A masquerade-like serine proteinase homologue is necessary for phenoloxidase activity in the coleopteran insect, Holotrichia diomphalia larvae

Prophenoloxidase system and its role in shrimp immune responses against major pathogens

The global shrimp industry still faces various serious disease-related problems that are mainly caused by pathogenic bacteria and viruses. Understanding the host defense mechanisms is likely to be beneficial in designing and implementing effective strategies to solve the current and future pathogen-related problems. Melanization, which is performed by phenoloxidase (PO) and controlled by the prophenoloxidase (proPO) activation cascade, plays an important role in the invertebrate immune system in allowing a rapid response to pathogen infection. The activation of the proPO system, by the specific recognition of microorganisms by pattern-recognition proteins (PRPs), triggers a serine proteinase cascade, eventually leading to the cleavage of the inactive proPO to the active PO that functions to produce the melanin and toxic reactive intermediates against invading pathogens. This review highlights the recent discoveries of the critical roles of the proPO system in the shrimp immune responses against major pathogens, and emphasizes the functional characterizations of four major groups of genes and proteins in the proPO cascade in penaeid shrimp, that is the PRPs, serine proteinases, proPO and inhibitors.

Cloning of a trypsin-like serine protease and expression patterns during Plasmodium falciparum invasion in the mosquito, Anopheles dirus (Peyton and Harrison)

Understanding specific gene regulation during responses to malaria infection is key to dissecting the mosquito defense mechanisms and host/parasite interactions. A full-length serine protease cDNA was isolated from the fat body of female Anopheles dirus, a major malaria vector in Thailand. The predicted amino acid sequence of SERF4 identifies it as a member of the serine protease family containing a single trypsin-like protease domain. Digestive trypsins function in the female mosquito midgut and are inducible in two phases in this tissue upon blood intake. However, the gene was highly upregulated in the midgut at day 3 postinfection by Plasmodium falciparum. In situ hybridization confirmed that SERF4 transcripts were located in the midgut epithelial cells rather than hemocytes or other tissues associated with the midgut. SERF4 was also strongly downregulated in the whole insects at day 16 after infection in comparison with the blood-fed control. Changes in the expression of the SERF4 gene in response to infection with this human malaria parasite suggest a role in parasite-specific innate immunity.

Cloning and characterization of trypsin- and chymotrypsin-like genes in the striped rice stem borer, Chilo suppressalis

Serine proteinases including trypsins and chymotrypsins play various important roles in insects, including food digestion, immune defense, and zymogen activation. Studies on insect serine proteinases could reveal their feeding preference (polyphagous and monophagous) and facilitate identification of protease inhibitors, which can be engineered for pest management. In this paper, 11 transcripts encoding trypsin- and chymotrypsin-like proteins were cloned from the striped rice stem borer, Chilo suppressalis (Walker). All the predicted proteins share high sequence similarity with known trypsin- and chymotrypsin-like proteins from either lepidopterans or dipterans, and most of the proteins have conserved motifs that are characteristics of serine proteinases. Among the 11 cloned genes, six were expressed predominantly and one exclusively in the midgut of the insect, three were expressed relatively evenly in examined tissues, and one was not expressed in either the gut or hemolymph based on RT–PCR results. The seven genes that were predominantly or exclusively expressed in the gut were also affected by feeding on different host plants. The genes that were expressed in the gut and were affected by host plants are likely to encode digestive proteinases. The identification of trypsin- and chymotrypsin-like genes in this insect species is the first step towards further comparative studies and for identification of insect-specific proteinase inhibitors, which might be engineered to protect rice plants against the striped rice stem borer, which is one of the destructive pests of rice.

Temporally variable selection on proteolysis-related reproductive tract proteins in Drosophila

In order to gain further insight into the processes underlying rapid reproductive protein evolution, we have conducted a population genetic survey of 44 reproductive tract-expressed proteases, protease inhibitors, and targets of proteolysis in Drosophila melanogaster and Drosophila simulans. Our findings suggest that positive selection on this group of genes is temporally heterogeneous, with different patterns of selection inferred using tests sensitive at different time scales. Such variation in the strength and targets of selection through time may be expected under models of sexual conflict and/or host-pathogen interaction. Moreover, available functional information concerning the genes that show evidence of selection suggests that both sexual selection and immune processes have been important in the evolutionary history of this group of molecules.

Toxicity and differential protein analysis following destruxin A treatment of Spodoptera litura (Lepidoptera: Noctuidae) SL-1 cells

The cytotoxicity of a destruxin A (DA) treatment of Spodoptera litura SL-1 cells was investigated. An MTT assay showed that DA was highly toxic to SL-1 cells in a concentration- and time-dependent manner. The IC(50) values of DA, after 24 h and 48 h of treatment, were 17.86 μg/mL and 7.80 μg/mL, respectively. Under inverted phase contrast microscopy (IPCM), it was found that prolonged treatment with DA could induce cell rounding, cellular membrane shrinking, formation of apoptotic bodies, vacuole appearance and cytoplasm leak out. Apoptosis induced by DA was further confirmed by fluorescence microscopy (FM) and flow cytometry (FCM) studies. SL-1 cells entered early apoptosis following a treatment with 2.5 μg/mL DA and entered late apoptosis following a treatment with increasing concentrations of DA. Furthermore, two-dimensional gel electrophoresis (2-DE) analysis was used to identify 22 proteins which were differentially expressed (≥2-fold difference) between control cells and DA-treated cells, and the expression level of these proteins was significantly different between the treated and untreated cells. Our results suggest that these differentially expressed proteins may help explain the diverse biological effects caused by the destruxin A treatment of cells; additionally, some of the identified proteins may have roles in SL-1 cellular proliferation and apoptosis.

Localization of the serine protease homolog BmSPH-1 in nodules of E. coli-injected Bombyx mori larvae and functional analysis of its role in nodule melanization

The molecular mechanisms underlying nodule formation and melanization, an important pathogen defense mechanism in insects, are poorly understood. In this study, we investigated the role of BmSPH-1, a catalytically inactive Bombyx mori serine protease homolog, in nodule melanization induced by injection of Escherichia coli cells into the B. mori larval hemocoel. Addition of the melanization substrate L-3,4-dihydroxyphenylalanine (DOPA) to newly formed nodules prompted nodule melanization, confirming that nodules contain activated prophenoloxidase needed for melanization. Immunoprecipitation and immunoblot studies demonstrated that BmSPH-1 interacts with BmLBP, a C-type lectin that binds Gram-negative bacteria, and that BmSPH-1 is present in a truncated, putatively activated form at the E. coli cell surface in nodules. Pretreatment of larvae with anti-BmSPH-1 serum inhibited nodule melanization in E. coli-injected larvae. These results suggest that BmSPH-1 regulates nodule melanization and is recruited into nodules from the hemolymph by BmLBP.

A serine proteinase homologue, SPH-3, plays a central role in insect immunity

Numerous vertebrate and invertebrate genes encode serine proteinase homologues (SPHs) similar to members of the serine proteinase family, but lacking one or more residues of the catalytic triad. These SPH proteins are thought to play a role in immunity, but their precise functions are poorly understood. In this study, we show that SPH-3 (an insect non-clip domain-containing SPH) is of central importance in the immune response of a model lepidopteran, Manduca sexta. We examine M. sexta infection with a virulent, insect-specific, Gram-negative bacterium Photorhabdus luminescens. RNA interference suppression of bacteria-induced SPH-3 synthesis severely compromises the insect's ability to defend itself against infection by preventing the transcription of multiple antimicrobial effector genes, but, surprisingly, not the transcription of immune recognition genes. Upregulation of the gene encoding prophenoloxidase and the activity of the phenoloxidase enzyme are among the antimicrobial responses that are severely attenuated on SPH-3 knockdown. These findings suggest the existence of two largely independent signaling pathways controlling immune recognition by the fat body, one governing effector gene transcription, and the other regulating genes encoding pattern recognition proteins.

