Characterization of the nonstructural protein NS80 of grass carp reovirus

Piscine Vitamin D Receptors Vdra/Vdrb in the Absence of Vitamin D Are Utilized by Grass Carp Reovirus for Promoting Viral Replication

The vitamin D receptor (VDR) plays a pivotal role in the biological actions of vitamin D (VitD). However, little is known about the functions of VDR in the production of viral inclusion bodies (VIBs). Using a representative strain of grass carp reovirus (GCRV) genotype I, GCRV-873, we show that GCRV-873 recruits grass carp Vdrs for promoting the production of VIBs in the absence of VitD. Inhibition of cholesterol synthesis by lovastatin impairs the production of VIBs and blocks the effects of grass carp Vdrs in promoting the production of VIBs in the absence of VitD. Furthermore, grass carp Vdrs are found to form the Vdra-Vdrb heterodimer, which is vital for 3-hydroxy-3-methylglutaryl-coenzyme A reductase (hmgcr)-dependent cholesterol synthesis and GCRV replication. Intriguingly in the presence of VitD, grass carp Vdra but not Vdrb forms the heterodimer with the retinoid X receptor beta b (Rxrbb), which induces the transcription of those genes involved in the RIG-I-like receptor (RLR) antiviral signaling pathway for inhibiting GCRV infection. Furthermore, the VitD-activated Vdra-Vdrb heterodimer attenuates the transcription of the RLR antiviral signaling pathway induced by VitD. In the presence of VitD, a balance between the Vdra-Rxrbb heterodimers as coactivators and Vdra-Vdrb heterodimers as corepressors in affecting the transcriptional regulation of the RLR antiviral signaling pathway may eventually determine the outcome of GCRV infection. Transfection with VitD can abolish the effect of grass carp Vdrs in promoting GCRV replication in a dose-dependent manner. Taken together, these findings demonstrate that GCRV utilizes host Vdrs to increase hmgcr-dependent cholesterol synthesis for promoting its replication, which can be prevented by VitD treatment. IMPORTANCE Grass carp reovirus (GCRV) is the causative agent of grass carp hemorrhagic disease, which seriously harms freshwater fish. Although many positive or negative regulators of GCRV infection have been identified in teleosts, little is known about the molecular mechanisms by which GCRV utilizes host factors to generate its infectious compartments beneficial for viral replication and infection. Here, we show that in the absence of VitD, the GCRV-873 strain utilizes host vitamin D receptors Vdra/Vdrb to increase hmgcr-dependent cholesterol synthesis for promoting the production of VIBs, which are important functional sites for aquareovirus replication and assembly. The negative regulation of Vdrs during viral infection can be prevented by VitD treatment. Thus, this present work broadens understanding of the pivotal roles of Vdrs in the interaction between the host and GCRV in the absence or presence of VitD, which might provide a rational basis for developing novel anti-GCRV strategies.

Structural and functional analysis of the small GTPase ARF1 reveals a pivotal role of its GTP-binding domain in controlling of the generation of viral inclusion bodies and replication of grass carp reovirus

Grass carp reovirus (GCRV) is the most pathogenic double-stranded (ds) RNA virus among the isolated aquareoviruses. The molecular mechanisms by which GCRV utilizes host factors to generate its infectious compartments beneficial for viral replication and infection are poorly understood. Here, we discovered that the grass carp ADP ribosylation factor 1 (gcARF1) was required for GCRV replication since the knockdown of gcARF1 by siRNA or inhibiting its GTPase activity by treatment with brefeldin A (BFA) significantly impaired the yield of infectious viral progeny. GCRV infection recruited gcARF1 into viral inclusion bodies (VIBs) by its nonstructural proteins NS80 and NS38. The small_GTP domain of gcARF1 was confirmed to be crucial for promoting GCRV replication and infection, and the number of VIBs reduced significantly by the inhibition of gcARF1 GTPase activity. The analysis of gcARF1-GDP complex crystal structure revealed that the ²⁷AAGKTT³² motif and eight amino acid residues (A²⁷, G²⁹, K³⁰, T³¹, T³², N¹²⁶, D¹²⁹ and A¹⁶⁰), which were located mainly within the GTP-binding domain of gcARF1, were crucial for the binding of gcARF1 with GDP. Furthermore, the ²⁷AAGKTT³² motif and the amino acid residue T³¹ of gcARF1 were indispensable for the function of gcARF1 in promoting GCRV replication and infection. Taken together, it is demonstrated that the GTPase activity of gcARF1 is required for efficient replication of GCRV and that host GTPase ARF1 is closely related with the generation of VIBs.

