The mechanism of Axl-mediated Ebola virus infection

Hippo signaling pathway regulates Ebola virus transcription and egress

Filovirus-host interactions play important roles in all stages of the virus lifecycle. Here, we identify LATS1/2 kinases and YAP, key components of the Hippo pathway, as critical regulators of EBOV transcription and egress. Specifically, we find that when YAP is phosphorylated by LATS1/2, it localizes to the cytoplasm (Hippo "ON") where it sequesters VP40 to prevent egress. In contrast, when the Hippo pathway is "OFF", unphosphorylated YAP translocates to the nucleus where it transcriptionally activates host genes and promotes viral egress. Our data reveal that LATS2 indirectly modulates filoviral VP40-mediated egress through phosphorylation of AMOTp130, a positive regulator of viral egress, but more surprisingly that LATS1/2 kinases directly modulate EBOV transcription by phosphorylating VP30, an essential regulator of viral transcription. In sum, our findings highlight the potential to exploit the Hippo pathway/filovirus axis for the development of host-oriented countermeasures targeting EBOV and related filoviruses.

Anexelekto (AXL) no more: microRNA-155 (miR-155) controls the "Uncontrolled" in SARS-CoV-2

AXL is the gene that encodes the Anexelekto (AXL) receptor tyrosine kinase that demonstrates significant roles in various cellular processes, including cell growth, survival, and migration. Anexelekto is a Greek word meaning excessive and uncontrolled, semantically implying the crucial involvement of AXL in cancer and immune biology, and in promoting cancer metastasis. AXL overexpression appears to drive epithelial to mesenchymal transition, tumor angiogenesis, decreased antitumor immune response, and resistance to therapeutic agents. Recently, AXL has been reported to play important roles in several viral infections, including SARS-CoV-2. We have previously outlined the importance of microRNAs (miRNAs, miRs) and especially miR-155 in SARS-CoV-2 pathophysiology through regulation of the Renin-Angiotensin Aldosterone System (RAAS) and influence on several aspects of host innate immunity. MiRNAs are negative regulators of gene expression, decreasing the stability of target RNAs or limiting their translation and, enthrallingly, miR-155 is also involved in AXL homeostasis-both endogenously and pharmaceutically using repurposed drugs (e.g., metformin)-highlighting thrifty evolutionary host innate immunity mechanisms that successfully can thwart viral entry and replication. Cancer, infections, and immune system disturbances will increasingly involve miRNA diagnostics and therapeutics in the future.

Role of TAM Receptors in Antimalarial Humoral Immune Response

Immune response against malaria and the clearance of Plasmodium parasite relies on germinal-center-derived B cell responses that are temporally and histologically layered. Despite a well-orchestrated germinal center response, anti-Plasmodium immune response seldom offers sterilizing immunity. Recent studies report that certain pathophysiological features of malaria such as extensive hemolysis, hypoxia as well as the extrafollicular accumulation of short-lived plasmablasts may contribute to this suboptimal immune response. In this review, we summarize some of those studies and attempt to connect certain host intrinsic features in response to the malarial disease and the resultant gaps in the immune response.

Molecular Characterization of the Recombinant Ig1 Axl Receptor Domain: An Intriguing Bait for Screening in Drug Discovery

Axl receptor tyrosine kinase and its ligand Gas6 regulate several biological processes and are involved in both the onset and progression of tumor malignancies and autoimmune diseases. Based on its key role in these settings, Axl is considered a promising target for the development of molecules with therapeutic and diagnostic purposes. In this paper, we describe the molecular characterization of the recombinant Ig1 domain of Axl (Ig1 Axl) and its biochemical properties. For the first time, an exhaustive spectroscopic characterization of the recombinant protein through circular dichroism and fluorescence studies is also reported, as well as a binding analysis to its natural ligand Gas6, paving the way for the use of recombinant Ig1 Axl as a bait in drug discovery screening procedures aimed at the identification of novel and specific binders targeting the Axl receptor.