Identification of three different types of serine proteases (one SP and two SPHs) in Chinese white shrimp

Serine proteases (SPs) and serine protease homologs (SPHs) participate in digestion, embryonic development, blood coagulation, and immune defense responses. In this paper, we identify one SP and two SPHs, including a masquerade SPH (FcMas), a CUB domain containing SP (FcCUBSP), and a single domain containing SPH (FcSPH2) in Chinese white shrimp, Fenneropenaeus chinensis. FcMas has a Gly-rich region formed by three repeats of LGGQGGG, a clip domain and a C-terminal SP-like domain. Absence of Ser catalytic residue results in the loss of serine protease activity of FcMas, which then functions as an SPH. FcCUBSP has a signal peptide, followed by a CUB domain and an SP domain. FcSPH2 has a signal peptide and an SP-like domain. Loss of one catalytic residue (H) makes FcSPH2 catalytically inactive, which is considered an SPH. Phylogenetic analysis shows that FcMas and other SPHs from shrimp or insect are classified into one group. FcSPH2 is grouped in the chymotrypsin family. RT-PCR results show that FcMas mRNA is mainly distributed in hemocytes and gills. FcCUBSP is only detected in gills, whereas FcSPH2 is found in hepatopancreas only. QRT-PCR is used to analyze changes of FcMas, FcCUBSP and FcSPH2 in some tissues challenged with white spot syndrome virus (WSSV) or Vibrio. FcMas in hemocytes is down-regulated by WSSV or Vibrio challenge, and down-regulated by WSSV in gills. However, it is up-regulated upon Vibrio challenge in gills. FcCUBSP in gills and FcSPH2 in hepatopancreas are up-regulated upon WSSV or Vibrio challenge. Results suggest the roles of FcMas, FcCUBSP and FcSPH2 in shrimp's innate immunity.

PmPPAE2, a new class of crustacean prophenoloxidase (proPO)-activating enzyme and its role in PO activation

The prophenoloxidase (proPO) activating system plays an important role in the defense against microbial invasion in invertebrates. In the present study, we report a second proPO-activating enzyme (designated PmPPAE2) from the hemocytes of the black tiger shrimp, Penaeus monodon. PmPPAE2 contained the structural features of the clip domain serine proteinase family and exhibited 51% amino acid sequence similarity to the insect Manduca sexta PAP-1. Amino acid sequence alignment with the available arthropod PPAE sequences demonstrated that PmPPAE2 is a new class of crustacean PPAE. Transcript expression analysis revealed that PmPPAE2 transcripts were mainly expressed in hemocytes. Double-stranded RNA-mediated suppression of PmPPAE2 transcript levels resulted in a significant decrease in the total hemolymph PO activity (41%) and also increased the shrimp's susceptibility to Vibrio harveyi infection. Genomic organization analysis revealed that PmPPAE1 and PmPPAE2 are encoded by different genomic loci. The PmPPAE1 gene consists of ten exons and nine introns, whilst PmPPAE2 comprises of eight exons interrupted by seven introns. Analysis of the larval developmental stage expression of the four key genes in the shrimp proPO system (PmPPAE1, PmPPAE2, PmproPO1 and PmproPO2) revealed that PmPPAE1 and PmproPO2 transcripts were expressed in all larval stages (nauplius, protozoea, mysis and post-larvae), whilst PmPPAE2 and PmproPO1 transcripts were mainly presented in the late larval developmental stages (mysis and post-larvae). These results suggest that the PmPPAE2 functions as a shrimp PPAE and possibly mediates the activation of PmproPO1.

Isolation, gene cloning and expression profile of a pathogen recognition protein: a serine proteinase homolog (Sp-SPH) involved in the antibacterial response in the crab Scylla paramamosain

To identify the frontline defense molecules against microbial infection in the crab Scylla paramamosain, a live crab pathogenic microbe, Vibrio parahaemolyticus, was recruited as an affinity matrix to isolate innate immune factors from crab hemocytes lysate. Interestingly, a serine proteinase homolog (Sp-SPH) was obtained together with an antimicrobial peptide-antilipopolysaccharide factor (Sp-ALF). We then determined the full-length cDNA sequence of Sp-SPH, which contained 1298bp with an open reading frame of 1107bp encoding 369 amino acid residues. Multiple alignment analysis showed that the deduced amino acid sequences of Sp-SPH shared overall identity (83.8%) with those of SPH-containing proteins from other crab species. Tissue distribution analysis indicated that the Sp-SPH transcripts were present in various tissues including eye stalk, subcuticular epidermis, gill, hemocyte, stomach, thorax ganglion, brain and muscle of S. paramamosain. The Sp-SPH was highly expressed in selected different development stages including embryo (I, II, III and V), zoea (I), megalopa, and juvenile. Importantly, the prophenoloxidase was also present in the embryos, zoea, juvenile and adult crabs, but relatively lower in megalopa compared to those of other stages. Furthermore, the Sp-SPH mRNA expression showed a statistically significant increase (P<0.05) in both hemocyte and subcuticular epidermis at 24h, and in gill at 96h after challenge of V. parahaemolyticus determined by quantitative real-time PCR. Taken together, the live-bacterial-binding activity and the acute-phase response against bacterial infection of Sp-SPH suggested that it might function as an innate immune recognition molecule and play a key role in host defense against microbe invasion in the crab S. paramamosain.

A clip domain serine proteinase plays a role in antibacterial defense but is not required for prophenoloxidase activation in shrimp

The clip domain serine proteinases (clip-SPs) play critical roles in the signaling processes during embryonic development and in the innate immunity of invertebrates. In the present study, we identified a homolog of the clip-SP, designated as PmClipSP1, by searching the Penaeus monodon EST database (http://pmonodon.biotec.or.th), and using RACE-PCR to obtain the complete gene which contained a 1101bp open reading frame encoding 366 amino acids with a 25 amino acid signal peptide. The deduced PmClipSP1 protein sequence, which shares a predicted structural similarity to the clip-SPs of other arthropod species, appears to possess a clip domain at the N-terminus and an enzymatically active serine proteinase domain at the C-terminus. Tissue distribution analyses revealed that, at the transcript level, PmClipSP1 is mainly expressed in shrimp hemocytes, whilst temporal gene expression analyses showed that the hemocyte PmClipSP1 transcript levels were upregulated at 3h and downregulated at 6-48h following systemic Vibrio harveyi infection. RNAi-mediated silencing of the PmClipSP1 gene, by injection of double-stranded RNA (dsRNA) corresponding to the PmClipSP1 gene into shrimp, significantly reduced PmClipSP1 transcript levels, but neither significantly altered the other clip-SP and clip-SPH transcript levels nor reduced the total phenoloxidase (PO) enzyme activity in shrimp hemocytes, compared to the levels seen in the GFP dsRNA control, suggesting that PmClipSP1 is not involved in the proPO system. However, suppression of the PmClipSP1 gene led to a significant increase in the number of viable bacteria in the hemolymph (approximately 2.4-fold) and in the mortality rate (59%) of shrimp systemically infected with V. harveyi. These findings suggest that PmClipSP1 plays a role in the antibacterial defense mechanism of P. monodon shrimp.

