GHSC70 is involved in the cellular entry of nervous necrosis virus

Slc43a2+ T cell metastasis from spleen to brain in RGNNV infected teleost

The origin of T cells in the teleost's brain is unclear. While viewing the central nervous system (CNS) as immune privileged has been widely accepted, previous studies suggest that T cells residing in the thymus but not in the spleen of the teleost play an essential role in communicating with the peripheral organs. Here, we identified nine T cell subpopulations in the thymus and spleen of orange-spotted grouper (Epinephelus coioices) through single-cell RNA-sequencing analysis. After viral CNS infection with red-spotted grouper nervous necrosis virus (RGNNV), the number of slc43a2+ T cells synchronously increased in the spleen and brain. During the infection tests in asplenic zebrafish (tlx1▲ zebrafish model), no increase in the number of slc43a2+ T cells was observed in the brain. Single-cell transcriptomic analysis indicated that slc43a2+ T cells mature and functionally differentiate within the spleen and then migrate into the brain to trigger an immune response. This study suggests a novel route for T cell migration from the spleen to the brain during viral infection in fish.

Involvement of transcriptional co-activator p300 in upregulated expression of HSP70 by aquareovirus non-structural protein NS31

Members of Aquareovirus genus, including grass carp reovirus (GCRV), contribute to a serious threat to aquaculture animals accompanied by stress response. Our previous reports revealed that GCRV nonstructural protein NS31 serves as a potent contributor for virus selectively up-regulating specific heat shock protein 70-kd gene（HSP70），however，the mechanism by which inducing HSP70 gene expression is unknown. In this study, we further found that either the N- or C-terminal domain of GCRV NS31 is responsible for enhancing fish HSP70 promoter transcription, and recombinant NS31 protein purified from baculovirus expression system seems to not directly bind HSP70 basic promoter in vitro by an electrophoretic mobility shift assay. However, the transcriptional co-activator p300 was identified as a potential interacting partner for NS31 by pull-down assay. Moreover, knock-down of p300 or addition of p300 inhibitor resulted in obviously reduced HSP70 expression by NS31 or GCRV infection suggesting that the well-characterized heat-shock-responsive HSF1/p300 transcriptional complex might involve in the induction of HSP70. These results collectively reveal this aquareovirus generates cell stress response through its nonstructural protein NS31 recruiting transcriptional co-activator p300.

In vitro infection efficiency of nervous necrosis virus alters depending on amount of viral particles adsorbed onto cells

Nervous necrosis virus (NNV) in the family Nodaviridae is one of the simplest spherical RNA viruses and is pathogenic to many fish species. We investigated the effect of purified NNV on striped snakehead cells (SSN-1) in terms of adsorption ratio and infection efficiency using the 96-well titration system. The proportion of cytopathic effect (CPE)-positive wells among total number of wells inoculated with the virus (CPE appearance ratio) reduced by 17% each time the NNV infectivity dose was halved (y = 55.7x + 50.6). Thus, subtle differences in NNV infectivity could be accurately detected using this system. Experiments performed to observe alteration of CPE appearance ratio with changing viral doses and adsorption times showed that NNV particles introduced into microplate wells as suspensions in ≤ 100 µl inoculum were adsorbed almost completely onto cells seeded on the wells within 4 days of incubation. Density profile analysis of NNV coat proteins revealed that the NNV suspension at 1 50% tissue culture infectious dose (TCID₅₀) contained 60 particles. Infection efficiency/NNV peaked at 20 particles (1.20%/particle) and then declined gradually with increasing NNV doses. Therefore, in vitro infection efficiency of NNV may alter depending on the quantity of viral particles adsorbed onto cells.

Cells and Fugu Response to Capsid of BFNNV Genotype

The nervous necrosis virus (NNV) of the BFNNV genotype is the causative agent of viral encephalopathy and retinopathy (VER) in cold water fishes. Similar to the RGNNV genotype, BFNNV is also considered a highly destructive virus. In the present study, the RNA2 of the BFNNV genotype was modified and expressed in the EPC cell line. The subcellular localization results showed that the capsid and N-terminal (1-414) were located in the nucleus, while the C-terminal (415-1014) of the capsid was located in the cytoplasm. Meanwhile, cell mortality obviously increased after expression of the capsid in EPC. EPC cells were transfected with pEGFP-CP and sampled at 12 h, 24 h and 48 h for transcriptome sequencing. There are 254, 2997 and 229 up-regulated genes and 387, 1611, and 649 down-regulated genes post-transfection, respectively. The ubiquitin-activating enzyme and ubiquitin-conjugating enzyme were up-regulated in the DEGs, indicating that cell death evoked by capsid transfection may be related to ubiquitination. The qPCR results showed that heat stock protein 70 (HSP70) is extremely up-regulated after expression of BFNNV capsid in EPC, and N-terminal is the key region to evoke the high expression. For further study, the immunoregulation of the capsid in fish pcDNA-3.1-CP was constructed and injected into the Takifugu rubripes muscle. pcDNA-3.1-CP can be detected in gills, muscle and head kidney, and lasted for more than 70 d post-injection. The transcripts of IgM and interferon inducible gene Mx were up-regulated after being immunized in different tissues, and immune factors, such as IFN-γ and C3, were also up-regulated in serum, while C4 was down-regulated one week after injection. It was suggested that pcDNA-3.1-CP can be a potential DNA vaccine in stimulating the immune system of T. rubripes; however, NNV challenge needs to be conducted in the following experiments.

The Alpha-1 Subunit of the Na+/K+-ATPase (ATP1A1) Is a Host Factor Involved in the Attachment of Porcine Epidemic Diarrhea Virus

Porcine epidemic diarrhea (PED) is an acute and severe atrophic enteritis caused by porcine epidemic diarrhea virus (PEDV) that infects pigs and makes huge economic losses to the global swine industry. Previously, researchers have believed that porcine aminopeptidase-N (pAPN) was the primary receptor for PEDV, but it has been found that PEDV can infect pAPN knockout pigs. Currently, the functional receptor for PEDV remains unspecified. In the present study, we performed virus overlay protein binding assay (VOPBA), found that ATP1A1 was the highest scoring protein in the mass spectrometry results, and confirmed that the CT structural domain of ATP1A1 interacts with PEDV S1. First, we investigated the effect of ATP1A1 on PEDV replication. Inhibition of hosts ATP1A1 protein expression using small interfering RNA (siRNAs) significantly reduced the cells susceptibility to PEDV. The ATP1A1-specific inhibitors Ouabain (a cardiac steroid) and PST2238 (a digitalis toxin derivative), which specifically bind ATP1A1, could block the ATP1A1 protein internalization and degradation, and consequently reduce the infection rate of host cells by PEDV significantly. Additionally, as expected, overexpression of ATP1A1 notably enhanced PEDV infection. Next, we observed that PEDV infection of target cells resulted in upregulation of ATP1A1 at the mRNA and protein levels. Furthermore, we found that the host protein ATP1A1 was involved in PEDV attachment and co-localized with PEDV S1 protein in the early stage of infection. In addition, pretreatment of IPEC-J2 and Vero-E6 cells with ATP1A1 mAb significantly reduced PEDV attachment. Our observations provided a perspective on identifying key factors in PEDV infection, and may provide valuable targets for PEDV infection, PEDV functional receptor, related pathogenesis, and the development of new antiviral drugs.

