High-throughput identification of compounds targeting influenza RNA-dependent RNA polymerase activity

Antiviral strategies against influenza virus: an update on approved and innovative therapeutic approaches

Influenza viruses still represent a great concern for Public Health by causing yearly seasonal epidemics and occasionally worldwide pandemics. Moreover, spillover events at the animal-human interface are becoming more frequent nowadays, also involving animal species not previously found as reservoirs. To restrict the effects of influenza virus epidemics, especially in at-risk population, and to prepare a drug arsenal for possible future pandemics, researchers worldwide have been working on the development of antiviral strategies since the 80's of the last century. One of the main obstacles is the considerable genomic variability of influenza viruses, which constantly poses the issues of drug-resistance emergence and immune evasion. This review summarizes the approved therapeutics for clinical management of influenza, promising new anti-flu compounds and monoclonal antibodies currently undergoing clinical evaluation, and molecules with efficacy against influenza virus in preclinical studies. Moreover, we discuss some innovative anti-influenza therapeutic approaches such as combination therapies and targeted protein degradation. Given the limited number of drugs approved for influenza treatment, there is a still strong need for novel potent anti-influenza drugs endowed with a high barrier to drug resistance and broad-spectrum activity against influenza viruses of animal origin that may be responsible of future large outbreaks and pandemics.

The evaluation of DNA-linked inhibitor antibody and AlphaScreen assays for high-throughput screening of compounds targeting the cap-binding domain in influenza a polymerase

The PB2 subunit of the influenza virus polymerase complex is essential for viral replication, primarily through a mechanism known as cap-snatching. In this process, PB2 binds to the 5' cap structure of host pre-mRNAs, enabling the viral polymerase to hijack the host transcriptional machinery. This binding facilitates the cleavage and integration of the capped RNA fragment into viral mRNA, thereby promoting efficient viral replication. Inhibiting the PB2-cap interaction is therefore crucial, as it directly disrupts the viral replication cycle. Consequently, targeting PB2 with specific inhibitors is a promising strategy for antiviral drug development against influenza. However, there are currently no available methods for the high-throughput screening of potential inhibitors. The development of new inhibitor screening methods of potential PB2 binders is the focus of this study. In this study, we present two novel methods, DIANA and AlphaScreen, for screening influenza PB2 cap-binding inhibitors and evaluate their effectiveness compared to the established differential scanning fluorimetry (DSF) technique. Using a diverse set of substrates and compounds based on the previously described PB2 binder pimodivir, we thoroughly assessed the capabilities of these new methods. Our findings demonstrate that both DIANA and AlphaScreen are highly effective for PB2 inhibitor screening, offering distinct advantages over traditional techniques such as isothermal titration calorimetry (ITC) and surface plasmon resonance (SPR). These advantages include improved scalability, reduced sample requirements, and the capacity for label-free detection. Notably, DIANA's ability to determine Ki values from a single-well measurement significantly enhances its practicality and efficiency in inhibitor screening. This research represents a significant step forward in the development of more efficient and scalable screening strategies, helping advance efforts in the discovery of antiviral drugs against influenza.

Antiviral drug development by targeting RNA binding site, oligomerization and nuclear export of influenza nucleoprotein

The quasispecies of the influenza virus poses a significant challenge for developing effective therapies. Current antiviral drugs such as oseltamivir, zanamivir, peramivir and baloxavir marboxil along with seasonal vaccines have limitations due to viral variability caused by antigenic drift and shift as well as the development of drug resistance. Therefore, there is a clear need for novel antiviral agents targeting alternative mechanisms, either independently or in combination with existing modalities, to reduce the impact of influenza virus-related infections. The influenza nucleoprotein (NP) is a key component of the viral ribonucleoprotein complex. The multifaceted nature of the NP makes it an attractive target for antiviral intervention. Recent reports have identified inhibitors that specifically target this protein. Recognizing the importance of developing influenza treatments for potential pandemics, this review explores the structural and functional aspects of NP and highlights its potential as an emerging target for anti-influenza drugs. We discuss various strategies for targeting NP, including RNA binding, oligomerization, and nuclear export, and also consider the potential of NP-based vaccines. Overall, this review provides insights into recent developments and future perspectives on targeting influenza NP for antiviral therapies.

Recent advances of phenotypic screening strategies in the application of anti-influenza virus drug discovery

Seasonal and pandemic influenza virus infections not only pose a serious threat to human health but also cause tremendous economic losses and social burdens. However, due to the inherent high variability of influenza virus RNA genomes, the existing anti-influenza virus drugs have been frequently faced with the clinical issue of emerging drug-resistant mutants. Therefore, there is an urgent need to develop efficient and broad-spectrum antiviral agents against wild-type and drug-resistant mutant strains. Phenotypic screening has been widely employed as a reliable strategy to evaluate antiviral efficacy of novel agents independent of their modes of action, either directly targeting viral proteins or regulating cellular factors involved in the virus life cycle. Here, from the point of view of medicinal chemistry, we review the research progress of phenotypic screening strategies by focusing direct acting antivirals against influenza virus. It could provide scientific insights into discovery of a distinctive class of therapeutic candidates that ensure high efficiency but low cytotoxicity, and address issues from circulation of drug-resistant influenza viruses in the future.

Identification of the Imidazo[1,2-a]pyrazine Derivative as a Potential Influenza Virus Nucleoprotein Inhibitor

Influenza A viruses (IAVs) have gradually developed resistance to FDA-approved drugs, which increases the need to discover novel antivirals with new mechanisms of action. Here, we used a phenotypic screening strategy and discovered that the imidazo[1,2-a]pyrazine derivative A4 demonstrates potent and broad-spectrum anti-influenza activity, especially for the oseltamivir-resistant H1N1/pdm09 strain. Indirect immunofluorescence assays revealed that A4 induces clustering of the viral nucleoprotein (NP) and prevents its nuclear accumulation. Furthermore, upon conducting binding analyses between A4 and the influenza NP using surface plasmon resonance assays and molecular docking simulations, we were able to confirm that A4 binds directly to the viral NP. Additionally, A4 exhibits high human plasma metabolic stability (remaining120 min > 90%, T1/2 = 990 min) and moderate inhibitory effects on CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4 as well as low acute toxicity in Kunming mice. Overall, this study provides valuable insights and lays the groundwork for future efforts in medicinal chemistry to identify effective drugs against influenza.

A high throughput antiviral screening platform for alphaviruses based on Semliki Forest virus expressing eGFP reporter gene

Alphaviruses, which contain a variety of mosquito-borne pathogens, are important pathogens of emerging/re-emerging infectious diseases and potential biological weapons. Currently, no specific antiviral drugs are available for the treatment of alphaviruses infection. For most highly pathogenic alphaviruses are classified as risk group-3 agents, the requirement of biosafety level 3 (BSL-3) facilities limits the live virus-based antiviral study. To facilitate the antiviral development of alphaviruses, we developed a high throughput screening (HTS) platform based on a recombinant Semliki Forest virus (SFV) which can be manipulated in BSL-2 laboratory. Using the reverse genetics approach, the recombinant SFV and SFV reporter virus expressing eGFP (SFV-eGFP) were successfully rescued. The SFV-eGFP reporter virus exhibited robust eGFP expression and remained relatively stable after four passages in BHK-21 ​cells. Using a broad-spectrum alphavirus inhibitor ribavirin, we demonstrated that the SFV-eGFP can be used as an effective tool for antiviral study. The SFV-eGFP reporter virus-based HTS assay in a 96-well format was then established and optimized with a robust Z' score. A section of reference compounds that inhibit highly pathogenic alphaviruses were used to validate that the SFV-eGFP reporter virus-based HTS assay enables rapid screening of potent broad-spectrum inhibitors of alphaviruses. This assay provides a safe and convenient platform for antiviral study of alphaviruses.

