PLC-γ1 signaling plays a subtype-specific role in postbinding cell entry of influenza A virus

Conserved sequence features in intracellular domains of viral spike proteins

Viral spike proteins mutate frequently, but conserved features within these proteins often have functional importance and can inform development of anti-viral therapies which circumvent the effects of viral sequence mutations. Through analysis of large numbers of viral spike protein sequences from several viral families, we found highly (>99%) conserved patterns within their intracellular domains. The patterns generally consist of one or more basic amino acids (arginine or lysine) adjacent to a cysteine, many of which are known to undergo acylation. These patterns were not enriched in cellular proteins in general. Molecular dynamics simulations show direct electrostatic and hydrophobic interactions between these conserved residues in hemagglutinin (HA) from influenza A and B and the phosphoinositide PIP2. Super-resolution microscopy shows nanoscale colocalization of PIP2 and several of the same viral proteins. We propose the hypothesis that these conserved viral spike protein features can interact with phosphoinositides such as PIP2.

Influenza virus uses mGluR2 as an endocytic receptor to enter cells

Influenza virus infection is initiated by the attachment of the viral haemagglutinin (HA) protein to sialic acid receptors on the host cell surface. Most virus particles enter cells through clathrin-mediated endocytosis (CME). However, it is unclear how viral binding signals are transmitted through the plasma membrane triggering CME. Here we found that metabotropic glutamate receptor subtype 2 (mGluR2) and potassium calcium-activated channel subfamily M alpha 1 (KCa1.1) are involved in the initiation and completion of CME of influenza virus using an siRNA screen approach. Influenza virus HA directly interacted with mGluR2 and used it as an endocytic receptor to initiate CME. mGluR2 interacted and activated KCa1.1, leading to polymerization of F-actin, maturation of clathrin-coated pits and completion of the CME of influenza virus. Importantly, mGluR2-knockout mice were significantly more resistant to different influenza subtypes than the wild type. Therefore, blocking HA and mGluR2 interaction could be a promising host-directed antiviral strategy.

Unveiling the role of host kinases at different steps of influenza A virus life cycle

Influenza viruses remain a major public health concern causing contagious respiratory illnesses that result in around 290,000-650,000 global deaths every year. Their ability to constantly evolve through antigenic shifts and drifts leads to the emergence of newer strains and resistance to existing drugs and vaccines. To combat this, there is a critical need for novel antiviral drugs through the introduction of host-targeted therapeutics. Influenza viruses encode only 14 gene products that get extensively modified through phosphorylation by a diverse array of host kinases. Reversible phosphorylation at serine, threonine, or tyrosine residues dynamically regulates the structure, function, and subcellular localization of viral proteins at different stages of their life cycle. In addition, kinases influence a plethora of signaling pathways that also regulate virus propagation by modulating the host cell environment thus establishing a critical virus-host relationship that is indispensable for executing successful infection. This dependence on host kinases opens up exciting possibilities for developing kinase inhibitors as next-generation anti-influenza therapy. To fully capitalize on this potential, extensive mapping of the influenza virus-host kinase interaction network is essential. The key focus of this review is to outline the molecular mechanisms by which host kinases regulate different steps of the influenza A virus life cycle, starting from attachment-entry to assembly-budding. By assessing the contributions of different host kinases and their specific phosphorylation events during the virus life cycle, we aim to develop a holistic overview of the virus-host kinase interaction network that may shed light on potential targets for novel antiviral interventions.

Pathogenicity and virulence of influenza

Influenza viruses, including four major types (A, B, C, and D), can cause mild-to-severe and lethal diseases in humans and animals. Influenza viruses evolve rapidly through antigenic drift (mutation) and shift (reassortment of the segmented viral genome). New variants, strains, and subtypes have emerged frequently, causing epidemic, zoonotic, and pandemic infections, despite currently available vaccines and antiviral drugs. In recent years, avian influenza viruses, such as H5 and H7 subtypes, have caused hundreds to thousands of zoonotic infections in humans with high case fatality rates. The likelihood of these animal influenza viruses acquiring airborne transmission in humans through viral evolution poses great concern for the next pandemic. Severe influenza viral disease is caused by both direct viral cytopathic effects and exacerbated host immune response against high viral loads. Studies have identified various mutations in viral genes that increase viral replication and transmission, alter tissue tropism or species specificity, and evade antivirals or pre-existing immunity. Significant progress has also been made in identifying and characterizing the host components that mediate antiviral responses, pro-viral functions, or immunopathogenesis following influenza viral infections. This review summarizes the current knowledge on viral determinants of influenza virulence and pathogenicity, protective and immunopathogenic aspects of host innate and adaptive immune responses, and antiviral and pro-viral roles of host factors and cellular signalling pathways. Understanding the molecular mechanisms of viral virulence factors and virus-host interactions is critical for the development of preventive and therapeutic measures against influenza diseases.

Associating bovine herpesvirus 1 envelope glycoprotein gD with activated phospho-PLC-γ1(S1248)