Identification of B-Cell Epitopes on Capsid Protein Reveals Two Potential Neutralization Mechanisms in Red-Spotted Grouper Nervous Necrosis Virus

Nervous necrosis virus (NNV), a formidable pathogen in marine and freshwater fish, has inflicted enormous financial tolls on the aquaculture industry worldwide. Although capsid protein (CP) is the sole structural protein with pathogenicity and antigenicity, public information on immunodominant regions remains extremely scarce. Here, we employed neutralizing monoclonal antibodies (MAbs) specific for red-spotted grouper NNV (RGNNV) CNPgg2018 in combination with partially overlapping truncated proteins and peptides to identify two minimal B-cell epitope clusters on CP, ¹²²GYVAGFL¹²⁸ and ²²⁷SLYNDSL²³³. Site-directed mutational analysis confirmed residues Y123, G126, and L128 and residues L228, Y229, N230, D231, and L233 as the critical residues responsible for the direct interaction with ligand, respectively. According to homologous modeling and bioinformatic evaluation, ¹²²GYVAGFL¹²⁸ is harbored at the groove of the CP junction with strict conservation among all NNV isolates, while ²²⁷SLYNDSL²³³ is localized in proximity to the tip of a viral protrusion having relatively high evolutionary dynamics in different genotypes. Additionally, ²²⁷SLYNDSL²³³ was shown to be a receptor-binding site, since the corresponding polypeptide could moderately suppress RGNNV multiplication by impeding virion entry. In contrast, ¹²²GYVAGFL¹²⁸ seemed dedicated only to stabilizing viral native conformation and not to assisting initial virus attachment. Altogether, these findings contribute to a novel understanding of the antigenic distribution pattern of NNV and the molecular basis for neutralization, thus advancing the development of biomedical products, especially epitope-based vaccines, against NNV. IMPORTANCE NNV is a common etiological agent associated with neurological virosis in multiple aquatic organisms, causing significant hazards to the host. However, licensed drugs or vaccines to combat NNV infection are very limited to date. Toward the advancement of broad-spectrum prophylaxis and therapeutics against NNV, elucidating the diversity of immunodominant regions within it is undoubtedly essential. Here, we identified two independent B-cell epitopes on NNV CP, followed by the confirmation of critical amino acid residues participating in direct interaction. These two sites were distributed on the shell and protrusion domains of the virion, respectively, and mediated the neutralization exerted by MAbs via drastically distinct mechanisms. Our work promotes new insights into NNV antigenicity as well as neutralization and, more importantly, offers promising targets for the development of antiviral countermeasures.

Hsp90 Regulates GCRV-II Proliferation by Interacting with VP35 as Its Receptor and Chaperone

The frequent outbreak of grass carp hemorrhagic disease caused by grass carp reovirus (GCRV), especially the mainly prevalent type II GCRV (GCRV-II), has seriously affected the grass carp culture in China. However, its pathogenic mechanism is still far from clear. In this study, the GCRV-II outer capsid protein VP35 was used as bait to capture interacting partners from Ctenopharyngon idellus kidney (CIK) cells, and heat shock protein 90 (Hsp90) was selected and confirmed interacting with VP35 through the C-terminal domain of Hsp90. Knockdown of Hsp90 or inhibition of Hsp90 activity suppressed GCRV-II proliferation, demonstrating that Hsp90 is an essential factor for GCRV-II proliferation. The confocal microscopy and flow cytometry showed that Hsp90 localized at both membrane and cytoplasm of CIK cells. The entry of GCRV-II into CIK cells was efficiently blocked by incubating the cells with Hsp90 antibody or by pretreating the virus with recombinant Hsp90 protein. Whereas overexpression of Hsp90 in CIK cells, grass carp ovary (GCO) cells, or 293T cells promoted GCRV-II entry, indicating that the membrane Hsp90 functions as a receptor of GCRV-II. Furthermore, Hsp90 interacted with clathrin and mediated GCRV-II entry into CIK cells through clathrin endocytosis pathway. In addition, we found that the cytoplasmic Hsp90 acted as a chaperone of VP35 because inhibition of Hsp90 activity enhanced VP35 polyubiquitination and degraded VP35 through the proteasome pathway. Collectively, our data suggest that Hsp90 functions both as a receptor for GCRV-II entry and a chaperone for the maturation of GCRV-II VP35, thus ensuring efficient proliferation of GCRV-II. IMPORTANCE Identification of viral receptors has always been the research hot spot in virus research field as receptor functions at the first stage of viral infection, which can be designed as efficient antiviral drug targets. GCRV-II, the causative agent of the grass carp epidemic hemorrhagic disease, has caused tremendous losses in grass carp culture in China. To date, the receptor of GCRV-II remains unknown. This study focused on identifying cellular receptor interacting with the GCRV-II outer capsid protein VP35, studying the effects of their interaction on GCRV-II proliferation, and revealing the underlying mechanisms. We demonstrated that Hsp90 acts both as a receptor of GCRV-II by interacting with VP35 and as a chaperone for the maturation of VP35, thus ensuring efficient proliferation of GCRV-II. Our data provide important insights into the role of Hsp90 in GCRV-II life cycle, which will help understand the mechanism of reovirus infection.

Computer-aided drug design of Azadirachta indica compounds against nervous necrosis virus by targeting grouper heat shock cognate protein 70 (GHSC70): quantum mechanics calculations and molecular dynamic simulation approaches

Nervous necrosis virus (NNV) is a deadly infectious disease that affects several fish species. It has been found that the NNV utilizes grouper heat shock cognate protein 70 (GHSC70) to enter the host cell. Thus, blocking the virus entry by targeting the responsible protein can protect the fishes from disease. The main objective of the study was to evaluate the inhibitory potentiality of 70 compounds of Azadirachta indica (Neem plant) which has been reported to show potential antiviral activity against various pathogens, but activity against the NNV has not yet been reported. The binding affinity of 70 compounds was calculated against the GHSC70 with the docking and molecular dynamics (MD) simulation approaches. Both the docking and MD methods predict 4 (PubChem CID: 14492795, 10134, 5280863, and 11119228) inhibitory compounds that bind strongly with the GHSC70 protein with a binding affinity of ‒9.7, ‒9.5, ‒9.1, and ‒9.0 kcal/mol, respectively. Also, the ADMET (absorption, distribution, metabolism, excretion, and toxicity) properties of the compounds confirmed the drug-likeness properties. As a result of the investigation, it may be inferred that Neem plant compounds may act as significant inhibitors of viral entry into the host cell. More in-vitro testing is needed to establish their effectiveness.

Multiple isoforms of HSP70 and HSP90 required for betanodavirus multiplication in medaka cells

Heat shock proteins (HSPs) are molecular chaperones that have recently been shown to function as host factors (HFs) for virus multiplication in fish as well as in mammals, plants, and insects. HSPs are classified into families, and each family has multiple isoforms. However, no comprehensive studies have been performed to clarify the biological importance of these multiple isoforms for fish virus multiplication. Betanodaviruses are the causative agents of viral nervous necrosis in cultured marine fish and cause very high mortality. Although the viral genome and encoded proteins have been characterized extensively, information on HFs for these viruses is limited. In this study, therefore, we focused on the HSP70 and HSP90 families to examine the importance of their isoforms for betanodavirus multiplication. We found that HSP inhibitors (17-AAG, radicicol, and quercetin) suppressed viral RNA replication and production of progeny virus in infected medaka (Oryzias latipes) cells. Thermal stress or virus infection resulted in increased expression of some isoform genes and facilitated virus multiplication. Furthermore, overexpression and knockdown of some isoform genes revealed that the isoforms HSP70-1, HSP70-2, HSP70-5, HSP90-α1, HSP90-α2, and HSP90-β play positive roles in virus multiplication in medaka. Collectively, these results suggest that multiple isoforms of fish HPSs serve as HFs for betanodavirus multiplication.

High-density lipoproteins negatively regulate innate immunity and facilitate red-spotted grouper nervous necrosis virus entry via scavenger receptor B type 1

Lipid metabolism plays an important role in viral infections, and it can directly or indirectly affect various stages of viral infection in cells. As an important component of lipid metabolism, high-density lipoprotein (HDL) plays crucial roles in inflammation, immunity, and viral infections. Scavenger receptor B type 1 (SR-B1), a receptor of HDL, cannot be ignored in the regulation of lipid metabolism. Here, we investigate, for the first time, the role of Epinephelus coioides SR-B1 (Ec-SR-B1) in red-spotted grouper nervous necrosis virus (RGNNV) infection. Our results indicate that Ec-SR-B1 could promote RGNNV infection. We also demonstrate that Ec-SR-B1 could facilitate viral entry and interact with capsid protein (CP) of RGNNV. As the natural ligand of SR-B1, HDL significantly increased RGNNV entry in a dose-dependent manner. However, we observed no effect of HDL on Ec-SR-B1 expression. The results of the micro-scale thermophoresis assay did not reveal an association between HDL and CP, suggesting that RGNNV does not enter target cells by using HDL as a ligand to bind to its receptor. In addition, block lipid transport-1, a compound that inhibits HDL-mediated cholesterol transfer, reduced the HDL-induced enhancement of RGNNV infection, indicating a role for lipid transfer in facilitating RGNNV entry. Furthermore, HDL inhibited the expression of pro-inflammatory factors and antiviral genes in a dose-dependent manner. These findings suggest that the HDL-induced enhancement of RGNNV entry involves the complex interplay between Ec-SR-B1, HDL, and RGNNV, as well as the regulation of innate antiviral responses by HDL. In summary, we highlight the crucial role of HDL in RGNNV entry, identify a possible molecular connection between RGNNV and lipoprotein metabolism, and indicate the role of Ec-SR-B1 in RGNNV infection.

