Identification of a nucleocapsid protein (VP35) gene of shrimp white spot syndrome virus and characterization of the motif important for targeting VP35 to the nuclei of transfected insect cells

Complete genome of the new bacilliform virus that causes Milky Hemolymph Syndrome in Chionoecetes bairdi (Rathbun, 1924)

The genome of a new member of the Nimaviridae family has been sequenced. The Chionoecetes bairdi bacilliform virus (CbBV) causes Milky Hemolymph Syndrome (MHS) in Chionoecetes bairdi populations of the Pacific coast of Kamchatka. The CbBV genome is represented by double-stranded DNA with a length of 245,567 nucleotides containing 120 ORFs. Of these, 85 proteins had significant matches in the NCBI database, and 57 genes encoded capsid, envelope, tegument and nonstructural proteins. Comparative analysis of the genomes of CbBV and a number of representatives of the class nuclear arthropod large DNA viruses (NALDVs) made it possible to isolate 49 evolutionarily conserved orthologue core genes. Among them, 5 were multicopy genes, and 44 were single-copy genes. There were ancestral genes characteristic of all Naldaviricetes - per os infectivity complex genes, one DNA polymerase gene and one thymidylate synthase gene. Phylogenetic analysis of representatives of the Nimaviridae family revealed that the CbBV and Chionoecetes opilio bacilliform virus (CoBV) form an independent clade within the family separate from the clade containing WSSV strains. This is supported by data on the order and arrangement of genes in the genomes of nimaviruses that were identical within each clade but differed between them. In addition, a high identity of the genomes and proteomes of CbBV and CoBV (approximately 99%) was shown, and their identity with WSSV strains was no more than 33%. The data on the structure of the genome of the new virus that causes MHS in C. bairdi indicate that it belongs to the family Nimaviridae, genus Whispovirus. Thus, the CbBV infecting the commercially important species of Tanner crab in populations of the Pacific coast of Kamchatka is the second "wild" representative of replicating nimaviruses whose genome has been characterized after the CoBV that causes MHS in C. opilio in populations of the Sea of Japan. The discovery of a new member of the family that infects decapods indicates the prevalence of nimaviruses in marine ecosystems. The information obtained is important for understanding the evolution of representatives of the class of nuclear arthropod large DNA viruses. The discovery of a new nimavirus that causes MHS in Chionoecetes crabs, in contrast to the white spot syndrome (WSS) caused by WSSV strains, makes it relevant to identify two variants and possibly species within the family, namely, WSSV and Milky Hemolymph Syndrome virus (MHSV).

Characterization and prevalence of a novel white spot syndrome viral genotype in naturally infected wild crayfish, Procambarus clarkii, in Shanghai, China

White spot syndrome virus (WSSV) infection is commonly detected by vp28-qPCR assay in wild crayfish, Procambarus clarkii, a widespread crustacean species in the aquatic environment in China. The virions of crayfish WSSV have been isolated and purified. Based on TEM observation, they exhibited morphological structures that are identical to known WSSV. In addition, the WSSV major envelope protein VP28 was observed based on Western blot analysis of the total structural proteins of crayfish WSSV. PCR amplification and sequencing analyses of variable regions of ORF14/15, ORF23/24 and ORF94, along with viral genomic sequencing and phylogenomic analysis, indicated that the crayfish WSSV, named WSSV-CN-Pc, represents a new WSSV genotype. Intramuscular injection bioassay revealed that WSSV-CN-Pc was as virulent as the WSSV Taiwan strain. The WSSV-CN-Pc exhibited characteristics of a dominant genotype, with high infection load (107-108 WSSV/mg) and high prevalence (91.7%, 110 of 120 crayfish samples) observed in the surveyed wild crayfish. WSSV-CN-Pc was also detected, with similar infection pattern as observed in crayfish, in farmed Litopenaeus vannamei shrimp that shared similar ecological niches with the sampled crayfish. Our results indicated that there was horizontal transmission of WSSV-CN-Pc between crayfish and shrimp in nature. Our findings also implicated that crayfish and shrimp farming should be integrated and managed with cautions in order to reduce the risk of spread and circulation of WSSV in the aquatic environment.

WSSV envelope protein VP51B links structural protein complexes and may mediate virus infection

White spot syndrome virus (WSSV), an enveloped double-stranded DNA virus, is the causative agent of a disease that has led to severe mortalities of cultured shrimps in Taiwan and many other countries. In the previous study, Penaeus monodon chitin-binding protein (CBP) and glucose transporter 1 (Glut1), two cell membrane proteins, were found to at least interact with other 10 WSSV envelope proteins including VP51B. These envelope proteins might form a protein complex. According to the known information, VP51B was used to identify its role in the protein complex. Western blotting of the intact viral particles and fractionation of the viral components confirmed that VP51B is one of WSSV envelope proteins. In this study, the protein-protein interaction between VP51B and other WSSV envelope proteins was identified by far-western blot experiment and VP51B was found to interact with VP24, VP31, VP32, VP39B and VP41A. Furthermore, the in vivo neutralization experiment using recombinant VP51B plus with VP39B showed the best inhibition. These data indicate that VP51B participates in the WSSV protein complex and plays an important role in WSSV infection.