Phospholipase C gamma 1 (PLC-γ1) may locate at distinct subcellular locations, such as cytosol, plasma membrane, and nucleus for varied biological functions. Bovine herpesvirus 1 (BoHV-1) productive infection activates PLC-γ1 signaling, as demonstrated by increased protein levels of phosphorylated-PLC-γ1 at Ser1248 [p-PLC-γ1(S1248)], which benefits virus productive infection. Here, for the first time, we reported that Golgi apparatus also contains activated p-PLC-γ1(S1248). And BoHV-1 productive infection at later stages (24 hpi) increased the accumulation of p-PLC-γ1(S1248) in the Golgi apparatus, where p-PLC-γ1(S1248) forms highlighted puncta observed via a confocal microscope. Coimmunoprecipitation studies demonstrated that the Golgi p-PLC-γ1(S1248) is specifically associated with the viral protein gD but not gC. In addition, we found that p-PLC-γ1(S1248) is consistently associated with both the plasma membrane-associated virions and the released virions. When the virus-infected cells were treated with PLC-γ1-specific inhibitor, U73122, for a short duration of 4 hours prior to the endpoint of virus infection, we found that the viral protein gD was trapped in the Golgi apparatus, suggesting that the PLC-γ1 signaling may facilitate trafficking of progeny virions out of this organelle. These findings provide a novel insight into the interplay between PLC-γ1 signaling and BoHV-1 replication. IMPORTANCE Bovine herpesvirus 1 (BoHV-1) productive infection increases protein levels of phosphorylated-phospholipase C gamma 1 at Ser1248 [p-PLC-γ1(S1248)]. However, whether it causes any variations to p-PLC-γ1(S1248) localization is not well understood. Here, for the first time, we found that partial p-PLC-γ1(S1248) is residing in the Golgi apparatus, where the accumulation is enhanced by virus infection. p-PLC-γ1(S1248) is consistently associated with virions, partially via binding to gD, in both the Golgi apparatus and cytoplasm membranes. Surprisingly, it also associates with the released virions. Of note, this is the first evidenced BoHV-1 virion-bound host protein. It seems that p-PLC-γ1(S1248) works as an escort during trafficking of progeny virions out of Golgi apparatus to the plasma membranes as well as releasing outside of the cell membranes. Furthermore, we showed that the activated p-PLC-γ1(S1248) is potentially implicated in the transport of virions out of Golgi apparatus, which may represent a novel mechanism to regulate virus productive infection.

Phospholipase C-γ1 potentially facilitates subcellular localization of activated β-catenin, p-β-catenin(S552), during bovine herpesvirus 1 productive infection in MDBK cells

Bovine herpesvirus 1 (BoHV-1) is a significant risk factor for the bovine respiratory disease complex (BRDC), a severe disease causing great economic losses to the cattle industry worldwide. Previous studies have reported that both phospholipase C-γ1 (PLC-γ1) and β-catenin are activated during BoHV-1 infection for efficient replication. However, the interplay between PLC-γ1 and β-catenin as a consequence of virus infection remains to be elucidated. Here, we reported that PLC-γ1 interacted with β-catenin, which was enhanced following virus infection. PLC-γ1-specific inhibitor, U73122, significantly reduced the mRNA levels of β-catenin in BoHV-1-infected cells; however, the steady-state protein levels were not affected due to the virus infection. Interestingly, the treatment of virus-infected cells with U73122 reduced the accumulation of activated β-catenin [p-β-catenin(S552)] in fractions of the cytoplasmic membrane as that observed with the treatment of methyl-β-cyclodextrin (MβCD), which can disrupt cytoplasmic membrane structure via sequestering cholesterol. Nucleus accumulation of p-β-catenin(S552) was increased following U73122 treatment in virus-infected cells. In addition, the association of p-β-catenin(S552) with cytoplasmic membrane induced by the virus infection was significantly disrupted by the treatment of U73122 and MβCD. These data indicated that the PLC-γ1 signaling is potentially involved in the regulation of β-catenin signaling stimulated by BoHV-1 infection partially via affecting the subcellular localization of p-β-catenin(S552).

Influenza A virus activates cellular Tropomyosin receptor kinase A (TrkA) signaling to promote viral replication and lung inflammation

Influenza A virus (IAV) infection causes acute respiratory disease with potential severe and deadly complications. Viral pathogenesis is not only due to the direct cytopathic effect of viral infections but also to the exacerbated host inflammatory responses. Influenza viral infection can activate various host signaling pathways that function to activate or inhibit viral replication. Our previous studies have shown that a receptor tyrosine kinase TrkA plays an important role in the replication of influenza viruses in vitro, but its biological roles and functional mechanisms in influenza viral infection have not been characterized. Here we show that IAV infection strongly activates TrkA in vitro and in vivo. Using a chemical-genetic approach to specifically control TrkA kinase activity through a small molecule compound 1NMPP1 in a TrkA knock-in (TrkA KI) mouse model, we show that 1NMPP1-mediated TrkA inhibition completely protected mice from a lethal IAV infection by significantly reducing viral loads and lung inflammation. Using primary lung cells isolated from the TrkA KI mice, we show that specific TrkA inhibition reduced IAV viral RNA synthesis in airway epithelial cells (AECs) but not in alveolar macrophages (AMs). Transcriptomic analysis confirmed the cell-type-specific role of TrkA in viral RNA synthesis, and identified distinct gene expression patterns under the TrkA regulation in IAV-infected AECs and AMs. Among the TrkA-activated targets are various proinflammatory cytokines and chemokines such as IL6, IL-1β, IFNs, CCL-5, and CXCL9, supporting the role of TrkA in mediating lung inflammation. Indeed, while TrkA inhibitor 1NMPP1 administered after the peak of IAV replication had no effect on viral load, it was able to decrease lung inflammation and provided partial protection in mice. Taken together, our results have demonstrated for the first time an important biological role of TrkA signaling in IAV infection, identified its cell-type-specific contribution to viral replication, and revealed its functional mechanism in virus-induced lung inflammation. This study suggests TrkA as a novel host target for therapeutic development against influenza viral disease.