Identification of West Nile virus RNA-dependent RNA polymerase non-nucleoside inhibitors by real-time high throughput fluorescence screening

West Nile virus (WNV) is a re-emergent mosquito-borne RNA virus that causes major outbreaks of encephalitis around the world. However, there is no therapeutic treatment to struggle against WNV, and the current treatment relies on alleviating symptoms. Therefore, due to the threat virus poses to animal and human health, there is an urgent need to come up with fast strategies to identify and assess effective antiviral compounds. A relevant target when developing drugs against RNA viruses is the viral RNA-dependent RNA polymerase (RdRp), responsible for the replication of the viral genome within a host cell. RdRps are key therapeutic targets based on their specificity for RNA and their essential role in the propagation of the infection. We have developed a fluorescence-based method to measure WNV RdRp activity in a fast and reliable real-time way. Interestingly, rilpivirine has shown in our assay inhibition of the WNV RdRp activity with an IC₅₀ value of 3.3 μM and its antiviral activity was confirmed in cell cultures. Furthermore, this method has been extended to build up a high-throughput screening platform to identify WNV polymerase inhibitors. By screening a small chemical library, novel RdRp inhibitors 1-4 have been identified. When their antiviral activity was tested against WNV in cell culture, 4 exhibited an EC₅₀ value of 2.5 μM and a selective index of 12.3. Thus, rilpivirine shows up as an interesting candidate for repurposing against flavivirus. Moreover, the here reported method allows the rapid identification of new WNV RdRp inhibitors.

Modifications in the piperazine ring of nucleozin affect anti-influenza activity

The infection caused by the influenza virus is a latent tret. The limited access to vaccines and approved drugs highlights the need for additional antiviral agents. Nucleozin and its analogs have gain attention for their promising anti-influenza activity. To contribute to the advancement of the discovery and design of nucleozin analogs, we analyzed piperazine-modified nucleozin analogs to increase conformational freedom. Also, we describe a new synthetic strategy to obtain nucleozin and its analogues, three molecules were synthesized and two of them were biologically evaluated in vitro. Although the analogues were less active than nucleozin, the loss of activity highlights the need for the piperazine ring to maintain the activity of nucleozin analogs. Interestingly, this result agrees with the prediction of anti-influenza activity made with a QSAR model presented in this work. The proposed model and the synthetic route will be useful for the further development of nucleozin analogs with antiviral activity.

Identification of Potent Zika Virus NS5 RNA-Dependent RNA Polymerase Inhibitors Combining Virtual Screening and Biological Assays

The Zika virus (ZIKV) epidemic poses a significant threat to human health globally. Thus, there is an urgent need for developing effective anti-ZIKV agents. ZIKV non-structural protein 5 RNA-dependent RNA polymerase (RdRp), a viral enzyme for viral replication, has been considered an attractive drug target. In this work, we screened an anti-infection compound library and a natural product library by virtual screening to identify potential candidates targeting RdRp. Then, five selected candidates were further applied for RdRp enzymatic analysis, cytotoxicity, and binding examination by SPR. Finally, posaconazole (POS) was confirmed to effectively inhibit both RdRp activity with an IC₅₀ of 4.29 μM and the ZIKV replication with an EC₅₀ of 0.59 μM. Moreover, POS was shown to reduce RdRp activity by binding with the key amino acid D666 through molecular docking and site-directed mutation analysis. For the first time, our work found that POS could inhibit ZIKV replication with a stronger inhibitory activity than chloroquine. This work also demonstrated fast anti-ZIKV screening for inhibitors of RdRp and provided POS as a potential anti-ZIKV agent.

Research progress on antitumor activity of XRP44X and analogues as microtubule targeting agents

Cancer threatens human health and life. Therefore, it is particularly important to develop safe and effective antitumor drugs. Microtubules, the main component of cytoskeleton, play an important role in maintaining cell morphology, mitosis, and signal transduction, which are one of important targets of antitumor drug research and development. Colchicine binding site inhibitors have dual effects of inhibiting proliferation and destroying blood vessels. In recent years, a series of inhibitors targeting this target have been studied and some progress has been made. XRP44X has a novel structure and overcomes some disadvantages of traditional inhibitors. It is also a multifunctional molecule that regulates not only the function of tubulin but also a variety of biological pathways. Therefore, the structure, synthesis, structure-activity relationship, and biological activity of XRP44X analogues reported in recent years were summarized in this paper, to provide a useful reference for the rational design of efficient colchicine binding site inhibitors.

Contemporary medicinal chemistry strategies for the discovery and optimization of influenza inhibitors targeting vRNP constituent proteins

Influenza is an acute respiratory infectious disease caused by the influenza virus, affecting people globally and causing significant social and economic losses. Due to the inevitable limitations of vaccines and approved drugs, there is an urgent need to discover new anti-influenza drugs with different mechanisms. The viral ribonucleoprotein complex (vRNP) plays an essential role in the life cycle of influenza viruses, representing an attractive target for drug design. In recent years, the functional area of constituent proteins in vRNP are widely used as targets for drug discovery, especially the PA endonuclease active site, the RNA-binding site of PB1, the cap-binding site of PB2 and the nuclear export signal of NP protein. Encouragingly, the PA inhibitor baloxavir has been marketed in Japan and the United States, and several drug candidates have also entered clinical trials, such as favipiravir. This article reviews the compositions and functions of the influenza virus vRNP and the research progress on vRNP inhibitors, and discusses the representative drug discovery and optimization strategies pursued.

Approaches for discovery of small-molecular antivirals targeting to influenza A virus PB2 subunit

Influenza is an acute respiratory infectious disease caused by influenza virus, leading to huge morbidity and mortality in humans worldwide. Despite the availability of antivirals in the clinic, the emergence of resistant strains calls for antivirals with novel mechanisms of action. The PB2 subunit of the influenza A virus polymerase is a promising target because of its vital role in the 'cap-snatching' mechanism. In this review, we summarize the technologies and medicinal chemistry strategies for hit identification, hit-to-lead and lead-to-candidate optimization, and current challenges in PB2 inhibitor development, as well as offering insights for the fight against drug resistance.

Inhibitory Potentiality of Secondary Metabolites Extracted from Marine Fungus Target on Avian Influenza Virus-A Subtype H5N8 (Neuraminidase) and H5N1 (Nucleoprotein): A Rational Virtual Screening

Highly contagious avian influenza virus’ (AIV) subtypes, including H5N1 and H5N8 are considered as serious threats for poultry industry. Despite its severity, treatment and mitigation attempts are fall into baffling. Though a few approved anti-influenza medications are available, the M2 channel blockers amantadine and rimantadine, as well as the neuraminidase inhibitor oseltamivir are being less effective due to widespread drug resistance. To cope up with these circumstances, scientists have found nucleoprotein as a novice drug targeting site for H5N1. Hence, the current study used a rational screening method to find the best candidates for nucleoprotein inhibitors of H5N1 subtype and neuraminidase inhibitors for H5N8 subtype against pathogenic AIV. Finding the best candidates, molecular docking method and computational pharmacokinetics and pharmacology was developed to estimate the potential of the multi-targeting fungal-derived natural compounds for the development of drug. Chevalone E compound was found as the best inhibitor for both nucleoprotein and neuraminidase of H5N1 and H5N8 subtypes respectively, whereas, Brevione F and Brocazine-A for nucleoprotein with Penilactone-A and Aspergifuranone for neuraminidase. In case of drug prediction, the study recommends Estramustine and Iloprost against both nucleoprotein and neuraminidase. Besides these, Butorphanol, Desvenlafaxine, Zidovudine and Nadolol are the best drug candidates for nucleoprotein inhibitors, meanwhile, Sitaxentan, Ergoloid mesylate, Capecitabine and Fenoterol act as speculated candidates against neuraminidase.

