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

Class B scavenger receptor resists WSSV replication by recognizing the viral lipid molecule and promoting phagocytosis

Class B scavenger receptors (SRBs) have been well-studied in bacteria-induced immune responses in invertebrates. However, the status of SRB-defending viruses remains unclear. In this study, we identified a scavenger receptor in Procambarus clarkii (crayfish), which is homologous to mammalian SRBs, and designated it as PcSRB. The expression of PcSRB was upregulated after the WSSV challenge. The survival rate of crayfish was decreased, but the WSSV copy number increased after PcSRB knockdown during virus invasion. In addition, PcSRB bound to WSSV. Furthermore, we detected how PcSRB interacted with WSSV, and we found that PcSRB could bind to cholesta-3,5-diene, (CD3,5), a novel WSSV lipid ligand, rather than dibutyl phthalate (DBP). Besides, PcSRB could bind to VP19, VP26, and VP28, rather than VP24. Mutant-binding experiments demonstrated that the hydrophobic domain (130-180 aa) of PcSRB is important for recognizing WSSV. Furthermore, PcSRB might promote lysosomal eliminating function to degrade WSSV. Altogether, we identified a new mechanism for scavenger receptor recognition and resistance to WSSV.IMPORTANCEPcSRB could bind to WSSV directly. PcSRB could interact with WSSV via binding to lipid molecule CD3,5 and viral envelope proteins. PcSRB could influence lysosomal activation.

White spot syndrome virus (WSSV) modulates lipid metabolism in white shrimp

In addition to the Warburg effect, which increases the availability of energy and biosynthetic building blocks in WSSV-infected shrimp, WSSV also induces both lipolysis at the viral genome replication stage (12 hpi) to provide material and energy for the virus replication, and lipogenesis at the viral late stage (24 hpi) to complete virus morphogenesis by supplying particular species of long-chain fatty acids (LCFAs). Here, we further show that WSSV causes a reduction in lipid droplets (LDs) in hemocytes at the viral genome replication stage, and an increase in LDs in the nuclei of WSSV-infected hemocytes at the viral late stage. In the hepatopancreas, lipolysis is triggered by WSSV infection, and this leads to fatty acids being released into the hemolymph. β-oxidation inhibition experiment reveals that the fatty acids generated by WSSV-induced lipolysis can be diverted into β-oxidation for energy production. At the viral late stage, WSSV infection leads to lipogenesis in both the stomach and hepatopancreas, suggesting that fatty acids are in high demand at this stage for virion morphogenesis. Our results demonstrate that WSSV modulates lipid metabolism specifically at different stages to facilitate its replication.

MD-2 Homologue Recognizes the White Spot Syndrome Virus Lipid Component and Induces Antiviral Molecule Expression in Shrimp

The myeloid differentiation factor 2 (MD-2)-related lipid-recognition (ML) domain is found in multiple proteins, including MD-2, MD-1, Niemann-Pick disease type C2, and mite major allergen proteins. The significance of ML proteins in antibacterial signal transduction and in lipid metabolism has been well studied. However, their function in host-virus interaction remains poorly understood. In the current study, we found that the ML protein family is involved in resistance against white spot syndrome virus in kuruma shrimp, Marsupenaeus japonicus One member, which showed a high similarity to mammalian MD-2/MD-1 and was designated as ML1, participated in the antiviral response by recognizing cholesta-3,5-diene (CD), a lipid component of the white spot syndrome virus envelope. After recognizing CD, ML1 induced the translocation of Rel family NF-κB transcription factor Dorsal into the nucleus, resulting in the expression of Vago, an IFN-like antiviral cytokine in arthropods. Overall, this study revealed the significance of an MD-2 homologue as an immune recognition protein for virus lipids. The identification and characterization of CD-ML1-Dorsal-Vago signaling provided new insights into invertebrate antiviral immunity.

Low-abundance envelope protein VP12 of white spot syndrome virus interacts with envelope protein VP150 and capsid protein VP51

VP12 and VP150 are two minor envelope proteins of white spot syndrome virus (WSSV). In our previous studies, VP12 was found to co-migrate with 53-kDa form of VP150 on two-dimensional Blue Native/SDS-PAGE, suggesting that there is an interaction between them. In this study, we confirmed the interaction by co-immunoprecipitation assay and demonstrated that the binding region with VP12 is located between residues 207 and 803 of VP150. Further studies found that VP12 can be attached to WSSV capsids by interacting with capsid protein VP51. These findings suggest that VP12 may function as a linker protein participating in the linkage between VP12/VP150 complex and viral nucleocapsid.

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.

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.

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

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

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

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