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.

Silencing of the Prophenoloxidase Gene BtPPO1 Increased the Ability of Acquisition and Retention of Tomato chlorosis virus by Bemisia tabaci

Tomato chlorosis virus (ToCV) has seriously impacted tomato production around the world. ToCV is semi-persistently transmitted by the whitefly, Bemisia tabaci, which is a serious agricultural pest in the world. However, the interaction mechanism between ToCV and its whitefly vector is still poorly understood. Our previous transcriptome analysis demonstrated that the expression level of an immune-related gene, prophenoloxidase (PPO), in B. tabaci increased after ToCV acquisition, which indicates that the PPO may be involved in the interaction mechanism between the ToCV and its vector. To determine the role of the PPO in the acquisition and retention of ToCV by B. tabaci, we cloned the complete Open Reading Frames (ORF) of the BtPPOs (BtPPO1 and BtPPO2), and then structure and phylogenetic analyses were performed. BtPPOs were closely related to the PPO genes of Hemiptera insects. Spatial-temporal expression detection was qualified by using reverse transcription quantitative PCR (RT-qPCR), and this revealed that BtPPOs were expressed in all tissues and developmental stages. We found that only BtPPO1 was significantly upregulated after B. tabaci acquired ToCV for 12 and 24 h. According to the paraffin-fluorescence probe-fluorescence in situ hybridization (FISH) experiment, we verified that ToCV and BtPPO1 were co-located in the thorax of B. tabaci, which further revealed the location of their interaction. Finally, the effects of the BtPPOs on ToCV acquisition and retention by B. tabaci were determined using RNA interference (RNAi). The results showed that the RNAi of the responsive gene (BtPPO1) significantly increased the titer of ToCV in B. tabaci. These results demonstrate that BtPPO1 participates in ToCV acquisition and retention by B. tabaci.

Haemocyte-mediated immunity in insects: Cells, processes and associated components in the fight against pathogens and parasites

The host defence of insects includes a combination of cellular and humoral responses. The cellular arm of the insect innate immune system includes mechanisms that are directly mediated by haemocytes (e.g., phagocytosis, nodulation and encapsulation). In addition, melanization accompanying coagulation, clot formation and wound healing, nodulation and encapsulation processes leads to the formation of cytotoxic redox-cycling melanin precursors and reactive oxygen and nitrogen species. However, demarcation between cellular and humoral immune reactions as two distinct categories is not straightforward. This is because many humoral factors affect haemocyte functions and haemocytes themselves are an important source of many humoral molecules. There is also a considerable overlap between cellular and humoral immune functions that span from recognition of foreign intruders to clot formation. Here, we review these immune reactions starting with the cellular mechanisms that limit haemolymph loss and participate in wound healing and clot formation and advancing to cellular functions that are critical in restricting pathogen movement and replication. This information is important because it highlights that insect cellular immunity is controlled by a multilayered system, different components of which are activated by different pathogens or during the different stages of the infection.

Silencing of prophenoloxidase (proPO) gene in freshwater prawn, Macrobrachium rosenbergii, makes them susceptible to white spot syndrome virus (WSSV)

Prophenoloxidase (proPO) is very important to protect the invertebrates from microbial infections. Our previous studies revealed that proPO was up-regulated in WSSV-injected Macrobrachium rosenbergii and is responsible for protecting M. rosenbergii from WSSV. In order to prove this mechanism, an attempt was made in the present study to silence the proPO gene in freshwater prawn by injection of dsRNA-proPO followed by WSSV challenge. Two partial fragments of proPO with the size of 251 and 331 bp were used to synthesize dsRNA using LITMUS38i vector and E. coli. The bacterially synthesized dsRNA-proPO was used to silence proPO gene to determine its involvement in developing resistance in prawn against WSSV. In proPO gene-silenced prawn, 100% mortality was observed after WSSV challenge whereas no mortality was observed in prawn injected with WSSV alone. The WSSV infection in gene-silenced prawn was confirmed by PCR, and its propagation was quantified by ELISA and real-time PCR at different time intervals. Real-time PCR assay revealed a significant reduction in the expression of proPO gene in WSSV-challenged proPO-silenced prawn when compared to normal prawn. Level of proPO was reduced significantly in the haemolymph of proPO-silenced prawn when compared to prawn injected with PBS.

