Antiviral Effect of Isoquercitrin against Influenza A Viral Infection via Modulating Hemagglutinin and Neuraminidase

Isoquercitrin (IQC) is a component abundantly present in many plants and is known to have an anti-viral effect against various viruses. In this study, we demonstrate that IQC exhibits strong anti-influenza A virus infection, and its effect is closely related to the suppression of hemagglutinin (HA) and neuraminidase (NA) activities. We used green fluorescent protein-tagged Influenza A/PR/8/34 (H1N1), A/PR/8/34 (H1N1), and HBPV-VR-32 (H3N2) to evaluate the anti-IAV effect of IQC. The fluorescence microscopy and fluorescence-activated cell sorting analysis showed that IQC significantly decreases the levels of GFP expressed by IAV infection, dose-dependently. Consistent with that, IQC inhibited cytopathic effects by H1N1 or H3N2 IAV infection. Immunofluorescence analysis confirmed that IQC represses the IAV protein expression. Time-of-addition assay showed that IQC inhibits viral attachment and entry and exerts a strong virucidal effect during IAV infection. Hemagglutination assay confirmed that IQC affects IAV HA. Further, IQC potently reduced the NA activities of H1N1 and H3N2 IAV. Collectively, IQC prevents IAV infection at multi-stages via virucidal effects, inhibiting attachment, entry and viral release. Our results indicate that IQC could be developed as a potent antiviral drug to protect against influenza viral infection.

Cellular Protein Phosphatase 2A Regulates Cell Survival Mechanisms in Influenza A Virus Infection

Influenza A viruses (IAVs) are respiratory pathogens that are able to hijack multiple cellular mechanisms to drive their replication. Consequently, several viral and cellular proteins undergo posttranslational modifications such as dynamic phosphorylation/dephosphorylation. In eukaryotic cells, dephosphorylation is mainly catalyzed by protein phosphatase 2A (PP2A). While the function of kinases in IAV infection is quite well studied, only little is known about the role of PP2A in IAV replication. Here, we show, by using knockdown and inhibition approaches of the catalytic subunit PP2Ac, that this phosphatase is important for efficient replication of several IAV subtypes. This could neither be attributed to alterations in the antiviral immune response nor to changes in transcription or translation of viral genes. Interestingly, decreased PP2Ac levels resulted in a significantly reduced cell viability after IAV infection. Comprehensive kinase activity profiling identified an enrichment of process networks related to apoptosis and indicated a synergistic action of hyper-activated PI3K/Akt, MAPK/JAK-STAT and NF-kB signaling pathways, collectively resulting in increased cell death. Taken together, while IAV seems to effectively tap leftover PP2A activity to ensure efficient viral replication, reduced PP2Ac levels fail to orchestrate cell survival mechanisms to protect infected cells from early cell death.

Isorhamnetin inhibits amplification of influenza A H1N1 virus inflammation mediated by interferon via the RIG-I/JNK pathway

Background

Interferon (IFN) amplifies the influenza virus-mediated inflammatory response by forming a paracrine signal feedback loop, which is considered an important cause of excessive inflammatory damage. Isorhamnetin has a wide spectrum of beneficial pharmacological properties, including anti-inflammatory and antiviral effects. The regulatory effect and mechanism of isorhamnetin on influenza virus-mediated inflammation have not yet been reported.

Methods

We pre-treated A549 cells with IFN-β (50 ng/mL) for 4 h followed by IAV (H1N1) infection to simulate the inflammation amplification effect caused by the paracrine effect of IFN-β. The anti-inflammation activity of isorhamnetin against amplification inflammation of interferon mediated by IAV (H1N1) was assessed by performing quantitative real-time polymerase chain reaction (qRT-PCR), western blotting, and enzyme-linked immunosorbent assay (ELISA) in A549 cells.

Results

Compared with the virus infection group, the IFN-β pretreatment virus infection group had an upregulated level of pro-inflammatory cytokine expression, which was inhibited by isorhamnetin significantly via the retinoic acid-induced gene I (RIG-I)/c-Jun N-terminal kinase (JNK) signaling pathway. Molecular docking studies further verified that isorhamnetin can interact with JNK.

