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

Nectin1 is a pivotal host factor involved in attachment and entry of red-spotted grouper nervous necrosis virus in the early stages of the viral life cycle

Nervous necrosis virus (NNV) is a highly neurotropic virus that poses a persistent threat to the survival of multiple fish species. However, its inimitable neuropathogenesis remains largely elusive. To rummage potential partners germane to the nervous system, we investigated the interaction between red-spotted grouper NNV (RGNNV) and grouper brain by immunoprecipitation coupled with mass spectrometry and discerned Nectin1 as a novel host factor subtly involved in viral early invasion events. Nectin1 was abundant in neural tissues and implicated in the inception of tunnel nanotubes triggered by RGNNV. Its overexpression not only dramatically potentiated the replication dynamics of RGNNV in susceptible cells, but also empowered non-sensitive cells to expeditiously capture free virions within 2 min. This potency was impervious to low temperatures but was dose-dependently suppressed by soluble protein or specific antibody of Nectin1 ectodomain, indicating Nectin1 as an attachment receptor for RGNNV. Mechanistically, efficient hijacking of virions by Nectin1 strictly depended on intricate linkages to different modules of viral capsid protein, especially the direct binding between the IgC1 loop and P-domain. More strikingly, despite abortive proliferation in Nectin1-reconstructed CHSE-214 cells, a non-sensitive cell, RGNNV could gain access to the intracellular compartment by capitalizing on Nectin1, thereby inducing canonical cytoplasmic vacuolation. Altogether, our findings delineate a candidate entrance for RGNNV infiltration into the nervous system, which may shed unprecedented insights into the exploration and elucidation of RGNNV pathogenesis.IMPORTANCENervous necrosis virus (NNV) is one of the most virulent pathogens in the aquaculture industry, which inflicts catastrophic damage to ecology, environment, and economy annually around the world. Nevertheless, its idiosyncratic invasion and latency mechanisms pose enormous hardships to epidemic prevention and control. In this study, deploying grouper brain as a natural screening library, a single-transmembrane glycoprotein, Nectin1, was first identified as an emergent functional receptor for red-spotted grouper NNV (RGNNV) that widely allocated in nervous tissues and directly interacted with viral capsid protein through distinct Ig-like loops to bridge virus-host crosstalk, apprehend free virions, and concomitantly propel viral entry. Our findings illuminate the critical role of Nectin1 in RGNNV attachment and entry and provide a potential target for future clinical intervention strategies in the therapeutic race against RGNNV.

Molecular Mechanism of pH-Induced Protrusion Configuration Switching in Piscine Betanodavirus Implies a Novel Antiviral Strategy

Many viruses contain surface spikes or protrusions that are essential for virus entry. These surface structures can thereby be targeted by antiviral drugs to treat viral infections. Nervous necrosis virus (NNV), a simple nonenveloped virus in the genus of betanodavirus, infects fish and damages aquaculture worldwide. NNV has 60 conspicuous surface protrusions, each comprising three protrusion domains (P-domain) of its capsid protein. NNV uses protrusions to bind to common receptors of sialic acids on the host cell surface to initiate its entry via the endocytic pathway. However, structural alterations of NNV in response to acidic conditions encountered during this pathway remain unknown, while detailed interactions of protrusions with receptors are unclear. Here, we used cryo-EM to discover that Grouper NNV protrusions undergo low-pH-induced compaction and resting. NMR and molecular dynamics (MD) simulations were employed to probe the atomic details. A solution structure of the P-domain at pH 7.0 revealed a long flexible loop (amino acids 311-330) and a pocket outlined by this loop. Molecular docking analysis showed that the N-terminal moiety of sialic acid inserted into this pocket to interact with conserved residues inside. MD simulations demonstrated that part of this loop converted to a β-strand under acidic conditions, allowing for P-domain trimerization and compaction. Additionally, a low-pH-favored conformation is attained for the linker connecting the P-domain to the NNV shell, conferring resting protrusions. Our findings uncover novel pH-dependent conformational switching mechanisms underlying NNV protrusion dynamics potentially utilized for facilitating NNV entry, providing new structural insights into complex NNV-host interactions with the identification of putative druggable hotspots on the protrusion.