Genome-wide association study of VHSV-resistance trait in Paralichthys olivaceus

In flounder aquaculture, selective breeding plays a vital role in the development of disease-resistant traits and animals with high growth rates. Moreover, superior animals are required to achieve high profits. Unlike growth-related traits, disease-resistant experiments need to be conducted in a controlled environment, as the improper measurement of traits often leads to low genetic correlation and incorrect estimation of breeding values. In this study, viral hemorrhagic septicemia virus (VHSV) resistance was studied using a genome-wide association study (GWAS), and the genetic parameters were estimated. Genotyping was performed using a high-quality 70 K single nucleotide polymorphism (SNP) Affymetrix® Axiom® myDesign™ Genotyping Array of olive flounder. A heritability of ∼0.18 for resistance to VHSV was estimated using genomic information of the fish. According to the GWAS, significant SNPs were detected in chromosomes 21, 24, and contig AGQT02032065.1. Three SNPs showed significance at the genome-wide level (p < 1 × 10^-6), while others showed significance above the suggestive cutoff (p < 1 × 10^-4). The 3% phenotypic variation was explained by the highest significant SNP, named AX-419319631. Of the important genes for disease resistance, SNPs were associated with plcg1, epha4, clstn2, pik3cb, hes6, meis3, prx6, cep164, siae, and kirrel3b. Most of the genes associated with these SNPs have been previously reported with respect to viral entry, propagation, and immune mechanisms. Therefore, our study provides helpful information regarding VHSV resistance in olive flounder, which can be used for breeding applications.

Differential gene expression reveals host factors for viral shedding variation in mallards (Anas platyrhynchos) infected with low-pathogenic avian influenza virus

Intraspecific variation in pathogen shedding impacts disease transmission dynamics; therefore, understanding the host factors associated with individual variation in pathogen shedding is key to controlling and preventing outbreaks. In this study, ileum and bursa of Fabricius tissues of wild-bred mallards (Anas platyrhynchos) infected with low-pathogenic avian influenza (LPAIV) were evaluated at various post-infection time points to determine genetic host factors associated with intraspecific variation in viral shedding. By analysing transcriptome sequencing data (RNA-seq), we found that LPAIV-infected wild-bred mallards do not exhibit differential gene expression compared to uninfected birds, but that gene expression was associated with cloacal viral shedding quantity early in the infection. In both tissues, immune gene expression was higher in high/moderate shedding birds compared to low shedding birds, and significant positive relationships with viral shedding were observed. In the ileum, expression for host genes involved in viral cell entry was lower in low shedders compared to moderate shedders at 1 day post-infection (DPI), and expression for host genes promoting viral replication was higher in high shedders compared to low shedders at 2 DPI. Our findings indicate that viral shedding is a key factor for gene expression differences in LPAIV-infected wild-bred mallards, and the genes identified in this study could be important for understanding the molecular mechanisms driving intraspecific variation in pathogen shedding.

Astrocyte inflammatory signaling mediates α-synuclein aggregation and dopaminergic neuronal loss following viral encephalitis

Viral infection of the central nervous system (CNS) can cause lasting neurological decline in surviving patients and can present with symptoms resembling Parkinson's disease (PD). The mechanisms underlying postencephalitic parkinsonism remain unclear but are thought to involve increased innate inflammatory signaling in glial cells, resulting in persistent neuroinflammation. We therefore studied the role of glial cells in regulating neuropathology in postencephalitic parkinsonism by studying the involvement of astrocytes in loss of dopaminergic neurons and aggregation of α-synuclein protein following infection with western equine encephalitis virus (WEEV). Infections were conducted in both wildtype mice and in transgenic mice lacking NFκB inflammatory signaling in astrocytes. For 2 months following WEEV infection, we analyzed glial activation, neuronal loss and protein aggregation across multiple brain regions, including the substantia nigra pars compacta (SNpc). These data revealed that WEEV induces loss of SNpc dopaminergic neurons, persistent activation of microglia and astrocytes that precipitates widespread aggregation of α-synuclein in the brain of C57BL/6 mice. Microgliosis and macrophage infiltration occurred prior to activation of astrocytes and was followed by opsonization of ⍺-synuclein protein aggregates in the cortex, hippocampus and midbrain by the complement protein, C3. Astrocyte-specific NFκB knockout mice had reduced gliosis, α-synuclein aggregate formation and neuronal loss. These data suggest that astrocytes play a critical role in initiating PD-like pathology following encephalitic infection with WEEV through innate immune inflammatory pathways that damage dopaminergic neurons, possibly by hindering clearance of ⍺-synuclein aggregates. Inhibiting glial inflammatory responses could therefore represent a potential therapy strategy for viral parkinsonism.

Compound Cocktail Inhibits Influenza Viral Pneumonia via Phospholipase Cγ1 Phosphorylation-Related Necroptosis and Partial Autophagy in Natural Killer Cells

Influenza viral infections are prone to global outbreaks and cause pneumonia in affected populations. High morbidity and mortality caused by pneumonia occur during an influenza pandemic. Antivirals or control of inflammation is the primary means of influenza treatment. A compound cocktail composed of arctiin, daidzein, glycyrrhizic acid, and liquiritin inhibited mouse pneumonia resulting from a PR8 viral infection and caused a weight gain after oral administration. Natural killer cell activating receptors, both Ly49D and Ly49H in the lungs, were increased in the treatment in mice. In H3N2 virus-infected natural killer-92MI cells, the cocktail treatment had different effects on phosphorylation sites of phospholipase Cγ1 (PLCγ1) and killed infected cells through necroptosis or late apoptosis, in which RIP3 was increased and both caspase-3 and phosphorylated-JNK in the cells were downregulated. Acid phosphatase activity in viral-infected natural killer-92MI cells was induced by the compound cocktail treatment, which could be related to the p62 decrease in natural killer-92MI cells. In addition, an autophagic flux induction was observed in alveolar basal epithelial cells (A549). Protein p65, but not phosphorylated-p65, was significantly decreased by the treatment. Our results indicate that the compound cocktail strengthened the phosphorylation of PLCγ1-related necroptosis and partial autophagy in natural killer cells, which could yield an inhibitory effect on viral pneumonia in influenza.