Establishing an effective gene knockdown system using cultured cells of the model fish medaka (Oryzias latipes)

In spite of the growing attention given to medaka (Oryzias latipes) as an excellent vertebrate model, an effective gene knockdown system has not yet been established using cultured cells of this fish species. In this study, a gene knockdown system using short interfering RNA (siRNA) in medaka cell lines was established through the optimization of transfection conditions. By extensive screening of several medaka cell lines and transfection reagents, OLHNI-2 cells and X-tremeGENE siRNA Transfection Reagent were selected as the best combination to achieve high transfection efficiency of siRNA without cytotoxic effect. Knockdown conditions were then refined using the endogenous heat shock protein 90 (Hsp90) genes as the siRNA targets. Among the parameters tested, cell density, serum concentration in the culture medium, and duration of transfection improved knockdown efficiency, where the target mRNA in cells transfected with each of the siRNAs was reduced from 12.0% to 26.7% of the control level. Our results indicate that the established knockdown system using siRNA is a promising tool for functional analysis of medaka genes in vitro.

Profile of Innate Immunity in Gilthead Seabream Larvae Reflects Mortality upon Betanodavirus Reassortant Infection and Replication

Historically, gilthead seabream (Sparus aurata) has been considered a fish species resistant to nervous necrosis virus (NNV) disease. Nevertheless, mortality in seabream hatcheries, associated with typical clinical signs of the viral encephalopathy and retinopathy (VER) disease has been confirmed to be caused by RGNNV/SJNNV reassortants. Because of this, seabream larvae at 37 and 86 days post-hatching (dph) were infected by immersion with RGNNV/SJNNV and SJNNV/RGNNV reassortants under laboratory conditions, and mortality, viral replication and immunity were evaluated. Our results show that gilthead seabream larvae, mainly those at 37 dph, are susceptible to infection with both NNV reassortant genotypes, with the highest impact from the RGNNV/SJNNV reassortant. In addition, viral replication occurs at both ages (37 and 86 dph) but the recovery of infective particles was only confirmed in 37 dph larvae,; this value was also highest with the RGNNV/SJNNV reassortant. Larvae immunity, including the expression of antiviral, inflammatory and cell-mediated cytotoxicity genes, was affected by NNV infection. Levels of the natural killer lysin (Nkl) peptide were increased in SJNNV/RGNNV-infected larvae of 37 dph, though hepcidin was not. Our results demonstrate that the seabream larvae are susceptible to both NNV reassortants, though mainly to RGNNV/SJNNV, in an age-dependent manner.

T-Cell Immunoglobulin and Mucin Domain 1 (TIM-1) Is a Functional Entry Factor for Tick-Borne Encephalitis Virus

Tick-borne encephalitis virus (TBEV) is the causative agent of a potentially fatal neurological infection affecting humans. The host factors required for viral entry have yet to be described. Here, we found that T-cell immunoglobulin and mucin domain 1 (TIM-1) acted as the cellular entry factor for TBEV. Using a virus overlay protein binding assay, TIM-1 was identified as a virion-interacting protein. Cells that were relatively resistant to TBEV infection became highly susceptible to infection when TIM-1 was ectopically expressed. TIM-1 knockout and viral RNA bypass assays showed that TIM-1 functioned in the entry phase of TBEV infection. TIM-1 mediated TBEV uptake and was cointernalized with virus particles into the cell. Antibodies for TIM-1, soluble TIM-1, or TIM-1 knockdown significantly inhibited TBEV infection in permissive cells. Furthermore, in TIM-1 knockout mice, TIM-1 deficiency markedly lowered viral burden and reduced mortality and morbidity, highlighting the functional relevance of TIM-1 in vivo. With TIM-1, we have identified a key host factor for TBEV entry and a potential target for antiviral intervention. IMPORTANCE TBEV is a tick-transmitted flavivirus that causes serious diseases in the human central nervous system in Eurasia. The host determinants required for viral entry remain poorly understood. Here, we found that TIM-1 is a cellular entry factor for TBEV. Antibodies directed at TIM-1 or soluble TIM-1 treatment decreased virus infection in cell cultures. TIM-1 was cointernalized with virus particles into cells. TIM-1 deficiency significantly lowered viral burden and attenuated pathogenesis in the murine TBEV infection model. The demonstration of TIM-1 as a cellular entry factor for TBEV will improve understanding of virus infection and provide a target for antiviral development.

Differential Nervous Necrosis Virus (NNV) Replication in Five Putative Susceptible Cell Lines

Viral encephalopathy and retinopathy caused by nervous necrosis virus (NNV), is one of the most threatening viral diseases affecting marine fish worldwide. In vitro propagation of NNV strains is essential for the design of effective control measures. In the present study we analysed both the susceptibility and the permissiveness of five fish cell lines (E-11, GF-1, SAF-1, DLB-1, and SaB-1) to three NNV strains (one RGNNV, one SJNNV, and one reassortant RGNNV/SJNNV). E-11 and DLB-1 were demonstrated to be highly susceptible to NNV strains, with average adsorption efficiency (AE) values higher than 90%. SAF-1 also showed high susceptibility (AE 88%), whereas GF-1 can be regarded as moderately susceptible (AE around 50%). On the contrary, SaB-1 can be considered a poorly susceptible cell line (AE values below 20%). E-11 and GF-1 cell lines provided the highest production rates for RGNNV and RG/SJ (around 10³) and both cell lines can be regarded as fully permissive for these viral types. However, the SJNNV production rate in GF-1 was only 17.8 and therefore this cell line should be considered semi-permissive for this genotype. In SAF-1 cells, moderate viral replication was recorded but differences in intracellular and extracellular production suggest that viral progeny was not efficiently released. In DLB-1 and SaB-1 the final viral titres obtained in E-11 were lower than those of the inoculum. However, RNA1 synthesis values seem to indicate that RGNNV replication in DLB-1 and SAF-1 could have been underestimated, probably due to a poor adaptation of the virus grown in these cell lines to E-11. Based on all these results, E-11 seems to be the most appropriate cell for in vitro culture of RGNNV, SJNNV, and reassortant strains.

HSC70 Inhibits Spring Viremia of Carp Virus Replication by Inducing MARCH8-Mediated Lysosomal Degradation of G Protein

As a fierce pathogen, spring viremia of carp virus (SVCV) can cause high mortality in the common carp, and its glycoprotein (G protein) is a component of the viral structure on the surface of virion, which is crucial in viral life cycle. This report adopted tandem affinity purification (TAP), mass spectrometry analysis (LC-MS/MS), immunoprecipitation, and confocal microscopy assays to identify Heat shock cognate protein 70 (HSC70) as an interaction partner of SVCV G protein. It was found that HSC70 overexpression dramatically inhibited SVCV replication, whereas its loss of functions elicited opposing effects on SVCV replication. Mechanistic studies indicate that HSC70 induces lysosomal degradation of ubiquitinated-SVCV G protein. This study further demonstrates that Membrane-associated RING-CH 8 (MARCH8), an E3 ubiquitin ligase, is critical for SVCV G protein ubiquitylation and leads to its lysosomal degradation. Furthermore, the MARCH8 mediated ubiquitylation of SVCV G protein required the participation of HSC70 through forming a multicomponent complex. Taken together, these results demonstrate that HSC70 serves as a scaffold for MARCH8 and SVCV G, which leads to the ubiquitylation and degradation of SVCV G protein and thus inhibits viral replication. These findings have established a novel host defense mechanism against SVCV.

