The epidermal growth factor receptor (EGFR) promotes uptake of influenza A viruses (IAV) into host cells

Application of Super-Resolution and Advanced Quantitative Microscopy to the Spatio-Temporal Analysis of Influenza Virus Replication

With an estimated three to five million human cases annually and the potential to infect domestic and wild animal populations, influenza viruses are one of the greatest health and economic burdens to our society, and pose an ongoing threat of large-scale pandemics. Despite our knowledge of many important aspects of influenza virus biology, there is still much to learn about how influenza viruses replicate in infected cells, for instance, how they use entry receptors or exploit host cell trafficking pathways. These gaps in our knowledge are due, in part, to the difficulty of directly observing viruses in living cells. In recent years, advances in light microscopy, including super-resolution microscopy and single-molecule imaging, have enabled many viral replication steps to be visualised dynamically in living cells. In particular, the ability to track single virions and their components, in real time, now allows specific pathways to be interrogated, providing new insights to various aspects of the virus-host cell interaction. In this review, we discuss how state-of-the-art imaging technologies, notably quantitative live-cell and super-resolution microscopy, are providing new nanoscale and molecular insights into influenza virus replication and revealing new opportunities for developing antiviral strategies.

Do some viruses use growth hormone, prolactin and their receptors to facilitate entry into cells?: Episodic evolution of hormones and receptors suggests host-virus arms races; related placental lactogens may provide protective viral decoys

The molecular evolution of pituitary growth hormone and prolactin in mammals shows two unusual features: episodes of markedly accelerated evolution and, in some species, complex families of related proteins expressed in placenta and resulting from multiple gene duplications. Explanations of these phenomena in terms of physiological adaptations seem unconvincing. Here, I propose an alternative explanation, namely that these evolutionary features reflect the use of the hormones (and their receptors) as viral receptors. Episodes of rapid evolution can then be explained as due to "arms races" in which changes in the hormone lead to reduced interaction with the virus, and subsequent changes in the virus counteract this. Placental paralogues of the hormones could provide decoys that bind viruses, and protect the foetus against infection. The hypothesis implies that the extensive changes introduced into growth hormone, prolactin and their receptors during the course of mammalian evolution reflect viral interactions, not endocrine adaptations.

Ebola virus triggers receptor tyrosine kinase-dependent signaling to promote the delivery of viral particles to entry-conducive intracellular compartments

Filoviruses, such as the Ebola virus (EBOV) and Marburg virus (MARV), are causative agents of sporadic outbreaks of hemorrhagic fevers in humans. To infect cells, filoviruses are internalized via macropinocytosis and traffic through the endosomal pathway where host cathepsin-dependent cleavage of the viral glycoproteins occurs. Subsequently, the cleaved viral glycoprotein interacts with the late endosome/lysosome resident host protein, Niemann-Pick C1 (NPC1). This interaction is hypothesized to trigger viral and host membrane fusion, which results in the delivery of the viral genome into the cytoplasm and subsequent initiation of replication. Some studies suggest that EBOV viral particles activate signaling cascades and host-trafficking factors to promote their localization with host factors that are essential for entry. However, the mechanism through which these activating signals are initiated remains unknown. By screening a kinase inhibitor library, we found that receptor tyrosine kinase inhibitors potently block EBOV and MARV GP-dependent viral entry. Inhibitors of epidermal growth factor receptor (EGFR), tyrosine protein kinase Met (c-Met), and the insulin receptor (InsR)/insulin like growth factor 1 receptor (IGF1R) blocked filoviral GP-mediated entry and prevented growth of replicative EBOV in Vero cells. Furthermore, inhibitors of c-Met and InsR/IGF1R also blocked viral entry in macrophages, the primary targets of EBOV infection. Interestingly, while the c-Met and InsR/IGF1R inhibitors interfered with EBOV trafficking to NPC1, virus delivery to the receptor was not impaired in the presence of the EGFR inhibitor. Instead, we observed that the NPC1 positive compartments were phenotypically altered and rendered incompetent to permit viral entry. Despite their different mechanisms of action, all three RTK inhibitors tested inhibited virus-induced Akt activation, providing a possible explanation for how EBOV may activate signaling pathways during entry. In sum, these studies strongly suggest that receptor tyrosine kinases initiate signaling cascades essential for efficient post-internalization entry steps.

Ebola virus (EBOV) and Marburg virus (MARV) are zoonotic pathogens that can cause severe hemorrhagic fevers in humans and non-human primates. They are members of the growing Filoviridae family that also includes three other species of Ebolaviruses known to be highly pathogenic in humans. While vaccines for EBOV are being deployed and showed high efficacy, pan-filoviral treatment is still lacking. To infect cells, EBOV requires the endosomal/lysosomal resident protein Niemann-Pick C1 (NPC1). Accordingly, viral particles require extensive trafficking within endosomal pathways for entry and delivery of the viral genome into the host cell cytoplasm. Here, we used chemical biology to reveal that EBOV triggers receptor tyrosine kinase (RTK)-dependent signaling to traffic to intracellular vesicles that contain the receptor and are conducive to entry. The characterization of host trafficking factors and signaling pathways that are potentially triggered by the virus are important as these could be targeted for antiviral therapies. In our study, we identified several RTK inhibitors, some of which are FDA-approved drugs, that potently block EBOV infection. Since all filoviruses known to date, even Měnglà virus that was recently discovered in bats in China, use NPC1 as their entry receptor, these inhibitors have the potential to be effective pan-filovirus antivirals.

The Two Sides of the Same Coin-Influenza Virus and Intracellular Signal Transduction

Cells respond to extracellular agents by activation of intracellular signaling pathways. Viruses can be regarded as such agents, leading to a firework of signaling inside the cell, primarily induced by pathogen-associated molecular patterns (PAMPs) that provoke safeguard mechanisms to defend from the invader. In the constant arms race between pathogen and cellular defense, viruses not only have evolved mechanisms to suppress or misuse supposedly antiviral signaling processes for their own benefit but also actively induce signaling to promote replication. This creates viral dependencies that may be exploited for novel strategies of antiviral intervention. Here, we will summarize the current knowledge of activation and function of influenza virus-induced signaling pathways with a focus on nuclear factor (NF)-κB signaling, mitogen-activated protein kinase cascades, and the phosphatidylinositol-3-kinase pathway. We will discuss the opportunities and drawbacks of targeting these signaling pathways for antiviral intervention.

Heparanase, cell signaling, and viral infections

Heparanase (HPSE) is a multifunctional protein endowed with many non-enzymatic functions and a unique enzymatic activity as an endo-β-D-glucuronidase. The latter allows it to serve as a key modulator of extracellular matrix (ECM) via a well-regulated cleavage of heparan sulfate side chains of proteoglycans at cell surfaces. The cleavage and associated changes at the ECM cause release of multiple signaling molecules with important cellular and pathological functions. New and emerging data suggest that both enzymatic as well as non-enzymatic functions of HPSE are important for health and illnesses including viral infections and virally induced cancers. This review summarizes recent findings on the roles of HPSE in activation, inhibition, or bioavailability of key signaling molecules such as AKT, VEGF, MAPK-ERK, and EGFR, which are known regulators of common viral infections in immune and non-immune cell types. Altogether, our review provides a unique overview of HPSE in cell-survival signaling pathways and how they relate to viral infections.

