The unique stacked rings in the nucleocapsid of the white spot syndrome virus virion are formed by the major structural protein VP664, the largest viral structural protein ever found

Insect cells of Spodoptera frugiperda support WSSV gene replication but not progeny virion assembly

The lacking of stable and susceptible cell lines has hampered research on pathogenic mechanism of crustacean white spot syndrome virus (WSSV). To look for the suitable cell line which can sustain WSSV infection, we performed the studies on WSSV infection in the Spodoptera frugiperda (Sf9) insect cells. In consistent with our previous study in vitro in crayfish hematopoietic tissue cells, the WSSV envelope was detached from nucleocapsid around 2 hpi in Sf9 cells, which was accompanied with the cytoplasmic transport of nucleocapsid toward the cell nucleus within 3 hpi. Furthermore, the expression profile of both gene and protein of WSSV was determined in Sf9 cells after viral infection, in which a viral immediate early gene IE1 and an envelope protein VP28 exhibited gradually increased presence from 3 to 24 hpi. Similarly, the significant increase of WSSV genome replication was found at 3-48 hpi in Sf9 cells after infection with WSSV, indicating that Sf9 cells supported WSSV genome replication. Unfortunately, no assembled progeny virion was observed at 24 and 48 hpi in Sf9 cell nuclei as determined by transmission electron microscope, suggesting that WSSV progeny could not be assembled in Sf9 cell line as the viral structural proteins could not be transported into cell nuclei. Collectively, these findings provide a cell model for comparative analysis of WSSV infection mechanism with crustacean cells.

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.

Three-Dimensional Investigations of Virus-Associated Structures in the Nuclei with White Spot Syndrome Virus (WSSV) Infection in Red Swamp Crayfish (Procambarus clarkii)

Simple Summary

Infections of white spot syndrome virus could be a lethal disease in shrimp culture systems. The 3D structures of the virus-associated structures were obtained through electron tomography. The relationships between the mature and immature particles were connected by the localizations of viral proteins. Our study provided new structural information on the virus through 3D, which extends the limited 2D knowledge of pathogen assembling in the white spot disease, further contributing to the development of a strategy for therapy or prevention.

Abstract

White spot syndrome virus (WSSV) has been reported to cause severe economic loss in the shrimp industry. With WSSV being a large virus still under investigation, the 3D structure of its assembly remains unclear. The current study was planned to clarify the 3D structures of WSSV infections in the cell nucleus of red swamp crayfish (Procambarus clarkii). The samples from various tissues were prepared on the seventh day post-infection. The serial sections of the intestinal tissue were obtained for electron tomography after the ultrastructural screening. After 3D reconstruction, the WSSV-associated structures were further visualized, and the expressions of viral proteins were confirmed with immuno-gold labeling. While the pairs of sheet-like structures with unknown functions were observed in the nucleus, the immature virions could be recognized by the core units of nucleocapsids on a piece of the envelope. The maturation of the particle could include the elongation of core units and the filling of empty nucleocapsids with electron-dense materials. Our observations may bring to light a possible order of WSSV maturation in the cell nucleus of the crayfish, while more investigations remain necessary to visualize the detailed viral–host interactions.

Invasion and Propagation of White Spot Syndrome Virus: Hijacking of the Cytoskeleton, Intracellular Transport Machinery, and Nuclear Import Transporters

