Susceptibility of the fish cell line SAF-1 to betanodavirus

Polystyrene nanoplastics as an ecotoxicological hazard: cellular and transcriptomic evidences on marine and freshwater in vitro teleost models

The contamination of marine and freshwater environments by nanoplastics is considered a global threat for aquatic biota. Taking into account the most recent concentration range estimates reported globally and recognizing a knowledge gap in polystyrene nanoplastics (PS-NPs) ecotoxicology, the present work investigated the harmful effects of 20 nm and 80 nm PS-NPs, at increasing biological complexity, on the rainbow trout Oncorhynchus mykiss RTG-2 and gilthead seabream Sparus aurata SAF-1 cell lines. Twenty nm PS-NPs exerted a greater cytotoxicity than 80 nm ones and SAF-1 were approximately 4-fold more vulnerable to PS-NPs than RTG-2. The engagement of PS-NPs with plasma membranes was accompanied by discernible uptake patterns and morphological alterations along with a nuclear translocation already within a 30-min exposure. Cells were structurally damaged only by the 20 nm PS-NPs in a time-dependent manner as indicated by distinctive features of the execution phase of the apoptotic cell death mechanism such as cell shrinkage, plasma membrane blebbing, translocation of phosphatidylserine to the outer leaflet of the cell membrane and DNA fragmentation. At last, functional analyses unveiled marked transcriptional impairment at both sublethal and lethal doses of 20 nm PS-NPs, with the latter impacting the "Steroid biosynthesis", "TGF-beta signaling pathway", "ECM-receptor interaction", "Focal adhesion", "Regulation of actin cytoskeleton" and "Protein processing in endoplasmic reticulum" pathways. Overall, a distinct ecotoxicological hazard of PS-NPs at environmentally relevant concentrations was thoroughly characterized on two piscine cell lines. The effects were demonstrated to depend on size, exposure time and model, emphasizing the need for a comparative evaluation of endpoints between freshwater and marine ecosystems.

Establishment of a brain cell line (SaB-1) from gilthead seabream and its application to fish virology

Gilthead seabream (Sparus aurata) is among the most important cultured fish species in the Mediterranean area and pathogen diseases one of the bottlenecks to the aquaculture sector. For this reason, generation of laboratory tools for diagnostic and research applications would be beneficial to improve the seabream aquaculture. In this sense, we aimed to generate a seabream cell line for biological studies. Thus, we have obtained a brain-derived cell line (SaB-1) that is continuously growing for more than 4 years. Cellular characterization of the SaB-1 cells shows that they express both neural and glial cell markers, suggesting they are neural-stem cells, have a neuron-like morphology and show a rapid growth in culture. We evaluated their susceptibility to the main fish viruses: nervous necrosis virus (NNV), spring viremia carp virus (SVCV), infectious pancreatic necrosis virus (IPNV) and viral haemorrhagic septicaemia virus (VHSV). SaB-1 cells are susceptible to all the tested viruses. In addition, the transcription of genes related to the type I interferon (IFN) is greatly up-regulated by the NNV infection whilst the viral infection with SVCV, IPNV or VHSV failed to do so. These data demonstrate that the seabream SaB-1 cell line is continuous, stable and could be useful, at least, for fish virology and immunity applications.

Innate Cell-Mediated Cytotoxic Activity of European Sea Bass Leucocytes Against Nodavirus-Infected Cells: A Functional and RNA-seq Study

Nervous necrosis virus (NNV) causes high mortalities in several marine species. We aimed to evaluate the innate cell-mediated cytotoxic (CMC) activity of head-kidney leucocytes (HKLs) isolated from naïve European sea bass (Dicentrarchus labrax) and gilthead seabream (Sparus aurata), a very susceptible and resistant fish species to NNV, respectively, against fish cell lines infected with NNV. Seabream HKLs showed significantly increased innate CMC activity against NNV-infected cells, compared to those uninfected, while sea bass HKLs failed to do so. Thus, we performed a RNA-seq study to identify genes related to the CMC activity of sea bass leucocytes. Thus, we found that sea bass HKLs incubated with DLB-1 cells alone (CMC_DLB1) or with NNV-infected DLB-1 cells (CMC_DLB1-NNV) showed very similar transcriptomic profiles and the GO analysis revealed that most of the up-regulated genes were related to immunity. Strikingly, when the CMC samples with and without NNV were compared, GO analysis revealed that most of the up-regulated genes in CMC_DLB1-NNV samples were related to metabolism and very few to immunity. This is also in agreement with the functional data. These data point to the escape of CMC activity by NNV infection as an important factor involved in the high susceptibility to nodavirus infections of European sea bass.