Growth Factor Receptor Signaling Inhibition Prevents SARS-CoV-2 Replication

SARS-CoV-2 infections are rapidly spreading around the globe. The rapid development of therapies is of major importance. However, our lack of understanding of the molecular processes and host cell signaling events underlying SARS-CoV-2 infection hinders therapy development. We use a SARS-CoV-2 infection system in permissible human cells to study signaling changes by phosphoproteomics. We identify viral protein phosphorylation and define phosphorylation-driven host cell signaling changes upon infection. Growth factor receptor (GFR) signaling and downstream pathways are activated. Drug-protein network analyses revealed GFR signaling as key pathways targetable by approved drugs. The inhibition of GFR downstream signaling by five compounds prevents SARS-CoV-2 replication in cells, assessed by cytopathic effect, viral dsRNA production, and viral RNA release into the supernatant. This study describes host cell signaling events upon SARS-CoV-2 infection and reveals GFR signaling as a central pathway essential for SARS-CoV-2 replication. It provides novel strategies for COVID-19 treatment.

Post-Translation Regulation of Influenza Virus Replication

Influenza virus exploits cellular factors to complete each step of viral replication. Yet, multiple host proteins actively block replication. Consequently, infection success depends on the relative speed and efficacy at which both the virus and host use their respective effectors. Post-translational modifications (PTMs) afford both the virus and the host means to readily adapt protein function without the need for new protein production. Here we use influenza virus to address concepts common to all viruses, reviewing how PTMs facilitate and thwart each step of the replication cycle. We also discuss advancements in proteomic methods that better characterize PTMs. Although some effectors and PTMs have clear pro- or antiviral functions, PTMs generally play regulatory roles to tune protein functions, levels, and localization. Synthesis of our current understanding reveals complex regulatory schemes where the effects of PTMs are time and context dependent as the virus and host battle to control infection.

[The potential of synthetic small interfering RNA-based antiviral drugs for influenza treatment]

Influenza is a worldwide public health problem. Annually, this infection affects up to 15% of the world population; and about half a million people die from this disease every year. Moreover, influenza A and B viruses tend to garner most of the attention, as these types are a major cause of the epidemics and pandemics. Although the influenza virus primarily affects the respiratory tract, it may also affect the cardiovascular and central nervous systems. Several antiviral drugs, that target various stages of viral reproduction, have been considered effective for the treatment and prevention of influenza, but some virus strains become resistant to these medications. Thus, new strategies and techniques should be developed to overcome the antiviral drug resistance. Recent studies suggest that new drugs based on RNA interference (RNAi) appear to be a promising therapeutic approach that regulates the activity of viral or cellular genes. As it is known, the RNAi is a eukaryotic gene regulatory mechanism that can be triggered by a foreign double-stranded RNA (dsRNA) and results in the cleavage of the target messenger RNA (mRNA). This review discusses the prospects, advantages, and disadvantages of using RNAi in carrying out a specific treatment for influenza infection. However, some viruses confer resistance to small interfering RNAs (siRNA) targeting viral genes. This problem can significantly reduce the effectiveness of RNAi. Therefore, applying siRNAs targeting host cell factors required for influenza virus reproduction can be a way to overcome the antiviral drug resistance.

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.

Repurposing of Kinase Inhibitors for Treatment of COVID-19

The outbreak of COVID-19, the pandemic disease caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has spurred an intense search for treatments by the scientific community. In the absence of a vaccine, the goal is to target the viral life cycle and alleviate the lung-damaging symptoms of infection, which can be life-threatening. There are numerous protein kinases associated with these processes that can be inhibited by FDA-approved drugs, the repurposing of which presents an alluring option as they have been thoroughly vetted for safety and are more readily available for treatment of patients and testing in clinical trials. Here, we characterize more than 30 approved kinase inhibitors in terms of their antiviral potential, due to their measured potency against key kinases required for viral entry, metabolism, or reproduction. We also highlight inhibitors with potential to reverse pulmonary insufficiency because of their anti-inflammatory activity, cytokine suppression, or antifibrotic activity. Certain agents are projected to be dual-purpose drugs in terms of antiviral activity and alleviation of disease symptoms, however drug combination is also an option for inhibitors with optimal pharmacokinetic properties that allow safe and efficacious co-administration with other drugs, such as antiviral agents, IL-6 blocking agents, or other kinase inhibitors.

The Role of Receptor Tyrosine Kinases in Lassa Virus Cell Entry

The zoonotic Old World mammarenavirus Lassa (LASV) causes severe hemorrhagic fever with high mortality and morbidity in humans in endemic regions. The development of effective strategies to combat LASV infections is of high priority, given the lack of a licensed vaccine and restriction on available treatment to off-label use of ribavirin. A better understanding of the fundamental aspects of the virus's life cycle would help to improve the development of novel therapeutic approaches. Host cell entry and restriction factors represent major barriers for emerging viruses and are promising targets for therapeutic intervention. In addition to the LASV main receptor, the extracellular matrix molecule dystroglycan (DG), the phosphatidylserine-binding receptors of the Tyro3/Axl/Mer (TAM), and T cell immunoglobulin and mucin receptor (TIM) families are potential alternative receptors of LASV infection. Therefore, the relative contributions of candidate receptors to LASV entry into a particular human cell type are a complex function of receptor expression and functional DG availability. Here, we describe the role of two receptor tyrosine kinases (RTKs), Axl and hepatocyte growth factor receptor (HGFR), in the presence and absence of glycosylated DG for LASV entry. We found that both RTKs participated in the macropinocytosis-related LASV entry and, regardless of the presence or absence of functional DG, their inhibition resulted in a significant antiviral effect.

Phosphatidylinositol-3-kinase-Akt pathway in negative-stranded RNA virus infection: a minireview

The PI3K/Akt signalling pathway is a crucial signalling cascade that regulates transcription, protein translation, cell growth, proliferation, cell survival, and metabolism. During viral infection, viruses exploit a variety of cellular pathways, including the well-known PI3K/Akt signalling pathway. Conversely, cells rely on this pathway to stimulate an antiviral response. The PI3K/Akt pathway is manipulated by a number of viruses, including DNA and RNA viruses and retroviruses. The aim of this review is to provide up-to-date information about the role of the PI3K-Akt pathway in infection with members of five different families of negative-sense ssRNA viruses. This pathway is hijacked for viral entry, regulation of endocytosis, suppression of premature apoptosis, viral protein expression, and replication. Although less common, the PI3K/Akt pathway can be downregulated as an immunomodulatory strategy or as a mechanism for inducing autophagy. Moreover, the cell activates this pathway as an antiviral strategy for interferon and cytokine production, among other strategies. Here, we present new data concerning the role of this pathway in infection with the paramyxovirus Newcastle disease virus (NDV). Our data seem to indicate that NDV uses the PI3K/Akt pathway to delay cell death and increase cell survival as a means of improving its replication. The interference of negative-sense ssRNA viruses with this essential pathway might have implications for the development of antiviral therapies.

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.

