Emerging cellular targets for influenza antiviral agents

KCNE4 is a crucial host factor for Orf virus infection by mediating viral entry

The orf virus (ORFV) poses a serious threat to the health of domestic small ruminants (i.e., sheep and goats) and humans on a global scale, causing around $150 million in annual losses to livestock industry. However, the host factors involved in ORFV infection and replication are still elusive. In this study, we compared the RNA-seq profiles of ORFV-infected or non-infected sheep testicular interstitial cells (STICs) and identified a novel host gene, potassium voltage-gated channel subfamily E member 4 (KCNE4), as a key host factor involved in the ORFV infection. Both RNA-seq data and RT-qPCR assay revealed a significant increase in the expression of KCNE4 in the infected STICs from 9 to 48 h post infection (hpi). On the other hand, the RT-qPCR assay detected a decrease in ORFV copy number in both the STICs transfected by KCNE4 siRNA and the KCNE4 knockout (KO) HeLa cells after the ORFV infection, together with a reduced fluorescence ratio of ORFV-GFP in the KO HeLa cells at 24 hpi, indicating KCNE4 to be critical for the ORFV infection. Furthermore, the attachment and internalization assays showed decreased ORFV attachment, internalization, replication, and release by the KO HeLa cells, demonstrating a potential inhibition of ORFV entry into the cells by KCNE4. Pretreatment with the KCNE4 inhibitors such as quinidine and fluoxetine significantly repressed the ORFV infection. All our findings reveal KCNE4 as a novel host regulator of the ORFV entry and replication, shedding new insight into the interactive mechanism of ORFV infection. The study also highlights the K+ channels as possible druggable targets to impede viral infection and disease.

Unravelling the interaction between Influenza virus and the nuclear pore complex: insights into viral replication and host immune response

Influenza viruses are known to cause severe respiratory infections in humans, often associated with significant morbidity and mortality rates. Virus replication relies on various host factors and pathways, which also determine the virus's infectious potential. Nonetheless, achieving a comprehensive understanding of how the virus interacts with host cellular components is essential for developing effective therapeutic strategies. One of the key components among host factors, the nuclear pore complex (NPC), profoundly affects both the Influenza virus life cycle and the host's antiviral defenses. Serving as the sole gateway connecting the cytoplasm and nucleoplasm, the NPC plays a vital role as a mediator in nucleocytoplasmic trafficking. Upon infection, the virus hijacks and alters the nuclear pore complex and the nuclear receptors. This enables the virus to infiltrate the nucleus and promotes the movement of viral components between the nucleus and cytoplasm. While the nucleus and cytoplasm play pivotal roles in cellular functions, the nuclear pore complex serves as a crucial component in the host's innate immune system, acting as a defense mechanism against virus infection. This review provides a comprehensive overview of the intricate relationship between the Influenza virus and the nuclear pore complex. Furthermore, we emphasize their mutual influence on viral replication and the host's immune responses.

Outlining recent updates on influenza therapeutics and vaccines: A comprehensive review

Influenza virus has presented a considerable healthcare challenge during the past years, particularly in vulnerable groups with compromised immune systems. Therapeutics and vaccination have always been in research annals since the spread of influenza. Efforts have been going on to develop an antiviral therapeutic approach that could assist in better disease management and reduce the overall disease complexity, resistance development, and fatality rates. On the other hand, vaccination presents a chance for effective, long-term, cost-benefit, and preventive response against the morbidity and mortality associated with the influenza. However, the issues of resistance development, strain mutation, antigenic variability, and inability to cure wide-spectrum and large-scale strains of the virus by available vaccines remain there. The article gathers the updated data for the therapeutics and available influenza vaccines, their mechanism of action, shortcomings, and trials under clinical experimentation. A methodological approach has been adopted to identify the prospective therapeutics and available vaccines approved and within the clinical trials against the influenza virus. Review contains influenza therapeutics, including traditional and novel antiviral drugs and inhibitor therapies against influenza virus as well as research trials based on newer drug combinations and latest technologies such as nanotechnology and organic and plant-based natural products. Most recent development of influenza vaccine has been discussed including some updates on traditional vaccination protocols and discussion on next-generation and upgraded novel technologies. This review will help the readers to understand the righteous approach for dealing with influenza virus infection and for deducing futuristic approaches for novel therapeutic and vaccine trials against Influenza.

Comparing the Effects of Rocaglates on Energy Metabolism and Immune Modulation on Cells of the Human Immune System

A promising new approach to broad spectrum antiviral drugs is the inhibition of the eukaryotic translation initiation factor 4A (elF4A), a DEAD-box RNA helicase that effectively reduces the replication of several pathogenic virus types. Beside the antipathogenic effect, modulation of a host enzyme activity could also have an impact on the immune system. Therefore, we performed a comprehensive study on the influence of elF4A inhibition with natural and synthetic rocaglates on various immune cells. The effect of the rocaglates zotatifin, silvestrol and CR-31-B (−), as well as the nonactive enantiomer CR-31-B (+), on the expression of surface markers, release of cytokines, proliferation, inflammatory mediators and metabolic activity in primary human monocyte-derived macrophages (MdMs), monocyte-derived dendritic cells (MdDCs), T cells and B cells was assessed. The inhibition of elF4A reduced the inflammatory potential and energy metabolism of M1 MdMs, whereas in M2 MdMs, drug-specific and less target-specific effects were observed. Rocaglate treatment also reduced the inflammatory potential of activated MdDCs by altering cytokine release. In T cells, the inhibition of elF4A impaired their activation by reducing the proliferation rate, expression of CD25 and cytokine release. The inhibition of elF4A further reduced B-cell proliferation, plasma cell formation and the release of immune globulins. In conclusion, the inhibition of the elF4A RNA helicase with rocaglates suppressed the function of M1 MdMs, MdDCs, T cells and B cells. This suggests that rocaglates, while inhibiting viral replication, may also suppress bystander tissue injury by the host immune system. Thus, dosing of rocaglates would need to be adjusted to prevent excessive immune suppression without reducing their antiviral activity.

B-cell lymphoma-2 family proteins-activated proteases as potential therapeutic targets for influenza A virus and severe acute respiratory syndrome coronavirus-2: Killing two birds with one stone?

The COVID-19 pandemic caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has led to a global health emergency. There are many similarities between SARS-CoV-2 and influenza A virus (IAV); both are single-stranded RNA viruses infecting airway epithelial cells and have similar modes of replication and transmission. Like IAVs, SARS-CoV-2 infections poses serious challenges due to the lack of effective therapeutic interventions, frequent appearances of new strains of the virus, and development of drug resistance. New approaches to control these infectious agents may stem from cellular factors or pathways that directly or indirectly interact with viral proteins to enhance or inhibit virus replication. One of the emerging concepts is that host cellular factors and pathways are required for maintaining viral genome integrity, which is essential for viral replication. Although IAVs have been studied for several years and many cellular proteins involved in their replication and pathogenesis have been identified, very little is known about how SARS-CoV-2 hijacks host cellular proteins to promote their replication. IAV induces apoptotic cell death, mediated by the B-cell lymphoma-2 (Bcl-2) family proteins in infected epithelia, and the pro-apoptotic members of this family promotes viral replication by activating host cell proteases. This review compares the life cycle and mode of replication of IAV and SARS-CoV-2 and examines the potential roles of host cellular proteins, belonging to the Bcl-2 family, in SARS-CoV-2 replication to provide future research directions.