A Plant Virus Ensures Viral Stability in the Hemolymph of Vector Insects through Suppressing Prophenoloxidase Activation

Large ratios of vector-borne plant viruses circulate in the hemolymph of their vector insects before entering the salivary glands to be transmitted to plants. The stability of virions in the hemolymph is vital in this process. Activation of the proteolytic prophenoloxidase (PPO) to produce active phenoloxidase (PO) is one of the major innate immune pathways in insect hemolymph. How a plant virus copes with the PPO immune reaction in its vector insect remains unclear. Here, we report that the PPO affects the stability of rice stripe virus (RSV), a notorious rice virus, in the hemolymph of a vector insect, the small brown planthopper. RSV suppresses PPO activation using viral nonstructural protein. Once the level of PO activity is elevated, RSV is melanized and eliminated from the hemolymph. Our work gives valuable clues for developing novel strategies for controlling the transmission of vector-borne plant viruses.

Most plant viruses require vector insects for transmission. Viral stability in the hemolymph of vector insects is a prerequisite for successful transmission of persistent plant viruses. However, knowledge of whether the proteolytic activation of prophenoloxidase (PPO) affects the stability of persistent plant viruses remains elusive. Here, we explored the interplay between rice stripe virus (RSV) and the PPO cascade of the vector small brown planthopper. Phenoloxidase (PO) activity was suppressed by RSV by approximately 60%. When the PPO cascade was activated, we found distinct melanization around RSV particles and serious damage to viral stability in the hemolymph. Viral suppression of PO activity was derived from obstruction of proteolytic cleavage of PPOs by binding of the viral nonstructural protein NS3. These results indicate that RSV attenuates the PPO response to ensure viral stability in the hemolymph of vector insects. Our research provides enlightening cues for controlling the transmission of vector-borne viruses.

Identification of two clip domain serine proteases involved in the pea aphid's defense against bacterial and fungal infection

Phenoloxidases (POs) are required for the pea aphid's defense against bacterial and fungal infection. Prophenoloxidases (PPOs) are proteolytically converted to its active form PO through a clip domain serine protease cascade. In this study, we identified five clip domain serine proteases in the pea aphids. The messenger RNA levels of two of them, Ap_SPLP and Ap_VP, were upregulated by Gram-positive bacterium Staphylococcus aureus and fungus Beauveria bassiana infections. Double-stranded RNA-based expression knockdown of these two genes resulted in reduced PO activity of the aphid hemolymph, higher loads of S. aureus and B. bassiana in the aphids, and lower survival rates of the aphids after infections. Our data suggest that Ap_SPLP and Ap_VP are involved in PPO activation pathway in the pea aphid.

Analysis of Cry1Ah Toxin-Binding Reliability to Midgut Membrane Proteins of the Asian Corn Borer

Evolution of insect resistance to Bt toxins challenges the use of Cry toxins to control agricultural pests. In lepidopterans, Cry toxin affinity towards multiple midgut epithelial receptors has become a matter of dispute. Cry1Ah toxin-binding proteins were identified in the larval midgut of susceptible (ACB-BtS) and resistant (ACB-AhR) strains of the Asian corn borer (ACB). A pull-down assay was performed using biotinylated Cry1Ah toxin, and the binding proteins were identified by employing liquid chromatography–tandem mass spectrometry (LC-MS/MS). This study aimed to find the binding consistency of the midgut epithelial protein to the Cry1Ah toxin. The binding proteins from different fractions of SDS-PAGE showed a different pattern. We observed an isoform of prophenoloxidase PPO1b (UniProt Acc No. A0A1Q1MKI0), which was found only in the ACB-AhR fractions. Prophenoloxidase (proPO) is an extraordinary defense molecule activated in insect species during pathogen invasion and the wound healing process. Importantly, this prophenoloxidase might have direct/indirect interaction with the Cry1Ah toxin. Our data also suggest that factors like techniques, enrichment of binding proteins in the sample and the reversible and irreversible nature of the brush border membrane vesicles (BBMVs) to Cry toxins could cause the inconsistency in the protein–protein interactions. Moreover, inside the larva midgut, the influence of the Cry toxins under physiological conditions might be different from the laboratory procedures.