A77 1726, the active metabolite of the anti-rheumatoid arthritis drug leflunomide, inhibits influenza A virus replication in vitro and in vivo by inhibiting the activity of Janus kinases

The newly reassorted IAV subtypes from zoonotic reservoirs respond poorly to current vaccines and antiviral therapy. There is an unmet need in developing novel antiviral drugs for better control of IAV infection. The cellular factors that are crucial for virus replication have been sought as novel molecular targets for antiviral therapy. Recent studies have shown that Janus kinases (JAK), JAK1, and JAK2, play an important role in IAV replication. Leflunomide is an anti-inflammatory drug primarily used for treating rheumatoid arthritis (RA). Prior studies suggest that A77 1726, the active metabolite of leflunomide, inhibits the activity of JAK1 and JAK3. Our current study aims to determine if A77 1726 can function as a JAK inhibitor to control IAV infection. Here, we report that A77 1726 inhibited the replication of three IAV subtypes（H5N1, H1N1, H9N2）in three cell types (chicken embryonic fibroblasts, A549, and MDCK). A77 1726 inhibited JAK1, JAK2, and STAT3 tyrosine phosphorylation. Similar observations were made with Ruxolitinib (Rux), a JAK-specific inhibitor. JAK2 overexpression enhanced H5N1 virus replication and compromised the antiviral activity of A77 1726. Leflunomide inhibited virus replication in the lungs of IAV-infected mice, alleviated their body weight loss, and prolonged their survival. Our study demonstrates for the first time the ability of A77 1726 to inhibit JAK2 activity and suggests that inhibition of JAK activity contributes to its antiviral activity.

Inhibition of porcine epidemic diarrhea virus (PEDV) replication by A77 1726 through targeting JAK and Src tyrosine kinases

Porcine epidemic diarrhea (PED) virus (PEDV) is a coronavirus that primarily infects porcine intestinal epithelial cells and causes severe diarrhea and high fatality in piglets. A77 1726 is the active metabolite of leflunomide, a clinically approved anti-rheumatoid arthritis (RA) drug. A77 1726 inhibits the activity of protein tyrosine kinases (PTKs), p70 S6 kinase (S6K1), and dihydroorotate dehydrogenase (DHO-DHase). Whether A77 1726 can control coronavirus infections has not been investigated. Here we report that A77 1726 effectively restricted PEDV replication by inhibiting Janus kinases (JAKs) and Src kinase activities but not by inhibiting DHO-DHase and S6K1 activities. Overexpression of Src, JAK2 or its substrate STAT3 enhanced PEDV replication and attenuated the antiviral activity of A77 1726. Our study demonstrates for the first time the ability of A77 1726 to control coronavirus replication by inhibiting PTK activities. Leflunomide has potential therapeutic value for the control of PEDV and other coronavirus infections.

Attenuating influenza a virus infection by heparin binding EGF-like growth factor

Cell entry of influenza A virus (IAV) was reported to be promoted by epidermal growth factor receptor (EGFR). On the other hand, binding of heparin-binding EGF-like growth factor (HB-EGF) to EGFR leads to internalisation and degradation of the receptors. This study aimed to testify whether or not HB-EGF-induced downregulation of EGFR could attenuate IAV cell entry and subsequently diminish the infection. Immunoblotting and plaque assay revealed that HB-EGF-induced degradation of EGFR led to reduction of viral matrix 1 protein level and suppressed virion production. In addition, immunoblotting and imaging flow cytometric analysis demonstrated that IAV-induced phosphorylation of STAT1 and its localisation to nucleus in the early stage of infection were inhibited by HB-EGF treatment. This suggested the potential of HB-EGF in modulating uncontrolled and exaggerated inflammatory response caused by IAV infection. Together these findings attest the potential of HB-EGF mediated endocytosis and degradation of EGFR as a novel anti-IAV strategy.

Epidermal Growth Factor Receptor and Abl2 Kinase Regulate Distinct Steps of Human Papillomavirus 16 Endocytosis

Human papillomavirus 16 (HPV16), the leading cause of cervical cancer, exploits a novel endocytic pathway during host cell entry. This mechanism shares many requirements with macropinocytosis but differs in the mode of vesicle formation. Previous work indicated a role of the epidermal growth factor receptor (EGFR) in HPV16 endocytosis. However, the functional outcome of EGFR signaling and its downstream targets during HPV16 uptake are not well characterized. Here, we analyzed the functional importance of signal transduction via EGFR and its downstream effectors for endocytosis of HPV16. Our findings indicate two phases of EGFR signaling as follows: a-likely dispensable-transient activation with or shortly after cell binding and signaling required throughout the process of asynchronous internalization of HPV16. Interestingly, EGFR inhibition interfered with virus internalization and strongly reduced the number of endocytic pits, suggesting a role for EGFR signaling in the induction of HPV16 endocytosis. Moreover, we identified the Src-related kinase Abl2 as a novel regulator of virus uptake. Inhibition of Abl2 resulted in an accumulation of misshaped endocytic pits, indicating Abl2's importance for endocytic vesicle maturation. Since Abl2 rather than Src, a regulator of membrane ruffling during macropinocytosis, mediated downstream signaling of EGFR, we propose that the selective effector targeting downstream of EGFR determines whether HPV16 endocytosis or macropinocytosis is induced.IMPORTANCE Human papillomaviruses are small, nonenveloped DNA viruses that infect skin and mucosa. The so-called high-risk HPVs (e.g., HPV16, HPV18, HPV31) have transforming potential and are associated with various anogenital and oropharyngeal tumors. These viruses enter host cells by a novel endocytic pathway with unknown cellular function. To date, it is unclear how endocytic vesicle formation occurs mechanistically. Here, we addressed the role of epidermal growth factor receptor signaling, which has previously been implicated in HPV16 endocytosis and identified the kinase Abl2 as a novel regulator of virus uptake. Since other viruses, such as influenza A virus and lymphocytic choriomeningitis virus, possibly make use of related mechanisms, our findings shed light on fundamental strategies of virus entry and may in turn help to develop new host cell-targeted antiviral strategies.

Lipid rafts and pathogens: the art of deception and exploitation

Lipid rafts, solid regions of the plasma membrane enriched in cholesterol and glycosphingolipids, are essential parts of a cell. Functionally, lipid rafts present a platform that facilitates interaction of cells with the outside world. However, the unique properties of lipid rafts required to fulfill this function at the same time make them susceptible to exploitation by pathogens. Many steps of pathogen interaction with host cells, and sometimes all steps within the entire lifecycle of various pathogens, rely on host lipid rafts. Such steps as binding of pathogens to the host cells, invasion of intracellular parasites into the cell, the intracellular dwelling of parasites, microbial assembly and exit from the host cell, and microbe transfer from one cell to another all involve lipid rafts. Interaction also includes modification of lipid rafts in host cells, inflicted by pathogens from both inside and outside the cell, through contact or remotely, to advance pathogen replication, to utilize cellular resources, and/or to mitigate immune response. Here, we provide a systematic overview of how and why pathogens interact with and exploit host lipid rafts, as well as the consequences of this interaction for the host, locally and systemically, and for the microbe. We also raise the possibility of modulation of lipid rafts as a therapeutic approach against a variety of infectious agents.