Beetle immunity

Genetic studies have elegantly characterized the innate immune response in Drosophila melanogaster. However, these studies have a limited ability to reveal the biochemical mechanisms underlying the innate immune response. To investigate the biochemical basis of how insects recognize invading microbes and how these recognition signals activate the innate immune response, it is necessary to use insects, from which larger amounts of hemolymph can be extracted. Using the larvae from two species of beetle, Tenebrio molitor and Holotrichia diomphalia, we elucidated the mechanisms underlying pathogenic microbe recognition. In addition, we studied the mechanism of host defense molecule amplification. In particular, we identified several pattern recognition proteins, serine proteases, serpins and antimicrobial peptides and examined how these molecules affect innate immunity.

Characterization of a serine proteinase homologous (SPH) in Chinese mitten crab Eriocheir sinensis

The serine protease homologous (SPH) is an important cofactor of prophenoloxidase-activating enzyme (PPAE). The gene of SPH of Chinese mitten crab Eriocheir sinensis (EsSPH) in hemocytes was cloned and characterized using reverse transcript polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE). The SPH cDNA consisted of 1386 bp with an open reading frame (ORF) encoded a protein of 378 amino acids, 154 bp 5'-untranslated region, and 95 bp 3'-untranslated region. Sequence comparisons against the GenBank database showed that EsSPH deduced amino acids had an overall identity to the gene of serine protease family from 41% to 70% of 15 invertebrate species. The protein had the structural characteristics of SPH, including the conserved six cysteine residues in the N-terminal clip domain and the functional activity (His157, Asp209, Gly311) in the C-terminal serine proteinase-like domain. To analyze the role of EsSPH in an acute infection, the temporal expression of the EsSPH gene after the Aeromonas hydrophila challenge was measured by real-time RT-PCR. The EsSPH transcripts in hemocytes significantly increased at 6 h, 12 h and 48 h over time after the A. hydrophila injection. This expression pattern shows that EsSPH has the potential to defend against invading microorganisms. The mRNA transcripts of EsSPH were detected in all tissues with the highest in the hepatopancreas. Interestingly, the mRNA transcripts of EsSPH and proPO were found in ova and expressed in oosperms, suggesting that the maternal transfer of EsSPH and proPO may exit in crab, but this warrants confirmation in further research.

A clip domain serine protease (cSP) from the Chinese mitten crab Eriocheir sinensis: cDNA characterization and mRNA expression

Clip domain serine protease (cSP), characterized by conserved clip domains, is a new serine protease family identified mainly in arthropod, and plays important roles in development and immunity. In the present study, the full-length cDNA of a cSP (designated EscSP) was cloned from Chinese mitten crab Eriocheir sinensis by expressed sequence tags (ESTs) and PCR techniques. The 1380 bp EscSP cDNA contained a 1152 bp open reading frame (ORF) encoding a putative cSP of 383 amino acids, a 5'-untranslated region (UTR) of 54 bp, and a 3'-UTR of 174 bp. Multiple sequence alignment presented twelve conserved cysteine residues and a canonical catalytic triad (His(185), Asp(235) and Ser(332)) critical for the fundamental structure and function of EscSP. Two types of cSP domains, the clip domain and tryp_spc domain, were identified in the deduced amino acids sequence of EscSP. The conservation characteristics and similarities with previously known cSPs indicated that EscSP was a member of the large cSP family. The mRNA expression of EscSP in different tissues and the temporal expression in haemocytes challenged by Listonella anguillarum were measured by real-time RT-PCR. EscSP mRNA transcripts could be detected in all examined tissues, and were higher expressed in muscle than that in hepatopancreas, gill, gonad, haemocytes and heart. The EscSP mRNA expression in haemocytes was up-regulated after L. anguillarum challenge and peaked at 2 h (4.96 fold, P < 0.05) and 12 h (9.90 fold, P < 0.05). Its expression pattern was similar to prophenoloxidase (EsproPO), one of the components of crab proPO system found in our previous report. These results implied that EscSP was involved in the processes of host-pathogen interaction probably as one of the proPO system members.

Gene silencing of a prophenoloxidase activating enzyme in the shrimp, Penaeus monodon, increases susceptibility to Vibrio harveyi infection

The prophenoloxidase (proPO) activating system is an important innate immune response against microbial infections in invertebrates. The major enzyme, phenoloxidase (PO), is synthesized as an inactive precursor and its activation to an active enzyme is mediated by a cascade of clip domain serine proteinases (clip-SPs). In this study, a cDNA encoding a proPO activating enzyme (PPAE) from the black tiger shrimp, Penaeus monodon, designated as PmPPAE1, was cloned and characterized. The full-length cDNA contains an open reading frame (ORF) of 1392bp encoding a predicted protein of 463 amino acids including an 18 amino acid signal peptide. The PmPPAE1 protein exhibits a characteristic sequence structure of clip-SPs consisting of the clip domain at the N-terminus and a SP domain at the C-terminus. Sequence analysis showed that PmPPAE1 exhibited the highest amino acid sequence similarity (70%) to a PPAE of the crayfish, Pacifastacus leniusculus. PmPPAE1 mRNA is abundantly expressed in hemocytes, and this is regulated after systemic Vibrio harveyi infection supporting that it is an immune-responsive gene. RNA interference-mediated suppression of PmPPAE1, performed by injection of double-stranded RNA (dsRNA) corresponding to the PmPPAE1 gene into shrimp, resulted in a significant reduction of PmPPAE1 but not other clip-SP and related gene transcript levels of P. monodon, suggesting gene-specific knockdown. RNAi-mediated silencing of PmPPAE1 gene significantly decreased the total PO activity (36.7%) in shrimp and additionally increased the mortality of V. harveyi infected shrimp, the latter of which correlated with an increase in the number of viable bacteria in the hemolymph. These results indicate that PmPPAE1 functions in the proPO system and is an important component in the shrimp immune system.

Clip domain serine protease and its homolog respond to Vibrio challenge in Chinese white shrimp, Fenneropenaeus chinensis

Clip domain serine proteases and their homologs are involved in invertebrate innate immunity, including hemolymph coagulation, antimicrobial peptide synthesis, cell adhesion, and melanization. Recognition of pathogens by pattern recognition receptors can trigger activation of a serine protease cascade. We report here the cDNA cloning of a serine protease (FcSP) and a serine protease homolog (FcSPH) from Chinese white shrimp, Fenneropenaeus chinensis. Both FcSP and FcSPH possess a clip domain at the N-terminal and an SP or SP-like domain at the C-terminal. In contrast to FcSP, FcSPH lacks a catalytic residue and is catalytically inactive. Tissue distribution and time course qRT-PCR analysis indicates that FcSP and FcSPH can respond to Vibrio anguillarum challenge in hemocytes, hepatopancreas and intestine. In situ hybridization analysis shows that FcSP is distributed in hemocytes and gills, and originated mainly from the hemocytes. FcSPH protein is expressed in gills and stomach of non-challenged shrimp. Its expression in gill mainly originates from the hemocytes in it. Two immunoreactive bands of FcSP can be detected in gills and stomach of non-challenged shrimp. FcSP protein is partially cleaved in non-challenged shrimp, while FcSPH protein is unprocessed in unchallenged shrimp and is partially cleaved after V. anguillarum challenge. Our results suggest that this Clip domain serine protease and its homolog may be involved in the serine protease cascade and play an important role in innate immunity of the shrimp.