The Role of Epidermal Growth Factor Receptor Signaling Pathway during Bovine Herpesvirus 1 Productive Infection in Cell Culture

Accumulating studies have shown that the epidermal growth factor receptor (EGFR) signaling pathway plays an essential role in mediating cellular entry of numerous viruses. In this study, we report that bovine herpesvirus 1 (BoHV-1) productive infection in both the human lung carcinoma cell line A549 and bovine kidney (MDBK) cells leads to activation of EGFR, as demonstrated by the increased phosphorylation of EGFR at Tyr1068 (Y1068), which in turn plays important roles in virus infection. A time-of-addition assay supported that virus replication at post-entry stages was affected by the EGFR specific inhibitor Gefitinib. Interestingly, both phospholipase C-γ1 (PLC-γ1) and Akt, canonical downstream effectors of EGFR, were activated following virus infection in A549 cells, while Gefitinib could inhibit the activation of PLC-γ1 but not Akt. In addition, virus titers in A549 cells was inhibited by chemical inhibition of PLC-γ1, but not by the inhibition of Akt. However, the Akt specific inhibitor Ly294002 could significantly reduce the virus titer in MDBK cells. Taken together, our data suggest that PLC-γ1 is stimulated in part through EGFR for efficient replication in A549 cells, whereas Akt can be stimulated by virus infection independent of EGFR, and is not essential for virus productive infection, indicating that Akt modulates BoHV-1 replication in a cell type-dependent manner. This study provides novel insights on how BoHV-1 infection activates EGFR signaling transduction to facilitate virus replication.

Fibroblast Growth Factor Receptor 4 Deficiency Mediates Airway Inflammation in the Adult Healthy Lung?

Fibroblast growth factor receptor (FGFR) 4 has been shown to mediate pro-inflammatory signaling in the liver and airway epithelium in chronic obstructive pulmonary disease. In past reports, FGFR4 knockout (Fgfr4 -/- ) mice did not show any lung phenotype developmentally or at birth, unless FGFR3 deficiency was present simultaneously. Therefore, we wanted to know whether the loss of FGFR4 had any effect on the adult murine lung. Our results indicate that adult Fgfr4 -/- mice demonstrate a lung phenotype consisting of widened airway spaces, increased airway inflammation, bronchial obstruction, and right ventricular hypertrophy consistent with emphysema. Despite downregulation of FGF23 serum levels, interleukin (IL) 1β and IL-6 in the Fgfr4 -/- lung, and abrogation of p38 signaling, primary murine Fgfr4 -/- airway cells showed increased expression of IL-1β and augmented secretion of IL-6, which correlated with decreased airway surface liquid depth as assessed by micro-optical coherence tomography. These findings were paralleled by increased ERK phosphorylation in Fgfr4 -/- airway cells when compared with their control wild-type cells. Analysis of a murine model with constitutive activation of FGFR4 showed attenuation of pro-inflammatory mediators in the lung and airway epithelium. In conclusion, we are the first to show an inflammatory and obstructive airway phenotype in the adult healthy murine Fgfr4 -/- lung, which might be due to the upregulation of ERK phosphorylation in the Fgfr4 -/- airway epithelium.

Influenza A viruses use multivalent sialic acid clusters for cell binding and receptor activation

Influenza A virus (IAV) binds its host cell using the major viral surface protein hemagglutinin (HA). HA recognizes sialic acid, a plasma membrane glycan that functions as the specific primary attachment factor (AF). Since sialic acid alone cannot fulfill a signaling function, the virus needs to activate downstream factors to trigger endocytic uptake. Recently, the epidermal growth factor receptor (EGFR), a member of the receptor-tyrosine kinase family, was shown to be activated by IAV and transmit cell entry signals. However, how IAV’s binding to sialic acid leads to engagement and activation of EGFR remains largely unclear. We used multicolor super-resolution microscopy to study the lateral organization of both IAV’s AFs and its functional receptor EGFR at the scale of the IAV particle. Intriguingly, quantitative cluster analysis revealed that AFs and EGFR are organized in partially overlapping submicrometer clusters in the plasma membrane of A549 cells. Within AF domains, the local AF concentration reaches on average 10-fold the background concentration and tends to increase towards the cluster center, thereby representing a multivalent virus-binding platform. Using our experimentally measured cluster characteristics, we simulated virus diffusion on a flat membrane. The results predict that the local AF concentration strongly influences the distinct mobility pattern of IAVs, in a manner consistent with live-cell single-virus tracking data. In contrast to AFs, EGFR resides in smaller clusters. Virus binding activates EGFR, but interestingly, this process occurs without a major lateral EGFR redistribution, indicating the activation of pre-formed clusters, which we show are long-lived. Taken together, our results provide a quantitative understanding of the initial steps of influenza virus infection. Co-clustering of AF and EGFR permit a cooperative effect of binding and signaling at specific platforms, thus linking their spatial organization to their functional role during virus-cell binding and receptor activation.

The plasma membrane is the major interface between a cell and its environment. This complex and dynamic organelle needs to protect, as a barrier, but also transmit subtle signals into and out of the cell. For the enveloped virus IAV, the plasma membrane represents both a major obstacle to overcome during infection, and the site for the assembly of progeny virus particles. However, the organisation of the plasma membrane–a key to understanding how viral entry works—at the scale of an infecting particle (length scales < 100 nm) remains largely unknown. Sialylated glycans serve as IAV attachment factors but are not able to transmit signals across the plasma membrane. Receptor tyrosine kinases were identified to be activated upon virus binding and serve as functional receptors. How IAV engages and activates its functional receptors while initially binding glycans still remains speculative. Here, we use super resolution microscopy to study the lateral organization of plasma membrane-bound molecules involved in IAV infection, as well as their functional relationship. We find that molecules are organized in submicrometer nanodomains and, in combination with virus diffusion simulations, present a mechanistic model for how IAV first engages with AFs in the plasma membrane to subsequently engage and trigger entry-associated membrane receptors.