Axin1: A novel scaffold protein joins the antiviral network of interferon

Acute respiratory infection by influenza virus is a persistent and pervasive public health problem. Antiviral innate immunity initiated by type I interferon (IFN) is the first responder to pathogen invasion and provides the first line of defense. We discovered that Axin1, a scaffold protein, was reduced during influenza virus infection. We also found that overexpression of Axin1 and the chemical stabilizer of Axin1, XAV939, reduced influenza virus replication in lung epithelial cells. This effect was also observed with respiratory syncytial virus and vesicular stomatitis virus. Axin1 boosted type I IFN response to influenza virus infection and activated JNK/c-Jun and Smad3 signaling. XAV939 protected mice from influenza virus infection. Thus, our studies provide new mechanistic insights into the regulation of the type I IFN response and present a new potential therapeutic of targeting Axin1 against influenza virus infection.

Characterization and functional analysis of GSK3β from Epinephelus coioides in Singapore grouper iridovirus infection

Glycogen synthase kinase 3β (GSK3β), a serine/threonine protein kinase, is a crucial regulator of several signaling pathways and plays a vital role in cell proliferation, growth, apoptosis, and immune responses. However, the role of GSK3β during viral infection in teleosts remains largely unknown. In the present study, a GSK3β homologue from Epinephelus coioides (EcGSK3β) was cloned and characterized. The open reading frame of EcGSK3β consists of 1323 bp, encoding a 440 amino acid protein, with a predicted molecular mass of 48.23 kDa. Similar to its mammalian counterpart, EcGSK3β contains an S_TKc domain. EcGSK3β shares 99.77% homology with the giant grouper (Epinephelus lanceolatus). Quantitative real-time PCR analysis indicated that EcGSK3β mRNA was broadly expressed in all tested tissues, with abundant expression in the skin, blood, and intestines. Additionally, the expression of EcGSK3β increased after Singapore grouper iridovirus (SGIV) infection in grouper spleen (GS) cells. Intracellular localization analysis demonstrated that EcGSK3β is mainly distributed in the cytoplasm. EcGSK3β overexpression promoted SGIV replication during viral infection in vitro. In contrast, silencing of EcGSK3β inhibited SGIV replication. EcGSK3β significantly downregulated the activities of interferon-β, interferon-sensitive response element, and NF-κB. Taken together, these findings are important for a better understanding of the function of GSK3β in fish and reveal its involvement in the host response to viral immune challenge.

In Vitro Safety, Off-Target and Bioavailability Profile of the Antiviral Compound Silvestrol

We characterized the in vitro safety and bioavailability profile of silvestrol, a compound effective against various viruses, such as corona- and Ebolaviruses, with an EC₅₀ value of about 5 nM. The cytotoxic profile of silvestrol was assessed in various cancer cell lines, as well as the mutagenic and genotoxic potential with Ames and micronuclei tests, respectively. To identify off-target effects, we investigated whether silvestrol modulates G-protein coupled receptor (GPCR) signaling pathways. To predict the bioavailability of silvestrol, its stability, permeability and cellular uptake were determined. Silvestrol reduced viability in a cell-type-dependent manner, mediated no off-target effects via GPCRs, had no mutagenic potential and minor genotoxic effects at 50 nM. Silvestrol did not disturb cell barrier integrity, showed low membrane permeability, was stable in liver microsomes and exhibited good cellular uptake. Efficient cellular uptake and increased cytotoxicity were observed in cell lines with a low expression level of the transport protein P-glycoprotein, the known efflux transporter of silvestrol. In conclusion, silvestrol showed low permeability but good cellular uptake and high stability. Cell-type-dependent cytotoxicity seems to be caused by the accumulation of silvestrol in cells lacking the ability to expel silvestrol due to low P-glycoprotein levels.

Kinases as Potential Therapeutic Targets for Anti-coronaviral Therapy

The global coronavirus disease-19 (COVID-19) has affected more than 140 million and killed more than 3 million people worldwide as of April 20, 2021. The novel human severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has been identified as an etiological agent for COVID-19. Several kinases have been proposed as possible mediators of multiple viral infections, including life-threatening coronaviruses like SARS-CoV-1, Middle East syndrome coronavirus (MERS-CoV), and SARS-CoV-2. Viral infections hijack abundant cell signaling pathways, resulting in drastic phosphorylation rewiring in the host and viral proteins. Some kinases play a significant role throughout the viral infection cycle (entry, replication, assembly, and egress), and several of them are involved in the virus-induced hyperinflammatory response that leads to cytokine storm, acute respiratory distress syndrome (ARDS), organ injury, and death. Here, we highlight kinases that are associated with coronavirus infections and their inhibitors with antiviral and potentially anti-inflammatory, cytokine-suppressive, or antifibrotic activity.

Heterogeneous Ribonucleoprotein A1 (hnRNPA1) Interacts with the Nucleoprotein of the Influenza a Virus and Impedes Virus Replication

Influenza A virus (IAV), like other viruses, depends on the host cellular machinery for replication and production of progeny. The relationship between a virus and a host is complex, shaped by many spatial and temporal interactions between viral and host proteome, ultimately dictating disease outcome. Therefore, it is imperative to identify host-virus interactions as crucial determinants of disease pathogenies. Heterogeneous ribonucleoprotein A1 (hnRNPA1) is an RNA binding protein involved in the life cycle of many DNA and RNA viruses; however, its role in IAV remains undiscovered. Here we report that human hnRNPA1 physically interacts with the nucleoprotein (NP) of IAV in mammalian cells at different time points of the viral replication cycle. Temporal distribution studies identify hnRNPA1 and NP co-localize in the same cellular milieu in both nucleus and mitochondria in NP-transfected and IAV-infected mammalian cells. Interestingly, hnRNPA1 influenced NP gene expression and affected viral replication. Most importantly, hnRNPA1 knockdown caused a significant increase in NP expression and enhanced viral replication (93.82%) in IAV infected A549 cells. Conversely, hnRNPA1 overexpression reduced NP expression at the mRNA and protein levels and impeded virus replication by (60.70%), suggesting antagonistic function. Taken together, results from this study demonstrate that cellular hnRNPA1 plays a protective role in the host hitherto unknown and may hold potential as an antiviral target to develop host-based therapeutics against IAV.

