Molecular cloning and characterization of prophenoloxidase in the black tiger shrimp, Penaeus monodon

Expression of polyphenol oxidase of Litopenaeus vannamei and its characterization

Polyphenol oxidase (PPO) plays a critical role in decrement of shrimp quality. To obtain active PPO and elucidate its enzymatic properties, PPO from Litopenaeus vannamei (Lv-PPO) was cloned, expressed in E. coli and purified by affinity column chromatography. The Lv-PPO gene was 2076 bp in length encoding 691 amino acids. The recombinant Lv-PPO (rLv-PPO) with a molecular mass of ∼85.0 kDa was successfully expressed and its sequence was verified by LC-MS/MS. rLv-PPO was biologically active with an optimal temperature of 40℃ and an optimal pH of 6.0. Metal ions Cu2+ and Zn2+ altered the activity of rLv-PPO by influencing its secondary and tertiary structures. rLv-PPO showed catalytic activity towards l-Dopa and catechol. A specific polyclonal antibody against rLv-PPO was prepared. Western blot analysis revealed that PPO levels were highest in hemolymph, followed by telson, carapace, and eyestalk. Expression of rLv-PPO will assist future studies on the mechanism in shrimp melanosis.

Cloning, identification, and functional characterization of novel prophenoloxidases (ShproPO) from the freshwater crab Sinopotamon henanense in response to cadmium exposure and Aeromonas hydrophila infection

Prophenoloxidase (proPO) is essential in the prophenoloxidase-activating system (proPO-AS) which is important for defense against foreign infection in crustaceans. However, most studies have focused on expression in the presence of a single pathogenic bacterium, and very few have addressed the presence of environmental contaminants simultaneously, such as cadmium (Cd) and Aeromonas hydrophila. Our study aimed to investigate the function of proPO in the freshwater crab Sinopotamon henanense and the changes in its expression by Cd and infection of A. hydrophila. A novel proPO from the hemocytes of S. henanense (ShproPO) was found in this research, the full-length cDNA of ShproPO was 2620 bp of encoding a protein of 678 amino acids containing three typical hemocyanin domains. The ShproPO protein could be found in both the granular (GHc) and the semi-granular hemocytes (SGHc). The ShproPO mRNA was found to be abundantly expressed in hemocytes and could be influenced by A. hydrophila infection. These results indicate that ShproPO could be involved in the antibacterial process. Further research found that low concentrations of Cd could promote its expression after infection with A. hydrophila. Therefore, it was hypothesized that Cd disrupted the response of crabs to A. hydrophila infection. Subsequently, PO enzyme activity was found to be significantly reduced through in vivo RNA interference with ShproPO, and the results suggested that ShproPO is likely to be a key enzyme in the melanization response. Finally, ShproPO was found to significantly enhance the phagocytosis of A. hydrophila-infected hemocytes by in vitro recombination, confirming that ShproPO is involved in hemocyte-mediated melanization and phagocytosis. Our findings reveal completely new insight into the immunotoxicity of Cd and the immune function of ShproPO in S. henanense.

Ergothioneine: a potential antioxidative and anti-melanosis agent for food quality preservation

The global population increase has increased the demand for food products. However, post-harvest deterioration due to oxidation and discoloration results in a drastic loss of food quality and supply. Thus, research has focused on developing strategies to minimize such losses. One of those strategies includes the application of ergothioneine (ET), a potent hydrophilic antioxidant, to several food products so as to overcome their short shelf-life. ET can be synthetic or derived from several species of edible mushrooms and their extracts, which are known sources of natural ET. Given the reported potential of ET in food quality preservation, this review compiles the recent applications of ET as a preservative for maintaining the quality of food commodities.

Study on Purification and Characterization of Polyphenol Oxidase from Acetes chinensis

Acetes chinensis (belonging to the Decapoda Sergestidae genus) is widely distributed in East Asian waters and is extremely widespread and present in the shallow coastal areas of China. Polyphenol oxidase (PPO), which was extracted from Acetes chinensis, was purified in a four-step procedure involving phosphate-buffered saline treatment, ammonium sulphate precipitation, DEAE-Cellulose chromatography, and Phenyl-Sepharose HP chromatography, and then, its biochemical characterization was measured. The specific activity of the purified enzyme was increased to 643.4 U/mg, which is a 30.35 times increase in purification, and the recovery rate was 17.9%. L-dopa was used as the substrate, the enzymatic reactions catalyzed by PPO conformed to the Michaelis equation, the maximum reaction velocity was 769.23 U/mL, and the Michaelis constant K_m was 0.846 mmol/L. The optimal pH of PPO from Acetes chinensis was 7.5, and the optimal temperature was 35 °C. The metal ions experiment showed that Mn²⁺ and K⁺ could enhance the activity of PPO; that Ba²⁺ and Ca²⁺ could inhibit the activity of PPO; and that Cu²⁺ had a double effect on PPO, increasing the PPO activity at low concentrations and inhibiting the PPO activity at high concentrations. The inhibitor experiment showed that the inhibitory effects of EDTA and kojic acid were weak and that ascorbic acid and sodium pyrophosphate had good inhibitory effects. The purification and characterization of Acetes chinensis serve as guidelines for the prediction of enzyme behavior, leading to effective prevention of enzymatic browning during processing.

Beta-glucan's varying structure characteristics modulate survival and immune-related genes expression from Vibrio harveyi-infected Artemia franciscana in gnotobiotic conditions

β-Glucans have long been used as an immunostimulant in aquaculture. However, the relationship of its structure to its immunomodulatory properties are poorly understood. In this study, the particle size and chemical structure of β-glucans extracted from wild-type strain of baker's yeast (Saccharomyces cerevisiae) and its null-mutant yeasts Gas1 were characterised. Using Sigma β-glucan as a reference, the immunomodulatory properties of these polysaccharides in the germ-free Artemia franciscana model system in the presence of Vibrio harveyi bacterial challenge were investigated. The survival of the A. franciscana nauplii, upon challenge with V. harveyi, was significantly higher in all three glucan-treated groups compared to the control. The glucan Gas1 with a lower degree of branching and shorter side chain length had the most prominent V. harveyi-protective effects. The particle size did not affect the nauplii survival when challenged with V. harveyi. Results also showed that the salutary effect of the tested glucans was associated with the upregulation of innate immune genes such as lipopolysaccharide and β-1,3-glucan-binding protein (lgbp), high mobility group box protein (hmgb), and prophenoloxidase (proPO). Interestingly, the up-regulation of superoxidase dismutase (sod) and glutathione-s-transferase (gst) was only observed in Gas1 treated group, indicating that Gas1 could function to induce higher reactive oxygen species and stronger immunomodulatory function in A. franciscana, and therefore higher survival rate. The expression of heat shock protein 70 (hsp70), peroxinectin (pxn), and down syndrome cell adhesion molecule (dscam) remain unaltered in response to glucan treatment. Taken together, this study provides insights into the structure-function relationship of β-glucan and the results confirmed that β-glucan can be an effective immunostimulant in aquaculture, especially the Gas1 glucan.