Grass Carp Reovirus Nonstructural Proteins Avoid Host Antiviral Immune Response by Targeting the RLR Signaling Pathway

Grass carp reovirus (GCRV) is a highly virulent RNA virus that mainly infects grass carp and causes hemorrhagic disease. The roles of nonstructural proteins NS38 and NS80 of GCRV-873 in the viral replication cycle and viral inclusion bodies have been established. However, the strategies that NS38 and NS80 used to avoid host antiviral immune response are still unknown. In this study, we report the negative regulations of NS38 and NS80 on the RIG-I-like receptors (RLRs) antiviral signaling pathway and the production of IFNs and IFN-stimulated genes. First, both in the case of overexpression and GCRV infection, NS38 and NS80 inhibited the IFN promoter activation induced by RIG-I, MDA5, MAVS, TBK1, IRF3, and IRF7 and mRNA abundance of key antiviral genes involved in the RLR-mediated signaling. Second, both in the case of overexpression and GCRV infection, NS38 interacted with piscine TBK1 and IRF3, but not with piscine RIG-I, MDA5, MAVS, and TNF receptor-associated factor (TRAF) 3. Whereas NS80 interacted with piscine MAVS, TRAF3, and TBK1, but not with piscine RIG-I, MDA5, and IRF3. Finally, both in the case of overexpression and GCRV infection, NS38 inhibited the formation of the TBK1-IRF3 complex, but NS80 inhibited the formation of the TBK1-TRAF3 complex. Most importantly, NS38 and NS80 could hijack piscine TBK1 and IRF3 into the cytoplasmic viral inclusion bodies and inhibit the translocation of IRF3 into the nucleus. Collectively, all of these data demonstrate that GCRV nonstructural proteins can avoid host antiviral immune response by targeting the RLR signaling pathway, which prevents IFN-stimulated gene production and facilitates GCRV replication.

Type II Grass Carp Reovirus Infects Leukocytes but Not Erythrocytes and Thrombocytes in Grass Carp (Ctenopharyngodon idella)

Grass carp reovirus (GCRV) causes serious losses to the grass carp industry. At present, infectious tissues of GCRV have been studied, but target cells remain unclear. In this study, peripheral blood cells were isolated, cultured, and infected with GCRV. Using quantitative real-time polymerase chain reaction (qRT-PCR), Western Blot, indirect immunofluorescence, flow cytometry, and transmission electron microscopy observation, a model of GCRV infected blood cells in vitro was established. The experimental results showed GCRV could be detectable in leukocytes only, while erythrocytes and thrombocytes could not. The virus particles in leukocytes are wrapped by empty membrane vesicles that resemble phagocytic vesicles. The empty membrane vesicles of leukocytes are different from virus inclusion bodies in C. idella kidney (CIK) cells. Meanwhile, the expression levels of IFN1, IL-1β, Mx2, TNFα were significantly up-regulated in leukocytes, indicating that GCRV could cause the production of the related immune responses. Therefore, GCRV can infect leukocytes in vitro, but not infect erythrocytes and thrombocytes. Leukocytes are target cells in blood cells of GCRV infections. This study lays a theoretical foundation for the study of the GCRV infection mechanism and anti-GCRV immunity.

Isolation, Identification, and Genomic Analysis of a Novel Reovirus from Healthy Grass Carp and Its Dynamic Proliferation In Vitro and In Vivo

A new grass carp reovirus (GCRV), healthy grass carp reovirus (HGCRV), was isolated from grass carp in 2019. Its complete genome sequence was determined and contained 11 dsRNAs with a total size of 23,688 bp and 57.2 mol% G+C content, encoding 12 proteins. All segments had conserved 5' and 3' termini. Sequence comparisons showed that HGCRV was closely related to GCRV-873 (GCRV-I; 69.57-96.71% protein sequence identity) but shared only 22.65-45.85% and 23.37-43.39% identities with GCRV-HZ08 and Hubei grass carp disease reovirus (HGDRV), respectively. RNA-dependent RNA-polymerase (RdRp) protein-based phylogenetic analysis showed that HGCRV clustered with Aquareovirus-C (AqRV-C) prior to joining a branch common with other aquareoviruses. Further analysis using VP6 amino acid sequences from Chinese GCRV strains showed that HGCRV was in the same evolutionary cluster as GCRV-I. Thus, HGCRV could be a new GCRV isolate of GCRV-I but is distantly related to other known GCRVs. Grass carp infected with HGCRV did not exhibit signs of hemorrhage. Interestingly, the isolate induced a typical cytopathic effect in fish cell lines, such as infected cell shrank, apoptosis, and plague-like syncytia. Further analysis showed that HGCRV could proliferate in grass carp liver (L28824), gibel carp brain (GiCB), and other fish cell lines, reaching a titer of up to 7.5 × 10⁴ copies/μL.

Development of a Mucus Gland Bioreactor in Loach Paramisgurnus dabryanus

Most currently available bioreactors have some defects in the expression, activity, or purification of target protein and peptide molecules, whereas the mucus gland of fish can overcome these defects to become a novel bioreactor for the biopharmaceutical industry. In this study, we have evaluated the practicability of developing a mucus gland bioreactor in loach (Paramisgurnus dabryanus). A transgenic construct pT2-krt8-IFN1 was obtained by subcloning the promoter of zebrafish keratin 8 gene and the type I interferon (IFN1) cDNA of grass carp into the SB transposon. The IFN1 expressed in CIK cells exhibited an antiviral activity against the replication of GCRV873 and activated two genes downstream of JAK-STAT signaling pathway. A transgenic loach line was then generated by microinjection of the pT2-krt8-IFN1 plasmids and in vitro synthesized capped SB11 mRNA. Southern blots indicated that a single copy of IFN1 gene was stably integrated into the genome of transgenic loach. The expression of grass carp IFN1 in transgenic loaches was detected with RT-PCR and Western blots. About 0.0825 µg of grass carp IFN1 was detected in 20 µL mucus from transgenic loaches. At a viral titer of 1 × 10³ PFU/mL, plaque numbers on plates containing mucus from transgenic loaches reduced by 18% in comparison with those of the control, indicating that mucus of IFN1-transgenic loaches exhibited an antiviral activity. Thus, we have successfully created a mucus gland bioreactor that has great potential for the production of various proteins and peptides.