The theoretical reliability of PCR-based fish viral diagnostic methods is critically affected when they are applied to fish populations with low prevalence and virus loads

AIMS

The reliability of polymerase chain reaction (PCR) techniques is an important issue in viral diagnosis, and it is even crucial when they must be applied for detection of viruses in asymptomatic carriers. The problems will arise when the aim is to study wild fish populations, where the viral loads and prevalence values are extremely low. We have evaluated several PCR procedures employed by two laboratories for monitoring fish captured in several oceanographic campaigns performed in the Gulf of Cádiz.

METHODS AND RESULTS

To evaluate the reliability of different diagnostics test used, we have re-analysed fish samples that had been previously subjected to diagnosis for a surveillance of viruses performed in 2010-2011 in wild fish populations. The following parameters were employed: the clinical sensitivity (Ss), the clinical specificity (Sp), the predictive positive value, the predictive negative value, and the positive and negative likelihood ratio (LR⁺ and LR^- ). For viral nervous necrosis virus, a RT-PCR procedure supplemented by nested PCR showed the highest values (100%) for all the parameters. For viral haemorrhagic septicaemia virus, the highest values were provided by RT-PCR supplemented by dot-blot hybridization. In the case of infectious pancreatic necrosis virus, none of the procedures yielded 100% for any parameter.

CONCLUSIONS

The results obtained for viral prevalence indicate: (i) that the conservation of the samples at -80°C did not affect to the capacity of detection of the virus in the tissues, and (ii) that the reproducibility of the diagnosis can be affected by factors including the staff experience and/or the materials employed. Finally, the use of a combination of procedures in advised to ensure the maximum reliability of the diagnosis when it is applied to asymptomatic fish populations.

SIGNIFICANCE AND IMPACT OF THE STUDY

This paper describes a strategy of combining diagnostic tests for the surveillance and monitoring of wild fish populations to reduce underestimation of the prevalence of viruses this type of populations.

In vitro effects of Italian Lavandula multifida L. leaf extracts on gilthead seabream (Sparus aurata) leucocytes and SAF-1 cells

Lavandula multifida is very appreciated by pharmaceutical and cosmetic industries. In Italy is only found in Calabria and Sicily and, at present, urge its valorization due to its high extinction and genetic erosion risks. Possible applications of L. multifida extracts as immunostimulant in fish aquaculture were assayed by using gilthead seabream (Sparus aurata) as a marine fish model, due to its importance in fish aquaculture. The in vitro effects of both aqueous and ethanolic leaf extracts obtained from two Italian populations of L. multifida on head kidney leucocyte activities (viability, phagocytosis, respiratory burst and peroxidase content) were assessed. Furthermore, the possible cytotoxic effects of the extracts on SAF-1 cells and their bactericidal effects on three fish pathogenic bacteria (Vibrio harveyi, Vibrio anguillarum, Aeromonas salmonicida) were also evaluated. All the assays were performed in comparison with leaf extracts obtained from a widely-distributed species as L. angustifolia. Results showed that water and ethanolic leaf extracts obtained from L. multifida enhanced innate immune activities of S. aurata HK leucocytes. Furthermore, SAF-1 cell viability was not affected significantly after being incubated with the extracts. These extracts did not exert any bactericidal activity on the pathogenic bacterial strains tested in the present study. Results obtained in the present work suggested the possibility of use such extracts in in vivo studies in order to corroborate the possibility of their use in aquaculture. Their use could prevent to improve fish defense against pathogenic infections through enhancement of the fish immune status.

Susceptibility of Chinese Perch Brain (CPB) Cell and Mandarin Fish to Red-Spotted Grouper Nervous Necrosis Virus (RGNNV) Infection

Nervous necrosis virus (NNV) is the causative agent of viral encephalopathy and retinopathy (VER), a neurological disease responsible for high mortality of fish species worldwide. Taking advantage of our established Chinese perch brain (CPB) cell line derived from brain tissues of Mandarin fish (Siniperca chuatsi), the susceptibility of CPB cell to Red-Spotted Grouper nervous necrosis virus (RGNNV) was evaluated. The results showed that RGNNV replicated well in CPB cells, resulting in cellular apoptosis. Moreover, the susceptibility of Mandarin fish to RGNNV was also evaluated. Abnormal swimming was observed in RGNNV-infected Mandarin fish. In addition, the cellular vacuolation and viral particles were also observed in brain tissues of RGNNV-infected Mandarin fish by Hematoxylin-eosin staining or electronic microscopy. The established RGNNV susceptible brain cell line from freshwater fish will pave a new way for the study of the pathogenicity and replication of NNV in the future.

