White spot syndrome virus: an overview on an emergent concern

Sf-PHB2, a new transcription factor, drives WSSV Ie1 gene expression via a 12-bp DNA element

BACKGROUND

The WSSV immediate early gene ie1 is highly expressed throughout viral infection cycle and may play a central role in initiating viral replication during infection.

RESULTS

Here, a detailed characterization of the ie1 promoter was performed using deletion and mutation analyses to elucidate the role of the individual promoter motifs. Three results were obtained: 1) the ie1 promoter is a classical eukaryotic promoter that contains the initiator element (Inr) and TATA box responsible for the basal promoter activity; 2) mutation or truncation of a predicted Sp1 site decreased the level of promoter activity by about 3-fold, indicating that the Sp1 site is an important cis-element of the promoter; and 3) truncation of a 12-bp sequence that resides at -78/-67 of the ie1 promoter decreased the level of promoter activity by about 14-fold, indicating that the 12-bp motif is a critical upstream element of the ie1 promoter for binding of a strong transcription factor to drive the ie1 gene expression in the cells. Further, the 12-bp DNA binding protein was purified from the nuclear proteins of Sf9 cells using DNA affinity chromatography, and was identified as a homologue of the prohibitin2 protein (named as Sf-PHB2) using mass spectrometry. Furthermore, the DNA binding activity of Sf-PHB2 was verified using a super shift analysis.

CONCLUSION

These results support that the Sf-PHB2 is a novel transcription factor that drives WSSV ie1 gene expression by binding to the 12-bp DNA element.

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.

Establishment of shrimp cell lines: perception and orientation

Development of continuous shrimp cell lines for effective investigation on shrimp viruses remains elusive with an arduous history of over 25 years. Despite presenting challenges to researchers in developing a cell line, the billion dollar aquaculture industry is under viral threat. Advances in molecular biology and various gene transfer technologies for immortalization of cells have resulted in the development of hundreds of cell lines from insects and mammals, but yet not a single cell line has been developed from shrimp and other marine invertebrates. Though improved growth and longevity of shrimp cells in vitro could be achieved by using modified growth media this did not make any leap to spontaneous transformation; probably due to the fact that shrimp cells inhibited neoplastic transformations. Oncogenic induction and immortalization are considered as the possible ways, and an exclusive medium for shrimp cell culture and an appropriate mode of transformation are crucial. In this review status of shrimp cell line development and its future orientation are discussed.

White spot syndrome virus: Genotypes, Epidemiology and Evolutionary Studies

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

Expression Profiling of WSSV ORF 199 and Shrimp Ubiquitin Conjugating Enzyme in WSSV Infected Penaeus monodon

White spot syndrome virus (WSSV) is one of the major viral pathogens affecting shrimp aquaculture. Four proteins, WSSV199, WSSV 222, WSSV 249 and WSSV 403, from WSSV are predicted to encode a RING-H2 domain, which in presence of ubiquitin conjugating enzyme (E2) in shrimp can function as viral E3 ligase and modulate the host ubiquitin proteasome pathway. Modulation of host ubiquitin proteasome pathway by viral proteins is implicated in viral pathogenesis. In the present study, a time course expression profile analysis of WSSV Open Reading Frame (ORF) 199 and Penaeus monodon ubiquitin conjugating enzyme (PmUbc) was carried out at 0, 3, 6, 12, 24, 48 and 72 h post WSSV challenge by semi-quantitative RT-PCR as well as Real Time PCR. EF1α was used as reference control to normalize the expression levels. A significant increase in PmUbc expression at 24 h post infection (h.p.i) was observed followed by a decline till 72 h.p.i. Expression of WSSV199 was observed at 24 h.p.i in WSSV infected P. monodon. Since the up-regulation of PmUbc was observed at 24 h.p.i where WSSV199 expression was detected, it can be speculated that these proteins might interact with host ubiquitination pathway for viral pathogenesis. However, further studies need to be carried out to unfold the molecular mechanism of interaction between host and virus to devise efficient control strategies for this chaos in the shrimp culture industry.

