Up-regulation of nuclear factor E2-related factor 2 (Nrf2) represses the replication of SVCV

The possible role of nuclear factor erythroid-2-related factor 2 activators in the management of Covid-19

COVID-19 is caused by a novel SARS-CoV-2 leading to pulmonary and extra-pulmonary manifestations due to oxidative stress (OS) development and hyperinflammation. COVID-19 is primarily asymptomatic though it may cause acute lung injury (ALI), acute respiratory distress syndrome (ARDS), systemic inflammation, and thrombotic events in severe cases. SARS-CoV-2-induced OS triggers the activation of different signaling pathways, which counterbalances this complication. One of these pathways is nuclear factor erythroid 2-related factor 2 (Nrf2), which induces a series of cellular interactions to mitigate SARS-CoV-2-mediated viral toxicity and OS-induced cellular injury. Nrf2 pathway inhibits the expression of pro-inflammatory cytokines and the development of cytokine storm in COVID-19. Therefore, Nrf2 activators may play an essential role in reducing SARS-CoV-2 infection-induced inflammation by suppressing NLRP3 inflammasome in COVID-19. Furthermore, Nrf2 activators can attenuate endothelial dysfunction (ED), renin-angiotensin system (RAS) dysregulation, immune thrombosis, and coagulopathy. Thus this mini-review tries to clarify the possible role of the Nrf2 activators in the management of COVID-19. Nrf2 activators could be an effective therapeutic strategy in the management of Covid-19. Preclinical and clinical studies are recommended in this regard.

Mangiferin and Taurine Ameliorate MSRV Infection by Suppressing NF-κB Signaling

The emergence or reemergence of viruses pose a substantial threat and challenge to the world population, livestock, and wildlife. However, the landscape of antiviral agents either for human or animal viral diseases is still underdeveloped. The far tougher actuality is the case that there are no approved antiviral drugs in the aquaculture industry, although there are diverse viral pathogens. In this study, using a novel epithelial cell line derived from the brain of Micropterus salmoides (MSBr), inflammation and oxidative stress were found to implicate the major pathophysiology of M. salmoides rhabdovirus (MSRV) through transcriptome analysis and biochemical tests. Elevated levels of proinflammatory cytokines (interleukin-1β [IL-1β], IL-6, IL-8, tumor necrosis factor alpha [TNF-α], and gamma interferon [IFN-γ]) and accumulated contents of reactive oxygen species (ROS) as well as biomarkers of oxidative damage (protein carbonyl and 8-OHdG) were observed after MSRV infection in the MSBr cells. Mangiferin or taurine dampened MSRV-induced inflammation and rescued the oxidative stress and, thus, inhibited the replication of MSRV in the MSBr cells with 50% effective concentration (EC50) values of 6.77 μg/mL and 8.02 μg/mL, respectively. Further, mangiferin or taurine hampered the activation of NF-κB1 and the NF-κB1 promoter as well as the increase of phosphorylated NF-κB (p65) protein level induced by MSRV infection, indicating their antiviral mechanism by suppressing NF-κB signaling. These findings exemplify a practice approach, aiming to dampen and redirect inflammatory responses, to develop broad-spectrum antivirals. IMPORTANCE Aquaculture now provides almost half of all fish for human food in 2021 and plays a significant role in eliminating hunger, promoting health, and reducing poverty. There are diverse viral pathogens that decrease production in aquaculture. We developed a novel epithelial cell line derived from the brain of Micropterus salmoides, which can be used for virus isolation, gene expressing, and drug screening. In this study, we focus on M. salmoides rhabdovirus (MSRV) and revealed its pathophysiology of inflammation and oxidative stress. Aiming to dampen and redirect inflammatory responses, mangiferin or taurine exhibited their antiviral capability by suppressing NF-κB signaling. Our findings exemplify a practice approach to develop broad-spectrum antivirals by dampening and redirecting inflammatory responses.

Keap1 recognizes EIAV early accessory protein Rev to promote antiviral defense

The Nrf2/Keap1 axis plays a complex role in viral susceptibility, virus-associated inflammation and immune regulation in host cells. However, whether or how the Nrf2/Keap1 axis is involved in the interactions between equine lentiviruses and their hosts remains unclear. Here, we demonstrate that the Nrf2/Keap1 axis was activated during EIAV infection. Mechanistically, EIAV-Rev competitively binds to Keap1 and releases Nrf2 from Keap1-mediated repression, leading to the accumulation of Nrf2 in the nucleus and promoting Nrf2 responsive genes transcription. Subsequently, we demonstrated that the Nrf2/Keap1 axis represses EIAV replication via two independent molecular mechanisms: directly increasing antioxidant enzymes to promote effective cellular resistance against EIAV infection, and repression of Rev-mediated RNA transport through direct interaction between Keap1 and Rev. Together, these data suggest that activation of the Nrf2/Keap1 axis mediates a passive defensive response to combat EIAV infection. The Nrf2/Keap1 axis could be a potential target for developing strategies for combating EIAV infection.