Antiviral plant-derived natural products to combat RNA viruses: Targets throughout the viral life cycle

There is a need for new effective antivirals, particularly in response to the development of antiviral drug resistance and emerging RNA viruses such as SARS‐CoV‐2. Plants are a significant source of structurally diverse bioactive compounds for drug discovery suggesting that plant‐derived natural products could be developed as antiviral agents. This article reviews the antiviral activity of plant‐derived natural products against RNA viruses, with a focus on compounds targeting specific stages of the viral life cycle. A range of plant extracts and compounds have been identified with antiviral activity, often against multiple virus families suggesting they may be useful as broad‐spectrum antiviral agents. The antiviral mechanism of action of many of these phytochemicals is not fully understood and there are limited studies and clinical trials demonstrating their efficacy and toxicity in vivo. Further research is needed to evaluate the therapeutic potential of plant‐derived natural products as antiviral agents.

Influenza A Virus (H1N1) Infection Induces Glycolysis to Facilitate Viral Replication

Viruses depend on host cellular metabolism to provide the energy and biosynthetic building blocks required for their replication. In this study, we observed that influenza A virus (H1N1), a single-stranded, negative-sense RNA virus with an eight-segmented genome, enhanced glycolysis both in mouse lung tissues and in human lung epithelial (A549) cells. In detail, the expression of hexokinase 2 (HK2), the first enzyme in glycolysis, was upregulated in H1N1-infected A549 cells, and the expression of pyruvate kinase M2 (PKM2) and pyruvate dehydrogenase kinase 3 (PDK3) was upregulated in H1N1-infected mouse lung tissues. Pharmacologically inhibiting the glycolytic pathway or targeting hypoxia-inducible factor 1 (HIF-1), the central transcriptional factor critical for glycolysis, significantly reduced H1N1 replication, revealing a requirement for glycolysis during H1N1 infection. In addition, pharmacologically enhancing the glycolytic pathway further promoted H1N1 replication. Furthermore, the change of H1N1 replication upon glycolysis inhibition or enhancement was independent of interferon signaling. Taken together, these findings suggest that influenza A virus induces the glycolytic pathway and thus facilitates efficient viral replication. This study raises the possibility that metabolic inhibitors, such as those that target glycolysis, could be used to treat influenza A virus infection in the future.

Active Components of Commonly Prescribed Medicines Affect Influenza A Virus-Host Cell Interaction: A Pilot Study

Background: Every year, millions of people are hospitalized and thousands die from influenza A virus (FLUAV) infection. Most cases of hospitalizations and death occur among the elderly. Many of these elderly patients are reliant on medical treatment of underlying chronic diseases, such as arthritis, diabetes, and hypertension. We hypothesized that the commonly prescribed medicines for treatment of underlying chronic diseases can affect host responses to FLUAV infection and thus contribute to the morbidity and mortality associated with influenza. Therefore, the aim of this study was to examine whether commonly prescribed medicines could affect host responses to virus infection in vitro. Methods: We first identified 45 active compounds from a list of commonly prescribed medicines. Then, we constructed a drug-target interaction network and identified the potential implication of these interactions for FLUAV-host cell interplay. Finally, we tested the effect of 45 drugs on the viability, transcription, and metabolism of mock- and FLUAV-infected human retinal pigment epithelial (RPE) cells. Results: In silico drug-target interaction analysis revealed that drugs such as atorvastatin, candesartan, and hydroxocobalamin could target and modulate FLUAV-host cell interaction. In vitro experiments showed that at non-cytotoxic concentrations, these compounds affected the transcription and metabolism of FLUAV- and mock-infected cells. Conclusion: Many commonly prescribed drugs were found to modulate FLUAV-host cell interactions in silico and in vitro and could therefore affect their interplay in vivo, thus contributing to the morbidity and mortality of patients with influenza virus infections.

Design of Neuraminidase-Targeted Imaging and Therapeutic Agents for the Diagnosis and Treatment of Influenza Virus Infections

The last step in influenza virus replication involves the assembly of viral components on the infected cell's plasma membrane followed by budding of intact virus from the host cell surface. Because viral neuraminidase and hemagglutinin are both inserted into the host cell's membrane during this process, influenza virus-infected cells are distinguished from uninfected cells by the presence of viral neuraminidase and hemagglutinin on their cell surfaces. In an effort to exploit this difference in cell surface markers for development of diagnostic and therapeutic agents, we have modified an influenza neuraminidase inhibitor, zanamivir, for targeting of attached imaging and therapeutic agents selectively to influenza viruses and virus-infected cells. We have designed here a zanamivir-conjugated rhodamine dye that allows visual monitoring of binding, internalization, and intracellular trafficking of the fluorescence-labeled neuraminidase in virus-infected cells. We also synthesize a zanamivir-^99mTc radioimaging conjugate that permits whole body imaging of the virus's biodistribution and abundance in infected mice. Finally, we create both a zanamivir-targeted cytotoxic drug (i.e., zanamivir-tubulysin B) and a viral neuraminidase-targeted CAR T cell and demonstrate that they are both able to kill viral neuraminidase-expressing cells without damaging healthy cells. Taken together, these data suggest that the influenza virus neuraminidase inhibitor, zanamivir, can be exploited to improve the diagnosis, imaging, and treatment of influenza virus infections.

Development and Effects of Influenza Antiviral Drugs

Influenza virus is a highly contagious zoonotic respiratory disease that causes seasonal outbreaks each year and unpredictable pandemics occasionally with high morbidity and mortality rates, posing a great threat to public health worldwide. Besides the limited effect of vaccines, the problem is exacerbated by the lack of drugs with strong antiviral activity against all flu strains. Currently, there are two classes of antiviral drugs available that are chemosynthetic and approved against influenza A virus for prophylactic and therapeutic treatment, but the appearance of drug-resistant virus strains is a serious issue that strikes at the core of influenza control. There is therefore an urgent need to develop new antiviral drugs. Many reports have shown that the development of novel bioactive plant extracts and microbial extracts has significant advantages in influenza treatment. This paper comprehensively reviews the development and effects of chemosynthetic drugs, plant extracts, and microbial extracts with influenza antiviral activity, hoping to provide some references for novel antiviral drug design and promising alternative candidates for further anti-influenza drug development.

GSK3β Plays a Negative Role During White Spot Syndrome Virus (WSSV) Infection by Regulating NF-κB Activity in Shrimp Litopenaeus vannamei

Glycogen synthase kinase-3 (GSK3), a cytoplasmic serine/threonine-protein kinase involved in a large number of key cellular processes, is a little-known signaling molecule in virus study. In this study, a GSK3 protein which was highly similar to GSK3β homologs from other species in Litopenaeus vannamei (designated as LvGSK3β) was obtained. LvGSK3β was expressed constitutively in the healthy L. vannamei, at the highest level in the intestine and the lowest level in the eyestalk. White spot syndrome virus (WSSV) reduced LvGSK3β expression was in immune tissues including the hemocyte, intestine, gill and hepatopancreas. The inhibition of LvGSK3β resulted in significantly higher survival rates of L. vannamei during WSSV infection than the control group, and significantly lower WSSV viral loads in LvGSK3β-inhibited L. vannamei were observed. Knockdown of LvGSK3β by RNAi resulted in increases in the expression of LvDorsal and several NF-κB driven antimicrobial peptide (AMP) genes (including ALF, PEN and crustin), but a decrease in LvCactus expression. Accordingly, overexpression of LvGSK3β could reduce the promoter activity of LvDorsal and several AMPs, while the promoter activity of LvCactus was increased. Electrophoretic mobility shift assays (EMSA) showed that LvDorsal could bind to the promoter of LvGSK3β. The interaction between LvGSK3β and LvDorsal or LvCactus was confirmed using co-immunoprecipitation (Co-IP) assays. In addition, the expression of LvGSK3β was dramatically reduced by knockdown of LvDorsal. In summary, the results presented in this study indicated that LvGSK3β had a negative effect on L. vannamei by mediating a feedback regulation of the NF-κB pathway when it is infected by WSSV.