Japanese medaka (Oryzias latipes) Nectin4 plays an important role against red spotted grouper nervous necrosis virus infection

Nectins are adhesion molecules that play a crucial role in the organization of epithelial and endothelial junctions and function as receptors for the entry of herpes simplex virus. However, the role of Nectin4 remains poorly understood in fish. In this study, nectin4 gene was cloned from medaka (OlNectin4). OlNectin4 was located on chromosome 18 and contained 11 exons, with a total genome length of 25754 bp, coding sequences of 1689 bp, coding 562 amino acids and a molecular weight of 65.5 kDa. OlNectin4 contained four regions, including an Immunoglobulin region, an Immunoglobulin C-2 Type region, a Transmembrane region and a Coiled coil region. OlNectin4 shared 47.18 % and 25.00 % identity to Paralichthys olivaceus and Mus musculus, respectively. In adult medaka, the transcript of nectin4 was predominantly detected in gill. During red spotted grouper nervous necrosis virus (RGNNV) infection, overexpression of OlNectin4 in GE cells significantly increased viral gene transcriptions. Meanwhile, Two mutants named OlNectin4△4 (+4 bp) and OlNectin4△7 (-7 bp) medaka were established using CRISPR-Cas9 system. Nectin4-KO medaka had higher mortality than WT after infected with RGNNV. Moreover, the expression of RGNNV RNA2 gene in different tissues of the Nectin4-KO were higher than WT medaka after challenged with RGNNV. The brain and eye of Nectin4-KO medaka which RGNNV mainly enriched, exhibited significantly higher expression of interferon signaling genes than in WT. Taken together, the OlNectin4 plays a complex role against RGNNV infection by inducing interferon responses for viral clearance.

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.

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.

New insights into β-glucan-enhanced immunity in largemouth bass Micropterus salmoides by transcriptome and intestinal microbial composition

β-glucan is widely used in aquaculture due to its immunostimulatory effects, but the specific effect and potential regulatory mechanism on largemouth bass (Micropterus salmoides) are still unclear. Here, we evaluated the effects of β-glucan on growth, resistance to Aeromonas schubertii, intestinal health, and transcriptome of largemouth bass to reveal the potential regulators, metabolic pathways, and altered differential microbiota. Four experimental diets were designed with β-glucan supplementation levels of 0 (control), 100 (LA-100), 200 (MA-200), and 300 (HA-300) mg kg^-1, and each diet was fed to largemouth bass (79.30 ± 0.50 g) in triplicate for 70 days, followed by a 3-day challenge experiment. Results showed that different β-glucan supplementations had no significant effects on growth performance and whole-body composition. Fish fed a diet with 300 mg kg^-1 β-glucan significantly increased the activity of lysozyme than those fed diets with 0 and 100 mg kg^-1 β-glucan. In addition, the survival rate of largemouth bass in β-glucan supplementation groups was significantly higher than the control group at 12- and 24-h challenge by Aeromonas schubertii. Transcriptome analysis showed that a total of 1,245 genes were differentially expressed [|log₂(fold change)| ≥1, q-value ≤0.05], including 109 immune-related differentially expressed genes (DEGs). Further analysis revealed that significantly upregulated and downregulated DEGs associated with immunity were mapped into 12 and 24 pathways, respectively. Results of intestinal microflora indicated that fish fed a diet with 300 mg kg^-1 β-glucan had higher bacterial richness and diversity as evaluated by Sobs, Chao, Ace, and Simpson indices, but no significant differences were found in the comparison groups. Furthermore, 300 mg kg^-1 β-glucan significantly increased the relative abundance of Mycoplasma and decreased Proteobacteria (mainly Escherichia-Shigella and Escherichia coli) and Bacillus anthracis in largemouth bass intestinal microflora. The findings of this study provided new insights that will be valuable in future studies to elucidate the mechanism of immunity enhancement by β-glucan.

Comprehensive Relationship Analysis of the Long Noncoding RNAs (lncRNAs) and the Target mRNAs in Response to the Infection of Edwardsiella anguillarum in European eel (Anguilla anguilla) Inoculated with Freund's Adjuvant