Genome dynamics in three different geographical isolates of white spot syndrome virus (WSSV)

White spot syndrome virus (WSSV), the sole member of the monotypic family Nimaviridae, is considered an extremely lethal shrimp pathogen. Despite its impact, some essential biological characteristics related to WSSV genome dynamics, such as the synonymous codon usage pattern and selection pressure in genes, remain to be elucidated. The results show that compositional limitations and mutational pressure determine the codon usage bias and base composition in WSSV. Furthermore, different forces of selective pressure are acting across various regions of the WSSV genome. Finally, this study points out the possible occurrence of two major recombination events.

Gene expression and protein levels of thioredoxin in the gills from the whiteleg shrimp (Litopenaeus vannamei) infected with two different viruses: the WSSV or IHHNV

The thioredoxin (TRX) system in crustaceans has demonstrated to act as a cell antioxidant being part of the immune response by dealing with the increased production of reactive oxygen species during bacterial or viral infection. Since the number of marine viruses has increased in the last years significantly affecting aquaculture practices of penaeids, and due to the adverse impact on wild and cultured shrimp populations, it is important to elucidate the dynamics of the shrimp response to viral infections. The role of Litopenaeus vannamei thioredoxin (LvTRX) was compared at both, mRNA and protein levels, in response to two viruses, the white spot syndrome virus (WSSV) and the infectious hypodermal and hematopoietic necrosis virus (IHHNV). The results confirmed changes in the TRX gene expression levels of WSSV-infected shrimp, but also demonstrated a more conspicuous response of TRX to WSSV than to IHHNV. While both the dimeric and monomeric forms of LvTRX were detected by Western blot analysis during the WSSV infection, the dimer on its reduced form was only detected through the IHHNV infectious process. These findings indicate that WSSV or IHHNV infected shrimp may induce a differential response of the LvTRX protein.

Feeding hermit crabs to shrimp broodstock increases their risk of WSSV infection

White spot syndrome virus (WSSV) is a serious shrimp pathogen that has spread globally to all major shrimp farming areas, causing enormous economic losses. Here we investigate the role of hermit crabs in transmitting WSSV to Penaeus monodon brooders used in hatcheries in Vietnam. WSSV-free brooders became PCR-positive for WSSV within 2 to 14 d, and the source of infection was traced to hermit crabs being used as live feed. Challenging hermit crabs with WSSV confirmed their susceptibility to infection, but they remained tolerant to disease even at virus loads equivalent to those causing acute disease in shrimp. As PCR screening also suggests that WSSV infection occurs commonly in hermit crab populations in both Vietnam and Taiwan, their use as live feed for shrimp brooders is not recommended.

The role of selective breeding and biosecurity in the prevention of disease in penaeid shrimp aquaculture

About 3.5 million metric tons of farmed shrimp were produced globally in 2009 with an estimated value greater than USD$14.6 billion. Despite the economic importance of farmed shrimp, the global shrimp farming industry continues to be plagued by disease. There are a number of strategies a shrimp farmer can employ to mitigate crop loss from disease, including the use of Specific Pathogen Free (SPF), selectively bred shrimp and the adoption of on-farm biosecurity practices. Selective breeding for disease resistance began in the mid 1990s in response to outbreaks of Taura syndrome, caused by Taura syndrome virus (TSV), which devastated populations of farmed shrimp (Litopenaeus vannamei) throughout the Americas. Breeding programs designed to enhance TSV survival have generated valuable information about the quantitative genetics of disease resistance in shrimp and have produced shrimp families which exhibit high survival after TSV exposure. The commercial availability of these selected shrimp has benefitted the shrimp farming industry and TSV is no longer considered a major threat in many shrimp farming regions. Although selective breeding has been valuable in combating TSV, this approach has not been effective for other viral pathogens and selective breeding may not be the most effective strategy for the long-term viability of the industry. Cost-effective, on-farm biosecurity protocols can be more practical and less expensive than breeding programs designed to enhance disease resistance. Of particular importance is the use of SPF shrimp stocked in biosecure environments where physical barriers are in place to mitigate the introduction and spread of virulent pathogens.