Adult Mosquitoes Infected with Bacteria Early in Life Have Stronger Antimicrobial Responses and More Hemocytes after Reinfection Later in Life

The immunological strategies employed by insects to overcome infection vary with the type of infection and may change with experience. We investigated how a bacterial infection in the hemocoel of the African malaria mosquito, Anopheles gambiae, prepares the immune system to face a subsequent bacterial infection. For this, adult female mosquitoes were separated into three groups—unmanipulated, injured, or infected with Escherichia coli—and five days later all the mosquitoes were infected with a different strain of E. coli. We found that an injury or a bacterial infection early in life enhances the ability of mosquitoes to kill bacteria later in life. This protection results in higher mosquito survival and is associated with an increased hemocyte density, altered phagocytic activity by individual hemocytes, and the increased expression of nitric oxide synthase and perhaps prophenoloxidase 6. Protection from a second infection likely occurs because of heightened immune awareness due to an already existing infection instead of memory arising from an earlier, cured infection. This study highlights the dynamic nature of the mosquito immune response and how one infection prepares mosquitoes to survive a subsequent infection.

Identification of a Conserved Prophenoloxidase Activation Pathway in Cotton Bollworm Helicoverpa armigera

Melanization is a prominent insect humoral response for encapsulation of and killing invading pathogens. It is mediated by a protease cascade composed of a modular serine protease (SP), and clip domain SPs (cSPs), which converts prophenoloxidase (PPO) into active phenoloxidase (PO). To date, melanization pathway in cotton bollworm Helicoverpa armigera, an important agricultural pest, remains largely unclear. To biochemically reconstitute the pathway in vitro, the putative proteases along with modified proteases containing the factor Xa cleavage site were expressed by Drosophila S2 cell expression system. Purified recombinant proteins were used to examine their role in activating PPO. It is revealed that cascade is initiated by a modular SP-SP41, followed by cSP1 and cSP6. The three-step SP41/cSP1/cSP6 cascade could further activate PPO, and the PO activity was significantly enhanced in the presence of two cSP homologs (cSPHs), cSPH11 and cSPH50, suggesting the latter are cofactors for PPO activation. Moreover, baculovirus infection was efficiently blocked by the reconstituted PPO activation cascade, and the effect was boosted by cSPH11 and cSPH50. Taken together, we unraveled a conserved PPO activation cascade in H. armigera, which is similar to that exists in lepidopteran biochemical model Manduca sexta and highlighted its role in antagonizing viral infection.

The Role of Lozenge in Drosophila Hematopoiesis

Drosophila hematopoiesis is comparable to mammalian differentiation of myeloid lineages, and therefore, has been a useful model organism in illustrating the molecular and genetic basis for hematopoiesis. Multiple novel regulators and signals have been uncovered using the tools of Drosophila genetics. A Runt domain protein, lozenge, is one of the first players recognized and closely studied in the hematopoietic lineage specification. Here, we explore the role of lozenge in determination of prohemocytes into a special class of hemocyte, namely the crystal cell, and discuss molecules and signals controlling the lozenge function and its implication in immunity and stress response. Given the highly conserved nature of Runt domain in both invertebrates and vertebrates, studies in Drosophila will enlighten our perspectives on Runx-mediated development and pathologies.

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.

White Spot Syndrome Virus-Induced Shrimp miR-315 Attenuates Prophenoloxidase Activation via PPAE3 Gene Suppression