Molecular cloning, characterization and expression analysis of a clip-domain serine protease from pearl oyster Pinctada fucata

The clip-domain serine proteases (SPs) are the essential components of extracellular signaling cascade in various biological processes, especially in embryonic development and the innate immune responses of invertebrate. Herein, we described the isolation and characterization of pearl oyster Pinctada fucata clip-domain SP gene (designated as poSP). The poSP cDNA was 1080 bp long and consisted of a 5'-untranslated region (UTR) of 13 bp, a 3'-UTR of 68 bp with a polyadenylation signal (AATAAA) at 22 nucleotides upstream of the poly(A) tail, and an open reading frame (ORF) of 999 bp encoding a polypeptide of 332 amino acids with an estimated molecular mass of 36.5 kDa and a theoretical isoelectric point of 7.3. A clip-domain and a trypsin-like serine protease domain were identified in the poSP using SMART analysis. Homology analysis of the deduced amino acid sequence of the poSP with other known SP sequences by MatGAT software revealed that the poSP shared 47.0-68.4% similarity to the other known SP sequences. The poSP mRNA was expressed in haemocytes, gonad, digestive gland and mantle, but not expressed in adductor muscle and gill. The poSP mRNA was up-regulated and increased nearly double-fold after LPS or Vibrio alginolyticus stimulation, respectively. These results suggested that the poSP was an inducible acute-phase protein that perhaps involved in the innate immune response of pearl oyster.

Purification and characterization of silkworm hemocytes by flow cytometry

Hemocyte functions are well-investigated in the silkworm, Bombyx mori, however, detailed analysis of each hemocyte subset has been hampered by the lack of appropriate separation method. Here we use an array of flow cytometric analyses to characterize silkworm hemocytes with various molecular probes, such as propidium iodide, green fluorescence protein, monoclonal antibodies, and fluorescent lectins. Of these, separation using propidium iodide was the simplest and provided most reliable results for the isolation of the hemocyte subsets. cDNAs were then synthesized from these sorted populations and subset-specific gene expression was examined by RT-PCR. Granulocytes, plasmatocytes, and oenocytoids expressed different classes of immune genes, suggesting that they have multiple roles in silkworm immunity. In contrast, a contribution of spherulocytes to immunity was not documented in that they failed to express most of the genes. The functions of spherulocytes are thus likely to be distinct from those of the other three hemocyte subsets.

Expression of four trypsin-like serine proteases from the Chinese shrimp, Fenneropenaeus chinensis, as regulated by pathogenic infection

Four trypsin-like serine proteases, designated as Fctry1, Fctry2, Fctry3, and Fctry4, were obtained from the hepatopancreas of the Chinese shrimp, Fenneropenaeus chinensis. Three trypsin-like serine proteases, specifically Fctry1, 2, and 3 were observed to have the conserved catalytic triad (H, D, and S). Regarding Fctry4, the catalytic triad S was substituted by F, and the mature peptide was found to be inactive. Further analysis for similarities indicated that Fctry1, 2 and 4 were 89-92% identical to trypsins from Pacific white shrimp (Litopenaeus vannamei); while Fctry3 was only 42% identical to trypsins from salmon louse (Lepeophtheirus salmonis). The genomic organizations of Fctry1, 2 and 4 are also quite different from Fctry3. So, Fctry3 may be a new member of the trypsin-like serine protease family. Moreover, a semi-quantitative reverse transcription polymerase chain reaction (PCR) and quantitative real-time PCR was carried out to analyze the distribution profiles and expression patterns after they were subjected to a bacterial and viral challenge. The results showed that the four trypsin-like serine proteases were upregulated in the hepatopancreas of shrimp infected with the white spot syndrome virus (WSSV), and Fctry3 increased after a bacteria challenge. Therefore, these trypsin-like serine proteases might be involved in the innate defense reactions against different pathogens in Chinese shrimp.

Endonuclease activity of phenol oxidase from Musca domestica larvae

Phenol oxidase (PO), a copper-containing enzyme with oxygenase activity, can convert mono- or diphenol into quinone and plays an important role in the arthropod melanization reaction. Here, we report a new property of PO from Musca domestica larvae: a thermotolerant endonuclease activity, by which PO can degrade plasmid DNA even after being heated to 80 degrees C for 20 min. We cloned PO cDNA, constructed the expression vector pVAX1-PO, and expressed it in HeLa cells. The expression product showed the same properties as purified PO. Our data indicate that PO is a bifunctional enzyme, exhibiting both oxygenase and endonuclease activity, suggesting new roles for this important molecule in the innate responses of M. domestica.

A positive feedback mechanism in the Manduca sexta prophenoloxidase activation system

In Manduca sexta, pathogen recognition triggers a branched serine proteinase cascade which generates active phenoloxidase (PO) in the presence of a proPO-activating proteinase (PAP) and two noncatalytic serine proteinase homologs (SPHs). PO then catalyzes the production of reactive compounds for microbe killing, wound healing, and melanin formation. In this study, we discovered that a minute amount of PAP1 (a final component of the proteinase pathway) caused a remarkable increase in PO activity in plasma from naïve larvae, which was significantly higher than that from the same amounts of PAP1, proPO and SPHs incubated in vitro. The enhanced proPO activation concurred with the proteolytic activation of HP6, HP8, PAP1, SPH1, SPH2 and PO precursors. PAP1 cleaved proSPH2 to yield bands with mobility identical to SPH2 generated in vivo. PAP1 partially hydrolyzed proHP6 and proHP8 at a bond amino-terminal to the one cut in the PAP1-added plasma. PAP1 did not directly activate proPAP1. These results suggest that a self-reinforcing mechanism is built into the proPO activation system and other plasma proteins are required for cleaving proHP6 and proHP8 at the correct site to strengthen the defense response, perhaps in the early stage of the pathway activation.

Molecular control of phenoloxidase-induced melanin synthesis in an insect

The melanization reaction induced by activated phenoloxidase in arthropods must be tightly controlled because of excessive formation of quinones and excessive systemic melanization damage to the hosts. However, the molecular mechanism by which phenoloxidase-induced melanin synthesis is regulated in vivo is largely unknown. It is known that the Spätzle-processing enzyme is a key enzyme in the production of cleaved Spätzle from pro-Spätzle in the Drosophila Toll pathway. Here, we provide biochemical evidence that the Tenebrio molitor Spätzle-processing enzyme converts both the 79-kDa Tenebrio prophenoloxidase and Tenebrio clip-domain SPH1 zymogen to an active melanization complex. This complex, consisting of the 76-kDa Tenebrio phenoloxidase and an active form of Tenebrio clip-domain SPH1, efficiently produces melanin on the surface of bacteria, and this activity has a strong bactericidal effect. Interestingly, we found the phenoloxidase-induced melanization reaction to be tightly regulated by Tenebrio prophenoloxidase, which functions as a competitive inhibitor of melanization complex formation. These results demonstrate that the Tenebrio Toll pathway and the melanization reaction share a common serine protease for the regulation of these two major innate immune responses.

The viral protein Egf1.0 is a dual activity inhibitor of prophenoloxidase-activating proteinases 1 and 3 from Manduca sexta

Some pathogens are capable of suppressing the melanization response of host insects, but the virulence factors responsible are largely unknown. The insect pathogen Microplitis demolitor bracovirus encodes the Egf family of small serine proteinase inhibitors. One family member, Egf1.0, was recently shown to suppress melanization of hemolymph in Manduca sexta in part by inhibiting the enzymatic activity of prophenoloxidase activating proteinase 3 (PAP3). However, other experiments suggested this viral protein suppresses melanization by more than one mechanism. Here we report that Egf1.0 inhibited the amidolytic activity of PAP1 and dose-dependently blocked processing of pro-PAP1 and pro-PAP3. Consistent with its PAP inhibitory activity, Egf1.0 also prevented processing of pro-phenoloxidase, serine proteinase homolog (SPH) 1, and SPH2. Isolation of Egf1.0-protein complexes from plasma indicated that Egf1.0 binds PAPs through its C-terminal repeat domain. Egf1.0 also potentially interacts with SPH2 and two other proteins, ferritin and gloverin, not previously associated with the phenoloxidase cascade. Overall, our results indicate that Egf1.0 is a dual activity PAP inhibitor that strongly suppresses the insect melanization response.