The pathogenesis of white spot syndrome virus (WSSV) is largely unclear. In this study, we found that actin nucleation and clathrin-mediated endocytosis were recruited for internalization of WSSV into crayfish hematopoietic tissue (Hpt) cells. This internalization was followed by intracellular transport of the invading virions via endocytic vesicles and endosomes. After envelope fusion within endosomes, the penetrated nucleocapsids were transported along microtubules toward the periphery of the nuclear pores. Furthermore, the nuclear transporter CqImportin α1/β1, via binding of ARM repeat domain within CqImportin α1 to the nuclear localization sequences (NLSs) of viral cargoes and binding of CqImportin β1 to the nucleoporins CqNup35/62 with the action of CqRan for docking to nuclear pores, was hijacked for both targeting of the incoming nucleocapsids toward the nuclear pores and import of the expressed viral structural proteins containing NLS into the cell nucleus. Intriguingly, dysfunction of CqImportin α1/β1 resulted in significant accumulation of incoming nucleocapsids on the periphery of the Hpt cell nucleus, leading to substantially decreased introduction of the viral genome into the nucleus and remarkably reduced nuclear import of expressed viral structural proteins with NLS; both of these effects were accompanied by significantly inhibited viral propagation. Accordingly, the survival rate of crayfish post-WSSV challenge was significantly increased after dysfunction of CqImportin α1/β1, also showing significantly reduced viral propagation, and was induced either by gene silencing or by pharmacological blockade via dietary administration of ivermectin per os. Collectively, our findings improve our understanding of WSSV pathogenesis and support future antiviral designing against WSSV. IMPORTANCE As one of the largest animal DNA viruses, white spot syndrome virus (WSSV) has been causing severe economical loss in aquaculture due to the limited knowledge on WSSV pathogenesis for an antiviral strategy. We demonstrate that the actin cytoskeleton, endocytic vesicles, endosomes, and microtubules are hijacked for WSSV invasion; importantly, the nuclear transporter CqImportin α1/β1 together with CqRan were recruited, via binding of CqImportin β1 to the nucleoporins CqNup35/62, for both the nuclear pore targeting of the incoming nucleocapsids and the nuclear import of expressed viral structural proteins containing the nuclear localization sequences (NLSs). This is the first report that NLSs from both viral structure proteins and host factor are elaborately recruited together to facilitate WSSV infection. Our findings provide a novel explanation for WSSV pathogenesis involving systemic hijacking of host factors, which can be used for antiviral targeting against WSSV disease, such as the blockade of CqImportin α1/β1 with ivermectin.

Antiviral therapy in shrimp through plant virus VLP containing VP28 dsRNA against WSSV

The white spot syndrome virus (WSSV), currently affecting cultured shrimp, causes substantial economic losses to the worldwide shrimp industry. An antiviral therapy using double-stranded RNA interference (dsRNAi) by intramuscular injection (IM) has proven the most effective shrimp protection against WSSV. However, IM treatment is still not viable for shrimp farms. The challenge is to develop an efficient oral delivery system that manages to avoid the degradation of antiviral RNA molecules. The present work demonstrates that VLPs (virus-like particles) allow efficient delivery of dsRNAi as antiviral therapy in shrimp. In particular, VLPs derived from a virus that infects plants, such as cowpea chlorotic mottle virus (CCMV), in which the capsid protein (CP) encapsidates the dsRNA of 563 bp, are shown to silence the WSSV glycoprotein VP28 (dsRNAvp28). In experimental challenges in vivo, the VLPs- dsRNAvp28 protect shrimp against WSSV up to 40% by oral administration and 100% by IM. The novel research demonstrates that plant VLPs, which avoid zoonosis, can be applied to pathogen control in shrimp and also other organisms, widening the application window in nanomedicine.

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.

Cloning and functional characterization of thioredoxin gene from kuruma shrimp Marsupenaeus japonicus

As an important disulfide reductase of the intracellular antioxidant system, Thioredoxin (Trx) plays an important role in maintaining oxidative stress balance and protecting cells from oxidative damage. In recent years, there is increasing evidence that Trx is a key molecule in the pathogenesis of various diseases and a potential therapeutic target for major diseases including lung, colon, cervical, gastric and pancreatic cancer. However, few knowledge is known about the function of Trx in virus infection. In this study, we reported the cloning and functional investigation of a Trx homologue gene, named MjTrx, in shrimp Marsupenaeus japonicus suffered white spot syndrome virus (WSSV) infection. MjTrx is a 105-amino acid polypeptide with a conservative Cys-Gly-Pro-Cys motif in the catalytic center. Phylogenetic trees analysis showed that MjTrx has a higher relationship with Trx from other invertebrate and clustered with Trx1 from arthropod. MjTrx transcripts is abundant in the gill and intestine tissues and can be detected in the hemocytes, heart, stomach, and hepatopancreas tissues. The transcription levels of MjTrx in hemocytes, gills and intestine tissues of shrimp were significantly up-regulated after white spot syndrome virus infection. MjTrx was recombinant expressed in vitro and exhibited obvious disulfide reductase activity. In addition, overexpression MjTrx in shrimp resulted in the increase of hydrogen peroxide (H₂O₂) concentration in vivo. All these results strongly suggested that MjTrx functioned in redox homeostasis regulating and played an important role in shrimp antiviral immunity.