Targeted inhibition of the endonuclease activity of influenza polymerase acidic proteins

Influenza is a type of acute respiratory virus infection caused by the influenza virus that occurs in epidemics worldwide every year. Due to the increasing incidence of influenza virus resistance to existing drugs, researchers are looking for novel antiviral drugs with new mechanisms. The endonuclease activity of polymerase acidic protein is essential in the process of influenza virus reproduction, and inhibiting it could prevent the virus from replicating. There are relatively few drugs that act on this protein, and only baloxavir marboxil has been approved for clinical use. In this article, the structure and function of influenza virus polymerase acidic protein endonuclease, mechanism of action of polymerase acidic endonuclease inhibitors and the research progress of inhibitors are reviewed.

SARS-CoV-2 RNA-dependent RNA polymerase as a target for high-throughput drug screening

The ongoing COVID-19 pandemic caused by the SARS-CoV-2 has necessitated rapid development of drug screening tools. RNA-dependent RNA polymerase (RdRp) is a promising target due to its essential functions in replication and transcription of viral genome. To date, through minimal RNA synthesizing machinery established from cryo-electron microscopy structural data, there has been development of high-throughput screening assays for directly screening inhibitors that target the SARS-CoV-2 RdRp. Here, we analyze and present verified techniques that could be used to discover potential anti-RdRp agents or repurposing of approved drugs to target the SARS-CoV-2 RdRp. In addition, we highlight the characteristics and application value of cell-free or cell-based assays in drug discovery.

Type-IInterferon-Inducible SERTAD3 Inhibits Influenza A Virus Replication by Blocking the Assembly of Viral RNA Polymerase Complex

Influenza A virus (IAV) infection stimulates a type I interferon (IFN-I) response in host cells that exerts antiviral effects by inducing the expression of hundreds of IFN-stimulated genes (ISGs). However, most ISGs are poorly studied for their roles in the infection of IAV. Herein, we demonstrate that SERTA domain containing 3 (SERTAD3) has a significant inhibitory effect on IAV replication in vitro. More importantly, Sertad3^-/- mice develop more severe symptoms upon IAV infection. Mechanistically, we find SERTAD3 reduces IAV replication through interacting with viral polymerase basic protein 2 (PB2), polymerase basic protein 1 (PB1), and polymerase acidic protein (PA) to disrupt the formation of the RNA-dependent RNA polymerase (RdRp) complex. We further identify an 8-amino-acid peptide of SERTAD3 as a minimum interacting motif that can disrupt RdRp complex formation and inhibit IAV replication. Thus, our studies not only identify SERTAD3 as an antiviral ISG, but also provide the mechanism of potential application of SERTAD3-derived peptide in suppressing influenza replication.

Discovery of a Novel Specific Inhibitor Targeting Influenza A Virus Nucleoprotein with Pleiotropic Inhibitory Effects on Various Steps of the Viral Life Cycle

Influenza A viruses (IAVs) continue to pose an imminent threat to humans due to annual influenza epidemic outbreaks and episodic pandemics with high mortality rates. In this context, the suboptimal vaccine coverage and efficacy, coupled with recurrent events of viral resistance against a very limited antiviral portfolio, emphasize an urgent need for new additional prophylactic and therapeutic options, including new antiviral targets and drugs with new mechanisms of action to prevent and treat influenza virus infection. Here, we characterized a novel influenza A virus nucleoprotein (NP) inhibitor, FA-6005, that inhibited a broad spectrum of human pandemic and seasonal influenza A and B viruses in vitro and protects mice against lethal influenza A virus challenge. The small molecule FA-6005 targeted a conserved NP I41 domain and acted as a potentially broad, multimechanistic anti-influenza virus therapeutic since FA-6005 suppressed influenza virus replication and perturbed intracellular trafficking of viral ribonucleoproteins (vRNPs) from early to late stages. Cocrystal structures of the NP/FA-6005 complex reconciled well with concurrent mutational studies. This study provides the first line of direct evidence suggesting that the newly identified NP I41 pocket is an attractive target for drug development that inhibits multiple functions of NP. Our results also highlight FA-6005 as a promising candidate for further development as an antiviral drug for the treatment of IAV infection and provide chemical-level details for inhibitor optimization.IMPORTANCE Current influenza antivirals have limitations with regard to their effectiveness and the potential emergence of resistance. Therefore, there is an urgent need for broad-spectrum inhibitors to address the considerable challenges posed by the rapid evolution of influenza viruses that limit the effectiveness of vaccines and lead to the emergence of antiviral drug resistance. Here, we identified a novel influenza A virus NP antagonist, FA-6005, with broad-spectrum efficacy against influenza viruses, and our study presents a comprehensive study of the mode of action of FA-6005 with the crystal structure of the compound in complex with NP. The influenza virus inhibitor holds promise as an urgently sought-after therapeutic option offering a mechanism of action complementary to existing antiviral drugs for the treatment of influenza virus infection and should further aid in the development of universal therapeutics.

Insights into RNA-dependent RNA Polymerase Inhibitors as Antiinfluenza Virus Agents

BACKGROUND

Influenza is a seasonal disease that affects millions of people every year and has a significant economic impact. Vaccines are the best strategy to fight this viral pathology, but they are not always available or administrable, prompting the search for antiviral drugs. RNA-dependent RNA polymerase (RdRp) recently emerged as a promising target because of its key role in viral replication and its high conservation among viral strains.

DISCUSSION

This review presents an overview of the most interesting RdRp inhibitors that have been discussed in the literature since 2000. Compounds already approved or in clinical trials and a selection of inhibitors endowed with different scaffolds are described, along with the main features responsible for their activity.

RESULTS

RdRp inhibitors are emerging as a new strategy to fight viral infections and the importance of this class of drugs has been confirmed by the FDA approval of baloxavir marboxil in 2018. Despite the complexity of the RdRp machine makes the identification of new compounds a challenging research topic, it is likely that in the coming years, this field will attract the interest of a number of academic and industrial scientists because of the potential strength of this therapeutic approach.

RNA-Dependent RNA Polymerase as a Target for COVID-19 Drug Discovery

COVID-19 respiratory disease caused by the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has rapidly become a global health issue since it emerged in December 2019. While great global efforts are underway to develop vaccines and to discover or repurpose therapeutic agents for this disease, as of this writing only the nucleoside drug remdesivir has been approved under Emergency Use Authorization to treat COVID-19. The RNA-dependent RNA polymerase (RdRP), a viral enzyme for viral RNA replication in host cells, is one of the most intriguing and promising drug targets for SARS-CoV-2 drug development. Because RdRP is a viral enzyme with no host cell homologs, selective SARS-CoV-2 RdRP inhibitors can be developed that have improved potency and fewer off-target effects against human host proteins and thus are safer and more effective therapeutics for treating COVID-19. This review focuses on biochemical enzyme and cell-based assays for RdRPs that could be used in high-throughput screening to discover new and repurposed drugs against SARS-CoV-2.

