Purification and characterization of White Spot syndrome virus (WSSV) produced in an alternate host: crayfish, Cambarus clarkii

Experimental Infection Models and Their Usefulness for White Spot Syndrome Virus (WSSV) Research in Shrimp

White spot syndrome virus (WSSV) is marked as one of the most economically devastating pathogens in shrimp aquaculture worldwide. Infection of cultured shrimp can lead to mass mortality (up to 100%). Although progress has been made, our understanding of WSSV's infection process and the virus-host-environment interaction is far from complete. This in turn hinders the development of effective mitigation strategies against WSSV. Infection models occupy a crucial first step in the research flow that tries to elucidate the infectious disease process to develop new antiviral treatments. Moreover, since the establishment of continuous shrimp cell lines is a work in progress, the development and use of standardized in vivo infection models that reflect the host-pathogen interaction in shrimp is a necessity. This review critically examines key aspects of in vivo WSSV infection model development that are often overlooked, such as standardization, (post)larval quality, inoculum type and choice of inoculation procedure, housing conditions, and shrimp welfare considerations. Furthermore, the usefulness of experimental infection models for different lines of WSSV research will be discussed with the aim to aid researchers when choosing a suitable model for their research needs.

Research progress of pattern recognition receptors in red swamp crayfish (Procambarus clarkii)

Though Procambarus clarkii (red swamp crayfish) is a lower invertebrate, it has nonetheless developed a complex innate immune system. The crayfish farming industry has suffered considerable economic losses in recent years as a consequence of bacterial and viral diseases. Hence, perhaps the most effective ways to prevent microbial infections in P. clarkii are to examine and elucidate its innate immunity. The first step in the immune response is to recognize pathogen-associated molecular patterns (PAMPs) through pattern recognition receptors (PRRs). PRRs are expressed mainly on immune cell surfaces and recognize at least one PAMP. Thence, downstream immune responses are activated and pathogens are phagocytosed. To date, the PRRs identified in P. clarkii include Toll-like receptors (TLRs), lectins, fibrinogen-related proteins (FREPs), and β-1,3-glucan-binding proteins (BGRPs). The present review addresses recent progress in research on PRRs and aims to provide guidance for improving immunity and preventing and treating infectious diseases in P. clarkii.

Multiple Nucleocapsid Structural Forms of Shrimp White Spot Syndrome Virus Suggests a Novel Viral Morphogenetic Pathway

White spot syndrome virus (WSSV) is a very large dsDNA virus. The accepted shape of the WSSV virion has been as ellipsoidal, with a tail-like extension. However, due to the scarcity of reliable references, the pathogenesis and morphogenesis of WSSV are not well understood. Here, we used transmission electron microscopy (TEM) and cryogenic electron microscopy (Cryo-EM) to address some knowledge gaps. We concluded that mature WSSV virions with a stout oval-like shape do not have tail-like extensions. Furthermore, there were two distinct ends in WSSV nucleocapsids: a portal cap and a closed base. A C14 symmetric structure of the WSSV nucleocapsid was also proposed, according to our Cryo-EM map. Immunoelectron microscopy (IEM) revealed that VP664 proteins, the main components of the 14 assembly units, form a ring-like architecture. Moreover, WSSV nucleocapsids were also observed to undergo unique helical dissociation. Based on these new results, we propose a novel morphogenetic pathway of WSSV.

Ring-stacked capsids of white spot syndrome virus and structural transitions with genome ejection

White spot syndrome virus (WSSV) is one of the largest DNA viruses and the major pathogen responsible for white spot syndrome in crustaceans. The WSSV capsid is critical for genome encapsulation and ejection and exhibits the rod-shaped and oval-shaped structures during the viral life cycle. However, the detailed architecture of the capsid and the structural transition mechanism remain unclear. Here, using cryo-electron microscopy (cryo-EM), we obtained a cryo-EM model of the rod-shaped WSSV capsid and were able to characterize its ring-stacked assembly mechanism. Furthermore, we identified an oval-shaped WSSV capsid from intact WSSV virions and analyzed the structural transition mechanism from the oval-shaped to rod-shaped capsids induced by high salinity. These transitions, which decrease internal capsid pressure, always accompany DNA release and mostly eliminate the infection of the host cells. Our results demonstrate an unusual assembly mechanism of the WSSV capsid and offer structural insights into the pressure-driven genome release.

Coumarin protects Cherax quadricarinatus (red claw crayfish) against white spot syndrome virus infection

Coumarin is a natural compound from plants with the molecular formula C₉H₆O₂. Cherax quadricarinatus (red claw crayfish) is an aquaculture species exhibiting high economic efficiency and quality that is mainly distributed and cultivated in the southeast provinces in China. In order to identify an effective herbal immunopotentiator against white spot syndrome virus (WSSV) infection, this study examined the effect of coumarin as a feed additive in protecting C. quadricarinatus against WSSV infection. The expression of immune-related genes and WSSV copies were analyzed by Q-PCR. Challenge experiments were conducted to analyze the survival rate and determine the optimal concentration of coumarin. The Phenoloxidase activity (PO), Acid phosphatase (ACP) and superoxide dismutase activity (SOD) activity and lysozyme activity were also analyzed. Total hemocyte count (THC) and apoptosis rate were determined by flow cytometry. The WSSV challenge results showed that 40 mg/kg coumarin reduced the mortality of C. quadricarinatus and delayed the WSSV infection process. Further investigation showed that coumarin treatment had a positive effect on the important immunity-related parameters THC, ACP activity, SOD activity, LZM and PO activity. Coumarin up-regulated the expression of proPO, JAK, STAT, ALF, Hsp70 and down-regulated the expression of caspase at the mRNA level. After WSSV infection, the hemocyte apoptosis rate was lower in the 40 mg/kg coumarin + WSSV group compared with the WSSV only group. These data illustrate that coumarin enhances innate immunity in C. quadricarinatus and exhibits a protective effect against WSSV infection by reducing the number of WSSV copies and slowing the process of infection, which provides a potential theoretical basis for studies of coumarin as a new aquatic feed additive in crustacean aquaculture.