Endosomes and Microtubles are Required for Productive Infection in Aquareovirus

Grass carp reovirus (GCRV), the genus Aquareovirus in family Reoviridae, is viewed as the most pathogenic aquareovirus. To understand the molecular mechanism of how aquareovirus initiates productive infection, the roles of endosome and microtubule in cell entry of GCRV are investigated by using quantum dots (QDs)-tracking in combination with biochemical approaches. We found that GCRV infection and viral protein synthesis were significantly inhibited by pretreating host cells with endosome acidification inhibitors NH₄Cl, chloroquine and bafilomycin A1 (Bafi). Confocal images indicated that GCRV particles could colocalize with Rab5, Rab7 and lysosomes in host cells. Further ultrastructural examination validated that viral particle was found in late endosomes. Moreover, disruption of microtubules with nocodazole clearly blocked GCRV entry, while no inhibitory effects were observed with cytochalasin D treated cells in viral infection, hinting that intracellular transportation of endocytic uptake in GCRV infected cells is via microtubules but not actin filament. Notably, viral particles were observed to transport along microtubules by using QD-labeled GCRV. Altogether, our results suggest that GCRV can use endosomes and microtubules to initiate productive infection.

Complete genome sequence and phylogenetic analysis of a novel aquareovirus isolated from a diseased marbled eel (Anguilla marmorata)

Marbled eel reovirus (MERV) is an aquareovirus (AQRV) isolated from diseased marbled eels (Anguilla marmorata) with petechial skin hemorrhage. In this study, we propagated MERV in a cell line derived from the brain of Aequidens rivulatus and purified viral particles by using a discontinuous cesium chloride gradient. Genomic RNA sequences were obtained through next-generation sequencing. MERV, similar to most other AQRVs, showed the presence of 11 double-stranded RNA segments encoding 12 proteins; however, the genome sequence displayed very little similarity to known AQRV sequences. Furthermore, the structural proteins of MERV were most closely related to American grass carp reovirus with sequence identity values of no more than 64.89%. Phylogenetic analysis based on the sequences of structural proteins indicated that MERV shows an evolutionary history between AQRV-B and -G, which belong to the saline and freshwater environment subgroups, respectively. We also observed that MERV showed a closer relationship to orthoreoviruses based on the protein sequences of NS38 and NS73. In summary, MERV is a novel AQRV that could be classified as a member of the new proposed AQRV species "Aquareovirus H". The taxonomic assignments and evolution of AQRVs thus warrant further investigation.

Identification and characterization of two cleavage fragments from the Aquareovirus nonstructural protein NS80

Aquareovirus species vary with respect to pathogenicity, and the nonstructural protein NS80 of aquareoviruses has been implicated in the regulation of viral replication and assembly, which can form viral inclusion bodies (VIBs) and recruit viral proteins to its VIBs in infected cells. NS80 consists of 742 amino acids with a molecular weight of approximately 80 kDa. Interestingly, a short specific fragment of NS80 has also been detected in infected cells. In this study, an approximately 58-kDa product of NS80 was confirmed in various infected and transfected cells by immunoblotting analyses using α-NS80C. Mutational analysis and time course expression assays indicated that the accumulation of the 58-kDa fragment was related to time and infection dose, suggesting that the fragment is not a transient intermediate of protein degradation. Moreover, another smaller fragment with a molecular mass of approximately 22 kDa was observed in transfected and infected cells by immunoblotting with a specific anti-FLAG monoclonal antibody or α-NS80N, indicating that the 58- kDa polypeptide is derived from a specific cleavage site near the amino terminus of NS80. Additionally, different subcellular localization patterns were observed for the 22-kDa and 58-kDa fragments in an immunofluorescence analysis, implying that the two cleavage fragments of NS80 function differently in the viral life cycle. These results provide a basis for additional studies of the role of NS80 played in replication and particle assembly of the Aquareovirus.

Disruption of clathrin-dependent trafficking results in the failure of grass carp reovirus cellular entry

Background

Grass carp reovirus (GCRV) is responsible for viral hemorrhagic disease in cultured grass carp (Ctenopharyngon idellus). GCRV is a non-enveloped, double-stranded RNA virus in the genus Aquareovirus, of the family Reoviridae, which encodes seven structural proteins (VP1-VP7) and five nonstructural proteins (NS80, NS38, NS31, NS26, and NS16). To date, the mechanism of GCRV entry into CIK Ctenopharyngon idellus kidney (CIK) cells remains poorly understood.