Dissecting the mechanism of signaling-triggered nuclear export of newly synthesized influenza virus ribonucleoprotein complexes

Influenza viruses (IV) replicate in the nucleus. Export of newly produced genomes, packaged in viral ribonucleoprotein (vRNP) complexes, relies on the nuclear CRM1 export pathway and appears to be timely controlled by virus-induced cellular signaling. However, the exact mechanism of the signaling-controlled complex assembly and export is enigmatic. Here we show that IV activates the Raf/MEK/ERK/RSK1 pathway, leading to phosphorylation at specific sites of the NP, which in turn, creates a docking site for binding of the M1 protein, an initial step in formation of vRNP export complexes. These findings are of broad relevance regarding the regulatory role of signaling pathways and posttranslational modifications in virus propagation and will strongly support ongoing development of an alternative anti-influenza therapy.

Influenza viruses (IV) exploit a variety of signaling pathways. Previous studies showed that the rapidly accelerated fibrosarcoma/mitogen-activated protein kinase/extracellular signal-regulated kinase (Raf/MEK/ERK) pathway is functionally linked to nuclear export of viral ribonucleoprotein (vRNP) complexes, suggesting that vRNP export is a signaling-induced event. However, the underlying mechanism remained completely enigmatic. Here we have dissected the unknown molecular steps of signaling-driven vRNP export. We identified kinases RSK1/2 as downstream targets of virus-activated ERK signaling. While RSK2 displays an antiviral role, we demonstrate a virus-supportive function of RSK1, migrating to the nucleus to phosphorylate nucleoprotein (NP), the major constituent of vRNPs. This drives association with viral matrix protein 1 (M1) at the chromatin, important for vRNP export. Inhibition or knockdown of MEK, ERK or RSK1 caused impaired vRNP export and reduced progeny virus titers. This work not only expedites the development of anti-influenza strategies, but in addition demonstrates converse actions of different RSK isoforms.

GSK-3-associated signaling is crucial to virus infection of cells

Signal transduction pathways play important roles in virus infection, replication, and associated pathogenesis. Some of the best understood cell signaling networks are crucial to virus infections such the mitogen-activated protein kinase (MAPK), phosphoinositide 3-kinase (PI3K), protein kinase C (PKC), and the WNT/β-catenin pathways. Glycogen synthase kinase-3 (GSK-3) is a lesser known signaling molecule in the field of virus research. Interestingly, GSK-3 forms the crux of multiple cell signaling pathways. However, recent studies indicate that GSK-3 may perform key roles in the response to viral infection, replication and pathogenesis. The effects of activated or inactivated forms of GSK-3 on virus infection are still not yet clearly understood phenomenon. The comprehension of the molecular mechanisms underlying the regulation of GSK-3-associated signaling pathways in terms of different stages of virus replication could be important not only to understand the pathogenesis of virus, but also possibly leading to new therapeutic targets. This review will focus on recent advances in understanding the roles of GSK-3 on viral replication, pathogenesis and the immune responses.

Natural antiviral compound silvestrol modulates human monocyte-derived macrophages and dendritic cells

Outbreaks of infections with viruses like Sars‐CoV‐2, Ebola virus and Zika virus lead to major global health and economic problems because of limited treatment options. Therefore, new antiviral drug candidates are urgently needed. The promising new antiviral drug candidate silvestrol effectively inhibited replication of Corona‐, Ebola‐, Zika‐, Picorna‐, Hepatis E and Chikungunya viruses. Besides a direct impact on pathogens, modulation of the host immune system provides an additional facet to antiviral drug development because suitable immune modulation can boost innate defence mechanisms against the pathogens. In the present study, silvestrol down‐regulated several pro‐ and anti‐inflammatory cytokines (IL‐6, IL‐8, IL‐10, CCL2, CCL18) and increased TNF‐α during differentiation and activation of M1‐macrophages, suggesting that the effects of silvestrol might cancel each other out. However, silvestrol amplified the anti‐inflammatory potential of M2‐macrophages by increasing expression of anti‐inflammatory surface markers CD206, TREM2 and reducing release of pro‐inflammatory IL‐8 and CCL2. The differentiation of dendritic cells in the presence of silvestrol is characterized by down‐regulation of several surface markers and cytokines indicating that differentiation is impaired by silvestrol. In conclusion, silvestrol influences the inflammatory status of immune cells depending on the cell type and activation status.

Host-Virus Interaction: How Host Cells Defend against Influenza A Virus Infection

Influenza A viruses (IAVs) are highly contagious pathogens infecting human and numerous animals. The viruses cause millions of infection cases and thousands of deaths every year, thus making IAVs a continual threat to global health. Upon IAV infection, host innate immune system is triggered and activated to restrict virus replication and clear pathogens. Subsequently, host adaptive immunity is involved in specific virus clearance. On the other hand, to achieve a successful infection, IAVs also apply multiple strategies to avoid be detected and eliminated by the host immunity. In the current review, we present a general description on recent work regarding different host cells and molecules facilitating antiviral defenses against IAV infection and how IAVs antagonize host immune responses.

The multifarious roles of heterogeneous ribonucleoprotein A1 in viral infections

Viruses are obligate parasites known to interact with a wide variety of host proteins at different stages of infection. Current antiviral treatments target viral proteins and may be compromised due to the emergence of drug resistant viral strains. Targeting viral-host interactions is now gaining recognition as an alternative approach against viral infections. Recent research has revealed that heterogeneous ribonucleoprotein A1, an RNA-binding protein, plays an essential functional and regulatory role in the life cycle of many viruses. In this review, we summarize the interactions between heterogeneous ribonucleoprotein A1 (hnRNPA1) and multiple viral proteins during the life cycle of RNA and DNA viruses. hnRNPA1 protein levels are modulated differently, in different viruses, which further dictates its stability, function, and intracellular localization. Multiple reports have emphasized that in Sindbis virus, enteroviruses, porcine endemic diarrhea virus, and rhinovirus infection, hnRNPA1 enhances viral replication and survival. However, in others like hepatitis C virus and human T-cell lymphotropic virus, it exerts a protective response. The involvement of hnRNPA1 in viral infections highlights its importance as a central regulator of host and viral gene expression. Understanding the nature of these interactions will increase our understanding of specific viral infections and pathogenesis and eventually aid in the development of novel and robust antiviral intervention strategies.