Upregulation of nuclear factor E2-related factor 2 (Nrf2) represses the replication of herpes simplex virus type 1

Background

Nuclear factor E2-related factor 2 (Nrf2) is an important transcription factor which plays a pivotal role in detoxifying reactive oxygen species (ROS) and has been more recently shown to regulate inflammatory and antiviral responses. However, the role of Nrf2 in Herpes Simplex Virus type 1 (HSV-1) infection is still unclear. In this study, the interaction between the Nrf2 and HSV-1 replication was investigated.

Methods

The levels of oxidative stress was monitored by using 8-hydroxy-2'-deoxyguanosine (8-OHdG) ELISA kits, and the dynamic changes of Nrf2-antioxidant response element (Nrf2-ARE) pathway were detected by Western Blot. The effect of Nrf2-ARE pathway on the regulation of HSV-1 proliferation was analyzed by Western Blot, Real-Time PCR and TCID₅₀ assay.

Results

HSV-1 infection induced oxidative stress. Nrf2 was activated, accompanied by the increase of its down-stream antioxidant enzyme heme oxygenase-1 (HO-1) and NAD(P)H quinone oxidoreductase 1 (NQO1) in the early stage of HSV-1 infection. The proliferation of HSV-1 was inhibited by overexpression of Nrf2 or treatment with its activator tert-Butylhydroquinone (tBHQ). On the contrary, silencing of Nrf2 promotes virus replication. HO-1 is involved in the regulation of IFN response, leading to efficient anti-HSV-1 effects.

Conclusion

Our observations indicate that the Nrf2-ARE pathway activates a passive defensive response in the early stage of HSV-1 infection. Targeting the Nrf2 pathway demonstrates the potential for combating HSV-1 infection.

Azoxystrobin increases the infection of spring viraemia of carp virus in fish

Azoxystrobin (AZ) has entered aquatic ecosystems and produced serious damages to fish associated with potentially increasing the susceptibility to pathogens. This study characterized the defense abilities of fish by exposed to AZ on challenging with the infection of spring viraemia of carp virus (SVCV). The results showed that SVCV replication increased significantly in EPC cells and zebrafish that were exposed to up to 50 μg/L of AZ at 3, 5, 7, and 14 d. Intracellular biochemical assays indicated that AZ at 5 and 50 μg/L inhibited the activation of Nrf2-ARE pathway including a decrease in Nrf2 expression, Nrf2 phosphorylation, HO-1 content, and three antioxidant activities. While no significant difference in ERK1/2 and JNK MAPKs in zebrafish was observed, P38 phosphorylation was significantly decreased at 7 and 14 d, and the changes in MAPKs were more evident in EPC cells previously exposed to AZ at 7 d. These results revealed that AZ initially induced low phosphorylation of MAPKs, triggering the attenuation of Nrf2 phosphorylation to weaken Nrf2 translocation into the nucleus in a longer exposure period (more than 5 d). The data in the cells and fish also showed that antioxidant activities were decreased to some extent at 5-7 d for the cells and 7-14 d for the fish. Furthermore, interferon-related factors were decreased in AZ-exposed zebrafish, explaining the reason that fish can't resist the virus infection. Overall, the present study provided a new adverse threat of AZ by amplifying the viral outbreak to endanger ecological safety in aquatic environment.

Emodin resists to Cyprinid herpesvirus 3 replication via the pathways of Nrf2/Keap1-ARE and NF-κB in the ornamental koi carp (Cyprinus carpio haematopterus)

Cyprinid herpesvirus 3 (CyHV-3) causes high mortality in carp. Emodin has been shown of the effects of antioxidant, anti-inflammatory and antiviral. In present study, we investigated the preventive effects and mechanism of emodin on CyHV-3 infection. The ornamental koi carp (Cyprinus carpio haematopterus) were intraperitoneally injected with emodin (10 mg/kg, 20 mg/kg, or 40 mg/kg). 72 h later, an intraperitoneal injection of CyHV-3 was administered, and collected the samples one week later to detect the antioxidant parameters, antioxidant genes, inflammatory genes and to perform histopathology assays. The results showed that emodin significantly suppressed CyHV-3 replication (P < 0.05), improved the koi survival rate and slowed the damage caused by CyHV-3. Emodin treatment increased the antioxidant activity and decreased the lipid peroxidation level of the koi. Compared to the CyHV-3 group, emodin treatment resulted in the same antioxidant parameters after CyHV-3 infection. Emodin treatment activated the Nuclear factorery throid 2-related factor 2/Kelch-like ECH-associated protein 1-antioxidatant response element (Nrf2/Keap1-ARE) pathway and upregulated the expression of heme oxygenase 1 (HO-1), superoxide dismutase (SOD), and catalase (CAT) in the hepatopancreas after CyHV-3 infection. Emodin activated the nuclear factor kappa-B (NF-κB) pathway and decreased the expression of interleukin-6 (IL-6), interleukin-8 (IL-8), and tumour necrosis factor-α (TNF-α) in the koi induced by CyHV-3. In conclusion, emodin treatment can suppress CyHV-3 replication and reduce the mortality of koi caused by CyHV-3. Emodin improves antioxidant function, relieves oxidative stress and inflammation cytokines via Nrf2/Keap1-ARE and NF-κB pathways, and protects against the adverse effects induced by CyHV-3.