Historic emergence, impact and current status of shrimp pathogens in Asia

It is estimated that approximately 60% of disease losses in shrimp aquaculture have been caused by viral pathogens and 20% by bacterial pathogens. By comparison, losses to fungi and parasites have been relatively small. For bacterial pathogens, Vibrio species are the most important while for viral pathogens importance has changed since 2003 when domesticated and genetically selected stocks of the American whiteleg shrimp Penaeus (Litopenaeus) vannamei (Boone 1931) replaced the formerly dominant giant tiger or black tiger shrimp Penaeus (Penaeus) monodon (Fabricius 1798) as the dominant cultivated species. For both species, white spot syndrome virus (WSSV) and yellow head virus (YHV) are the most lethal. Next most important for P. vannamei is infectious myonecrosis virus (IMNV), originally reported from Brazil, but since 2006 from Indonesia where it was probably introduced by careless importation of shrimp aquaculture stocks. So far, IMNV has not been reported from other countries in Asia. Former impacts of Taura syndrome virus (TSV) and infectious hypodermal and hematopoietic necrosis virus (IHHNV) on this species have dramatically declined due to the introduction of tolerant stocks and to implementation of good biosecurity practices. Another problem recently reported for P. vannamei in Asia is abdominal segment deformity disease (ASDD), possibly caused by a previously unknown retrovirus-like agent. Next most important after WSSV and YHV for P. monodon is monodon slow growth syndrome (MSGS) for which component causes appear to be Laem Singh virus (LSNV) and a cryptic integrase containing element (ICE). Hepatopancreatic parvovirus (HPV) and monodon baculovirus (MBV) may be problematic when captured P. monodon are used to produce larvae, but only in the absence of proper preventative measures. Since 2009 increasing losses with P. vannamei in China, Vietnam and now Thailand are associated with acute hepatopancreatic necrosis syndrome (AHPNS) of presently unknown cause. Despite these problems, total production of cultivated penaeid shrimp from Asia will probably continue to rise as transient disease problems are solved and use of post larvae originating from domesticated SPF shrimp stocks in more biosecure settings expands.

Co-interactive DNA-binding between a novel, immunophilin-like shrimp protein and VP15 nucleocapsid protein of white spot syndrome virus

White spot syndrome virus (WSSV) is one of the most serious pathogens of penaeid shrimp. Although its genome has been completely characterized, the functions of most of its putative proteins are not yet known. It has been suggested that the major nucleocapsid protein VP15 is involved in packaging of the WSSV genome during virion formation. However, little is known in its relationship with shrimp host cells. Using the yeast two-hybrid approach to screen a shrimp lymphoid organ (LO) cDNA library for proteins that might interact with VP15, a protein named PmFKBP46 was identified. It had high sequence similarity to a 46 kDa-immunophilin called FKBP46 from the lepidopteran Spodoptera frugiperda (the fall armyworm). The full length PmFKBP46 consisted of a 1,257-nucleotide open reading frame with a deduced amino acid sequence of 418 residues containing a putative FKBP-PPIase domain in the C-terminal region. Results from a GST pull-down assay and histological co-localization revealed that VP15 physically interacted with PmFKBP46 and that both proteins shared the same subcellular location in the nucleus. An electrophoretic mobility shift assay indicated that PmFKBP46 possessed DNA-binding activity and functionally co-interacted with VP15 in DNA binding. The overall results suggested that host PmFKBP46 might be involved in genome packaging by viral VP15 during virion assembly.

Analysis of variable genomic loci in white spot syndrome virus to predict its origins in Procambarus clarkii crayfish farmed in China

Variable genomic loci were examined in 4 white spot syndrome virus (WSSV) isolates (08HB, 09HB, 08JS and 09JS) from Procambarus clarkii crayfish collected from Jiangsu and Hubei Provinces in China in 2008 and 2009. In ORF75, sequence variation detected in the 4 isolates, as well as in isolates sequenced previously, suggested that WSSV might have segregated into 2 lineages since first emerging as a serious pathogen of farmed shrimp in East Asia in the early-mid 1990s, with one lineage remaining in East Asia and the other separating to South Asia. In ORF23/24, deletions of 9.31, 10.97, or 11.09 kb were evident compared to a reference isolate from Taiwan (WSSV-TW), and, in ORF14/15, deletions of 5.14 or 5.95 kb were evident compared to a reference isolate from Thailand with the largest genome size (TH-96-II). With respect to these genome characteristics, the crayfish isolates 08HB, 09HB and 08JS were similar to WSSV-TW and the isolate 09JS was similar to a reference isolate from China (WSSV-CN). In addition to these loci, sequence variation was evident in ORF94 and ORF125 that might be useful for differentiating isolates and in epidemiological tracing of WSSV spread in crayfish farmed in China. However, as all 4 crayfish isolates possessed a Homologous Region 9 sequence identical to isolate WSSV-TW and another Thailand isolate (WSSV-TH), and as their transposase sequence was identical to isolates WSSV-CN and WSSV-TH, these 2 loci were not useful in predicting their origins.