Freund's complete adjuvant (FCA) and incomplete adjuvant (FIA), generally applied in subunit fishery vaccine, have not been explored on the molecular mechanism of the non-specific immune enhancement. As long noncoding RNAs (lncRNAs) play vital regulating roles in various biological activities, in this study, we examined the genome-wide expression of transcripts in the liver of European eel (Anguilla anguilla, Aa) inoculated with FCA and FIA (FCIA) to elucidate the regulators of lncRNAs in the process of Edwardsiella anguillarum (Ea) infection and Aa anti-Ea infection using strand-specific RNA-seq. After eels were challenged by Ea at 28 days post the first inoculation (dpi), compared to the control uninfected eels (Li group), the control infected eels (Con_Li group) showed severe bleeding, hepatocyte atrophy, and thrombi formed in the hepatic vessels of the liver, although eels inoculated with FCIA (FCIA_Li group) also formed slight thrombi in the hepatic vessels. Compared to the FCIA_Li group, there was about 10 times colony-forming unit (cfu) in the Con_Li group per 100 μg liver tissue, and the relative percent survival (RPS) of eels was 50% in FCIA_Li vs Con_Li. Using high-throughput transcriptomics, differential expressed genes (DEGs) and transcripts were identified and the results were verified using fluorescence real-time polymerase chain reaction (qRT-PCR). Interactions between the differential expressed lncRNAs (DE-lncRNAs) and the target DEGs were explored using Cytoscape according to their co-expression and co-location relationship. We found 13,499 lncRNAs (10,176 annotated and 3423 novel lncRNAs) between 3 comparisons of Con_Li vs Li, FCIA_Li vs Li, and FCIA_Li vs Con_Li, of which 111, 110, and 129 DE-lncRNAs were ascertained. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis of DEGs targeted by DE-lncRNAs revealed these DEGs mainly involved in single-organism cellular process in BP, membrane in CC and binding in MF, and KEGG pathways showed that the target DEGs in co-expression and co-location enriched in cell adhesion molecules. Finally, 118 DE-lncRNAs target 1161 DEGs were involved in an interaction network of 8474 co-expression and 333 co-location-related links, of which 16 DE-lncRNAs play vital roles in anti-Ea infection. Taken together, the interaction networks revealed that DE-lncRNAs underlies the process of Ea infection and Aa anti-Ea infection.

NLRC3 attenuates antiviral immunity and activates inflammasome responses in primary grouper brain cells following nervous necrosis virus infection

NLRC3 is identified as a unique regulatory NLR involved in the modulation of cellular processes and inflammatory responses. In this study, a novel Nod like receptor C3 (NLRC3) was functionally characterized from seven band grouper in the context of nervous necrosis virus infection. The grouper NLRC3 is highly conserved and homologous with other vertebrate proteins with a NACHT domain and a C-terminal leucine-rich repeat (LRR) domain and an N-terminal CARD domain. Quantitative gene expression analysis revealed the highest mRNA levels of NLRC3 were in the brain and gill followed by the spleen and kidney following NNV infection. Overexpression of NLRC3 augmented the NNV replication kinetics in primary grouper brain cells. NLRC3 attenuated the interferon responses in the cells following NNV infection by impacting the TRAF6/NF-κB activity and exhibited reduced IFN sensitivity, ISRE promoter activity, and IFN pathway gene expression. In contrast, NLRC3 expression positively regulated the inflammasome response and pro-inflammatory gene expression during NNV infection. NLRC3 negatively regulates the PI3K-mTOR axis and activated the cellular autophagic response. Delineating the complexity of NLRC3 regulation of immune response in the primary grouper brain cells following NNV infection suggests that the protein acts as a virally manipulated host factor that negatively regulated the antiviral immune response to augment the NNV replication.

Nectin4 antagonises type I interferon production by targeting TRAF3 for autophagic degradation and disrupting TRAF3-TBK1 complex formation

Autophagy, a conserved cellular degradative process, plays a crucial role in innate immunity during viral infections. Nervous necrosis virus (NNV), a leading cause of fish diseases with morbidity and mortality, triggers cell autophagy to promote viral replication; however, the details of how NNV utilises autophagy to facilitate its own replication remain largely unexplored. Here, we investigated the mechanism by which the sea perch Nectin4 (LjNectin4), a receptor of NNV, regulates autophagy and the innate immune system by targeting TNFR-associated factor 3 (TRAF3). Our data demonstrated that LjNectin4 directly binds to the NNV capsid protein and facilitates NNV entry, indicating that LjNectin4 functions as an NNV receptor. Moreover, LjNectin4 promoted NNV replication by inhibiting key elements of the RLR signalling pathway (MDA5, MAVS, TRAF3, TBK1, and IRF3)-induced IFN response. Mechanistically, LjNectin4 directly interacted with TRAF3 and promoted its autophagy-mediated lysosomal degradation. Domain mapping of the interaction between TRAF3 and LjNectin4 or TBK1 showed that both LjNectin4 and TBK1 interacted with the ZF2 and TRAF-C domains of TRAF3, suggesting that LjNectin4 blocked TRAF3-TBK1 complex formation. Collectively, our study revealed that NNV utilises LjNectin4 to suppress IFN production by mediating TRAF3 autophagic degradation and disrupting the TRAF3-TBK1 complex, thereby promoting NNV replication.