Hydroxycoumarin efficiently inhibits spring viraemia of carp virus infection in vitro and in vivo

Spring viremia of carp virus (SVCV) causes devastating losses in aquaculture. Coumarin has an advantageous structure for the design of novel antiviral agents with high affinity and specificity. In this study, we evaluated a hydroxycoumarin medicine, i.e., 7-(6-benzimidazole) coumarin (C10), regarding its anti-SVCV effects in vitro and in vivo. Results showed that up to 12.5 mg/L C10 significantly inhibited SVCV replication in the epithelioma papulosum cyprini (EPC) cell line, with a maximum inhibitory rate of >97%. Furthermore, C10 significantly reduced cell death and relieved cellular morphological damage in SVCV-infected cells. Decreased mitochondrial membrane potential (ΔΨm) also suggested that C10 not only protected mitochondria, but also reduced apoptosis in SVCV-infected cells. For in vivo studies, intraperitoneal injection of C10 resulted in an anti-SVCV effect and substantially enhanced the survival rate of virus-infected zebrafish. Furthermore, C10 significantly enhanced antioxidant enzyme activities and decreased reactive oxygen species (ROS) to maintain antioxidant-oxidant balance within the host, thereby contributing to inhibition of SVCV replication. The up-regulation of six interferon (IFN)-related genes also demonstrated that C10 indirectly activated IFNs for the clearance of SVCV in zebrafish. This was beneficial for the continuous maintenance of antiviral effects because of the low viral loads in fish. Thus, C10 is suggested as a therapeutic agent with great potential against SVCV infection in aquaculture.

Evaluation on the antiviral effect of a hydroxycoumarin against infectious hematopoietic necrosis virus infection in vitro and in vivo

Infectious hematopoietic necrosis (IHN) caused by the viral pathogen infectious hematopoietic necrosis virus (IHNV) is a highly contagious disease of salmonid species, resulting in significant economic impact. The previous study showed a hydroxycoumarin derivative 7-[6-(2-methylimidazole) hexyloxy] coumarin (D5) significantly inhibited spring viraemia of carp virus (SVCV) infection, suggesting that D5 may be useful as a potential anti-IHNV agent. In this study, D5 at the concentration of up to 10 mg/L significantly inhibited IHNV replication in epithelioma papulosum cyprini (EPC) cells with a maximum inhibitory rate of >90%, maintained mitochondrial membrane potential (ΔΨm) levels, and decreased IHNV-induced apoptosis in virus-infected cells. As the consequence of protection on mitochondria, D5 enhanced antioxidant enzyme activities and decreased reactive oxygen species (ROS) to maintain the antioxidant-oxidant balance of IHNV-infected EPC cells. For in vivo study, D5 via intraperitoneal injection exhibited an anti-IHNV effect in the virus-infected fish by substantially enhancing the survival rate. Meanwhile, up-regulation of six interferon (IFN) related gene expressions demonstrated that D5 may activate IFN-related expressions for inhibiting IHNV replication during the early stage of viral infection, which is beneficial for the continuous antiviral action on controlling low viral loads in rainbow trout juvenile. Thus, D5 effective regulated IHNV-induced undesirable conditions to be an excellent potential therapeutic agent against IHNV infection.

Progressive Rotavirus Infection Downregulates Redox-Sensitive Transcription Factor Nrf2 and Nrf2-Driven Transcription Units