The immune function of prophenoloxidase from red swamp crayfish (Procambarus clarkii) in response to bacterial infection

Prophenoloxidase (proPO) is the zymogen form of phenoloxidase (PO), a key enzyme in melanization cascade that has been co-opted in invertebrate immune reactions. There have been reported that proPO plays many essential roles in the crustacean immune system. However, little is known about the function of proPO from red swamp crayfish (Procambarus clarkii) which is an important cultured species worldwide. Here, we cloned and expressed proPO gene from red swamp crayfish (PcproPO). Subsequently, specific antibody against PcproPO was generated. The immune function of PcproPO was further characterized in vitro and in vivo. The results showed that the expression of PcproPO mRNA could be significantly up-regulated during the challenge of Gram-positive-negative (Vibrio parahaemolyticus) and Gram-positive-positive bacterial (Staphylococcus aureus). Furthermore, the purified recombinant PcproPO protein had a strong affinity binding to both bacteria and polysaccharides. In vivo knockdown of PcproPO could significantly reduce the crayfish bacterial clearance ability, resulting in the higher mortality of the crayfish during V. parahaemolyticus infection. In addition, in vitro knockdown of PcproPO in the hemocytes significantly reduced the phenoloxidase (PO) activity and the bacterial clearance ability, indicating that PcproPO might involve in hemocyte-mediated melanization. Our results will shed a new light on the immune function of PcproPO in the crayfish.

Molecular characterization of prophenoloxidase-1 (PPO1) and the inhibitory effect of kojic acid on phenoloxidase (PO) activity and on the development of Zeugodacus tau (Walker) (Diptera: Tephritidae)

Phenoloxidase (PO) plays a key role in melanin biosynthesis during insect development. Here, we isolated the 2310-bp full-length cDNA of PPO1 from Zeugodacus tau, a destructive horticultural pest. qRT-polymerase chain reaction showed that the ZtPPO1 transcripts were highly expressed during larval-prepupal transition and in the haemolymph. When the larvae were fed a 1.66% kojic acid (KA)-containing diet, the levels of the ZtPPO1 transcripts significantly increased by 2.79- and 3.39-fold in the whole larvae and cuticles, respectively, while the corresponding PO activity was significantly reduced; in addition, the larval and pupal durations were significantly prolonged; pupal weights were lowered; and abnormal phenotypes were observed. An in vitro inhibition experiment indicated that KA was an effective competitive inhibitor of PO in Z. tau. Additionally, the functional analysis showed that 20E could significantly up-regulate the expression of ZtPPO1, induce lower pupal weight, and advance pupation. Knockdown of the ZtPPO1 gene by RNAi significantly decreased mRNA levels after 24 h and led to low pupation rates and incomplete pupae with abnormal phenotypes during the larval-pupal interim period. These results proved that PO is important for the normal growth of Z. tau and that KA can disrupt the development of this pest insect.

WSSV-host interaction: Host response and immune evasion

As invertebrates, shrimps rely on multiple innate defense reactions, including humoral immunity and cellular immunity to recognize and eliminate various invaders, such as viruses. White spot syndrome virus (WSSV) causes the most prevalent and devastating viral disease in penaeid shrimps, which are the most widely cultured species in the coastal waters worldwide. In the last couple of decades, studies about WSSV implicate a dual role of the immune system in protecting shrimps against the infection; these studies also explore on the pathogenesis of WSSV infection. Herein, we review our current knowledge of the innate immune responses of shrimps to WSSV, as well as the molecular mechanisms used by this virus to evade host immune responses or actively subvert them for its own benefit. Deciphering the interactions between WSSV and the shrimp host is paramount to understanding the mechanisms that regulate the balance between immune-mediated protection and pathogenesis during viral infection and to the development of a safe and effective WSSV defensive strategy.

Two host gut-derived lactic acid bacteria activate the proPO system and increase resistance to an AHPND-causing strain of Vibrio parahaemolyticus in the shrimp Litopenaeus vannamei

Lactic acid bacteria (LAB) are group of beneficial bacteria that have been proposed as relevant probiotics with immunomodulatory functions. In this study, we initially isolated and identified host-derived LAB from the gut of the Pacific white shrimp Litopenaeus vannamei. Analysis of the bacterial 16S rRNA gene sequence revealed two candidate LAB, the Lactobacillus plantarum strain SGLAB01 and the Lactococcus lactis strain SGLAB02, which exhibited 99% identity to the L. plantarum strain LB1-2 and the L. lactis strain R-53658, which were isolated from bee gut, respectively. The two LAB displayed antimicrobial activities against gram-positive and gram-negative bacteria, including the virulent acute hepatopancreatic necrosis disease (AHPND)-causing strain of Vibrio parahaemolyticus (VP_AHPND). Viable colony count and SEM analysis showed that the two candidate LAB, administered via oral route as feed supplement, could reside and adhere in the shrimp gut. Double-stranded RNA-mediated gene silencing of LvproPO1 and LvproPO2 revealed a significant role of two LvproPOs in the proPO system as well as in the immune response against VP_AHPND infection in L. vannamei shrimp. The effect of LAB supplementation on modulation of the shrimp proPO system was investigated in vivo, and the results showed that administration of the two candidate LAB significantly increased hemolymph PO activity, the relative mRNA expression of LvproPO1 and LvproPO2, and resistance to VP_AHPND infection. These findings suggest that administration of L. plantarum and L. lactis could modulate the immune system and increase shrimp resistance to VP_AHPND infection presumably via upregulation of the two LvproPO transcripts.

Transcriptional Identification of Related Proteins in the Immune System of the Crayfish Procambarus clarkii

The freshwater crayfish Procambarus clarkii is an animal model employed for physiological and immunological studies and is also of great economic importance in aquaculture. Although it is a species of easy husbandry, a high percentage of its production is lost annually as a result of infectious diseases. Currently, genetic information about the immune system of crustaceans is limited. Therefore, we used the abdominal nerve cord from P. clarkii to obtain its transcriptome using Next Generation Sequencing (NGS) to identify proteins that participate in the immune system. The reads were assembled de novo and consensus sequences with more than 3000 nucleotides were selected for analysis. The transcripts of the sequences of RNA were edited for annotation and sent to the GenBank database of the National Center for Biotechnology Information (NCBI). We made a list of accession numbers of the sequences which were organized by the putative role of the immune system pathway in which they participate. In this work, we report on 80 proteins identified from the transcriptome of crayfish related to the immune system, 74 of them being the first reported for P. clarkii. We hope that the knowledge of these sequences will contribute significantly to the development of future studies of the immune system in crustaceans.