Microbial pathogens of freshwater crayfish: A critical review and systematization of the existing data with directions for future research

Despite important ecological role and growing commercial value of freshwater crayfish, their diseases are underresearched and many studies examining potential crayfish pathogens do not thoroughly address their epizootiology, pathology or biology. This study reviews over 100 publications on potentially pathogenic viruses, bacteria, fungi and fungal-like microorganisms reported in crayfish and systematizes them based on whether pathogenicity has been observed in an analysed species. Conclusions on pathogenicity were based on successful execution of infectivity trials. For 40.6% of examined studies, microbes were successfully systematized, while for more than a half (59.4%) no conclusion on pathogenicity could be made. Fungi and fungal-like microorganisms were the most studied group of microbes with the highest number of analysed hosts, followed by bacteria and viruses. Our analysis demonstrated the need for: (a) inclusion of higher number of potential host species in the case of viruses, (b) research of bacterial effects in tissues other than haemolymph, and (c) more research into potential fungal and fungal-like pathogens other than Aphanomyces astaci. We highlight the encountered methodological challenges and biases and call for a broad but standardized framework for execution of infectivity trials that would enable systematic data acquisition on interactions between microbes and the host.

Metabolic responses of whiteleg shrimp to white spot syndrome virus (WSSV)

Outbreaks of white spot syndrome virus (WSSV) have caused serious damage to penaeid shrimp aquaculture worldwide. Despite great efforts to characterize the virus, the conditions that lead to infection and the infection mechanisms, there is still a lack of understanding regarding these complex virus-host interactions, which is needed to develop consistent and effective treatment methods for WSSV. In this study, we used a gas chromatography - mass spectrometry (GC-MS)-based metabolomics approach to compare the metabolite profiles of gills, haemolymph and hepatopancreas from whiteleg shrimp (Penaeus vannamei) exposed to WSSV and corresponding controls. The results revealed clear discriminations between metabolite profiles of WSSV-challenged shrimp and controlled shrimp in each tissue. The responses of shrimp gills to WSSV infection were characterized by increases of many fatty acids and amino acids in WSSV-challenged shrimp compared to the controls. Changes in haemolymph metabolite profiles include the increased levels of itaconic acid, energy-related metabolites, metabolites in glutathione cycle and decrease of amino acids. The WSSV challenge led to the decreases of several fatty acids and amino acids and increases of other amino acids, lactic acid and other organic compounds (levulinic acid, malonic acid and putrescine) in hepatopancreas. These alterations of shrimp metabolites suggest several immune responses of shrimp to WSSV in a tissue-specific manner, including upregulation of osmoregulation, antimicrobial activity, metabolic rate, gluconeogenesis, glutathione pathway in control of oxidative stress and shift from aerobic to anaerobic metabolism in shrimp which indicates the Warburg effect. The findings from this study provide a better understanding of molecular process of shrimp response against WSSV invasion which may be useful for development of disease management strategies.

WSSV proteins and DNA genome released by ultrasonic rupture can infect crayfish as effectively as intact virions

Proteins and nucleic acids from ultrasonically ruptured white spot syndrome virus (WSSV) can infect crayfish and cause death as effectively as intact WSSV virions. In this study, ultrasound was used to rupture the virus and the resulting suspension was filtered through a 50 nm membrane. Analysis by PCR and SDS-PAGE showed that both viral genes (VP19, VP26, VP28 and DNA polymerase) and proteins (VP15, VP19, VP26 and VP28) were present in the filtered solution. Electron microscopy showed that there were no intact virions in the filtered solution. When crayfish were injected with the filtered solution or with intact WSSV, the mortality in each group was 100 %. The same result was seen when crayfish were challenged orally with the filtered solution and intact WSSV. The filtered solution of ultrasonically ruptured virus, which contains viral proteins and residual DNA genome, can thus infect the host as effectively as intact virions. When the solution of viral proteins and residual DNA genome was digested with DNase I and then injected into crayfish, the survival rate was 100 %. We also found that, although viral proteins (except VP15) in the solution of ruptured virus were destroyed by treatment with DNase I, DNase I did not destroy the structural proteins of intact virions. A remaining viral protein in the DNase I-treated solution protects the DNA genome from degradation and we concluded that this protein is VP15, which is a DNA-binding protein. Our study highlights the extreme danger in producing vaccines from proteins obtained by ultrasonic rupture of viruses sincethe viral DNA genome is difficult to degrade and, if present, will lead to viral infection.

The miRNAs profiling revealed by high-throughput sequencing upon WSSV infection in mud crab Scylla paramamosain

microRNAs (miRNAs) are known to regulate various immune functions by silencing the target genes in both vertebrates and invertebrates. However, in mud crab Scylla paramamosain, the role of miRNAs during the response to virus invasion remains unclear. To investigate the roles of miRNAs in S. paramamosain during virus infection, the mud crab was challenged with white spot syndrome virus (WSSV) and then subjected to the transcriptional analysis at different conditions. The results of high-throughput sequencing revealed that 940,379 and 1,306,023 high-quality mappable reads were detected in the hemocyte of normal and WSSV-infected mud crabs, respectively. Besides, the total number of 261 unique miRNAs were identified. Among them, 131 miRNAs were specifically expressed in the hemocytes of normal mud crabs, 46 miRNAs were specifically transcribed in those of WSSV-infected individuals, the other 84 miRNAs were expressed in both normal and WSSV-infected individuals. Furthermore, a number of 152 (89 down-regulated and 63 up-regulated) miRNAs were found to be differentially expressed in the WSSV-infected hemocytes, normalized to the controls. The identified miRNAs were subjected to GO analysis and target gene prediction and the results suggested that the differentially regulated miRNAs were mainly correlated with the changes of the immune responses of the hemocytes, including phagocytosis, melanism, and apoptosis as well. Taken together, the results demonstrated that the expressed miRNAs during the virus infection were mainly involved in the regulation of immunological pathways in mud crabs. Our findings not only enrich the understanding of the functions of miRNAs in the innate immune system but also provide some novel potential targets for the prevention of WSSV infection in crustaceans.