Single-cell RNA-seq landscape midbrain cell responses to red spotted grouper nervous necrosis virus infection

Viral nervous necrosis (VNN) is an acute and serious fish disease caused by nervous necrosis virus (NNV) which has been reported massive mortality in more than fifty teleost species worldwide. VNN causes damage of necrosis and vacuolation to central nervous system (CNS) cells in fish. It is difficult to identify the specific type of cell targeted by NNV, and to decipher the host immune response because of the functional diversity and highly complex anatomical and cellular composition of the CNS. In this study, we found that the red spotted grouper NNV (RGNNV) mainly attacked the midbrain of orange-spotted grouper (Epinephelus coioides). We conducted single-cell RNA-seq analysis of the midbrain of healthy and RGNNV-infected fish and identified 35 transcriptionally distinct cell subtypes, including 28 neuronal and 7 non-neuronal cell types. An evaluation of the subpopulations of immune cells revealed that macrophages were enriched in RGNNV-infected fish, and the transcriptional profiles of macrophages indicated an acute cytokine and inflammatory response. Unsupervised pseudotime analysis of immune cells showed that microglia transformed into M1-type activated macrophages to produce cytokines to reduce the damage to nerve tissue caused by the virus. We also found that RGNNV targeted neuronal cell types was GLU1 and GLU3, and we found that the key genes and pathways by which causes cell cytoplasmic vacuoles and autophagy significant enrichment, this may be the major route viruses cause cell death. These data provided a comprehensive transcriptional perspective of the grouper midbrain and the basis for further research on how viruses infect the teleost CNS.

Sites responsible for infectivity and antigenicity on nervous necrosis virus (NNV) appear to be distinct

Nervous necrosis virus (NNV) is a pathogenic fish-virus belonging to the genus Betanodavirus (Nodaviridae). Surface protrusions on NNV particles play a crucial role in both antigenicity and infectivity. We exposed purified NNV particles to different physicochemical conditions to investigate the effects on antigenicity and infectivity, in order to reveal information regarding the conformational stability and spatial relationships of NNV neutralizing-antibody binding sites and cell receptor binding sites. Treatment with PBS at 37 °C, drastically reduced NNV antigenicity by 66–79% on day one, whereas its infectivity declined gradually from 10^7.6 to 10^5.8 TCID₅₀/ml over 10 days. When NNV was treated with carbonate/bicarbonate buffers at different pHs, both antigenicity and infectivity of NNV declined due to higher pH. However, the rate of decline with respect to antigenicity was more moderate than for infectivity. NNV antigenicity declined 75–84% after treatment with 2.0 M urea, however, there was no reduction observed in infectivity. The antibodies used in antigenicity experiments have high NNV-neutralizing titers and recognize conformational epitopes on surface protrusions. The maintenance of NNV infectivity means that receptor binding sites are functionally preserved. Therefore, it seems highly likely that NNV neutralizing-antibody binding sites and receptor binding sites are independently located on surface protrusions.

Protein Disulfide Isomerase A4 Is Involved in Genome Uncoating during Human Astrovirus Cell Entry

Although human astroviruses (HAstVs) are important agents of gastroenteritis in young children, the studies aimed at characterizing their biology have been limited, in particular regarding their cell entry process. It has been shown that HAstV serotype 8 enters human cells by a classical clathrin-mediated endocytosis pathway; however, the cell receptor or other cell entry factors that may be relevant for an efficient viral infection are unknown. In this work we used a far-Western blotting approach to identify cellular proteins that interact with the recombinant capsid spike proteins of HAstV serotypes 1, 2, and 8, synthesized in Escherichia coli. We identified the 72 kDa protein disulfide isomerase A4 (PDIA4) as a binding partner for HAstV-1 and -8 spikes, but not for the HAstV-2 spike. In agreement with this observation, the PDI inhibitor 16F16 strongly blocked infection by HAstV serotypes 1 and 8, but not serotype 2. RNA interference of PDIA4 expression selectively blocked HAstV-8 infectivity. We also showed that the PDI activity does not affect virus binding or internalization but is required for uncoating of the viral genome.

Roles of extracellular matrix components in Tiger frog virus attachment to fathead minnow (Pimephales promelas) cells

Tiger frog virus (TFV) belongs to the genus Ranavirus (family Iridoviridae) and causes significant harm in cultured frogs, resulting in substantial losses in ecological and economic field in Southern China. Attachment is the first step in viral life cycle, which is dependent on the interactions of virions with extracellular matrix (ECM) components. Studying this process will help in understanding virus infection and controlling viral diseases. In this study, the roles of primary ECM components in TFV attachment were investigated. The results on the kinetics of virus attachment showed TFV successful attachment to the cell surface as a relatively rapid process after TFV was used to inoculate cells for 10 min at 4 °C. Western blot and quantitative PCR analyses results showed that soluble fibronectin, collagen IV, laminin, or hyaluronic acid treatment with TFV caused no significant effect on virus attachment. Soluble heparin, heparan sulfate and chondroitin sulfate A/B could inhibit TFV attachment in a dose-dependent manner. Enzymic digestion by cell surface heparin/heparan sulfate using heparinase I, II, and III could significantly prevent TFV attachment, suggesting that heparan sulfate plays an important role in TFV attachment. Furthermore, the binding assays of heparin-agarose beads and virion showed that TFV virions specifically bound with heparin in a dose-dependent manner. Given that heparin is a structural analogue of heparan sulfate, the above results suggest that heparan sulfate might serve as an attachment factor of TFV infection. Our work would be beneficial to understand the mechanisms of TFV attachment and the interactions of TFV with cellular receptor(s).

Screening for the Proteins That Can Interact with Grouper Nervous Necrosis Virus Capsid Protein

Nervous necrosis virus (NNV) can infect many species of fish and has an 80-100% mortality rate. NNV capsid protein (NNVCP) is the only structural protein of NNV, but there are few studies on the protein-protein interaction between NNVCP and the host cell. To investigate NNV morphogenesis, native NNV capsid protein (NNVCP) was used to screen for protein-protein interactions in this study. The results identified that 49 grouper optic nerve proteins can interact with NNVCP and may function as putative receptor or co-receptor, cytoskeleton, glucose metabolism and ATP generation, immunity, mitochondrial ion regulation, and ribosomal proteins. Creatine kinase B-type (CKB) is one of those 49 optic nerve proteins. CKB, a kind of enzyme of ATP generation, was confirmed to interact with NNVCP by far-Western blot and showed to colocalize with NNVCP in GF-1 cells. Compared to the control, the expression of CKB was significantly induced in the brain and eyes infected with NNV. Moreover, the amount of replication of NNV is relatively high in cells expressing CKB. In addition to providing the database of proteins that can interact with NNVCP for subsequent analysis, the results of this research also verified that CKB plays an important role in the morphogenesis of NNV.

Macrobrachium rosenbergii nodavirus virus-like particles attach to fucosylated glycans in the gills of the giant freshwater prawn

The Macrobrachium rosenbergii nodavirus (MrNV), the causative agent of white-tail disease (WTD) in many species of shrimp and prawn, has been shown to infect hemocytes and tissues such as the gills and muscles. However, little is known about the host surface molecules to which MrNV attach to initiate infection. Therefore, the present study investigated the role of glycans as binding molecules for virus attachment in susceptible tissues such as the gills. We established that MrNV in their virus-like particle (MrNV-VLP) form exhibited strong binding to gill tissues and lysates, which was highly reduced by the glycan-reducing periodate and PNGase F. The broad, fucose-binding Aleuria Aurantia lectin (AAL) highly reduced MrNV-VLPs binding to gill tissue sections and lysates, and efficiently disrupted the specific interactions between the VLPs and gill glycoproteins. Furthermore, mass spectroscopy revealed the existence of unique fucosylated LacdiNAc-extended N-linked and O-linked glycans in the gill tissues, whereas beta-elimination experiments showed that MrNV-VLPs demonstrated a binding preference for N-glycans. Therefore, the results from this study highly suggested that MrNV-VLPs preferentially attach to fucosylated N-glycans in the susceptible gill tissues, and these findings could lead to the development of strategies that target virus-host surface glycan interactions to reduce MrNV infections.

Early viral uptake and host-associated immune response in the tissues of seven-band grouper following a bath challenge with nervous necrosis virus

In the present study, early uptake of nervous necrosis virus (NNV) in the tissues (gill, brain, skin, eye, heart) and immune response associated with the uptake in the gill and brain of seven-band grouper was investigated. The gill was found to act as a primary portal of entry for NNV during the initial phase of the water-borne infection. The presence of viral genome and infectious particles was demonstrated using quantitative (qPCR, viral titer) and qualitative (ISH) approach. Initially, an increased viral uptake was noticed, but the virus got cleared from the gills at the later phase of infection. Localization in the brain was evident at the blood-brain barrier followed by the brain parenchyma in the latter stage of infection. Nectin-4, an established NNV receptor, and GHSC70 showed an up-regulated expression throughout the challenge period initially in the gill and at latter phase in brain; however, it seems that the virus does not use gill as a primary replication site but brain as a permissive tissue. Combined activity as reflected by the up-regulation of cytokine, interferon, antigen-presenting cell, and immunoglobulin genes restricts early NNV replication in gill. Observations from the present study provide a better understanding of early NNV entry and also opens a window for further elucidating the modes of NNV neuro-invasion through systemic circulation.