A probiotic Bacillus strain containing amorphous poly-beta-hydroxybutyrate (PHB) stimulates the innate immune response of Penaeus monodon postlarvae

In this study, the PHB-accumulating Bacillus sp. JL47 strain (capable of accumulating 55% PHB on cell dry weight) was investigated for its effects on the immune response of giant tiger shrimp (Penaeus monodon) postlarvae (PL) before and after the Vibrio campbellii challenge. Briefly, shrimp PL were cultured and fed with Artemia nauplii enriched with Bacillus sp. JL47. Shrimp receiving the Artemia nauplii without JL47 enrichment were used as control. After 15 days of feeding, the shrimp were challenged with pathogenic V. campbellii LMG 21363 at 10⁶ cells mL^-1 by immersion. Relative expression of the immune related genes encoding for prophenoloxidase (proPO), transglutaminase (TGase) and heat shock protein 70 (Hsp70) in the shrimp were measured before (0 h) and after (3, 6, 9, 12, 24 h) the Vibrio challenge by quantitative real-time PCR using β-actin as the reference gene. The expressions of TGase and proPO were significantly up-regulated (p < 0.05) within 9 h and 12 h, respectively after challenge in shrimp receiving the Bacillus sp. JL47 as compared to the challenged and non-challenged controls. Hsp70 expression was significantly increased (p < 0.05) at 3 h post-challenge in all challenged shrimp. Interestingly, proPO and TGase genes were significantly up-regulated (p < 0.05) in Bacillus sp. JL47 treated shrimp even before the Vibrio challenge was applied. No up-regulation in the Hsp70 gene, however, was observed under these conditions. The data suggest that the protective effect of the PHB-accumulating Bacillus sp. JL47 in shrimp was due to its capacity to stimulate the innate immune related genes of the shrimp, specifically the proPO and TGase genes. The application of probiotic Bacillus species, capable of accumulating a significant amount of PHB, is suggested as potential immunostimulatory strategy for aquaculture.

Immune indices and identical functions of two prophenoloxidases from the haemolymph of green tiger shrimp Penaeus semisulcatus and its antibiofilm activity

In the present study, we purified two prophenoloxidases (proPO) from haemolymph of green tiger shrimp, Penaeus semisulcatus by gel fermentation chromatography using blue Sepharose matrix. The two purified prophenoloxidase macromolecules are of about 76 and 75 kDa determined through SDS-PAGE and named as Penaeus semisulcatus prophenoloxidase I (PSproPO I) and Penaeus semisulcatus prophenoloxidase II (PSproPO II). It was further characterized by X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Circular Dichroism (CD) and High Performance Liquid Chromatography (HPLC) analysis. The purified PSproPO I and PSproPO II showed the strongest agglutination titre against human erythrocytes compared to goat RBC. The PSproPO I and PSproPO II showed phagocytic activity against yeast Saccharomyces cerevisiae and encapsulation activity against Sepharose CL 6B beads compared to CM Sepharose and Sodium alginate beads. The functional analysis of purified PSproPO I and PSproPO II showed enhanced PO activity when added with the triggering molecules such as pathogen associated molecular patterns (PAMPs), metals and chemicals. In addition, eluted fraction containing PSproPO I and PSproPO II showed antibiofilm activity against Gram positive and Gram negative bacteria. The above results concluded that no significant differences were found between the purified PSproPO I and PSproPO II immune indices and functions. This study might provide a sensitive platform to understand more about the critical roles of PSproPO I and PSproPO II in crustacean immune system.

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.

The Involvement of Hemocyte Prophenoloxidase in the Shell-Hardening Process of the Blue Crab, Callinectes sapidus

Cuticular structures of arthropods undergo dramatic molt-related changes from being soft to becoming hard. The shell-hardening process of decapod crustaceans includes sclerotization and mineralization. Hemocyte PPO plays a central role in melanization and sclerotization particularly in wound healing in crustaceans. However, little is known about its role in the crustacean initial shell-hardening process. The earlier findings of the aggregation of heavily granulated hemocytes beneath the hypodermis during ecdysis imply that the hemocytes may be involved in the shell-hardening process. In order to determine if hemocytes and hemocyte PPO have a role in the shell-hardening of crustaceans, a knockdown study using specific CasPPO-hemo-dsRNA was carried out with juvenile blue crabs, Callinectes sapidus. Multiple injections of CasPPO-hemo-dsRNA reduce specifically the levels of CasPPO-hemo expression by 57% and PO activity by 54% in hemocyte lysate at the postmolt, while they have no effect on the total hemocyte numbers. Immunocytochemistry and flow cytometry analysis using a specific antiserum generated against CasPPO show granulocytes, semigranulocytes and hyaline cells as the cellular sources for PPO at the postmolt. Interestingly, the type of hemocytes, as the cellular sources of PPO, varies by molt stage. The granulocytes always contain PPO throughout the molt cycle. However, semigranulocytes and hyaline cells become CasPPO immune-positive only at early premolt and postmolt, indicating that PPO expression in these cells may be involved in the shell-hardening process of C. sapidus.

Immune gene expression for diverse haemocytes derived from pacific white shrimp, Litopenaeus vannamei

In this study, diverse haemocytes from Pacific white shrimp Litopenaeus vannamei were spread by flow cytometer sorting system. Using the two commonly flow cytometric parameters FSC and SSC, the haemocytes could be divided into three populations. Microscopy observation of L. vannamei haemocytes in anticoagulant buffer revealed three morphologically distinct cell types designated as granular cell, hyaline cell and semigranular cell. Immune genes, which includes prophenoloxidase (proPO), lipopolysaccharide-β-glucan binding protein (LGBP), peroxinectin, crustin, lysozyme, penaeid-3a and transglutaminase (TGase), expressed from different haemocyte were analysed by quantitative real time PCR (qPCR). Results from the mRNA expression was estimated by relative level of each gene to β-actin gene. Finally, the seven genes could be grouped by their dominant expression sites. ProPO, LGBP and peroxinectin were highly expressed in granular cells, while LGBP, crustin, lysozyme and P-3a were highly expressed in semigranular cells and TGase was highly expressed in hyaline cells. In this study, L. vannamei haemocytes were firstly grouped into three different types and the immune related genes expression in grouped haemocytes were estimated.