VP19 is important for the envelope coating of white spot syndrome virus

VP19 is a major envelope protein of white spot syndrome virus (WSSV), an important pathogen of farmed shrimp. However, the exact function of VP19 in WSSV assembly and infection is unknown. To understand the function of VP19, the gene was knocked down by RNA interference. We found that the dsRNA specific for vp19 gene dramatically reduced the replication of WSSV genomic DNA in infected animals. Further investigation by transmission electron microscopy showed that inhibition of VP19 prevented envelope coating of progeny virions, resulting in a high amount of immature virus particles without outer layer (envelope) in the host cells. This finding was further confirmed by SDS-PAGE analysis, which showed the loss of VP19 and other envelope proteins from the improperly assembled virions. These results suggest that VP19 is essential for WSSV envelope coating.

Proliferation dynamics of WSSV in crayfish, Procambarus clarkii, and the host responses at different temperatures

The replication profile of white spot syndrome virus (WSSV) in crayfish, Procambarus clarkii, at different water temperature was investigated in this study. The WSSV detections were negative at 15 ± 1°C, and the natural infection ratio increased at 19 ± 1°C (24.2% ± 2.25%), reached 100% at 25 ± 1°C and decreased at 30 ± 1°C (93.2% ± 3.37%). The WSSV genome copies number was much higher at 25 ± 1°C (≥5 × 10^{6.45 ± 0.35} /mg) than at 15 ± 1°C (≤5 × 10^{1.13 ± 0.12} /mg), 19 ± 1°C (≤5 × 10^{2.74 ± 0.48} /mg) and 32 ± 1°C (≤5 × 10^{3.18 ± 0.27} /mg). Meanwhile, the significant transcription signals of immediate early gene ie1 and late gene vp28 and a large number of virus particles were detected in epitheliums of stomach, gut and gill, hepatopancreas, heart and muscle cells at 25 ± 1°C by using in situ hybridization (ISH) and transmission electron microscopy. The experimental infection of P. clarkii with WSSV infection showed reduced mortality and lower virus copies number at 19 ± 1°C (23.51% ± 0.84%, ≤5 × 10^{3.41 ± 0.11} /mg) and 32 ± 1°C (38.42% ^±  1.21%, ≤5 × 10^{3.72 ± 0.13} /mg) compared to 25 ± 1°C (100%, ≥5 × 10^{4.99 ± 0.24} /mg). The water temperature regulated the transcription of immune-related genes (crustin2, prophenoloxidase (proPO) and heat shock protein70 (Hsp70)), with some differences between WSSV treatments and control treatments. These results demonstrate that water temperature has effect on WSSV proliferation, which may due to transcriptional response of immune-related genes to temperature.

A transcriptome study on Macrobrachium nipponense hepatopancreas experimentally challenged with white spot syndrome virus (WSSV)

White spot syndrome virus (WSSV) is one of the most devastating pathogens of cultured shrimp, responsible for massive loss of its commercial products worldwide. The oriental river prawn Macrobrachium nipponense is an economically important species that is widely farmed in China and adult prawns can be infected by WSSV. However, the molecular mechanisms of the host pathogen interaction remain unknown. There is an urgent need to learn the host pathogen interaction between M. nipponense and WSSV which will be able to offer a solution in controlling the spread of WSSV. Next Generation Sequencing (NGS) was used in this study to determin the transcriptome differences by the comparison of control and WSSV-challenged moribund samples, control and WSSV-challenged survived samples of hepatopancreas in M. nipponense. A total of 64,049 predicted unigenes were obtained and classified into 63 functional groups. Approximately, 4,311 differential expression genes were identified with 3,308 genes were up-regulated when comparing the survived samples with the control. In the comparison of moribund samples with control, 1,960 differential expression genes were identified with 764 genes were up-regulated. In the contrast of two comparison libraries, 300 mutual DEGs with 95 up-regulated genes and 205 down-regulated genes. All the DEGs were performed GO and KEGG analysis, overall a total of 85 immune-related genes were obtained and these gene were groups into 13 functions and 4 KEGG pathways, such as protease inhibitors, heat shock proteins, oxidative stress, pathogen recognition immune receptors, PI3K/AKT/mTOR pathway, MAPK signaling pathway and Ubiquitin Proteasome Pathway. Ten genes that valuable in immune responses against WSSV were selected from those DEGs to furture discuss the response of host to WSSV. Results from this study contribute to a better understanding of the immune response of M. nipponense to WSSV, provide information for identifying novel genes in the absence of genome of M. nipponense. Furthermore, large number of transcripts obtained from this study could provide a strong basis for future genomic research on M. nipponense.

An optimized double-antibody sandwich ELISA for quantitative detection of WSSV in artificially infected crayfish

Developing a rapid, accurate and quantitative method for detecting white spot syndrome virus (WSSV) is extremely urgent and critical for reducing the risk of white spot disease outbreaks. In the present work, an optimized double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) was developed for quantitative detection of WSSV. The method employed rabbit polyclonal antibodies against WSSV as the capture antibody and previously produced anti-WSSV monoclonal antibodies as the detector antibody. A standard curve of the log concentration of WSSV versus OD value was established, which was linear in the concentration range of 120-7680ng/mL, and the linear regression equation was y=0.166x-0.151. Viral proteins in different tissues of crayfish (Procambarus clarkia) post artificial infection with WSSV were quantitatively measured using the DAS-ELISA. WSSV proliferated quickly within 60h post infection and gradually slowed down afterwards. According to the linear regression relationship, the viral proteins in hemolymph, gut and gonad were firstly able to be quantified at 24h post infection with the concentrations of 186, 158 and 128ng/mL, respectively. These three tissues also contained higher viral proteins than the gill, heart, hepatopancreas and muscle during the entire infection period. The viral protein concentration in gut reached the highest level of 6220ng/mL at 72h post infection. Real time quantitative PCR was also used to detect the dynamic change of viral copies in crayfish hemolymph post WSSV infection, with similar results for both assays. The developed DAS-ELISA could detect WSSV propagation from initial to moribund stage in infected crayfish and demonstrated potential application for diagnosis of WSSV.