Breaking the Convention: Sialoglycan Variants, Coreceptors, and Alternative Receptors for Influenza A Virus Entry

The influenza A virus (IAV) envelope protein hemagglutinin binds α2,6- or α2,3-linked sialic acid as a host cell receptor. Bat IAV subtypes H17N10 and H18N11 form an exception to this rule and do not bind sialic acid but enter cells via major histocompatibility complex (MHC) class II. Here, we review current knowledge on IAV receptors with a focus on sialoglycan variants, protein coreceptors, and alternative receptors that impact IAV attachment and internalization beyond the well-described sialic acid binding.

The G Protein-Coupled Receptor FFAR2 Promotes Internalization during Influenza A Virus Entry

Influenza A virus (IAV) coopts numerous host factors to complete its replication cycle. Here, we identify free fatty acid receptor 2 (FFAR2) as a cofactor for IAV entry into host cells. We found that downregulation of FFAR2 or Ffar2 expression significantly reduced the replication of IAV in A549 or RAW 264.7 cells. The treatment of A549 cells with small interfering RNA (siRNA) targeting FFAR2 or the FFAR2 pathway agonists 2-(4-chlorophenyl)-3-methyl-N-(thiazol-2-yl)butanamide (4-CMTB) and compound 58 (Cmp58) [(S)-2-(4-chlorophenyl)-3,3-dimethyl-N-(5-phenylthiazol-2-yl)butanamide] dramatically inhibited the nuclear accumulation of viral nucleoprotein (NP) at early time points postinfection, indicating that FFAR2 functions in the early stage of the IAV replication cycle. FFAR2 downregulation had no effect on the expression of sialic acid (SA) receptors on the cell membrane, the attachment of IAV to the SA receptors, or the activity of the viral ribonucleoprotein (vRNP) complex. Rather, the amount of internalized IAVs was significantly reduced in FFAR2-knocked-down or 4-CMTB- or Cmp58-treated A549 cells. Further studies showed that FFAR2 associated with β-arrestin1 and that β-arrestin1 interacted with the β2-subunit of the AP-2 complex (AP2B1), the essential adaptor of the clathrin-mediated endocytosis pathway. Notably, siRNA knockdown of either β-arrestin1 or AP2B1 dramatically impaired IAV replication, and AP2B1 knockdown or treatment with Barbadin, an inhibitor targeting the β-arrestin1/AP2B1 complex, remarkably decreased the amount of internalized IAVs. Moreover, we found that FFAR2 interacted with three G protein-coupled receptor (GPCR) kinases (i.e., GRK2, GRK5, and GRK6) whose downregulation inhibited IAV replication. Together, our findings demonstrate that the FFAR2 signaling cascade is important for the efficient endocytosis of IAV into host cells.IMPORTANCE To complete its replication cycle, IAV hijacks the host endocytosis machinery to invade cells. However, the underlying mechanisms of how IAV is internalized into host cells remain poorly understood, emphasizing the need to elucidate the role of host factors in IAV entry into cells. In this study, we identified FFAR2 as an important host factor for the efficient replication of both low-pathogenic and highly pathogenic IAV. We revealed that FFAR2 facilitates the internalization of IAV into target cells during the early stage of infection. Upon further characterization of the role of FFAR2-associated proteins in virus replication, we found that the FFAR2-β-arrestin1-AP2B1 signaling cascade is important for the efficient endocytosis of IAV. Our findings thus further our understanding of the biological details of IAV entry into host cells and establish FFAR2 as a potential target for antiviral drug development.

Influenza virus entry via the GM3 ganglioside-mediated platelet-derived growth factor receptor β signalling pathway

The possible resistance of influenza virus against existing antiviral drugs calls for new therapeutic concepts. One appealing strategy is to inhibit virus entry, in particular at the stage of internalization. This requires a better understanding of virus-host interactions during the entry process, including the role of receptor tyrosine kinases (RTKs). To search for cellular targets, we evaluated a panel of 276 protein kinase inhibitors in a multicycle antiviral assay in Madin-Darby canine kidney cells. The RTK inhibitor Ki8751 displayed robust anti-influenza A and B virus activity and was selected for mechanistic investigations. Ki8751 efficiently disrupted the endocytic process of influenza virus in different cell lines carrying platelet-derived growth factor receptor β (PDGFRβ), an RTK that is known to act at GM3 ganglioside-positive lipid rafts. The more efficient virus entry in CHO-K1 cells compared to the wild-type ancestor (CHO-wt) cells indicated a positive effect of GM3, which is abundant in CHO-K1 but not in CHO-wt cells. Entering virus localized to GM3-positive lipid rafts and the PDGFRβ-containing endosomal compartment. PDGFRβ/GM3-dependent virus internalization involved PDGFRβ phosphorylation, which was potently inhibited by Ki8751, and desialylation of activated PDGFRβ by the viral neuraminidase. Virus uptake coincided with strong activation of the Raf/MEK/Erk cascade, but not of PI3K/Akt or phospholipase C-γ. We conclude that influenza virus efficiently hijacks the GM3-enhanced PDGFRβ signalling pathway for cell penetration, providing an opportunity for host cell-targeting antiviral intervention.

The evaluation of an immunoperoxidase assay applicable in antiviral drug screening

Bovine viral diarrhea virus (BVDV) fall into cytopathic (CP) and noncytopathic (NCP) biotypes, based on their ability to kill cultured cells. NCP-BVDV can not be titrated by conventional means as used for CP-BVDV, which has impeded the identification of antiviral drugs targeting NCP-BVDV virus strains. In this study, the application of an immunoperoxidase assay in the screening of antiviral drugs was tested using two known BVDV inhibitors, ribavirin and ammonium chloride (NH4Cl). Phospholipase C inhibitor U73122 was identified to affect BVDV infection by using this immunoperoxidase assay. In addition, the results of immunoperoxidase assay were validated by real-time PCR. Taken together, the immunoperoxidase assay is a useful and versatile method suitable for antiviral drug screening targeting NCP-BVDV.