Conclusions

Our work was the first to demonstrate the anti-inflammatory activity and mechanism of isorhamnetin during influenza virus infection. Isorhamnetin significantly improves the excessive inflammatory response mediated by IAV (H1N1) infection mainly via the RIG-I/JNK pathway. Additionally, isorhamnetin exhibited an apparent antiviral effect of H1N1 in vitro.

In Vitro Inhibition of Influenza Virus Using CRISPR/Cas13a in Chicken Cells

Advances in the field of CRISPR/Cas systems are expanding our ability to modulate cellular genomes and transcriptomes precisely and efficiently. Here, we assessed the Cas13a-mediated targeted disruption of RNA in chicken fibroblast DF1 cells. First, we developed a Tol2 transposon vector carrying the Cas13a-msGFP-NLS (pT-Cas13a) transgene, followed by a stable insertion of the Cas13a transgene into the genome of DF1 cells to generate stable DF1-Cas13a cells. To assess the Cas13a-mediated functional knockdown, DF1-Cas13a cells were transfected with the combination of a plasmid encoding DsRed coding sequence (pDsRed) and DsRed-specific crRNA (crRNA-DsRed) or non-specific crRNA (crRNA-NS). Fluorescence-activated cell sorting (FACS) and a microscopy analysis showed reduced levels of DsRed expression in cells transfected with crRNA-DsRed but not in crRNA-NS, confirming a sequence-specific Cas13a mediated mRNA knockdown. Next, we designed four crRNAs (crRNA-IAV) against the PB1, NP and M genes of influenza A virus (IAV) and cloned in tandem to express from a single vector. DF1-Cas13a cells were transfected with plasmids encoding the crRNA-IAV or crRNA-NS, followed by infection with WSN or PR8 IAV. DF1 cells transfected with crRNA-IAV showed reduced levels of viral titers compared to cells transfected with crRNA-NS. These results demonstrate the potential of Cas13a as an antiviral strategy against highly pathogenic strains of IAV in chickens.

The Role of Lipid Metabolism in Influenza A Virus Infection

Influenza A virus (IAV) is an important zoonotic pathogen that can cause disease in animals such as poultry and pigs, and it can cause infection and even death in humans, posing a serious threat to public health. IAV is an enveloped virus that relies on host cell metabolic systems, especially lipid metabolism systems, to complete its life cycle in host cells. On the other side, host cells regulate their metabolic processes to prevent IAV replication and maintain their normal physiological functions. This review summarizes the roles of fatty acid, cholesterol, phospholipid and glycolipid metabolism in IAV infection, proposes future research challenges, and looks forward to the prospective application of lipid metabolism modification to limit IAV infection, which will provide new directions for the development of anti-influenza drugs.

Anti-Influenza Virus Activity and Chemical Components from the Parasitic Plant Cuscuta japonica Choisy on Dimocarpus longans Lour

Dodder (Cuscuta spp.) is a parasitic weed damaging many plants and agricultural production. The native obligate parasite Cuscuta japonica Choisy (Japanese dodder) parasitizes Dimocarpus longans Lour., Ficus septica Burm. F., Ficus microcarpa L.f., Mikania micrantha H.B.K. and Melia azedarach Linn, respectively. Five Japanese dodders growing on different plants exhibit slightly different metabolites and amounts which present different pharmacological effects. Among these plants, a significant antiviral activity against influenza A virus (IAV) was found in Japanese dodder parasitizing on D. longans Lour. (CL). To further explore methanol extract components in Japanese dodder (CL), four undescribed aromatic glycosides, cuscutasides A–D (compounds 1–4) were isolated, together with twenty-six known compounds 5–30. The chemical structures of 1–4 were elucidated using a combination of spectroscopic techniques. The eighteen isolated compounds were evaluated for antiviral activity against IAV activity. Among them, 1-monopalmitin (29) displayed potent activity against influenza A virus (A/WSN/1933(H1N1)) with EC₅₀ 2.28 ± 0.04 μM and without noteworthy cytotoxicity in MDCK cells. The interrupt step of 29 on the IAV life cycle was determined. These data provide invaluable information for new applications for this otherwise harmful weed.