Expanding the activity spectrum of antiviral agents

Broad-spectrum antivirals (BSAs) are agents that inhibit replication of several human viruses. Here, we review 108 approved, investigational, and experimental BSAs, for which safety profiles in humans are available. The most effective and tolerable BSAs could reinforce the arsenal of available antiviral therapeutics pending the results of further pre-clinical and clinical studies.

Model-based analysis of influenza A virus replication in genetically engineered cell lines elucidates the impact of host cell factors on key kinetic parameters of virus growth

The best measure to limit spread of contagious diseases caused by influenza A viruses (IAVs) is annual vaccination. The growing global demand for low-cost vaccines requires the establishment of high-yield production processes. One possible option to address this challenge is the engineering of novel vaccine producer cell lines by manipulating gene expression of host cell factors relevant for virus replication. To support detailed characterization of engineered cell lines, we fitted an ordinary differential equation (ODE)-based model of intracellular IAV replication previously established by our group to experimental data obtained from infection studies in human A549 cells. Model predictions indicate that steps of viral RNA synthesis, their regulation and particle assembly and virus budding are promising targets for cell line engineering. The importance of these steps was confirmed in four of five single gene overexpression cell lines (SGOs) that showed small, but reproducible changes in early dynamics of RNA synthesis and virus release. Model-based analysis suggests, however, that overexpression of the selected host cell factors negatively influences specific RNA synthesis rates. Still, virus yield was rescued by an increase in the virus release rate. Based on parameter estimations obtained for SGOs, we predicted that there is a potential benefit associated with overexpressing multiple host cell genes in one cell line, which was validated experimentally. Overall, this model-based study on IAV replication in engineered cell lines provides a step forward in the dynamic and quantitative characterization of IAV-host cell interactions. Furthermore, it suggests targets for gene editing and indicates that overexpression of multiple host cell factors may be beneficial for the design of novel producer cell lines.

Luteolin decreases the yield of influenza A virus in vitro by interfering with the coat protein I complex expression

Influenza is an acute transmissible respiratory infectious disease in humans and animals with high morbidity and mortality. It was reported that luteolin, extracted from Chinese herbs, could potently inhibit influenza virus replication in vitro. To assess the effect and explore the fundamental mechanism of luteolin, we infected several cell lines with two subtypes of influenza A virus (IAV), including A/Jiangxi/312/2006 (H3N2) and A/Fort Monmouth/1/1947 (H1N1) and demonstrated that luteolin suppressed the replication of IAV by cytopathic effect reduction method, qRT-PCR, immunofluorescence and Western blot assays. A time-of-addition assay indicated that this compound interfered with viral replication at the early stage of infection. In addition, we found that luteolin suppressed coat protein I complex expression, which was related to influenza virus entry and endocytic pathway. Overall, our findings demonstrated the antiviral effect of luteolin against IAV and its novel antiviral mechanism.

Annexins in Influenza Virus Replication and Pathogenesis

Influenza A viruses (IAVs) are important human respiratory pathogens which cause seasonal or periodic endemic infections. IAV can result in severe or fatal clinical complications including pneumonia and respiratory distress syndrome. Treatment of IAV infections is complicated because the virus can evade host immunity through antigenic drifts and antigenic shifts, to establish infections making new treatment options desirable. Annexins (ANXs) are a family of calcium and phospholipid binding proteins with immunomodulatory roles in viral infections, lung injury, and inflammation. A current understanding of the role of ANXs in modulating IAV infection and host responses will enable the future development of more effective antiviral therapies. This review presents a comprehensive understanding of the advances made in the field of ANXs, in particular, ANXA1 and IAV research and highlights the importance of ANXs as a suitable target for IAV therapy.

Metabolic conversion of CI-1040 turns a cellular MEK-inhibitor into an antibacterial compound

Influenza virus (IV) infections cause severe respiratory illnesses that can be complicated by bacterial super-infections. Previously, we identified the cellular Raf-MEK-ERK cascade as a promising antiviral target. Inhibitors of MEK, such as CI-1040, showed potent antiviral activity. However, it remained unclear if this inhibitor and its active form, ATR-002, might sensitize host cells to either IV or secondary bacterial infections. To address these questions, we studied the anti-pathogen activity of ATR-002 in comparison to CI-1040, particularly, its impact on Staphylococcus aureus (S. aureus), which is a major cause of IV super-infections. We analysed IV and S. aureus titres in vitro during super-infection in the presence and absence of the drugs and characterized the direct impact of ATR-002 on bacterial growth and phenotypic changes. Importantly, neither CI-1040 nor ATR-002 treatment led to increased bacterial titres during super-infection, indicating that the drug does not sensitize cells for bacterial infection. In contrast, we rather observed reduced bacterial titres in presence of ATR-002. Surprisingly, ATR-002 also led to reduced bacterial growth in suspension cultures, reduced stress- and antibiotic tolerance without resistance induction. Our data identified for the first time that a particular MEK-inhibitor metabolite exhibits direct antibacterial activity, which is likely due to interference with the bacterial PknB kinase/Stp phosphatase signalling system.

Inhibitors of Influenza A Virus Polymerase

The propensity of influenza virus to develop resistance to commonly prescribed drugs highlights the need for continuing development of new therapeutics. Biological and structural investigations of the enzymatic and interaction domains among influenza A virus polymerase subunits have broadened the target reservoir for drug screening. With the wealth of knowledge from these studies, identification of small-molecule and peptidic inhibitors that specifically abrogate polymerase activity or disrupt the polymerase assembly has emerged as an innovative and promising approach. Importantly, those domains are highly conserved among influenza subtypes and thus minimize the emergence of drug resistant mutants. An overview of the reported enzymatic inhibitors and protein-protein disruptors has been provided, in our effort to facilitate the development of next-generation anti-influenza therapeutics.

Advanced nanotechnologies in avian influenza: Current status and future trends - A review

In the last decade, the control of avian influenza virus has experienced many difficulties, which have caused major global agricultural problems that have also led to public health consequences. Conventional biochemical methods are not sufficient to detect and control agricultural pathogens in the field due to the growing demand for food and subsidiary products; thus, studies aiming to develop potent alternatives to conventional biochemical methods are urgently needed. In this review, emerging detection systems, their applicability to diagnostics, and their therapeutic possibilities in view of nanotechnology are discussed. Nanotechnology-based sensors are used for rapid, sensitive and cost-effective diagnostics of agricultural pathogens. The application of different nanomaterials promotes interactions between these materials and the virus, which enables researchers to construct portable electroanalytical biosensing analyser that should effectively detect the influenza virus. The present review will provide insights into the guidelines for future experiments to develop better techniques to detect and control influenza viruses.