A transcriptome study on Macrobrachium rosenbergii hepatopancreas experimentally challenged with white spot syndrome virus (WSSV)

The world production of shrimp such as the Malaysian giant freshwater prawn, Macrobrachium rosenbergii is seriously affected by the white spot syndrome virus (WSSV). There is an urgent need to understand the host pathogen interaction between M. rosenbergii and WSSV which will be able to provide a solution in controlling the spread of this infectious disease and lastly save the aquaculture industry. Now, using Next Generation Sequencing (NGS), we will be able to capture the response of the M. rosenbergii to the pathogen and have a better understanding of the host defence mechanism. Two cDNA libraries, one of WSSV-challenged M. rosenbergii and a normal control one, were sequenced using the Illumina HiSeq™ 2000 platform. After de novo assembly and clustering of the unigenes from both libraries, 63,584 standard unigenes were generated with a mean size of 698bp and an N50 of 1137bp. We successfully annotated 35.31% of all unigenes by using BLASTX program (E-value <10-5) against NCBI non-redundant (Nr), Swiss-Prot, Kyoto Encyclopedia of Genes and Genome pathway (KEGG) and Orthologous Groups of proteins (COG) databases. Gene Ontology (GO) assessment was conducted using BLAST2GO software. Differentially expressed genes (DEGs) by using the FPKM method showed 8443 host genes were significantly up-regulated whereas 5973 genes were significantly down-regulated. The differentially expressed immune related genes were grouped into 15 animal immune functions. The present study showed that WSSV infection has a significant impact on the transcriptome profile of M. rosenbergii's hepatopancreas, and further enhanced the knowledge of this host-virus interaction. Furthermore, the high number of transcripts generated in this study will provide a platform for future genomic research on freshwater prawns.

VP24 Is a Chitin-Binding Protein Involved in White Spot Syndrome Virus Infection

Oral ingestion is the major route of infection for the white spot syndrome virus (WSSV). However, the mechanism by which virus particles in the digestive tract invade host cells is unknown. In the present study, we demonstrate that WSSV virions can bind to chitin through one of the major envelope proteins (VP24). Mutagenesis analysis indicated that amino acids (aa) 186 to 200 in the C terminus of VP24 were required for chitin binding. Moreover, the P-VP24186-200 peptide derived from the VP24 chitin binding region significantly inhibited the VP24-chitin interaction and the WSSV-chitin interaction, implying that VP24 participates in WSSV binding to chitin. Oral inoculation experiments showed that P-VP24186-200 treatment reduced the number of virus particles remaining in the digestive tract during the early stage of infection and greatly hindered WSSV proliferation in shrimp. These data indicate that binding of WSSV to chitin through the viral envelope protein VP24 is essential for WSSV per os infection and provide new ideas for preventing WSSV infection in shrimp farms.

IMPORTANCE

In this study, we show that WSSV can bind to chitin through the envelope protein VP24. The chitin-binding domain of VP24 maps to amino acids 186 to 200 in the C terminus. Binding of WSSV to chitin through the viral envelope protein VP24 is essential for WSSV per os infection. These findings not only extend our knowledge of WSSV infection but also provide new insights into strategies to prevent WSSV infection in shrimp farms.

Crayfish hematopoietic tissue cells but not hemocytes are permissive for white spot syndrome virus replication

Hemocytes are the major immune cells of crustaceans which are believed to be essential for the pathogenesis of white spot syndrome virus (WSSV) infection. Crayfish hemocytes and hematopoietic tissue (HPT) cells have been found to be susceptible to WSSV infection, but the procedure of WSSV infection to both cell types has not yet been carefully investigated. In this study, we analyzed the infection and proliferation of WSSV in crayfish hemocytes as well as HPT cells in detail through transmission electronic microscopy (TEM). The results showed that WSSV could enter both hemocytes and HPT cells through endocytosis, but the production of progeny virus was only achieved in HPT cells. Further investigation demonstrated that although WSSV could transcribe its genes in both cell types, viral genome replication and structural protein expression were unsuccessful in hemocytes, which may be responsible for the failure of progeny production. Therefore, we propose that both hemocytes and HPT cells are susceptible to WSSV infection but only HPT cells are permissive to WSSV replication. These findings will extend our knowledge of the interaction between WSSV and the host immune system.

Tissue distribution of hepatopancreatic parvo-like virus of shrimp in freshwater rice-field crab, Paratelphusa hydrodomous (Herbst)

An attempt was made to determine the replication efficiency of hepatopancreatic parvo-like virus (HPV) of shrimp in different organs of freshwater rice-field crab Paratelphusa hydrodomous (Herbst) using bioassay, PCR, RT-PCR, ELISA, Western blot and q-PCR analyses. Another attempt was made to use this crab as an alternative to penaeid shrimp for the large-scale production of HPV. This crab was found to be highly susceptible to HPV by intramuscular injection. The systemic HPV infection was confirmed by PCR and Western blot analyses in freshwater crab. The expression of capsid protein gene in different organs of infected crab was revealed by RT-PCR analysis. Indirect ELISA was used to quantify the capsid protein in different organs of the crab. The copy number of HPV in different organs of the infected crab was quantified by q-PCR. The results revealed a steady decrease in CT values in different organs of the infected crab during the course of infection. The viral inoculum that was prepared from different organs of the infected crab caused significant mortality in post-larvae of tiger prawn, Penaeus monodon (Fabricius). The results revealed that this rice-field crab could be used as an alternative host for HPV replication and also for large-scale production of HPV.

A new anti-lipopolysaccharide factor (ALF) gene with its SNP polymorphisms related to WSSV-resistance of Litopenaeus vannamei

Anti-lipopolysaccharide factors (ALFs) of crustacean play an important role against bacteria or virus infection. In this study, the cDNA sequence and genomic sequence of one new isoform of ALF designated as nLvALF1 were reported. The open reading frame (ORF) of nLvALF1 consisted of 369 bp encoding 123 amino acids and the genomic structure of nLvALF1 comprised four introns and three exons. The predicted pI of the deduced protein was 8.82 and the molecular weight (MW) was 13.72 KDa. The deduced amino acid sequence of nLvALF1 contained a typical functional domain of ALF: LPS-binding domain. Phylogenetic analysis showed that nLvALF1 had the closest relationship with FcALF1 from Fenneropenaeus chinensis. The nLvALF1 was specifically expressed in lymphoid organ (Oka) of shrimp. Its transcriptional level was significantly up-regulated after white spot syndrome virus (WSSV) challenge, suggesting that nLvALF1 might participate in defense against WSSV in Litopenaeus vannamei. In order to search potential genetic markers associated with WSSV-resistance, we scanned the polymorphisms of the genomic fragment with 397 bp where the LPS-binding domain encoding sequence located and 18 SNPs were found. The distribution frequency of these SNPs was analyzed in WSSV susceptible shrimp and resistant shrimp separately. Significant differences existed in allelic frequencies at loci g.1361-T > C, g.1370-T > C, g.1419-T > A between the WSSV-resistant group and the WSSV-susceptible/susceptible group (P < 0.05). The specific haplotype CT consisted of g.1415-C > A and g.1419-T > A was associated with susceptibility to WSSV (P < 0.05). These findings provide theoretical support for selection of WSSV-resistant varieties of L. vannamei.