(-)-Epigallocatechin-3-gallate induces interferon-λ2 expression to anti-influenza A virus in human bronchial epithelial cells (BEAS-2B) through p38 MAPK signaling pathway

Background

(-)-Epigallocatechin-3-gallate (EGCG), a major component of green tea, has been found to inhibit the influenza virus. However, the mechanism of EGCG anti-influenza virus effect needs to be further explored.

Methods

BEAS-2B cells were treated with different concentrations of EGCG or were treated with EGCG for different times. CCK8 assay was used to detect the cell viability, and quantitative real time polymerase chain reaction (qRT-PCR) and enzyme-linked immunosorbent assay were employed to measure the interferon (IFN)-λ2 mRNA and protein expression levels. The phospho-p38 mitogen-activated protein kinase (P-p38 MAPK), phospho-extracellular signal-regulated kinase (P-ERK), and phospho-c-Jun N-terminal kinase (P-JNK) expression were tested by western blot. Then, p38 MAPK, ERK, and JNK inhibitor were used to study the effect of p38 MAPK, ERK, and JNK signaling pathways on IFN-λ2 expression. The BEAS-2B cells were treated with EGCG, EGCG and IFN λ2 neutralizing antibody or control antibody for 12 h, and were infected with influenza A virus (IAV) (H1N1) for 1 h. After 12 h, nucleoprotein (NP) mRNA and protein expression levels of H1N1 were assessed by qRT-PCR and western blot.

Results

The IFN-λ2 mRNA and protein expression levels in BEAS-2B cells were up-regulated after EGCG (treatment in time- and dose-dependent manners the concentration range from 0 to 50 µg/mL had no cytotoxicity). Meanwhile, the P-p38 MAPK, P-ERK, and P-JNK expression levels were up-regulated. IFN-λ2 mRNA and protein expression was inhibited after p38 MAPK inhibitor pre-treatment, but not by ERK and JNK inhibitors. Furthermore, the expression of H1N1 NP gene and protein decreased after EGCG pre-treatment, while IFN-λ2 neutralizing antibody attenuated the effect of EGCG inhibiting the expression of H1N1 NP gene and protein.

Conclusions

EGCG inhibited IAV H1N1 by inducing the expression of IFN-λ2 in BEAS-2B cells through the p38 MAPK signaling pathway.

An Oleanolic Acid Derivative Inhibits Hemagglutinin-Mediated Entry of Influenza A Virus

Influenza A viruses (IAV) have been a major public health threat worldwide, and options for antiviral therapy become increasingly limited with the emergence of drug-resisting virus strains. New and effective anti-IAV drugs, especially for highly pathogenic influenza, with different modes of action, are urgently needed. The influenza virus glycoprotein hemagglutinin (HA) plays critical roles in the early stage of virus infection, including receptor binding and membrane fusion, making it a potential target for the development of anti-influenza drugs. In this study, we show that OA-10, a newly synthesized triterpene out of 11 oleanane-type derivatives, exhibited significant antiviral activity against four different subtypes of IAV (H1N1, H5N1, H9N2 and H3N2) replications in A549 cell cultures with EC₅₀ ranging from 6.7 to 19.6 μM and a negligible cytotoxicity (CC₅₀ > 640 μM). It inhibited acid-induced hemolysis in a dose-dependent manner, with an IC₅₀ of 26 µM, and had a weak inhibition on the adsorption of H5 HA to chicken erythrocytes at higher concentrations (≥40 µM). Surface plasmon resonance (SPR) analysis showed that OA-10 interacted with HA in a dose-dependent manner with the equilibrium dissociation constants (KD) of the interaction of 2.98 × 10^-12 M. Computer-aided molecular docking analysis suggested that OA-10 might bind to the cavity in HA stem region which is known to undergo significant rearrangement during membrane fusion. Our results demonstrate that OA-10 inhibits H5N1 IAV replication mainly by blocking the conformational changes of HA2 subunit required for virus fusion with endosomal membrane. These findings suggest that OA-10 could serve as a lead for further development of novel virus entry inhibitors to prevent and treat IAV infections.