Crustacean Genome Exploration Reveals the Evolutionary Origin of White Spot Syndrome Virus

White spot syndrome virus (WSSV) is a crustacean-infecting, double-stranded DNA virus and is the most serious viral pathogen in the global shrimp industry. WSSV is the sole recognized member of the family Nimaviridae, and the lack of genomic data on other nimaviruses has obscured the evolutionary history of WSSV. Here, we investigated the evolutionary history of WSSV by characterizing WSSV relatives hidden in host genomic data. We surveyed 14 host crustacean genomes and identified five novel nimaviral genomes. Comparative genomic analysis of Nimaviridae identified 28 "core genes" that are ubiquitously conserved in Nimaviridae; unexpected conservation of 13 uncharacterized proteins highlighted yet-unknown essential functions underlying the nimavirus replication cycle. The ancestral Nimaviridae gene set contained five baculoviral per os infectivity factor homologs and a sulfhydryl oxidase homolog, suggesting a shared phylogenetic origin of Nimaviridae and insect-associated double-stranded DNA viruses. Moreover, we show that novel gene acquisition and subsequent amplification reinforced the unique accessory gene repertoire of WSSV. Expansion of unique envelope protein and nonstructural virulence-associated genes may have been the key genomic event that made WSSV such a deadly pathogen.IMPORTANCE WSSV is the deadliest viral pathogen threatening global shrimp aquaculture. The evolutionary history of WSSV has remained a mystery, because few WSSV relatives, or nimaviruses, had been reported. Our aim was to trace the history of WSSV using the genomes of novel nimaviruses hidden in host genome data. We demonstrate that WSSV emerged from a diverse family of crustacean-infecting large DNA viruses. By comparing the genomes of WSSV and its relatives, we show that WSSV possesses an expanded set of unique host-virus interaction-related genes. This extensive gene gain may have been the key genomic event that made WSSV such a deadly pathogen. Moreover, conservation of insect-infecting virus protein homologs suggests a common phylogenetic origin of crustacean-infecting Nimaviridae and other insect-infecting DNA viruses. Our work redefines the previously poorly characterized crustacean virus family and reveals the ancient genomic events that preordained the emergence of a devastating shrimp pathogen.

Identification of microRNAs with heat stress responsive and immune properties in Marsupenaeus japonicus based on next-generation sequencing and bioinformatics analysis: Essential regulators in the heat stress-host interactions

Summer mortality syndrome is one of the most serious issue for Marsupenaeus japonicus aquaculture in China. Since it causes massive economic loss and threatens sustainability of M. japonicus aquaculture industry, thus, there is an urgent desire to reveal the heat stress-host interactions mechanisms that lead to mass mortalities of M. japonicus in hot summer months. MicroRNAs (miRNAs) are small noncoding RNAs that involved in regulation of diverse biological processes, including stress and immune response, and might serve as potential regulators in the heat stress-host interactions. In the present study, miRNAs with heat stress responsive and immune properties were identified and characterized in M. japonicus by small RNA sequencing and bioinformatics analysis. In total, 79 host miRNAs were identified, among which 15 miRNAs were differentially expressed in response to heat stress. Target genes prediction and function annotation revealed that a variety of host cellular processes, such as signal transduction, transcription, anti-stress response, ribosomal biogenesis, lipid metabolism, cytoskeleton, etc, were potentially subject to miRNA-mediated regulation in response to heat stress. Furthermore, a total of 30 host miRNAs that potentially involved in interaction with white spot syndrome virus (WSSV) were obtained via predicting and analyzing the target genes from WSSV. The results showed that a batch of WSSV genes that code for structural proteins and enzymes that are essential for WSSV infection and proliferation, such as envelope proteins, capsid proteins, immediate-early proteins, collagen-like protein, protein kinase, thymidylate synthetase, TATA-box bind protein, etc, were predicted to be targeted by host miRNAs. Several of the host miRNAs with predicted antiviral capacity were down-regulated under heat stress, indicating a repression of host miRNA-mediated antiviral immune response. This study highlighted the essential roles of host miRNAs in the heat stress-host interactions and provided valuable information for further investigation on the mechanism of miRNA-mediated heat stress and immune response of shrimp.

Variable RNA expression from recently acquired, endogenous viral elements (EVE) of white spot syndrome virus (WSSV) in shrimp