Depletion of Host and Viral Sphingomyelin Impairs Influenza Virus Infection

Influenza A virus (IAV) is a major human respiratory pathogen causing annual epidemics as well as periodic pandemics. A complete understanding of the virus pathogenesis and host factors involved in the viral lifecycle is crucial for developing novel therapeutic approaches. Sphingomyelin (SM) is the most abundant membrane sphingolipid. It preferentially associates with cholesterol to form distinct domains named lipid rafts. Sphingomyelinases, including acid sphingomyelinase (ASMase), catalyzes the hydrolysis of membrane SM and consequently transform lipid rafts into ceramide-enriched membrane platforms. In this study, we investigated the effect of SM hydrolysis on IAV propagation. Depleting plasma membrane SM by exogenous bacterial SMase (bSMase) impaired virus infection and reduced virus entry, whereas exogenous SM enhanced infection. Moreover, the depletion of virus envelope SM also reduced virus infectivity and impaired its attachment and internalization. Nonetheless, inhibition of ASMase by desipramine did not affect IAV infection. Similarly, virus replication was not impaired in Niemann-Pick disease type A (NPA) cells, which lack functional ASMase. IAV infection in A549 cells was associated with suppression of ASMase activity starting at 6 h post-infection. Our data reveals that intact cellular and viral envelope SM is required for efficient IAV infection. Therefore, SM metabolism can be a potential target for therapeutic intervention against influenza virus infection.

HCC-derived EGFR mutants are functioning, EGF-dependent, and erlotinib-resistant

Background

Epidermal growth factor receptor (EGFR) has emerged as an important therapeutic target. Overexpression of EGFR is frequently observed in hepatocellular carcinoma (HCC) and EGFR activation has been proven to be a potential determinant of primary resistance of HCC cells to sorafenib. In our previous study, we found 13 missense mutations in EGFR exon 19-23 from hepatocellular carcinoma (HCC) tissues, but the functions of these mutations have not been determined. This study aims to determine the kinase activity and sensitivity to erlotinib, a 1st-generation EGFR-tyrosine kinase inhibitor (TKI), of seven HCC-derived mutants (K757E, N808S, R831C, V897A, P937L, T940A, and M947T).

Results

Using transduction of pBabe-puro retroviral vector with or without EGFR, we constructed and determined the function of EGFRs in NIH-3T3 cells stably harboring each of the seven mutants, as well as the erlotinib-sensitive L858R-mutant, the erlotinib-resistant T790M-mutant, and EGFR wild type (WT). Our results indicate that the seven mutants are functioning, EGF-dependent, EGFRs. Cells harboring six of the seven mutants could generate some level of EGFR phosphorylation in the absence of EGF, indicating some constitutive kinase activity, but all of the seven mutants remain primarily EGF-dependent. Our results demonstrate that erlotinib induces differential degree of apoptosis and autophagy among cells harboring different EGFRs: complete apoptosis and autophagy (cleavage of both caspase-3 and PARP, and marked LC3-II increment) in L858R-mutant; partial apoptosis and autophagy (only cleavage of caspase-3, and moderate LC3-II increment) in WT and HCC-derived mutants; and no apoptosis and minimal autophagy (no cleavage of caspase-3 and PARP, and minimal LC3-II increment) in T790M-mutant. The seven HCC-derived mutants are erlotinib-resistant, as treatment with erlotinib up to high concentration could only induce partial inhibition of EGFR phosphorylation, partial or no inhibition of AKT and ERK phosphorylation, and partial apoptosis and autophagy.

Conclusion

The seven HCC-derived EGFR mutants in this study are functioning, EGF-dependent, and erlotinib-resistant. Erlotinib induces differential degree of apoptosis and autophagy among cells harboring different EGFRs. The degree of inhibition of EGFR phosphorylation by erlotinib is the determining factor for the degree of apoptosis and autophagy amongst cells harboring EGFR mutants. This study paves the way for further investigation into the sensitivity of these HCC-derived mutants to the 3rd-generation irreversible EGFR-TKI, osimertinib.

Inhibition of herpes simplex virus by myricetin through targeting viral gD protein and cellular EGFR/PI3K/Akt pathway

Myricetin, a common dietary flavonoid, was reported to possess many different biological activities such as anti-oxidant, anti-inflammatory, and antiviral effects. In this study, we explored the anti-HSV effects and mechanisms of myricetin both in vitro and in vivo. The results showed that myricetin possessed anti-HSV-1 and HSV-2 activities with very low toxicity, superior to the effects of acyclovir. Myricetin may block HSV infection through direct interaction with virus gD protein to interfere with virus adsorption and membrane fusion, which was different from the nucleoside analogues such as acyclovir. Myricetin also down-regulate the cellular EGFR/PI3K/Akt signaling pathway to further inhibit HSV infection and its subsequent replication. Most importantly, intraperitoneal therapy of myricetin markedly improved mice survival and reduced virus titers in both lungs and spinal cord. Therefore, the natural dietary flavonoid myricetin has potential to be developed into a novel anti-HSV agent targeting both virus gD protein and cellular EGFR/PI3K/Akt pathway.

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Myricetin possessed anti-HSV-1 and HSV-2 activities in vitro with low toxicity.

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Myricetin may be able to block HSV binding and entry process in HeLa cells.

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Myricetin may directly bind to virus gD protein rather than cellular receptors of HSV.

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The EGFR/PI3K/Akt pathway may be involved in the anti-HSV actions of myricetin.

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Myricetin markedly improved survival and reduced virus titers in HSV infected mice.

Reovirus uses macropinocytosis-mediated entry and fast axonal transport to infect neurons

Several barriers protect the central nervous system (CNS) from pathogen invasion. Yet viral infections of the CNS are common and often debilitating. Understanding how neurotropic viruses co-opt host machinery to overcome challenges to neuronal entry and transmission is important to combat these infections. Neurotropic reovirus disseminates through neural routes and invades the CNS to cause lethal encephalitis in newborn animals. To define mechanisms of reovirus neuronal entry and directional transport, we used primary neuron cultures, which reproduce in vivo infection patterns displayed by different reovirus serotypes. Treatment of neurons with small-molecule inhibitors of different endocytic uptake pathways allowed us to discover that the cellular machinery mediating macropinocytosis is required for reovirus neuronal entry. This mechanism of reovirus entry differs from clathrin-mediated endocytosis, which is used by reovirus to invade non-neuronal cells. Analysis of reovirus transport and release from isolated soma or axonal termini of neurons cultivated in microfluidic devices indicates that reovirus is capable of retrograde but only limited anterograde neuronal transmission. The dynamics of retrograde reovirus movement are consistent with fast axonal transport coordinated by dynein along microtubules. Further analysis of viral transport revealed that multiple virions are transported together in axons within non-acidified vesicles. Reovirus-containing vesicles acidify after reaching the soma, where disassembly of virions and release of the viral core into the cytoplasm initiates replication. These results define mechanisms of reovirus neuronal entry and transport and establish a foundation to identify common host factors used by neuroinvasive viruses. Furthermore, our findings emphasize consideration of cell type-specific entry mechanisms in the tailored design of neurotropic viruses as tracers, oncolytic agents, and delivery vectors.

Viral infections of the central nervous system (CNS) cause a significant health burden globally and compel a better mechanistic understanding of neural invasion by viruses to develop effective interventions. Neurotropic reovirus disseminates through neural routes to infect the CNS and serves as a tractable model to study neural invasion by viruses. Despite knowledge of reovirus neurotropism for decades, mechanisms mediating reovirus neuronal infection remain undefined. We used primary neurons cultured in microfluidic devices to study entry and directional transport of reovirus. We discovered that reovirus uses macropinocytosis for neuronal entry as opposed to the use of a clathrin-mediated pathway in non-neuronal cells. We are unaware of another virus using macropinocytosis to enter neurons. Following internalization, reovirus spreads in the retrograde direction using dynein-mediated fast axonal transport but exhibits limited anterograde spread. We further demonstrate that reovirus disassembly and replication occur in the neuronal soma subsequent to axonal transport. Remarkably, these entry and transport mechanisms mirror those used by misfolded proteins implicated in neurodegenerative diseases. Our findings establish the mechanics of reovirus neuronal uptake and spread and provide clues about therapeutic targets to limit neuropathology inflicted by pathogens and misfolded proteins.