PmPPAF is a pro-phenoloxidase activating factor involved in innate immunity response of the shrimp Penaeus monodon

One of the major steps in the innate immune response of shrimp includes the activation of serine proteinases of the pro-phenoloxidase pathway by the prophenoloxidase activation enzyme (PPAF). In this study, the cDNA encoding a serine proteinase homologue (SPH) with prophenoloxidase activating activity of Penaeus monodon (PmPPAF) was cloned and characterized. PmPPAF cDNA consists of 1444 nucleotides encoding a protein with 394 amino acid residues. The estimated molecular weight of PmPPAF is 43.5 kDa with an isoelectric point of 5.19. PmPPAF consists of a signal peptide, a CLIP domain and a carboxyl-terminal trypsin-like serine protease domain. It is highly similar to the masquerade-like protein 2A (61% similarity) of the crayfish Pacifastacus leniusculus, other serine proteases (42.9-67% identity) of P. monodon, and the PPAF of the crab (61% similarity). Unlike other SPH of P. monodon, which express mainly in the hemocytes, PmPPAF transcripts were detected in the hemocytes, eyestalk, hypodermis, gill, swimming leg and brain. Similar to the crab PPAF, PmPPAF transcript level is high in shrimp at the premolt stages and PmPPAF expression is up-regulated in shrimp infected with white spot syndrome virus (WSSV). Gene silencing of PmPPAF decreased expression of a prophenoloxidase-like gene and injection of Anti-PmPPAF antibody causes a decrease in PO activity. Taken together, these results provided evidence that PmPPAF is a serine proteinase homologue, and is involved in the pro-PO activation pathway of the shrimp innate immune system.

Cloning of prophenoloxidase from hemocytes of the blue crab, Callinectes sapidus and its expression and enzyme activity during the molt cycle

The arthropods cuticle undergoes dramatic morphological and biochemical changes from being soft to hardness through each molting process. Prophenoloxidase (PPO) known as a key enzyme in the arthropod innate immune system involved in the melanization reaction, has been related with the initial shell-hardening process, specifically in the sclerotization of the protein matrix in the new cuticle. Since hemocytes have been reported as the main PPO source in arthropods, the transport of hemocyte PPO into the newly laid, soft cuticle has been proposed for shell-hardening occurring during and immediately after ecdysis. In order to define the role of hemocyte PPO in the shell-hardening of crustaceans, the full-length cDNA sequence (2806 nt) of hemocytes PPO of the blue crab Callinectes sapidus (CasPPO-hemo) is isolated using degenerate PCR and 5'-3' RACE. CasPPO-hemo encodes a putative PPO (672 aa) showing three hemocyanin domains: N, M, and C in order and two copper binding sites (CuA & CuB). The sequence analysis identifies the putative CasPPO-hemo as zymogen which requires the cleavage at the N-terminus for its activation. Hemocyte extract (CasHLS) contains the PO, the activity of which depends on the in vitro activation of trypsin. The expression levels of CasPPO-hemo are kept constant during the molt cycle. The increase in the number of hemocytes at early premolt correlates with the elevated PO activity, while at late premolt, the increment in hemocyte numbers does not reflect on the PO activity. The functional importance of the changes in the levels of CasHLS-PO activity during molt cycle is discussed in relation to cuticle hardening process.

The trypsin inhibitor panulirin regulates the prophenoloxidase-activating system in the spiny lobster Panulirus argus

The melanization reaction promoted by the prophenoloxidase-activating system is an essential defense response in invertebrates subjected to regulatory mechanisms that are still not fully understood. We report here the finding and characterization of a novel trypsin inhibitor, named panulirin, isolated from the hemocytes of the spiny lobster Panulirus argus with regulatory functions on the melanization cascade. Panulirin is a cationic peptide (pI 9.5) composed of 48 amino acid residues (5.3 kDa), with six cysteine residues forming disulfide bridges. Its primary sequence was determined by combining Edman degradation/N-terminal sequencing and electrospray ionization-MS/MS spectrometry. The low amino acid sequence similarity with known proteins indicates that it represents a new family of peptidase inhibitors. Panulirin is a competitive and reversible tight-binding inhibitor of trypsin (Ki = 8.6 nm) with a notable specificity because it does not inhibit serine peptidases such as subtilisin, elastase, chymotrypsin, thrombin, and plasmin. The removal of panulirin from the lobster hemocyte lysate leads to an increase in phenoloxidase response to LPS. Likewise, the addition of increasing concentrations of panulirin to a lobster hemocyte lysate, previously depleted of trypsin-inhibitory activity, decreased the phenoloxidase response to LPS in a concentration-dependent fashion. These results indicate that panulirin is implicated in the regulation of the melanization cascade in P. argus by inhibiting peptidase(s) in the pathway toward the activation of the prophenoloxidase enzyme.

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.

Transcriptome analysis on Chinese shrimp Fenneropenaeus chinensis during WSSV acute infection

Previous studies have discovered a lot of immune-related genes responding to white spot syndrome virus (WSSV) infection in crustacean. However, little information is available in relation to underlying mechanisms of host responses during the WSSV acute infection stage in naturally infected shrimp. In this study, we employed next-generation sequencing and bioinformatic techniques to observe the transcriptome differences of the shrimp between latent infection stage and acute infection stage. A total of 64,188,426 Illumina reads, including 31,685,758 reads from the latent infection group and 32,502,668 reads from the acute infection group, were generated and assembled into 46,676 unigenes (mean length: 676 bp; range: 200-15,094 bp). Approximately 24,000 peptides were predicted and classified based on homology searches, gene ontology, clusters of orthologous groups of proteins, and biological pathway mapping. Among which, 805 differentially expressed genes were identified and categorized into 11 groups based on their possible function. Genes in the Toll and IMD pathways, the Ras-activated endocytosis process, the RNA interference pathway, anti-lipopolysaccharide factors and many other genes, were found to be activated in shrimp from latent infection stage to acute infection stage. The anti-bacterially proPO-activating cascade was firstly uncovered to be probably participated in antiviral process. These genes contain not only members playing function in host defense against WSSV, but also genes utilized by WSSV for its rapid proliferation. In addition, the transcriptome data provides detail information for identifying novel genes in absence of the genome database of shrimp.

Comparative proteomic identification of the hemocyte response to cold stress in white shrimp, Litopenaeus vannamei

To understand molecular responses of crustacean hemocytes to cold stress, flow cytometer analysis and two-dimensional electrophoresis proteomic approach were used to investigate altered proteins in hemocytes of Litopenaeus vannamei during cold stress treatment. Through flow cytometer analysis, 13°C for 24h post-cold stress was selected as the suitable temperature and condition for cold stress treatment. MALDI-TOF/TOF MS analysis revealed that 6 forms of 6 proteins were significantly up-regulated, including three enzymes (cystathionase, glyceraldehyde 3-phosphate dehydrogenase and glyoxalase 1) and one immune-related protein (oncoprotein nm23), whereas 24 forms of 3 proteins were significantly down-regulated in the treated shrimp (hemocyanin, hemocyte transglutaminase and transketolase). There were 20 spots identified as hemocyanin meaning that it may play important roles in environmental regulation in shrimp. Real-time fluorescence quantitative PCR confirmed that the levels of transcription of the hemocyanin, partial mRNA for hemocyanin, cystathionase, glyoxalase 1 and oncoprotein nm23 genes were found to relate well with that of their translation products after cold stress treated, while only the levels of hemocyte transglutaminase transcripts were not corresponded with that of their translation products. Further investigation of these data may lead to better understanding of the molecular responses of crustacean hemocytes to cold stress.