MicroRNAs (miRNAs), the small non-coding RNAs, play a pivotal role in post-transcriptional gene regulation in various cellular processes. However, the miRNA function in shrimp antiviral response is not clearly understood. This research aims to uncover the function of pmo-miR-315, a white spot syndrome virus (WSSV)-responsive miRNAs identified from Penaeus monodon hemocytes during WSSV infection. The expression of the predicted pmo-miR-315 target mRNA, a novel PmPPAE gene called PmPPAE3, was negatively correlated with that of the pmo-miR-315. Furthermore, the luciferase assay indicated that the pmo-miR-315 directly interacted with the target site in PmPPAE3 suggesting the regulatory role of pmo-miR-315 on PmPPAE3 gene expression. Introducing the pmo-miR-315 into the WSSV-infected shrimp caused the reduction of the PmPPAE3 transcript level and, hence, the PO activity activated by the PmPPAE3 whereas the WSSV copy number in the shrimp hemocytes was increased. Taken together, our findings state a crucial role of pmo-miR-315 in attenuating proPO activation via PPAE3 gene suppression and facilitating the WSSV propagation in shrimp WSSV infection.

Effect of the insect phenoloxidase on the metabolism of l-DOPA

Insect prophenoloxidase (PPO) induces melanization around pathogens. Before melanization, PPO is cleaved into phenoloxidase (PO) by serine proteases. Insect PPO can also be activated by exogenous proteases secreted by pathogens as well as by other compounds, such as ethanol and cetylpyridinium chloride (CPC). However, the effect of these activators on the activity of PO is unclear. In this study, the insect endogenous serine protease AMM1, α-chymotrypsin, and ethanol were used to activate recombinant Drosophila PPO1 (rPPO1), and the PO activity differed depending on the activator applied. The PO-induced intermediates during melanization also varied markedly in their numbers and abundances. Therefore, this study indicates that the mechanism of PPO activation influences PO activity. It also suggests that PO-induced different intermediates may affect the antibacterial activity during melanization due to their toxicity.

Identification of three prophenoloxidase-activating factors (PPAFs) from an invasive beetle Octodonta nipae Maulik (Coleoptera: Chrysomelidae) and their roles in the prophenoloxidase activation

A typical characteristic of the insect innate immune system is the activation of the serine protease cascade in the hemolymph. As being the terminal component of the extracellular serine protease cascade in the prophenoloxidase (proPO) activating system, proPO-activating factors (PPAFs) activated by the upstream cascade may generate active phenoloxidase, which then induces downstream melanization. In the present study, we reported three PPAFs from the nipa palm hispid beetle Octodonta nipae (Maulik) (designated as OnPPAF1, OnPPAF2, OnPPAF3). All three OnPPAFs contained a single clip domain at the amino-terminus followed by a trypsin-like serine protease domain at the carboxyl-terminus, except the Ser in the active sites of OnPPAF2 and OnPPAF3 was substituted with Gly. Transcript expression analysis revealed that all OnPPAFs were highly expressed in hemolymph, whereas OnPPAF2 showed an extremely low mRNA abundance compared with that of OnPPAF1 and OnPPAF3, and that the abundance of all three OnPPAFs was dramatically increased upon bacterial challenge. Knockdown of OnPPAF1 or OnPPAF3 resulted in a reduction of hemolymph phenoloxidase activity and an inhibition of hemolymph melanization, whereas the knockdown of OnPPAF2 did not affect the proPO cascade. Our work thus implies that the three OnPPAFs may have different functions and regulation during immune responses in O. nipae.

Prophenoloxidase-Mediated Ex Vivo Immunity to Delay Fungal Infection after Insect Ecdysis

Skin immunity protects animals from airborne pathogen infection. Unlike mammals, arthropods, including insects, undergo periodic ecdysis to grow and develop. Newly molted insects emerge with unsclerotized thin cuticles but successfully escape pathogenic infections during the post-molt period. Here we show that prophenoloxidases (PPOs) in molting fluids remain bioactive on the integument and impede fungal infection after ecdysis. We found that the purified plasma PPOs or recombinant PPOs could effectively bind to fungal spores (conidia) by targeting the cell wall components chitin and β-1,3-glucan. Pretreatment of the spores of the fungal pathogen Beauveria bassiana with PPOs increased spore hydrophilicity and reduced spore adhesion activity, resulting in a significant decrease in virulence as compared with mock infection. We also identified a spore-secreted protease BPS8, a member of peptidase S8 family of protease that degrade PPOs at high levels to benefit fungal infection, but which at lower doses activate PPOs to inhibit spore germination after melanization. These data indicate that insects have evolved a distinct strategy of ex vivo immunity to survive pathogen infections after ecdysis using PPOs in molting fluids retained on the underdeveloped and tender integument of newly molted insects for protection against airborne fungal infection.

