Visualization of Assembly Intermediates and Budding Vacuoles of Singapore Grouper Iridovirus in Grouper Embryonic Cells

Decapod iridescent virus 1 (DIV1) 168L can target cuticle protein 8 from Litopenaeus vannamei

Decapod iridescent virus 1 (DIV1) stands as a significant pathogen affecting crustaceans, posing a grave threat to the shrimp industries in aquaculture dependent nations. Within the Iridoviridae family, the conserved envelope protein DIV1-168L plays a pivotal role in virion entry. Nonetheless, the host factors that interact with 168L remain unidentified. To address this gap, we established a cDNA library derived from Litopenaeus vannamei gill tissue and conducted yeast two-hybrid screening to identify host factors that interact with 168L. Additionally, we performed co-immunoprecipitation assays to verify the interaction between cuticle protein 8 (CP8) and 168L. Expression pattern analysis revealed the presence of CP8 transcripts in the gill and epidermis. Furthermore, immunohistochemistry results demonstrated the expression of CP8 in gill cells and its localization in the gill filament epithelium. Fluorescence analysis indicated that full-length CP8 colocalized with 168L in the cytoplasm of Sf9 cells. Removal of the signal peptide from the N-terminal of CP8 eliminated its concentration in the cytoplasm. Additionally, CP8 expression was significantly inhibited during DIV1 infection. Therefore, our research contributes to a better understanding of the entry mechanism of iridovirids. The GenBank accession number for the DIV1 sequence is MF197913.1.

A Collagen10a1 mutation disrupts cell polarity in a medaka model for metaphyseal chondrodysplasia type Schmid

Heterozygous mutations in COL10A1 lead to metaphyseal chondrodysplasia type Schmid (MCDS), a skeletal disorder characterized by epiphyseal abnormalities. Prior analysis revealed impaired trimerization and intracellular retention of mutant collagen type X alpha 1 chains as cause for elevated endoplasmic reticulum (ER) stress. However, how ER stress translates into structural defects remained unclear. We generated a medaka (Oryzias latipes) MCDS model harboring a 5 base pair deletion in col10a1, which led to a frameshift and disruption of 11 amino acids in the conserved trimerization domain. col10a1Δ633a heterozygotes recapitulated key features of MCDS and revealed early cell polarity defects as cause for dysregulated matrix secretion and deformed skeletal structures. Carbamazepine, an ER stress-reducing drug, rescued this polarity impairment and alleviated skeletal defects in col10a1Δ633a heterozygotes. Our data imply cell polarity dysregulation as a potential contributor to MCDS and suggest the col10a1Δ633a medaka mutant as an attractive MCDS animal model for drug screening.

Near-atomic architecture of Singapore grouper iridovirus and implications for giant virus assembly

Singapore grouper iridovirus (SGIV), one of the nucleocytoviricota viruses (NCVs), is a highly pathogenic iridovirid. SGIV infection results in massive economic losses to the aquaculture industry and significantly threatens global biodiversity. In recent years, high morbidity and mortality in aquatic animals have been caused by iridovirid infections worldwide. Effective control and prevention strategies are urgently needed. Here, we present a near-atomic architecture of the SGIV capsid and identify eight types of capsid proteins. The viral inner membrane-integrated anchor protein colocalizes with the endoplasmic reticulum (ER), supporting the hypothesis that the biogenesis of the inner membrane is associated with the ER. Additionally, immunofluorescence assays indicate minor capsid proteins (mCPs) could form various building blocks with major capsid proteins (MCPs) before the formation of a viral factory (VF). These results expand our understanding of the capsid assembly of NCVs and provide more targets for vaccine and drug design to fight iridovirid infections.

Characterization of scavenger receptor MARCO in orange-spotted grouper, Epinephelus coioides

Macrophage receptor with collagenous structure (MARCO) is a scavenger receptor that plays a crucial role in the immune response against microbial infections. To clarify the roles of fish MARCO in Singapore grouper iridovirus (SGIV) infection, we identified and characterized Ec-MARCO in the orange-spotted grouper (Epinephelus coioides). The Ec-MARCO encoded a 370-amino acid protein with transmembrane region, coiled coil region and SR domain, which shared high identities with reported MARCO. The abundant transcriptional level of Ec-MARCO was found in spleen, head kidney and blood. And the Ec-MARCO expression was significantly up-regulated in grouper spleen (GS) cells after infection with SGIV in vitro. Subcellular localization analysis revealed that Ec-MARCO was mainly distributed in the cytoplasm and on the cell membrane. Ec-MARCO knockdown in vitro significantly inhibited SGIV infection in GS cells, as evidenced by reduced decreased SGIV major capsid protein (MCP) transcription and MCP protein expression. Further studies showed that Ec-MARCO knockdown positively regulated proinflammatory cytokines and interferon-stimulated genes, and enhanced IFN and ISRE promoter activities. However, overexpression of Ec-MARCO did not affect SGIV entry into host cells. In summary, our results suggested that Ec-MARCO affected SGIV infection by regulating antiviral innate immune response.