Plant-Derived Epi-Nutraceuticals as Potential Broad-Spectrum Anti-Viral Agents

Although the COVID-19 pandemic appears to be diminishing, the emergence of SARS-CoV-2 variants represents a threat to humans due to their inherent transmissibility, immunological evasion, virulence, and invulnerability to existing therapies. The COVID-19 pandemic affected more than 500 million people and caused over 6 million deaths. Vaccines are essential, but in circumstances in which vaccination is not accessible or in individuals with compromised immune systems, drugs can provide additional protection. Targeting host signaling pathways is recommended due to their genomic stability and resistance barriers. Moreover, targeting host factors allows us to develop compounds that are effective against different viral variants as well as against newly emerging virus strains. In recent years, the globe has experienced climate change, which may contribute to the emergence and spread of infectious diseases through a variety of factors. Warmer temperatures and changing precipitation patterns can increase the geographic range of disease-carrying vectors, increasing the risk of diseases spreading to new areas. Climate change may also affect vector behavior, leading to a longer breeding season and more breeding sites for disease vectors. Climate change may also disrupt ecosystems, bringing humans closer to wildlife that transmits zoonotic diseases. All the above factors may accelerate the emergence of new viral epidemics. Plant-derived products, which have been used in traditional medicine for treating pathological conditions, offer structurally novel therapeutic compounds, including those with anti-viral activity. In addition, plant-derived bioactive substances might serve as the ideal basis for developing sustainable/efficient/cost-effective anti-viral alternatives. Interest in herbal antiviral products has increased. More than 50% of approved drugs originate from herbal sources. Plant-derived compounds offer diverse structures and bioactive molecules that are candidates for new drug development. Combining these therapies with conventional drugs could improve patient outcomes. Epigenetics modifications in the genome can affect gene expression without altering DNA sequences. Host cells can use epigenetic gene regulation as a mechanism to silence incoming viral DNA molecules, while viruses recruit cellular epitranscriptomic (covalent modifications of RNAs) modifiers to increase the translational efficiency and transcript stability of viral transcripts to enhance viral gene expression and replication. Moreover, viruses manipulate host cells' epigenetic machinery to ensure productive viral infections. Environmental factors, such as natural products, may influence epigenetic modifications. In this review, we explore the potential of plant-derived substances as epigenetic modifiers for broad-spectrum anti-viral activity, reviewing their modulation processes and anti-viral effects on DNA and RNA viruses, as well as addressing future research objectives in this rapidly emerging field.

Apoptotic cell clearance components in inflammatory arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory disease of the synovial joints that affects ~1% of the human population. Joint swelling and bone erosion, hallmarks of RA, contribute to disability and, sometimes, loss of life. Mechanistically, disease is driven by immune dysregulation characterized by circulating autoantibodies, inflammatory mediators, tissue degradative enzymes, and metabolic dysfunction of resident stromal and recruited immune cells. Cell death by apoptosis has been therapeutically explored in animal models of RA due to the comparisons drawn between synovial hyperplasia and paucity of apoptosis in RA with the malignant transformation of cancer cells. Several efforts to induce cell death have shown benefits in reducing the development and/or severity of the disease. Apoptotic cells are cleared by phagocytes in a process known as efferocytosis, which differs from microbial phagocytosis in its "immuno-silent," or anti-inflammatory, nature. Failures in efferocytosis have been linked to autoimmune disease, whereas administration of apoptotic cells in RA models effectively inhibits inflammatory indices, likely though efferocytosis-mediated resolution-promoting mechanisms. However, the nature of signaling pathways elicited and the molecular identity of clearance mediators in RA are understudied. Furthermore, canonical efferocytosis machinery elements also play important non-canonical functions in homeostasis and pathology. Here, we discuss the roles of efferocytosis machinery components in models of RA and discuss their potential involvement in disease pathophysiology.

Identification of AXL as a co-receptor for human parvovirus B19 infection of human erythroid progenitors

Parvovirus B19 (B19V) infects human erythroid progenitor cells (EPCs) and causes several hematological disorders and fetal hydrops. Amino acid (aa) 5-68 of minor capsid protein VP1 (VP1u5-68aa) is the minimal receptor binding domain for B19V to enter EPCs. Here, we carried out a genome-wide CRISPR-Cas9 guide RNA screen and identified tyrosine protein kinase receptor UFO (AXL) as a proteinaceous receptor for B19V infection of EPCs. AXL gene silencing in ex vivo expanded EPCs remarkably decreased B19V internalization and replication. Additions of the recombinant AXL extracellular domain or a polyclonal antibody against it upon infection efficiently inhibited B19V infection of ex vivo expanded EPCs. Moreover, B19V VP1u interacted with the recombinant AXL extracellular domain in vitro at a relatively high affinity (KD = 103 nM). Collectively, we provide evidence that AXL is a co-receptor for B19V infection of EPCs.

Gas6/TAM Axis Involvement in Modulating Inflammation and Fibrosis in COVID-19 Patients

Gas6 (growth arrest-specific gene 6) is a widely expressed vitamin K-dependent protein that is involved in many biological processes such as homeostatic regulation, inflammation and repair/fibrotic processes. It is known that it is the main ligand of TAMs, a tyrosine kinase receptor family of three members, namely MerTK, Tyro-3 and Axl, for which it displays the highest affinity. Gas6/TAM axis activation is known to be involved in modulating inflammatory responses as well as fibrotic evolution in many different pathological conditions. Due to the rapidly evolving COVID-19 pandemic, this review will focus on Gas6/TAM axis activation in SARS-CoV-2 infection, where de-regulated inflammatory responses and fibrosis represent a relevant feature of severe disease manifestation. Furthermore, this review will highlight the most recent scientific evidence supporting an unsuspected role of Axl as a SARS-CoV-2 infection driver, and the potential therapeutic advantages of the use of existing Axl inhibitors in COVID-19 management. From a physiological point of view, the Gas6/TAM axis plays a dual role, fostering the tissue repair processes or leading to organ damage and loss of function, depending on the prevalence of its anti-inflammatory or profibrotic properties. This review makes a strong case for further research focusing on the Gas6/TAM axis as a pharmacological target to manage different disease conditions, such as chronic fibrosis or COVID-19.