Activation of Aedes aegypti prophenoloxidase-3 and its role in the immune response against entomopathogenic fungi

Serine protease cascade-mediated melanization is an important innate immune response in insects and crustaceans, which involves the proteolytic activation of prophenoloxidase (PPO). In this study, we investigated the role of Aedes aegypti PPO3 in antifungal immune defence. We expressed and purified recombinant PPO3 (rPPO3) in Escherichia coli and demonstrated that rPPO3 was activated by ethanol and, to a lesser extent, by cetylpyridinium chloride. In the presence of Cu²⁺ , rPPO3 exhibited enzyme activity. Immunoblot results revealed that the rPPO3 was cleaved by the haemolymph from immune-challenged mosquitoes or purified Ostrinia furnacalis serine protease 105 in vitro. The cleaved rPPO3 converted dopamine to toxic intermediates that killed fungal conidia of Beauveria bassiana in vitro. In mosquitoes challenged with Be. bassiana, cleavage of rPPO3 produced a 50 kDa phenoloxidase (PO) fragment. Further analysis revealed that the survival rate of mosquitoes with fungal infection increased significantly following injection of rPPO3 into the haemocoel. Taken together, our results suggest that proteolytic cleavage of the mosquito PPO3 plays an important role in the antifungal immune response. This has led to a better understanding of the mechanism of PPO activation in the mosquito and the role of melanization in the antifungal immune response.

Solution Structure and Expression Profile of an Insect Cytokine: Manduca sexta Stress Response Peptide-2

Manduca sexta stress response peptide-2 (SRP2) is predicted to be a 25-residue peptide (FGVKDGKCPSGRVRRLGICVPDDDY), which may function as an insect cytokine to regulate immune responses. Produced as an inactive precursor, endogenous proSRP2 is probably converted to active SRP2 by limited proteolysis in response to invading pathogens, along with prophenoloxidase and pro-Spätzle activation. In addition to immunity, SRP2 may control head morphogenesis or other developmental processes in the lepidopteran insect. We have examined the profiles of SRP2 gene expression in terms of immune induction capacity, tissue specificity, and developmental changes. To gain insights into its functions, we chemically synthesized SRP2, injected the peptide solution into naïve larvae, and detected significant up-regulation of several antimicrobial peptide genes. We determined the 3D molecular structure in solution of SRP2 by two-dimensional 1H-1H NMR spectroscopy. SRP2 has an ordered structure, which is composed of two short β-strands at regions R12 - R15 and I18 - V20, one type-I' β-turn at region R15 - I18, and a half turn at region C8 - S10 in its welldefined core stabilized by a covalent disulfide bond between C8 and C19. The secondary and tertiary structures are further stabilized by hydrogen bonds. Possible relationships between the structure and function are also discussed.

Molecular Characterization and Bioinformatics Analysis of a Prophenoloxidase-1 (PPO1) in Plutella xylostella

Phenoloxidase (PO) is an important enzyme in insect life, which is involved in important physical functions, such as defensive encapsulation and melanization of foreign organisms and wound healing. In this study, we obtained a cDNA sequence of 2838 bp with 2049 open reading frames encoding 682 amino acids. The protein sequence deduced from the cDNA has high homology with the known PPO1 sequences of other lepidopterous insects. There were three conserved regions, including the two copper-binding sites characteristic of arthropod PPOs. The whole PxPPO1 DNA was also obtained with 7202 bp when the five fragments were stitched together and the overlapping sequences were deleted. The PxPPO1 DNA consists of 11 introns and 12 exons, and the homology is 99.9% when the exons are compared with the above cDNA. Moreover, the gene expression levels were also determined by semiquantitative polymerase chain reaction (PCR), Western blotting, and real-time quantitative PCR; the results indicated that PxPPO1 transcripts in the eggs and the fourth instar larvae were more abundant, followed by the second and the third instar larvae, prepupae, and pupa.