Singapore Grouper Iridovirus Disturbed Glycerophospholipids Homeostasis: Cytosolic Phospholipase A2 Was Essential for Virus Replication

Singapore grouper iridovirus (SGIV), belonging to genus Ranavirus, family Iridoviridae, causes great economic losses in the aquaculture industry. Previous studies demonstrated the lipid composition of intracellular unenveloped viruses, but the changes in host-cell glyceophospholipids components and the roles of key enzymes during SGIV infection still remain largely unknown. Here, the whole cell lipidomic profiling during SGIV infection was analyzed using UPLC-Q-TOF-MS/MS. The lipidomic data showed that glycerophospholipids (GPs), including phosphatidylcholine (PC), phosphatidylserine (PS), glycerophosphoinositols (PI) and fatty acids (FAs) were significantly elevated in SGIV-infected cells, indicating that SGIV infection disturbed GPs homeostasis, and then affected the metabolism of FAs, especially arachidonic acid (AA). The roles of key enzymes, such as cytosolic phospholipase A2 (cPLA2), 5-Lipoxygenase (5-LOX), and cyclooxygenase (COX) in SGIV infection were further investigated using the corresponding specific inhibitors. The inhibition of cPLA2 by AACOCF3 decreased SGIV replication, suggesting that cPLA2 might play important roles in the process of SGIV infection. Consistent with this result, the ectopic expression of EccPLA2α or knockdown significantly enhanced or suppressed viral replication in vitro, respectively. In addition, the inhibition of both 5-LOX and COX significantly suppressed SGIV replication, indicating that AA metabolism was essential for SGIV infection. Taken together, our results demonstrated for the first time that SGIV infection in vitro disturbed GPs homeostasis and cPLA2 exerted crucial roles in SGIV replication.

Epitope screening of the major capsid protein within grouper iridovirus of Taiwan and the immunoprotective effect with SWCNTs as the vaccine carrier

Iridovirus can cause a mass of death in grouper, leading to huge economic loss in recent years. At present, practical vaccine is still the best way to control the outbreak of this virus. Many researches had indicated that the major capsid protein (MCP) of grouper iridovirus of Taiwan (TGIV) is an effective antigen to induce a specific immune response in grouper. However, these traditional vaccines that based on large proteins or whole organisms are faced with challenges because of the unnecessary antigenic load. Thus, in this study, we screened the dominant linear epitope within the MCP of TGIV and then, a new peptide vaccine (P2) was developed via prokaryotic expression system. Furthermore, SWCNTs was used as a vaccine carrier to enhance the immunoprotective effect. To evaluate the immunoprotective effect of this vaccine, a total of 245 fish were vaccinated with P2 (5, 10, 20 mg L^-1) and SWCNTs-P2 (5, 10, 20 mg L^-1) via immersion before being challenged with live TGIV at 28 days post immunization (d.p.i.). Results showed that the serum antibody titer, enzymatic activity, expression level of some immune-related genes (CC chemokine, IgM and TNF-α) and survival rate were significantly increased (SWCNTs-P2, 20 mg L^-1, 100%) compared to the control group (0%). These results indicated that this peptide vaccine could effectively induce specific immune response in vaccinated groupers. Functionalized SWCNTs could serve as a carrier of the peptide vaccine to enhance the immunoprotective effect via immersion. To sum up, epitope screening might be a potential way to develop an effective vaccine nowadays, and SWCNTs might provide a practical method that can be used in large-scale vaccination, especially for juvenile fish, to fight against diseases in aquaculture industry.