The viral accommodation hypothesis proposes that endogenous viral elements (EVE) from both RNA and DNA viruses are being continually integrated into the shrimp genome by natural host processes and that they can result in tolerance to viral infection by fortuitous production of antisense, immunospecific RNA (imRNA). Thus, we hypothesized that previously reported microarray results for the presence of white spot syndrome virus (WSSV) open reading frames (ORFs) formerly called 151, 366 and 427 in a domesticated giant tiger shrimp (Penaeus monodon) breeding stock might have represented expression from EVE, since the stock had shown uninterrupted freedom from white spot disease (WSD) for many generations. To test this hypothesis, 128 specimens from a current stock generation were confirmed for freedom from WSSV infection using two nested PCR detection methods. Subsequent nested-PCR testing revealed 33/128 specimens (26%) positive for at least one of the ORF at very high sequence identity (95-99%) to extant WSSV. Positive results for ORF 366 (now known to be a fragment of the WSSV capsid protein gene) dominated (28/33 = 84.8%), so 9 arbitrarily selected 366-positive specimens were tested by strand-specific, nested RT-PCR using DNase-treated RNA templates. This revealed variable RNA expression in individual shrimp including no RNA transcripts (n = 1), sense transcripts only (n = 1), antisense transcripts only (n = 2) or transcripts of both sense (n = 5). The latter 7 expression products indicated specimens producing putative imRNA. The variable types and numbers of the EVE and the variable RNA expression (including potential imRNA) support predictions of the viral accommodation hypothesis that EVE are randomly produced and expressed. Positive nested PCR test results for EVE of ORF 366 using DNA templates derived from shrimp sperm (germ cells), indicated that they were heritable.

Molecular docking and simulation studies of Phyllanthus amarus phytocompounds against structural and nucleocapsid proteins of white spot syndrome virus

White spot disease caused by white spot syndrome virus (WSSV) is a lethal disease for shrimp. Envelope structural proteins play a major role in viral attachment and are believed to be the initial molecules to interact with the host cell. Thus, the envelope proteins have been preferred as a potential molecular target for drug discovery. In the present investigation, molecular docking and simulation analysis were performed to predict the binding efficiency of phytocompounds identified from Phyllanthus amarus with major envelope proteins, VP26, VP28, and VP110, and a nucleocapsid protein VP664 of WSSV. The docking result reveals that the compounds 2H-1-benzopyran-6-ol, 3,4-dihydro-2,5,7,8-tetramethyl-2-(4,8,12-trimethyltridecyl)-acetate and 1,4-benzenediamine, N,N'-diphenyl exhibited highest binding energy with the envelope proteins. The mobility of protein-ligand binding complex at various time intervals was validated by molecular dynamics and simulation study. Therefore, P. amarus phytocompounds were found to be most suitable inhibitors for the antiviral treatment for WSSV infection.

Kinetic analysis of internalization of white spot syndrome virus by haemocyte subpopulations of penaeid shrimp, Litopenaeus vannamei (Boone), and the outcome for virus and cell

Little is known about the innate antiviral defence of shrimp haemocytes. In this context, the haemocytes of penaeid shrimp Litopenaeus vannamei (Boone) were separated by iodixanol density gradient centrifugation into five subpopulations (sub): sub 1 (hyalinocytes), sub 2 and 3 (prohyalinocytes), sub 4 (semigranulocytes) and sub 5 (granulocytes) and exposed to beads, white spot syndrome virus (WSSV) and ultraviolet (UV)-killed WSSV. In a first experiment, the uptake of beads, white spot syndrome virus (WSSV) and UV-killed WSSV by these different haemocyte subpopulations was investigated using confocal microscopy. Only haemocytes of sub 1, 4 and 5 were internalizing beads, WSSV and UV-killed WSSV. Beads were engulfed by a much larger percentage of cells (91.2 in sub 1; 84.1 in sub 4 and 58.1 in sub 5) compared to WSSV (9.6 in sub 1; 10.5 in sub 4 and 7.9 in sub 5) and UV-killed WSSV (12.9 in sub 1; 13.3 in sub 4; and 11.8 in sub 5). In a second experiment, it was shown that upon internalization, WSS virions lost their envelope most probably by fusion with the cellular membrane of the endosome (starting between 30 and 60 min post-inoculation) and that afterwards the capsid started to become disintegrated (from 360 min post-inoculation). Expression of new viral proteins was not observed. Incubation of haemocyte subpopulations with WSSV but not with UV-killed WSSV and polystyrene beads resulted in a significant drop in haemocyte viability. To find the underlying mechanism, a third experiment was performed in which haemocyte subpopulations were exposed to a short WSSV DNA fragment (VP19) and CpG ODNs. These small DNA fragments induced cell death. In conclusion, WSSV is efficiently internalized by hyalinocytes, semigranulocytes and granulocytes, after which the virus loses its envelope; as soon as the capsids start to disintegrate, cell death is activated, which in part may be explained by the exposure of viral DNA to cellular-sensing molecules.