Eukaryotic cells adopt highly tuned stress response physiology under threats of exogenous stressors including viruses to maintain cellular homeostasis. Not surprisingly, avoidance of cellular stress response pathways is an essential facet of virus-induced obligatory host reprogramming to invoke a cellular environment conducive to viral perpetuation. Adaptive cellular responses to oxidative and electrophilic stress are usually taken care of by an antioxidant defense system, core to which lies the redox-responsive transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) and Nrf2-driven transcriptional cascade. Deregulation of host redox balance and redox stress-sensitive Nrf2 antioxidant defense have been reported for many viruses. In the current study, we aimed to study the modulation of the Nrf2-based host cellular redox defense system in response to Rotavirus (RV) infection in vitro. Interestingly, we found that Nrf2 protein levels decline sharply with progression of RV infection beyond an initial upsurge. Moreover, Nrf2 decrease as a whole was found to be accompanied by active nuclear vacuity of Nrf2, resulting in lowered expression of stress-responsive Nrf2 target genes heme oxygenase-1 (HO-1), NAD(P)H quinone dehydrogenase 1, and superoxide dismutase 1 both in the presence and absence of Nrf2-driven transcriptional inducers. Initial induction of Nrf2 concurred with RV-induced early burst of oxidative stress and therefore was sensitive to treatments with antioxidants. Reduction of Nrf2 levels beyond initial hours, however, was found to be independent of the cellular redox status. Furthermore, increasing the half-life of Nrf2 through inhibition of the Kelch-like erythroid cell-derived protein with CNC homology- (ECH-) associated protein 1/Cullin3-RING Box1-based canonical Nrf2 turnover pathway could not restore Nrf2 levels post RV-SA11 infection. Depletion of the Nrf2/HO-1 axis was subsequently found to be sensitive to proteasome inhibition with concurrent observation of increased K48-linked ubiquitination associated with Nrf2. Together, the present study describes robust downregulation of Nrf2-dependent cellular redox defense beyond initial hours of RV infection, justifying our previous observation of potent antirotaviral implications of Nrf2 agonists.

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.

Ursolic acid from Prunella vulgaris L. efficiently inhibits IHNV infection in vitro and in vivo

Infectious hematopoietic necrosis virus (IHNV) is a fish viral pathogen that causes severe disease and huge economic losses in the salmonid aquaculture industry. However, anti-IHNV drugs currently are scarce. For the purpose of seeking out anti-IHNV drugs, the anti-IHNV activities of 32 medicinal plants were investigated by using epithelioma papulosum cyprini (EPC) cells. Among these plants, Prunella vulgaris L. (PVL) showed the strongest inhibition on IHNV replication with an inhibitory percentage of 99.3% at the concentration 100 mg/L. Further studies demonstrated that ursolic acid (UA), a major constituent of PVL, also showed a highly effective anti-IHNV activity. The half-maximal inhibitory concentration (IC50) at 72 h of UA on IHNV was 8.0 μM. Besides, UA could significantly decrease cytopathic effect (CPE) and the viral titer induced by IHNV in EPC cells. More importantly, UA also showed a strong anti-IHNV activity in vivo, as indicated by increasing the survival rate of rainbow trout and inhibiting viral gene expression. Intraperitoneal injection of UA increased the relative percentage of survival of rainbow trout by 18.9% and inhibited IHNV glycoprotein mRNA expression by > 90.0% in the spleen at the 1st-day post-infection. Altogether, UA was expected to be a therapeutic agent against IHNV infection in aquaculture.

Bovine Herpesvirus 1 Productive Infection Led to Inactivation of Nrf2 Signaling through Diverse Approaches

Bovine herpesvirus type 1 (BoHV-1) is a significant cofactor for bovine respiratory disease complex (BRDC), the most important inflammatory disease in cattle. BoHV-1 infection in cell cultures induces overproduction of pathogenic reactive oxygen species (ROS) and the depletion of nuclear factor erythroid 2 p45-related factor 2 (Nrf2), a master transcriptional factor regulating a panel of antioxidant and cellular defense genes in response to oxidative stress. In this study, we reported that the virus productive infection in MDBK cells at the later stage significantly decreased the expression levels of heme oxygenase-1 (HO-1) and NAD(P)H quinone oxidoreductase-1 (NQO1) proteins, the canonical downstream targets regulated by Nrf2, inhibited Nrf2 acetylation, reduced the accumulation of Nrf2 proteins in the nucleus, and relocalized nuclear Nrf2 proteins to form dot-like staining patterns in confocal microscope assay. The differential expression of Kelch-like ECH associated protein 1 (KEAP1) and DJ-1 proteins as well as the decreased association between KEAP1 and DJ-1 promoted Nrf2 degradation through the ubiquitin proteasome pathway. These data indicated that the BoHV-1 infection may significantly suppress the Nrf2 signaling pathway. Moreover, we found that there was an association between Nrf2 and LaminA/C, H3K9ac, and H3K18ac, and the binding ratios were altered following the virus infection. Taken together, for the first time, we provided evidence showing that BoHV-1 infection inhibited the Nrf2 signaling pathway by complicated mechanisms including promoting Nrf2 degradation, relocalization of nuclear Nrf2, and inhibition of Nrf2 acetylation.