A Single Amino Acid at Position 431 of the PB2 Protein Determines the Virulence of H1N1 Swine Influenza Viruses in Mice

The frequent reassortment among different influenza viruses in pigs adds complexity to the epidemiology of swine influenza. The diverse viral virulence phenotypes underline the need to investigate the possible genetic determinants for evaluating the pandemic potential to human public health. Here, we found that multiple genotypes of influenza viruses cocirculate in the swine population in Liaoning Province, China. Furthermore, we pinpointed a single amino acid at position 431 in the PB2 protein which plays a critical role in the virulence of H1N1 viruses in mice and found that the alteration of viral polymerase activities is the cause of the different virulence. Our study further indicated that the virulence of influenza virus is a polygenic trait, and the newly identified virulence-related residue in the PB2 provides important information for broadening knowledge on the genetic basis of viral virulence of influenza viruses.

Genetic reassortments occurred continuously among multiple subtypes or genotypes of influenza viruses prevalent in pigs. Of note, some reassortant viruses bearing the internal genes of the 2009 pandemic H1N1 (2009/H1N1) virus sporadically caused human infection, which highlights their potential threats to human public health. In this study, we performed phylogenetic analysis on swine influenza viruses (SIVs) circulating in Liaoning Province, China. A total of 22 viruses, including 18 H1N1 and 4 H1N2 viruses, were isolated from 5,750 nasal swabs collected from pigs in slaughterhouses from 2014 to 2016. H1N1 viruses formed four genotypes, which included Eurasian avian-like H1N1 (EA H1N1) and double/triple reassortant H1N1 derived from EA H1N1, 2009/H1N1, and triple reassortant H1N2 (TR H1N2) viruses. H1N1 SIVs with different genotypes and even those within the same genotypes represented different pathogenicities in mice. We further characterized two naturally isolated H1N1 SIVs that had similar viral genomes but differed substantially in their virulence in mice and found that a single amino acid at position 431 in the basic polymerase 2 (PB2) protein significantly affected the viral replication capacity and virulence of these two viruses. Taken together, our findings revealed the diverse genomic origins and virulence of the SIVs prevalent in Liaoning Province during 2014 to 2016, which highlights that continuous surveillance is essential to monitor the evolution of SIVs. We identified a naturally occurring amino acid mutation in the PB2 protein of H1N1 SIVs that impacts the viral replication and virulence in mice by altering the viral polymerase activity.

IMPORTANCE The frequent reassortment among different influenza viruses in pigs adds complexity to the epidemiology of swine influenza. The diverse viral virulence phenotypes underline the need to investigate the possible genetic determinants for evaluating the pandemic potential to human public health. Here, we found that multiple genotypes of influenza viruses cocirculate in the swine population in Liaoning Province, China. Furthermore, we pinpointed a single amino acid at position 431 in the PB2 protein which plays a critical role in the virulence of H1N1 viruses in mice and found that the alteration of viral polymerase activities is the cause of the different virulence. Our study further indicated that the virulence of influenza virus is a polygenic trait, and the newly identified virulence-related residue in the PB2 provides important information for broadening knowledge on the genetic basis of viral virulence of influenza viruses.

Design, synthesis and in vitro biological evaluation of isoxazol-4-carboxa piperidyl derivatives as new anti-influenza A agents targeting virus nucleoprotein

Influenza infection is a major cause of morbidity and mortality during seasonal epidemics and sporadic pandemics. It is important and urgent to develop new anti-influenza agents with a new mechanism of action. Nucleozin has been reported as a potent antagonist of nucleoprotein accumulation in the nucleus. In this study, a new series of isoxazol-4-carboxa piperidyl derivatives 1a-j were synthesized and their chemical structures were confirmed by 1H, 13C NMR and mass spectral data. Furthermore, all the synthesized compounds were evaluated for in vitro anti-influenza virus activity against influenza virus (A/PR/8/34 H1N1). Among all the compounds, 1a, 1b, 1c, 1f and 1g exhibited more potent activity than the standard drug, and compound 1b has showed most promising anti-influenza virus activity. These results are also consistent with the docking study results in terms of the design of compounds targeting influenza A via viral nucleoprotein.

Disrupting the Conserved Salt Bridge in the Trimerization of Influenza A Nucleoprotein

Antiviral drug resistance in influenza infections has been a major threat to public health. To develop a broad-spectrum inhibitor of influenza to combat the problem of drug resistance, we previously identified the highly conserved E339...R416 salt bridge of the nucleoprotein trimer as a target and compound 1 as an inhibitor disrupting the salt bridge with an EC₅₀ = 2.7 μM against influenza A (A/WSN/1933). We have further modified this compound via a structure-based approach and performed antiviral activity screening to identify compounds 29 and 30 with EC₅₀ values of 110 and 120 nM, respectively, and without measurable host cell cytotoxicity. Compared to the clinically used neuraminidase inhibitors, these two compounds showed better activity profiles against drug-resistant influenza A strains, as well as influenza B, and improved survival of influenza-infected mice.

A Parallel Phenotypic Versus Target-Based Screening Strategy for RNA-Dependent RNA Polymerase Inhibitors of the Influenza A Virus

Influenza A virus infections cause significant morbidity and mortality, and novel antivirals are urgently needed. Influenza RNA-dependent RNA polymerase (RdRp) activity has been acknowledged as a promising target for novel antivirals. In this study, a phenotypic versus target-based screening strategy was established to identify the influenza A virus inhibitors targeting the virus RNA transcription/replication steps by sequentially using an RdRp-targeted screen and a replication-competent reporter virus-based approach using the same compounds. To demonstrate the utility of this approach, a pilot screen of a library of 891 compounds derived from natural products was carried out. Quality control analysis indicates that the primary screen was robust for identification of influenza A virus inhibitors targeting RdRp activity. Finally, two hit candidates were identified, and one was validated as a putative RdRp inhibitor. This strategy can greatly reduce the number of false positives and improve the accuracy and efficacy of primary screening, thereby providing a powerful tool for antiviral discovery.

Antivirals targeting the polymerase complex of influenza viruses

Current influenza antivirals have limitations with regard to their effectiveness and the potential emergence of resistance. Encouragingly, several new compounds which inhibit the polymerase of influenza viruses have recently been shown to have enhanced pre-clinical and clinical effectiveness compared to the neuraminidase inhibitors, the mainstay of influenza antiviral therapy over the last two decades. In this review we focus on four compounds which inhibit polymerase function, baloxavir marboxil, favipiravir, pimodivir and AL-794 and discuss their clinical and virological effectiveness, their propensity to select for resistance and their potential for future combination therapy with the most commonly used neuraminidase inhibitor, oseltamivir.