WSV399, a viral tegument protein, interacts with the shrimp protein PmVRP15 to facilitate viral trafficking and assembly

Viral responsive protein 15 (PmVRP15) has been identified as a highly up-regulated gene in the hemocyte of white spot syndrome virus (WSSV)-infected shrimp Penaeus monodon. However, the function of PmVRP15 in host-viral interaction was still unclear. To elucidate PmVRP15 function, the interacting partner of PmVRP15 from WSSV was screened by yeast two-hybrid assay and then confirmed by co-immunoprecipitation (Co-IP). Only WSV399 protein was identified as a PmVRP15 binding protein; however, the function of WSV399 has not been characterized. Localization of WSV399 on the WSSV virion was revealed by immunoblotting analysis (in vitro) and immunoelectron microscopy (in vivo). The results showed that WSV399 is a structural protein of the WSSV virion and is particularly located on the tegument. Gene silencing of wsv399 in WSSV-infected shrimp reduced the percentage of cumulative mortality by 74%, although the expression level of a viral replication marker gene, vp28, was not changed suggesting that WSV399 might not involved in viral replication but viral assembly. Because it has already been known that tegument proteins function in capsid transport during viral trafficking and assembly, interaction between PmVRP15 on hemocyte nuclear membrane and the WSV399 viral tegument protein suggests that PmVRP15 might be required for trafficking and assembly of WSSV during infection.

Two host microRNAs influence WSSV replication via STAT gene regulation

MicroRNAs (miRNAs) have important roles in post-transcriptional regulation of gene expression. During viral infection, viruses utilize hosts to enhance their replication by altering cellular miRNAs. The Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway plays crucial roles in the antiviral responses. In this study, two miRNAs (miR-9041 and miR-9850) from Macrobrachium rosenbergii were found to promote white spot syndrome virus (WSSV) replication. The up-regulation of miR-9041 or miR-9850 suppresses STAT expression in the gills of M. rosenbergii, which subsequently down-regulates the expression of its downstream dynamin (Dnm) genes: Dnm1, Dnm2, and Dnm3. Knockdown of miR-9041 and miR-9850 restricts WSSV replication by up-regulating STAT and Dnm gene expression. The silencing of STAT, Dnm1, Dnm2, or Dnm3 led to an increase of the number of WSSV copies in shrimp. The injection of recombinant Dnm1, Dnm2, or Dnm3 proteins could inhibit WSSV replication in vivo. Overall, our research indicates the roles of host miRNAs in the enhancement of WSSV replication by regulating the host JAK/STAT pathway.

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.

Immunological-based assays for specific detection of shrimp viruses

Among shrimp viral pathogens, white spot syndrome virus (WSSV) and yellow head virus (YHV) are the most lethal agents, causing serious problems for both the whiteleg shrimp, Penaeus (Litopenaeus) vannamei, and the black tiger shrimp, Penaeus (Penaeus) monodon. Another important virus that infects P. vannamei is infectious myonecrosis virus (IMNV), which induces the white discoloration of affected muscle. In the cases of taura syndrome virus and Penaeus stylirostris densovirus (PstDNV; formerly known as infectious hypodermal and hematopoietic necrosis virus), their impacts were greatly diminished after the introduction of tolerant stocks of P. vannamei. Less important viruses are Penaeus monodon densovirus (PmDNV; formerly called hepatopancreatic parvovirus), and Penaeus monodon nucleopolyhedrovirus (PemoNPV; previously called monodon baculovirus). For freshwater prawn, Macrobrachium rosenbergii nodavirus and extra small virus are considered important viral pathogens. Monoclonal antibodies (MAbs) specific to the shrimp viruses described above have been generated and used as an alternative tool in various immunoassays such as enzyme-linked immunosorbent assay, dot blotting, Western blotting and immunohistochemistry. Some of these MAbs were further developed into immunochromatographic strip tests for the detection of WSSV, YHV, IMNV and PemoNPV and into a dual strip test for the simultaneous detection of WSSV/YHV. The strip test has the advantages of speed, as the result can be obtained within 15 min, and simplicity, as laboratory equipment and specialized skills are not required. Therefore, strip tests can be used by shrimp farmers for the pond-side monitoring of viral infection.