ICP35 Is a TREX-Like Protein Identified in White Spot Syndrome Virus

ICP35 is a non-structural protein from White spot syndrome virus believed to be important in viral replication. Since ICP35 was found to localize in the host nucleus, it has been speculated that the function of ICP35 might be involved in the interaction of DNA. In this study, we overexpressed, purified and characterized ICP35. The thioredoxin-fused ICP35 (thio-ICP35) was strongly expressed in E. coli and be able to form itself into dimers. Investigation of the interaction between ICP35 and DNA revealed that ICP35 can perform DNase activity. Structural model of ICP35 was successfully built on TREX1, suggesting that ICP35 might adopt the folding similar to that of TREX1 protein. Several residues important for dimerization in TREX1 are also conserved in ICP35. Residue Asn126 and Asp132, which are seen to be in close proximity to metal ions in the ICP35 model, were shown through site-directed mutagenesis to be critical for DNase activity.

Molecular Mechanisms of White Spot Syndrome Virus Infection and Perspectives on Treatments

Since its emergence in the 1990s, White Spot Disease (WSD) has had major economic and societal impact in the crustacean aquaculture sector. Over the years shrimp farming alone has experienced billion dollar losses through WSD. The disease is caused by the White Spot Syndrome Virus (WSSV), a large dsDNA virus and the only member of the Nimaviridae family. Susceptibility to WSSV in a wide range of crustacean hosts makes it a major risk factor in the translocation of live animals and in commodity products. Currently there are no effective treatments for this disease. Understanding the molecular basis of disease processes has contributed significantly to the treatment of many human and animal pathogens, and with a similar aim considerable efforts have been directed towards understanding host-pathogen molecular interactions for WSD. Work on the molecular mechanisms of pathogenesis in aquatic crustaceans has been restricted by a lack of sequenced and annotated genomes for host species. Nevertheless, some of the key host-pathogen interactions have been established: between viral envelope proteins and host cell receptors at initiation of infection, involvement of various immune system pathways in response to WSSV, and the roles of various host and virus miRNAs in mitigation or progression of disease. Despite these advances, many fundamental knowledge gaps remain; for example, the roles of the majority of WSSV proteins are still unknown. In this review we assess current knowledge of how WSSV infects and replicates in its host, and critique strategies for WSD treatment.

The DNA virus white spot syndrome virus uses an internal ribosome entry site for translation of the highly expressed nonstructural protein ICP35

Although shrimp white spot syndrome virus (WSSV) is a large double-stranded DNA virus (∼300 kbp), it expresses many polycistronic mRNAs that are likely to use internal ribosome entry site (IRES) elements for translation. A polycistronic mRNA encodes the gene of the highly expressed nonstructural protein ICP35, and here we use a dual-luciferase assay to demonstrate that this protein is translated cap independently by an IRES element located in the 5' untranslated region of icp35. A deletion analysis of this region showed that IRES activity was due to stem-loops VII and VIII. A promoterless assay, a reverse transcription-PCR together with quantitative real-time PCR analysis, and a stable stem-loop insertion upstream of the Renilla luciferase open reading frame were used, respectively, to rule out the possibility that cryptic promoter activity, abnormal splicing, or read-through was contributing to the IRES activity. In addition, a Northern blot analysis was used to confirm that only a single bicistronic mRNA was expressed. The importance of ICP35 to viral replication was demonstrated in a double-stranded RNA (dsRNA) interference knockdown experiment in which the mortality of the icp35 dsRNA group was significantly reduced. Tunicamycin was used to show that the α subunit of eukaryotic initiation factor 2 is required for icp35 IRES activity. We also found that the intercalating drug quinacrine significantly inhibited icp35 IRES activity in vitro and reduced the mortality rate and viral copy number in WSSV-challenged shrimp. Lastly, in Sf9 insect cells, we found that knockdown of the gene for the Spodoptera frugiperda 40S ribosomal protein RPS10 decreased icp35 IRES-regulated firefly luciferase activity but had no effect on cap-dependent translation.

Effects of high salinity, high temperature and pH on capsid structure of white spot syndrome virus

The structural stability of white spot syndrome virus (WSSV) capsids at high salinity, high temperature and various pH values was studied. To obtain the viral capsids, the nucleocapsids were treated with high salinity. The results showed that high salinity treatment can cause the dissociation of VP15 and most of VP95 from the nucleocapsid, but there were no noticeable alterations in morphology and ultrastructure of the nucleocapsid and capsid. The capsids retained morphological integrity at temperatures <45°C but became aberrant at >60°C. In addition, the capsids were relatively resistant to strong acid conditions and were tolerant to a broad pH range of 1 to 10. However, morphological change occurred at pH 10.5. The capsids broke up into small pieces at pH 11 and completely degraded in 0.1 and 1.0 M NaOH. These results indicated that the WSSV capsid is acid-stable and alkali-labile.

Penaeus monodon thioredoxin restores the DNA binding activity of oxidized white spot syndrome virus IE1

AIMS

In this study we identified viral gene targets of the important redox regulator thioredoxin (Trx), and explored in depth how Trx interacts with the immediate early gene #1 (IE1) of the white spot syndrome virus (WSSV).

RESULTS

In a pull-down assay, we found that recombinant Trx bound to IE1 under oxidizing conditions, and a coimmunoprecipitation assay showed that Trx bound to WSSV IE1 when the transfected cells were subjected to oxidative stress. A pull-down assay with Trx mutants showed that no IE1 binding occurred when cysteine 62 was replaced by serine. Electrophoretic mobility shift assay (EMSA) showed that the DNA binding activity of WSSV IE1 was downregulated under oxidative conditions, and that Penaeus monodon Trx (PmTrx) restored the DNA binding activity of the inactivated, oxidized WSSV IE1. Another EMSA experiment showed that IE1's Cys-X-X-Cys motif and cysteine residue 55 were necessary for DNA binding. Measurement of the ratio of reduced glutathione to oxidized glutathione (GSH/GSSG) in WSSV-infected shrimp showed that oxidative stress was significantly increased at 48 h postinfection. The biological significance of Trx was also demonstrated in a double-strand RNA Trx knockdown experiment where suppression of shrimp Trx led to significant decreases in mortality and viral copy numbers.