Comparative genomic analysis of the Tribolium immune system

The annotation, and comparison with homologous genes in other species, of immunity-related genes in the Tribolium castaneum genome allowed the identification of around 300 candidate defense proteins, and revealed a framework of information on Tribolium immunity.

Background

Tribolium castaneum is a species of Coleoptera, the largest and most diverse order of all eukaryotes. Components of the innate immune system are hardly known in this insect, which is in a key phylogenetic position to inform us about genetic innovations accompanying the evolution of holometabolous insects. We have annotated immunity-related genes and compared them with homologous molecules from other species.

Results

Around 300 candidate defense proteins are identified based on sequence similarity to homologs known to participate in immune responses. In most cases, paralog counts are lower than those of Drosophila melanogaster or Anopheles gambiae but are substantially higher than those of Apis mellifera. The genome contains probable orthologs for nearly all members of the Toll, IMD, and JAK/STAT pathways. While total numbers of the clip-domain serine proteinases are approximately equal in the fly (29), mosquito (32) and beetle (30), lineage-specific expansion of the family is discovered in all three species. Sixteen of the thirty-one serpin genes form a large cluster in a 50 kb region that resulted from extensive gene duplications. Among the nine Toll-like proteins, four are orthologous to Drosophila Toll. The presence of scavenger receptors and other related proteins indicates a role of cellular responses in the entire system. The structures of some antimicrobial peptides drastically differ from those in other orders of insects.

Conclusion

A framework of information on Tribolium immunity is established, which may serve as a stepping stone for future genetic analyses of defense responses in a nondrosophiline genetic model insect.

Expression of Manduca sexta serine proteinase homolog precursors in insect cells and their proteolytic activation

Phenoloxidase (PO)-catalyzed reactions are crucial to the survival of insects after a pathogen or parasite infection. In Manduca sexta, active PO is generated from its precursor by a prophenoloxidase activating proteinase (PAP) in the presence of non-catalytic serine proteinase homologs (SPHs). The PAP and SPHs, located at the ends of a branched proteinase cascade, also require limited proteolysis to become functional. While the processing enzyme of M. sexta proPAP-2 and proPAP-3 is known, we are now investigating the proteolytic activation of proSPH-1 and proSPH-2. Here, we report the development of a series of Bac-to-Bac plasmid vectors for co-expression, secretion, and affinity purification of proSPH-1 and proSPH-2 from insect cells infected by one baculovirus. The purified proteins were characterized and used as substrates in a search for their activating enzymes in plasma of the larvae injected with microorganisms. Proteolytic processing occurred after the proSPHs had been incubated with hydroxyapatite or gel filtration column fractions. The cleaved proteins were active as a cofactor for proPO activation by PAP, and coexistence of SPH-1 and SPH-2 is essential for manifesting the auxiliary effect.

Reconstitution of a branch of the Manduca sexta prophenoloxidase activation cascade in vitro: snake-like hemolymph proteinase 21 (HP21) cleaved by HP14 activates prophenoloxidase-activating proteinase-2 precursor

Upon wounding or infection, a serine proteinase cascade in insect hemolymph leads to prophenoloxidase (proPO) activation and melanization, a defense response against invading microbes. In the tobacco hornworm Manduca sexta, this response is initiated via hemolymph proteinase 14 (HP14), a mosaic protein that interacts with bacterial peptidoglycan or fungal beta-1,3-glucan to autoactivate. In this paper, we report the expression, purification, and functional analysis of M. sexta HP21 precursor, an HP14 substrate similar to Drosophila snake. The recombinant proHP21 is a 51.1 kDa glycoprotein with an amino-terminal clip domain, a linker region, and a carboxyl-terminal serine proteinase domain. HP14, generated by incubating proHP14 with beta-1,3-glucan and beta-1,3-glucan recognition protein-2, activated proHP21 by limited proteolysis between Leu(152) and Ile(153). Active HP21 formed an SDS-stable complex with M. sexta serpin-4, a physiological regulator of the proPO activation system. We determined the P1 site of serpin-4 to be Arg(355) and, thus, confirmed our prediction that HP21 has trypsin-like specificity. After active HP21 was added to the plasma, there was a major increase in PO activity. HP21 cleaved proPO activating proteinase-2 precursor (proPAP-2) after Lys(153) and generated an amidase activity, which activated proPO in the presence of serine proteinase homolog-1 and 2. In summary, we have discovered and reconstituted a branch of the proPO activation cascade in vitro: beta-1,3-glucan recognition--proHP14 autoactivation--proHP21 cleavage--PAP-2 generation--proPO activation--melanin formation.

Broad-spectrum antimicrobial activity of the reactive compounds generated in vitro by Manduca sexta phenoloxidase

Although quinone production and melanin formation are widely recognized as an integral part of the insect defense system, experimental evidence is lacking that the proteolytic activation of prophenoloxidase participates in the direct killing of invading microbes-active phenoloxidase generates quinones that polymerize to form melanin. Here, we report the antimicrobial effect of reactive intermediates produced in phenoloxidase-catalyzed reactions. After being treated with Manduca sexta phenoloxidase and dopamine, Escherichia coli and Bacillus subtilis ceased to grow, whereas the growth of Pichia pastoris was slightly affected. Microscopic analysis showed melanin deposition on cell surface, aggregation of bacteria, and loss of cell mobility. Viability tests revealed major decreases in the bacterial colony counts and, since the decrease remained significant after dispersion of the cell clumps, the reactive compounds were surmised to have aggregated and killed E. coli and B. subtilis cells. Under the experimental conditions, 60-94% of the Gram-negative bacteria (E. coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Salmonella typhimurium) and 52-99% of the Gram-positive bacteria (Bacillus cereus, B. subtilis, Micrococcus luteus, and Staphylococcus aureus) were killed. In the presence of phenoloxidase, dopamine or 5,6-dihydroxyindole (DHI) exhibited much higher antibacterial activity than L-dopa, N-acetyldopamine (NADA) or N-beta-alanyldopamine (NBAD) did, suggesting that DHI and its oxidation products were cytotoxic. The antifungal activity of DHI was detected using P. pastoris, Saccharomyces cerevisiae, Candida albicans, and Beauveria bassiana. These results established that prophenoloxidase activation is an integral component of the insect defense system involving a multitude of enzymes (e.g. proteinases, oxidases, and dopachrome conversion enzyme (DCE)), which immobilizes and kills invading microorganisms.

The melanization reaction is not required for survival of Anopheles gambiae mosquitoes after bacterial infections

The melanization reaction of insects requires activation of pro-phenoloxidase by a proteolytic cascade leading to melanin production. Studies in adult mosquitoes have shown that bacteria are efficiently melanized in the hemocoel, but the contribution of melanization to survival after bacterial infections has not been established. Here we show that the Anopheles gambiae noncatalytic serine protease CLIPA8, an essential factor for Plasmodium ookinete melanization, is also required for melanization of bacteria in adult mosquitoes. CLIPA8 silencing by RNA interference inhibits pro-phenoloxidase activation and melanization of bacteria in the hemolymph following microbial challenge. However, CLIPA8 is not required for wound melanization nor for melanotic pseudotumor formation in serpin2 knockdown mosquitoes, suggesting a specific role for pathogen melanization. Surprisingly, CLIPA8 knockdown mosquitoes are as resistant to bacterial challenge as controls, indicating that melanization is not essential for defense against bacteria and questions its precise role in mosquito immunity.