A novel effect of imidazole derivative KK-42 on increasing survival of Aeromonas hydrophila challenged prawn Macrobrachium nipponense

Imidazole derivative KK-42 is well known as the insect growth regulator. Here we find that KK-42 pretreatment could promote the survival of Macrobrachium nipponense infected with Aeromonas hydrophila, which is considered to be possibly related to the prophenoloxidase (proPO), a conserved copper-containing enzyme that plays an important role in defense against pathogens. In this study, a full-length of proPO gene from M. nipponense haemocytes, designated as MnproPO, was firstly cloned and characterized. The full-length cDNA contained 2428 bp with a 2013 bp open reading frame encoding a putative proPO protein of 671 amino acids with a predicted molecular mass of 76.5 kDa and pI of 7.31. It was predicted to possess all the expected features of proPO members, including two putative copper-binding sites with six histidine residues and a thiol ester-like motif. Sequence analysis showed that MnproPO exhibited the highest amino acid sequence similarity (93%) to a proPO of Macrobrachium rosenbergii. The gene was expressed highly in haemocytes and weakly in hepatopancreas. Real-time PCR analysis revealed that the MnproPO expression increased significantly at 3, 12 and 24 h after KK-42 treatment, the PO activity also importantly rose from 6 to 48 h in KK-42-treated prawns and reached the maximum at 24 h with a 2.3-fold higher than that in control group. Injection of A. hydrophila could stimulate the MnproPO transcription and PO activity whether or not the prawns were pretreated by KK-42, the mRNA level increased obviously only at 3 h and 6 h after the bacterium injection (challenged control), but increased constantly during the phase of experiment except at 6 h under the condition of KK-42 pretreatment (challenged treatment group). The change trend of PO activity was basically similar to that of MnproPO expression. Our present results demonstrate that the MnproPO expression as well as PO activity may be induced by KK-42, which is likely one of the molecular mechanisms of KK-42 acts for increasing survival of the prawn infected with A. hydrophila.

Effects of depleted uranium on oxidative stress, detoxification, and defence parameters of zebrafish Danio rerio

In this study, we investigated the effects of depleted uranium (DU), the by-product of nuclear enrichment of uranium, on several parameters related to oxidative stress, detoxification, and the defence system in the zebrafish Danio rerio. Several parameters were recorded: phenoloxidase-like (PO) activity, reactive oxygen species (ROS) production, and 7-ethoxyresrufin-O-deethylase (EROD) activity. Experiments were performed on adult and larvae D. rerio. Adult fish were exposed for 28 days at 20 μg U/L followed by a 27-day depuration period. Eggs of D. rerio were exposed for 4 days at 0, 20, 100, 250, 500, and 1,000 μg U/L. Results showed that DU increased ROS production both in adult and in larvae even at the low concentrations tested and even during the depuration period for adult D. rerio. DU also modified PO-like activity, both in the D. rerio adult and larvae experiments, but in a more transient manner. EROD activity was not modified by DU, but sex effects were shown. Results are discussed by way of comparison with other known effects of uranium in fish. Overall, these results show that the mechanisms of action of DU in fish tend to be similar to the ones existing for mammals. These results encourage the development and use of innate immune biomarkers to understand the effects of uranium and, more generally, radionuclides on the fish immune system.

Immune gene discovery by expressed sequence tag (EST) analysis of hemocytes in the ridgetail white prawn Exopalaemon carinicauda

The ridgetail white prawn Exopalaemon carinicauda is one of the most important commercial species in eastern China. However, little information of immune genes in E. carinicauda has been reported. To identify distinctive genes associated with immunity, an expressed sequence tag (EST) library was constructed from hemocytes of E. carinicauda. A total of 3411 clones were sequenced, yielding 2853 ESTs and the average sequence length is 436 bp. The cluster and assembly analysis yielded 1053 unique sequences including 329 contigs and 724 singletons. Blast analysis identified 593 (56.3%) of the unique sequences as orthologs of genes from other organisms (E-value < 1e-5). Based on the COG and Gene Ontology (GO), 593 unique sequences were classified. Through comparison with previous studies, 153 genes assembled from 367 ESTs have been identified as possibly involved in defense or immune functions. These genes are categorized into seven categories according to their putative functions in shrimp immune system: antimicrobial peptides, prophenoloxidase activating system, antioxidant defense systems, chaperone proteins, clottable proteins, pattern recognition receptors and other immune-related genes. According to EST abundance, the major immune-related genes were thioredoxin (141, 4.94% of all ESTs) and calmodulin (14, 0.49% of all ESTs). The EST sequences of E. carinicauda hemocytes provide important information of the immune system and lay the groundwork for development of molecular markers related to disease resistance in prawn species.

Molecular cloning of prophenoloxidase and the effects of dietary β-glucan and rutin on immune response in hemocytes of the fleshy shrimp, Fenneropenaeus chinensis

To investigate the effects of dietary β-glucan (0.5 or 1 g kg⁻¹ diet: 0.5-BG, 1-BG) and rutin (0.5 or 1 g kg⁻¹ diet: 0.5-RT, 1-RT) after 10 days in the absence of pathogen challenge on the immune response of Fenneropenaeus chinensis, we determined total hemocyte count (THC) and the expression of four immune-related genes in hemocytes: those for prophenoloxidase (proPO), peroxinectin (PX), lipopolysaccharide and/or β-glucan binding protein (LGBP), and c-type lectin (CL). As a prerequisite for subsequent experiments, cDNA encoding proPO of the fleshy shrimp, Fenneropenaeus chinensis (f-proPO) was obtained from hemocytes; it had a full length of 3023 bp, with an open reading frame (ORF) of 2061bp, a 105-bp 5'-untranslated region, and a 906-bp 3'-untranslated region containing the poly A signal. The THCs of shrimp fed β-glucan of 1 g kg⁻¹ diet, and rutin of 1 g kg⁻¹ diet were significantly higher than that of the control (P < 0.05). The expression of proPO mRNA was slightly downregulated and that of LGBP mRNA was upregulated (except in 1-RT). PX and CL mRNA remained constitutively expressed in all groups. Our results reveal that β-glucan and rutin dietary supplements have minimal effect on immune response in the absence of pathogen challenge.

The expression of prophenoloxidase mRNA in red swamp crayfish, Procambarus clarkii, when it was challenged

The expression of the prophenoloxidase (proPO) gene was investigated in nine tissues of red swamp crayfish Procambarus clarkii, by real-time PCR after challenges by CpG oligodeoxynucleotide (ODN), Aeromonas hydrophila and white spot syndrome virus (WSSV). The results can be summarized as follows: (i) the expression level of the proPO gene in haemocytes was highest among nine studied tissues before the challenge; (ii) the expression of proPO increased in all studied tissues after stimulation by CpG ODN and WSSV, and also increased in all tissues, except the ovary, after the A. hydrophila challenge; (iii) the whole expression profiles were different, suggesting that different immune mechanisms may exist for crayfish that are resistant to WSSV and A. hydrophila, although the expression in haemocytes was similar before and after the WSSV and A. hydrophila challenges.