Results

Here, we present a study of the GCRV internalization mechanism in CIK cells. Our results indicated that GCRV infection was inhibited by chlorpromazine, the specific inhibitor for clathrin-mediated endocytosis. Colocalization of GCRV virions with endogenous clathrin was observed during early infection by confocal microscopy. Moreover, GCRV infection of CIK cells depended on acidification of the endosome. This was indicated by significant inhibition of viral infection following prophylactic treatment with the lysosomotropic drugs chloroquine or ammonium chloride. In addition, the disturbance of dynamin activity blocked GCRV entry, which confirmed the dynamin-dependent nature of clathrin-mediated endocytosis.

Conclusion

Our findings suggest that GCRV might enter CIK cells via clathrin-mediated endocytosis in a pH-dependent manner. Additionally, dynamin is critical for efficient viral entry.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-016-0485-7) contains supplementary material, which is available to authorized users.

The N-Terminal of Aquareovirus NS80 Is Required for Interacting with Viral Proteins and Viral Replication

Reovirus replication and assembly occurs within viral inclusion bodies that formed in specific intracellular compartments of cytoplasm in infected cells. Previous study indicated that aquareovirus NS80 is able to form inclusion bodies, and also can retain viral proteins within its inclusions. To better understand how NS80 performed in viral replication and assembly, the functional regions of NS80 associated with other viral proteins in aquareovirus replication were investigated in this study. Deletion mutational analysis and rotavirus NSP5-based protein association platform were used to detect association regions. Immunofluorescence images indicated that different N-terminal regions of NS80 could associate with viral proteins VP1, VP4, VP6 and NS38. Further co-immunoprecipitation analysis confirmed the interaction between VP1, VP4, VP6 or NS38 with different regions covering the N-terminal amino acid (aa, 1–471) of NS80, respectively. Moreover, removal of NS80 N-terminal sequences required for interaction with proteins VP1, VP4, VP6 or NS38 not only prevented the capacity of NS80 to support viral replication in NS80 shRNA-based replication complementation assays, but also inhibited the expression of aquareovirus proteins, suggesting that N-terminal regions of NS80 are necessary for viral replication. These results provided a foundational basis for further understanding the role of NS80 in viral replication and assembly during aquareovirus infection.

Identification of (-)-epigallocatechin-3-gallate as a potential agent for blocking infection by grass carp reovirus

Grass carp reovirus (GCRV), the representative strain of the species Aquareovirus C, serves as a model for studying the pathogenesis of aquareoviruses. Previously, epigallocatechin gallate (EGCG) was shown to inhibit orthoreovirus infection. The aim of this study was to test its potential in blocking infection by GCRV. We show that adhesion to the CIK (Ctenopharyngodon idellus kidney) cell surface by GCRV particles is inhibited in a dose-dependent manner by EGCG, as well as by a crude extract of green tea. We also evaluated the safety of EGCG and green tea extract using CIK cells, and the results suggest that EGCG is a promising compound that may be developed as a plant-derived small molecular therapeutic agent against grass carp hemorrhagic disease caused by GCRV infection. As the ligand for the 37/67-kDa laminin receptor (LamR), EGCG's blocking effect on GCRV attachment was associated with the binding potential of GCRV particles to LamR, which was inferred from a VOPBA assay.

Proteomic analysis of cellular protein expression profiles in response to grass carp reovirus infection

Grass carp (Ctenopharyngodon idella) hemorrhagic disease, caused by grass carp reovirus (GCRV), is emerging as a serious problem in grass carp aquaculture. To better understand the molecular responses to GCRV infection, two-dimensional electrophoresis (2-DE) and matrix-assisted laser desorption/ionization tandem mass spectroscopy were performed to investigate altered proteins in C. idella kidney (CIK) cells. Differentially expressed proteins in mock infected CIK cells and GCRV-infected CIK cells were compared. Twenty-three differentially expressed spots were identified (22 upregulated spots and 1 downregulated spot), which included cytoskeleton proteins, macromolecular biosynthesis-associated proteins, stress response proteins, signal transduction proteins, energy metabolism-associated proteins and ubiquitin proteasome pathway-associated proteins. Moreover, 10 of the corresponding genes of the differentially expressed proteins were quantified by real-time reverse transcription polymerase chain reaction to examine their transcriptional profiles. The T cell internal antigen 1 (TIA1) and Ras-GTPase-activating SH3-domain-binding protein1 (G3BP1) of the cellular stress granule pathway from grass carp C. idella (designated as CiTIA1 and CiG3BP1) were upregulated and downregulated during GCRV infection, respectively. The full-length cDNA of CiTIA1 was 2753 bp, with an open reading frame (ORF) of 1155bp, which encodes a putative 385-amino acid protein. The 2271 bp full-length cDNA of CiG3BP1 comprised an ORF of 1455 bp that encodes a putative 485-amino acid protein. Phylogenetic analysis revealed that the complete ORFs of CiTIA1 and CiG3BP1 were very similar to zebrafish and well-characterized mammalian homologs. The expressions of the cellular proteins CiTIA1 and CiG3BP1 in response to GCRV were validated by western blotting, which indicated that the GCRV should unlink TIA1 aggregation and stress granule formation. This study provides useful information on the proteomic and cellular stress granule pathway's responses to GCRV infection, which adds to our understanding of viral pathogenesis.