The interactome of Singapore grouper iridovirus protein ICP18 as revealed by proximity-dependent BioID approach

Singapore grouper iridovirus (SGIV) is a large double-stranded DNA virus that is a major threat to grouper aquaculture. The pathogenesis of SGIV is not well understood so far. Previous studies have revealed that ICP18, an immediate early protein encoded by SGIV ORF086R gene, promotes viral replication by regulating cell proliferation and virus assembly. In the present study, the potential functions of ICP18 were further explored by probing into its interactors using a proximity-dependent BioID method. Since our in-house grouper embryonic cells (a natural host cell of SGIV) could not be efficiently transfected with the plasmid DNA, and the grouper genome data for mass spectrometry-based protein identification is not currently available, we chosen a non-permissive cell (HEK293 T) as a substitute for this study. A total of 112 cellular proteins that potentially bind to ICP18 were identified by mass spectrometry analysis. Homology analysis showed that among these identified proteins, 110 candidate ICP18-interactors had homologous proteins in zebrafish (a host of SGIV), and shared high sequence identity. Further analysis revealed that the identified ICP18-interacting proteins modulate various cellular processes such as cell cycle and cell adhesion. In addition, the interaction between ICP18 and its candidate interactor, i.e., cyclin-dependent kinase1 (CDK1), was confirmed using Co-immunoprecipitation (Co-IP) and Pull-down assays. Collectively, our present data provides additional insight into the biological functions of ICP18 during viral infection, which could help in further unraveling the pathogenesis of SGIV.

Fluorescent protein tagging of adenoviral proteins pV and pIX reveals 'late virion accumulation compartment'

The human adenovirus type 5 (HAdV5) causes disease of the upper and lower respiratory tract. The early steps of HAdV5 entry up to genome replication in the host nucleus have been extensively studied. However, late stages of infection remain poorly understood. Here, we set out to elucidate the spatiotemporal orchestration of late adenovirus nuclear remodeling in living cells. We generated virus mutants expressing fluorescently tagged protein IX (pIX) and protein V (pV), a capsid and viral genome associated protein, respectively. We found that during progeny virion production both proteins localize to a membrane-less, nuclear compartment, which is highly impermeable such that in immunofluorescence microscopy antibodies can hardly penetrate it. We termed this compartment ‘late virion accumulation compartment’ (LVAC). Correlation between light- and electron microscopy revealed that the LVAC contains paracrystalline arrays of viral capsids that arrange tightly packed within a honeycomb-like organization of viral DNA. Live-cell microscopy as well as FRAP measurements showed that the LVAC is rigid and restricts diffusion of larger molecules, indicating that capsids are trapped inside.

Understanding the regulation of adenovirus morphogenesis is not only of interest to cell biologists but is also key to define novel drug targets as well as to optimize adenoviruses as tools for gene therapy. While early steps of the adenovirus ‘life cycle’ are well understood, it is currently debated how, when and where capsid components associate with viral DNA. Here we used a combination of imaging methods to detail virus-induced spatiotemporal changes at late stages of infection. We found that HAdV5 induces a structured, membrane-less nuclear compartment. In this compartment capsids are closely packed within a honeycomb-like organization of replicated DNA, such that the newly formed particles appear to be trapped and show very little motility. Interestingly, we found a clear discrepancy between immunostaining and fluorescent fusion tagging, indicating a limited penetration of immunostains into this compartment. Since other pathogens induce similar compartments during replication, interpretation of immunostaining data requires careful evaluation.

Genetic and codon usage bias analyses of major capsid protein gene in Ranavirus

The Ranavirus (one genus of Iridovidae family) is an emerging pathogen that infects fish, amphibian, and reptiles, and causes great economical loss and ecological threat to farmed and wild animals globally. The major capsid protein (MCP) has been used as genetic typing marker and as target to design vaccines. Herein, the codon usage pattern of 73 MCP genes of Ranavirus and Lymphocystivirus are studied by calculating effective number of codons (ENC), relative synonymous codon usage (RSCU), codon adaptation index (CAI), and relative codon deoptimization index (RCDI), and similarity index (SiD). The Ranavirus are confirmed to be classified into five groups by using phylogenetic analysis, and varied nucleotide compositions and hierarchical cluster analysis based on RSCU. The results revealed different codon usage patterns among Lymphocystivirus and five groups of Ranavirus. Ranavirus had six over-represented codons ended with G/C nucleotide, while Lymphocystivirus had six over-represented codons ended with A/T nucleotide. A comparative analysis of parameters that define virus and host relatedness in terms of codon usage were analyzed indicated that Amphibian-like ranaviruses (ALRVs) seem to possess lower ENC values and higher CAIs in contrast to other ranaviruses isolated from fishes, and two groups (FV3-like and CMTV-like group) of them had received higher selection pressure from their hosts as having higher relative codon deoptimization index (RCDI) and similarity index (SiD). The correspondence analysis (COA) and Spearman's rank correlation analyses revealed that nucleotide compositions, relative dinucleotide frequency, mutation pressure, and natural translational selection shape the codon usage pattern in MCP genes and the ENC-GC_3S and neutrality plots indicated that the natural selection is the predominant factor. These results contribute to understanding the evolution of Ranavirus and their adaptions to their hosts.