HATRIC-based identification of receptors for orphan ligands

Cellular responses depend on the interactions of extracellular ligands, such as nutrients, growth factors, or drugs, with specific cell-surface receptors. The sensitivity of these interactions to non-physiological conditions, however, makes them challenging to study using in vitro assays. Here we present HATRIC-based ligand receptor capture (HATRIC-LRC), a chemoproteomic technology that successfully identifies target receptors for orphan ligands on living cells ranging from small molecules to intact viruses. HATRIC-LRC combines a click chemistry-based, protein-centric workflow with a water-soluble catalyst to capture ligand-receptor interactions at physiological pH from as few as 1 million cells. We show HATRIC-LRC utility for general antibody target validation within the native nanoscale organization of the surfaceome, as well as receptor identification for a small molecule ligand. HATRIC-LRC further enables the identification of complex extracellular interactomes, such as the host receptor panel for influenza A virus (IAV), the causative agent of the common flu.

Technologies for identifying receptor-ligand pairs on living cells at physiological conditions remain scarce. Here, the authors develop a mass spectrometry-based ligand receptor capture technology that can identify receptors for a diverse range of ligands at physiological pH with as few as a million cells.

Wolbachia-mediated virus blocking in mosquito cells is dependent on XRN1-mediated viral RNA degradation and influenced by viral replication rate

Wolbachia is currently being developed as a novel tool to block the transmission of dengue viruses (DENV) by Aedes aegypti. A number of mechanisms have been proposed to explain the DENV-blocking phenotype in mosquitoes, including competition for fatty acids like cholesterol, manipulation of host miRNAs and upregulation of innate immune pathways in the mosquito. We examined the various stages in the DENV infection process to better understand the mechanism of Wolbachia-mediated virus blocking (WMVB). Our results suggest that infection with Wolbachia does not inhibit DENV binding or cell entry, but reduces virus replication. In contrast to a previous report, we also observed a similar reduction in replication of West Nile virus (WNV). This reduced replication is associated with rapid viral RNA degradation in the cytoplasm. We didn't find a role for host miRNAs in WMVB. Further analysis showed that the 3' end of the virus subgenomic RNA was protected and accumulated over time suggesting that the degradation is XRN1-mediated. We also found that sub genomic flavivirus RNA accumulation inactivated XRN1 in mosquito cells in the absence of Wolbachia and led to enhancement of RNA degradation in its presence. Depletion of XRN1 decreased WMVB which was associated with a significant increase in DENV RNA. We also observed that WMVB is influenced by virus MOI and rate of virus replication. A comparatively elevated blocking was observed for slowly replicating DENV, compared to WNV. Similar results were obtained while analysing different DENV serotypes.

The Role of Phospholipase C Signaling in Macrophage-Mediated Inflammatory Response

Macrophages are crucial members of the mononuclear phagocyte system essential to protect the host from invading pathogens and are central to the inflammatory response with their ability to acquire specialized phenotypes of inflammatory (M1) and anti-inflammatory (M2) and to produce a pool of inflammatory mediators. Equipped with a broad range of receptors, such as Toll-like receptor 4 (TLR4), CD14, and Fc gamma receptors (FcγRs), macrophages can efficiently recognize and phagocytize invading pathogens and secrete cytokines by triggering various secondary signaling pathways. Phospholipase C (PLC) is a family of enzymes that hydrolyze phospholipids, the most significant of which is phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]. Cleavage at the internal phosphate ester generates two second messengers, inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG), both of which mediate in diverse cellular functions including the inflammatory response. Recent studies have shown that some PLC isoforms are involved in multiple stages in TLR4-, CD14-, and FcγRs-mediated activation of nuclear factor kappa B (NF-κB), mitogen-activated protein kinase (MAPK), and interferon regulatory factors (IRFs), all of which are associated with the regulation of the inflammatory response. Therefore, secondary signaling by PLC is implicated in the pathogenesis of numerous inflammatory diseases. This review provides an overview of our current knowledge on how PLC signaling regulates the macrophage-mediated inflammatory response.

Soloxolone methyl inhibits influenza virus replication and reduces virus-induced lung inflammation

Highly pathogenic influenza viruses pose a serious public health threat to humans. Although vaccines are available, new antivirals are needed to efficiently control disease progression and virus transmission due to the emergence of drug-resistant viral strains. In this study, we describe the anti-viral properties of Soloxolone methyl (SM) (methyl 2-cyano-3,12-dioxo-18βH-olean-9(11),1(2)-dien-30-oate, a chemical derivative of glycyrrhetinic acid) against the flu virus. Anti-flu efficacy studies revealed that SM exhibits antiviral activity against the H1N1 influenza A virus in a dose-dependent manner causing a more than 10-fold decrease in virus titer and a reduction in the expression of NP and M2 viral proteins. In a time-of-addition study, SM was found to act at an early stage of infection to exhibit an inhibitory effect on both the attachment step and virus uptake into cells. Also, in infected cells SM downregulates the expression of the inflammatory cytokines IL-6 and TNF-α. In infected mice, SM administered intranasally prior to and after infection significantly decreases virus titers in the lung and prevents post-challenge pneumonia. Together, these results suggest that Soloxolone methyl might serve as an effective therapeutic agent to manage influenza outbreaks and virus-associated complications, and further preclinical and clinical investigation may be warranted.