INNOVATION AND CONCLUSION

WSSV's pathogenicity is enhanced by the virus' use of host Trx to rescue the DNA binding activity of WSSV IE1 under oxidizing conditions.

Multiplex real-time PCR and high-resolution melting analysis for detection of white spot syndrome virus, yellow-head virus, and Penaeus monodon densovirus in penaeid shrimp

A multiplex real-time PCR and high-resolution melting (HRM) analysis was developed to detect simultaneously three of the major viruses of penaeid shrimp including white spot syndrome virus (WSSV), yellow-head virus (YHV), and Penaeus monodon densovirus (PmDNV). Plasmids containing DNA/cDNA fragments of WSSV and YHV, and genomic DNAs of PmDNV and normal shrimp were used to test sensitivity of the procedure. Without the need of any probe, the products were identified by HRM analysis after real-time PCR amplification using three sets of viral specific primers. The results showed DNA melting curves that were specific for individual virus. No positive result was detected with nucleic acids from shrimp, Penaeus monodon nucleopolyhedrovirus (PemoNPV), Penaeus stylirostris densovirus (PstDNV), or Taura syndrome virus (TSV). The detection limit for PmDNV, YHV and WSSV DNAs were 40fg, 50fg, and 500fg, respectively, which was 10 times more sensitive than multiplex real-time PCR analyzed by agarose gel electrophoresis. In viral nucleic acid mixtures, HRM analysis clearly identified each virus in dual and triple infection. To test the capability to use this method in field, forty-one of field samples were examined by HRM analysis in comparison with agarose gel electrophoresis. For HRM analysis, 11 (26.83%), 9 (21.95%), and 4 (9.76%) were infected with WSSV, PmDNV, and YHV, respectively. Agarose gel electrophoresis detected lesser number of PmDNV infection which may due to the limit of sensitivity. No multiple infection was found in these samples. This method provides a rapid, sensitive, specific, and simultaneous detection of three major viruses making it as a useful tool for diagnosis and epidemiological studies of these viruses in shrimp and carriers.

Shrimp molecular responses to viral pathogens

From almost negligible amounts in 1970, the quantity of cultivated shrimp (~3 million metric tons in 2007) has risen to approach that of the capture fishery and it constitutes a vital source of export income for many countries. Despite this success, viral diseases along the way have caused billions of dollars of losses for shrimp farmers. Desire to reduce the losses to white spot syndrome virus in particular, has stimulated much research since 2000 on the shrimp response to viral pathogens at the molecular level. The objective of the work is to develop novel, practical methods for improved disease control. This review covers the background and limitations of the current work, baseline studies and studies on humoral responses, on binding between shrimp and viral structural proteins and on intracellular responses. It also includes discussion of several important phenomena (i.e., the quasi immune response, viral co-infections, viral sequences in the shrimp genome and persistent viral infections) for which little or no molecular information is currently available, but is much needed.

Shrimp Pm-fortilin inhibits the expression of early and late genes of white spot syndrome virus (WSSV) in an insect cell model

Fortilin plays an important role in anti-apoptotic mechanisms and cell proliferation in many eukaryotic organisms. This work confirmed previous reports that Sf9 can support the replication of white spot syndrome virus (WSSV) genomic material by using immunohistochemistry with a specific antibody to detect the immediate early gene 1 (ie1) and by amplification of WSSV DNA and mRNA products. Using this insect-cell model system, we show that overexpression of Pm-fortilin in Sf9 cells inhibited the expression of WSSV early genes and late genes (WSSV-DNA polymerase, VP15 and VP28) but not an immediate early gene ie1. This is the first time that an insect cell line has been used to demonstrate interaction between a shrimp gene and genes of a shrimp virus.

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.

Polycistronic mRNAs and internal ribosome entry site elements (IRES) are widely used by white spot syndrome virus (WSSV) structural protein genes

The genome of the white spot syndrome virus (WSSV) Taiwan isolate has many structural and non-structural genes that are arranged in clusters. Screening with Northern blots showed that at least four of these clusters produce polycistronic mRNA, and one of these (vp31/vp39b/vp11) was studied in detail. The vp31/vp39b/vp11 cluster produces two transcripts, including a large 3.4-kb polycistronic transcript of all three genes. No monocistronic vp39b mRNA was detected. TNT and in vitro translation assays showed that vp39b translation was independent of vp31 translation, and that ribosomal reinitiation was not a possible mechanism for vp39b. An unusually located IRES (internal ribosome entry site) element was identified in the vp31/vp39b coding region, and this region was able to promote the expression of a downstream firefly luciferase reporter. We show that vp31/vp39b/vp11 is representative of many other WSSV structural/non-structural gene clusters, and argue that these are also likely to produce polycistronic mRNAs and that use an IRES mechanism to regulate their translation.