Co-Expression Network Analysis of Spleen Transcriptome in Rock Bream (Oplegnathus fasciatus) Naturally Infected with Rock Bream Iridovirus (RBIV)

Rock bream iridovirus (RBIV) is a notorious agent that causes high mortality in aquaculture of rock bream (Oplegnathus fasciatus). Despite severity of this virus, no transcriptomic studies on RBIV-infected rock bream that can provide fundamental information on protective mechanism against the virus have been reported so far. This study aimed to investigate physiological mechanisms between host and RBIV through transcriptomic changes in the spleen based on RNA-seq. Depending on infection intensity and sampling time point, fish were divided into five groups: uninfected healthy fish at week 0 as control (0C), heavy infected fish at week 0 (0H), heavy mixed RBIV and bacterial infected fish at week 0 (0MH), uninfected healthy fish at week 3 (3C), and light infected fish at week 3 (3L). We explored clusters from 35,861 genes with Fragments Per Kilo-base of exon per Million mapped fragments (FPKM) values of 0.01 or more through signed co-expression network analysis using WGCNA package. Nine of 22 modules were highly correlated with viral infection (|gene significance (GS) vs. module membership (MM) |> 0.5, p-value < 0.05). Expression patterns in selected modules were divided into two: heavy infected (0H and 0MH) and control and light-infected groups (0C, 3C, and 3L). In functional analysis, genes in two positive modules (5448 unigenes) were enriched in cell cycle, DNA replication, transcription, and translation, and increased glycolysis activity. Seven negative modules (3517 unigenes) built in this study showed significant decreases in the expression of genes in lymphocyte-mediated immune system, antigen presentation, and platelet activation, whereas there was significant increased expression of endogenous apoptosis-related genes. These changes lead to RBIV proliferation and failure of host defense, and suggests the importance of blood cells such as thrombocytes and B cells in rock bream in RBIV infection. Interestingly, a hub gene, pre-mRNA processing factor 19 (PRPF19) showing high connectivity (kME), and expression of this gene using qRT-PCR was increased in rock bream blood cells shortly after RBIV was added. It might be a potential biomarker for diagnosis and vaccine studies in rock bream against RBIV. This transcriptome approach and our findings provide new insight into the understanding of global rock bream-RBIV interactions including immune and pathogenesis mechanisms.

Virus susceptibility of a new cell line derived from the fin of black carp Mylopharyngodon piceus

A continuous cell line MPF derived from the fin of black carp Mylopharyngodon piceus was established and characterised in this study. Mylopharyngodon piceus fin (MPF) cells were subcultured for more than 80 passages with high viability recovery after long-term storage. The karyotyping analysis revealed that MPF had a modal diploid chromosome number (2n = 48) and identical ribosomal RNA sequence with black carp. In addition, the expression of pluripotency-associated markers including nanog, oct4 and vasa, were detected in MPF. The transient transfection efficiency of MPF reached 23% with a fluorescent reporter by modified electroporation and stable expression of red fluorescent MPF was established by the baculovirus system, indicating that MPF is an ideal platform for studying gene functions in vitro. Lastly, cytopathic effects were also observed and RNA transcripts of a viral gene increased after infection by spring viremia of carp virus (SVCV), suggesting that MPF could be an alternative tool for investigating pathogen-host interactions in black carp. In conclusion, a fin cell line that is susceptible to SVCV was established as a potential adult stem-cell line, providing a suitable tool for future genetic analyses and pathogen-host studies in black carp.

Virus capsid assembly across different length scales inspire the development of virus-based biomaterials

In biology, there are an abundant number of self-assembled structures organized according to hierarchical levels of complexity. In some examples, the assemblies formed at each level exhibit unique properties and behaviors not present in individual components. Viruses are an example of such where first individual subunits come together to form a capsid structure, some utilizing a scaffolding protein to template or catalyze the capsid formation. Increasing the level of complexity, the viral capsids can then be used as building blocks of higher-level assemblies. This has inspired scientists to design and construct virus capsid-based functional nano-materials. This review provides some insight into the assembly of virus capsids across several length scales, and certain properties that arise at different levels, providing examples found in naturally occurring systems and those that are synthetically designed.