A novel type of prophenoloxidase from the kuruma prawn Marsupenaeus japonicus contributes to the melanization of plasma in crustaceans

Melanization is one of the major immune responses in arthropods. Prophenoloxidases (proPOs) catalyze the oxidation of mono- or o-diphenols, a reaction that is the key initial step of melanin formation. Well-characterized proPOs from crustaceans are synthesized in haemocytes and are released into plasma in response to microbial attack. However, PO activity does exist in the plasma of haemolymph without pathogenic infections. Here, we demonstrate that a novel type of proPO contributes to such PO activity in the plasma fraction of haemolymph of crustaceans. The novel enzyme, which was purified from the plasma of the kuruma prawn (Marsupenaeus japonicus), possessed strong and specific monophenol and o-diphenol oxidation activity compared with that of known haemocyte-type proPO. Amino acid sequence analyses indicated that this enzyme was distinct from the known proPO. The cDNA sequence and deduced amino acid sequence of this enzyme has a putative binuclear copper center, and showed approximately 30% and 20% identity with the primary structures of reported proPO and haemocyanin sequences of the kuruma prawn, respectively. Reverse transcription PCR analysis showed that this enzyme was synthesized in the hepatopancreas rather than in haemocytes. Although the primary structure and enzymatic properties of this novel enzyme suggested that it is a phenoloxidase, its biogenesis, tissue distribution, and oligomeric state resemble those of haemocyanin, which belongs to the same protein family (type III copper protein). This novel proPO enzyme may share a role with the already characterized version, itself a major component of the innate immune system in crustaceans.

Selectively enhanced expression of prophenoloxidase activating enzyme 1 (PPAE1) at a bacteria clearance site in the white shrimp, Litopenaeus vannamei

BACKGROUND

The prophenoloxidase-activating (PO activating) system plays an important role in the crustacean innate immunity, particularly in wound healing and pathogen defense. A key member of this system is prophenoloxidase-activating enzyme (PPAE), which is the direct activator of prophenoloxidase (proPO). Despite their importance in crustacean PO activating system, the studies on them remain limited.

RESULTS

Here we report on a PPAE of white shrimp, Litopenaeus vannamei (lvPPAE1), which showed 94% similarity to PPAE1 of Penaeus monodon. We found that lvPPAE1 in fluid hemocytes was down regulated after challenge by Vibrio harveyi but was enhanced when shrimps were exposed to a bacteria-rich environment for long-term. In vivo gene silence of lvPPAE1 by RNAi can significantly reduce the phenoloxidase activity (PO) and increase the susceptibility of shrimps to V. harveyi. Although lvPPAE1 was down-regulated in fluid hemocytes by Vibrio challenge, its expression increased significantly in gill after bacteria injection, which is the primary bacteria-clearance tissue.

CONCLUSION

Suppressed expression in fluid hemocytes and enhanced expression in gill indicates selectively enhanced expression at the bacterial clearance site. This is a novel feature for PPAE expression. The results will contribute to our understanding of the PO activating system in crustaceans.

A prophenoloxidase from Artemia sinica: cDNA cloning, expression and activity analysis during early development

To understand the defense mechanisms of Crustacean animals, brine shrimp Artemia sinica prophenoloxidase (AsproPO) cDNA was cloned and its expression at early developmental stages was examined by reverse-transcription PCR (RT-PCR) and semi-quantitative RT-PCR, and activity of phenoloxidase (PO) at different developmental stages was further detected by using l-3,4-dihydroxyphenylalanine (l-DOPA) as a specific substrate in this study. It was found that the full-length of AsproPO cDNA is 2125 bp and it contains an open reading frame of 2100 bp encoding a protein of 699 amino acids. The deduced amino acid sequence of AsproPO has two putative copper binding sites highly conserved in Arthropods. Semi-quantitative RT-PCR analyses showed that the gene of AsproPO expressed at Emergence, Instar I and Instar II stages but did not at 0 h and 6 h stages. Activity measurement showed that PO activity could only be detected at Instar II stage but the other measured stages. All these implied that Artemia proPO immune system was complexly modulated during early development.

Reactive oxygen system plays an important role in shrimp Litopenaeus vannamei defense against Vibrio parahaemolyticus and WSSV infection

The present study investigated the in vivo hemocytic and hepatopancreatic response to Vibrio parahaemolyticus and white spot syndrome virus (WSSV) injection in shrimp Litopenaeus vannamei. The proliferation of bacteria and virus in shrimp, animal mortality, total hemocyte counts (THCs), phenoloxidase (PO) activity, respiratory burst, and gene expression of immune factors associated with immune recognition (lectin), prophenoloxidase (proPO) activation, and the anti-microorganism (lysozyme) and active oxygen defense response (including respiratory burst, cytosolic manganese superoxide dismutase [C-MnSOD], and catalase [CAT]) were quantified. Shrimp death rate increased significantly and was time-dependent after V. parahaemolyticus or WSSV injection. The production of superoxide anion, and the gene expression including lectin in hemocytes, proPO in the hepatopancreas, lysozyme, C-MnSOD and CAT could be induced by injection with V parahaemolyticus and WSSV. The highest value of lysozyme was in the hemocytes with 66.59 times (at 3 h) greater expression than in the control group after WSSV injection and 3.69 times (24 h) greater than in the control group after V parahaemolyticus injection. In the hepatopancreas, CAT expression showed a significant increase, with up to 16 times greater expression than in the control group at 6 h postinjection with WSSV and 7.02 times greater expression than in the control group at 48 h postinjection with V parahaemolyticus (p < 0.05). However, significant decreases in PO activity and proPO transcripts in hemocytes and lectin transcripts in the hepatopancreas were detected after V parahaemolyticus and WSSV injection (p < 0.05). The results suggest that lysozyme, the antioxidase system, and reactive oxygen species might play a crucial role in shrimp defense against bacterial and viral infection.

Priming the prophenoloxidase system of Artemia franciscana by heat shock proteins protects against Vibrio campbellii challenge

Like other invertebrates, the brine shrimp Artemia franciscana relies solely on innate immunity, which by definition lacks adaptive characteristics, to combat against invading pathogens. One of the innate mechanisms is melanisation of bacteria mediated by the activation of the prophenoloxidase (proPO) system. The 70 kDa heat shock proteins (Hsp70) derived from either prokaryote (Escherichia coli) or eukaryote (Artemia), well conserved and immune-dominant molecules, protect Artemia against Vibrio campbellii. However, the molecular mechanisms by which these proteins protect Artemia against Vibrio campbellii infection are unknown. Here we demonstrated that feeding gnotobiotically grown Artemia with either Artemia Hsp70 or the E. coli Hsp70 equivalent DnaK, each overproduced in E. coli, followed by V. campbellii challenge enhanced the proPO system, at both mRNA and protein activity levels. Additionally, the Artemia fed with these proteins survived well in a Vibrio challenge assay. These results indicated that Hsp70s derived from either prokaryotic or eukaryotic sources generate protective immunity in the crustacean Artemia against V. campbellii infection by priming the proPO system. This is apparently the first in vivo report on priming activity of Hsp70 in an invertebrate.