Four major envelope proteins of white spot syndrome virus bind to form a complex

Early events in white spot syndrome virus (WSSV) morphogenesis, particularly the formation of viral membranes, are poorly understood. The major envelope proteins of WSSV are VP28, VP26, VP24, and VP19. Our previous results indicated that VP28 interacts with VP26 and VP24. In the present study, we used coimmunoprecipitation assays and pull-down assays to confirm that the four major proteins in the WSSV envelope can form a multiprotein complex. Yeast two-hybrid assays were also used to test for interactions among the four proteins. In summary, three pairwise protein interactions (VP19-VP28, VP19-VP24, and VP24-VP26) and one self-association (VP24-VP24) were identified for the first time.

Lipid of white-spot syndrome virus originating from host-cell nuclei

The hypothesis that white-spot syndrome virus (WSSV) generates its envelope in the nucleoplasm is based on electron microscopy observations; however, as yet there is no direct evidence for this. In the present study, the lipids of WSSV and the nuclei of its host, the crayfish Procambarus clarkii, were extracted and the neutral lipid and phospholipid contents were analysed by high-performance liquid chromatography, thin-layer chromatography and gas chromatography/mass spectrometry. Phosphatidylcholine (PC) and phosphatidylethanolamine comprised 62.9 and 25.8 %, respectively, of WSSV phospholipids, whereas they comprised 58.5 and 30 %, respectively, of crayfish nuclei phospholipids. These two phospholipids were the dominant phospholipids, and amounts of other phospholipids were very low in the total WSSV and crayfish nuclei phospholipids. The data indicate that the phospholipid profile of WSSV and crayfish nuclei are similar, which is in agreement with the model that the lipids of WSSV are from the host-cell nuclei. However, the fatty acid chains of PC were different between the WSSV virions and crayfish nuclei, and the viral neutral lipid component was also found to be somewhat more complicated than that of the host nuclei. The number of species of cholesterol and hydrocarbon in virus neutral lipid was increased compared with that in host-cell nuclei neutral lipid. It is suggested that the differences between WSSV and its host are either due to selective sequestration of lipids or reflect the fact that the lipid metabolism of the host is changed by WSSV infection.

The C-terminal region of envelope protein VP38 from white spot syndrome virus is indispensable for interaction with VP24

White spot syndrome virus (WSSV) is a large, rod-shaped, enveloped double-stranded DNA virus. In this study, VP38, a viral envelope protein, was expressed as a glutathione S-transferase (GST) fusion protein, and a polyclonal antibody against VP38 was obtained. Far-Western blotting and GST pull-down showed that VP38 interacted directly with VP24, a major WSSV envelope protein. In addition, to delineate the interaction region of VP38 with VP24, GST-VP38n (aa 1-142) and GST-VP38c (aa 143-309) were expressed. The GST pull-down assay revealed that VP38 binds via its C-terminal region to VP24. The result implies that VP38 may participate in the formation of the WSSV envelope.

VP26 of white spot syndrome virus functions as a linker protein between the envelope and nucleocapsid of virions by binding with VP51

The envelopment of the nucleocapsid is an important step in white spot syndrome virus (WSSV) assembly. Previous studies showed that VP26, a major envelope protein of WSSV, can interact with viral nucleocapsid. In this study, using the biotin label transfer technique, we found that the biotin label was transferred from Bio-rVP26 to the viral capsid protein VP51 or from Bio-MBP-VP51 to VP26. Far-Western analyses provided further evidence for direct interaction between VP26 and VP51. Therefore, we conclude that VP26 functions as a matrix-like linker protein between the viral envelope and nucleocapsid, which suggests that VP26 is a key factor in the envelopment of WSSV virion.

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.

Detection of White spot syndrome virus DNA in pond soil using a 2-step nested PCR

White spot syndrome virus (WSSV) continues to be the most pathogenic among the penaeid shrimp viruses. In this study, WSSV DNA was detected in pond soil samples using a 2-step nested PCR. Primers described previously were used for the first round of amplification and based on the sequenced amplicon, an inner primer was designed for the 2nd round of amplification. Using plasmid DNA (pET 100) containing the 211 bp target WSSV sequence, analytical sensitivity showed that the 2-step nested PCR protocol was able to detect down to 0.015 fg of the plasmid DNA, or approximately 2 copies of the target DNA sequence. Persistence of WSSV DNA in pond soil samples after various time intervals was determined. WSSV-specific PCR product (161 bp) was still present in the soil samples even after 10 months of storage. The effect of soil heat treatment on the WSSV DNA was also examined. Soils were subjected to 25, 37, 50 and 70 degrees C for 1, 3 and 5 days. The results showed that PCR amplifiable WSSV DNA was still present even after 5 days at 70 degrees C. To our knowledge, this is the first report on the detection of WSSV DNA in soil samples. Based on these findings, it is concluded that the persistence of viral DNA in soil habitats may be an important aspect of WSSV ecology and may have an implication for viral transmissibility.

Nucleotide sequence variations of the major structural proteins (VP15, VP19, VP26 and VP28) of white spot syndrome virus (WSSV), a pathogen of cultured Litopenaeus vannamei in Mexico

White spot syndrome virus (WSSV) was first reported in farmed Litopenaeus vannamei stocks in Sinaloa and Sonora, Mexico during 1999 and continues to cause severe shrimp losses. WSSV genes encoding nucleocapsid (VP26 and VP15) and envelope proteins (VP19 and VP28) of a Mexican isolate were cloned in the pMosBlue vector. The nucleotide sequences of these genes were compared with WSSV isolates in GenBank. VP15 is highly conserved, and VP26 showed 99% homology to a Chinese isolate. The VP28 fragment demonstrated 100% homology to the majority of the isolates analysed (UniProt accession no. Q91CB7), differing from two Indian WSSV and one Chinese WSSV isolates by two non-conserved and one conserved replacements, respectively. Because of their highly conserved nature, these three structural proteins are good candidates for the development of antibody-based WSSV diagnostic tools or for the production of recombinant protein vaccines to stimulate the quasi-immune response of shrimp. In contrast, VP19 of the Mexican isolate was distinguishable from almost all isolates tested, including an American strain of WSSV (US98/South Carolina, GenBank accession no. AAP14086). Although homology was found with isolates from Taiwan (GenBank accession no. AAL89341) and India (GenBank accession no. AAW67477), VP19 may have application as a genetic marker.