Description of a Natural Infection with Decapod Iridescent Virus 1 in Farmed Giant Freshwater Prawn, Macrobrachium rosenbergii

Macrobrachium rosenbergii is a valuable freshwater prawn in Asian aquaculture. In recent years, a new symptom that was generally called "white head" has caused high mortality in M. rosenbergii farms in China. Samples of M. rosenbergii, M. nipponense, Procambarus clarkii, M. superbum, Penaeus vannamei, and Cladocera from a farm suffering from white head in Jiangsu Province were collected and analyzed in this study. Pathogen detection showed that all samples were positive for Decapod iridescent virus 1 (DIV1). Histopathological examination revealed dark eosinophilic inclusions and pyknosis in hematopoietic tissue, hepatopancreas, and gills of M. rosenbergii and M. nipponense. Blue signals of in situ digoxigenin-labeled loop-mediated isothermal amplification appeared in hematopoietic tissue, hemocytes, hepatopancreatic sinus, and antennal gland. Transmission electron microscopy of ultrathin sections showed a large number of DIV1 particles with a mean diameter about 157.9 nm. The virogenic stromata and budding virions were observed in hematopoietic cells. Quantitative detection with TaqMan probe based real-time PCR of different tissues in naturally infected M. rosenbergii showed that hematopoietic tissue contained the highest DIV1 load with a relative abundance of 25.4 ± 16.9%. Hepatopancreas and muscle contained the lowest DIV1 loads with relative abundances of 2.44 ± 1.24% and 2.44 ± 2.16%, respectively. The above results verified that DIV1 is the pathogen causing white head in M. rosenbergii. M. nipponense and Pr. clarkii are also species susceptible to DIV1.

Assessment of fish iridoviruses using a novel cell line GS-1, derived from the spleen of orange-spotted grouper Epinephelus coioides (Hamilton) and susceptible to ranavirus and megalocytivirus

A new cell line (GS-1) was developed from the spleen tissue of the orange-spotted grouper, Epinephelus coioides applied for viral infection studies of fish ranavirus and megalocytivirus. The cells proficiently multiplied in Leibovitz’s L-15 medium supplemented with 10% fetal bovine serum at temperatures between 20°C and 32°C. Morphologically, the cell line comprised fibroblast-like cells, and this was confirmed by immunostaining with vimentin, fibronectin, and desmin antibodies. The optimal temperature for grouper iridovirus (GIV) and infectious spleen and kidney necrosis virus (ISKNV) proliferation in GS-1 cells was 25°C, and the highest titer of GIV was 10^8.4 TCID₅₀/ml, and the highest titer of ISKNV was 10^5.2 TCID₅₀/ml. Electron micrographs showed that the mean diameter of GIV virions was 180−220 nm, which was larger than ISKNV virions (160−200 nm). Negatively stained GIV particles possessed an envelope structure that was assembled by the three-layered structure with an inner electron-dense core surrounded by a lighter coat (mean diameter, 27 ± 3 nm). The highest GIV-induced mortality of groupers occurred at 25°C, whereas the highest ISKNV-induced mortality occurred at 30°C. In summary, GS-1 cell line is a valuable tool for isolating and investigating fish ranavirus and megalocytivirus in the same host system.

Organelle dynamics and viral infections: at cross roads

Viruses are obligate intracellular parasites of the host cells. A commonly accepted view is the requirement of internal membranous structures for various aspects of viral life cycle. Organelles enable favourable intracellular environment for several viruses. However, studies reporting organelle dynamics upon viral infections are scant. In this review, we aim to summarize and highlight modulations caused to various organelles upon viral infection or expression of its proteins.

Invertebrate Iridoviruses: A Glance over the Last Decade

Members of the family Iridoviridae (iridovirids) are large dsDNA viruses that infect both invertebrate and vertebrate ectotherms and whose symptoms range in severity from minor reductions in host fitness to systemic disease and large-scale mortality. Several characteristics have been useful for classifying iridoviruses; however, novel strains are continuously being discovered and, in many cases, reliable classification has been challenging. Further impeding classification, invertebrate iridoviruses (IIVs) can occasionally infect vertebrates; thus, host range is often not a useful criterion for classification. In this review, we discuss the current classification of iridovirids, focusing on genomic and structural features that distinguish vertebrate and invertebrate iridovirids and viral factors linked to host interactions in IIV6 (Invertebrate iridescent virus 6). In addition, we show for the first time how complete genome sequences of viral isolates can be leveraged to improve classification of new iridovirid isolates and resolve ambiguous relations. Improved classification of the iridoviruses may facilitate the identification of genus-specific virulence factors linked with diverse host phenotypes and host interactions.