Influenza A virus uncoating

Influenza A virus (IAV) is an enveloped virus of the Orthomyxoviridae with a negative-sense single-stranded RNA genome. During virus cell entry, viral and cellular cues are delivered in a stepwise manner within two distinct cellular compartments-the endosomes and the cytosol. Endosome maturation primes the viral core for uncoating by cytosolic host proteins and host-mediated virus disaggregation is essential for genome import and replication in the nucleus. Recent evidence shows that two well-known cellular proteins-histone deacetylase 6 (HDAC6) and karyopherin-β2 (kapβ2)-uncoat influenza virus. HDAC6 is 1 of 11 HDACs and an X-linked, cytosolic lysine deacetylase. Under normal cellular conditions HDAC6 is the tubulin deacetylase. Under proteasomal stress HDAC6 binds unanchored ubiquitin, dynein and myosin II to sequester misfolded protein aggregates for autophagy. Kapβ2 is a member of the importin β family that transports RNA-binding proteins into the nucleus by binding to disordered nuclear localization signals (NLSs) known as PY-NLS. Kapβ2 is emerging as a universal uncoating factor for IAV and human immunodeficiency virus type 1 (HIV-1). Kapβ2 can also reverse liquid-liquid phase separation (LLPS) of RNA-binding proteins by promoting their disaggregation. Thus, it is becoming evident that key players in the management of cellular condensates and membraneless organelles are potent virus uncoating factors. This emerging concept reveals implications in viral pathogenesis, as well as, the promise for cell-targeted therapeutic strategies to block universal virus uncoating pathways hijacked by enveloped RNA viruses.

Respiratory Barrier as a Safeguard and Regulator of Defense Against Influenza A Virus and Streptococcus pneumoniae

The primary function of the respiratory system of gas exchange renders it vulnerable to environmental pathogens that circulate in the air. Physical and cellular barriers of the respiratory tract mucosal surface utilize a variety of strategies to obstruct microbe entry. Physical barrier defenses including the surface fluid replete with antimicrobials, neutralizing immunoglobulins, mucus, and the epithelial cell layer with rapidly beating cilia form a near impenetrable wall that separates the external environment from the internal soft tissue of the host. Resident leukocytes, primarily of the innate immune branch, also maintain airway integrity by constant surveillance and the maintenance of homeostasis through the release of cytokines and growth factors. Unfortunately, pathogens such as influenza virus and Streptococcus pneumoniae require hosts for their replication and dissemination, and prey on the respiratory tract as an ideal environment causing severe damage to the host during their invasion. In this review, we outline the host-pathogen interactions during influenza and post-influenza bacterial pneumonia with a focus on inter- and intra-cellular crosstalk important in pulmonary immune responses.

Hepatitis C Virus Entry: Protein Interactions and Fusion Determinants Governing Productive Hepatocyte Invasion

Hepatitis C virus (HCV) entry is among the best-studied uptake processes for human pathogenic viruses. Uptake follows a spatially and temporally tightly controlled program. Numerous host factors including proteins, lipids, and glycans promote productive uptake of HCV particles into human liver cells. The virus initially attaches to surface proteoglycans, lipid receptors such as the scavenger receptor BI (SR-BI), and to the tetraspanin CD81. After lateral translocation of virions to tight junctions, claudin-1 (CLDN1) and occludin (OCLN) are essential for entry. Clathrin-mediated endocytosis engulfs HCV particles, which fuse with endosomal membranes after pH drop. Uncoating of the viral RNA genome in the cytoplasm completes the entry process. Here we systematically review and classify HCV entry factors by their mechanistic role, relevance, and level of evidence. Finally, we report on more recent knowledge on determinants of membrane fusion and close with an outlook on future implications of HCV entry research.

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.

Prostasin Impairs Epithelial Growth Factor Receptor Activation to Suppress Dengue Virus Propagation

BACKGROUND

Dengue virus (DENV), a common and widely spread arbovirus, causes life-threatening diseases, such as dengue hemorrhagic fever or dengue shock syndrome. There is currently no effective therapeutic or preventive treatment for DENV infection.

METHODS

Next-generation sequencing analysis revealed that prostasin expression was decreased upon DENV infection. Prostasin expression levels were confirmed by real-time quantitative polymerase chain reaction in patients with dengue fever and a DENV-infected mice model. Short hairpin RNA against EGFR and LY294002 were used to investigate the molecular mechanism.

RESULTS

Based on clinical studies, we first found relatively low expression of prostasin, a glycosylphosphatidyl inositol-anchored membrane protease, in blood samples from patients with dengue fever compared with healthy individuals and a high correlation of prostasin expression and DENV-2 RNA copy number. DENV infection significantly decreased prostasin RNA levels of in vivo and in vitro models. By contrast, exogenous expression of prostasin could protect ICR suckling mice from life-threatening DENV-2 infection. Mechanistic studies showed that inhibition of DENV propagation by prostasin was due to reducing expression of epithelial growth factor receptor, leading to suppression of the Akt/NF-κB-mediated cyclooxygenase-2 signaling pathway.

CONCLUSION

Our results demonstrate that prostasin expression is a noteworthy clinical feature and a potential therapeutic target against DENV infection.

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.

Introducing APOA1 as a key protein in COVID-19 infection: a bioinformatics approach

AIM

Introducing possible diagnostic and therapeutic biomarker candidates via the identification of chief dysregulated proteins in COVID-19 patients is the aim of this study.

BACKGROUND

Molecular studies, especially proteomics, can be considered as suitable approaches for discovering the hidden aspect of the disease.

METHODS

Differentially expressed proteins (DEPs) of three patients with demonstrated severe condition (S-COVID-19) were compared to healthy cases by a proteomics study. Cytoscape software and STRING database were used to construct the protein-protein interaction (PPI) network. The central DEPs were identified through topological analysis of the network. ClueGO+CluePedia were applied to find the biological processes related to the central nodes. MCODE molecular complex detection (MCODE) was used to discover protein complexes.

RESULTS

A total of 242 DEPs from among 256 query ones were included in the network. Centrality analysis of the network assigned 16 hub-bottlenecks, nine of which were presented in the highest-scored protein complex. Ten protein complexes were determined. APOA1 was identified as the protein complex seed, and APP, EGF, and C3 were the top hub-bottlenecks of the network. The results specify that up-regulation of C3 and down-regulation of APOA1 in urine play a role in the stiffness in respiration and, accordingly, the severity of COVID-19. Moreover, dysregulation of APP and APOA1 could both contribute to the possible adverse effects of COVID-19 on the nervous system.

CONCLUSION

The introduced central proteins of the S-COVID-19 interaction network, particularly APOA1, can be considered as diagnostic and therapeutic targets related to the coronavirus disease after being approved with complementary studies.