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.

Application of spectrophotometry to evaluate the concentration of purified White Spot Syndrome Virus

White Spot Syndrome Virus (WSSV) is a highly virulent pathogen of shrimp. In previous work, a simple and efficient method has been established in our laboratory to purify intact WSSV virions from infected crayfish tissues. To perform studies of WSSV infection mechanism, pathogenesis and gene function by using this purified virion, quantitative assay for the virus becomes increasingly important. In this study, the optical density of the purified virion samples was measured at 600nm wavelength using spectrophotometer and the corresponding concentration was counted by transmission electron microscopy. The statistical results revealed a high correlation between optical density and concentration of WSSV virions (r=0.993; n=5). Finally, a conversion coefficient "f" (3.34x10(8)virions/microl) was obtained and a formula was established: C (virions/microl)=fOD(600)=3.34x10(8)xOD(600), which can be conveniently used to convert the optical density of purified WSSV preparation into the virion concentration.

Beta-integrin mediates WSSV infection

White Spot Syndrome Virus (WSSV) is a virulent and widespread dsDNA virus with a wide range of hosts. Although remarkable progress has been made on virus characterization, however, its mechanism of infection is poorly understood. In this study, by analyzing the phage display library of the WSSV genome, a WSSV envelope protein VP187 (wsv209) was found to interact with shrimp integrin. VP187 possesses the RGD motif. The interaction between integrin and VP187 was confirmed with coimmunoprecipitation. These results demonstrate for the first time an interaction between the WSSV envelope protein and a cell surface molecule. Soluble integrin, integrin-specific antibody and an RGD-containing peptide were found to block the WSSV infection in vivo and in vitro. Gene silencing using a sequence-specific dsRNA targeting beta-integrin effectively inhibited the virus infection. These findings suggest that beta-integrin may function as a cellular receptor for WSSV infection.

Shotgun identification of the structural proteome of shrimp white spot syndrome virus and iTRAQ differentiation of envelope and nucleocapsid subproteomes

White spot syndrome virus (WSSV) is a major pathogen that causes severe mortality and economic losses to shrimp cultivation worldwide. The genome of WSSV contains a 305-kb double-stranded circular DNA, which encodes 181 predicted ORFs. Previous gel-based proteomics studies on WSSV have identified 38 structural proteins. In this study, we applied shotgun proteomics using off-line coupling of an LC system with MALDI-TOF/TOF MS/MS as a complementary and comprehensive approach to investigate the WSSV proteome. This approach led to the identification of 45 viral proteins; 13 of them are reported for the first time. Seven viral proteins were found to have acetylated N termini. RT-PCR confirmed the mRNA expression of these 13 newly identified viral proteins. Furthermore iTRAQ (isobaric tags for relative and absolute quantification), a quantitative proteomics strategy, was used to distinguish envelope proteins and nucleocapsid proteins of WSSV. Based on iTRAQ ratios, we successfully identified 23 envelope proteins and six nucleocapsid proteins. Our results validated 15 structural proteins with previously known localization in the virion. Furthermore the localization of an additional 12 envelope proteins and two nucleocapsid proteins was determined. We demonstrated that iTRAQ is an effective approach for high throughput viral protein localization determination. Altogether WSSV is assembled by at least 58 structural proteins, including 13 proteins newly identified by shotgun proteomics and one identified by iTRAQ. The localization of 42 structural proteins was determined; 33 are envelope proteins, and nine are nucleocapsid proteins. A comprehensive identification of WSSV structural proteins and their localization should facilitate the studies of its assembly and mechanism of infection.

Characterization of a homologous-region-binding protein from white spot syndrome virus by phage display

Homologous regions (hrs) of white spot syndrome virus (WSSV) might serve as origins of DNA replication or be involved in transcriptional regulation. To characterize the interaction between hrs of WSSV and the viral proteins, in this investigation, phage display technology was used. WSSV genomic DNA was sheared by sonication to generate fragments in lengths between 0.5 and 2.0 kb. Then these fragments were blunt-ended with T4 DNA polymerase and cloned into the EcoRV site of rebuilt vector pCANTAB 5 EE to obtain WSSV genome phage display library. Using a 210 bp DNA from the b minifragment of WSSV hr2 as the bait, biopanning of WSSV genome phage display library for five rounds resulted in the isolation of a recombinant phage clone containing an exogenous DNA fragment of 306 bp. This DNA fragment was identified to be the 5' terminus of the wsv021 open reading frame in WSSV genome. Temporal transcription analysis revealed that the wsv021 gene was transcribed at the early stage of WSSV infection. The gene was expressed as a fusion protein in Escherichia coli XL1-Blue. The electrophoretic mobility shift assay indicated that the recombinant WSV021 protein (rWSV021) could bind specifically to the 210 bp DNA from the b minifragment of WSSV hr2. The wsv021 gene might be a functional gene involved in WSSV replication and transcriptional regulation.