Prophenoloxidase activating enzyme-III from giant freshwater prawn Macrobrachium rosenbergii: characterization, expression and specific enzyme activity

The prophenoloxidase activating system is an important innate immune response against microbial infections in invertebrates. The major enzyme, phenoloxidase, is synthesized as an inactive precursor and its activation to an active enzyme is mediated by a cascade of clip domain serine proteinases. In this study, a cDNA encoding a prophenoloxidase activating enzyme-III from the giant freshwater prawn Macrobrachium rosenbergii, designated as MrProAE-III, was identified and characterized. The full-length cDNA contains an open reading frame of 1110 base pair (bp) encoding a predicted protein of 370 amino acids including an 22 amino acid signal peptide. The MrProAE-III protein exhibits a characteristic sequence structure of a long serine proteases-trypsin domain and an N- and C-terminal serine proteases-trypsin family histidine active sites, respectively, which together are the characteristics of the clip-serin proteases. Sequence analysis showed that MrProAE-III exhibited the highest amino acid sequence similarity (63%) to a ProAE-III from Atlantic blue crab, Callinectes sapidus. MrProAE-III mRNA and enzyme activity of MrProAE-III were detectable in all examined tissues, including hepatopancreas, hemocytes, pleopods, walking legs, eye stalk, gill, stomach, intestine, brain and muscle with the highest level of both in hepatopancreas. This is regulated after systemic infectious hypodermal and hematopoietic necrosis virus infection supporting that it is an immune-responsive gene. These results indicate that MrProAE-III functions in the proPO system and is an important component in the prawn immune system.

Prophenoloxidase from Pieris rapae: gene cloning, activity, and transcription in response to venom/calyx fluid from the endoparasitoid wasp Cotesia glomerata

Prophenoloxidase (PPO) plays an important role in melanization, necessary for defense against intruding parasitoids. Parasitoids have evolved to inject maternal virulence factors into the host hemocoel to suppress hemolymph melanization for the successful development of their progeny. In this study, the full-length complementary DNA (cDNA) of a Pieris rapae PPO was cloned. Its cDNA contained a 2 076-base pair (bp) open reading frame (ORF) encoding 691 amino acids (aa). Two putative copper-binding sites, a proteolytic activation site, three conserved hemocyanin domains, and a thiol ester motif were found in the deduced amino acid sequence. According to both multiple alignment and phylogenetic analysis, P. rapae PPO gene cloned here is a member of the lepidopteran PPO-2 family. Injection of Cotesia glomerata venom or calyx fluid resulted in reduction of P. rapae hemolymph phenoloxidase activity, demonstrating the ability to inhibit the host's melanization. Real-time reverse transcriptase polymerase chain reaction (RT-PCR) showed that transcripts of P. rapae PPO-2 in the haemocytes from larvae had not significantly changed following venom injection, suggesting that the regulation of PPO messenger RNA (mRNA) expression by venom was not employed by C. glomerata to cause failure of melanization in parasitized host. While decreased P. rapae PPO-2 gene expression was observed in the haemocytes after calyx fluid injection, no detectable transcriptional change was induced by parasitization, indicating that transcriptional down-regulation of PPO by calyx fluid might play a minor role involved in inhibiting the host's melanization.

Expression of immune-related genes in the digestive organ of shrimp, Penaeus monodon, after an oral infection by Vibrio harveyi

In all previous studies, to study shrimp immune response, bacteria were directly injected into the shrimp body and as a consequence the initial step of a natural interaction was omitted. In this study we have instead used an immersion technique, which is a more natural way of establishing an infection, to study immune responses in black tiger shrimp (Penaeus monodon). Normally, Vibrio harveyi (Vh) is highly pathogenic to post-larval shrimp, but not to juveniles which usually resist an infection. In post-larvae, Vh causes a massive destruction of the digestive system, especially in the hepatopancreas and in the anterior gut. We have therefore investigated changes in transcription levels of fifteen immune-related genes and morphological changes in juvenile shrimp following an immersion of shrimp in Vh suspension. We found that a pathogenic bacterium, Vh, has the capacity to induce a local expression of some immune-related genes in shrimp after such a bacterial immersion. Our results show that in the juvenile gut small changes in expression of the antimicrobial peptide (AMP) genes such as antilipopolysaccharide factor isoform 3, crustin and penaeidin were observed. However some other genes were more strongly induced in their expression compared to the AMP genes. C-type lectin, Tachylectin 5a1 and mucin-like peritrophic membrane were increased in their expression and the C-type lectin was affected most in its expression. Several other examined genes did not change their expression levels. By performing histology studies it was found that Vh infection induced a strong perturbation of the midgut epithelium in some regions. As a consequence, the epithelial cells and basement membrane of the infected site were completely damaged and necrotic and massive hemocyte infiltration occurred underneath the affected tissue to combat the infection.

Immune response and gene expression in shrimp (Litopenaeus vannamei) hemocytes and hepatopancreas against some pathogen-associated molecular patterns

The effects of some pathogen-associated molecular patterns (PAMPs) (laminarin, LPS and poly I:C) on total hemocyte counts (THC), phenoloxidase (PO) activity, superoxide anion production and lectin, prophenoloxidase, lysozyme, cytosolic manganese superoxide dismutase (C-MnSOD) and catalase (CAT) gene expression were studied. The results showed that the production or activity of most tested immune factors and the expression of most tested genes were up-regulated after stimulation with PAMPs, among which the highest value of lectin with 4.4 times as much as that of the control group appeared at 6 h in hemocytes, of CAT with 47 times as much as that of the control group appeared at 12 h in hepatopancreas, and with 2.7 times higher than that of the control group at 24 h of C-MnSOD in hepatopancreas after laminarin injection. The peak value of proPO, lysozyme and C-MnSOD appeared at 6 h in hepatopancreas, 24 h in hepatopancreas and 24 h in hemocytes after LPS injection, respectively. The highest expression level of lysozyme appeared at 12 h in hemocytes after poly I:C injection. However, significant decreases of PO activity in hemocytes and lectin expression in hepatopancreas were found after poly I:C injection, and a dramatic down-regulation of proPO expression from 3 h to 48 h was found in hemocytes after injection with laminarin, LPS and poly I:C. The results suggest that the shrimp immune response could be activated or inhibited by different PAMPs, and that the hepatopancreas also plays a key role by synthesizing immune factors.