Purification of white spot syndrome virus by iodixanol density gradient centrifugation

Up to now, only a few brief procedures for purifying white spot syndrome virus (WSSV) have been described. They were mainly based on sucrose, NaBr and CsCl density gradient centrifugation. This work describes for the first time the purification of WSSV through iodixanol density gradients, using virus isolated from infected tissues and haemolymph of Penaeus vannamei (Boone). The purification from tissues included a concentration step by centrifugation (2.5 h at 60,000 g) onto a 50% iodixanol cushion and a purification step by centrifugation (3 h at 80,000 g) through a discontinuous iodixanol gradient (phosphate-buffered saline, 5%, 10%, 15% and 20%). The purification from infected haemolymph enclosed a dialysis step with a membrane of 1,000 kDa (18 h) and a purification step through the earlier iodixanol gradient. The gradients were collected in fractions and analysed. The number of particles, infectivity titre (in vivo), total protein and viral protein content were evaluated. The purification from infected tissues gave WSSV suspensions with a very high infectivity and an acceptable purity, while virus purified from haemolymph had a high infectivity and a very high purity. Additionally, it was observed that WSSV has an unusually low buoyant density and that it is very sensitive to high external pressures.

Binding of white spot syndrome virus to Artemia sp. cell membranes

Using differential velocity centrifugation, cell membranes of Artemia sp. were prepared, and their binding to white spot syndrome virus (WSSV) was analyzed in vitro. The results indicated that WSSV can specifically bind to Artemia cell membranes, and that WSSV receptor very likely existed in this membrane, which suggested that Artemia sp. may be a reservoir of WSSV. This study investigated the specific WSSV binding site by performing competitive inhibition experiments using shrimp gill cell membranes to bind WSSV to Artemia cell membranes. The results showed that shrimp gill cell membranes had a distinct inhibition effect on the specific binding of Artemia cell membranes to WSSV. Thus, potentially similar WSSV receptors or binding sites existed on Artemia sp. cell membranes and shrimp gill cell membranes. Taken together, these findings may provide experimental basis for the development of an effective approach to controlling WSSV, and theoretical basis for the study of WSSV receptors.

Recent advances in researches on the innate immunity of shrimp in China

The annual production of shrimp culture in mainland of China has been over one million tons for several years. The major cultivated penaeidae species are Litopenaeus vannamei, Fenneropenaeus chinensis, Penaeus monodon and Marsupenaeus japonicus. Due to the importance of shrimp aquaculture in China, researchers have paid more attention to the molecular mechanism of shrimp disease occurrence and tried to develop an efficient control strategy for disease. This paper summarizes the research progress related to innate immunity of penaeid shrimp made in the last decade in Mainland China. Several pattern recognition receptors, such as lectin, toll, lipopolysaccharide and β-1,3-glucan binding protein (LGBP) and tetraspanin were identified. The major signal transduction pathways, including Toll pathway, IMD pathway, which might be involved in the immune response of shrimp, were focused on and most of the components in Toll pathway were identified. Also, cellular immune responses such as phagocytosis and apoptosis were regarded playing very important roles in anti-WSSV infection to shrimp. The molecules involved in the maintenance of the immune homeostasis of shrimp and the progress on molecular structure and pathogenic mechanism of WSSV were summarized. Therefore, the brief outline about the immune system of shrimp is drawn based on the recent data which will help us to understand the immune responses of shrimp to different pathogens.

In vitro propagation of hepatopancreatic parvo-like virus (HPV) of shrimp in C6/36 (Aedes albopictus) cell line

Hepatopancreatic parvovirus (HPV) which causes infection in many species of penaeid shrimp is a serious viral pathogen in the young life stages of shrimp. An attempt was made to develop an in vitro system using C6/36 subclone of Aedes albopictus cell line for propagation of HPV. The results revealed that C6/36 cells were susceptible to this virus and the infected cells showed CPE in the form of vacuole formation. The results of PCR, immunocytochemistry and Western blot revealed the HPV-infection in C6/36 cell line. The RT-PCR analysis confirmed the replication of HPV in C6/36 cell line. The HPV load was quantified at different time intervals by ELISA and real time PCR, and the results showed the increase of viral load in C6/36 cell line in time course of infection. HPV propagated in C6/36 cell line was used to infect post-larvae of shrimp and the results showed that the twentieth passage of HPV propagated in C6/36 cell line caused 100% mortality in post-larvae after 6 weeks post infection (d.p.i.). The infected post-larvae showed clinical signs of reduced growth, reduced preening, muscle opacity and atrophy of hepatopancreas. The HPV-infection was confirmed by PCR. The results of the present study showed that C6/36 cell line can be used as an in vitro model for HPV replication instead of whole animal.

High efficacy of white spot syndrome virus replication in tissues of freshwater rice-field crab, Paratelphusa hydrodomous (Herbst)

An attempt was made to determine the replication efficiency of white spot syndrome virus (WSSV) of shrimp in different organs of freshwater rice-field crab, Paratelphusa hydrodomous (Herbst), using bioassay, PCR, RT-PCR, ELISA, Western blot and real-time PCR analyses, and also to use this crab instead of penaeid shrimp for the large-scale production of WSSV. This crab was found to be highly susceptible to WSSV by intramuscular injection. PCR and Western blot analyses confirmed the systemic WSSV infection in freshwater crab. The RT-PCR analysis revealed the expression of VP28 gene in different organs of infected crab. The indirect ELISA was used to quantify the VP28 protein in different organs of crab. It was found that there was a high concentration of VP28 protein in gill tissue, muscle, haemolymph and heart tissue. The copy number of WSSV in different organs of infected crab was quantified by real-time PCR, and the results revealed a steady increase in copy number in different organs of infected crab during the course of infection. The viral inoculum prepared from different organs of infected crab caused significant mortality in tiger prawn, Penaeus monodon (Fabricius). The results revealed that this crab can be used as an alternate host for WSSV replication and production.