Beta glucan induced immune priming protects against nervous necrosis virus infection in sevenband grouper

In the present study, we studied the effect of β-glucan on the activation of antiviral immune responses against nervous necrosis virus (NNV) taking into consideration the role of innate immune training. Sevenband grouper primary macrophages showed an attenuated proinflammatory response and elevated antiviral response to NNV infection. In vitro, priming of β-glucan enhanced macrophage viability against NNV infection which is associated with the activation of sustained inflammatory cytokines gene expression. Observations were clear to understand that NLR Family CARD Domain Containing 3 (NLRC3) and caspase-1 activation and subsequent IL-1β production were reduced in β-glucan-primed macrophages. Subsequent markers for training including Lactate and abundance of HIF-1α were elevated in the cells following training. However, the lactate dehydrogenase (LDH) concentrations remained stable among the β-glucan stimulated infected and uninfected groups suggesting similar macrophage health in both groups. In vivo, the NNV-infected fish primed with β-glucan had a higher survival rate (60%) than the control NNV-infected group (40%). Our findings demonstrate that β-glucan induced protective responses against NNV infection and studies are underway to harness its potential applicability for prime and boost vaccination strategies.

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.

Transcriptome RNA-seq revealed lncRNAs activated by Edwardsiella anguillarum post the immunization of OmpA protecting European eel (Anguilla anguilla) from being infected

Long noncoding RNAs (lncRNAs) play important roles in various biological activities as vital regulators. However, no study has focused on the lncRNA regulation of Outer membrane protein (OMP) immunization against aquatic bacterial infection. In this study, we examined the genome-wide expression of lncRNAs in the liver of European eel (Anguilla anguilla, Aa) administrated by a recombinant OmpA (rOmpA) from Edwardsiella anguillarum (Ea) to elucidate the functions of lncRNAs in the process of Ea infection and Aa anti-Ea infection using strand specific RNA-seq. Eels were challenged by Ea at 28 d post the immunization (dpi) of OmpA, and the result showed, compared to uninfected livers in the PBS group (Con group), the infected livers in the PBS group (Con_inf group) showed severe bleeding, hepatocyte atrophy and thrombi formed in the hepatic vessels; livers in the OmpA group (OmpA_inf) also formed slight thrombi in the hepatic vessels. The relative percent survival of eels in OmpA_inf vs Con_inf was 78.6%. Using high-throughput transcriptomics, we found 13405 lncRNAs in 3 compares of Con_inf vs Con, OmpA_inf vs Con and OmpA_inf vs Con_inf, of which 111, 129 and 158 DE-lncRNAs were ascertained. GO analysis of the DE-lncRNAs revealed the targeting DEGs were mainly involved in single-organism process, signaling, biological process and response to stimulus in BP, component of membrane in CC and binding in MF; KEGG pathways showed that the targeting DEGs in co-expression and co-location enriched in cell adhesion molecules. Finally, 54 DE-lncRNAs targeting 1675 DEGs were involved in an interaction network of 21692 co-expression and 483 co-location related links, of which 18 DE-lncRNAs appear to play crucial roles in anti-Ea infection. Thus, the interaction networks revealed crucial DE-lncRNAs underlying the process of Ea infection and Aa anti-Ea infection pre and post the immunization of OmpA.

Proteasome subunit beta type-8 from sevenband grouper negatively regulates cytokine responses by interfering NF-κB signaling upon nervous necrosis viral infection

During viral infection, proper regulation of immune signaling is essential to ensure successful clearance of virus. Immunoproteasome is constitutively expressed and gets induced during viral infection by interferon signaling and contributes to regulate proinflammatory cytokine production and activation of the NF-κB pathway. In this study, we identified Hs-PSMB8, a member of the proteasome β-subunits (PSMB) family, as a negative regulator of NF-κB responses during NNV infection. The transient expression of Hs-PSMB8 delayed the appearance of cytopathic effect (CPE) and showed a higher viral load. The Hs-PSMB8 interacted with NNV which was confirmed using immunocolocalization and co-IP. Overexpression of Hs-PSMB8 diminished virus induced activation of the NF-κB promoters and downregulated the activation of IL-1β, TNFα, IL6, IL8, IFNγ expression upon NNV infection. Collectively, our results demonstrate that PSMB8 is an important regulator of NF-κB signaling during NNV infection in sevenband grouper.

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.

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.

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.