key humoral immune responses in insects. Induced synthesis of AMPs results from Toll and IMD signal transduction whereas melanization depends on prophenoloxidase (PPO) activation system. During invasion, pathogens produce toxins and other virulent factors to counteract host immune responses. Here we show that the pathways leading to PPO activation and AMP synthesis in the silkworm Bombyx mori are affected by a metalloprotease, named elastase B, secreted by Pseudomonas aeruginosa (PAO1). The metalloprotease gene (lasB) was expressed shortly after PAO1 cells had been injected into the larval silkworm hemocoel, leading to an increase of elastase activity. Injection of the purified PAO1 elastase B into silkworm hemolymph compromised PPO activation. In contrast, the protease caused a level increase of gloverin, an AMP in the hemolymph. To verify our results obtained using the purified elastase B, we infected B. mori with PAO1 ΔlasB mutant and found that PO activity in hemolymph of the PAO1 ΔlasB-infected larvae was significantly higher than that in the wild type-infected. The mutant-inhabited hemolymph had lower levels of gloverin and antimicrobial activity. PAO1 ΔlasB showed a decreased viability in the silkworm hemolymph whereas the host had a lower mortality. In addition, the effects caused by the ΔlasB mutant were restored by a complementary strain. These data collectively indicated that the elastase B produced by PAO1 is an important virulent factor that manipulates the silkworm immune system during infection.

RNA interference-mediated silencing of genes involved in the immune responses of the soybean pod borer Leguminivora glycinivorella (Lepidoptera: Olethreutidae)

RNA interference (RNAi) technology may be useful for developing new crop protection strategies against the soybean pod borer (SPB; Leguminivora glycinivorella), which is a critical soybean pest in northeastern Asia. Immune-related genes have been recently identified as potential RNAi targets for controlling insects. However, little is known about these genes or mechanisms underlying their expression in the SPB. In this study, we completed a transcriptome-wide analysis of SPB immune-related genes. We identified 41 genes associated with SPB microbial recognition proteins, immune-related effectors or signalling molecules in immune response pathways (e.g., Toll and immune deficiency pathways). Eleven of these genes were selected for a double-stranded RNA artificial feeding assay. The down-regulated expression levels of LgToll-5-1a and LgPGRP-LB2a resulted in relatively high larval mortality rates and abnormal development. Our data represent a comprehensive genetic resource for immune-related SPB genes, and may contribute to the elucidation of the mechanism regulating innate immunity in Lepidoptera species. Furthermore, two immune-related SPB genes were identified as potential RNAi targets, which may be used in the development of RNAi-mediated SPB control methods.

Immune responses to bacterial and fungal infections in the silkworm, Bombyx mori

The silkworm Bombyx mori, an economically important insect that is usually reared indoors, is susceptible to various pathogens, including bacteria, fungi, viruses, and microsporidia. As with other insects, the silkworm lacks an adaptive immune system and relies solely on innate immunity to defend itself against infection. Compared to other intensively studied insects, such as the fruit fly and tobacco hornworm, the principal immune pathways in the silkworm remain unclear. In this article, we review the literature concerning silkworm immune responses to bacteria and fungi and present our perspectives on future research into silkworm immunity.

Insights into the molecular evolution of peptidase inhibitors in arthropods

Peptidase inhibitors are key proteins involved in the control of peptidases. In arthropods, peptidase inhibitors modulate the activity of peptidases involved in endogenous physiological processes and peptidases of the organisms with which they interact. Exploring available arthropod genomic sequences is a powerful way to obtain the repertoire of peptidase inhibitors in every arthropod species and to understand the evolutionary mechanisms involved in the diversification of this kind of proteins. A genomic comparative analysis of peptidase inhibitors in species belonging to different arthropod taxonomic groups was performed. The results point out: i) species or clade-specific presence is shown for several families of peptidase inhibitors; ii) multidomain peptidase inhibitors are commonly found in many peptidase inhibitor families; iii) several families have a wide range of members in different arthropod species; iv) several peptidase inhibitor families show species-specific (or clade-specific) gene family expansions; v) functional divergence may be assumed for particular clades; vi) passive expansions may be used by natural selection to fix adaptations. In conclusion, conservation and divergence of duplicated genes and the potential recruitment as peptidase inhibitors of proteins from other families are the main mechanisms used by arthropods to fix diversity. This diversity would be associated to the control of target peptidases and, as consequence, to adapt to specific environments.

Inhibition of melanization by serpin-5 and serpin-9 promotes baculovirus infection in cotton bollworm Helicoverpa armigera

Melanization, an important insect defense mechanism, is mediated by clip-domain serine protease (cSP) cascades and is regulated by serpins. Here we show that proteolytic activation of prophenoloxidase (PPO) and PO-catalyzed melanization kill the baculovirus in vitro. Our quantitative proteomics and biochemical experiments revealed that baculovirus infection of the cotton bollworm, Helicoverpa armigera, reduced levels of most cascade members in the host hemolymph and PO activity. By contrast, serpin-9 and serpin-5 were sequentially upregulated after the viral infection. The H. armigera serpin-5 and serpin-9 regulate melanization by directly inhibiting their target proteases cSP4 and cSP6, respectively and cSP6 activates PPO purified from hemolymph. Furthermore, serpin-5/9-depleted insects exhibited high PO activities and showed resistance to baculovirus infection. Together, our results characterize a part of the melanization cascade in H. armigera, and suggest that natural insect virus baculovirus has evolved a distinct strategy to suppress the host immune system.