Aquareovirus NS80 Initiates Efficient Viral Replication by Retaining Core Proteins within Replication-Associated Viral Inclusion Bodies

Viral inclusion bodies (VIBs) are specific intracellular compartments for reoviruses replication and assembly. Aquareovirus nonstructural protein NS80 has been identified to be the major constituent for forming globular VIBs in our previous study. In this study, we investigated the role of NS80 in viral structural proteins expression and viral replication. Immunofluorescence assays showed that NS80 could retain five core proteins or inner-capsid proteins (VP1-VP4 and VP6), but not outer-capsid proteins (VP5 and VP7), within VIBs in co-transfected or infected cells. Further co-immunoprecipitation analysis confirmed that NS80 could interact with each core protein respectively. In addition, we found that newly synthesized viral RNAs co-localized with VIBs. Furthermore, time-course analysis of viral structural proteins expression showed that the expression of NS80 was detected first, followed by the detection of inner shell protein VP3, and then of other inner-capsid proteins, suggesting that VIBs were essential for the formation of viral core frame or progeny virion. Moreover, knockdown of NS80 by shRNA not only inhibited the expression of aquareovirus structural proteins, but also inhibited viral infection. These results indicated that NS80-based VIBs were formed at earlier stage of infection, and NS80 was able to coordinate the expression of viral structural proteins and viral replication.

Insights into the antiviral immunity against grass carp (Ctenopharyngodon idella) reovirus (GCRV) in grass carp

Global fish production from aquaculture has rapidly grown over the past decades, and grass carp shares the largest portion. However, hemorrhagic disease caused by grass carp reovirus (GCRV) results in tremendous loss of grass carp (Ctenopharyngodon idella) industry. During the past years, development of molecular biology and cellular biology technologies has promoted significant advances in the understanding of the pathogen and the immune system. Immunoprophylaxis based on stimulation of the immune system of fish has also got some achievements. In this review, authors summarize the recent progresses in basic researches on GCRV; viral nucleic acid sensors, high-mobility group box proteins (HMGBs); pattern recognition receptors (PRRs), Toll-like receptors (TLRs) and retinoic acid inducible gene I- (RIG-I-) like receptors (RLRs); antiviral immune responses induced by PRRs-mediated signaling cascades of type I interferon (IFN-I) and IFN-stimulated genes (ISGs) activation. The present review also notices the potential applications of molecule genetic markers. Additionally, authors discuss the current preventive and therapeutic strategies (vaccines, RNAi, and prevention medicine) and highlight the importance of innate immunity in long term control for grass carp hemorrhagic disease.

Identification of a functional motif in the AqRV NS26 protein required for enhancing the fusogenic activity of FAST protein NS16

Aquareoviruses AqRVs have a close relationship with orthoreoviruses. However, they contain an additional genome segment, S11, which encodes nonstructural protein NS26. We previously showed that NS26 can enhance the fusogenic activity of the fusion-associated small transmembrane FAST protein NS16 from AqRV. In this study, a TLPK motif in NS26 was identified as being important for the enhancement. When the TLPK motif was deleted from NS26, the enhanced efficiency of the NS16-mediated cellcell fusion was significantly impaired. Further mutational analysis showed that the lysine K residue in the TLPK motif was critical for the enhancement. Additionally, deletion of the TLPK motif prevented NS26 from interacting with lysosomes. These findings suggested that the TLPK motif is important for NS26 to enhance the fusogenic activity of NS16, and NS26 may utilize the lysosome to benefit the fusion process.

Dynamic localization and the associated translocation mechanism of HMGBs in response to GCRV challenge in CIK cells

High-mobility group box (HMGB) proteins, a family of chromatin-associated nuclear proteins, play amazingly multifaceted roles in the immune system of mammals. Thus far, little is known about the nucleocytoplasmic distribution of HMGBs in teleosts. The present study systematically investigated the dynamic localization of all six HMGB proteins in Ctenopharyngodon idella kidney (CIK) cells. Under basal conditions, all HMGBs exclusively localized to the nucleus. Grass carp reovirus (GCRV), polyinosinic-polycytidylic (poly(I∶C)) potassium salt and lipopolysaccharide (LPS) challenge evoked the nuclear export of HMGBs to various degrees: GCRV challenge induced the highest nuclear export of CiHMGB2b, and poly(I∶C) and LPS evoked the highest nucleocytoplasmic shuttling of CiHMGB1b. Overall, the nucleocytoplasmic shuttling of CiHMGB2a and CiHMGB3b was rarely induced by these challenges. Dynamic imaging uncovered that the nucleocytoplasmic GCRV-induced relocation of CiHMGB2b occurred in cells undergoing karyotheca rupture, apoptosis or proliferation. Western blot analyses were used to examine HMGB-EGFP fusion proteins in whole cell lysates, cytosol, nuclear fractions and culture medium. Further investigation demonstrated the nuclear retention of N-terminal HMG-boxes and the nucleocytoplasmic distribution of the C-terminal acidic tails. Comparative analyses of the dynamic relocation of full-length, truncated or chimeric HMGBs confirmed that the intramolecular interaction between HMG-boxes and C-tail domains mediated the nucleocytoplasmic translocation of HMGBs. These results not only provide an overall understanding of the subcellular localization of HMGBs, but also reveal the induction mechanism of the nucleocytoplasmic translocation of HMGBs by GCRV challenge, which lays a foundation for further studies on the interactions among pathogens, HMGBs and pattern recognition receptors in the innate immunity of teleosts.