Influenza Virus Infection, Interferon Response, Viral Counter-Response, and Apoptosis

Human influenza A viruses (IAVs) cause global pandemics and epidemics, which remain serious threats to public health because of the shortage of effective means of control. To combat the surge of viral outbreaks, new treatments are urgently needed. Developing new virus control modalities requires better understanding of virus-host interactions. Here, we describe how IAV infection triggers cellular apoptosis and how this process can be exploited towards the development of new therapeutics, which might be more effective than the currently available anti-influenza drugs.

The natural compound silvestrol is a potent inhibitor of Ebola virus replication

The DEAD-box RNA helicase eIF4A, which is part of the heterotrimeric translation initiation complex in eukaryotes, is an important novel drug target in cancer research because its helicase activity is required to unwind extended and highly structured 5'-UTRs of several proto-oncogenes. Silvestrol, a natural compound isolated from the plant Aglaia foveolata, is a highly efficient, non-toxic and specific inhibitor of eIF4A. Importantly, 5'-capped viral mRNAs often contain structured 5'-UTRs as well, which may suggest a dependence on eIF4A for their translation by the host protein synthesis machinery. In view of the recent Ebola virus (EBOV) outbreak in West Africa, the identification of potent antiviral compounds is urgently required. Since Ebola mRNAs are 5'-capped and harbor RNA secondary structures in their extended 5'-UTRs, we initiated a BSL4 study to analyze silvestrol in EBOV-infected Huh-7 cells and in primary human macrophages for its antiviral activity. We observed that silvestrol inhibits EBOV infection at low nanomolar concentrations, as inferred from large reductions of viral titers. This correlated with an almost complete disappearance of EBOV proteins, comparable in effect to the translational shutdown of expression of the proto-oncoprotein PIM1, a cellular kinase known to be affected by silvestrol. Effective silvestrol concentrations were non-toxic in the tested cell systems. Thus, silvestrol appears to be a promising first-line drug for the treatment of acute EBOV and possibly other viral infections.

Identification of a novel small-molecule compound targeting the influenza A virus polymerase PB1-PB2 interface

The PB1 C-terminal domain and PB2 N-terminal domain interaction of the influenza A polymerase, which modulates the assembly of PB1 and PB2 subunits, may serve as a valuable target for the development of novel anti-influenza therapeutics. In this study, we performed a systematic screening of a chemical library, followed by the antiviral evaluation of primary hits and their analogues. Eventually, a novel small-molecule compound PP7 that abrogated the PB1-PB2 association and impaired viral polymerase activity was identified. PP7 exhibited antiviral activities against influenza virus subtypes A (H1N1)pdm09, A(H7N9) and A(H9N2) in cell cultures and partially protected mice against lethal challenge of mouse-adapted influenza A (H1N1)pdm09 virus. Surprisingly, a panel of other subtypes of influenza virus, including A(H5N1) and A(H7N7), showed various degrees of resistance to the compound. Biochemical studies revealed a similar pattern of resistance on the impairment of polymerase activity. Molecular docking analyses suggested a PP7-binding site that appeared to be completely conserved among the subtypes of the virus mentioned above. Thus, we propose that alternative/additional binding site (s) may exist for the regulation of PB1-PB2 subunits assembly of influenza A virus.

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A novel small-molecule compound was identified to provide anti-influenza effect in vitro and in vivo.

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An RT-qPCR based assay was modified to evaluate the polymerase activity of various subtypes of influenza viruses.

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The PB1-PB2 assembly strategies of the trimeric polymerase complex might be stain/subtype specific.

Multi-Omics Studies towards Novel Modulators of Influenza A Virus-Host Interaction

Human influenza A viruses (IAVs) cause global pandemics and epidemics. These viruses evolve rapidly, making current treatment options ineffective. To identify novel modulators of IAV–host interactions, we re-analyzed our recent transcriptomics, metabolomics, proteomics, phosphoproteomics, and genomics/virtual ligand screening data. We identified 713 potential modulators targeting 199 cellular and two viral proteins. Anti-influenza activity for 48 of them has been reported previously, whereas the antiviral efficacy of the 665 remains unknown. Studying anti-influenza efficacy and immuno/neuro-modulating properties of these compounds and their combinations as well as potential viral and host resistance to them may lead to the discovery of novel modulators of IAV–host interactions, which might be more effective than the currently available anti-influenza therapeutics.

JNJ872 inhibits influenza A virus replication without altering cellular antiviral responses

JNJ-63623872 (formally known as VX-787; referred to here as JNJ872) is an orally bioavailable compound, which is in phase II clinical trials for the treatment of influenza A virus (IAV) infections. Here we show that JNJ872 inhibits at nanomolar concentrations the transcription of viral RNA in IAV-infected human macrophages by targeting a highly conserved site on the cap-snatching domain of influenza polymerase basic 2 protein (PB2). Furthermore, even lower concentrations of JNJ872 protected macrophages from IAV-mediated death when given in combination with 100 nM gemcitabine, which also attenuated transcription and replication of viral RNA. Importantly, treating human macrophages with JNJ872 allowed expression of many immune-related and other genes, involved in antiviral responses, such as indoleamine 2,3-dioxygenase 1 (IDO), and cytosolic 5'-nucleotidase 3A (NT5C3A). Moreover, our targeted metabolomics analysis indicate that treatment with JNJ782 did not interfere with metabolic responses to infection, which further supported our transcriptomics results. Thus, VX-737 alone or in combination with other drugs could be beneficial for treating IAV infected patients, because it would allow the development of antiviral responses and, thereby, protect individuals from current and future infections with closely related IAV strains.

Networks of Host Factors that Interact with NS1 Protein of Influenza A Virus

Pigs are an important host of influenza A viruses due to their ability to generate reassortant viruses with pandemic potential. NS1 protein of influenza A viruses is a key virulence factor and a major antagonist of innate immune responses. It is also involved in enhancing viral mRNA translation and regulation of virus replication. Being a protein with pleiotropic functions, NS1 has a variety of cellular interaction partners. Hence, studies on swine influenza viruses (SIV) and identification of swine influenza NS1-interacting host proteins is of great interest. Here, we constructed a recombinant SIV carrying a Strep-tag in the NS1 protein and infected primary swine respiratory epithelial cells (SRECs) with this virus. The Strep-tag sequence in the NS1 protein enabled us to purify intact, the NS1 protein and its interacting protein complex specifically. We identified cellular proteins present in the purified complex by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and generated a dataset of these proteins. 445 proteins were identified by LC-MS/MS and among them 192 proteins were selected by setting up a threshold based on MS parameters. The selected proteins were analyzed by bioinformatics and were categorized as belonging to different functional groups including translation, RNA processing, cytoskeleton, innate immunity, and apoptosis. Protein interaction networks were derived using these data and the NS1 interactions with some of the specific host factors were verified by immunoprecipitation. The novel proteins and the networks revealed in our study will be the potential candidates for targeted study of the molecular interaction of NS1 with host proteins, which will provide insights into the identification of new therapeutic targets to control influenza infection and disease pathogenesis.