Molecular modeling and expression of the Litopenaeus vannamei proliferating cell nuclear antigen (PCNA) after white spot syndrome virus shrimp infection

Proliferating cell nuclear antigen (PCNA) is the eukaryotic sliding clamp that tethers DNA polymerase to DNA during replication. The full-length cDNA of the Pacific white shrimp Litopenaeus vannamei PCNA (LvPCNA) was cloned and encoded a protein of 260 amino acids that is highly similar to other Crustacean PCNAs. The theoretical shrimp PCNA structure has all the domains that are necessary for its interaction with template DNA and DNA polymerase. RT-PCR analysis showed that LvPCNA is expressed mainly in muscle and hemocytes and much less in hepatopancreas and gills. LvPCNA mRNA levels are not statistically different in muscle from healthy and challenged shrimp with the white spot syndrome virus (WSSV). In contrast, the mRNA levels of the viral DNA polymerase show a biphasic pattern with expression at 6 h post-infection and later at 24 and 48 h. These results suggest that in shrimp muscle LvPCNA levels are steadily kept to allow viral replication and that WSSV DNA polymerase (WSSV-DNApol) is more responsive towards later stages of infection. More knowledge of the DNA replication machinery would result in a better understanding of the mechanism and components of viral replication, since the WSSV genome does not have all the components required for assembly of a fully functional replisome.

The infectious hypodermal and haematopoietic necrosis virus: a brief review of what we do and do not know

Given its high prevalence, its wide distribution and its remarkable capacity to cause severe mortality in shrimp, the infectious hypodermal and haematopoietic necrosis virus (IHHNV) may deserve far more attention than it has received, as it remains considered as one of the most serious problems plaguing the global shrimp farming industry. Furthermore, its real measurable impact over wild shrimp populations remains unknown. Undeniably, the progress that we have reached today on the knowledge of its geographical distribution, clinical signs, genetic diversity, transmission and virulence may help to identify and understand important aspects of its biology and pathogenesis. However, the information regarding the molecular events that occur during the infection process is scarce. Thus, it may not be surprising to find that there are no therapeutic options available for the prophylaxis or treatments to reduce the deleterious impact of this viral pathogen to date. The aim of this review is to integrate and discuss the current state of knowledge concerning several aspects of the biology of IHHNV and to highlight potential future directions for this area of research.

Penaeus monodon TATA box-binding protein interacts with the white spot syndrome virus transactivator IE1 and promotes its transcriptional activity

We show here that the white spot syndrome virus (WSSV) immediate-early protein IE1 interacts with the Penaeus monodon TATA box-binding protein (PmTBP) and that this protein-protein interaction occurs in the absence of any other viral or cellular proteins or nucleic acids, both in vitro and in vivo. Mapping studies using enhanced green fluorescent protein (EGFP) fusion proteins containing truncations of IE1 and PmTBP delimited the interacting regions to amino acids (aa) 81 to 180 in IE1 and, except for aa 171 to 230, to aa 111 to 300 in PmTBP. A WSSV IE1 transactivation assay showed that large quantities (>800 ng) of the GAL4-IE1 plasmid caused "squelching" of the GAL4-IE1 activity and that this squelching effect was alleviated by the overexpression of PmTBP. Gene silencing of WSSV ie1 and PmTBP by pretreatment with double-stranded RNAs (dsRNAs) prior to WSSV challenge showed that the expression of these two target genes was specifically inhibited by their corresponding dsRNAs 72 and 96 h after dsRNA treatment. dsRNA silencing of ie1 and PmTBP expression also significantly reduced WSSV replication and the expression of the viral early gene dnapol (DNA polymerase gene). These results suggest that WSSV IE1 and PmTBP work cooperatively with each other during transcription initiation and, furthermore, that PmTBP is an important target for WSSV IE1's transactivation activity that can enhance viral gene expression and help in virus replication.