Naproxen Exhibits Broad Anti-influenza Virus Activity in Mice by Impeding Viral Nucleoprotein Nuclear Export

Naproxen is a non-steroidal anti-inflammatory drug that has previously been shown to exert antiviral activity against influenza A virus by inhibiting nucleoprotein (NP) binding to RNA. Here, we show that naproxen is a potential broad, multi-mechanistic anti-influenza virus therapeutic, as it inhibits influenza B virus replication both in vivo and in vitro. The anti-influenza B virus activity of naproxen is more efficient than that of the commonly used neuraminidase inhibitor oseltamivir in mice. Furthermore, the NP of influenza B virus (BNP) has a higher binding affinity to naproxen than influenza A virus NP (ANP). Specifically, naproxen targets the NP at residues F209 (BNP) and Y148 (ANP). This interaction antagonizes the nuclear export of NP normally mediated by the host export protein CRM1. This study reveals a crucial mechanism of broad-spectrum anti-influenza virus activity of naproxen, suggesting that the existing drug naproxen may be used as an anti-influenza drug.

Development of a fluorescence-based method for the rapid determination of Zika virus polymerase activity and the screening of antiviral drugs

Zika virus (ZIKV) is an emerging pathogen that has been associated with large numbers of cases of severe neurologic disease, including Guillain-Barré syndrome and microcephaly. Despite its recent establishment as a serious global public health concern there are no licensed therapeutics to control this virus. Accordingly, there is an urgent need to develop methods for the high-throughput screening of antiviral agents. We describe here a fluorescence-based method to monitor the real-time polymerization activity of Zika virus RNA-dependent RNA polymerase (RdRp). By using homopolymeric RNA template molecules, de novo RNA synthesis can be detected with a fluorescent dye, which permits the specific quantification and kinetics of double-strand RNA formation. ZIKV RdRp activity detected using this fluorescence-based assay positively correlated with traditional assays measuring the incorporation of radiolabeled nucleotides. We also validated this method as a suitable assay for the identification of ZIKV inhibitors targeting the viral polymerase using known broad-spectrum inhibitors. The assay was also successfully adapted to detect RNA polymerization activity by different RdRps, illustrated here using purified RdRps from hepatitis C virus and foot-and-mouth disease virus. The potential of fluorescence-based approaches for the enzymatic characterization of viral polymerases, as well as for high-throughput screening of antiviral drugs, are discussed.

Novel influenza inhibitors designed to target PB1 interactions with host importin RanBP5

In search of novel targets for influenza inhibitors, a site on PB1 was selected for its high conservation and probable interaction with a host protein, RanBP5, that is key to nuclear import of PB1, where it complexes with PB2, PA, and NP to transcribe viral RNA. Docking with libraries of drug-like compounds led to a selection of five candidates that bound tightly and with a pose likely to inhibit protein binding. These were purchased and tested in vitro, found to be active, and then one was synthetically expanded to explore the structure-activity relationship. The top candidates had a carboxylic acid converted to an ester and electron-withdrawing substituents added to a phenyl group in the original structure. Resistance was slow to develop, but cytotoxicity was moderately high. Nuclear localization of PB1 and in vitro polymerase activity were both strongly inhibited.

Temperature Sensitive Mutations in Influenza A Viral Ribonucleoprotein Complex Responsible for the Attenuation of the Live Attenuated Influenza Vaccine

Live attenuated influenza vaccines (LAIV) have prevented morbidity and mortality associated with influenza viral infections for many years and represent the best therapeutic option to protect against influenza viral infections in humans. However, the development of LAIV has traditionally relied on empirical methods, such as the adaptation of viruses to replicate at low temperatures. These approaches require an extensive investment of time and resources before identifying potential vaccine candidates that can be safely implemented as LAIV to protect humans. In addition, the mechanism of attenuation of these vaccines is poorly understood in some cases. Importantly, LAIV are more efficacious than inactivated vaccines because their ability to mount efficient innate and adaptive humoral and cellular immune responses. Therefore, the design of potential LAIV based on known properties of viral proteins appears to be a highly appropriate option for the treatment of influenza viral infections. For that, the viral RNA synthesis machinery has been a research focus to identify key amino acid substitutions that can lead to viral attenuation and their use in safe, immunogenic, and protective LAIV. In this review, we discuss the potential to manipulate the influenza viral RNA-dependent RNA polymerase (RdRp) complex to generate attenuated forms of the virus that can be used as LAIV for the treatment of influenza viral infections, one of the current and most effective prophylactic options for the control of influenza in humans.

The effect of P38 MAP kinase inhibition in a mouse model of influenza

PURPOSE

Influenza viruses are a common cause of human respiratory infections, resulting in epidemics of high morbidity and mortality. We investigated the effect of a novel mitogen-activated protein kinase (MAPK) inhibitor in vitro and in a murine influenza model to further explore whether p38 MAPK inhibition could reduce viral replication.

METHODS

In vitro, the antiviral effect of p38 MAPK inhibitor BCT194 was evaluated in differentiated human bronchial epithelial cells (HBECs); in vivo, female BALB/c mice were infected intranasally with 150 pfu of influenza H1N1 A/Puerto Rico/8/34 and treated with BCT197 (a closely related p38 MAPK inhibitor with an IC50 value of<1 µM, currently in clinical testing), dexamethasone or oseltamivir (Tamiflu) starting 24 h post infection. Body weight, bronchoalveolar lavage cells, cytokines, total protein and lactate dehydrogenase as well as serum cytokines were measured; a subset of animals was evaluated histopathologically.Results/Key findings. p38MAP kinase inhibition with BCT194 had no impact on influenza replication in HBECs. When examining BCT197 in vivo, and comparing to vehicle-treated animals, reduced weight loss, improvement in survival and lack of impaired viral control was observed at BCT197 concentrations relevant to those being used in clinical trials of acute exacerbations of chronic obstructive pulmonary disease; at higher concentrations of BCT197 these effects were reduced.

CONCLUSIONS

Compared to vehicle treatment, BCT197 (administered at a clinically relevant concentration) improved outcomes in a mouse model of influenza. This is encouraging given that the use of innate inflammatory pathway inhibitors may raise concerns of negative effects on infection regulation.

Verdinexor Targeting of CRM1 is a Promising Therapeutic Approach against RSV and Influenza Viruses

Two primary causes of respiratory tract infections are respiratory syncytial virus (RSV) and influenza viruses, both of which remain major public health concerns. There are a limited number of antiviral drugs available for the treatment of RSV and influenza, each having limited effectiveness and each driving selective pressure for the emergence of drug-resistant viruses. Novel broad-spectrum antivirals are needed to circumvent problems with current disease intervention strategies, while improving the cytokine-induced immunopathology associated with RSV and influenza infections. In this review, we examine the use of Verdinexor (KPT-335, a novel orally bioavailable drug that functions as a selective inhibitor of nuclear export, SINE), as an antiviral with multifaceted therapeutic potential. KPT-335 works to (1) block CRM1 (i.e., Chromosome Region Maintenance 1; exportin 1 or XPO1) mediated export of viral proteins critical for RSV and influenza pathogenesis; and (2) repress nuclear factor κB (NF-κB) activation, thus reducing cytokine production and eliminating virus-associated immunopathology. The repurposing of SINE compounds as antivirals shows promise not only against RSV and influenza virus but also against other viruses that exploit the nucleus as part of their viral life cycle.

Influenza A virus polymerase: an attractive target for next-generation anti-influenza therapeutics

The influenza RNA-dependent RNA polymerase (RdRP) is conserved among different types of influenza virus, playing an important part in transcription and replication. In this regard, influenza RdRP is an attractive target for novel anti-influenza drug discovery. Herein, we will introduce the structural and functional information of influenza polymerase; and an overview of inhibitors targeting the PA endonuclease and PB2 cap-binding site is provided, along with the approaches utilized for identification of these inhibitors. The protein-protein interactions (PPIs) of the three polymerase subunits: PA, PB1 and PB2, are described based on the published crystal structures, and inhibitors targeting the PA-PB1 interaction are introduced briefly.