Chemical Genomics Identifies the PERK-Mediated Unfolded Protein Stress Response as a Cellular Target for Influenza Virus Inhibition

Influenza A viruses generate annual epidemics and occasional pandemics of respiratory disease with important consequences for human health and the economy. Therefore, a large effort has been devoted to the development of new anti-influenza virus drugs directed to viral targets, as well as to the identification of cellular targets amenable to anti-influenza virus therapy. Here we have addressed the identification of such potential cellular targets by screening collections of drugs approved for human use. We reasoned that screening with a green fluorescent protein-based recombinant replicon system would identify cellular targets involved in virus transcription/replication and/or gene expression and hence address an early stage of virus infection. By using such a strategy, we identified Montelukast (MK) as an inhibitor of virus multiplication. MK inhibited virus gene expression but did not alter viral RNA synthesis in vitro or viral RNA accumulation in vivo. The low selectivity index of MK prevented its use as an antiviral, but it was sufficient to identify a new cellular pathway suitable for anti-influenza virus intervention. By deep sequencing of RNA isolated from mock- and virus-infected human cells, treated with MK or left untreated, we showed that it stimulates the PERK-mediated unfolded protein stress response. The phosphorylation of PERK was partly inhibited in virus-infected cells but stimulated in MK-treated cells. Accordingly, pharmacological inhibition of PERK phosphorylation led to increased viral gene expression, while inhibition of PERK phosphatase reduced viral protein synthesis. These results suggest the PERK-mediated unfolded protein response as a potential cellular target to modulate influenza virus infection.

Influenza A viruses are responsible for annual epidemics and occasional pandemics with important consequences for human health and the economy. The unfolded protein response is a defense mechanism fired by cells when the demand of protein synthesis and folding is excessive, for instance, during an acute virus infection. In this report, we show that influenza virus downregulates the unfolded protein response mediated by the PERK sensor, while Montelukast, a drug used to treat asthma in humans, specifically stimulated this response and downregulated viral protein synthesis and multiplication. Accordingly, we show that PERK phosphorylation was reduced in virus-infected cells and increased in cells treated with Montelukast. Hence, our studies suggest that modulation of the PERK-mediated unfolded protein response is a target for influenza virus inhibition.

Immuno-modulating properties of saliphenylhalamide, SNS-032, obatoclax, and gemcitabine

Influenza A viruses (IAVs) impact the public health and global economy by causing yearly epidemics and occasional pandemics. Several anti-IAV drugs are available and many are in development. However, the question remains which of these antiviral agents may allow activation of immune responses and protect patients against co- and re-infections. To answer to this question, we analysed immuno-modulating properties of the antivirals saliphenylhalamide (SaliPhe), SNS-032, obatoclax, and gemcitabine, and found that only gemcitabine did not impair immune responses in infected cells. It also allowed activation of innate immune responses in lipopolysaccharide (LPS)- and interferon alpha (IFNα)-stimulated macrophages. Moreover, immuno-mediators produced by gemcitabine-treated IAV-infected macrophages were able to prime immune responses in non-infected cells. Thus, we identified an antiviral agent which might be beneficial for treatment of patients with severe viral infections.

Learning from structure-based drug design and new antivirals targeting the ribonucleoprotein complex for the treatment of influenza

INTRODUCTION

Influenza viruses are a threat to human health. There are presently only two methods for treating influenza: vaccines, which require yearly updates, and two classes of antivirals that suffer with the problem of resistance by current human influenza viruses; this is especially the case with amantadine and rimantadine. Consequently, there is an urgent need for the development of new antivirals with new mechanisms of action.

AREAS COVERED

In this review, the authors focus on viral protein domains, their associated activity and their inhibition by small molecules defined by a structure-based design with a special emphasis on the ribonucleoprotein complex and its inhibitors. Several new classes of antiviral candidates targeting viral replication through individual domains of the polymerase and the nucleoprotein (NP) have been developed through structure-based design.

EXPERT OPINION

To date, the antivirals targeting neuraminidase are by far the most developed and potent. Antiviral candidates targeting the NP and polymerase domains are in the pipeline but their pharmacokinetics needs further studies. The recently published structures of the polymerase expand the possibilities for development of new antivirals. Combination therapies targeting conserved viral targets and new cellular proteins or exploiting drug promiscuity hold promises to fight against the emergence of resistance.

Influenza virus-host interactomes as a basis for antiviral drug development

Currently, antiviral drugs that target specific viral protein functions are available for the treatment of influenza; however, concern regarding the emergence of drug-resistant viruses is warranted, as is the urgent need for new antiviral targets, including non-viral targets, such as host cellular factors. Viruses rely on host cellular functions to replicate, and therefore a thorough understanding of the roles of virus-host interactions during influenza virus replication is essential to develop novel anti-influenza drugs that target the host factors involved in virus replication. Here, we review recent studies that used several approaches to identify host factors involved in influenza virus replication. These studies have permitted the construction of an interactome map of virus-host interactions in the influenza virus life cycle, clarifying the entire life cycle of this virus and accelerating the development of new antiviral drugs with a low propensity for the development of resistance.

Targeting Importin-α7 as a Therapeutic Approach against Pandemic Influenza Viruses

Viral drug resistance is believed to be less likely to occur if compounds are directed against cellular rather than viral proteins. In this study, we analyzed the feasibility of a crucial viral replication factor, namely, importin-α7, as a cellular drug target to combat pandemic influenza viruses. Surprisingly, only five viral lung-to-lung passages were required to achieve 100% lethality in importin-α7⁻/⁻ mice that otherwise are resistant. Viral escape from importin-α7 requirement was mediated by five mutations in the viral ribonucleoprotein complex and the surface glycoproteins. Moreover, the importin-α7⁻/⁻ mouse-adapted strain became even more virulent for wild-type mice than the parental strain. These studies show that targeting host proteins may still result in viral escape by alternative pathways, eventually giving rise to even more virulent virus strains. Thus, therapeutic intervention strategies should consider a multitarget approach to reduce viral drug resistance. IMPORTANCE Here, we investigated the long-standing hypothesis based on in vitro studies that viral drug resistance occurrence is less likely if compounds are directed against cellular rather than viral proteins. Here, we challenged this hypothesis by analyzing, in an in vivo animal model, the feasibility of targeting the cellular factor importin-α7, which is crucial for human influenza virus replication and pathogenesis, as an efficient antiviral strategy against pandemic influenza viruses. In summary, our studies suggest that resistance against cellular factors is possible in vivo, and the emergence of even more virulent viral escape variants calls for particular caution. Thus, therapeutic intervention strategies should consider a multitarget approach using compounds against viral as well as cellular factors to reduce the risk of viral drug resistance and potentially increased virulence.

