White spot syndrome virus Orf514 encodes a bona fide DNA polymerase

A review on molecular detection techniques of white spot syndrome virus: Perspectives of problems and solutions in shrimp farming

This review aims to provide an update on the current scientific understanding of various aspects of White Spot Syndrome Virus (WSSV) formation, diagnostic procedures, transmission, ecological effects, pathophysiology and management strategies. In terms of production and financial benefits, the WSSV has been the most virulent in shrimp and several other crustacean sectors around the globe. It spreads vertically from diseased broodstock to post-larvae and horizontally by cannibalism, invertebrate vectors, freshwater and sediments. In the transfer of white spot disease (WSD) in newly stocked ponds, the survivability of WSSV in sediment is the most important variable. In typical cultural conditions, it is a highly infectious pathogen capable of inflicting total death within 3-10 days after an outbreak. Some of the current biosecurity strategies used to keep diseases out of shrimp ponds such as pond water disinfection, quarantine of new stocks before stocking and broader usage of specific pathogen-free shrimp. The sequencing and characterisation of various WSSV strains have provided details about pathogen biology, pathogenicity and disease. To develop successful control methods, knowledge of these characteristics is essential. In several shrimp-producing countries in Asia and the Americas, the infections produced by the WSSV have had disastrous socio-economic consequences. As a result of international trade or migration of diseased species, the World Animal Health Organization recognised several illnesses as posing a substantial hazard to farmed shrimp. WSD is receiving much scientific research due to the potential economic effects of the virus. Research is now being done to understand better the molecular biology and pathophysiology of WSSV, as well as how to treat and prevent the virus. However, further study should be conducted in countries with more resilient host species to understand their role in mitigating disease impacts since these revelations may aid in developing a WSD treatment.

Recent insights into anti-WSSV immunity in crayfish

White spot syndrome virus (WSSV) is currently the most severely viral pathogen for farmed crustaceans such as shrimp and crayfish, which has been causing huge economic losses for crustaceans farming worldwide every year. Unfortunately, study on the molecular mechanisms of WSSV has been restricted by the lack of crustacean cell lines for WSSV propagation as well as the incompletely annotated genomes for host species, resulting in limited elucidation for WSSV pathogenesis at present. In addition to the findings of anti-WSSV response in shrimp, some of novel cellular events involved in WSSV infection have been recently revealed in crayfish, including endocytosis and intracellular transport of WSSV, innate immune pathways in response to WSSV infection, and regulation of viral gene expression by host genes. Despite these advances, many fundamental gaps in WSSV pathogenesis are still remaining, for example, how WSSV genome enters into nucleus and how the progeny virions are fully assembled in the host cell nucleus. In this review, recent findings in WSSV infection mechanism and the antiviral immunity against WSSV in crayfish are summarized and discussed, which may provide us a better understanding of the WSSV pathogenesis as well as new ideas for the target design of antiviral drugs against WSSV in crustaceans farming.

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.

A novel viral thymidylate kinase with dual kinase activity

Nucleotide phosphorylation is a key step in DNA replication and viral infections, since suitable levels of nucleotide triphosphates pool are required for this process. Deoxythymidine monophosphate (dTMP) is produced either by de novo or salvage pathways, which is further phosphorylated to deoxythymidine triphosphate (dTTP). Thymidyne monophosphate kinase (TMK) is the enzyme in the junction of both pathways, which phosphorylates dTMP to yield deoxythymidine diphosphate (dTDP) using adenosine triphosphate (ATP) as a phosphate donor. White spot syndrome virus (WSSV) genome contains an open reading frame (ORF454) that encodes a thymidine kinase and TMK domains in a single polypeptide. We overexpressed the TMK ORF454 domain (TMKwssv) and its specific activity was measured with dTMP and dTDP as phosphate acceptors. We found that TMKwssv can phosphorylate dTMP to yield dTDP and also is able to use dTDP as a substrate to produce dTTP. Kinetic parameters K M and k cat were calculated for dTMP (110 μM, 3.6 s(-1)), dTDP (251 μM, 0.9 s(-1)) and ATP (92 μM, 3.2 s(-1)) substrates, and TMKwssv showed a sequential ordered bi-bi reaction mechanism. The binding constants K d for dTMP (1.9 μM) and dTDP (10 μM) to TMKwssv were determined by Isothermal Titration Calorimetry. The affinity of the nucleotidic analog stavudine monophosphate was in the same order of magnitude (K d 3.6 μM) to the canonical substrate dTMP. These results suggest that nucleotide analogues such as stavudine could be a suitable antiviral strategy for the WSSV-associated disease.

Crystal structure of the shrimp proliferating cell nuclear antigen: structural complementarity with WSSV DNA polymerase PIP-box

DNA replication requires processivity factors that allow replicative DNA polymerases to extend long stretches of DNA. Some DNA viruses encode their own replicative DNA polymerase, such as the white spot syndrome virus (WSSV) that infects decapod crustaceans but still require host replication accessory factors. We have determined by X-ray diffraction the three-dimensional structure of the Pacific white leg shrimp Litopenaeus vannamei Proliferating Cell Nuclear Antigen (LvPCNA). This protein is a member of the sliding clamp family of proteins, that binds DNA replication and DNA repair proteins through a motif called PIP-box (PCNA-Interacting Protein). The crystal structure of LvPCNA was refined to a resolution of 3 Å, and allowed us to determine the trimeric protein assembly and details of the interactions between PCNA and the DNA. To address the possible interaction between LvPCNA and the viral DNA polymerase, we docked a theoretical model of a PIP-box peptide from the WSSV DNA polymerase within LvPCNA crystal structure. The theoretical model depicts a feasible model of interaction between both proteins. The crystal structure of shrimp PCNA allows us to further understand the mechanisms of DNA replication processivity factors in non-model systems.

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.