Synthesis and antiviral activity of a new arctigenin derivative against IHNV in vitro and in vivo

Viral diseases in aquaculture were challenging because there are few preventative measures and/or treatments. Our previous study indicated that imidazole arctigenin derivatives possessed antiviral activities against infectious hematopoietic necrosis virus (IHNV). Based on the structure-activity relationship in that study, a new imidazole arctigenin derivative, 4-(8-(2-ethylimidazole)octyloxy)-arctigenin (EOA), was designed, synthesized and its anti-IHNV activity was evaluated. By comparing inhibitory concentration at half-maximal activity (IC₅₀), we found that EOA (IC₅₀ = 0.56 mg/L) possessed a higher antiviral activity than those imidazole arctigenin derivatives in our previous study. Besides, EOA could significantly decrease cytopathic effect (CPE) and viral titer induced by IHNV in epithelioma papulosum cyprinid (EPC) cells. In addition, EOA significantly inhibited apoptosis induced by IHNV in EPC cells. Further data verified that EOA inhibited IHNV replication in rainbow trout, with reducing 32.0% mortality of IHNV-infected fish. The results suggested that EOA was more stable with a prolonged inhibitory half-life in the early stage of virus infection (1-4 days). Consistent with above results, EOA repressed IHNV glycoprotein gene expression in virus sensitive tissues (kidney and spleen) in the early stage of virus infection. Moreover, histopathological evaluation showed that tissues from the spleen and kidney of fish infected with IHNV exhibited pathological changes. But there were no lesions in any of the tissues from the control group and EOA-treaten group. In accordance with the histopathological assay, EOA could elicited anti-inflammation response in non-viral infected rainbow trout by down-regulating the expression of cytokine genes (IL-8, IL-12p40, and TNF-α). Altogether, EOA was expected to be a therapeutic agent against IHNV infection in the field of aquaculture.

PMA-triggered PKCε activity enhances Nrf2-mediated antiviral response on fish rhabdovirus infection

Viral infection is often accompanied with alteration of intracellular redox state, especially an imbalance between reactive oxygen species (ROS) production and antioxidant cellular defenses. The previous studies showed that an antioxidant cellular defense system, the transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2), played an important role against spring viraemia of carp virus (SVCV) infection in fish. To further reveal the mediated mechanism that Nrf2 active state was affected by protein kinase C (PKC), here we evaluated SVCV replication in host cells by treated with a strong activator of PKC phorbol-12-myristate-13-acetate (PMA) and an inhibitor staurosporine. Our results showed that PMA significantly repressed SVCV replication and viral-induced apoptosis in Epithelioma papulosum cyprini (EPC) cell, suggesting that PKC may exhibit an anti-SVCV effect. Likewise, PMA resulted in a higher phosphorylation levels of PKCε rather than PKCα/β to participate in the activation of Nrf2, mainly involved in the activation of Nrf2 phosphorylation of Ser40 to favor Nrf2 translocation to nucleus. Furthermore, the data revealed that PMA up-regulated an antiviral response heme oxygenase-1 (HO1) gene expression that was confirmed as the key player against SVCV infection by HO1 specific siRNA. Overall, this study provided a new therapeutic target for the treatment of SVCV infection, and modulating PKC activity could be used for the prevention and treatment of SVCV.

Antiviral effect of 7-(4-benzimidazole-butoxy)-coumarin on rhabdoviral clearance via Nrf2 activation regulated by PKCα/β phosphorylation

Coumarin forms an elite class of naturally occurring compounds that possess promising antiviral therapeutic perspectives. In the previous study, we designed and synthesized a coumarin derivative, 7-(4-benzimidazole-butoxy)-coumarin (BBC), to evaluate its antiviral activity on spring viraemia of carp virus (SVCV). In this study, our results show that BBC does not affect viral adhesion and delivery from endosomes to the cytosol, indicating BBC has no inhibitory activity in the early stage of viral infection. Further data are determined that BBC significantly declines SVCV-infected apoptosis and recovers caspase-3/8/9 activity. To reveal the pathway that affects Nrf2 translocation by BBC, we examine changes in protein kinase C (PKC) in EPC cells treated with BBC. We observe that BBC results in a higher phosphorylation of PKCα/β that is involved in the activation of erythroid 2-related factor 2 (Nrf2) phosphorylation to favor Nrf2 translocation to nucleus at 24 and 48 h. In addition, the results show that BBC also up-regulates both antiviral responses, heme oxygenase-1 (HO-1) expression and cellular IFN response. Overall, this mechanism of action provides a new therapeutic target for the treatment of SVCV infection, and these results suggest that treatment with BBC is effective in reducing SVCV infection and differently regulates SVCV-induced undesirable conditions.