Identification of fatty acid synthase from the Pacific white shrimp, Litopenaeus vannamei and its specific expression profiles during white spot syndrome virus infection

Fatty acid synthase (FAS) in animal tissues consists of two identical monomers and is known to be a complex multi-functional enzyme that plays an important role in energy homeostasis. However, there are few reports of studies focused on the relationship between FAS and virus infection in invertebrates. In the present study, we cloned the FAS gene from an economically important invertebrate, the Pacific white shrimp Litopenaeus vannamei. The full-length FAS cDNA is 8268 bp, including a 5'-terminal untranslated region of 137 bp, a 3'-terminal untranslated region of 601 bp and an open reading frame of 7530 bp. FAS cDNA encodes a polypeptide of 2509 amino acid residues that contains a typical β-ketoacyl synthase (KS) domain at the N-terminus, next to a malonyl/acetyltransferase (MAT) domain, a dehydrase domain, an enoyl reductase domain, a ketoacyl reductase domain, a phosphopantetheine attachment site domain and a thioesterase domain at the C-terminus. Quantitative real-time RT-PCR revealed the up-regulated expression of FAS in L. vannamei hepatopancreas and muscle after white spot syndrome virus (WSSV) infection. The expression of FAS in muscle was 13.03-fold greater than that in the control (p<0.05) and 2.93-fold greater in hepatopancreas (p>0.05). Meanwhile, expression of the fatty acid-binding protein (FABP), another important factor in lipid metabolism, was increased in muscle to 19.20-fold greater than that in the control (p<0.05) but only 0.76-fold in hepatopancreas (p>0.05). These results implied that WSSV infected body surface tissues, but there was very little infection of internal organs. We suggest that the increase of FAS expression is induced in WSSV-infected shrimps, and the virus changes the lipid metabolism of the host, which directly affects virus assembly or defense against virus infection.

White spot syndrome virus Orf514 encodes a bona fide DNA polymerase

White spot syndrome virus (WSSV) is the causative agent of white spot syndrome, one of the most devastating diseases in shrimp aquaculture. The genome of WSSV includes a gene that encodes a putative family B DNA polymerase (ORF514), which is 16% identical in amino acid sequence to the Herpes virus 1 DNA polymerase. The aim of this work was to demonstrate the activity of the WSSV ORF514-encoded protein as a DNA polymerase and hence a putative antiviral target. A 3.5 kbp fragment encoding the conserved polymerase and exonuclease domains of ORF514 was overexpressed in bacteria. The recombinant protein showed polymerase activity but with very low level of processivity. Molecular modeling of the catalytic protein core encoded in ORF514 revealed a canonical polymerase fold. Amino acid sequence alignments of ORF514 indicate the presence of a putative PIP box, suggesting that the encoded putative DNA polymerase may use a host processivity factor for optimal activity. We postulate that WSSV ORF514 encodes a bona fide DNA polymerase that requires accessory proteins for activity and maybe target for drugs or compounds that inhibit viral DNA replication.

Horizontal gene transfers with or without cell fusions in all categories of the living matter

This article reviews the history of widespread exchanges of genetic segments initiated over 3 billion years ago, to be part of their life style, by sphero-protoplastic cells, the ancestors of archaea, prokaryota, and eukaryota. These primordial cells shared a hostile anaerobic and overheated environment and competed for survival. "Coexist with, or subdue and conquer, expropriate its most useful possessions, or symbiose with it, your competitor" remain cellular life's basic rules. This author emphasizes the role of viruses, both in mediating cell fusions, such as the formation of the first eukaryotic cell(s) from a united crenarchaeon and prokaryota, and the transfer of host cell genes integrated into viral (phages) genomes. After rising above the Darwinian threshold, rigid rules of speciation and vertical inheritance in the three domains of life were established, but horizontal gene transfers with or without cell fusions were never abolished. The author proves with extensive, yet highly selective documentation, that not only unicellular microorganisms, but the most complex multicellular entities of the highest ranks resort to, and practice, cell fusions, and donate and accept horizontally (laterally) transferred genes. Cell fusions and horizontally exchanged genetic materials remain the fundamental attributes and inherent characteristics of the living matter, whether occurring accidentally or sought after intentionally. These events occur to cells stagnating for some 3 milliard years at a lower yet amazingly sophisticated level of evolution, and to cells achieving the highest degree of differentiation, and thus functioning in dependence on the support of a most advanced multicellular host, like those of the human brain. No living cell is completely exempt from gene drains or gene insertions.