Manduca sexta hemolymph proteinase 21 activates prophenoloxidase-activating proteinase 3 in an insect innate immune response proteinase cascade

Melanization, an insect immune response, requires a set of hemolymph proteins including pathogen recognition proteins that initiate the response, a cascade of mostly unknown serine proteinases, and phenoloxidase. Until now, only initial and final proteinases in the pathways have been conclusively identified. Four such proteinases have been purified from the larval hemolymph of Manduca sexta: hemolymph proteinase 14 (HP14), which autoactivates in the presence of microbial surface components, and three prophenoloxidase-activating proteinases (PAP1-3). In this study, we have used two complementary approaches to identify a serine proteinase that activates proPAP3. Partial purification from hemolymph of an activator of proPAP3 resulted in an active fraction with two abundant polypeptides of approximately 32 and approximately 37 kDa. Labeling of these polypeptides with a serine proteinase inhibitor, diisopropyl fluorophosphate, indicated that they were active serine proteinases. N-terminal sequencing revealed that both were cleaved forms of the previously identified hemolymph serine proteinase, HP21. Surprisingly, cleavage of proHP21 had occurred not at the predicted activation site but more N-terminal to it. In vitro reactions carried out with purified HP14 (which activates proHP21), proHP21, proPAP3, and site-directed mutant forms of the latter two proteinases confirmed that HP21 activates proPAP3 by limited proteolysis. Like the HP21 products purified from hemolymph, HP21 that was activated by HP14 in the in vitro reactions was not cleaved at its predicted activation site.

Crystal Structure of the Serine Protease Domain of Prophenoloxidase Activating Factor-I

A family of serine proteases (SPs) mediates the proteolytic cascades of embryonic development and immune response in invertebrates. These proteases, called easter-type SPs, consist of clip and chymotrypsin-like SP domains. The SP domain of easter-type proteases differs from those of typical SPs in its primary structure. Herein, we report the first crystal structure of the SP domain of easter-type proteases, presented as that of prophenoloxidase activating factor (PPAF)-I in zymogen form. This structure reveals several important structural features including a bound calcium ion, an additional loop with a unique disulfide linkage, a canyon-like deep active site, and an exposed activation loop. We subsequently show the role of the bound calcium and the proteolytic susceptibility of the activation loop, which occurs in a clip domain-independent manner. Based on biochemical study in the presence of heparin, we suggest that PPAF-III, highly homologous to PPAF-I, contains a surface patch that is responsible for enhancing the catalytic activity through interaction with a nonsubstrate region of a target protein. These results provide insights into an activation mechanism of easter-type proteases in proteolytic cascades, in comparison with the well studied blood coagulation enzymes in mammals.

Analysis of the immune-inducible genes of Plutella xylostella using expressed sequence tags and cDNA microarray

In the present study, the complex gene expression responses of Plutella xylostella to microbial challenges and injury were surveyed using a newly constructed expressed sequence tag (EST) clone collection and cDNA microarray analysis. A total of 1132 P. xylostella ESTs were cloned, annotated and categorized by their putative functions; these included proteases, protease inhibitors, recognition molecules and anti-microbial peptides. GeneOntology revealed that 4% of the P. xylostella ESTs corresponded to immunity-related genes potentially involved in innate immunity. We then used microarray analysis to identify 44 genes that were differentially expressed with at least a two-fold expression difference in P. xylostella before and after pathogen challenge. Together, our EST categorization and microarray profiling analyses allowed us to identify 70 genes that should be considered candidate immune response genes, providing important new insights into the molecular events that occur during the innate immune response in P. xylostella.

Characterization of a shrimp serine protease homolog, a binding protein of yellow head virus

A serine protease homolog (SPH) cDNA namely SPH516 was identified via a yeast two-hybrid screen between yellow head virus (YHV) proteins and hemocyte proteins of the black tiger shrimp Penaeus monodon. Initially, the C-terminal region of SPH516 (SPH516-C) was found to interact with a putative metal ion-binding domain (MIB) encoded by open-reading of frame ORF1b of the YHV genome. Subsequently, the full-length of SPH516 cDNA was obtained using 5' rapid amplification of cDNA ends (5' RACE) and it also bound specifically to the MIB domain only. Primers designed based on the SPH516 coding region amplified not only SPH516 but also an additional SPH named SPH509 from shrimp hemocytes using reverse transcriptase-polymerase chain reaction (RT-PCR). These new SPHs had high homology to MasSPH previously reported from P. monodon. All shared the same domain features including a putative signal peptide, glycine-rich repeat motifs, a clip domain, an HDG triad and a trypsin-like serine protease domain. It is interesting that these sequences were phylogenetically closer to a prophenoloxidase-activating factor (PPAF) from blue crab than to another SPH from the black tiger shrimp reported to be involved in cell adhesion. Our SPH transcripts were highly expressed in hemocytes and gills and were found to be down-regulated after YHV infection. Immunohistochemistry using a polyclonal antibody raised against shrimp protein SPH516-C heterologously expressed in Escherichia coli revealed that SPH516 was present almost exclusively in the shrimp hemolymph.

Comparative analysis of serine protease-related genes in the honey bee genome: possible involvement in embryonic development and innate immunity

We have identified 44 serine protease (SP) and 13 serine protease homolog (SPH) genes in the genome of Apis mellifera. Most of these genes encode putative secreted proteins, but four SPs and three SPHs may associate with the plasma membrane via a transmembrane region. Clip domains represent the most abundant non-catalytic structural units in these SP-like proteins -12 SPs and six SPHs contain at least one clip domain. Some of the family members contain other modules for protein-protein interactions, including disulphide-stabilized structures (LDL(r)A, SRCR, frizzled, kringle, Sushi, Wonton and Pan/apple), carbohydrate-recognition domains (C-type lectin and chitin-binding), and other modules (such as zinc finger, CUB, coiled coil and Sina). Comparison of the sequences with those from Drosophila led to a proposed SP pathway for establishing the dorsoventral axis of honey bee embryos. Multiple sequence alignments revealed evolutionary relationships of honey bee SPs and SPHs with those in Drosophila melanogaster, Anopheles gambiae, and Manduca sexta. We identified homologs of D. melanogaster persephone, M. sexta HP14, PAP-1 and SPH-1. A. mellifera genome includes at least five genes for potential SP inhibitors (serpin-1 through -5) and three genes of SP putative substrates (prophenoloxidase, spätzle-1 and spätzle-2). Quantitative RT-PCR analyses showed an elevation in the mRNA levels of SP2, SP3, SP9, SP10, SPH41, SPH42, SP49, serpin-2, serpin-4, serpin-5, and spätzle-2 in adults after a microbial challenge. The SP41 and SP6 transcripts significantly increased after an injection of Paenibacillus larva, but there was no such increase after injection of saline or Escherichia coli. mRNA levels of most SPs and serpins significantly increased by 48 h after the pathogen infection in 1st instar larvae. On the contrary, SP1, SP3, SP19 and serpin-5 transcript levels reduced. These results, taken together, provide a framework for designing experimental studies of the roles of SPs and related proteins in embryonic development and immune responses of A. mellifera.