Marine medaka heat shock protein 90ab1 is a receptor for red-spotted grouper nervous necrosis virus and promotes virus internalization through clathrin-mediated endocytosis

Nervous necrosis virus (NNV) can infect many species of fish and causes serious acute or persistent infection. However, its pathogenic mechanism is still far from clear. Specific cellular surface receptors are crucial determinants of the species tropism of a virus and its pathogenesis. Here, the heat shock protein 90ab1 of marine model fish species marine medaka (MmHSP90ab1) was identified as a novel receptor of red-spotted grouper NNV (RGNNV). MmHSP90ab1 interacted directly with RGNNV capsid protein (CP). Specifically, MmHSP90ab1 bound to the linker region (LR) of CP through its NM domain. Inhibition of MmHSP90ab1 by HSP90-specific inhibitors or MmHSP90ab1 siRNA caused significant inhibition of viral binding and entry, whereas its overexpression led to the opposite effect. The binding of RGNNV to cultured marine medaka hMMES1 cells was inhibited by blocking cell surface-localized MmHSP90ab1 with anti-HSP90β antibodies or pretreating virus with recombinant MmHSP90ab1 or MmHSP90ab1-NM protein, indicating MmHSP90ab1 was an attachment receptor for RGNNV. Furthermore, we found that MmHSP90ab1 formed a complex with CP and marine medaka heat shock cognate 70, a known NNV receptor. Exogenous expression of MmHSP90ab1 independently facilitated the internalization of RGNNV into RGNNV impenetrable cells (HEK293T), which was blocked by chlorpromazine, an inhibitor of clathrin-dependent endocytosis. Further study revealed that MmHSP90ab1 interacted with the marine medaka clathrin heavy chain. Collectively, these data suggest that MmHSP90ab1 is a functional part of the RGNNV receptor complex and involved in the internalization of RGNNV via the clathrin endocytosis pathway.

Cloning and characterisation of type I interferon receptor 1 in orange-spotted grouper (Epinephelus coioides) for response to nodavirus infection

Grouper is known as a highly economical teleost species in the Asian aquaculture industry; however, intensive culture activities easily cause disease outbreak, especially viral disease. For the prevention of viral outbreaks, interferon (IFN) is among the major defence systems being studied in different species. Fish type I IFNs are known to possess antiviral properties similar to mammalian type I IFNs. In order to stimulate antiviral function, IFN will bind to its cognate receptor, the type I interferon receptor (IFNAR), composed of heterodimeric receptor subunits known as IFNAR1 and IFNΑR2. The binding of type I interferon to receptors assists in the transduction of signals from the external to internal environments of cells to activate biological responses. In order to study the function of IFN, we first need to understand IFN receptors. In this study, we cloned and identified IFNAR1 in orange-spotted grouper (osgIFNAR1) and noted the up-regulated mRNA expression of the receptor and downstream effectors in the head kidney cells with cytokine treatment. The transcriptional expression of osgIFNAR1, which is characterised using polyinosinic-polycytidylic acid (poly[I:C]) and lipopolysaccharide (LPS) treatments, indicated the involvement of osgIFNAR1 in the immune response of grouper. The subcellular localisation of osgIFNAR1 demonstrated scattering across the grouper cell. Viral infection showed the negative feedback regulation of osgIFNAR1 in grouper larvae. Further loss of function of IFNAR1 showed a decreased expression of the virus. This study reported the identification of osgIFNAR1 and characterisation of receptor sensitivity towards immunostimulants, cytokine response, and viral challenge in the interferon pathway of orange-spotted grouper and possible different role of the receptor in viral production. Together, these results provide a frontline report of the potential function of osgIFNAR1 in the innate immunity of teleost.

Indigenous versus Lessepsian Hosts: Nervous Necrosis Virus (NNV) in Eastern Mediterranean Sea Fish

Viruses are among the most abundant and diverse biological components in the marine environment. In finfish, viruses are key drivers of host diversity and population dynamics, and therefore, their effect on the marine environment is far-reaching. Viral encephalopathy and retinopathy (VER) is a disease caused by the marine nervous necrosis virus (NNV), which is recognized as one of the main infectious threats for marine aquaculture worldwide. For over 140 years, the Suez Canal has acted as a conduit for the invasion of Red Sea marine species into the Mediterranean Sea. In 2016–2017, we evaluated the prevalence of NNV in two indigenous Mediterranean species, the round sardinella (Sardinella aurita) and the white steenbras (Lithognathus mormyrus) versus two Lessepsian species, the Randall’s threadfin bream (Nemipterus randalli) and the Lessepsian lizardfish (Saurida lessepsianus). A molecular method was used to detect NNV in all four fish species tested. In N. randalli, a relatively newly established invasive species in the Mediterranean Sea, the prevalence was significantly higher than in both indigenous species. In S. lessepsianus, prevalence varied considerably between years. While the factors that influence the effective establishment of invasive species are poorly understood, we suggest that the susceptibility of a given invasive fish species to locally acquired viral pathogens such as NVV may be important, in terms of both its successful establishment in its newly adopted environment and its role as a reservoir ‘host’ in the new area.

Establishment of a brain cell line (FuB-1) from mummichog (Fundulus heteroclitus) and its application to fish virology, immunity and nanoplastics toxicology

The marine fish mummichog (Fundulus heteroclitus), extensively used as research model, including in ecotoxicology, for over a century has been surpassed by other fish species. This fact may be associated with the lack of cell lines from this species, excellent models for the comprehension of fish physiology, immunology, toxicology and virology, that contribute to the reduction in the number of animals used in research. We have generated, for the first time, a brain-derived cell line from mummichog, FuB-1, and evaluated its application to the fields of fish virology, immunity and toxicology. First, FuB-1 cells show epithelial morphology and neural stem/astroglial origin. Secondly, FuB-1 cells effectively supports the replication of both spring viremia carp (SVCV) and infectious pancreatic necrosis (IPNV) viruses, but not nodavirus (NNV), indicating its potential use for fish virology. Related to this, FuB-1 cells infected with NNV up-regulate the transcription of genes related to the antiviral immune response, leading to cell resistance; while they are unaltered when infected with IPNV and SVCV, facilitating viral replication. Finally, FuB-1 cells were used for toxicological purposes and we demonstrated that exposure to either polystyrene nanoplastics (PS-100) or several human-usage pharmaceuticals are cytotoxic. Additionally, PS-100 particles increase the antioxidant catalase and glutathione S-transferase activities and decrease the total non-protein thiols in FuB-1 cells. However, PS-100 particles are able to reduce the cytotoxic effects induced by the pharmaceuticals. In conclusion, we have generated a cell line from mummichog, which might represent a valuable model for fish studies in the fields of virology, immunology and toxicology.

Betanodavirus and VER Disease: A 30-year Research Review

The outbreaks of viral encephalopathy and retinopathy (VER), caused by nervous necrosis virus (NNV), represent one of the main infectious threats for marine aquaculture worldwide. Since the first description of the disease at the end of the 1980s, a considerable amount of research has gone into understanding the mechanisms involved in fish infection, developing reliable diagnostic methods, and control measures, and several comprehensive reviews have been published to date. This review focuses on host–virus interaction and epidemiological aspects, comprising viral distribution and transmission as well as the continuously increasing host range (177 susceptible marine species and epizootic outbreaks reported in 62 of them), with special emphasis on genotypes and the effect of global warming on NNV infection, but also including the latest findings in the NNV life cycle and virulence as well as diagnostic methods and VER disease control.

Functional characterization of seven-band grouper immunoglobulin like cell adhesion molecule, Nectin4 as a cellular receptor for nervous necrosis virus

Nectin-4/PVRL4 belonging to the family of immunoglobulin-like cell adhesion molecules was identified as a potential cellular receptor for several animal viruses. Here we show that nervous necrosis virus that causes viral nervous necrosis in teleosts uses the same receptor in its life cycle. Transfection of SSN-1 cell lines with an expression vector encoding Nectin-4 rendered them to be more susceptible to NNV. Immunofluorescence microscopy on Nectin-4 expressing cells revealed that the protein interacted with NNV specifically. A virus binding assay indicated that Nectin-4 was a bonafide receptor that supported virus attachment to the host cell whereas siRNA directed against Nectin-4 blocked NNV infections in grouper primary brain cells. Results of the present study will improve our understanding of the pathogenesis of NNV infection and provide a target for the development of novel antiviral interventions in marine finfish aquaculture.