Phylogenetic analysis of newly isolated grass carp reovirus

Grass carp reovirus (GCRV) is a causative agent of haemorrhagic disease in grass carp that drastically affects grass carp aquaculture. Here we report a novel GCRV isolate isolated from sick grass carp that induces obvious cytopathic effect in CIK cells and name it as GCRV096. A large number of GCRV 096 viral particles were found in the infected CIK cells by electron microscope. The shape, size and the arrangement of this virus were similar to those of grass carp reovirus. With the primers designed according to GCRV 873 genome sequences, specific bands were amplified from sick grass carp and the infected CIK cells. The homology rates among vp4, vp6 and vp7 gene in GCRV 096 and those of some GCRV isolates were over 89%. In this study, the sequences of vp4, vp6 and vp7 were used to analyse sequence variation, phylogenetic relationships and genotypes in twenty five GCRV isolates. The results indicated these twenty five GCRV isolates should be attributed to four genotypes. And there were no obvious characteristics in the geographical distribution of GCRV genotype. The study should provide the exact foundation for developing more effective prevention strategies of grass carp haemorrhagic disease.

Identification and genomic characterization of a novel fish reovirus, Hubei grass carp disease reovirus, isolated in 2009 in China

A novel fish reovirus, Hubei grass carp disease reovirus (HGDRV; formerly grass carp reovirus strain 104, GCRV104), was isolated from diseased grass carp in China in 2009 and the full genome sequence was determined. This reovirus was propagated in a grass carp kidney cell line with a typical cytopathic effect. The total size of the genome was 23 706 bp with a 51 mol% G+C content, and the 11 dsRNA segments encoded 12 proteins (two proteins encoded by segment 11). A nucleotide sequence similarity search using blastn found no significant matches except for segment 2, which partially matched that of the RNA-dependent RNA polymerase (RdRp) from several viruses in the genera Aquareovirus and Orthoreovirus of the family Reoviridae. At the amino acid level, seven segments (Seg-1 to Seg-6, and Seg-8) matched with species in the genera Aquareovirus (15-46 % identities) and Orthoreovirus (12-44 % identities), while for four segments (Seg-7, Seg-9, Seg-10 and Seg-11) no similarities in these genera were found. Conserved terminal sequences, 5'-GAAUU----UCAUC-3', were found in each HGDRV segment at the 5' and 3' ends, and the 5'-terminal nucleotides were different from any known species in the genus Aquareovirus. Phylogenetic analysis based on RdRp amino acid sequences from members of the family Reoviridae showed that HGDRV clustered with aquareoviruses prior to joining a branch common with orthoreoviruses. Based on these observations, we propose that HGDRV is a new species in the genus Aquareovirus that is distantly related to any known species within this genus.

Aquareovirus protein VP6 colocalizes with NS80 protein in infected and transfected cells

Background

Aquareovirus particle is comprised of central core and outer capsid, which is built by seven structural proteins (VP1-VP7). The protein VP6 has been identified to be a clamp protein of stabilizing inner core frame VP3, and bridging outer shell protein VP5. However, the biological properties of VP6 in viral life cycle remain unknown.

Results

The recombinant VP6 (rVP6) of aquareovirus was expressed in E. coli, and the polyclonal antibody against VP6 was generated by using purified rVP6 in this study. Following the preparation of VP6 antibody, the VP6 component in aquareovirus infected cells and purified viral particles was detected by Immunoblotting (IB) assay. Furthermore, using Immunofluorescence (IF) microscopy, singly transfected VP6 protein was observed to exhibit a diffuse distribution mainly in the cytoplasm, while it appeared inclusion phenotype in infected cells. Meanwhile, inclusion structures were also identified when VP6 was coexpressed with nonstructural protein NS80 in cotransfected cells.

Conclusions

VP6 can be recruited by NS80 to its inclusions in both infected and transfected cells. The colocalization of VP6 and NS80 is corresponding to their homologous proteins σ2 and μNS in MRV. Our results suggest that VP6 may play a significant role in viral replication and particle assembly.