A genetic module regulates the melanization response of Anopheles to Plasmodium

Two modes of refractoriness to Plasmodium, ookinete lysis and melanization, are known in the malaria vector, Anopheles gambiae. Melanization, a potent insect immune response, is manifested in a genetically selected refractory strain and in susceptible mosquitoes that are depleted of specific C-type lectins (CTLs). Here we use a systematic in vivo RNA interference-mediated reverse genetic screen and other recent results to define a melanization-regulating genetic module or network. It encompasses at least 14 genes, including those that encode five Easter-like clip domain serine proteases and four Masquerade-like serine protease homologues of the mosquito CLIPB and CLIPA subfamilies respectively. We show that several but not all CLIPB genes promote Plasmodium melanization, exhibiting partial functional overlap and synergy. We also report that several CLIPA genes have contrasting roles: CLIPA8 is essential for parasite melanization, while three other CLIPAs are novel synergistic inhibitors of this response. Importantly, the roles of certain CLIPAs and CLIPBs are strain specific, indicating that this network may differ between strains. Finally, we provide evidence that in susceptible mosquitoes melanization induced by knockdown of either CTL4 or CLIPA2/CLIPA5 directly kills ookinetes, in contrast to refractory mosquitoes where it merely disposes of dead parasites.

Two Proteases Defining a Melanization Cascade in the Immune System of Drosophila

The melanization reaction is used as an immune mechanism in arthropods to encapsulate and kill microbial pathogens. In Drosophila, the serpin Spn27A regulates melanization apparently by inhibiting the protease that activates phenoloxidase, the key enzyme in melanin synthesis. Here, we have described the genetic characterization of two immune inducible serine proteases, MP1 and MP2, which act in a melanization cascade regulated by Spn27A. MP1 is required to activate melanization in response to both bacterial and fungal infection, whereas MP2 is mainly involved during fungal infection. Pathogenic bacteria and fungi may therefore trigger two different melanization cascades that use MP1 as a common downstream protease to activate phenoloxidase. We have also shown that the melanization reaction activated by MP1 and MP2 plays an important role in augmenting the effectiveness of other immune reactions, thereby promoting resistance of Drosophila to microbial infection.

Preliminary X-ray crystallographic analysis of the catalytic domain of prophenoloxidase activating factor-I

Clip-domain serine proteases (SPs) have been identified in invertebrates as crucial enzymes that are involved in diverse extracellular signalling pathways. Prophenoloxidase (proPO) activating factor-I (PPAF-I), a catalytically active clip-domain SP, cleaves proPO. To date, no crystal structures of a catalytically active clip-domain SP have been determined. Here, the results of crystallization and preliminary X-ray analysis of the SP domain of PPAF-I are reported. The crystal of the PPAF-I SP domain was obtained using the hanging-drop vapour-diffusion method in a precipitant solution containing 0.15 M lithium sulfate, 30% polyethylene glycol 4000 and 0.1 M Tris-HCl pH 8.0. The crystal diffracts X-rays to 1.7 angstroms resolution using a synchrotron-radiation source. The crystal belongs to space group P2(1)2(1)2(1), with one molecule in the asymmetric unit and unit-cell parameters a = 38.3, b = 53.3, c = 116.6 angstroms, alpha = beta = gamma = 90 degrees. A molecular-replacement solution has been found using kallikrein as a starting model, resulting in an interpretable electron-density map.

A Novel 40-kDa Protein Containing Six Repeats of an Epidermal Growth Factor-Like Domain Functions as a Pattern Recognition Protein for Lipopolysaccharide

Determination of structures and functions of pattern recognition proteins are important for understanding pathogen recognition mechanisms in host defense and for elucidating the activation mechanism of innate immune reactions. In this study, a novel 40-kDa protein, named LPS recognition protein (LRP), was purified to homogeneity from the cell-free plasma of larvae of the large beetle, Holotrichia diomphalia. LRP exhibited agglutinating activities on Escherichia coli, but not on Staphylococcus aureus and Candida albicans. This E. coli-agglutinating activity was preferentially inhibited by the rough-type LPS with a complete core oligosaccharide. LRP consists of 317 aa residues and six repeats of an epidermal growth factor-like domain. Recombinant LRP expressed in a baculovirus system also showed E. coli agglutination activity in vitro and was able to neutralize LPS by inhibition of LPS-induced IL-6 production in mouse bone marrow mast cells. Furthermore, E. coli coated with the purified LRP were more rapidly cleared in the Holotrichia larvae than only E. coli, indicating that this protein participates in the clearance of E. coli in vivo. The three amino-terminal epidermal growth factor-like domains of LRP, but not the three carboxyl epidermal growth factor-like domains, are involved in the LPS-binding activity. Taken together, this LRP functions as a pattern recognition protein for LPS and plays a role as an innate immune protein.

Cell-mediated immunity in arthropods: Hematopoiesis, coagulation, melanization and opsonization

The functions of hemocytes in innate immune response are reviewed with emphasized on their roles in coagulation, melanization and opsonization. Also the ways in which hemocytes are produced in and released from hematopoietic tissue are discussed.

Venom proteins from polydnavirus-producing endoparasitoids: their role in host-parasite interactions

Endoparasitoid wasps have evolved various mechanisms to ensure successful development of their progeny, including co-injection of a cocktail of maternal secretions into the host hemocoel, including venom, calyx fluid, and polydnaviruses. The components of each type of secretion may influence host physiology and development independently or in a synergistic fashion. For example, venom fluid consists of several peptides and proteins that promote expression of polydnavirus genes in addition to other activities, such as inhibition of prophenoloxidase activation, inhibition of hemocytes spreading and aggregation, and inhibition of development. This review provides a brief overview of advances and prospects in the study of venom proteins from polydnavirus-producing endoparasitoid wasps with a special emphasis on the role of C. rubecula venom proteins in host-parasitoid interactions.

Cathepsin B homologue at the interface between a parasitic nematode and its intermediate host

Parelaphostrongylus tenuis is a parasitic nematode that causes a debilitating neurologic disease in many North American cervids and domestic livestock species. We produced a PCR-based cDNA library from infective larvae (L3) in order to identify molecules that mediate parasitism. A dominant 1,250-bp amplicon encoded a homologue of cathepsin B cysteine proteases. The sequence incorporated a C29G substitution in the putative active site. Antibodies generated against a recombinant form detected the native protein (PtCPR-1) in Western blot assays of L3, but not adult worm, extracts. Immunohistochemical methods revealed that PtCPR-1 synthesis was restricted to larval stages within the snail intermediate host (Triodopsis sp.), beginning as early as 2 days postinfection (dpi) of snails. The protein was present in the intestine and luminal contents and was lost from larvae over time. Concurrent studies showed that larvae induced an immune response in snails beginning at 1 dpi. Layers of hemocytes encapsulated larvae immediately after infection, and granuloma-like structures formed around parasites in chronic infections. Loss of PtCPR-1 from L3 and its accumulation in host tissues coincided with degeneration of granuloma architecture 90 to 105 dpi. Fully developed L3 emerged from the snail at this time. Our data implicate PtCPR-1 in larval development and possibly in the emergence of P. tenuis from the intermediate host. Emerged L3 survived desiccation and cold stress, suggesting that they could remain infectious in the environment. Molecules promoting emergence would facilitate dispersal of L3 and increase the likelihood of transmission to definitive hosts.