Screening for Antiviral Medaka Haploid Embryonic Stem Cells by Genome Wide Mutagenesis

Nervous necrosis virus (NNV), one of the most prevalent fish pathogens, has caused significant losses in both yield and economy to the aquaculture. Host factors involved in NNV infection remain to be identified due to the lack of ideal model for the study of NNV and host interaction. Haploid stem cells have proven to be ideal materials in genetic screens. Here, we generated a cell line HX1G1 (simply named G1) with the activity against red-spotted grouper nervous necrosis virus (RGNNV) by N-ethyl-N-nitrosourea (ENU)-mediated whole genome random mutagenesis from the haploid embryonic stem cell HX1a, a cell clone from haploid cell line HX1 that we previously derived from the medaka fish. G1 cells retained the characteristics of haploidy and pluripotency as indicated by the EBs differentiation ability after genetic mutagenesis. Compared with HX1a cells, no typical cytopathic effects were observed, and the expression of RNA-dependent RNA polymerase (RDRP) was significantly reduced in G1 cells post RGNNV infection, indicating the enhanced anti-RGNNV activity of G1. Furthermore, we demonstrated that RGNNV entry into G1 cells was partially inhibited, and this inhibition might be relevant to the induced mutation of heat shock cognate protein 70 (HSC70) which was decisive for NNV entry. Interestingly, G1 cells were to some extent permissive to RGNNV infection, but RGNNV was spontaneously cleared in G1 cells during serial passage. In addition, we also found that the expression levels of interferon (IFN)-related genes were higher in G1 cells than those in HX1a cells, suggesting that viral clearance might be associated with the elevated expression of IFN-related genes in G1 cells.

European sea bass brain DLB-1 cell line is susceptible to nodavirus: A transcriptomic study

Viral diseases are responsible for high rates of mortality and subsequent economic losses in modern aquaculture. The nervous necrosis virus (NNV) produces viral encephalopathy and retinopathy (VER), which affects the fish central nervous system. It is considered one of the most serious viral diseases in marine aquaculture, the European sea bass (Dicentrarchus labrax) being amongst the most susceptible. We have evaluated the European sea bass brain derived cell line (DLB-1) susceptibility to NNV genotypes and evaluated its transcriptomic profile. DLB-1 cells supported NNV gene transcription and replication since strains belonging to the four NNV genotypes produce cytopathic effects. Afterwards, DLB-1 cells were infected with an RGNNV strain, the one which showed the highest replication, for 12 and 72 h and an RNA-seq analysis was performed to identify potential genes involved in the host-NNV interactions. Differential expression analysis showed the up-regulation of many genes related to immunity, heat-shock proteins or apoptosis but not to proteasome or autophagy processes. These data suggest that the immune response, mainly the interferon (IFN) pathway, is not powerful enough to abrogate the infection, and cells finally suffer stress and die by apoptosis liberating infective particles. GO enrichment also revealed, for the first time, the down-regulation of terms related to brain/neuron biology indicating molecular mechanisms causing the pathogenic effect of NNV. This study opens the way to understand key elements in sea bass brain and NNV interactions.

VER-155008 induced Hsp70 proteins expression in fish cell cultures while impeding replication of two RNA viruses

The heat-shock protein 70 (Hsp70) inhibitor, VER-155008 (VER), was explored as a potential antiviral agent for two RNA viruses important to fish aquaculture, viral hemorrhagic septicemia virus (VHSV) and infectious pancreatic necrosis virus (IPNV). Studies were done at a temperature of 14 °C, and with cell lines commonly used to propagate these viruses. These were respectively EPC from fathead minnow for VHSV and CHSE-214 from Chinook salmon embryo for IPNV. Additionally, both viruses were studied with the Atlantic salmon heart endothelial cell line ASHe. For both VHSV and IPNV, 25 μM VER impeded replication. This was evidenced by delays in the development of cytopathic effect (CPE) and the expression of viral proteins, N for VHSV and VP2 for IPNV, and by less production of viral RNA and of viral titre. As VER inhibits the activity of Hsp70 family members, these results suggest that VHSV and IPNV utilize one or more Hsp70s in their life cycles. Yet neither virus induced Hsp70. Surprisingly VER alone induced Hsp70, but whether this induction modulated VER's antiviral effects is unknown. Exploring this apparent paradox in the future should improve the usefulness of VER as an antiviral agent.

Development of sesbania mosaic virus nanoparticles for imaging

The capsids of viruses have a high degree of symmetry. Therefore, virus nanoparticles (VNPs) can be programmed to display many imaging agents precisely. Plant VNPs are biocompatible, biodegradable and non-infectious to mammals. We have carried out bioconjugation of sesbania mosaic virus (SeMV), a well characterized plant virus, with fluorophores using reactive lysine-N-hydroxysuccinimide ester and cysteine-maleimide chemistries. Monitoring of cellular internalization of labelled SeMV nanoparticles (NPs) by confocal microscopy and flow cytometry showed that the particles have a natural preference for entry into MDA-MB-231 (breast cancer) cells, although they could also enter various other cell lines. The fluorescence of SeMV NPs labelled via the cysteines with Cy5.5 dye was found to be more stable and was detectable with greater sensitivity than that of particles labelled via the lysines with Alexa Fluor. Live-cell imaging using SeMV internally labelled with Cy5.5 showed that it could bind to MDA-MB-231 cells in less than 5 minutes and enter the cells within 15 minutes. The particles undergo endolysosomal degradation by 6 h as evidenced by their co-localization with LAMP-1. Far-western blot analysis with a HeLa cell membrane protein fraction showed that SeMV interacts with 54-, 35- and 33-kDa proteins, which were identified by mass spectrometry as vimentin, voltage-dependent anion-selective channel protein (VDAC1), and annexin A2 isoform 2 (ANXA2), respectively, suggesting that the particles may bind and enter the cell through these proteins. The results presented here demonstrate that the SeMV NPs provide a new platform technology that could be used to develop in vivo imaging and targeted drug delivery agents for cancer diagnosis and therapy.

Quantitative Flow Cytometry to Measure Viral Production Using Infectious Pancreatic Necrosis Virus as a Model: A Preliminary Study

In recent decades, flow cytometry (FCM) has become an important tool in virology, due to its applications in viral replication and viral-cell interactions, as well as its capacity to quantify proteins (qFCM). In the present study, we have designed and evaluated a qFCM procedure for the in vitro analysis and quantification of fish viral proteins, using the infectious pancreatic necrosis virus (IPNV) as a model. We have also tested its use for viral titration and adapted the MARIS (method for analysing RNA following intracellular sorting) method for simultaneous quantification of viral RNA expression in infected cells. The procedure has proved to be repeatable and reproducible to an acceptable level, although to ensure reproducibility, the repetition of standard curves is inevitable. Regarding its use for viral quantification, a direct relationship (by a second-degree polynomial regression) between viral titres and Molecules of Equivalent Soluble Fluorochrome (MESF) was observed. Finally, the results support the use of this technology, not only for virus quantification, but also to study viral replication from a quantitative approach.

Betanodavirus infection in primary neuron cultures from sole

Nervous necrosis virus (NNV), G. Betanodavirus, is the causative agent of viral encephalopathy and retinopathy, a disease that causes mass mortalities in a wide range of fish species. Betanodaviruses are neurotropic viruses and their replication in the susceptible fish species seems to be almost entirely restricted to nerve tissue. However, none of the cell lines used for NNV propagation has a nervous origin. In this study, first we established a protocol for the primary culture of neurons from Senegalese sole, which made it possible to further study virus-host cell interactions. Then, we compared the replication of three NNV strains with different genotypes (SJNNV, RGNNV and a RGNNV/SJNNV reassortant strain) in sole neuron primary cultures and E-11 cells. In addition, to study how two amino acid substitutions at the c-terminal of the capsid protein (positions 247 and 270) affect the binding to cell receptors, a recombinant strain was also tested. The results show that sole neural cells enabled replication of all the tested NNV strains. However, the recombinant strain shows a clearly delayed replication when compared with the wt strain. This delay was not observed in virus replicating in E-11 cells, suggesting a viral interaction with different cell receptors. The establishment of a sole primary neuronal culture protocol provides an important tool for research into betanodavirus infection in sole.