Identification of a novel compound targeting the nuclear export of influenza A virus nucleoprotein

Although antiviral drugs are available for the treatment of influenza infection, it is an urgent requirement to develop new antiviral drugs regarding the emergence of drug‐resistant viruses. The nucleoprotein (NP) is conserved among all influenza A viruses (IAVs) and has no cellular equivalent. Therefore, NP is an ideal target for the development of new IAV inhibitors. In this study, we identified a novel anti‐influenza compound, ZBMD‐1, from a library of 20,000 compounds using cell‐based influenza A infection assays. We found that ZBMD‐1 inhibited the replication of H1N1 and H3N2 influenza A virus strains in vitro, with an IC₅₀ ranging from 0.41–1.14 μM. Furthermore, ZBMD‐1 inhibited the polymerase activity and specifically impaired the nuclear export of NP. Further investigation indicated that ZBMD‐1 binds to the nuclear export signal 3 (NES3) domain and the dimer interface of the NP pocket. ZBMD‐1 also protected mice that were challenged with lethal doses of A/PR/8/1934 (H1N1) virus, effectively relieving lung histopathology changes, as well as strongly inhibiting the expression of pro‐inflammatory cytokines/chemokines, without inducing toxicity effects in mice. These results suggest that ZBMD‐1 is a promising anti‐influenza compound which can be further investigated as a useful strategy against IAVs in the future.

Influenza A Virus Nucleoprotein: A Highly Conserved Multi-Functional Viral Protein as a Hot Antiviral Drug Target

Prevention and treatment of influenza virus infection is an ongoing unmet medical need. Each year, thousands of deaths and millions of hospitalizations are attributed to influenza virus infection, which poses a tremendous health and economic burden to the society. Aside from the annual influenza season, influenza viruses also lead to occasional influenza pandemics as a result of emerging or re-emerging influenza strains. Influenza viruses are RNA viruses that exist in quasispecies, meaning that they have a very diverse genetic background. Such a feature creates a grand challenge in devising therapeutic intervention strategies to inhibit influenza virus replication, as a single agent might not be able to inhibit all influenza virus strains. Both classes of currently approved anti-influenza drugs have limitations: the M2 channel blockers amantadine and rimantadine are no longer recommended for use in the U.S. due to predominant drug resistance, and resistance to the neuraminidase inhibitor oseltamivir is continuously on the rise. In pursuing the next generation of antiviral drugs with broad-spectrum activity and higher genetic barrier of drug resistance, the influenza virus nucleoprotein (NP) stands out as a high-profile drug target. This review summarizes recent developments in designing inhibitors targeting influenza NP and their mechanisms of action.

CNOT4-Mediated Ubiquitination of Influenza A Virus Nucleoprotein Promotes Viral RNA Replication

Influenza A virus (IAV) RNA segments are individually packaged with viral nucleoprotein (NP) and RNA polymerases to form a viral ribonucleoprotein (vRNP) complex. We previously reported that NP is a monoubiquitinated protein which can be deubiquitinated by a cellular ubiquitin protease, USP11. In this study, we identified an E3 ubiquitin ligase, CNOT4 (Ccr4-Not transcription complex subunit 4), which can ubiquitinate NP. We found that the levels of viral RNA, protein, viral particles, and RNA polymerase activity in CNOT4 knockdown cells were lower than those in the control cells upon IAV infection. Conversely, overexpression of CNOT4 rescued viral RNP activity. In addition, CNOT4 interacted with the NP in the cell. An in vitro ubiquitination assay also showed that NP could be ubiquitinated by in vitro-translated CNOT4, but ubiquitination did not affect the protein stability of NP. Significantly, CNOT4 increased NP ubiquitination, whereas USP11 decreased it. Mass spectrometry analysis of ubiquitinated NP revealed multiple ubiquitination sites on the various lysine residues of NP. Three of these, K184, K227, and K273, are located on the RNA-binding groove of NP. Mutations of these sites to arginine reduced viral RNA replication. These results indicate that CNOT4 is a ubiquitin ligase of NP, and ubiquitination of NP plays a positive role in viral RNA replication.

Influenza virus, particularly influenza A virus, causes severe and frequent outbreaks among human and avian species. Finding potential target sites for antiviral agents is of utmost importance from the public health point of view. We previously found that viral nucleoprotein (NP) is ubiquitinated, and ubiquitination enhances viral RNA replication. In this study, we found a cellular ubiquitin ligase, CNOT4, capable of ubiquitinating NP. The ubiquitination sites are scattered on the surface of the NP molecule, which is critical for RNA replication. CNOT4 and a ubiquitin protease, USP11, together regulate the extent of NP ubiquitination and thereby the efficiency of RNA replication. This study thus identifies a potential antiviral target site and reveals a novel posttranslational mechanism for regulating viral replication. This represents a novel finding in the literature of influenza virus research.

Progress of small molecular inhibitors in the development of anti-influenza virus agents

The influenza pandemic is a major threat to human health, and highly aggressive strains such as H1N1, H5N1 and H7N9 have emphasized the need for therapeutic strategies to combat these pathogens. Influenza anti-viral agents, especially active small molecular inhibitors play important roles in controlling pandemics while vaccines are developed. Currently, only a few drugs, which function as influenza neuraminidase (NA) inhibitors and M2 ion channel protein inhibitors, are approved in clinical. However, the acquired resistance against current anti-influenza drugs and the emerging mutations of influenza virus itself remain the major challenging unmet medical needs for influenza treatment. It is highly desirable to identify novel anti-influenza agents. This paper reviews the progress of small molecular inhibitors act as antiviral agents, which include hemagglutinin (HA) inhibitors, RNA-dependent RNA polymerase (RdRp) inhibitors, NA inhibitors and M2 ion channel protein inhibitors etc. Moreover, we also summarize new, recently reported potential targets and discuss strategies for the development of new anti-influenza virus drugs.

Activities of JNJ63623872 and oseltamivir against influenza A H1N1pdm and H3N2 virus infections in mice

JNJ63623872 (formerly known as VX-787) is an inhibitor of influenza A virus polymerases through interaction with the viral PB2 subunit. This interaction blocks the cap-snatching activity of the virus that is essential for virus replication. Previously published work has documented antiviral activity of JNJ63623872 in cell culture and mouse infection studies. In this report, we extend the in vivo observations by comparing the efficacies of JNJ63623872 and oseltamivir in mice infected with influenza A/California/04/2009 (H1N1pdm) and A/Victoria/3/75 (H3N2) viruses. Animals received JNJ63623872 or oseltamivir orally twice daily for 10 days starting 2 h pre-infection. JNJ63623872 (2, 6, and 20 mg/kg/day) and oseltamivir (20 mg/kg/day) completely prevented death in the H1N1pdm virus infection. Weight loss at nadir was only 12% in mice receiving 2 mg/kg/day of JNJ63623872 compared to 23% and 32%, respectively, in oseltamivir-treated (20 mg/kg/day) and placebo groups. Lung hemorrhage scores, lung weights, and lung virus titers on day 6 were reduced in a dose-responsive manner by JNJ63623872 treatments, whereas oseltamivir treatments were not as effective. JNJ63623872 was less active against H3N2 virus infection, with more body weight loss occurring and only 30% survival at the 2-mg/kg/day dose. Lung scores, lung weights, and H3N2 viral titers in lungs of mice were reduced less by JNJ63623872 treatments compared to the H1N1pdm infection. Nevertheless, the 20-mg/kg/day dose of JNJ63623872 was more effective than oseltamivir (20 mg/kg/day) in improving body weight and reducing the severity of lung infection. JNJ63623872 appears to be an important new drug candidate to treat influenza A H1N1pdm and H3N2 virus infections.