The role of phospholipase C signaling in bovine herpesvirus 1 infection

Bovine herpesvirus 1 (BoHV-1) infection enhanced the generation of inflammatory mediator reactive oxidative species (ROS) and stimulated MAPK signaling that are highly possibly related to virus induced inflammation. In this study, for the first time we show that BoHV-1 infection manipulated phospholipase C (PLC) signaling, as demonstrated by the activation of PLCγ-1 at both early stages [at 0.5 h post-infection (hpi)] and late stages (4-12 hpi) during the virus infection of MDBK cells. Viral entry, and de novo protein expression and/or DNA replication were potentially responsible for the activation of PLCγ-1 signaling. PLC signaling inhibitors of both U73122 and edelfosine significantly inhibited BoHV-1 replication in both bovine kidney cells (MDBK) and rabbit skin cells (RS-1) in a dose-dependent manner by affecting the virus entry stage(s). In addition, the activation of Erk1/2 and p38MAPK signaling, and the enhanced generation of ROS by BoHV-1 infection were obviously ameliorated by chemical inhibition of PLC signaling, implying the requirement of PLC signaling in ROS production and these MAPK pathway activation. These results suggest that the activation of PLC signaling is a potential pathogenic mechanism for BoHV-1 infection.

Japanese Encephalitis Virus Exploits Dopamine D2 Receptor-phospholipase C to Target Dopaminergic Human Neuronal Cells

Despite the availability of vaccines for Japanese encephalitis virus (JEV), the re-emerging virus remains a clinically important pathogen that causes acute encephalitis and permanent neuropsychiatric sequels. JEV highly targets dopaminergic neuron-rich brain regions including the thalamus and midbrain. The molecular mechanism contributing to the high susceptibility of these particular brain regions remains largely unclear. This study addressed whether this tissue tropism of JEV is associated with signaling of dopaminergic neurons. Three pieces of evidence indicate that JEV exploits dopamine signaling to facilitate its infection: (1) JEV infection modulates dopamine level; (2) a selective dopamine D2 receptor (D2R) agonist enhances JEV infection; and (3) stimulation of D2R activates phospholipase C (PLC) to enhance the surface expression of JEV binding/entry molecules, integrin β3 and vimentin. Overall, JEV may exploit dopamine-mediated neuronal communication to increase the susceptibility of D2R-expressing cells to JEV infection. This study identifies a potential underlying mechanism of viral invasiveness in the dopaminergic brain regions and suggests antiviral strategies against viral infection by targeting D2R-PLC signaling.

Role of Host Genes in Influenza Virus Replication

At every step of their replication cycle influenza viruses depend heavily on their host cells. The multifaceted interactions that occur between the virus and its host cell determine the outcome of the infection, including efficiency of progeny virus production, tropism, and pathogenicity. In order to understand viral disease and develop therapies for influenza it is therefore pertinent to study the intricate interplay between influenza viruses and their required host factors. Here, we review the current knowledge on host cell factors required by influenza virus at the different stages of the viral replication cycle. We also discuss the roles of host factors in zoonotic transmission of influenza viruses and their potential for developing novel antivirals.

PLC-γ1 is involved in the inflammatory response induced by influenza A virus H1N1 infection

We have previously reported that phosphoinositide-specific phospholipase γ1 (PLC-γ1) signaling is activated by influenza virus H1N1 infection and mediates efficient viral entry in human epithelial cells. In this study, we show that H1N1 also activates PLCγ-1 signaling in human promonocytic cell line -derived macrophages. Surprisingly, the activated PLCγ-1 signaling is not important for viral replication in macrophages, but is involved in the virus-induced inflammatory responses. PLC-γ1-specific inhibitor U73122 strongly inhibits the H1N1 virus-induced NF-κB signaling, blocking the up-regulation of TNF-α, IL-6, MIP-1α, and reactive oxidative species. In a positive feedback loop, IL-1β and TNF-α activate the PLCγ-1 signaling in both epithelial and macrophage cell lines. In summary, we have shown for the first time that the PLCγ-1 signaling plays an important role in the H1N1-induced inflammatory responses. Our study suggests that targeting the PLCγ-1 signaling is a potential antiviral therapy against H1N1 by inhibiting both viral replication and excessive inflammation.

Xijiao Dihuang Decoction combined with Yinqiao Powder reverses influenza virus-induced F-actin reorganization in PMVECs by inhibiting ERM phosphorylation

Objective

It has been documented that ezrin/radixin/moesin (ERM) phosphorylation by the p38 mitogen-activated protein kinase (MAPK), Rho/ROCK, and protein kinase C (PKC) pathways leads to filamentous actin (F-actin) reorganization and microvascular endothelial cell hyperpermeability. In this study, we investigated the effects of Xijiao Dihuang Decoction combined with Yinqiao Powder (XDY) on influenza virus (IV)-induced F-actin restructuring and ERM phosphorylation regulated by the Rho/Rho kinase 1 (ROCK), p38 MAPK, and PKC signaling pathways in pulmonary microvascular endothelial cells (PMVECs).

Methods

Serum containing XDY (XDY-CS; 13.8 g/kg) was acquired using standard protocols for serum pharmacology. Primary PMVECs were obtained from male Wistar rats and cultured. After adsorption of IV A (multiplicity of infection, 0.01) for 1 h, medium with 20% XDY-CS was added to the PMVECs. The distributions of F-actin and phosphorylated ERM were determined by confocal microscopy, and F-actin expression was measured by flow cytometry. The expression levels of ROCK1, phosphorylated myosin phosphatase target-subunit (p-MYPT), phosphorylated MAPK kinase, phosphorylated p38 (p-p38), phosphorylated PKC (p-PKC), and phosphorylated ERM (p-ERM) were determined by western blotting.