Influenza A Virus Hemagglutinin-Neuraminidase-Receptor Balance: Preserving Virus Motility

Influenza A viruses (IAVs) occasionally cross the species barrier and adapt to novel host species. This requires readjustment of the functional balance of the sialic acid receptor-binding hemagglutinin (HA) and the receptor-destroying neuraminidase (NA) to the sialoglycan-receptor repertoire of the new host. Novel techniques have revealed mechanistic details of this HA-NA-receptor balance, emphasizing a previously underappreciated crucial role for NA in driving the motility of receptor-associated IAV particles. Motility enables virion penetration of the sialylated mucus layer as well as attachment to, and uptake into, underlying epithelial cells. As IAVs are essentially irreversibly bound in the absence of NA activity, the fine-tuning of the HA-NA-receptor balance rather than the binding avidity of IAV particles per se is an important factor in determining host species tropism.

EPS8 Facilitates Uncoating of Influenza A Virus

All viruses balance interactions between cellular machinery co-opted to support replication and host factors deployed to halt the infection. We use gene correlation analysis to perform an unbiased screen for host factors involved in influenza A virus (FLUAV) infection. Our screen identifies the cellular factor epidermal growth factor receptor pathway substrate 8 (EPS8) as the highest confidence pro-viral candidate. Knockout and overexpression of EPS8 confirm its importance in enhancing FLUAV infection and titers. Loss of EPS8 does not affect virion attachment, uptake, or fusion. Rather, our data show that EPS8 specifically functions during virion uncoating. EPS8 physically associates with incoming virion components, and subsequent nuclear import of released ribonucleoprotein complexes is significantly delayed in the absence of EPS8. Our study identifies EPS8 as a host factor important for uncoating, a crucial step of FLUAV infection during which the interface between the virus and host is still being discovered.

Metabolic machinery encrypted in the Raman spectrum of influenza A virus-inoculated mammalian cells

Raman spectroscopy was applied with a high spectral resolution to a structural study of Influenza (type A) virus before and after its inoculation into Madin-Darby canine kidney cells. This study exploits the fact that the major virus and cell constituents, namely DNA/RNA, lipid, and protein molecules, exhibit peculiar fingerprints in the Raman spectrum, which clearly differed between cells and viruses, as well as before and after virus inoculation into cells. These vibrational features, which allowed us to discuss viral assembly, membrane lipid evolution, and nucleoprotein interactions of the virus with the host cells, reflected the ability of the virus to alter host cells' pathways to enhance its replication efficiency. Upon comparing Raman signals from the host cells before and after virus inoculation, we were also able to discuss in detail cell metabolic reactions against the presence of the virus in terms of compositional variations of lipid species, the formation of fatty acids, dephosphorylation of high-energy adenosine triphosphate molecules, and enzymatic hydrolysis of the hemagglutinin glycoprotein.

G protein-coupled receptor kinase 2 (GRK2) as a multifunctional signaling hub

Accumulating evidence indicates that G protein-coupled receptor kinase 2 (GRK2) is a versatile protein that acts as a signaling hub by modulating G protein-coupled receptor (GPCR) signaling and also via phosphorylation or scaffolding interactions with an extensive number of non-GPCR cellular partners. GRK2 multifunctionality arises from its multidomain structure and from complex mechanisms of regulation of its expression levels, activity, and localization within the cell, what allows the precise spatio-temporal shaping of GRK2 targets. A better understanding of the GRK2 interactome and its modulation mechanisms is helping to identify the GRK2-interacting proteins and its substrates involved in the participation of this kinase in different cellular processes and pathophysiological contexts.

An Influenza Virus Entry Inhibitor Targets Class II PI3 Kinase and Synergizes with Oseltamivir

Two classes of antivirals targeting the viral neuraminidase (NA) and endonuclease are currently the only clinically useful drugs for the treatment of influenza. However, resistance to both antivirals has been observed in clinical isolates, and there was widespread resistance to oseltamivir (an NA inhibitor) among H1N1 viruses prior to 2009. This potential for resistance and lack of diversity for antiviral targets highlights the need for new influenza antivirals with a higher barrier to resistance. In this study, we identified an antiviral compound, M85, that targets host kinases, epidermal growth factor receptor (EGFR), and phosphoinositide 3 class II β (PIK3C2β) and is not susceptible to resistance by viral mutations. M85 blocks endocytosis of influenza viruses and inhibits a broad-spectrum of viruses with minimal cytotoxicity. In vitro, we found that combinations of M85 and oseltamivir have strong synergism. In the mouse model for influenza, treatment with the combination therapy was more protective against a lethal viral challenge than oseltamivir alone, indicating that development of M85 could lead to combination therapies for influenza. Finally, through this discovery of M85 and its antiviral mechanism, we present the first description of PIK3C2β as a necessary host factor for influenza virus entry.

The Role of EGFR in Influenza Pathogenicity: Multiple Network-Based Approaches to Identify a Key Regulator of Non-lethal Infections

Despite high sequence similarity between pandemic and seasonal influenza viruses, there is extreme variation in host pathogenicity from one viral strain to the next. Identifying the underlying mechanisms of variability in pathogenicity is a critical task for understanding influenza virus infection and effective management of highly pathogenic influenza virus disease. We applied a network-based modeling approach to identify critical functions related to influenza virus pathogenicity using large transcriptomic and proteomic datasets from mice infected with six influenza virus strains or mutants. Our analysis revealed two pathogenicity-related gene expression clusters; these results were corroborated by matching proteomics data. We also identified parallel downstream processes that were altered during influenza pathogenesis. We found that network bottlenecks (nodes that bridge different network regions) were highly enriched in pathogenicity-related genes, while network hubs (highly connected network nodes) were significantly depleted in these genes. We confirmed that this trend persisted in a distinct virus: Severe Acute Respiratory Syndrome Coronavirus (SARS). The role of epidermal growth factor receptor (EGFR) in influenza pathogenesis, one of the bottleneck regulators with corroborating signals across transcript and protein expression data, was tested and validated in additional mouse infection experiments. We demonstrate that EGFR is important during influenza infection, but the role it plays changes for lethal versus non-lethal infections. Our results show that by using association networks, bottleneck genes that lack hub characteristics can be used to predict a gene's involvement in influenza virus pathogenicity. We also demonstrate the utility of employing multiple network approaches for analyzing host response data from viral infections.

Targeting of host cell receptor tyrosine kinases by intracellular pathogens

Intracellular pathogens use complex and tightly regulated processes to enter host cells. Upon initial interactions with signaling proteins at the surface of target cells, intracellular microbes activate and co-opt specific host signaling pathways that mediate cell surface-cytosol communications to facilitate pathogen internalization. Here, we discuss the roles of host receptor tyrosine kinases (RTKs) in the establishment of productive infections by major intracellular pathogens. We evaluate the gaps in the current understanding of this process and propose a comprehensive approach for assessing the role of host cell signaling in the biology of intracellular microorganisms and viruses. We also discuss RTK-targeting strategies for the treatment of various infections.