Properties of Litopenaeus vannamei Dscam (LvDscam) isoforms related to specific pathogen recognition

Arthropod Down syndrome cell adhesion molecules (Dscam) may sometimes function as hypervariable pathogen recognition receptors. They consist of an extracellular region and a cytoplasmic tail, both of which are highly variable. In shrimp, tail-less Dscam proteins (Dscams) have recently been identified, and these appear to be unlike other arthropod extracellular Dscams that are released from the cell membrane by proteolytic cleavage. Here we investigate the properties of these unique shrimp proteins and show that they can be directly secreted from transfected cells. We also investigate the diverse cytoplasmic tail variants of membrane-bound shrimp Dscams, and show that elements E1A and E3 seem to be related to Dscam immune function. Challenge with Vibrio harveyi not only enhanced total Dscam and the immune-related cytoplasmic tail variants, but also induced expression of certain Ig2 + Ig3 combinations. A pathogen binding assay with these Ig2 + Ig3 extracellular variants showed that both the V. harveyi-induced Dscams and Dscams induced by buffer injection could be either pathogen-specific or specific only for Gram-negative pathogens, while other "general" Dscam variants were sensitive to a wide range of pathogens. The same assay also suggested that shrimp Dscam isoforms show a stronger response to the host's natural pathogens.

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.

Biology, Host Range, Pathogenesis and Diagnosis of White spot syndrome virus

White spot syndrome virus (WSSV) is the most serious viral pathogen of cultured shrimp. It is a highly virulent virus that can spread quickly and can cause up to 100 % mortality in 3-10 days. WSSV is a large enveloped double stranded DNA virus belonging to genus Whispovirus of the virus family Nimaviridae. It has a wide host range among crustaceans and mainly affects commercially cultivated marine shrimp species. The virus infects all age groups causing large scale mortalities and the foci of infection are tissues of ectodermal and mesodermal origin, such as gills, lymphoid organ and cuticular epithelium. The whole genome sequencing of WSSV from China, Thailand and Taiwan have revealed minor genetic differences among different strains. There are varying reports regarding the factors responsible for WSSV virulence which include the differences in variable number of tandem repeats, the genome size and presence or absence of different proteins. Aim of this review is to give current information on the status, host range, pathogenesis and diagnosis of WSSV infection.

Nucleocapsid protein VP15 of White spot syndrome virus colocalizes with the nucleolar proteins nucleolin and fibrillarin

The core nucleocapsid protein VP15 of White spot syndrome virus (WSSV) was shown to interact with DNA and predicted to be involved in the packaging of the WSSV genome. In the present study, we explored the colocalization of VP15 with several nuclear proteins in insect cells. The results showed that the VP15 completely colocalized with nucleolin and fibrillarin, suggesting that VP15 is a nucleolar localization protein and plays an important role in the life cycle of WSSV in host cells.

Epitope analysis of white spot syndrome virus of Penaeus monodon by in vivo neutralization assay employing a panel of monoclonal antibodies

A panel of six monoclonal antibodies (MAbs) against the major envelope proteins VP18, VP26 and VP28 of white spot syndrome virus (WSSV) was evaluated for neutralization of the virus in vivo in Penaeus monodon. WSSV stock diluted to 1 x 10⁻⁶ resulting in 100% mortality on 12 day post injection (dpi) was used as optimum infectious dose of virus for challenge. Constant quantity (100 μg/ml) of MAbs C-5, C-14, C-33, C-38, C-56 and C-72 was incubated separately with WSSV (1 x 10⁻⁶ dilution) at 27 °C for 90 min and injected to shrimp. WSSV infection was neutralized by the MAbs C-5, C-14 and C-33 with a relative percent survival (RPS) of 60, 80 and 60 on 12 dpi, respectively compared to 100% mortality in positive control injected with WSSV alone. MAbs C-38, C-56 and C-72 could neutralize WSSV infection with RPS on 12 dpi of 40, 30 and 30, respectively. Shrimp injected with WSSV (1 x 10⁻⁶ dilution) incubated with panel of the MAbs at 100 μg/ml separately were subjected to nested PCR analysis at 0, 8, 12, 24, 36, 48 and 72 hour post injection (hpi) to provide further evidence for neutralization. MAbs C-5, C-14 and C-33 showed delay in WSSV positivity by 24 and 48 hpi by 2nd and 1st step PCR, respectively. MAbs C-38, C-56 and C-72 showed WSSV positivity by 12 and 24 hpi by 2nd and 1st step PCR, respectively. Shrimp injected with WSSV alone showed WSSV positivity by 8 and 12 hpi by 2nd and 1st step PCR, respectively. The study clearly shows that infectivity of WSSV could be delayed by MAbs C-14, C-5 and C-33.