Design and evaluation of reverse transcription nested PCR primers for the detection of betanodavirus in finfish

Viral encephalopathy and retinopathy otherwise known as viral nervous necrosis (VNN) is a neuropathological condition affecting more than 50 fish species worldwide, mostly marine. Different PCR protocols with specific primers were reported from many countries for confirmation of VNN in fishes. In the present study, two pairs of primers were designed and evaluated for the diagnosis of clinical and subclinical cases of infections from field. These primers designated as BARL-F1/BARL-R1 amplified a 902 bp product in the variable region (T4) of the coat protein gene by first step PCR. Nested PCR primers BARL-F2/BARL-R2 amplified a fragment of 313 bp. The results were comparable with other commonly used primer sets such as F2/R3 and RG668f/RG919r primers. These new primers could detect betanodavirus in standard reference samples containing low, moderate and high viral load. Known positive and negative control samples of fish also revealed a predictive value of 100 % by RT-PCR diagnosis.

Cell Culture Isolation of Piscine Nodavirus (Betanodavirus) in Fish-Rearing Seawater

Piscine nodavirus (betanodavirus) is the causative agent of viral nervous necrosis (VNN) in a variety of cultured fish species, particularly marine fish. In the present study, we developed a sensitive method for cell culture isolation of the virus from seawater and applied the method to a spontaneous fish-rearing environment. The virus in seawater was concentrated by an iron-based flocculation method and subjected to isolation with E-11 cells. A real-time reverse transcriptase PCR (RT-PCR) assay was used to quantify the virus in water. After spiking into seawater was performed, a betanodavirus strain (red spotted grouper nervous necrosis virus [RGNNV] genotype) was effectively recovered in the E-11 cells at a detection limit of approximately 10(5)copies (equivalent to 10(2)50% tissue culture infective doses [TCID50])/liter seawater. In an experimental infection of juvenile sevenband grouper (Epinephelus septemfasciatus) with the virus, the virus was isolated from the drainage of a fish-rearing tank when the virus level in water was at least approximately 10(5)copies/liter. The application of this method to seven band grouper-rearing floating net pens, where VNN prevailed, resulted in the successful isolation of the virus from seawater. No differences were found in the partial sequences of the coat protein gene (RNA2) between the clinical virus isolates of dead fish and the cell-cultured virus isolates from seawater, and the viruses were identified as RGNNV. The infection experiment showed that the virus isolates from seawater were virulent to seven band grouper. These results showed direct evidence of the horizontal transmission of betanodavirus via rearing water in marine aquaculture.

Characterization of the IFN pathway in the teleost fish gonad against vertically transmitted viral nervous necrosis virus

One of the most powerful innate immune responses against viruses is mediated by type I IFN. In teleost fish, it is known that virus infection triggers the expression of ifn and many IFN-stimulated genes, but the viral RNA sensors and mediators leading to IFN production are scarcely known. Thus, we have searched for the presence of these genes in gilt-head sea bream (Sparus aurata) and European sea bass (Dicentrarchus labrax), and evaluated their expression after infection with viral nervous necrosis virus (VNNV) in the brain, the main viral target tissue, and the gonad, used to transmit the virus vertically. In sea bream, a fish species resistant to the VNNV strain used, we found an upregulation of the genes encoding MDA5 (melanoma differentiation-associated gene 5), TBK1 (TANK-binding kinase 1), IRF3 (IFN regulatory factor 3), IFN, Mx [myxovirus (influenza) resistance protein] and PKR (dsRNA-dependent protein kinase receptor) proteins in the brain, which were unaltered in the gonad and could favour the dissemination by gonad fluids or gametes. Strikingly, in European sea bass, a very susceptible species, we also identified, transcripts coding for LGP2 (Laboratory of Genetics and Physiology 2), MAVS (mitochondrial antiviral signalling), TRAF3 (TNF receptor-associated factor 3), TANK (TRAF family member-associated NFκB activator) and IRF7 (IFN regulatory factor 7), and found that all the genes analysed were upregulated in the gonad, but only mda5, lgp2, irf3, mx and pkr were upregulated in the brain. These findings supported the notion that the European sea bass brain innate immune response is unable to clear the virus and pointed to the importance of gonad immunity to control the dissemination of VNNV to the progeny--an aspect that is worth investigating in aquatic animals.