Functional Characterization and Direct Comparison of Influenza A, B, C, and D NS1 Proteins in vitro and in vivo

Influenza viruses are important pathogens that affect multiple animal species, including humans. There are four types of influenza viruses: A, B, C, and D (IAV, IBV, ICV, and IDV, respectively). IAV and IBV are currently circulating in humans and are responsible of seasonal epidemics (IAV and IBV) and occasional pandemics (IAV). ICV is known to cause mild infections in humans and pigs, while the recently identified IDV primarily affect cattle and pigs. Influenza non-structural protein 1 (NS1) is a multifunctional protein encoded by the NS segment in all influenza types. The main function of NS1 is to counteract the host antiviral defense, including the production of interferon (IFN) and IFN-stimulated genes (ISGs), and therefore is considered an important viral pathogenic factor. Despite of homologous functions, the NS1 protein from the diverse influenza types share little amino acid sequence identity, suggesting possible differences in their mechanism(s) of action, interaction(s) with host factors, and contribution to viral replication and/or pathogenesis. In addition, although the NS1 protein of IAV, IBV and, to some extent ICV, have been previously studied, it is unclear if IDV NS1 has similar properties. Using an approach that allow us to express NS1 independently of the nuclear export protein from the viral NS segment, we have generated recombinant IAV expressing IAV, IBV, ICV, and IDV NS1 proteins. Although recombinant viruses expressing heterotypic (IBV, ICV, and IDV) NS1 proteins were able to replicate similarly in canine MDCK cells, their viral fitness was impaired in human A549 cells and they were highly attenuated in vivo. Our data suggest that despite the similarities to effectively counteract innate immune responses in vitro, the NS1 proteins of IBV, ICV, or IDV do not fully complement the functions of IAV NS1, resulting in deficient viral replication and pathogenesis in vivo.

Cellular Proteostasis During Influenza A Virus Infection-Friend or Foe?

In order to efficiently replicate, viruses require precise interactions with host components and often hijack the host cellular machinery for their own benefit. Several mechanisms involved in protein synthesis and processing are strongly affected and manipulated by viral infections. A better understanding of the interplay between viruses and their host-cell machinery will likely contribute to the development of novel antiviral strategies. Here, we discuss the current knowledge on the interactions between influenza A virus (IAV), the causative agent for most of the annual respiratory epidemics in humans, and the host cellular proteostasis machinery during infection. We focus on the manipulative capacity of this virus to usurp the cellular protein processing mechanisms and further review the protein quality control mechanisms in the cytosol and in the endoplasmic reticulum that are affected by this virus.

Emerging Influenza D Virus Threat: What We Know so Far!

Influenza viruses, since time immemorial, have been the major respiratory pathogen known to infect a wide variety of animals, birds and reptiles with established lineages. They belong to the family Orthomyxoviridae and cause acute respiratory illness often during local outbreaks or seasonal epidemics and occasionally during pandemics. Recent studies have identified a new genus within the Orthomyxoviridae family. This newly identified pathogen, D/swine/Oklahoma/1334/2011 (D/OK), first identified in pigs with influenza-like illness was classified as the influenza D virus (IDV) which is distantly related to the previously characterized human influenza C virus. Several other back-to-back studies soon suggested cattle as the natural reservoir and possible involvement of IDV in the bovine respiratory disease complex was established. Not much is known about its likelihood to cause disease in humans, but it definitely poses a potential threat as an emerging pathogen in cattle-workers. Here, we review the evolution, epidemiology, virology and pathobiology of influenza D virus and the possibility of transmission among various hosts and potential to cause human disease.

Development and application of bioluminescence imaging for the influenza A virus

Influenza A viruses (IAVs) cause seasonal epidemics and intermittent pandemics which threaten human health. Conventional assays cannot meet the demands for rapid and sensitive detection of viral spread and pathogenesis in real time cannot be used for high-throughput screens of novel antivirals. Bioluminescence imaging (BLI) has emerged as a powerful tool in the study of infectious diseases in animal models. The advent of influenza reverse genetics has enabled the incorporation of bioluminescent reporter proteins into replication-competent IAVs. This review briefly describes the current development and applications of bioluminescence in the study of viral infections and antiviral therapeutics for IAVs. BLI is expected to substantially accelerate the basic and applied research of IAV both in vitro and in vivo.