Results

F-actin reorganization in IV-infected PMVECs was reversed by XDY-CS treatment, which was accompanied by reduced p-ERM production. The p-ERM protein accumulated at plasma membrane of PMVECs infected with IV, which was also inhibited by XDY-CS treatment. In addition, XDY-CS treatment drastically reduced the levels of p-p38, ROCK1, p-MYPT, and p-PKC induced by IV infection in PMVECs.

Conclusion

These results show that XDY-CS inhibited influenza-induced F-actin reorganization in PMVECs by down-regulating p-ERM expression via inhibition of the Rho/ROCK, p38 MAPK, and PKC pathways. In conclusion, XDY could reduce the damage to endothelial cytoskeleton induced by IV infection, thus protecting the barriers of PMVECs.

Intracellular redox state as target for anti-influenza therapy: are antioxidants always effective?

Influenza virus infections represent a big issue for public health since effective treatments are still lacking. In particular, the emergence of strains resistant to drugs limits the effectiveness of anti-influenza agents. For this reason, many efforts have been dedicated to the identification of new therapeutic strategies aimed at targeting the virus-host cell interactions. Oxidative stress is a characteristic of some viral infections including influenza. Because antioxidants defend cells from damage caused by reactive oxygen species induced by different stimuli including pathogens, they represent interesting molecules to fight infectious diseases. However, most of the available studies have found that these would-be panaceas could actually exacerbate the diseases they claim to prevent, and have thus revealed "the dark side" of these molecules. This review article discusses the latest opportunities and drawbacks of the antioxidants used in anti-influenza therapy and new perspectives.

A Functional Role of Fibroblast Growth Factor Receptor 1 (FGFR1) in the Suppression of Influenza A Virus Replication

Influenza A virus causes annual epidemics and occasional pandemics in humans. Here, we investigated four members of the fibroblast growth factor receptor (FGFR) family; FGFR1 to 4, and examined their expression patterns in human lung epithelial cells A549 with influenza A virus infection. We identified a functional role of FGFR1 in influenza A/Puerto Rico/8/1934 (PR8) and A/Anhui/01/2005 (H5N1) virus replication. Our results showed that FGFR1 silencing by siRNA interference promoted influenza A/PR8 and H5N1 virus replication in A549 cells, while lentivirus-mediated exogenous FGFR1 expression significantly suppressed influenza A virus replication; however, FGFR4 did not have the same effects. Moreover, FGFR1 phosphorylation levels were downregulated in A549 cells by influenza A virus infection, while the repression of FGFR1 kinase using PD173074, a potent and selective FGFR1 inhibitor, could enhance virus replication. Furthermore, we found that FGFR1 inhibits influenza virus internalization, but not binding, during viral entry. These results suggested that FGFR1 specifically antagonizes influenza A virus replication, probably by blocking viral entry.

Cryptoporus volvatus extract inhibits influenza virus replication in vitro and in vivo

Influenza virus is the cause of significant morbidity and mortality, posing a serious health threat worldwide. Here, we evaluated the antiviral activities of Cryptoporus volvatus extract on influenza virus infection. Our results demonstrated that the Cryptoporus volvatus extract inhibited different influenza virus strain replication in MDCK cells. Time course analysis indicated that the extract exerted its inhibition at earlier and late stages in the replication cycle of influenza virus. Subsequently, we confirmed that the extract suppressed virus internalization into and released from cells. Moreover, the extract significantly reduced H1N1/09 influenza virus load in lungs and dramatically decreased lung lesions in mice. And most importantly, the extract protected mice from lethal challenge with H1N1/09 influenza virus. Our results suggest that the Cryptoporus volvatus extract could be a potential candidate for the development of a new anti-influenza virus therapy.

Viruses exploit the function of epidermal growth factor receptor

Epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase that regulates cellular homeostatic processes. Following ligand binding, EGFR activates different downstream signalling cascades that promote cell survival, proliferation, motility, and angiogenesis and induces F-actin-dependent EGFR endocytosis, which relocalises the activated receptors for degradation or recycling. The responses that are induced by ligand binding to EGFR, including cell signalling activation, protein kinase phosphorylation and cytoskeletal network rearrangement, resemble those induced by virus infection. Increasing evidence demonstrates that many viruses usurp EGFR endocytosis or EGFR-mediated signalling for entry, replication, inflammation, and viral antagonism to the host antiviral system. In addition, viruses have acquired sophisticated mechanisms to regulate EGFR functions by interrupting the EGFR-recycling process and modulating EGFR expression. In this review, we provide an overview of the mechanisms by which viruses alter EGFR signalling in favour of their continued survival.

Entry of influenza A virus: host factors and antiviral targets

Influenza virus is a major human pathogen that causes annual epidemics and occasional pandemics. Moreover, the virus causes outbreaks in poultry and other animals, such as pigs, requiring costly and laborious countermeasures. Therefore, influenza virus has a substantial impact on health and the global economy. Here, we review entry of this important pathogen into target cells, an essential process by which viral genomes are delivered from extracellular virions to sites of transcription/replication in the cell nucleus. We summarize current knowledge on the interaction of influenza viruses with their receptor, sialic acid, and highlight the ongoing search for additional receptors. We describe receptor-mediated endocytosis and the recently discovered macropinocytosis as alternative virus uptake pathways, and illustrate the subsequent endosomal trafficking of the virus with advanced live microscopy techniques. Release of virus from the endosome and import of the viral ribonucleoproteins into the host cell nucleus are also outlined. Although a focus has been on viral protein function during entry, recent studies have revealed exciting information on cellular factors required for influenza virus entry. We highlight these, and discuss established entry inhibitors targeting viral and host factors, as well as the latest prospects for designing novel 'anti-entry' compounds. New entry inhibitors are of particular importance for current efforts to develop the next generation of anti-influenza drugs - entry is the first essential step of virus replication and is an ideal target to block infection efficiently.