Cloning and characterization of a shrimp clip domain serine protease homolog (c-SPH) as a cell adhesion molecule

Clip domain serine protease homologs (c-SPHs) are involved in various innate immune functions in arthropods such as antimicrobial activity, cell adhesion, pattern recognition, opsonization, and regulation of the prophenoloxidase system. In the present study, we cloned a c-SPH cDNA from tiger shrimp (Penaeus monodon) hemocytes. It is 1337 bp in length with a coding region of 1068 bp consisting a protein of 355 amino acid residues. The deduced protein includes one clip domain and one catalytically inactive serine protease-like (SP-like) domain. Its molecular weight is estimated to be 38 kDa with an isoelectric point of 7.9. The predicted cutting site of the signal peptide is located between Gly(21) and Gln(22). We aligned 15 single clip domain SPH protein sequences from 12 arthropod species; the identity of these clip domains is low and that of SP-like domains is from 34% to 46%. The conserved regions are located near the amino acid residues which served as substrate interaction sites in catalytically active serine protease. Phylogenetically, the tiger shrimp c-SPH is most similar to a low molecular mass masquerade-like protein of crayfish, but less similar to c-SPHs in Chelicerata and Insecta. Nested reverse transcription polymerase chain reaction (RT-PCR) revealed that c-SPH mRNA is expressed most in tissues with the highest hemocyte abundance. Antimicrobial and opsonization activities of the molecule were not detected. The expression of c-SPH mRNA in hemocytes was up-regulated at the 12-day post beta-glucan immersion. Recombinant c-SPH could significantly enhance hemocyte adhesion. The result suggests that the shrimp c-SPH protein plays a role in innate immunity.

Crystal structure of a clip-domain serine protease and functional roles of the clip domains

Clip-domain serine proteases (SPs) are the essential components of extracellular signaling cascades in various biological processes, especially in embryonic development and the innate immune responses of invertebrates. They consist of a chymotrypsin-like SP domain and one or two clip domains at the N-terminus. Prophenoloxidase-activating factor (PPAF)-II, which belongs to the noncatalytic clip-domain SP family, is indispensable for the generation of the active phenoloxidase leading to melanization, a major defense mechanism of insects. Here, the crystal structure of PPAF-II reveals that the clip domain adopts a novel fold containing a central cleft, which is distinct from the structures of defensins with a similar arrangement of cysteine residues. Ensuing studies demonstrated that PPAF-II forms a homo-oligomer upon cleavage by the upstream protease and that the clip domain of PPAF-II functions as a module for binding phenoloxidase through the central cleft, while the clip domain of a catalytically active easter-type SP plays an essential role in the rapid activation of its protease domain.

Overexpression and preliminary X-ray crystallographic analysis of prophenoloxidase activating factor II, a clip domain family of serine proteases

A clip domain family of serine proteases has been identified in invertebrates as a crucial enzyme involved in diverse biological processes including immune responses and embryonic development. Although these proteins contain at least one clip domain at the N-terminal of the serine protease domain, the roles and three-dimensional structure of the clip domain are unknown. Prophenoloxidase activating factor-II (PPAF-II), a clip domain family of serine proteases, derived from the beetle Holotrichia diomphalia larvae, was overexpressed in the baculovirus system, and crystallized using the hanging-drop vapor-diffusion method. High-quality single crystals of PPAF-II were obtained in a precipitant solution containing 0.15 M ammonium sulfate, 1.25 M lithium sulfate monohydrate, and 0.1 M sodium citrate dehydrate (pH 5.5). These crystals belong to space group C2 with unit-cell parameters a=107.84, b=76.78, c=70.49 A and beta=113.93 degrees , and contain one or two molecules in the asymmetric unit. Determination of the three-dimensional structure of PPAF-II would clarify the functions of the clip domains.

Recent advances in the innate immunity of invertebrate animals

Invertebrate animals, which lack adaptive immune systems, have developed other systems of biological host defense, so called innate immunity, that respond to common antigens on the cell surfaces of potential pathogens. During the past two decades, the molecular structures and functions of various defense components that participated in innate immune systems have been established in Arthropoda, such as, insects, the horseshoe crab, freshwater crayfish, and the protochordata ascidian. These defense molecules include phenoloxidases, clotting factors, complement factors, lectins, protease inhibitors, antimicrobial peptides, Toll receptors, and other humoral factors found mainly in hemolymph plasma and hemocytes. These components, which together compose the innate immune system, defend invertebrate from invading bacterial, fungal, and viral pathogens. This review describes the present status of our knowledge concerning such defensive molecules in invertebrates.

Identification of plasma proteases inhibited by Manduca sexta serpin-4 and serpin-5 and their association with components of the prophenol oxidase activation pathway

One innate immune response pathway of insects is a serine protease cascade that activates prophenol oxidase (pro-PO) in plasma. However, details of this pathway are not well understood, including the number and order of proteases involved. Protease inhibitors from the serpin superfamily appear to regulate the proteases in the pathway. Manduca sexta serpin-4 and serpin-5 suppress pro-PO activation in plasma, apparently by inhibiting proteases upstream of the direct activator of pro-PO. To identify plasma proteases inhibited by these serpins, we used immunoaffinity chromatography with serpin antibodies to isolate serpin-protease complexes that formed after activation of the cascade by exposure of plasma to bacteria or lipopolysaccharide. Covalent complexes of serpin-4 with hemolymph proteases HP-1 and HP-6 appeared in plasma activated by Gram-positive or Gram-negative bacteria, whereas serpin-4 complexes with HP-21 and two unidentified proteases were unique to plasma treated with Gram-positive bacteria. HP-1 and HP-6 were also identified as target proteases of serpin-5, forming covalent complexes after bacterial activation of the cascade. These results suggest that HP-1 and HP-6 may be components of the pro-PO activation pathway, which are activated in response to infection and regulated by serpin-4 and serpin-5. HP-21 and two unidentified proteases may participate in a Gram-positive bacteria-specific branch of the pathway. Several plasma proteins that co-purified with serpin-protease complexes, most notably immulectins and serine protease homologs, are known to be components of the pro-PO activation pathway. Our results suggest that after activation by exposure to bacteria, components of the pro-PO pathway associate to form a large noncovalent complex, which localizes the melanization reaction to the surface of invading microorganisms.

Expression of a serine proteinase homolog prophenoloxidase-activating factor from the blue crab, Callinectes sapidus

Tanning, or sclerotization, of crustacean cuticle provides initial reinforcement by cross linking cuticular proteins attached to the cuticle chitin-fiber matrix. This process is catalyzed in part by phenoloxidase, which is under the control of a serine protease activation cascade. The cDNA of a prophenoloxidase-activating factor (PPAF) was cloned and sequenced from the hypodermal tissue of the blue crab, Callinectes sapidus. It codes for a serine proteinase homolog containing a single clip domain. If it is involved in sclerotization, its transcription might be expected to be molt-cycle related. Expression patterns were determined by quantitative PCR and Northern blotting in hypodermis underlying both arthrodial and dorsal (calcifying) cuticles. Transcript levels in pre-molt RNA from both hypodermis types were high, suggesting that the PPAF produced may be incorporated into the pre-ecdysial cuticle layers and then activated at ecdysis to regulate tanning. After a decrease at ecdysis, a second increase in PPAF mRNA occurred at three to four hours post-molt in arthrodial membrane hypodermis but not dorsal hypodermis. This suggests that cuticle deposited after ecdysis may tan in the non-calcifying regions but may not tan where calcification occurs. The PPAF gene is also transcribed at low levels in the hemocytes of intermolt crabs, but not in the hepatopancreas.

Characterisation of a serine proteinase from Penaeus vannamei haemocytes

Serine proteinases are involved, besides digestive role, in immune response processes. In addition to the typical serine proteinase domain, proteinases from arthropod haemocytes contain so-called clip domains which are believed to exert regulatory functions. Clones coding for clip domain-containing serine proteinases were isolated from both Penaeus vannamei and Penaeus monodon haemocyte cDNA libraries. These proteins have most of the structural characteristics of serine proteinase domain, but in the clip domain there are only four cysteines, whereas in most other clip domains there are six. Such structures are named pseudo-clip domains and apparently seem to be widely distributed in Penaeid shrimp. These proteinases were only expressed in haemocytes and not in muscles, hypodermis, heart, tail stalk, pleopods or hepatopancreas.