Characterization of a novel envelope protein WSV010 of shrimp white spot syndrome virus and its interaction with a major viral structural protein VP24

White spot syndrome virus is one of the most serious viral pathogens causing huge mortality in shrimp farming. Here we report characterization of WSV010, a novel structural protein identified by our recent shotgun proteomics study. Its ORF contains 294 nucleotides encoding 97 amino acids. Transcription analysis using RT-PCR showed that wsv010 is a late gene. Localization analyses by Western blot and immunoelectron microscopy demonstrated that WSV010 is a viral envelope protein. Furthermore, the pull-down assay revealed that WSV010 could interact with VP24, which is a major envelope protein. Since WSV010 lacks a transmembrane domain, these results suggest that WSV010 may anchor to the envelope through interaction with VP24. Previous studies indicated that VP24 could also interact with VP28 and VP26. Therefore, we propose that VP24 may act as a linker protein to associate these envelope proteins together to form a complex, which may play an important role in viral morphogenesis and viral infection.

Silencing shrimp white spot syndrome virus (WSSV) genes by siRNA

White spot syndrome virus (WSSV) is a major shrimp pathogen causing large economic losses all over the world. So far, however, there is no efficient approach to control this virus. RNA interference (RNAi), which has been applied to silence virus genes in eukaryotic organisms. In this investigation, a specific 21bp short interfering RNA (vp28-siRNA) targeting a major envelope protein gene (vp28) of WSSV was used to induce gene silencing in vivo in Penaeus japonicus shrimp. It was found that the transcription and expression of vp28 gene were silenced by the sequence-specific vp28-siRNA. However, the RNAi effect disappeared or significantly weakened even if one-nucleotide mutation existed in the vp28-siRNA. As revealed by quantitative PCR, the vp28-siRNA caused a significant reduction in viral DNA production of WSSV-infected shrimp. When treated with the vp28-siRNA, WSSV-infected shrimp had a reduced mortality rate. After three injections of the vp28-siRNA, the virus was completely eradicated from WSSV-infected shrimp. These findings suggest that RNAi is capable of silencing sequence-specific genes of WSSV and might constitute a new therapeutic strategy for WSSV infection in shrimp.

PmRab7 is a VP28-binding protein involved in white spot syndrome virus infection in shrimp

Our aim was to isolate and characterize white spot syndrome virus (WSSV)-binding proteins from shrimp. After a blot of shrimp hemocyte membrane proteins was overlaid with a recombinant WSSV envelope protein (rVP28), the reactive bands on the blot were detected using anti-VP28 antibody. Among three membrane-associated molecules identified by liquid chromatography-tandem mass spectrometry, there was a 25-kDa protein that bound to both rVP28 and WSSV. Since it had a primary structure with high homology to the small GTP-binding protein Rab7, we named it Penaeus monodon Rab7 (PmRab7). The full-length PmRab7 cDNA was obtained, and results from a glutathione S-transferase pull-down assay confirmed specific binding to rVP28. Reverse transcriptase PCR analysis revealed PmRab7 expression in many tissues, and real-time PCR analysis revealed that expression was constitutive. Binding of PmRab7 to rVP28 or WSSV occurred in a dose-dependent manner and was inhibited by anti-Rab7 antibody. In an in vivo neutralization assay, the number of dead shrimp after challenge with WSSV plus PmRab7 (15%) or WSSV plus anti-Rab7 antibody (5%) was significantly lower than after challenge with WSSV alone (95%). In contrast to the WSSV-injected group, shrimp injected with WSSV plus PmRab7 or WSSV plus anti-Rab7 showed no WSSV-type histopathology. We conclude that PmRab7 is involved in WSSV infection in shrimp. This is the first study to identify a shrimp protein that binds directly to a major viral envelope protein of WSSV.

Isolation and identification of a novel WSSV nucleocapsid protein by cDNA phage display using an scFv antibody

In a previous study, a scFv phage display library against white spot syndrome virus (WSSV) was constructed and yielded a clone designated A1 with conformational specificity against native but not denatured viral antigen. Although the clone A1 has been used successfully as a diagnostic antibody, its precise target antigen has not been elucidated. A different strategy was adopted involving the construction of a second T7 phage display library utilizing mRNA isolated from shrimp infected with WSSV. Following RT-PCR and T7 phage library construction, phages displaying the candidate epitope were selected with A1 scFv. Since successive enrichment steps were not associated with an increased titer of the phages, enrichment after successive tests was confirmed by PCR resulting in the preferred selection of a specific DNA sequence encoding a novel nucleocapsid protein WSSV388. Immune electron microscopy revealed that WSSV388 is located on the nucleocapsid. This result demonstrated that unknown antigen could be identified by phage display using the epitope conformation dependent scFv.

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 the nucleocapsid, tegument, and envelope proteins of the shrimp white spot syndrome virus virion

The protein components of the white spot syndrome virus (WSSV) virion have been well established by proteomic methods, and at least 39 structural proteins are currently known. However, several details of the virus structure and assembly remain controversial, including the role of one of the major structural proteins, VP26. In this study, Triton X-100 was used in combination with various concentrations of NaCl to separate intact WSSV virions into distinct fractions such that each fraction contained envelope and tegument proteins, tegument and nucleocapsid proteins, or nucleocapsid proteins only. From the protein profiles and Western blotting results, VP26, VP36A, VP39A, and VP95 were all identified as tegument proteins distinct from the envelope proteins (VP19, VP28, VP31, VP36B, VP38A, VP51B, VP53A) and nucleocapsid proteins (VP664, VP51C, VP60B, VP15). We also found that VP15 dissociated from the nucleocapsid at high salt concentrations, even though DNA was still present. These results were confirmed by CsCl isopycnic centrifugation followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and liquid chromatography-nanoelectrospray ionization-tandem mass spectrometry, by a trypsin sensitivity assay, and by an immunogold assay. Finally, we propose an assembly process for the WSSV virion.