The C terminus of NS1 protein of influenza A/WSN/1933(H1N1) virus modulates antiviral responses in infected human macrophages and mice

Non-structural protein NS1 of influenza A viruses interacts with cellular factors through its N-terminal RNA-binding, middle effector and C-terminal non-structured domains. NS1 attenuates antiviral responses in infected cells and thereby secures efficient virus replication. Some influenza strains express C-terminally truncated NS1 proteins due to nonsense mutations in the NS1 gene. To understand the role of the NS1 C-terminal region in regulation of antiviral responses, we engineered influenza viruses expressing C-terminally truncated NS1 proteins using A/WSN/33(H1N1) reverse genetics and tested them in human macrophages and in mice. We showed that a WSN virus expressing NS1 with a 28 aa deletion from its C terminus is a more powerful inducer of antiviral responses than the virus expressing full-length NS1, or one with a 10 aa truncation of NS1 in vitro. Thus, our findings suggest that the C-terminal region of NS1 is essential for regulation of antiviral responses. Moreover, viruses expressing truncated NS1 proteins could be good vaccine candidates.

A host susceptibility gene, DR1, facilitates influenza A virus replication by suppressing host innate immunity and enhancing viral RNA replication

Influenza A virus (IAV) depends on cellular factors to complete its replication cycle; thus, investigation of the factors utilized by IAV may facilitate antiviral drug development. To this end, a cellular transcriptional repressor, DR1, was identified from a genome-wide RNA interference (RNAi) screen. Knockdown (KD) of DR1 resulted in reductions of viral RNA and protein production, demonstrating that DR1 acts as a positive host factor in IAV replication. Genome-wide transcriptomic analysis showed that there was a strong induction of interferon-stimulated gene (ISG) expression after prolonged DR1 KD. We found that beta interferon (IFN-β) was induced by DR1 KD, thereby activating the JAK-STAT pathway to turn on ISG expression, which led to a strong inhibition of IAV replication. This result suggests that DR1 in normal cells suppresses IFN induction, probably to prevent undesired cytokine production, but that this suppression may create a milieu that favors IAV replication once cells are infected. Furthermore, biochemical assays of viral RNA replication showed that DR1 KD suppressed viral RNA replication. We also showed that DR1 associated with all three subunits of the viral RNA-dependent RNA polymerase (RdRp) complex, indicating that DR1 may interact with individual components of the viral RdRp complex to enhance viral RNA replication. Thus, DR1 may be considered a novel host susceptibility gene for IAV replication via a dual mechanism, not only suppressing the host defense to indirectly favor IAV replication but also directly facilitating viral RNA replication.

IMPORTANCE

Investigations of virus-host interactions involved in influenza A virus (IAV) replication are important for understanding viral pathogenesis and host defenses, which may manipulate influenza virus infection or prevent the emergence of drug resistance caused by a high error rate during viral RNA replication. For this purpose, a cellular transcriptional repressor, DR1, was identified from a genome-wide RNAi screen as a positive regulator in IAV replication. In the current studies, we showed that DR1 suppressed the gene expression of a large set of host innate immunity genes, which indirectly facilitated IAV replication in the event of IAV infection. Besides this scenario, DR1 also directly enhanced the viral RdRp activity, likely through associating with individual components of the viral RdRp complex. Thus, DR1 represents a novel host susceptibility gene for IAV replication via multiple functions, not only suppressing the host defense but also enhancing viral RNA replication. DR1 may be a potential target for drug development against influenza virus infection.

A dual character of flavonoids in influenza A virus replication and spread through modulating cell-autonomous immunity by MAPK signaling pathways

Flavonoids are well known as a large class of polyphenolic compounds, which have a variety of physiological activities, including anti-influenza virus activity. The influenza A/WSN/33 infected A549 cells have been used to screen anti-influenza virus drugs from natural flavonoid compounds library. Unexpectedly, some flavonoid compounds significantly inhibited virus replication, while the others dramatically promoted virus replication. In this study, we attempted to understand these differences between flavonoid compounds in their antivirus mechanisms. Hesperidin and kaempferol were chosen as representatives of both sides, each of which exhibited the opposite effects on influenza virus replication. Our investigation revealed that the opposite effects produced by hesperidin and kaempferol on influenza virus were due to inducing the opposite cell-autonomous immune responses by selectively modulating MAP kinase pathways: hesperidin up-regulated P38 and JNK expression and activation, thus resulting in the enhanced cell-autonomous immunity; while kaempferol dramatically down-regulated p38 and JNK expression and activation, thereby suppressing cell-autonomous immunity. In addition, hesperidin restricted RNPs export from nucleus by down-regulating ERK activation, but kaempferol promoted RNPs export by up-regulating ERK activation. Our findings demonstrate that a new generation of anti-influenza virus drugs could be developed based on selective modulation of MAP kinase pathways to stimulate cell-autonomous immunity.

Inhibition of influenza virus replication by targeting broad host cell pathways

Antivirals that are currently used to treat influenza virus infections target components of the virus which can mutate rapidly. Consequently, there has been an increase in the number of resistant strains to one or many antivirals in recent years. Here we compared the antiviral effects of lysosomotropic alkalinizing agents (LAAs) and calcium modulators (CMs), which interfere with crucial events in the influenza virus replication cycle, against avian, swine, and human viruses of different subtypes in MDCK cells. We observed that treatment with LAAs, CMs, or a combination of both, significantly inhibited viral replication. Moreover, the drugs were effective even when they were administered 8 h after infection. Finally, analysis of the expression of viral acidic polymerase (PA) revealed that both drugs classes interfered with early events in the viral replication cycle. This study demonstrates that targeting broad host cellular pathways can be an efficient strategy to inhibit influenza replication. Furthermore, it provides an interesting avenue for drug development where resistance by the virus might be reduced since the virus is not targeted directly.

Antiviral strategies against influenza virus: towards new therapeutic approaches

Influenza viruses are major human pathogens responsible for respiratory diseases affecting millions of people worldwide and characterized by high morbidity and significant mortality. Influenza infections can be controlled by vaccination and antiviral drugs. However, vaccines need annual updating and give limited protection. Only two classes of drugs are currently approved for the treatment of influenza: M2 ion channel blockers and neuraminidase inhibitors. However, they are often associated with limited efficacy and adverse side effects. In addition, the currently available drugs suffer from rapid and extensive emergence of drug resistance. All this highlights the urgent need for developing new antiviral strategies with novel mechanisms of action and with reduced drug resistance potential. Several new classes of antiviral agents targeting viral replication mechanisms or cellular proteins/processes are under development. This review gives an overview of novel strategies targeting the virus and/or the host cell for counteracting influenza virus infection.

A technique to increase protein yield in a rabbit reticulocyte lysate translation system

Rabbit reticulocyte lysate (RRL) is a mammalian cell-free system for protein production. However, one of the limitations of this system is its low protein yield. Inclusion of recombinant virus proteins and specific viral structures on target mRNA could enhance protein production in RRL. Here we show that simultaneous addition of influenza A virus NS1 protein and inclusion of the encephalomyocarditis virus (EMCV) internal ribosome entry site (IRES) in the target mRNA facilitate translation initiation and increase protein yield over 10-fold, improving the translation capacity of RRL.