Melanization is one of important modules in insect defense system. It consists of a cascade of clip-domain serine proteases (cSPs) that converts the zymogen prophenoloxidase (PPO) to active phenoloxidase (PO), which is negatively regulated by serpins. PO then catalyses the formation of melanin that physically encapsulates certain pathogens. Parasites and bacteria have evolved to produce specific proteins or antibiotic to suppress the melanization response of host insects for survival. However, the mechanisms by which virus persists in the face of the insect melanization are poorly understood. In this study, we show that a DNA virus baculovirus infection of the cotton bollworm, Helicoverpa armigera, reduced the levels of most cascade members in the host hemolymph and PO activity. By contrast, serpin-9 and serpin-5 were sequentially upregulated after the viral infection. Our results also reveal that melanization kills baculovirus in vitro. Serpin-5 and serpin-9 regulate melanization by directly inhibiting their target proteases cSP4 and cSP6, respectively and cSP6 activates PPO purified from hemolymph. Moreover, serpin-5/9-depleted insects show resistance to baculovirus infection. Our findings have enriched the understanding of molecular mechanisms by which pathogens suppress the melanization response of host insect for survival.

A starvation-induced regulator, RovM, acts as a switch for planktonic/biofilm state transition in Yersinia pseudotuberculosis

The transition between the planktonic state and the biofilm-associated state is a key developmental decision for pathogenic bacteria. Biofilm formation by Yersinia pestis is regulated by hmsHFRS genes (β-1, 6-N-acetyl-D-glucosamine synthesis operon) in its flea vector and in vitro. However, the mechanism of biofilm formation in Yersinia pseudotuberculosis remains elusive. In this study, we demonstrate that the LysR-type regulator RovM inversely regulates biofilm formation and motility in Y. pseudotuberculosis by acting as a transcriptional regulator of these two functions. RovM is strongly induced during growth in minimal media but strongly repressed in complex media. On one hand, RovM enhances bacterial motility by activating the expression of FlhDC, the master regulator of flagellar genes, via the recognition of an operator upstream of the flhDC promoter. On the other hand, RovM represses β-GlcNAc production under nutrition-limited conditions, negatively regulating hmsHFRS expression by directly binding to the -35 element of its promoter. Compared to wild-type bacteria, the rovM mutant established denser biofilms and caused more extensive mortality in mice and silkworm larvae. These results indicate that RovM acts as a molecular switch to coordinate the expression of genes involved in biofilm formation and motility in response to the availability of nutrients.

Characterization and expression profiling of serine protease inhibitors in the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae)

BACKGROUND

Serine protease inhibitors (SPIs) have been found in all living organisms and play significant roles in digestion, development and innate immunity. In this study, we present a genome-wide identification and expression profiling of SPI genes in the diamondback moth, Plutella xylostella (L.), a major pest of cruciferous crops with global distribution and broad resistance to different types of insecticides.

RESULTS

A total of 61 potential SPI genes were identified in the P. xylostella genome, and these SPIs were classified into serpins, canonical inhibitors, and alpha-2-macroglobulins based on their modes of action. Sequence alignments showed that amino acid residues in the hinge region of known inhibitory serpins from other insect species were conserved in most P. xylostella serpins, suggesting that these P. xylostella serpins may be functionally active. Phylogenetic analysis confirmed that P. xylostella inhibitory serpins were clustered with known inhibitory serpins from six other insect species. More interestingly, nine serpins were highly similar to the orthologues in Manduca sexta which have been demonstrated to participate in regulating the prophenoloxidase activation cascade, an important innate immune response in insects. Of the 61 P.xylostella SPI genes, 33 were canonical SPIs containing seven types of inhibitor domains, including Kunitz, Kazal, TIL, amfpi, Antistasin, WAP and Pacifastin. Moreover, some SPIs contained additional non-inhibitor domains, including spondin_N, reeler, and other modules, which may be involved in protein-protein interactions. Gene expression profiling showed gene-differential, stage- and sex-specific expression patterns of SPIs, suggesting that SPIs may be involved in multiple physiological processes in P. xylostella.

CONCLUSIONS

This is the most comprehensive investigation so far on SPI genes in P. xylostella. The characterized features and expression patterns of P. xylostella SPIs indicate that the SPI family genes may be involved in innate immunity of this species. Our findings provide valuable information for uncovering further biological roles of SPI genes in P. xylostella.

Identification and functional characterizations of serpin8, a potential prophenoloxidase-activating protease inhibitor in Pacific white shrimp, Litopenaeus vannamei

Serpins have been characterized from varieties of organisms by their inhibitory roles on serine or cysteine proteases. However, research for the functional study of serpins in crustacean is relatively small. To fully clarify the immune characterizations of serpin, a novel serpin (named Lvserpin8) encoding 414 amino acids with a 19-amino acid signal peptide and a serpin domain was identified from the Pacific white shrimp Litopenaeus vannamei. Sequence analysis indicated that the genomic Lvserpin8 gene contains 5 exons and 4 introns. The P1 residues of the predicted scissile bond in the reactive center loop (RCL) region represented for Lysine (Lys), which is in accordance with Pmserpin8, Dmserpin27A, Ofserpin3, Bmserpin3 and Msserpin3. Quantitative results showed that high mRNA expression of Lvserpin8 was detected in hepatopancreas and testis. Notably, a significant increase of Lvserpin8 was appeared post injection of Vibrio anguillarum, and Micrococcus lysodeikticus. Moreover, Lvserpin8 was knocked down in vivo by double-stranded RNA (dsRNA) mediated RNA interference (RNAi). Suppression of Lvserpin8 led to a significant increase in the transcripts of LvPPAE2 (Prophenoloxidase-activating Enzyme 2) and cumulative mortality. What's more, recombinant Lvserpin8 protein (rLvserpin8) displayed inhibition roles on trypsin activity, and prophenoloxidase activation. Taken together, the results implied that Lvserpin8 may be involved in shrimp innate immunity via the inhibition of prophenoloxidase-activating proteases.