Identification of Litopenaeus vannamei BiP as a novel cellular attachment protein for white spot syndrome virus by using a biotinylation based affinity chromatography method

White spot syndrome virus (WSSV) is a dangerous threat to shrimp farming that also attacks a wide range of crustaceans. Knowledge of the surface protein-protein interactions between the pathogen and host is very crucial to unraveling the molecular pathogenesis mechanisms of WSSV. In this study, LvBiP (Litopenaeus vannamei immunoglobulin heavy-chain-binding protein) was identified as a novel WSSV binding protein of L. vannamei by a biotinylation based affinity chromatography method. By using pull-down and ELISA assays, the binding of recombinant LvBiP to WSSV was proved to be specific and ATP- dependent. The interaction was also confirmed by the result of co-immunoprecipitation assay. Immunofluorescence studies revealed the co-localization of LvBiP with WSSV on the cell surface of shrimp haemocytes. Additionally, LvBiP is likely to play an important role in WSSV infection. Treatment of gill cellular membrane proteins (CMPs) with purified rLvBiP and antibody that specifically recognizes LvBiP, led to a significant reduction in the binding of WSSV to gill CMPs. In the in vivo neutralization assay, rLvBiP and anti-LvBiP polyclonal antibody partially blocked the infection of WSSV. Taken together, the results indicate that LvBiP, a molecular chaperon of the HSP70 family, is a novel host factor involved at the step of attachment of the WSSV to the host cells and a potential candidate of therapeutic target.

Surface IgM λ light chain is involved in the binding and infection of infectious bursal disease virus (IBDV) to DT40 cells

Infectious bursal disease virus (IBDV) is an important immunosuppressive virus in chickens. Surface immunoglobulin M (sIgM)-bearing B lymphocytes act as the major targets of IBDV in the bursa of Fabricius, and sIgM may function as one of the membrane binding sites responsible for IBDV infection. Recently, using the virus overlay protein binding assay, the chicken λ light chain of sIgM was identified to specifically interact with IBDV in a virulence-independent manner in vitro. To further investigate sIgM λ light chain-mediated IBDV binding and infection in pre-B cells, the cell line DT40, which is susceptible to both pathogenic and attenuated IBDV, was used. Based on the RNA interference strategy, the DT40 cell line whose λ light chain of sIgM was stably knocked down, herein termed DT40LKD, was generated by the genomic integration of a specific small hairpin RNA and a green fluorescence protein co-expression construct. Flow cytometry analysis indicated that the binding of IBDV to DT40LKD cells was significantly reduced due to the loss of sIgM λ light chain. In particular, reduced viral replication was observed in IBDV-incubated DT40LKD cells, and no viral release into cell culture medium was detected by the IBDV rapid diagnostic strips. In addition, the rescue of sIgM λ light chain expression restored viral binding and replication in DT40LKD cells. These results show that sIgM λ light chain appears to be beneficial for IBDV attachment and infection, suggesting that sIgM acts as a binding site involved in IBDV infection.

Betanodavirus: Dissection of the viral life cycle

Progressive research has been recently made in dissecting the molecular biology of Betanodavirus life cycle, the causative pathogen of viral encephalopathy and retinopathy in economic important marine fish species. Establishment of betanodavirus infectious clone allows the manipulation of virus genome for functional genomic study, which elucidates the biological event of the viral life cycle at molecular level. The betanodavirus strategizes its replication by expressing anti-apoptosis/antinecrotic proteins to maintain the cell viability during early infection. Subsequently utilizes and controls the biological machinery of the infected cells for viral genome replication. Towards the late phase of infection, mass production of capsid protein for virion assembly induces the activation of host apoptosis pathway. It eventually leads to the cell lysis and death, which the lysis of cell contributes to the accomplishment of viral shedding that completes a viral life cycle. The recent efforts to dissect the entire betanodavirus life cycle are currently reviewed.

Characterization of microRNAs in orange-spotted grouper (Epinephelus coioides) fin cells upon red-spotted grouper nervous necrosis virus infection

Nervous necrosis virus (NNV), one of the most prevalent fish pathogens, has caused fatal disease of viral nervous necrosis (VNN) in many marine and freshwater fishes, and resulted in heavy economic losses in aquaculture industry worldwide. However, the molecular mechanisms underlying the pathogenicity of NNV remain elusive. In this study, the expression profiles of microRNA (miRNA) were investigated in grouper fin (GF-1) cells infected with red-spotted grouper nervous necrosis virus (RGNNV) via deep sequencing technique. The results showed that a total of 220 miRNAs were identified by aligning the small RNA sequences with the miRNA database of zebrafish, and 18 novel miRNAs were predicted using miRDeep2 software. Compared with the non-infected groups, 51 and 16 differentially expressed miRNAs (DE-miRNAs) were identified in the samples infected with RGNNV at 3 and 24 h, respectively. Six DE-miRNAs were randomly selected to validate their expressions using quantitative reverse transcription polymerase chain reaction (qRT-PCR), the results showed that their expression profiles were consistent with those obtained by deep sequencing. The target genes of the DE-miRNAs covered a wide range of functions, such as regulation of transcription, oxidation-reduction process, proteolysis, regulation of apoptotic process, and immune response. In addition, the effects of four DE-miRNAs including miR-1, miR-30b, miR-150, and miR-184 on RGNNV replication were evaluated, and the results showed that over-expression of each of the four miRNAs promoted the replication of RGNNV. These data provide insight into the molecular mechanism of RGNNV infection, and will benefit for the development of effective strategies to control RGNNV infection.

Betanodavirus-like particles enter host cells via clathrin-mediated endocytosis in a cholesterol-, pH- and cytoskeleton-dependent manner

Betanodavirus, also referred to nervous necrosis virus (NNV), is the causative agent of the fatal disease, viral nervous necrosis and has brought significant economic losses in marine and freshwater cultured fish, especially larvae and juveniles. Here, we used an established invasion model with virus-like particle (VLP)-cells, mimicking orange-spotted grouper nervous necrosis virus (OGNNV), to investigate the crucial events of virus entry. VLP were observed in the perinuclear regions of Asian sea bass (SB) cells within 1.5 h after attachment. VLP uptake was strongly inhibited when cells were pretreated with biochemical inhibitors (chlorpromazine and dynasore) blocking clathrin-mediated endocytosis (CME) or transfected with siRNA against clathrin heavy and light chains. Inhibitors against key regulators of caveolae/raft-dependent endocytosis and macropinocytosis had no effect on VLP uptake. In contrast, disruption of cellular cholesterol by methyl-β-cyclodextrin or reduction of cholesterol fluidity by Cholera toxin B subunit significantly decreased VLP entry. Furthermore, VLP entry is dependent on low pH and cytoskeleton, demonstrated by inhibitor (chloroquine, ammonia chloride, cytochalasin D, wiskostatin, and nocodazole) perturbation. Therefore, OGNNV VLP enter SB cells via CME depending on dynamin-2, cholesterol and its fluidity, low pH, and cytoskeleton. In addition, ten more cell lines were screened for VLP entry and VLP can only enter NNV-sensitive cells, GB and SSN-1, via CME, indicating that CME is the common endocytosis pathway for VLP. These results may provide the data for NNV entry without the influence of the viral genome, an ideal model for exploring the behaviour of betanodavirus in cells, and valuable references to vaccine development.

Evidence that Hsc70 Is Associated with Cucumber Necrosis Virus Particles and Plays a Role in Particle Disassembly

Uncoating of a virus particle to expose its nucleic acid is a critical aspect of the viral multiplication cycle, as it is essential for the establishment of infection. In the present study, we investigated the role of plant HSP70 homologs in the uncoating process of Cucumber necrosis virus (CNV), a nonenveloped positive-sense single-stranded RNA [(+)ssRNA] virus having a T=3 icosahedral capsid. We have found through Western blot analysis and mass spectrometry that the HSP70 homolog Hsc70-2 copurifies with CNV particles. Virus overlay and immunogold labeling assays suggest that Hsc70-2 is physically bound to virions. Furthermore, trypsin digestion profiles suggest that the bound Hsc70-2 is partially protected by the virus, indicating an intimate association with particles. In investigating a possible role of Hsc70-2 in particle disassembly, we showed that particles incubated with Hsp70/Hsc70 antibody produce fewer local lesions than those incubated with prebleed control antibody on Chenopodium quinoa In conjunction, CNV virions purified using CsCl and having undetectable amounts of Hsc70-2 produce fewer local lesions. We also have found that plants with elevated levels of HSP70/Hsc70 produce higher numbers of local lesions following CNV inoculation. Finally, incubation of recombinant Nicotiana benthamiana Hsc70-2 with virus particles in vitro leads to conformational changes or partial disassembly of capsids as determined by transmission electron microscopy, and particles are more sensitive to chymotrypsin digestion. This is the first report suggesting that a cellular Hsc70 chaperone is involved in disassembly of a plant virus.