Synthesis and in vitro activities evaluation of arctigenin derivatives against spring viraemia of carp virus

Spring viraemia of carp virus (SVCV) is a viral fish pathogen causing high mortality in several carp species and other cultivated fish. However, robust anti-SVCV drugs currently are extremely scarce. For the purpose of seeking out anti-SVCV drugs, here a total of 35 arctigenin derivatives were designed, synthesized and evaluated for their anti-viral activities. By comparing the inhibitory concentration at half-maximal activity (IC₅₀) of the 15 screened candidate drugs (max inhibitory response surpassing 90%) in epithelioma papulosum cyprini (EPC) cells infected with SVCV, 2Q and 6 A were chosen for additional validation studies, with an IC₅₀ of 0.077 μg/mL and 0.095 μg/mL, respectively. Further experiments revealed that 2Q and 6 A could significantly decrease SVCV-induced apoptosis and have a protective effect on cell morphology at 48 and 72 h post-infection. Moreover, the reactive oxygen species (ROS) induced upon SVCV infection could be obviously inhibited by 2Q and 6 A, while SVCV-infected cells were clearly observed. On account of these findings, 2Q and 6 A could have a promising application for the treatment of infection of SVCV and provide a considerable reference for novel antivirals in aquaculture.

Therapeutic Modulation of Virus-Induced Oxidative Stress via the Nrf2-Dependent Antioxidative Pathway

Virus-induced oxidative stress plays a critical role in the viral life cycle as well as the pathogenesis of viral diseases. In response to reactive oxygen species (ROS) generation by a virus, a host cell activates an antioxidative defense system for its own protection. Particularly, a nuclear factor erythroid 2p45-related factor 2 (Nrf2) pathway works in a front-line for cytoprotection and detoxification. Recently, a series of studies suggested that a group of clinically relevant viruses have the capacity for positive and negative regulations of the Nrf2 pathway. This virus-induced modulation of the host antioxidative response turned out to be a crucial determinant for the progression of several viral diseases. In this review, virus-specific examples of positive and negative modulations of the Nrf2 pathway will be summarized first. Then a number of successful genetic and pharmacological manipulations of the Nrf2 pathway for suppression of the viral replication and the pathogenesis-associated oxidative damage will be discussed later. Understanding of the interplay between virus-induced oxidative stress and antioxidative host response will aid in the discovery of potential antiviral supplements for better management of viral diseases.

Synthesis and antiviral activity of a new coumarin derivative against spring viraemia of carp virus

Coumarin as a lead structure have received a considerable attention in the last three decades for the discovery of antiviral agents. Our previous study indicated that imidazole coumarins possessed antiviral activities against SVCV. Based on the structure-activity relationship in that study, a new imidazole coumarin derivative, 7-(4-benzimidazole-butoxy)-coumarin (BBC), was designed, synthesized and its anti-SVCV activity was evaluated. By comparing inhibitory concentration at half-maximal activity (IC₅₀), we found that BBC (IC₅₀ = 0.56 mg/L) possessed a higher antiviral activity than those imidazole coumarins in our previous study. Besides, BBC can significantly inhibit cell death and reduce cellular morphological damage induced by SVCV. Our further data indicated that intraperitoneal injection of BBC increased the survival rate of zebrafish by 17.5%, decreased viral titer in fish body and inhibited SVCV glycoprotein expression in kidney and spleen. In uninfected zebrafish, the expression levels of ifnγ, ifnφ1, ifnφ2 and rig1 genes were up-regulated after BBC treatment, which indicated that BBC could activate interferon response. In addition, data of the antioxidant enzymes activities and results of the antioxidant enzymes-related genes expressions suggested BBC could reduce SVCV-induced oxidative damage in infected zebrafish. Altogether, BBC is expected to be a therapeutic agent against SVCV infection in the field of aquaculture.

Heme Oxygenase-1 inhibits spring viremia of carp virus replication through carbon monoxide mediated cyclic GMP/Protein kinase G signaling pathway

Spring viremia of carp virus (SVCV) is the etiological agent of spring viremia of carp (SVC) and causes mass mortality in common carp (Cyprinus carpio). Currently, no effective treatments or commercial vaccines against SVCV are available. Heme oxygenase-1 (HO-1), an enzyme that catalyzes the degradation of heme to produce carbon monoxide (CO), biliverdin and ferrous iron (Fe²⁺), exerts anti-oxidant, antiinflammatory and anti-apoptotic properties. Previous studies demonstrated that nuclear factor-erythroid 2 related factor 2 (Nrf2) functions as an important upstream regulator of HO-1 and exhibits robust activity against SVCV infection. In this study, we further examined the antiviral activity of HO-1 against SVCV infection. The elevated expression of HO-1 was induced upon cobalt protoporphyrin (CoPP) treatment in EPC cells without affecting cell viability and thus inhibited SVCV replication in a dose dependent manner. Knocking down of HO-1 rescued SVCV replication. Thereby, the antiviral activity of ROS/Nrf2/HO-1 axis was confirmed in EPC cells. Furthermore, HO-1 enzymatic products CO, but not biliverdin, markedly inhibited SVCV replication via the activation of cyclic GMP/protein kinase G signaling pathway. Collectively, these findings suggest potential drug or therapy that induced the Nrf2/HO-1/CO/cGMP/PKG signaling pathway as a promising strategy for treating SVC.