Outbreaks of ulcerative disease associated with ranavirus infection in barcoo grunter, Scortum barcoo (McCulloch & Waite)

In 2013, an outbreak of ulcerative disease associated with ranavirus infection occurred in barcoo grunter, Scortum barcoo (McCulloch & Waite), farms in Thailand. Affected fish exhibited extensive haemorrhage and ulceration on skin and muscle. Microscopically, the widespread haemorrhagic ulceration and necrosis were noted in gill, spleen and kidney with the presence of intracytoplasmic eosinophilic inclusion bodies. When healthy barcoo grunter were experimentally challenged via intraperitoneal and oral modes with homogenized tissue of naturally infected fish, gross and microscopic lesions occurred with a cumulative mortality of 70-90%. Both naturally and experimentally infected fish yielded positive results to the ranavirus-specific PCR. The full-length nucleotide sequences of major capsid protein gene of ranaviral isolates were similar to largemouth bass virus (LMBV) and identical to largemouth bass ulcerative syndrome virus (LBUSV), previously reported in farmed largemouth bass (Micropterus salmoides L.), which also produced lethal ulcerative skin lesions. To the best of our knowledge, this is the first report of a LMBV-like infection associated with skin lesions in barcoo grunter, adding to the known examples of ranavirus infection associated with skin ulceration in fish.

Subcellular redistribution and sequential recruitment of macromolecular components during SGIV assembly

Virus infection consists of entry, synthesis of macromolecular components, virus assembly and release. Understanding of the mechanisms underlying each event is necessary for the intervention of virus infection in human healthcare and agriculture. Here we report the visualization of Singapore grouper iridovirus (SGIV) assembly in the medaka haploid embryonic stem (ES) cell line HX1. SGIV is a highly infectious DNA virus that causes a massive loss in marine aquaculture. Ectopic expression of VP88GFP, a fusion between green fluorescent protein and the envelope protein VP088, did not compromise the ES cell properties and susceptibility to SGIV infection. Although VP88GFP disperses evenly in the cytoplasm of non-infected cells, it undergoes aggregation and redistribution in SGIV-infected cells. Real-time visualization revealed multiple key stages of VP88GFP redistribution and the dynamics of viral assembly site (VAS). Specifically, VP88GFP entry into and condensation in the VAS occurred within a 6-h duration, a similar duration was observed also for the release of VP88GFP-containing SGIV out of the cell. Taken together, VP088 is an excellent marker for visualizing the SGIV infection process. Our results provide new insight into macromolecular component recruitment and SGIV assembly.

Recombinant Ranaviruses for Studying Evolution of Host-Pathogen Interactions in Ectothermic Vertebrates

Ranaviruses (Iridoviridae) are large DNA viruses that are causing emerging infectious diseases at an alarming rate in both wild and captive cold blood vertebrate species all over the world. Although the general biology of these viruses that presents some similarities with poxvirus is characterized, many aspects of their replication cycles, host cell interactions and evolution still remain largely unclear, especially in vivo. Over several years, strategies to generate site-specific ranavirus recombinant, either expressing fluorescent reporter genes or deficient for particular viral genes, have been developed. We review here these strategies, the main ranavirus recombinants characterized and their usefulness for in vitro and in vivo studies.

Endoplasmic Reticulum: The Favorite Intracellular Niche for Viral Replication and Assembly

The endoplasmic reticulum (ER) is the largest intracellular organelle. It forms a complex network of continuous sheets and tubules, extending from the nuclear envelope (NE) to the plasma membrane. This network is frequently perturbed by positive-strand RNA viruses utilizing the ER to create membranous replication factories (RFs), where amplification of their genomes occurs. In addition, many enveloped viruses assemble progeny virions in association with ER membranes, and viruses replicating in the nucleus need to overcome the NE barrier, requiring transient changes of the NE morphology. This review first summarizes some key aspects of ER morphology and then focuses on the exploitation of the ER by viruses for the sake of promoting the different steps of their replication cycles.