IMPORTANCE

Virus particles must disassemble and release their nucleic acid in order to establish infection in a cell. Despite the importance of disassembly in the ability of a virus to infect its host, little is known about this process, especially in the case of nonenveloped spherical RNA viruses. Previous work has shown that host HSP70 homologs play multiple roles in the CNV infection cycle. We therefore examined the potential role of these cellular components in the CNV disassembly process. We show that the HSP70 family member Hsc70-2 is physically associated with CNV virions and that HSP70 antibody reduces the ability of CNV to establish infection. Statistically significantly fewer lesions are produced when virions having undetectable HSc70-2 are used as an inoculum. Finally incubation of Hsc70-2 with CNV particles results in conformational changes in particles. Taken together, our data point to an important role of the host factor Hsc70-2 in CNV disassembly.

Nervous necrosis virus capsid protein exploits nucleolar phosphoprotein Nucleophosmin (B23) function for viral replication

Nucleolar proteins facilitate the replication of certain human and animal viruses through interaction with viral proteins. In this study, an interaction between nervous necrosis virus capsid protein and nucleolar phosphoprotein B23 was identified using in vitro experimental approaches. The capsid protein binds to B23 early during the viral infection and accumulates in the nucleus, particularly in the nucleolus. However, over the course of the infection B23 is redistributed from the nucleoli to the nucleoplasm. siRNA-mediated knockdown of B23 reduced viral replication and cytopathic effect. Thus, B23 targets capsid protein to the nucleus and facilitates NNV replication. The results provide the first demonstration that nucleolar protein B23 has a direct role in the nodavirus replication process.

Fish TRIM8 exerts antiviral roles through regulation of the proinflammatory factors and interferon signaling

The tripartite motif (TRIM)-containing proteins usually exert important regulatory roles during multiple biological processes. TRIM8 has been demonstrated to be a RING domain-containing E3 ubiquitin ligase which plays critical roles in inflammation and cancer. In this study, a TRIM8 homolog from grouper, Epinephelus coioides (EcTRIM8) was cloned, and its effects on fish virus replication were investigated. The full-length EcTRIM8 cDNA encoded a polypeptide of 568 amino acids with 92% identity to TRIM8 homolog from large yellow croaker (Larimichthys crocea). Sequence alignment analysis indicated that EcTRIM8 contained conserved RING finger, B-box and coiled-coil domain. Expression patterns analysis showed that EcTRIM8 was predominant in kidney, gill, fin, liver, spleen and brain. After challenging with Singapore grouper iridovirus (SGIV) or polyinosin-polycytidylic acid (poly I:C), the EcTRIM8 transcript was significantly increased at the early stage of injection. Under fluorescence microscopy, we observed different distribution patterns of EcTRIM8 in grouper spleen (GS) cells, including punctate fluorescence evenly situated throughout the cytoplasm and bright aggregates. The ectopic expression of EcTRIM8 in vitro significantly inhibited the replication of SGIV and red spotted grouper nervous necrosis virus (RGNNV), evidenced by the obvious reduction in the severity of cytopathic effect (CPE) and the significant decrease in viral gene transcription and protein synthesis. Moreover, the transcription of the proinflammatory factors and interferon related immune factors were differently regulated by EcTRIM8 during SGIV or RGNNV infection. In addition, overexpression of EcTRIM8 significantly increased the transcription of interferon regulator factor 3 (IRF3) and IRF7, and enhanced IRF3 or IRF7 induced interferon-stimulated response element (ISRE) promoter activity. Together, our results firstly demonstrated that fish TRIM8 could exert antiviral function through the regulation of the expression of proinflammatory cytokines and interferon related transcription factors in response to fish viruses.

Fish TRIM39 regulates cell cycle progression and exerts its antiviral function against iridovirus and nodavirus

The tripartite motif (TRIM)-containing proteins exert important immune regulatory roles through regulating different signaling pathways in response to different stimuli. TRIM39, a member of the TRIM family, is a RING domain-containing E3 ubiquitin ligase which could regulate cell cycle progression and apoptosis. However, the antiviral activity of TRIM39 is not explored. Here, a TRIM39 homolog from grouper, Epinephelus coioides (EcTRIM39) was cloned, and its effects on cell cycle progression and fish virus replication were investigated. The full-length EcTRIM39 cDNA was composed of 2535 bp and encoded a polypeptide of 543 amino acids with 70% identity with TRIM39 homologs from bicolor damselfish. Amino acid alignment analysis indicated that EcTRIM39 contained a RING finger, B-box and SPRY domain. Expression profile analysis revealed that EcTRIM39 was abundant in intestine, spleen and skin. Upon different stimuli in vivo, the EcTRIM39 transcript was obviously up-regulated after challenging with Singapore grouper iridovirus (SGIV), and polyinosinic-polycytidylic acid (poly I:C). Using fluorescence microscopy, we found that EcTRIM39 localized in the cytoplasm and formed aggregates in grouper spleen (GS) cells. The ectopic expression of EcTRIM39 in vitro affected the cell cycle progression via mediating G1/S transition. Moreover, the RING domain was essential for its accurate localization and effect on cell cycle. In addition, overexpression of EcTRIM39 significantly inhibited viral gene transcription of SGIV and red-spotted grouper nervous necrosis virus (RGNNV) in vitro, and the mutant of RING exerted the opposite effect. Together, our results demonstrated that fish TRIM39 not only regulated the cell cycle progression, but also acted as an important regulator of fish innate immune response against viruses.

Laminin receptor is an interacting partner for viral outer capsid protein VP5 in grass carp reovirus infection

Grass carp reovirus (GCRV) is responsible for viral hemorrhagic disease in cultured grass carp Ctenopharyngon idellus. Through yeast two-hybrid screen, laminin receptor (LamR) was identified as a potential interacting partner for the outer capsid protein VP5 of GCRV. We cloned and sequenced the gene encoding grass carp LamR. Viral attachment assay demonstrated the involvement of membrane-associated LamR in GCRV infection. Solid-phase overlay assays demonstrated that GCRV interacted with GST-tagged LamR in vitro. In contrast to VP7, GST-tagged VP5 was shown to associate with LamR in both pull-down and solid-phase blot overlay assays. With the reduction of LamR expression in CIK cells achieved by RNAi, remarkably reduced infection efficiency of GCRV was observed. CIK cells pretreated with polyclonal antibody against LamR resulted in dose-dependent inhibition of GCRV infection. These results collectively indicated that grass carp LamR was involved in GCRV infection by interacting with viral outer capsid protein VP5.

Grouper voltage-dependent anion selective channel protein 2 is required for nervous necrosis virus infection

Nervous necrosis virus (NNV) is a non-enveloped virus with 2 segmented positive-sense single-stranded RNAs. NNV-induced mass mortality has occurred worldwide in many species of cultured marine fish and resulted in substantial economic losses. In our previous study, we cloned the gene of voltage-dependent anion selective channel protein 2 (GVDAC2), and the NNV RNA2 expression level decreased in GVDAC2-knockdown GF-1 cells 24 h after infection. Here, we investigated the role of GVDAC2 in the NNV infection in GF-1 cells. NNV infection did not considerably affect GVDAC2 gene expression. After performing immunostaining, we detected GVDAC2 at the mitochondria and GVDAC2 was colocalized with NNV-RNA-dependent RNA polymerase. However, these 2 proteins did not interact with each other in immunoprecipitation assay. The cellular ATP level in GVDAC2-downregulated cells was lower than that in control cells, and NNV-induced apoptosis was delayed in GVDAC2-siRNA-transfected cells. Therefore, we suggest that GVDAC2 is required for NNV infection for maintaining the cellular ATP level and has positive impact on virus-induced apoptosis.