The Influenza Virus Polymerase Complex: An Update on Its Structure, Functions, and Significance for Antiviral Drug Design

Influenza viruses cause seasonal epidemics and pandemic outbreaks associated with significant morbidity and mortality, and a huge cost. Since resistance to the existing anti‐influenza drugs is rising, innovative inhibitors with a different mode of action are urgently needed. The influenza polymerase complex is widely recognized as a key drug target, given its critical role in virus replication and high degree of conservation among influenza A (of human or zoonotic origin) and B viruses. We here review the major progress that has been made in recent years in unravelling the structure and functions of this protein complex, enabling structure‐aided drug design toward the core regions of the PA endonuclease, PB1 polymerase, or cap‐binding PB2 subunit. Alternatively, inhibitors may target a protein–protein interaction site, a cellular factor involved in viral RNA synthesis, the viral RNA itself, or the nucleoprotein component of the viral ribonucleoprotein. The latest advances made for these diverse pharmacological targets have yielded agents in advanced (i.e., favipiravir and VX‐787) or early clinical testing, besides several experimental inhibitors in various stages of development, which are all covered here.

Antiviral therapies on the horizon for influenza

Adequate response to severe influenza infections or pandemic outbreaks requires two complementary strategies: preventive vaccination and antiviral therapy. The existing influenza drugs, M2 blockers and neuraminidase inhibitors, show modest clinical efficacy and established or potential resistance. In the past three years, several new agents have entered the clinical pipeline and already yielded some promising data from Phase 2 trials. For two main categories, that is, the broadly neutralizing anti-hemagglutinin antibodies and small-molecule inhibitors of the viral polymerase complex, crystallography was instrumental to guide drug development. These structural insights also aid to expand the activity spectrum towards influenza A plus B viruses, or conceive nucleoprotein or polymerase assembly inhibitors. The practice of influenza therapy should radically change in the next decade.

Open reading frames 1a and 1b of the porcine reproductive and respiratory syndrome virus (PRRSV) collaboratively initiate viral minus-strand RNA synthesis

The porcine reproductive and respiratory syndrome virus (PRRSV) causes a persistent threat to the swine industry, especially when highly pathogenic PRRSV (HP-PRRSV) emerges. Previous studies have indicated that PRRSV RNA synthesis was correlated with HP-PRRSV virulence. PRRSV RNA synthesis includes genomic RNA and sub-genomic mRNA, and these processes require minus-strand RNA as a template. However, the mechanisms involved in PRRSV minus-strand RNA synthesis are not fully understood. A mini-genome system can be used to assess viral replication mechanisms and to evaluate the effects of potential antiviral drugs on viral replicase activities. In this study, we developed a mini-genome system that uses firefly luciferase as a reporter. Based on this system, we found that PRRSV RNA-dependent RNA polymerase nsp9 alone failed to activate virus minus-strand RNA synthesis. We also demonstrated that combinations of open reading frames 1a (ORF1a) and ORF1b are necessary for viral minus-strand RNA synthesis.

A High Throughput Assay for Screening Host Restriction Factors and Antivirals Targeting Influenza A Virus

Influenza A virus (IAV) is a human respiratory pathogen that causes seasonal epidemics and occasional global pandemics with devastating levels of morbidity and mortality. Currently approved treatments against influenza are losing effectiveness, as new viral strains are often refractory to conventional treatments. Thus, there is an urgent need to find new therapeutic targets with which to develop novel antiviral drugs. The common strategy to discover new drug targets and antivirals is high throughput screening. However, most current screenings for IAV rely on the engineered virus carrying a reporter, which prevents the application to newly emerging wild type flu viruses, such as 2009 pandemic H1N1 flu. Here we developed a simple and sensitive screening assay for wild type IAV by quantitatively analyzing viral protein levels using a Dot Blot Assay in combination with the LI-COR Imaging System (DBALIS). We first validated DBALIS in overexpression and RNAi assays, which are suitable methods for screening host factors regulating viral infection. More importantly, we also validated and initiated drug screening using DBALIS. A pilot compound screening identified a small molecule that inhibited IAV infection. Taken together, our method represents a reliable and convenient high throughput assay for screening novel host factors and antiviral compounds.

Development and applications of single-cycle infectious influenza A virus (sciIAV)

The diverse host range, high transmissibility, and rapid evolution of influenza A viruses justify the importance of containing pathogenic viruses studied in the laboratory. Other than physically or mechanically changing influenza A virus containment procedures, modifying the virus to only replicate for a single round of infection similarly ensures safety and consequently decreases the level of biosafety containment required to study highly pathogenic members in the virus family. This biological containment is more ideal because it is less apt to computer, machine, or human error. With many necessary proteins that can be deleted, generation of single-cycle infectious influenza A viruses (sciIAV) can be achieved using a variety of approaches. Here, we review the recent burst in sciIAV generation and summarize the applications and findings on this important human pathogen using biocontained viral mimics.

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.

Using mutagenesis to explore conserved residues in the RNA-binding groove of influenza A virus nucleoprotein for antiviral drug development

Nucleoprotein (NP) is the most abundant type of RNA-binding viral protein in influenza A virus-infected cells and is necessary for viral RNA transcription and replication. Recent studies demonstrated that influenza NP is a valid target for antiviral drug development. The surface of the groove, covered with numerous conserved residues between the head and body domains of influenza A NP, plays a crucial role in RNA binding. To explore the mechanism by which NP binds RNA, we performed a series of site-directed mutagenesis in the RNA-binding groove, followed by surface plasmon resonance (SPR), to characterize the interactions between RNA and NP. Furthermore, a role of Y148 in NP stability and NP-RNA binding was evaluated. The aromatic residue of Y148 was found to stack with a nucleotide base. By interrupting the stacking interaction between Y148 and an RNA base, we identified an influenza virus NP inhibitor, (E, E)-1,7-bis(4-hydroxy-3-methoxyphenyl) -1,6-heptadiene-3,5-dione; this inhibitor reduced the NP's RNA-binding affinity and hindered viral replication. Our findings will be useful for the development of new drugs that disrupt the interaction between RNA and viral NP in the influenza virus.

New small-molecule drug design strategies for fighting resistant influenza A

Influenza A virus is the major cause of seasonal or pandemic flu worldwide. Two main treatment strategies-vaccination and small molecule anti-influenza drugs are currently available. As an effective vaccine usually takes at least 6 months to develop, anti-influenza small molecule drugs are more effective for the first line of protection against the virus during an epidemic outbreak, especially in the early stage. Two major classes of anti-influenza drugs currently available are admantane-based M2 protein blockers (amantadine and rimantadine) and neuraminidase (NA) inhibitors (oseltamivir, zanamivir, and peramivir). However, the continuous evolvement of influenza A virus and the rapid emergence of resistance to current drugs, particularly to amantadine, rimantadine, and oseltamivir, have raised an urgent need for developing new anti-influenza drugs against resistant forms of influenza A virus. In this review, we first give a brief introduction of the molecular mechanisms behind resistance, and then discuss new strategies in small-molecule drug development to overcome influenza A virus resistance targeting mutant M2 proteins and neuraminidases, and other viral proteins not associated with current drugs.