Aquareovirus NS80 recruits viral proteins to its inclusions, and its C-terminal domain is the primary driving force for viral inclusion formation

Cytoplasmic inclusion bodies formed in reovirus-infected cells are the sites of viral replication and assembly. Previous studies have suggested that the NS80 protein of aquareovirus may be involved in the formation of viral inclusion bodies. However, it remains unknown whether other viral proteins are involved in the process, and what regions of NS80 may act coordinately in mediating inclusion formation. Here, we observed that globular cytoplasmic inclusions were formed in virus-infected cells and viral proteins NS80 and NS38 colocalized in the inclusions. During transfection, singly expressed NS80 could form cytoplasmic inclusions and recruit NS38 and GFP-tagged VP4 to these structures. Further treatment of cells with nocodazole, a microtubule inhibitor, did not disrupt the inclusion, suggesting that inclusion formation does not rely on microtubule network. Besides, we identified that the region 530–742 of NS80 was likely the minimal region required for inclusion formation, and the C-tail, coiled-coil region as well as the conserved linker region were essential for inclusion phenotype. Moreover, with series deletions from the N-terminus, a stepwise conversion occurred from large condensed cytoplasmic to small nuclear inclusions, then to a diffused intracellular distribution. Notablely, we found that the nuclear inclusions, formed by NS80 truncations (471 to 513–742), colocalized with cellular protein β-catenin. These data indicated that NS80 could be a major mediator in recruiting NS38 and VP4 into inclusion structures, and the C-terminus of NS80 is responsible for inclusion formation.

The NS16 protein of aquareovirus-C is a fusion-associated small transmembrane (FAST) protein, and its activity can be enhanced by the nonstructural protein NS26

Orthoreoviruses and aquareoviruses represent two different genera in the family Reoviridae, but they share many common characteristics in structural organization and pathogenesis. Similar to fusogenic orthoreoviruses, aquareoviruses can induce cell-cell fusion and multinucleated syncytium formation. Sequence analysis indicated that the nonstructural protein NS16 might be a fusion protein responsible for aquareovirus-C (AqRV-C) syncytiogenesis. To understand the basis of AqRV-C in syncytium formation, the properties of NS16 in mediating cell-cell fusion were investigated in this study. Bioinformatics analysis indicated that NS16 shares basic structural motifs with reovirus fusion-associated small transmembrane (FAST) proteins. However, the relative arrangement of these predicted structural motifs is different from the identified FAST proteins, suggesting that NS16 may present a new member of the FAST protein family. Further transfection assays showed that NS16 was able to induce cell-cell fusion. Nevertheless, the fusion activity was less efficient in comparison with that of the viral infection. In addition, NS16 was defined to display an N-terminus-outside/C-terminus-inside orientation, and the N-terminal ectodomain was critical for effective fusion. Moreover, immunofluorescence assays revealed that NS16 colocalized with nonstructural protein NS26 in cotransfected cells. And the enhanced fusion efficiency could be detected when NS16 was coexpressed with NS26, implying that NS26 may participate in cell-cell fusion through cooperation with NS16 in aquareovirus infection. Our study provided a basis for further characterization of cell-cell fusion mediated by AqRV-C.

Characterization of grass carp reovirus minor core protein VP4

BACKGROUND

Grass Carp Reovirus (GCRV), a tentative member in the genus Aquareovirus of family Reoviridae, contains eleven segmented (double-stranded RNA) dsRNA genome which encodes 12 proteins. A low-copy core component protein VP4, encoded by the viral genome segment 5(S5), has been suggested to play a key role in viral genome transcription and replication.

RESULTS

To understand the role of minor core protein VP4 played in molecular pathogenesis during GCRV infection, the recombinant GCRV VP4 gene was constructed and expressed in both prokaryotic and mammalian cells in this investigation. The recombinant His-tag fusion VP4 products expressed in E.coli were identified by Western blotting utilizing His-tag specific monoclonal and GCRV polyclonal antibodies. In addition, the expression of VP4 in GCRV infected cells, appeared in granules structure concentrated mainly in the cytoplasm, can be detected by Immunofluorescence (IF) using prepared anti-VP4 polyclonal antibody. Meanwhile, VP4 protein in GCRV core and infected cell lysate was identified by Immunoblotting (IB) assay. Of particular note, the VP4 protein was exhibited a diffuse distribution in the cytoplasm and nucleus in transfected cells, suggesting that VP4 protein may play a partial role in the nucleus by regulating cell cycle besides its predicted cytoplasmic function in GCRV infection.

CONCLUSIONS

Our results indicate the VP4 is a core component in GCRV. The cellular localization of VP4 is correlated with its predicted function. The data provide a foundation for further studies aimed at understanding the role of VP4 in viroplasmic inclusion bodies (VIB) formation during GCRV replication and assembly.