White spot syndrome virus: Genotypes, Epidemiology and Evolutionary Studies

White spot syndrome virus (WSSV) is a pathogen that has emerged globally affecting shrimp populations. Comparison of WSSV genome have shown the virus to share a high genetic similarity except for a few variable genomic loci that has been employed as markers in molecular epidemiology studies for determining the origin, evolution and spread in different geographical regions. Molecular genotyping of WSSV are based on genomic deletions associated with ORF23/24 and ORF14/15 variable regions and the three variable number of tandem repeat regions, ORF75, ORF94 and ORF125. Studies show the prevalence of several genotypes for WSSV with particular genotypes being more prevalent than others in a given geographical area. Deletions associated with ORF23/24 and ORF14/15 variable regions have proven to be of evolutionary significance. Fitness and virulence studies on different genotypes of WSSV suggest that all the strains of WSSV are equally virulent, but the one with smaller genomic size is the fittest. Studies also have shown that mixed genotype infection of WSSV correlates with lower disease outbreaks. This review focuses on the genotyping studies that were undertaken in elucidating WSSV evolution and epidemiology.

Crassostrea gigas oysters as a shrimp farm bioindicator of white spot syndrome virus

This study explored whether Crassostrea gigas oysters can be used as a bioindicator of white spot syndrome virus (WSSV) in shrimp farm water canals. Bioassays showed that C. gigas can accumulate WSSV in their gills and digestive glands but do not become infected, either by exposure to seawater containing WSSV or by cohabitation with infected shrimp. The use of a WSSV nested PCR to screen oysters placed in water canals at the entry of a shrimp farm allowed WSSV to be detected 16 d prior to the disease occurring. The finding that C. gigas can concentrate small amounts of WSSV present in seawater without being harmed makes it an ideal sentinel species at shrimp farms.

Stimulating the immune response of Litopenaeus vannamei using the phagocytosis activating protein (PAP) gene

High mortality in the shrimp farming industry is caused by several pathogens such as white spot syndrome virus (WSSV), yellow head virus (YHV) and Vibrio harveyi (V. harveyi). A PAP (Phagocytosis activating protein) gene able to activate phagocytosis of shrimp hemocytes was cloned into the eukaryotic expression vector phMGFP. In vitro expression was confirmed by transfection of PAP-phMGFP into CHO (Chinese Hamster Ovary) cells and the expression of the Green Fluorescent Protein (GFP) was observed. In order to activate the phagocytic activity of shrimp, 20, 40 and 80 μg/shrimp of this PAP-phMGFP vector were injected into Litopenaeus vannamei muscle. After challenged with WSSV, 40 μg/shrimp produced the highest relative percent survival (77.78 RPS). Analysis for the expression of the GFP gene in various tissues showed the expression mostly in the hemolymph of the immunized shrimp. The expression level of PAP and proPO (Prophenoloxidase) gene were highest at 7 days after immunization. This agreed with the efficiency of protection against WSSV that also occurred 7 days after immunization with the highest RPS of 86.61%. However there was no protection 30 days after immunization. Hemocytes of shrimp injected with PAP-phMGFP had 1.9 folds and 3 folds higher percentage phagocytosis and phagocytic index than the shrimp injected with PBS. Accordingly, copies of WSSV reduced in the PAP-phMGFP injected shrimp. In addition, PAP-phMGFP also protected shrimp against several pathogens: WSSV, YHV and V. harveyi, with RPS values of 86.61%, 63.34% and 50% respectively. This finding shows that the immune cellular defense mechanisms in shrimp against pathogens can be activated by injection of PAP-phMGFP and could indicate possible useful ways to begin to control this process.

White spot syndrome virus (WSSV) concentrations in crustacean tissues: a review of data relevant to assess the risk associated with commodity trade

We have reviewed the available peer reviewed literature on pathogen load for white spot syndrome virus (WSSV) in species susceptible to infection. Data on pathogen load in traded commodities are relevant for undertaking import risk assessments for a specific pathogen. Data were available for several of the major penaeid shrimp species farmed for aquaculture and for one crab and crayfish species. Most data are based on experimental infection, but some data were available for farmed or wild shrimp. Owing to the unavailability of immortal cell lines to determine viral load of viable virus, quantitative PCR was the main method used for quantification. The viral loads measured in shrimp at the onset of mortality events were extremely high (in the order of 10(9) -10(10) copy numbers gram(-1) of tissue). In a farm setting, the onset of increased mortalities will often trigger emergency harvests. Therefore, shrimp obtained from emergency harvests are likely to carry substantial concentrations of viral particles. Viral load did not vary greatly with tissue type. The WSSV load in wild crustaceans, farmed crustaceans not undergoing a mortality event or survivors of a mortality event was significantly lower (usually by multiple logs). Studies have also been undertaken in 'vaccinated' shrimp. One of the 'vaccines' led to a significant reduction of viral load in WSSV-exposed animals. The data obtained from the literature review are put into context with published information on minimal infectious dose and WSSV survival in frozen commodity shrimp.

A 3D model of the membrane protein complex formed by the white spot syndrome virus structural proteins

BACKGROUND

Outbreaks of white spot disease have had a large negative economic impact on cultured shrimp worldwide. However, the pathogenesis of the causative virus, WSSV (whit spot syndrome virus), is not yet well understood. WSSV is a large enveloped virus. The WSSV virion has three structural layers surrounding its core DNA: an outer envelope, a tegument and a nucleocapsid. In this study, we investigated the protein-protein interactions of the major WSSV structural proteins, including several envelope and tegument proteins that are known to be involved in the infection process.