Cell surface α2,3-linked sialic acid facilitates Zika virus internalization

The emergence of neurotropic Zika virus (ZIKV) raised a public health emergency of global concern. ZIKV can cross the placental barrier and infect foetal brains, resulting in microcephaly, but the pathogenesis of ZIKV is poorly understood. With recent findings reporting AXL as a type I interferon antagonist rather than an entry receptor, the exact entry mechanism remains unresolved. Here we report that cell surface sialic acid plays an important role in ZIKV infection. Removal of cell surface sialic acid by neuraminidase significantly abolished ZIKV infection in Vero cells and human induced-pluripotent stem cells-derived neural progenitor cells. Furthermore, knockout of the sialic acid biosynthesis gene encoding UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase resulted in significantly less ZIKV infection of both African and Asian lineages. Huh7 cells deficient in α2,3-linked sialic acid through knockout of ST3 β-galactoside-α2,3-sialyltransferase 4 had significantly reduced ZIKV infection. Removal of membrane-bound, un-internalized virus with pronase treatment revealed the role of sialic acid in ZIKV internalization but not attachment. Sialyllactose inhibition studies showed that there is no direct interaction between sialic acid and ZIKV, implying that sialic acid could be mediating ZIKV-receptor complex internalization. Identification of α2,3-linked sialic acid as an important host factor for ZIKV internalization provides new insight into ZIKV infection and pathogenesis.

Toxoplasma gondii induces prolonged host epidermal growth factor receptor signalling to prevent parasite elimination by autophagy: Perspectives for in vivo control of the parasite

Toxoplasma gondii causes retinitis and encephalitis. Avoiding targeting by autophagosomes is key for its survival because T. gondii cannot withstand lysosomal degradation. During invasion of host cells, T. gondii triggers epidermal growth factor receptor (EGFR) signalling enabling the parasite to avoid initial autophagic targeting. However, autophagy is a constitutive process indicating that the parasite may also use a strategy operative beyond invasion to maintain blockade of autophagic targeting. Finding that such a strategy exists would be important because it could lead to inhibition of host cell signalling as a novel approach to kill the parasite in previously infected cells and treat toxoplasmosis. We report that T. gondii induced prolonged EGFR autophosphorylation. This effect was mediated by PKCα/PKCβ ➔ Src because T. gondii caused prolonged activation of these molecules and their knockdown or incubation with inhibitors of PKCα/PKCβ or Src after host cell invasion impaired sustained EGFR autophosphorylation. Addition of EGFR tyrosine kinase inhibitor (TKI) to previously infected cells led to parasite entrapment by LC3 and LAMP-1 and pathogen killing dependent on the autophagy proteins ULK1 and Beclin 1 as well as lysosomal enzymes. Administration of gefitinib (EGFR TKI) to mice with ocular and cerebral toxoplasmosis resulted in disease control that was dependent on Beclin 1. Thus, T. gondii promotes its survival through sustained EGFR signalling driven by PKCα/β ➔ Src, and inhibition of EGFR controls pre-established toxoplasmosis.

Immunological implications of epidermal growth factor receptor signaling in persistent infections

Infectious diseases account for a large proportion of global health emergencies and are rising more so owing to the paucity of effective vaccination and chemotherapeutic strategies. The severity is compounded by the development of antibiotic resistance among major pathogenic strains, capable of residing in the hostile host microenvironment by hijacking its signaling mechanisms and molecular circuitry. Among such processes, studies on epidermal growth factor receptor (EGFR) have revealed specific contributions of this classical oncogenic signaling axis during distinct infection conditions. Here, we review the current status of EGFR family members in the context of host-pathogen interactions and speculate the possible dimensions of exploration and manipulation of the EGFR pathway for host-directed therapeutic purposes.

Entry of Challenge Virus Standard (CVS) -11 into N2a cells via a clathrin-mediated, cholesterol-, dynamin-, pH-dependent endocytic pathway

Background

Rabies virus (RABV), a member of Lyssavirus of Rhabdoviridae family, is a kind of negative-strand RNA virus. The zoonosis caused by RABV leads to high mortality in animals and humans. Though with the extensive investigation, the mechanisms of RABV entry into cells have not been well characterized.

Methods

Chemical inhibitors and RNA interference (RNAi) were used to analysis RABV internalization pathway. The expression level of viral N protein was examined by quantitative PCR and western blot, and the virus infection in the cells was visualized by fluorescence microscopy.

Results

We firstly examined the endocytosis pathway of the challenge virus standard (CVS) -11 strain in N2a cells. Chlorpromazine treatment and knockdown of clathrin heavy chain (CHC) significantly reduced viral entry, which proved clathrin was required. Meanwhile neither nystatin nor knocking down caveolin-1 (Cav1) in N2a cells had an effect on CVS-11 infection, suggesting that caveolae was independent for CVS-11 internalization. And when cholesterol of cell membrane was extracted by MβCD, viral infection was strongly impacted. Additionally by using the specific inhibitor dynasore and ammonium chloride, we verified that dynamin and a low-pH environment were crucial for RABV infection, which was confirmed by confocal microscopy.

Conclusions

Our results demonstrated that CVS-11 entered N2a cells through a clathrin-mediated, cholesterol-, pH-, dynamin-required, and caveolae-independent endocytic pathway.

Electronic supplementary material

The online version of this article (10.1186/s12985-019-1186-9) contains supplementary material, which is available to authorized users.

Competitive Cooperation of Hemagglutinin and Neuraminidase during Influenza A Virus Entry

The hemagglutinin (HA) and neuraminidase (NA) of influenza A virus possess antagonistic activities on interaction with sialic acid (SA), which is the receptor for virus attachment. HA binds SA through its receptor-binding sites, while NA is a receptor-destroying enzyme by removing SAs. The function of HA during virus entry has been extensively investigated, however, examination of NA has long been focused to its role in the exit of progeny virus from infected cells, and the role of NA in the entry process is still under-appreciated. This review summarizes the current understanding of the roles of HA and NA in relation to each other during virus entry.

RNAi-based small molecule repositioning reveals clinically approved urea-based kinase inhibitors as broadly active antivirals

Influenza viruses (IVs) tend to rapidly develop resistance to virus-directed vaccines and common antivirals targeting pathogen determinants, but novel host-directed approaches might preclude resistance development. To identify the most promising cellular targets for a host-directed approach against influenza, we performed a comparative small interfering RNA (siRNA) loss-of-function screen of IV replication in A549 cells. Analysis of four different IV strains including a highly pathogenic avian H5N1 strain, an influenza B virus (IBV) and two human influenza A viruses (IAVs) revealed 133 genes required by all four IV strains. According to gene enrichment analyses, these strain-independent host genes were particularly enriched for nucleocytoplasmic trafficking. In addition, 360 strain-specific genes were identified with distinct patterns of usage for IAVs versus IBV and human versus avian IVs. The strain-independent host genes served to define 43 experimental and otherwise clinically approved drugs, targeting reportedly fourteen of the encoded host factors. Amongst the approved drugs, the urea-based kinase inhibitors (UBKIs) regorafenib and sorafenib exhibited a superior therapeutic window of high IV antiviral activity and low cytotoxicity. Both UBKIs appeared to block a cell signaling pathway involved in IV replication after internalization, yet prior to vRNP uncoating. Interestingly, both compounds were active also against unrelated viruses including cowpox virus (CPXV), hantavirus (HTV), herpes simplex virus 1 (HSV1) and vesicular stomatitis virus (VSV) and showed antiviral efficacy in human primary respiratory cells. An in vitro resistance development analysis for regorafenib failed to detect IV resistance development against this drug. Taken together, the otherwise clinically approved UBKIs regorafenib and sorafenib possess high and broad-spectrum antiviral activity along with substantial robustness against resistance development and thus constitute attractive host-directed drug candidates against a range of viral infections including influenza.