Proteomic analysis of Glossina pallidipes salivary gland hypertrophy virus virions for immune intervention in tsetse fly colonies

Many species of tsetse flies (Diptera: Glossinidae) can be infected by a virus that causes salivary gland hypertrophy (SGH). The genomes of viruses isolated from Glossina pallidipes (GpSGHV) and Musca domestica (MdSGHV) have recently been sequenced. Tsetse flies with SGH have reduced fecundity and fertility which cause a serious problem for mass rearing in the frame of sterile insect technique (SIT) programmes to control and eradicate tsetse populations in the wild. A potential intervention strategy to mitigate viral infections in fly colonies is neutralizing of the GpSGHV infection with specific antibodies against virion proteins. Two major GpSGHV virion proteins of about 130 and 50&emsp14;kDa, respectively, were identified by Western analysis using a polyclonal rabbit antibody raised against whole GpSHGV virions. The proteome of GpSGHV, containing the antigens responsible for the immune-response, was investigated by liquid chromatography tandem mass spectrometry and 61 virion proteins were identified by comparison with the genome sequence. Specific antibodies were produced in rabbits against seven candidate proteins, including the ORF10/C-terminal fragment, ORF47 and ORF96 as well as proteins involved in peroral infectivity PIF-1 (ORF102), PIF-2 (ORF53), PIF-3 (ORF76) and P74 (ORF1). Antiserum against ORF10 specifically reacted to the 130&emsp14;kDa protein in a Western blot analysis and to the envelope protein of GpSGHV, detected by using immunogold-electron microscopy. This result suggests that immune intervention of viral infections in colonies of G. pallidipes is a realistic option.

The Marsupenaeus japonicus voltage-dependent anion channel (MjVDAC) protein is involved in white spot syndrome virus (WSSV) pathogenesis

Voltage-dependent anion channel (VDAC) proteins abound in the outer membrane of mitochondria. They play an important role in mitochondrial membrane permeabilization (MMP), which can lead to stress-induced cellular apoptosis and necrosis. Several pathogens regulate this MMP in their host cells to benefit their replication cycle, while in other cases, the host can use the same mechanism to combat pathogenesis. In this study, the first shrimp VDAC gene was identified and characterized from Marsupenaeus japonicus (MjVDAC). Its open reading frame (ORF) contained 849 bp encoding 282 amino acids. The deduced MjVDAC protein includes the 4-element eukaryotic porin signature motif, the conserved ATP binding motif (the GLK motif) and a VKAKV-like sequence known in other organisms to be involved in the protein's incorporation in the mitochondrial outer membrane. Tissue tropism analysis indicated that MjVDAC is abundant in the heart, muscle, stomach and pleopod. MjVDAC proteins colocalized with mitochondria in transiently transfected Sf9 cells and in shrimp hemocytes. dsRNA silencing of shrimp VDAC delayed white spot syndrome virus (WSSV) infection by 1 day in different shrimp organs. Taken together, these findings suggest that MjVDAC is likely to be involved in WSSV pathogenesis.

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.

Viral disease emergence in shrimp aquaculture: origins, impact and the effectiveness of health management strategies

Shrimp aquaculture has grown rapidly over several decades to become a major global industry that serves the increasing consumer demand for seafood and has contributed significantly to socio-economic development in many poor coastal communities. However, the ecological disturbances and changes in patterns of trade associated with the development of shrimp farming have presented many of the pre-conditions for the emergence and spread of disease. Shrimp are displaced from their natural environments, provided artificial or alternative feeds, stocked in high density, exposed to stress through changes in water quality and are transported nationally and internationally, either live or as frozen product. These practices have provided opportunities for increased pathogenicity of existing infections, exposure to new pathogens, and the rapid transmission and transboundary spread of disease. Not surprisingly, a succession of new viral diseases has devastated the production and livelihoods of farmers and their sustaining communities. This review examines the major viral pathogens of farmed shrimp, the likely reasons for their emergence and spread, and the consequences for the structure and operation of the shrimp farming industry. In addition, this review discusses the health management strategies that have been introduced to combat the major pathogens and the reasons that disease continues to have an impact, particularly on poor, small-holder farmers in Asia.

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.

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.

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.

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.

Protein expression profiling of the shrimp cellular response to white spot syndrome virus infection

To better understand the pathogenesis of white spot syndrome virus (WSSV) and to determine which cell pathways might be affected after WSSV infection, two-dimensional gel electrophoresis (2-DE) was used to produce protein expression profiles from samples taken at 48 h post-infection (hpi) from the stomachs of Litopenaeus vannamei (also called Penaeus vannamei) that were either specific pathogen free or else infected with WSSV. Seventy-five protein spots that consistently showed either a marked change (>50%) in accumulated levels or else were highly expressed throughout the course of WSSV infection were selected for further study. After in-gel trypsin digestion followed by LC-nanoESI-MS/MS, bioinformatics databases were searched for matches. A total of 53 proteins were identified, with functions that included energy production, calcium homeostasis, nucleic acid synthesis, signaling/communication, oxygen carrier/transportation, and SUMO-related modification. 2-DE results were shown to be consistent with relative EST database data from a previously developed EST database of two Penaeus monodon cDNA libraries. For seven selected genes, 2-DE and EST data were also compared with transcriptional time-course RT-PCR data. This study is the first global analysis of differentially expressed proteins in WSSV-infected shrimp, and in addition to increasing our understanding of the molecular pathogenesis of this virus-associated shrimp disease, the results presented here should be useful both for identifying potential biomarkers and for developing antiviral measures.