Transcriptional profile of shrimp white spot syndrome virus (WSSV) genes with DNA microarray

In an attempt to obtain the transcriptional profile of shrimp white spot syndrome virus (WSSV) genes, DNA microarray analysis was performed using amplified DNA fragments of the WSSV genome that covered most of the presumptive open reading frames (ORFs). Total RNAs were extracted from WSSV-infected crayfish (Cambarus clarkii) and reverse transcribed into cDNA and labeled with 32P-dATP. The DNA microarray detected 81.1% of the ORFs, and 47 of these were transcribed at 6 h post infection, suggesting they may be early genes. Some early genes were further confirmed by temporal analyses of gene transcription. Results in this study indicated the effectiveness of DNA microarray for discovering early viral genes.

Multiple envelope proteins are involved in white spot syndrome virus (WSSV) infection in crayfish

White spot syndrome virus (WSSV) is a devastating viral pathogen of cultured shrimp worldwide. Previous studies have shown that the intact virion consists of at least 39 structural proteins and, among them, six were identified as envelope proteins involved in the virus infection. In this paper, the structural proteins VP36A, VP36B and VP31 (J Virol 2004; 78: 11360-11370), containing the RGD motif, were expressed in Escherichia coli and used to produce specific antibodies. Western blot confirmed that VP36A is a newly reported envelope protein. A neutralization assay with these three antibodies demonstrated that VP36A, VP36B and VP31 could significantly delay the initial infection of crayfish, but mortality still reached 100% at day 11 post-injection. However, a neutralization assay with the combination of antibodies against different envelope proteins showed that a combination of VP36B and VP31 antibodies could strongly inhibit WSSV infection in crayfish. These results revealed that multiple envelope proteins are involved in WSSV infection in crayfish and that VP36B and VP31 play a key role during this process.

Inactivation of White Spot Syndrome Virus (WSSV) by normal rabbit serum: implications for the role of the envelope protein VP28 in WSSV infection of shrimp

White Spot Syndrome Virus (WSSV) is a highly pathogenic and prevalent virus affecting crustacea. A number of WSSV envelope proteins, including vp28, have been proposed to be involved in viral infectivity based on the ability of specific antibodies to attenuate WSSV-induced mortality in vivo. In the present study, a series of monoclonal and polyclonal antibodies targeting vp28 were tested for their ability to neutralize WSSV infectivity, with the purpose of identifying epitopes potentially involved in vp28-mediated infection of shrimp. Surprisingly, when used as protein A-purified immunoglobulin, none of the antibodies tested were capable of inhibiting WSSV infectivity. This included one polyclonal preparation that has been previously shown to inactivate WSSV, when used as whole rabbit serum. Moreover, strong inactivation of WSSV by some rabbit sera was observed, in a manner independent of anti-vp28 antibodies. These results underscore the problems associated with using heterogeneous reagents (e.g. whole rabbit antiserum) in viral neutralization experiments aimed at defining proteins involved in infection by WSSV. In light of this, the potential of anti-vp28 antibodies to specifically neutralize WSSV should be reconsidered.

Activities of membrane bound phosphatases, transaminases and mitochondrial enzymes in white spot syndrome virus infected tissues of Fenneropenaeus indicus

Disease caused by viruses, especially white spot syndrome virus (WSSV), present the greatest challenge to shrimp aquaculture worldwide. Massive tissue disintegration occurs in WSSV-infected ectodermal and mesodermal tissues of penaeid shrimp. The activities of membrane bound phosphatases (Na(+)K(+)ATPase, Ca(2+)ATPase, Mg(2+)ATPase and Total ATPase), transaminases (alanine transaminase (ALT) and aspartate transaminase (AST)) and mitochondrial enzymes (isocitrate dehydrogenase (ICDH), succinate dehydrogenase (SDH), malate dehydrogenase (MDH), alpha-ketoglutarate dehydrogenase (KGDH), NADH dehydrogenase, cytochrome C oxidase) in WSSV-infected tissues (hemolymph, hepatopancreas, gills and muscle) of Fenneropenaeus indicus were determined at intervals after WSSV infection (0, 24, 48, 72 and after 72 h (moribund)). The activities of phosphatases, transaminases and mitochondrial enzymes in healthy as compared with WSSV-infected hemolymph, hepatopancreas, gills and muscle showed marked divergence throughout the course of infection. WSSV infected hemolymph, hepatopancreas, gills and muscle exhibited significantly reduced activity of membrane bound phosphatases compared with the uninfected animals. Inactivation of these enzymes may occur due to increased production of free radicals, that cause conformational change by oxidation of 'SH' groups present at the active site. Significantly marked elevation in the activities of transaminases (ALT and AST) was observed in WSSV-infected hemolymph, hepatopancreas, gills and muscle compared to the uninfected tissues. This may be due to leakage of these enzymes from the damaged tissues. The activities of mitochondrial enzymes in WSSV-infected tissues were significantly decreased compared to the activities in uninfected animals. WSSV-infected animals showed reduced feeding that may have led to decreased oxidation of glucose via the TCA cycle. Excessive production of free radicals in WSSV-infected animals may have affected aerobic oxidation leading to lower production of ATP. It is concluded that membrane dynamics play a major role in the pathogenesis of WSSV infection.