PRINCIPAL FINDINGS

In the present report, we used coimmunoprecipitation and yeast two-hybrid assays to elucidate and/or confirm all the interactions that occur among the WSSV structural (envelope and tegument) proteins VP51A, VP19, VP24, VP26 and VP28. We found that VP51A interacted directly not only with VP26 but also with VP19 and VP24. VP51A, VP19 and VP24 were also shown to have an affinity for self-interaction. Chemical cross-linking assays showed that these three self-interacting proteins could occur as dimers.

CONCLUSIONS

From our present results in conjunction with other previously established interactions we construct a 3D model in which VP24 acts as a core protein that directly associates with VP26, VP28, VP38A, VP51A and WSV010 to form a membrane-associated protein complex. VP19 and VP37 are attached to this complex via association with VP51A and VP28, respectively. Through the VP26-VP51C interaction this envelope complex is anchored to the nucleocapsid, which is made of layers of rings formed by VP664. A 3D model of the nucleocapsid and the surrounding outer membrane is presented.

White spot syndrome virus: an overview on an emergent concern

Viruses are ubiquitous and extremely abundant in the marine environment. One of such marine viruses, the white spot syndrome virus (WSSV), has emerged globally as one of the most prevalent, widespread and lethal for shrimp populations. However, at present there is no treatment available to interfere with the unrestrained occurrence and spread of the disease. The recent progress in molecular biology techniques has made it possible to obtain information on the factors, mechanisms and strategies used by this virus to infect and replicate in susceptible host cells. Yet, further research is still required to fully understand the basic nature of WSSV, its exact life cycle and mode of infection. This information will expand our knowledge and may contribute to developing effective prophylactic or therapeutic measures. This review provides a state-of-the-art overview of the topic, and emphasizes the current progress and future direction for the development of WSSV control strategies.

Purification of Bionanoparticles

The recent demand for nanoparticulate products such as viruses, plasmids, protein nanoparticles, and drug delivery systems have resulted in the requirement for predictable and controllable production processes. Protein nanoparticles are an attractive candidate for gene and molecular therapy due to their relatively easy production and manipulation. These particles combine the advantages of both viral and non‐viral vectors while minimizing the disadvantages. However, their successful application depends on the availability of selective and scalable methodologies for product recovery and purification. Downstream processing of nanoparticles depends on the production process, producer system, culture media and on the structural nature of the assembled nanoparticle, i.e., mainly size, shape and architecture. In this paper, the most common processes currently used for the purification of nanoparticles, are reviewed.

A review on the morphology, molecular characterization, morphogenesis and pathogenesis of white spot syndrome virus

Since it first appeared in 1992, white spot syndrome virus (WSSV) has become the most threatening infectious agent in shrimp aquaculture. Within a decade, this pathogen has spread to all the main shrimp farming areas and has caused enormous economic losses amounting to more than seven billion US dollars. At present, biosecurity methods used to exclude pathogens in shrimp farms include disinfecting ponds and water, preventing the entrance of animals that may carry infectious agents and stocking ponds with specific pathogen-free post-larvae. The combination of these practices increases biosecurity in shrimp farming facilities and may contribute to reduce the risk of a WSSV outbreak. Although several control methods have shown some efficacy against WSSV under experimental conditions, no therapeutic products or strategies are available to effectively control WSSV in the field. Furthermore, differences in virulence and clinical outcome of WSSV infections have been reported. The sequencing and characterization of different strains of WSSV has begun to determine aspects of its biology, virulence and pathogenesis. Knowledge on these aspects is critical for developing effective control methods. The aim of this review is to present an update of the knowledge generated so far on different aspects of WSSV organization, morphogenesis, pathology and pathogenesis.

Expression, purification and crystallization of two major envelope proteins from white spot syndrome virus

White spot syndrome virus (WSSV) is a major virulent pathogen known to infect penaeid shrimp and other crustaceans. VP26 and VP28, two major envelope proteins from WSSV, have been identified and overexpressed in Escherichia coli. In order to facilitate purification and crystallization, predicted N-terminal transmembrane regions of approximately 35 amino acids have been truncated from both VP26 and VP28. Truncated VP26 and VP28 and their corresponding SeMet-labelled proteins were purified and the SeMet proteins were crystallized by the hanging-drop vapour-diffusion method. Crystals of SeMet-labelled VP26 were obtained using a reservoir consisting of 0.1 M citric acid pH 3.5, 3.0 M sodium chloride and 1%(w/v) polyethylene glycol 3350, whereas SeMet VP28 was crystallized using a reservoir solution consisting of 25% polyethylene glycol 8000, 0.2 M calcium acetate, 0.1 M Na HEPES pH 7.5 and 1.5%(w/v) 1,2,3-heptanetriol. Crystals of SeMet-labelled VP26 diffract to 2.2 A resolution and belong to space group R32, with unit-cell parameters a = b = 73.92, c = 199.31 A. SeMet-labelled VP28 crystallizes in space group P2(1)2(1)2(1), with unit-cell parameters a = 105.33, b = 106.71, c = 200.37 A, and diffracts to 2.0 A resolution.

Crystal structures of major envelope proteins VP26 and VP28 from white spot syndrome virus shed light on their evolutionary relationship

White spot syndrome virus (WSSV) is a virulent pathogen known to infect various crustaceans. It has bacilliform morphology with a tail-like appendage at one end. The envelope consists of four major proteins. Envelope structural proteins play a crucial role in viral infection and are believed to be the first molecules to interact with the host. Here, we report the localization and crystal structure of major envelope proteins VP26 and VP28 from WSSV at resolutions of 2.2 and 2.0 A, respectively. These two proteins alone account for approximately 60% of the envelope, and their structures represent the first two structural envelope proteins of WSSV. Structural comparisons among VP26, VP28, and other viral proteins reveal an evolutionary relationship between WSSV envelope proteins and structural proteins from other viruses. Both proteins adopt beta-barrel architecture with a protruding N-terminal region. We have investigated the localization of VP26 and VP28 using immunoelectron microscopy. This study suggests that VP26 and VP28 are located on the outer surface of the virus and are observed as a surface protrusion in the WSSV envelope, and this is the first convincing observation for VP26. Based on our studies combined with the literature, we speculate that the predicted N-terminal transmembrane region of VP26 and VP28 may anchor on the viral envelope membrane, making the core beta-barrel protrude outside the envelope, possibly to interact with the host receptor or to fuse with the host cell membrane for effective transfer of the viral infection. Furthermore, it is tempting to extend this host interaction mode to other structural viral proteins of similar structures. Our finding has the potential to extend further toward drug and vaccine development against WSSV.