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.

White spot syndrome virus annexes a shrimp STAT to enhance expression of the immediate-early gene ie1

Although the Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling pathway is part of the antiviral response in arthropods such as Drosophila, here we show that white spot syndrome virus (WSSV) uses a shrimp STAT as a transcription factor to enhance viral gene expression in host cells. In a series of deletion and mutation assays using the WSSV immediate-early gene ie1 promoter, which is active in shrimp cells and also in insect Sf9 cells, an element containing a STAT binding motif was shown to be important for the overall level of WSSV ie1 promoter activity. In the Sf9 insect cell line, a specific protein-DNA complex was detected by using electrophoresis mobility shift assays (EMSA) with the 32P-labeled STAT binding motif of the WSSV ie1 promoter as the probe. When recombinant Penaeus monodon STAT (rPmSTAT) was overexpressed in Sf9 cells, EMSA with specific antibodies confirmed that the STAT was responsible for the formation of the specific protein-DNA complex. Another EMSA showed that in WSSV-infected P. monodon, levels of activated PmSTAT were higher than in WSSV-free P. monodon. A transactivation assay of the WSSV ie1 promoter demonstrated that increasing the level of rPmSTAT led to dose-dependent increases in ie1 promoter activity. These results show that STAT directly transactivates WSSV ie1 gene expression and contributes to its high promoter activity. We conclude that WSSV successfully annexes a putative shrimp defense mechanism, which it uses to enhance the expression of viral immediate-early genes.

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.

Proteomic analysis of the major envelope and nucleocapsid proteins of white spot syndrome virus

White spot syndrome virus (WSSV) virions were purified from the tissues of infected Procambarus clarkii (crayfish) isolates. Pure WSSV preparations were subjected to Triton X-100 treatment to separate into the envelope and nucleocapsid fractions, which were subsequently separated by 12% sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The major envelope and nucleocapsid proteins were identified by either matrix-assisted laser desorption ionization-time of flight mass spectrometry or defined antibody. A total of 30 structural proteins of WSSV were identified in this study; 22 of these were detected in the envelope fraction, 7 in the nucleocapsid fraction, and 1 in both the envelope and the nucleocapsid fractions. With the aid of specific antibodies, the localizations of eight proteins were further studied. The analysis of posttranslational modifications revealed that none of the WSSV structural proteins was glycosylated and that VP28 and VP19 were threonine phosphorylated. In addition, far-Western and coimmunoprecipitation experiments showed that VP28 interacted with both VP26 and VP24. In summary, the data obtained in this study should provide an important reference for future molecular studies of WSSV morphogenesis.

In silico identification of putative promoter motifs of White Spot Syndrome Virus

Background

White Spot Syndrome Virus, a member of the virus family Nimaviridae, is a large dsDNA virus infecting shrimp and other crustacean species. Although limited information is available on the mode of transcription, previous data suggest that WSSV gene expression occurs in a coordinated and cascaded fashion. To search in silico for conserved promoter motifs (i) the abundance of all 4 through 8 nucleotide motifs in the upstream sequences of WSSV genes relative to the complete genome was determined, and (ii) a MEME search was performed in the upstream sequences of either early or late WSSV genes, as assigned by microarray analysis. Both methods were validated by alignments of empirically determined 5' ends of various WSSV mRNAs.

Results

The collective information shows that the upstream region of early WSSV genes, containing a TATA box and an initiator, is similar to Drosophila RNA polymerase II core promoter sequences, suggesting utilization of the cellular transcription machinery for generating early transcripts. The alignment of the 5' ends of known well-established late genes, including all major structural protein genes, identified a degenerate motif (ATNAC) which could be involved in WSSV late transcription. For these genes, only one contained a functional TATA box. However, almost half of the WSSV late genes, as previously assigned by microarray analysis, did contain a TATA box in their upstream region.

Conclusion

The data may suggest the presence of two separate classes of late WSSV genes, one exploiting the cellular RNA polymerase II system for mRNA synthesis and the other generating messengers by a new virus-induced transcription mechanism.

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.