Synthesis and inhibitory effects of novel pyrimido-pyrrolo-quinoxalinedione analogues targeting nucleoproteins of influenza A virus H1N1

The influenza nucleoprotein (NP) is a single-strand RNA-binding protein and the core of the influenza ribonucleoprotein (RNP) particle that serves many critical functions for influenza replication. NP has been considered as a promising anti-influenza target. A new class of anti-influenza compounds, nucleozin and analogues were reported recently in several laboratories to inhibit the synthesis of influenza macromolecules and prevent the cytoplasmic trafficking of the influenza RNP. In this study, pyrimido-pyrrolo-quinoxalinedione (PPQ) analogues as a new class of novel anti-influenza agents are reported. Compound PPQ-581 was identified as a potential anti-influenza lead with EC50 value of 1 μM for preventing virus-induced cytopathic effects. PPQ produces similar anti-influenza effects as nucleozin does in influenza-infected cells. Treatment with PPQ at the beginning of H1N1 infection inhibited viral protein synthesis, while treatment at later times blocked the RNP nuclear export and the appearance of cytoplasmic RNP aggregation. PPQ resistant H1N1 (WSN) viruses were isolated and found to have a NPS377G mutation. Recombinant WSN carrying the S377G NP is resistant to PPQ in anti-influenza and RNA polymerase assays. The WSN virus with the NPS377G mutation also is devoid of the PPQ-mediated RNP nuclear retention and cytoplasmic aggregation. The NPS377G expressing WSN virus is not resistant to the reported NP inhibitors nucleozin. Similarly, the nucleozin resistant WSN viruses are not resistant to PPQ, suggesting that PPQ targets a different site from the nucleozin-binding site. Our results also suggest that NP can be targeted through various binding sites to interrupt the crucial RNP trafficking, resulting in influenza replication inhibition.

Establishment of a High-Throughput Assay to Monitor Influenza A Virus RNA Transcription and Replication

Influenza A virus (IAV) poses significant threats to public health because of the recent emergence of highly pathogenic strains and wide-spread resistance to available anti-influenza drugs. Therefore, new antiviral targets and new drugs to fight influenza virus infections are needed. Although IAV RNA transcription/replication represents a promising target for antiviral drug development, no assay ideal for high-throughput screening (HTS) application is currently available to identify inhibitors targeting these processes. In this work, we developed a novel HTS assay to analyze the transcription and replication of IAV RNA using an A549 cell line stably expressing IAV RNA-dependent RNA polymerase (RdRp) complex, NP and a viral mini-genomic RNA. Both secreted Gaussia luciferase (Gluc) and blasticidin resistance gene (Bsd) were encoded in the viral minigenome and expressed under the control of IAV RdRp. Gluc serves as a reporter to monitor the activity of IAV RdRp, and Bsd is used to maintain the expression of all foreign genes. Biochemical studies and the statistical analysis presented herein demonstrate the high specificity, sensitivity and reproducibility of the assay. This work provides an ideal HTS assay for the identification of inhibitors targeting the function of IAV RdRp and a convenient reporting system for mechanism study of IAV RNA transcription / replication.

Phosphorylation at the homotypic interface regulates nucleoprotein oligomerization and assembly of the influenza virus replication machinery

Negative-sense RNA viruses assemble large ribonucleoprotein (RNP) complexes that direct replication and transcription of the viral genome. Influenza virus RNPs contain the polymerase, genomic RNA and multiple copies of nucleoprotein (NP). During RNP assembly, monomeric NP oligomerizes along the length of the genomic RNA. Regulated assembly of the RNP is essential for virus replication, but how NP is maintained as a monomer that subsequently oligomerizes to form RNPs is poorly understood. Here we elucidate a mechanism whereby NP phosphorylation regulates oligomerization. We identified new evolutionarily conserved phosphorylation sites on NP and demonstrated that phosphorylation of NP decreased formation of higher-order complexes. Two phosphorylation sites were located on opposite sides of the NP:NP interface. In both influenza A and B virus, mutating or mimicking phosphorylation at these residues blocked homotypic interactions and drove NP towards a monomeric form. Highlighting the central role of this process during infection, these mutations impaired RNP formation, polymerase activity and virus replication. Thus, dynamic phosphorylation of NP regulates RNP assembly and modulates progression through the viral life cycle.

Replication and transcription by negative-sense RNA viruses occurs in large macromolecular complexes. These complexes contain the viral polymerase, genomic RNA, and multiple copies of nucleoprotein that bind RNA and oligomerize to coat the genome. For influenza virus, nucleoprotein (NP) non-specifically binds nucleic acids and spontaneously oligomerizes. It is essential that a portion of NP be maintained as a monomer so that it can selectively oligomerize into replication complexes. Despite the fact that this process must be tightly regulated during the viral life cycle, how this regulation is achieved is largely unknown. Here we show that phosphorylation of NP negatively regulates assembly of the influenza virus replication machinery. We identified two phosphorylation sites on opposite sides of the NP:NP interface and showed that phosphorylation at either site blocks homotypic interactions, distorting the monomer:oligomer balance of NP in cells and severely impairing virus replication. Our findings show that the phospho-regulated conversion of NP between mono- and oligomeric states is important for RNP formation, gene expression and viral replication. Moreover, we showed that these critical phosphorylation sites play the same role in influenza B virus and are likely present in influenza C and D viruses, suggesting our results are broadly applicable across viral strains and genera and reveal a global regulatory strategy for Orthomyxoviridae.

Intracellular expression of camelid single-domain antibodies specific for influenza virus nucleoprotein uncovers distinct features of its nuclear localization

Perturbation of protein-protein interactions relies mostly on genetic approaches or on chemical inhibition. Small RNA viruses, such as influenza A virus, do not easily lend themselves to the former approach, while chemical inhibition requires that the target protein be druggable. A lack of tools thus constrains the functional analysis of influenza virus-encoded proteins. We generated a panel of camelid-derived single-domain antibody fragments (VHHs) against influenza virus nucleoprotein (NP), a viral protein essential for nuclear trafficking and packaging of the influenza virus genome. We show that these VHHs can target NP in living cells and perturb NP's function during infection. Cytosolic expression of NP-specific VHHs (αNP-VHHs) disrupts virus replication at an early stage of the life cycle. Based on their specificity, these VHHs fall into two distinct groups. Both prevent nuclear import of the viral ribonucleoprotein (vRNP) complex without disrupting nuclear import of NP alone. Different stages of the virus life cycle thus rely on distinct nuclear localization motifs of NP. Their molecular characterization may afford new means of intervention in the virus life cycle.

IMPORTANCE

Many proteins encoded by RNA viruses are refractory to manipulation due to their essential role in replication. Thus, studying their function and determining how to disrupt said function through pharmaceutical intervention are difficult. We present a novel method based on single-domain-antibody technology that permits specific targeting and disruption of an essential influenza virus protein in the absence of genetic manipulation of influenza virus itself. Characterization of such interactions may help identify new targets for pharmaceutical intervention. This approach can be extended to study proteins encoded by other viral pathogens.