Pre-exposure to infectious hypodermal and haematopoietic necrosis virus or to inactivated white spot syndrome virus (WSSV) confers protection against WSSV in Penaeus vannamei (Boone) post-larvae

Larvae and post-larvae of Penaeus vannamei (Boone) were submitted to primary challenge with infectious hypodermal and haematopoietic necrosis virus (IHHNV) or formalin-inactivated white spot syndrome virus (WSSV). Survival rate and viral load were evaluated after secondary per os challenge with WSSV at post-larval stage 45 (PL45). Only shrimp treated with inactivated WSSV at PL35 or with IHHNV infection at nauplius 5, zoea 1 and PL22 were alive (4.7% and 4%, respectively) at 10 days post-infection (p.i.). Moreover, at 9 days p.i. there was 100% mortality in all remaining treatments, while there was 94% mortality in shrimp treated with inactivated WSSV at PL35 and 95% mortality in shrimp previously treated with IHHNV at N5, Z1 and PL22. Based on viral genome copy quantification by real-time PCR, surviving shrimp previously challenged with IHHNV at PL22 contained the lowest load of WSSV (0-1x10(3) copies microg-1 of DNA). In addition, surviving shrimp previously exposed to inactivated WSSV at PL35 also contained few WSSV (0-2x10(3) copies microg-1 of DNA). Consequently, pre-exposure to either IHHNV or inactivated WSSV resulted in slower WSSV replication and delayed mortality. This evidence suggests a protective role of IHHNV as an interfering virus, while protection obtained by inactivated WSSV might result from non-specific antiviral immune response.

Localization studies of two white spot syndrome virus structural proteins VP51 and VP76

VP51 and VP76 are two structural proteins of white spot syndrome virus (WSSV). However, there is some controversy about their localization in the virion at present. In this study, we employ multiple approaches to reevaluate the location of VP51 and VP76. Firstly, we found VP51 and VP76 presence in viral nucleocapsids fraction by Western blotting. Secondly, after the high-salt treatment of nucleocapsids, VP51 and VP76 were still exclusively present in viral capsids by Western blotting and immunoelectron microscopy, suggesting two proteins are structural components of the viral capsid. To gather more evidence, we developed a method based on immunofluorescence flow cytometry. The results revealed that the mean fluorescence intensity of the viral capsids group was significantly higher than that of intact virions group after incubation with anti-VP51 or anti-VP76 serum and fluorescein isothiocyanate conjugated secondary antibody. All these results indicate that VP51 and VP76 are both capsid proteins of WSSV.

Identification of a novel nonstructural protein, VP9, from white spot syndrome virus: its structure reveals a ferredoxin fold with specific metal binding sites

White spot syndrome virus (WSSV) is a major pathogen in shrimp aquaculture. VP9, a full-length protein of WSSV, encoded by open reading frame wsv230, was identified for the first time in the infected Penaeus monodon shrimp tissues, gill, and stomach as a novel, nonstructural protein by Western blotting, mass spectrometry, and immunoelectron microscopy. Real-time reverse transcription-PCR demonstrated that the transcription of VP9 started from the early to the late stage of WSSV infection as a major mRNA species. The structure of full-length VP9 was determined by both X-ray and nuclear magnetic resonance (NMR) techniques. It is the first structure to be reported for WSSV proteins. The crystal structure of VP9 revealed a ferredoxin fold with divalent metal ion binding sites. Cadmium sulfate was found to be essential for crystallization. The Cd2+ ions were bound between the monomer interfaces of the homodimer. Various divalent metal ions have been titrated against VP9, and their interactions were analyzed using NMR spectroscopy. The titration data indicated that VP9 binds with both Zn2+ and Cd2+. VP9 adopts a similar fold as the DNA binding domain of the papillomavirus E2 protein. Based on our present investigations, we hypothesize that VP9 might be involved in the transcriptional regulation of WSSV, a function similar to that of the E2 protein during papillomavirus infection of the host cells.

Transcription and identification of a novel envelope protein (VP124) gene of shrimp white spot syndrome virus

White spot syndrome virus (WSSV) is one of the most virulent pathogens in shrimp culture worldwide. Combining SDS-PAGE with mass spectrometry, a novel envelope protein from WSSV was identified to match an open reading frame (ORF) of WSSV genome. This ORF contained 3582nt, encoding 1194 aa, and was termed the vp124 gene. One part of the whole gene (named vp124p) was cloned into pET-GST vector and expressed as a fusion protein with glutathione S-transferase (GST) in Escherichia coli strain BL21 (DE3). Specific antibodies were raised using the purified fusion protein (GST-VP124P). Temporal transcription analysis revealed that the vp124 gene was a late gene. Western blot analysis showed that the mouse anti-GST-VP124P antibodies reacted specifically with VP124 present either in the WSSV virions or in the viral envelopes, and did not react with the proteins of the viral nucleocapsids. VP124 was located in the WSSV virions as an envelope protein using immunoelectron microscopy.

A simple and efficient method for purification of intact white spot syndrome virus (WSSV) viral particles

A new simple and efficient method for isolation of intact WSSV viral particles from infected crayfish tissues with high yield was developed. Abundant viral particles could be obtained with only a few steps of conventional differential centrifugations, while no density gradient centrifugation or ultracentrifugation was required. The concentrated virus preparations were further studied by transmission electron microscopy and polyacrylamide gel electrophoresis. Using negative-staining TEM, we found that purified viral particles were coated with integral envelope. At least 23 major structural proteins from purified WSSV virions could be observed by SDS-PAGE. By this method, about 10(12) viral particles could be recovered from 10 g of infected crayfish tissues. Moreover, purified virus does not lose its biological activity. Using purified virus, the minimal amount of WSSV that could initiate a successful virus proliferation in crayfish was determined.