Conventional medications against influenza infections, including vaccination and antiviral drug therapy, are targeted against viral determinants–an approach collectively referred to as pathogen-directed. However, influenza viruses mutate fast and quickly develop resistance to these pathogen-directed treatments. An alternative, yet not well established, is to block host cellular molecules required by the virus to successfully multiply. Such a host-directed approach is anticipated to be more robust against the development of drug resistance. This notion is founded on the different modes of action of the two principal approaches: Virus-directed therapeutics target the virus itself. Thus, just a single mutation could abrogate sensitivity to a virus-directed therapeutic. In contrast, it is unlikely that viruses can easily circumvent a pharmacological blockage of a cellular factor by means of just a few mutations. Instead, the virus needs to either exploit an immediate parallel cellular pathway or adjust its replication cycle to a different cellular factor–the latter being a process likely to require multiple mutations, if possible at all. To identify the most promising targets for a host-directed therapy, we performed a small interfering RNA (siRNA) screen with four different influenza virus strains using a lung epithelial cell line. Subsequently, we tested a series of drugs, specific for the products of the genes that are required for replication of all four influenza virus strains tested. Regorafenib and sorafenib, two chemically related urea-based kinase inhibitors already clinically approved for cancer treatment, turned out to be effective inhibitors of all influenza viruses and displayed low cytotoxicity. These drugs blocked viral replication at an early stage of the life cycle not only in cell lines but also in human primary respiratory cells. Moreover, these drugs exhibited high efficacy even against unrelated viruses. In addition, no development of resistance was observed against regorafenib, which was used in an in vitro assay representatively of urea-based kinase inhibitors. Our results suggest that regorafenib and sorafenib are promising drug candidates for a host-directed therapy of influenza and other viral infections.

Macropinocytosis contributes to hantavirus entry into human airway epithelial cells

Hantaviruses are emerging rodent-borne negative-strand RNA viruses associated with severe human diseases. Zoonotic transmission occurs via aerosols of contaminated rodent excreta and cells of the human respiratory epithelium represent likely early targets. Here we investigated cellular factors involved in entry of the pathogenic Old and New World hantaviruses Hantaan virus (HTNV) and Andes virus (ANDV) into human respiratory epithelial cells. Screening of a kinase inhibitor library using a biocontained recombinant vesicular stomatitis virus pseudotype platform revealed differential requirement for host kinases for HTNV and ANDV entry and provided first hints for an involvement of macropinocytosis. Examination of a selected panel of well-defined inhibitors of endocytosis confirmed that both HTNV and ANDV enter human respiratory epithelial cells via a pathway that critically depends on sodium proton exchangers and actin, hallmarks of macropinocytosis. However, HTNV and ANDV differed in their individual requirements for regulatory factors of macropinocytosis, indicating virus-specific differences.

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.

IFITM3 directly engages and shuttles incoming virus particles to lysosomes

Interferon-induced transmembrane proteins (IFITMs 1, 2 and 3) have emerged as important innate immune effectors that prevent diverse virus infections in vertebrates. However, the cellular mechanisms and live-cell imaging of these small membrane proteins have been challenging to evaluate during viral entry of mammalian cells. Using CRISPR–Cas9-mediated IFITM-mutant cell lines, we demonstrate that human IFITM1, IFITM2 and IFITM3 act cooperatively and function in a dose-dependent fashion in interferon-stimulated cells. Through site-specific fluorophore tagging and live-cell imaging studies, we show that IFITM3 is on endocytic vesicles that fuse with incoming virus particles and enhances the trafficking of this pathogenic cargo to lysosomes. IFITM3 trafficking is specific to restricted viruses, requires S-palmitoylation and is abrogated with loss-of-function mutants. The site-specific protein labeling and live-cell imaging approaches described here should facilitate the functional analysis of host factors involved in pathogen restriction as well as their mechanisms of regulation.

Live-cell imaging and virus trafficking studies show that the host innate immune receptor IFITM3 localizes with endocytic vesicles that fuse with incoming viruses to ultimately enhance their traffic to lysosomes.

Host Lipid Rafts Play a Major Role in Binding and Endocytosis of Influenza A Virus

Influenza still remains one of the most challenging diseases, posing a significant threat to public health. Host lipid rafts play a critical role in influenza A virus (IAV) assembly and budding, however, their role in polyvalent IAV host binding and endocytosis had remained elusive until now. In the present study, we observed co-localization of IAV with a lipid raft marker ganglioside, GM1, on the host surface. Further, we isolated the lipid raft micro-domains from IAV infected cells and detected IAV protein in the raft fraction. Finally, raft disruption using Methyl-β-Cyclodextrin revealed significant reduction in IAV host binding, suggesting utilization of host rafts for polyvalent binding on the host cell surface. In addition to this, cyclodextrin mediated inhibition of raft-dependent endocytosis showed significantly reduced IAV internalization. Interestingly, exposure of cells to cyclodextrin two hours post-IAV binding showed no such reduction in IAV entry, indicating use of raft-dependent endocytosis for host entry. In summary, this study demonstrates that host lipid rafts are selected by IAV as a host attachment factors for multivalent binding, and IAV utilizes these micro-domains to exploit raft-dependent endocytosis for host internalization, a virus entry route previously unknown for IAV.

Viral journeys on the intracellular highways

Viruses are obligate intracellular pathogens that are dependent on cellular machineries for their replication. Recent technological breakthroughs have facilitated reliable identification of host factors required for viral infections and better characterization of the virus-host interplay. While these studies have revealed cellular machineries that are uniquely required by individual viruses, accumulating data also indicate the presence of broadly required mechanisms. Among these overlapping cellular functions are components of intracellular membrane trafficking pathways. Here, we review recent discoveries focused on how viruses exploit intracellular membrane trafficking pathways to promote various stages of their life cycle, with an emphasis on cellular factors that are usurped by a broad range of viruses. We describe broadly required components of the endocytic and secretory pathways, the Endosomal Sorting Complexes Required for Transport pathway, and the autophagy pathway. Identification of such overlapping host functions offers new opportunities to develop broad-spectrum host-targeted antiviral strategies.

Single Particle Imaging of Polarized Hepatoma Organoids upon Hepatitis C Virus Infection Reveals an Ordered and Sequential Entry Process

Hepatitis C virus (HCV) enters hepatocytes via various entry factors, including scavenger receptor BI (SR-B1), cluster of differentiation 81 (CD81), epidermal growth factor receptor (EGFR), claudin-1 (CLDN1), and occludin (OCLN). As CLDN1 and OCLN are not readily accessible due to their tight junctional localization, HCV likely accesses them by either disrupting cellular polarity or migrating to the tight junction. In this study, we image HCV entry into a three-dimensional polarized hepatoma system and reveal that the virus sequentially engages these entry factors through actin-dependent mechanisms. HCV initially localizes with the early entry factors SR-B1, CD81, and EGFR at the basolateral membrane and then accumulates at the tight junction in an actin-dependent manner. HCV associates with CLDN1 and then OCLN at the tight junction and is internalized via clathrin-mediated endocytosis by an active process requiring EGFR. Thus, HCV uses a dynamic and multi-step process to engage and enter host cells.