Origins and activation of prophenoloxidases in the digestive tract of the cricket, Gryllus bimaculatus

The function of Phenoloxidases (POs) in sclerotization and defense in insects is well understood, but little is known concerning their occurrence, origins, and function in the digestive tract. In Gyrllus bimaculatus gut all of the PO activity is found in the lumen of the digestive tract, and no detectible activity is found in homogenates of the gut epithelium or secretions from incubated epithelial tissues. Prophenoloxidases (PPOs) are synthesized in the hemocytes of  Bombyx mori and are transported into the cuticle. It is suggested that the PPOs in the caecal lumen of G. bimaculatus likewise are synthesized in hemocytes and are transported by unknown means into the caecal lumen, where they are activated to POs by trypsin. Peristalsis transports the POs both forward into the crop and posterior within the peritrophic membrane into the hind gut. The PPOs in the hemolymph consist of a trimer (270-280 kDa) and a tetramer (340-370 kDa). The active POs in the gut lumen consist of a monomer (85-95 kDa) in addition to an activated trimer and tetramer.

A novel type of hemocytes localizing melanization with high-spreading behavior in Mythimna separata

Lepidopteran larvae show a cellular response to invading foreign substances that are larger than hemocytes, for example, parasitoid eggs or larvae. This response is called hemocyte encapsulation and is often accompanied by phenoloxidase (PO)-catalyzed melanization. In the present study, we artificially transplanted endoparasitoid larvae and small glass fragments into the hemocoel of the common armyworm, Mythimna separata. We observed that the host larva showed a cellular response and that, 2-4 h after transplantation, melanin formation was spatially confined to the surface of the encapsulated substances. We further noted that specific morphological hemocytes surrounded by melanin formation became attached to the surface of the foreign substances. We designated these hemocytes hyperspread cells (HSCs) on the basis of their specific characteristics and circumferential spread. We confirmed the occurrence of prophenoloxidase (PPO)/phenoloxidase (PO) on the periphery of the HSCs and in the substance secreted around the HSCs by using anti-PPO antibody. We were unable to detect PPO-mRNA in HSCs by using in situ hybridization, although we showed that oenocytoids contained PPO-mRNA and PPO protein. We used light microscopy and scanning electron microscopy to discriminate five main types of circulating M. separata hemocytes. We observed that HSCs differed from plasmatocytes, but spread out well. Further, during the encapsulation process, HSCs appeared to provide a localized melanization spot on the surface of foreign invaders.

Insect prophenoloxidase: the view beyond immunity

Insect prophenoloxidase (PPO) is an important innate immunity protein due to its involvement in cellular and humoral defense. It belongs to a group of type-3 copper-containing proteins that occurs in almost all organisms. Insect PPO has been studied for over a century, and the PPO activation cascade is becoming clearer. The insect PPO activation pathway incorporates several important proteins, including pattern-recognition receptors (PGRP, β GRP, and C-type lectins), serine proteases, and serine protease inhibitors (serpins). Due to their complexity, PPO activation mechanisms vary among insect species. Activated phenoloxidase (PO) oxidizes phenolic molecules to produce melanin around invading pathogens and wounds. The crystal structure of Manduca sexta PPO shows that a conserved amino acid, phenylalanine (F), can block the active site pocket. During activation, this blocker must be dislodged or even cleaved at the N-terminal sequence to expose the active site pockets and allow substrates to enter. Thanks to the crystal structure of M. sexta PPO, some domains and specific amino acids that affect PPO activities have been identified. Further studies of the relationship between PPO structure and enzyme activities will provide an opportunity to examine other type-3 copper proteins, and trace when and why their various physiological functions evolved. Recent researches show that insect PPO has a relationship with neuron activity, longevity, feces melanization (phytophagous insects) and development, which suggests that it is time for us to look back on insect PPO beyond the view of immunity in this review.