Complete genomic sequence of a reovirus isolated from grass carp in China

A reovirus was isolated from sick grass carp in Guangdong, China in 2009, and tentatively named 'grass carp reovirus Guangdong 108 strain' (GCRV-GD108). This reovirus was propagated in grass carp snout fibroblast cell line PSF with no obvious cytopathic effects. Its genome was 24,703bp in length with a 50% G+C content and 11 dsRNA segments encoding 11 proteins instead of 12 proteins. It has been classified as an Aquareovirus (AQRV). Sequence comparisons showed that it possessed only 7 homologous proteins to grass carp reovirus (GCRV) (with 17.6-45.8% identities), but 9 homologous proteins to mammalian orthoreoviruses (MRV) (with 15-46% identities). GCRV-GD108 lacked homology to VP7, NS4&NS5 and NS3 of GCRV, while it had sigma1 and sigma NS homology to MRV. VP2 of GCRV-GD108 shared high amino acid sequence identity (44-47%) with AQRVs, whereas VP5 did not exhibit much identity (24-25%) to AQRVs. Conserved terminal sequences, 5'-GUAAUUU and UUCAUC-3', were found in all of the 11 genomic segments of GCRV-GD108 at the 5' and 3' non-coding regions (NCRs) of the segments. The 5' NCRs of GCRV-GD108 was similar to GCRV, but differed from other species of AQRV or Orthoreoviruses (ORV). Phylogenetic analysis of coat proteins belonging to Reoviridae, VP1-VP6, showed that GCRV-GD108 clustered with AQRVs and grouped with ORVs, suggesting that GCRV-GD108 belonged to the genus Aquareovirus but was distinctive from any known species of AQRV. Morphological and pathological analyses, and genetic characterization of GCRV-GD108 suggested that it may be a new species of AQRV and it was more closely related with ORVs than other AQRVs. In addition, RT-PCR analysis of diseased grass carp samples collected from different regions of China indicated that these viruses displayed high similarities to each other (95.3-99.4%). They also shared high sequence similarities to GCRV-GD108 (96.7-99.4%), indicating that GCRV-GD108 is representative of the prevalence strain in southern China.

Functional investigation of grass carp reovirus nonstructural protein NS80

BACKGROUND

Grass Carp Reovirus (GCRV), a highly virulent agent of aquatic animals, has an eleven segmented dsRNA genome encased in a multilayered capsid shell, which encodes twelve proteins including seven structural proteins (VP1-VP7), and five nonstructural proteins (NS80, NS38, NS31, NS26, and NS16). It has been suggested that the protein NS80 plays an important role in the viral replication cycle that is similar to that of its homologous protein μNS in the genus of Orthoreovirus.

RESULTS

As a step to understanding the basis of the part played by NS80 in GCRV replication and particle assembly, we used the yeast two-hybrid (Y2H) system to identify NS80 interactions with proteins NS38, VP4, and VP6 as well as NS80 and NS38 self-interactions, while no interactions appeared in the four protein pairs NS38-VP4, NS38-VP6, VP4-VP4, and VP4-VP6. Bioinformatic analyses of NS80 with its corresponding proteins were performed with all currently available homologous protein sequences in ARVs (avian reoviruses) and MRVs (mammalian reoviruses) to predict further potential functional domains of NS80 that are related to VFLS (viral factory-like structures) formation and other roles in viral replication. Two conserved regions spanning from aa (amino acid) residues of 388 to 433, and 562 to 580 were discovered in this study. The second conserved region with corresponding conserved residues Tyr565, His569, Cys571, Asn573, and Glu576 located between the two coiled-coils regions (aa ~513-550 and aa ~615-690) in carboxyl-proximal terminus were supposed to be essential to form VFLS, so that aa residues ranging from 513 to 742 of NS80 was inferred to be the smallest region that is necessary for forming VFLS. The function of the first conserved region including Ala395, Gly419, Asp421, Pro422, Leu438, and Leu443 residues is unclear, but one-third of the amino-terminal region might be species specific, dominating interactions with other viral components.

CONCLUSIONS

Our results in this study together with those from previous investigations indicate the protein NS80 might play a central role in VFLS formation and viral components recruitment in GCRV particle assembly, similar to the μNS protein in ARVs and MRVs.

The use of an in vitro microneutralization assay to evaluate the potential of recombinant VP5 protein as an antigen for vaccinating against Grass carp reovirus

Background

Grass carp reovirus (GCRV) is the causative pathogen of grass carp hemorrhagic disease, one of the major diseases damaging grass carp Ctenopharyngon idellus breeding industry in China. Prevention and control of the disease is impeded largely due to the lack of research in economic subunit vaccine development. This study aimed to evaluate the potential of viral outer shell protein VP5 as subunit vaccine.

Methods

The vp5 gene was isolated from the viral genome through RT-PCR and genetically engineered to express the recombinant VP5 protein in E coli. The viral origin of the recombinant protein was confirmed by Western blot analysis with a monoclonal antibody against viral VP5 protein. Polyclonal antibody against the recombinant VP5 protein was prepared from mice. A microneutralization assay was developed to test its neutralizing ability against GCRV infection in cell culture.

Results

The GST-VP5 fusion protein (rVP5) was produced from E. Coli with expected molecular weight of 90 kDa. The protein was purified and employed to prepare anti-VP5 polyclonal antibody from mice. The anti-VP5 antibody was found to neutralize GCRV through in vitro microneutralization assay and viral progeny quantification analysis.

Conclusions

The present study showed that the viral VP5 protein was involved in viral infection and bacterially-expressed VP5 could be suitable for developing subunit vaccine for the control of GCRV infection.