ProPhenolOxidase in Daphnia magna: cDNA sequencing and expression in relation to resistance to pathogens

Invertebrates utilise the innate immune system when defending against pathogenic attack. However, except for some effectors as proPhenolOxidase (proPO), the innate immune response is less well understood outside model insect species, and its role in natural host-pathogen systems is generally not well documented. We have therefore initiated studies on the immune response of the crustacean Daphnia when exposed to the specialist endobacterial pathogen, Pasteuria ramosa. This study was focused on the proPO gene of Daphnia magna. D. magna possesses a single copy of proPO (as does its congener, D. pulex), but there was some evidence of alternative splicing. Analyses of sequence similarity in a range of arthropod taxa suggested that the proPO gene in Daphnia was as dissimilar to other crustaceans as it was to insects, while analysis on intraspecific variation indicated that the gene is highly conserved. ProPO was found to be significantly up-regulated within 1-4h following exposure to the bacteria. This is the first evidence of a Daphnia immune response, and our observations raise the possibility that the PhenolOxidase (PO) cascade is involved in the defence against pathogenic gram-positive bacteria.

Two prophenoloxidases are important for the survival of Vibrio harveyi challenged shrimp Penaeus monodon

Phenoloxidase (PO) plays an important role in arthropod melanization. Previously, a prophenoloxidase (PmproPO1) gene was cloned and characterized from the hemocytes of the black tiger shrimp, Penaeus monodon. In the present study, we report a novel proPO gene (PmproPO2) belonging to the proPO family identified from the P. monodon EST database (http://pmonodon.biotec.or.th). The full-length sequence of PmproPO2 consists of 2513bp encoding a predicted 689 amino acid residues with a calculated molecular mass and pI of 79.21kDa and 6.69, respectively. It is predicted to possess all the expected features of proPO members, including two putative tyrosinase copper-binding motifs with six histidine residues and a thiol ester-like motif, sharing 67% amino acid sequence identity with PmproPO1. Tissue distribution analyses revealed that the two proPO genes are primarily expressed in the hemocyte. Gene silencing of either PmproPO1 or PmproPO2 or both by RNA interference (RNAi) resulted in a significant decrease in the respective endogenous proPO mRNA level in hemocytes and a reduction of total PO enzyme activity by 75, 73 and 88%, respectively. Experimental infection of P. monodon with the pathogenic bacterium, Vibrio harveyi, revealed that PmproPO silenced shrimps were more susceptible to bacterial infection than the control GFP injected shrimps, and suggesting that the two proPOs are important components in the shrimp immune defense.

A novel prophenoloxidase 2 exists in shrimp hemocytes

The prophenoloxidase (proPO)-activating system in crustaceans and other arthropods is regarded as a constituent of the immune system and plays an important role in defense against pathogens. Hitherto in crustaceans, only one proPO gene per species has been identified. Here we report the identification of a novel proPO-2 (LvproPO-2) from the hemocytes of Litopenaeus vannamei, which shows 72% identity to proPO-1 (LvproPO-1) cloned previously. Northern blotting analysis and quantitative real-time PCR reveal that LvproPO-2 is mainly expressed in the hemocytes, and its expression is down-regulated in shrimp challenged with white spot syndrome virus (WSSV). Western blotting analysis shows that most LvproPO-2/LvPO-2 (L. vannamei phenoloxidase-2) exists in the hemocytes, but not in plasma of L. vannamei. LvproPO-2/LvPO-2 could be detected on the hemocyte surface and the nucleus of hemocytes by indirect immunofluorescence assay (IFA). These findings provide insight into the molecular biological basis for further studying on the defense mechanism of shrimp innate immunity, especially on the proPO-activating system and melanization cascade of shrimp.

The immune response of white shrimp Litopenaeus vannamei following Vibrio alginolyticus injection

White shrimp Litopenaeus vannamei injected with saline, and injected with tryptic soy broth (TSB)-grown Vibrio alginolyticus at 1.0 x 10(5) and 1.8 x 10(5) colony-forming units (cfu) shrimp(-1) were examined for hyaline cell (HC) counts, granular cell (GC) counts, total haemocyte counts (THCs), phenoloxidase (PO) activity, respiratory burst (RB) and superoxide dismutase (SOD) activity after 1-168 h. Shrimp that received no injection served as the control. The shrimps which received V. alginolyticus at both doses showed significant decreases in these parameters after 6-96 h. The values for HC and SOD activity decreased earlier and then RB. The time to cause maximum depletion of haemocytes (haemocytopenia), PO activity, RB, and SOD activity were 12, 72, 48, and 24 h post-injection, respectively. The HC, GC, and RB returned to the original values earlier at 72 h, followed by SOD activity at 96 h, and then PO activity at 168 h post-infection. It was concluded that an injection of V. alginolyticus rapidly reduced the shrimp's immunity by decreasing HC, GC, SOD activity, RB, and PO activity within 3-24 h, followed by a slow recovery during 72-168 h post-injection. Furthermore, white shrimp L. vannamei which received V. alginolyticus showed a 6-9 h later response in PO activity, and a 72-96 h later recovery of PO activity, compared to the responses in RB and SOD activity indicating their roles in shrimp defence and immunity.

Characterization of a prophenoloxidase from hemocytes of the shrimp Litopenaeus vannamei that is down-regulated by white spot syndrome virus

Previously, a prophenoloxidase (proPO) gene (named proPO-a here) from hemocytes of Litopenaeus vannamei was isolated. Here, a proPO-b gene was also identified and characterized from hemocytes of L. vannamei. The cDNA sequences of proPO-a and proPO-b were compared, and it was found that both proPOs had a microsatellite DNA site near the 3' end of the open reading frame (ORF). However, the microsatellite DNA of proPO-b contained a compound imperfect simple sequence repeats (SSR) ((CT)(38)(CA)(8)(AA)(CA)(3)(TA)(CA)(14)), which was different from the perfect one ((CT)(20)) of proPO-a, and the cDNA sequences of proPO-a and proPO-b prior to the microsatellite DNA were almost identical, but differed after the microsatellite DNA. ProPO-b (3232 bp) was longer than proPO-a (2471 bp). The 3' UTR sequence after SSR of proPO-a was not detected in shrimp randomly collected from five different geographically separate populations by reverse-transcription polymerase chain reaction (RT-PCR). On the contrary, the 3' UTR sequence of proPO-b was detected in all five groups of shrimps. Northern blot analysis showed that a transcript at approximately 3.2kb, but not 2.5kb, was detected mainly in hemocytes, and also present in midgut, gill, heart, stomach, posterior midgut cecum, and cuticular epidermis, but no signal was detected in hepatopancreas and musculature. RT-PCR and quantitative real-time RT-PCR analysis showed similar results of the proPO-b expression profile in these shrimp tissues. We also observed that proPO-b expression was down-regulated in shrimp challenged with white spot syndrome virus (WSSV). Our results suggest that proPO-b is a main transcript form of proPO gene in L. vannamei, and it may play a role in defence against WSSV virus.