Factors functioning in nodule melanization of insects and their mechanisms of accumulation in nodules

Nodules consisting of hemocytes and trapped microorganisms are important targets for melanization, which is best known in the insect immune system. We investigated factors functioning in nodule melanization and the mechanism by which these factors congregate in the nodule. BmHP21, BmSPH1 and BmSPH2, Bombyx mori orthologs of Manduca sexta serine protease HP21, serine protease homologs (SPH1 and SPH2), and a prophenoloxidase, BmPO1 were observed as inactive forms in the plasma, but as putatively active forms in the nodule. Production of prophenoloxidase-activating proteinases, BmPAP1 and BmPAP3/PPAE and BmPO1 were confirmed in hemocytes. BmSPH1 and BmSPH2 were observed on trapped bacterial cells in the nodule and were isolated from the surface of bacterial cells incubated with plasma. BmSPH1 and BmSPH2 were found in plasma in complex with a pattern recognition receptor, BmLBP. These data suggest that melanization-regulating factors congregate in nodules through a combination of microorganism-dependent and hemocyte-dependent routes.

Peptidoglycan recognition proteins in Drosophila immunity

Innate immunity is the front line of self-defense against infectious non-self in vertebrates and invertebrates. The innate immune system is mediated by germ-line encoding pattern recognition molecules (pathogen sensors) that recognize conserved molecular patterns present in the pathogens but absent in the host. Peptidoglycans (PGN) are essential cell wall components of almost all bacteria, except mycoplasma lacking a cell wall, which provides the host immune system an advantage for detecting invading bacteria. Several families of pattern recognition molecules that detect PGN and PGN-derived compounds have been indentified, and the role of PGRP family members in host defense is relatively well-characterized in Drosophila. This review focuses on the role of PGRP family members in the recognition of invading bacteria and the activation and modulation of immune responses in Drosophila.

Pathological analysis of hemolymphs of Charybdis japonica infected with Vibrio alginolyticus

We explored the pathogenic mechanism of Vibrio alginolyticus in the stone crab Charybdis japonica by studying the hemolymph of C. japonica artificially infected by V. alginolyticus. To this end, Wright-Geimsa staining and electron microscopy were used, and phenoloxidase (PO) activity and the immune protection rate of C. japonica injected with immune polysaccharide during infection were analyzed. The results indicated that the total hemocyte and hyaline hemocyte (HH) counts in diseased crabs were significantly lower than those in healthy crabs (P < 0.05), whereas the large granule hemocytes (LGHs) were significantly higher in diseased crabs than in healthy crabs (P < 0.05). The cellular sizes of HHs and LGHs showed an increasing trend after V. alginolyticus infection, while the nuclear/cytoplasmic ratio (NP) of these cells showed a sharp decline after V. alginolyticus infection (P < 0.05). Micro-pathological analysis of hemocytes revealed fewer hemocytes in the hemolymph of diseased crabs and the presence of disintegrated cells. Ultrastructural and micro-pathological analyses showed damage in all types of hemocytes. The mitochondria were damaged and incomplete in structure, parts of the nuclear membrane were anamorphic and parts of the nuclei had shrunk, hematocyte nuclei exhibited heterochromatinization, hemocyte granules were increased in the polysaccharide-treated group infected with V. alginolyticus, and the numbers of mitochondria and rough endoplasmic reticulum were also increased. PO activity in the two Vibrio-infected groups peaked at 6 h and 24 h after infection, respectively, and PO activity increased in the hemolymph of infected crabs but sharply decreased with prolonged infection. Finally, the PO activities in the two Vibrio-infected groups were significantly lower than controls at 120 h post-infection (P < 0.05). Interestingly, PO activity was higher in polysaccharide-treated crabs than non-polysaccharide-challenged infected crabs, resulting in an immunoprotective rate of 69.64% at 7 days post-infection. This phenomenon suggests that polysaccharides could enhance the organism's antibacterial defenses by improving immune-related enzyme activity.

Inhibition of Melanization by a Parasitoid Serine Protease Homolog Venom Protein Requires Both the Clip and the Non-Catalytic Protease-Like Domains

Endoparasitoid wasps inject a variety of components into their host hemocoel at oviposition to facilitate successful development of their progeny. Among these are venom proteins which have been shown to play crucial roles in host regulation. A serine protease homolog (SPH)-like venom protein from Cotesia rubecula was previously shown to inhibit melanization in the host hemolymph by blocking activation of prophenoloxidase to phenoloxidase, a key enzyme in melanin formation. Similar to other SPHs, Vn50 consists of a clip and a protease-like (SPL) domain. Protein modeling demonstrated that Vn50 has a very similar structure to known SPHs and functional analysis of Vn50 domains expressed in insect cells indicated that neither of the domains on its own has an inhibitory effect on melanization.