SVCV impairs mitochondria complex Ⅲ resulting in accumulation of hydrogen peroxide

Spring viraemia of carp virus (SVCV) is a deadly pathogen of common carp. SVCV infection is found to be associated with excess reactive oxygen species (ROS) generation and induces oxidative stress in EPC and FHM cells, which contributes to its pathogenesis. In this study, ROS production and mitochondria function as well as antioxidant enzymes in mitochondria were investigated during SVCV infection in EPC cells. Dysfunction of mitochondria and inactivation of mitochondria electron transport chain complex Ⅲ to augment O₂^-∙ and H₂O₂ accumulation were observed in SVCV infected EPC cells. Treatment of Antimycin A reduced the activity of mitochondria complex Ⅲ in EPC cells, which also inhibited the transcription of SVCV glycoprotein gene (SVCV-G) and production of SVCV. Our studies explain the production of ROS following SVCV infection and also suggest that integrate mitochondrial function is important for SVCV infection.

Establishment of a cell line from the kidney of black carp and its susceptibility to spring viremia of carp virus

In this study, we established and characterized a cell line derived from the kidney of black carp (Mylopharyngodon piceus), which is an important freshwater aquaculture species. The cell line was designated as MPK and subcultured for more than 70 passages in DMEM medium containing 10% fetal bovine serum (FBS) at 28°C. MPK had a modal diploid chromosome number of 48. Moreover, a transient MPK transfection efficiency was up to 18% using a green fluorescent protein plasmid by a modified electroporation. In addition, the MPK cells showed susceptibility to spring viremia of carp virus (SVCV), as demonstrated by the presence of severe cytopathic effects (CPEs) and increased viral RNA. Unexpectedly, the MPK cells expressed pluripotency-associated genes such as nanog, oct4 and vasa, indicating that these are possibly adult stem cells. Taken together, we have established a stable cell line from kidney that may potentially be utilized as an in vitro platform for genetic modifications and host-pathogen analysis in black carp.

Evaluation on the antiviral activity of arctigenin against spring viraemia of carp virus

Spring viraemia of carp virus (SVCV) causes high morality in several economically important cyprinid fishes, but there is no approved therapy up to now. To address the urgent need for therapeutics to combat SVCV infection, we investigated the anti-SVCV activities of 12 natural compounds and 7 common antiviral agents using epithelioma papulosum cyprini (EPC) cells in this study. From the 19 compounds, we identified arctigenin (ARG) has the highest inhibition on SVCV replication, with maximum inhibitory percentage on SVCV > 90%. And the 48 h half maximal inhibitory concentrations (IC₅₀) of ARG on SVCV glycoprotein and nucleoprotein were 0.29 (0.22-0.39) and 0.35 (0.29-0.41) mg/L respectively. In addition, ARG significantly reduced SVCV-induced apoptosis and recovered SVCV-activated caspase-3/8/9 activity. Further, cellular morphological damage induced by SVCV was blocked by ARG treatment. Mechanistically, ARG did not affect SVCV infectivity. Moreover, ARG could not induce reactive oxygen species (ROS) generation, which plays an antiviral role on SVCV. Interestingly, SVCV-induced autophagy which is necessary for virus replication was inhibited by ARG treatment. These results indicated that the inhibition of ARG on SVCV replication was, at least in part, via blocking SVCV-induced autophagy. Taken together, ARG has the potential to work as an agent for protecting economically important fishes against SVCV.

Identification of diverse defense mechanisms in rainbow trout red blood cells in response to halted replication of VHS virus

Background: It has been described that fish nucleated red blood cells (RBCs) generate a wide variety of immune-related gene transcripts when viruses highly replicate inside them and are their main target cell. The immune response and mechanisms of fish RBCs against viruses targeting other cells or tissues has not yet been explored and is the objective of our study. Methods: Rainbow trout RBCs were obtained from peripheral blood, ficoll purified and exposed to Viral Haemorrhagic Septicaemia virus (VHSV). Immune response was evaluated by means of RT-qPCR, flow cytometry, immunofluorescence and isobaric tag for relative and absolute quantification (iTRAQ) protein profiling. Results: VHSV N gene transcripts incremented early postexposure and were drastically decreased after 6 hours postexposure (hpe). The expression of type I interferon ( ifn1) gene was significantly downregulated at early postexposure (3 hpe), together with a gradual downregulation of interferon-inducible mx and pkr genes until 72 hpe. Type I IFN protein was downregulated and interferon-inducible Mx protein was maintained at basal levels. Co-culture assays of RBCs, previously exposed to UV-inactivated VHSV, and TSS (stromal cell line from spleen) revealed IFN crosstalk between both cell types. On the other hand, anti-microbial peptide β-defensin 1 and neutrophil chemotactic factor interleukin 8 were slightly upregulated in VHSV-exposed RBCs. iTRAQ profiling revealed that VHSV exposure can induce a global protein downregulation in rainbow trout RBCs, mainly related to RNA stability and proteasome pathways. Antioxidant/antiviral response is also suggested to be involved in the response of rainbow trout RBCs to VHSV. Conclusions: A variety of mechanisms are proposed to be implicated in the antiviral response of rainbow trout RBCs against VHSV halted infection. Ongoing research is focused on understanding the mechanisms in detail.