Optimization of Betanodavirus culture and enumeration in striped snakehead fish cells

An optimized culture method for detection of infection of fish with the Red spotted grouper nervous necrosis virus (RGNNV) genotype of betanodavirus in striped snakehead (SSN-1, Channa striatus) cells is described. Inoculation of fish tissue homogenates at the same time or within 4 hr of seeding the SSN-1 cells was as sensitive as the method recommended by the World Organization for Animal Health, where homogenates were adsorbed onto an established cell monolayer. Such modification halved the time required and the costs of consumables, and reduced the potential for error when processing large numbers of samples. Positive culture results were obtained from 88.3% of 392 fish tissue homogenates in which RGNNV was detected using a quantitative reverse transcription polymerase chain reaction (qRT-PCR) assay; 99.7% of 943 tissue homogenates, which were qRT-PCR negative, were cell culture negative. Cytopathic effect (CPE) was characterized by large intracytoplasmic vacuoles in 0.1–60% of cells. Detachment of affected cells from the culture surface resulting in progressive disruption of the monolayer occurred in 46.4% of primary cultures and 96.0% of subcultures of positive samples. Identification of CPE that did not disrupt the cell monolayer increased estimates of the 50% tissue culture infective dose (TCID50) by 1.07–2.79 logs (95% confidence interval). The predicted mean TCID50/ml was 3.3 logs higher when cells were inoculated less than 36 hr after subculture at less than 80% confluence compared to cells inoculated at greater than 80% confluence and more than 36 hr after subculture ( P < 0.05).

Antemortem versus postmortem methods for detection of betanodavirus in Senegalese sole (Solea senegalensis)

The suitability of nested reverse transcription polymerase chain reaction (nRT-PCR) to detect betanodavirus in blood samples from naturally infected Senegalese sole (Solea senegalensis) was evaluated in comparison with other diagnostic methods. Results indicated that histologic examination of brain lesions could be regarded as the most consistent indicator of nodavirus infection in this species. The nRT-PCR showed low to moderate levels of detection; the best values were obtained in brain samples followed by blood samples. Inoculation of SSN-1 and SAF-1 cells with fish samples did not cause cytopathic effect, although virus was detected by reverse transcription polymerase chain reaction in approximately 25% of the SSN-1 inoculated wells. The efficiency of detection of the viral genome was dramatically increased by the use of nRT-PCR, reaching 90.6% of positives in brain samples and 84.4% in blood samples. The sensitivity and the negative predictive value of nRT-PCR in blood samples were slightly lower than those obtained using brain samples. Nevertheless, it is suggested that the advantage of being able to perform diagnosis on live fish adequately counterbalances the slightly lower sensitivity of nRT-PCR on blood samples. This technique is proposed as a useful tool, not only for the selection of nodavirus-free breeders but also to check the fish status during ongrowing.

Expression analysis of Mx protein and evaluation of its antiviral activity against sole aquabirnavirus in SAF-1 and TV-1 cell lines

The transcription of Mx mRNA after poly I:C induction and sole aquabirnavirus infection has been analysed in SAF-1 and TV-1 cells (derived from gilt-head seabream and turbot, respectively). Both cell lines were stimulated with 10 microg ml(-1) poly I:C and Mx mRNA was analysed by a specific RT-PCR at several times post-induction. The results showed a high level of Mx expression from 12 to 120 h after induction in SAF-1 cells, whereas in TV-1 cells Mx mRNA was only detected at 12 and 24h. The treatment with different concentrations of poly I:C showed that TV-1 cells are less sensitive to this inductor than the SAF-1 cell line. The antiviral activity derived from poly I:C induction has been clearly demonstrated against sole aquabirnavirus on both cell lines. The inoculation of sole aquabirnavirus resulted in the Mx mRNA transcription at 48, 72, and 96 h post-infection (p.i.) in SAF-1 cells. On the contrary, inoculated TV-1 cells only showed a faint Mx mRNA band at 24 and 48 h p.i. This study has established different patterns of Mx expression in both cells under study as a consequence of the poly I:C induction and sole aquabirnavirus infection, and it shows that gilt-head seabream and turbot Mx inhibit sole aquabirnavirus replication.