Identification of a novel envelope protein (VP187) gene from shrimp white spot syndrome virus

A novel protein from white spot syndrome virus (WSSV) was identified to match an open reading frame (wsv209) of WSSV genome by combining SDS-PAGE with mass spectrometry. This ORF contained 4,818 bp, encoding 1,606 aa. The apparent molecular mass of the protein from WSSV virions on SDS-PAGE gel is 187 kDa, so it was named VP187 and its gene was termed vp187. Temporal transcription analysis revealed that vp187 was a late gene. To characterize VP187, a segment of the vp187 gene (vp187p) was cloned into pET-GST vector and expressed as a fusion protein with glutathione S-transferase (GST) in Escherichia coli strain BL21 (DE3). Specific antibody was raised using the purified fusion protein (GST-VP187P). Western blot analysis showed that the mouse anti-GST-VP187P serum reacted specifically with VP187 present either in the WSSV virions or in the viral envelopes, and did not react with fractions of the viral nucleocapsids. VP187 was proved to locate in the WSSV virions as an envelope protein using immunoelectron microscopy.

Identification and characterization of a prawn white spot syndrome virus gene that encodes an envelope protein VP31

Based on a combination of SDS-PAGE and mass spectrometry, a protein with an apparent molecular mass of 31 kDa (termed as VP31) was identified from purified shrimp white spot syndrome virus (WSSV) envelope fraction. The resulting amino acid (aa) sequence matched an open reading frame (WSV340) of the WSSV genome. This ORF contained 783 nucleotides (nt), encoding 261 aa. A fragment of WSV340 was expressed in Escherichia coli as a glutathione S-transferase (GST) fusion protein with a 6His-tag, and then specific antibody was raised. Western blot analysis and the immunoelectron microscope method (IEM) confirmed that VP31 was present exclusively in the viral envelope fraction. The neutralization experiment suggested that VP31 might play an important role in WSSV infectivity.

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.

Vp28 of shrimp white spot syndrome virus is involved in the attachment and penetration into shrimp cells

White spot disease (WSD) is caused by the white spot syndrome virus (WSSV), which results in devastating losses to the shrimp farming industry around the world. However, the mechanism of virus entry and spread into the shrimp cells is unknown. A binding assay in vitro demonstrated VP28-EGFP (envelope protein VP28 fused with enhanced green fluorescence protein) binding to shrimp cells. This provides direct evidence that VP28-EGFP can bind to shrimp cells at pH 6.0 within 0.5 h. However, the protein was observed to enter the cytoplasm 3 h post-adsorption. Meanwhile, the plaque inhibition test showed that the polyclonal antibody against VP28 (a major envelope protein of WSSV) could neutralize the WSSV and block an infection with the virus. The result of competition ELISA further confirmed that the envelope protein VP28 could compete with WSSV to bind to shrimp cells. Overall, VP28 of the WSSV can bind to shrimp cells as an attachment protein, and can help the virus enter the cytoplasm.

Identification of an envelope protein (VP39) gene from shrimp white spot syndrome virus

White spot syndrome virus (WSSV) was purified from the tissues of experimentally infected crayfish (Procambarus clarkii) with high yield. Based on SDS-PAGE of purified WSSV and mass spectrometry analysis, a protein with the molecular mass of 39 kDa was identified to match an open reading frame (ORF), WSV339, of WSSV genome. This ORF was 849 bp in length, encoding a 283 amino acid polypeptide. The protein was named VP39 and its gene was termed as vp39. Temporal transcription analysis revealed that vp39 was a late gene. The gene was cloned into pET-GST vector and expressed as a fusion protein with glutathione S-transferase (GST) in Escherichia coli strain BL21 (DE3). Western blot analysis indicated that VP39 could be detected in the extracts of both the WSSV virions and the viral envelopes. It was further 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.

RING-H2 protein WSSV249 from white spot syndrome virus sequesters a shrimp ubiquitin-conjugating enzyme, PvUbc, for viral pathogenesis

Modification of proteins by ubiquitin is essential for numerous cellular processes. The RING-H2 finger motif has been implicated in ubiquitin-conjugating enzyme (E2)-dependent ubiquitination. Four proteins, WSSV199, WSSV222, WSSV249, and WSSV403, from white spot syndrome virus (WSSV) contain the RING-H2 motif. Here we report that WSSV249 physically interacts with a shrimp ubiquitin-conjugating enzyme, PvUbc, and mediates ubiquitination through its RING-H2 motif in the presence of E1 and PvUbc. Mutations of the putative zinc coordination residues in the RING-H2 domain of WSSV249, however, ablate ubiquitination efficiency. In addition, the RING-H2 domain of WSSV249 is capable of ubiquitination with UbcH1, UbcH2, UbcH5a, UbcH5b, UbcH5c, UbcH6, and UbcH10, respectively, exhibiting a low degree of E2 specificity. Significantly, the expression of WSSV249 and PvUbc increased during infection, as revealed by real-time PCR. Furthermore, in situ hybridization showed that WSSV249 and PvUbc display similar expression patterns in infected shrimps, and immunofluorescence and immunohistochemistry assays showed an increase of PvUbc in infected shrimp cells. These results suggest that the RING-H2 protein WSSV249 from WSSV may function as an E3 ligase via sequestration of PvUbc for viral pathogenesis in shrimp.