The role of lysozyme in the prophenoloxidase activation system of Manduca sexta: an in vitro approach

Activation of the prophenoloxidase (proPO) system and synthesis of antimicrobial peptides (including lysozyme) are two key defense mechanisms in arthropods. Activation of proPO involves a cascade of serine proteinases that eventually converts proPO to active phenoloxidase (PO). However, a trade-off between lysozyme/antibacterial activity and PO activity has been observed in some insects, and a mosquito lysozyme can inhibit melanization. It is not clear whether lysozyme can inhibit PO activity and/or proPO activation. In this study, we used in vitro assays to investigate the role of lysozyme in proPO activation in the tobacco hornworm Manduca sexta. We showed that lysozymes from M. sexta, human milk and hen egg white did not inhibit PO activity in the pre-activated naïve plasma of M. sexta larvae, but significantly inhibited proPO activation in the naïve plasma. Western blot analysis showed that direct incubation of M. sexta lysozyme with the naïve plasma prevented conversion of proPO to PO, but stimulated degradation of precursor proteins for serine proteinase homolog-2 (SPH2) and proPO-activating proteinase-1 (PAP1), two key components required for proPO activation. Far-western blot analysis showed that M. sexta lysozyme and proPO interacted with each other. Altogether, our results suggest that lysozymes may inhibit the proPO activation system by preventing conversion of proPO to PO via direct protein interaction with proPO.

Purification and characterization of a small cationic protein from the tobacco hornworm Manduca sexta

The prophenoloxidase (proPO) activation system is an important defense mechanism in arthropods, and activation of proPO to active phenoloxidase (PO) involves a serine proteinase cascade. Here, we report the purification and characterization of a small cationic protein CP8 from the tobacco hornworm, Manduca sexta, which can stimulate proPO activation. BLAST search showed that Manduca CP8 is similar to a fungal proteinase inhibitor-1 (AmFPI-1), an inducible serine proteinase inhibitor-1 (ISPI-1), and other small cationic proteins with unknown functions. However, we showed that Manduca CP8 did not inhibit proteinase activity, but stimulated proPO activation in plasma. When small amount (0.1 microg) of purified native CP8 or BSA was added to cell-free plasma samples and incubated for 20 min, low PO activity was observed in both groups. But significantly higher PO activity was observed in the CP8-group than in the BSA-group when more proteins (0.5 microg) were added and incubated for 20 min. However, when the plasma samples were incubated with proteins for 30 min, high PO activity was observed in both the CP8 and BSA groups regardless of the amount of proteins added. Moreover, when PO in the plasma was pre-activated with Micrococcus luteus, addition of CP8 did not have an effect on PO activity, and CP8/bacteria mixture did not stimulate PO activity to a higher level than did BSA/bacteria. These results suggest that CP8 helps activate proPO more rapidly at the initial stage. CP8 mRNA was specifically expressed in fat body and its mRNA level decreased when larvae were injected with saline or bacteria. However, CP8 protein concentration in hemolymph did not change significantly in larvae injected with saline or microorganisms.

Identification and molecular characterization of a prophenoloxidase involved in Aedes aegypti chorion melanization

Chorion melanization is a vital biochemical event for the survival of mosquito eggs in the environment. This study describes the identification and molecular characterization of a prophenoloxidase (proPO) involved in chorion melanization in Aedes aegypti by various biochemical and molecular techniques. Results revealed that transcription of the chorion proPO occurs only in adults, blood feeding greatly stimulated its transcription and haemocytes are responsible for its transcription. Our study provides a solid basis for suggesting an essential role of the isolated proPO in chorion melanization during chorion hardening and also raises fundamental questions regarding its transportation and distribution in the chorion. This study should serve as a useful reference towards functional elucidation of other proPOs in mosquitoes.