Identification of diverse defense mechanisms in trout red blood cells in response to VHSV halted viral replication

Background: It has been described that fish nucleated red blood cells (RBCs) generate a wide variety of immune-related gene transcripts when viruses highly replicate inside them and are their main target cell. The immune response and mechanisms of fish RBCs against viruses targeting other cells or tissues has not yet been explored and is the objective of our study. Methods: Trout RBCs were obtained from peripheral blood, ficoll purified and exposed to Viral Haemorrhagic Septicaemia virus (VHSV). Immune response was evaluated by means of RT-qPCR, flow cytometry, immunofluorescence and isobaric tag for relative and absolute quantification (iTRAQ) protein profiling Results: VHSV N gene transcripts incremented early postexposure and were drastically decreased after 6 hours postexposure (hpe). The expression of the type I interferon ( ifn1) gene was significantly downregulated at early postexposure (3 hpe), together with a gradual downregulation of interferon-inducible mx and pkr genes until 72 hpe. Type I IFN protein was downregulated and interferon-inducible Mx protein was maintained at basal levels. Co-culture assays of RBCs with TSS (stromal cell line from spleen) revealed the IFN crosstalk between both cell types. On the other hand, anti-microbial peptide β-defensin 1 and neutrophil chemotactic factor interleukin 8 were slightly upregulated in VHSV-exposed RBCs Isobaric tag for relative and absolute quantification (iTRAQ) revealed that VHSV exposure can induce a global protein downregulation in trout RBCs, mainly related to RNA stability and proteasome pathways. The antioxidant/antiviral response is also suggested to be involved in the response of trout RBCs to VHSV. Conclusions: A variety of mechanisms are proposed to be implicated in the antiviral response of trout RBCs against VHSV halted infection. Ongoing research is focused on understanding the mechanisms in detail. To our knowledge, this is the first report that implicates fish RBCs in the antiviral response against viruses not targeting RBCs.

The replication of spring viraemia of carp virus can be regulated by reactive oxygen species and NF-κB pathway

Different viruses could induced ROS generation to alter intracellular redox state in the host cells, and unbalanced redox state was suggested to have various effects on viral replication. In this study, we investigated the influence of reactive oxygen species (ROS) on replication of spring viraemia of carp virus (SVCV) in fish cells. After SVCV infection, there existed a time-dependent increase in ROS generation. The present results revealed that antioxidant N-acetyl-l-cysteine (NAC) resulted in a lower ROS levels and increased SVCV replication in EPC cell. In contrast, a GSH synthesis inhibitor buthionine sulfoximine (BSO) induced ROS generation and decreased SVCV replication. In addition, activation of NF-κB suppressed SVCV replication by using two inhibitors of cytokine-induced IκBα phosphorylation. More importantly, enhancement of the activity of NF-κB was found in BSO treatment, which indicated that dropped SVCV replication likely occurred via ROS activation of NF-κB. Overall, our results revealed that the SVCV infection and replication could generate ROS and be affected by the redox state, where this progression was associated with the alteration in NF-κB pathway induced by oxidative stress.

Evaluation on antiviral activity of coumarin derivatives against spring viraemia of carp virus in epithelioma papulosum cyprini cells

As one of the most serious pathogens in the freshwater aquatic environment, spring viraemia of carp virus (SVCV) induces a high mortality rate in several cyprinid fishes. In this study, we designed and synthesized a total of 44 coumarin derivatives to evaluate the anti-SVCV activity. By comparing the inhibitory concentration at half-maximal activity (IC₅₀), two imidazole coumarins (B4 and C2) were selected, with maximum inhibitory rates on SVCV more than 90%. Mechanistically, B4 or C2 did not affect viral adhesion and delivery from endosomes to the cytosol. Further, B4 and C2 could decline the apoptosis in SVCV-infected cells and the viral activated caspase-3, 8, 9 activities. Other results showed that SVCV induced the cytoskeletal structure to be a circumferential ring of microtubules near the nucleus, with occurring a disrupted microfilament organization. In comparison, cytoskeleton structure in drug-treated cells kept complete. In addition, the cellular microstructure in drug treatments showed no significant change; while SVCV-infected cells were seriously shrunk, and observed typical apoptotic features including cell shrinkage, volume reduction and cell blebbing. More importantly, B4 and C2 enhanced anti-oxidative enzyme gene expression and triggered the Nrf-2 pathway to keep balance of intracellular redox state. Therefore, the use of two imidazole coumarins (B4 and C2) could be a viable way of preventing and controlling SVCV infection.