Pseudotyped Viruses for Marburgvirus and Ebolavirus

Marburg virus (MARV) and Ebola virus (EBOV) of the Filoviridae family are the most lethal viruses in terms of mortality rate. However, the development of antiviral treatment is hampered by the requirement for biosafety level-4 (BSL-4) containment. The establishment of BSL-2 pseudotyped viruses can provide important tools for the study of filoviruses. This chapter summarizes general information on the filoviruses and then focuses on the construction of replication-deficient pseudotyped MARV and EBOV (e.g., lentivirus system and vesicular stomatitis virus system). It also details the potential applications of the pseudotyped viruses, including neutralization antibody detection, the study of infection mechanisms, the evaluation of antibody-dependent enhancement, virus entry inhibitor screening, and glycoprotein mutation analysis.

Anti-SARS-CoV-2 activity of targeted kinase inhibitors: Repurposing clinically available drugs for COVID-19 therapy

Coronavirus disease 2019 (COVID-19) remains a major public health concern, and vaccine unavailability, hesitancy, or failure underscore the need for discovery of efficacious antiviral drug therapies. Numerous approved drugs target protein kinases associated with viral life cycle and symptoms of infection. Repurposing of kinase inhibitors is appealing as they have been vetted for safety and are more accessible for COVID-19 treatment. However, an understanding of drug mechanism is needed to improve our understanding of the factors involved in pathogenesis. We tested the in vitro activity of three kinase inhibitors against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), including inhibitors of AXL kinase, a host cell factor that contributes to successful SARS-CoV-2 infection. Using multiple cell-based assays and approaches, gilteritinib, nintedanib, and imatinib were thoroughly evaluated for activity against SARS-CoV-2 variants. Each drug exhibited antiviral activity, but with stark differences in potency, suggesting differences in host dependency for kinase targets. Importantly, for gilteritinib, the amount of compound needed to achieve 90% infection inhibition, at least in part involving blockade of spike protein-mediated viral entry and at concentrations not inducing phospholipidosis (PLD), approached a clinically achievable concentration. Knockout of AXL, a target of gilteritinib and nintedanib, impaired SARS-CoV-2 variant infectivity, supporting a role for AXL in SARS-CoV-2 infection and supporting further investigation of drug-mediated AXL inhibition as a COVID-19 treatment. This study supports further evaluation of AXL-targeting kinase inhibitors as potential antiviral agents and treatments for COVID-19. Additional mechanistic studies are needed to determine underlying differences in virus response.

Influenza A Virus Agnostic Receptor Tropism Revealed Using a Novel Biological System with Terminal Sialic Acid Knockout Cells

Avian or human influenza A viruses bind preferentially to avian- or human-type sialic acid receptors, respectively, indicating that receptor tropism is an important factor for determining the viral host range. However, there are currently no reliable methods for analyzing receptor tropism biologically under physiological conditions. In this study, we established a novel system using MDCK cells with avian- or human-type sialic acid receptors and with both sialic acid receptors knocked out (KO). When we examined the replication of human and avian influenza viruses in these KO cells, we observed unique viral receptor tropism that could not be detected using a conventional solid-phase sialylglycan binding assay, which directly assesses physical binding between the virus and sialic acids. Furthermore, we serially passaged an engineered avian-derived H4N5 influenza virus, whose PB2 gene was deleted, in avian-type receptor KO cells stably expressing PB2 to select a mutant with enhanced replication in KO cells; however, its binding to human-type sialylglycan was undetectable using the solid-phase binding assay. These data indicate that a panel of sialic acid receptor KO cells could be a useful tool for determining the biological receptor tropism of influenza A viruses. Moreover, the PB2KO virus experimental system could help to safely and efficiently identify the mutations required for avian influenza viruses to adapt to human cells that could trigger a new influenza pandemic. IMPORTANCE The acquisition of mutations that allow avian influenza A virus hemagglutinins to recognize human-type receptors is mandatory for the transmission of avian viruses to humans, which could lead to a pandemic. In this study, we established a novel system using a set of genetically engineered MDCK cells with knocked out sialic acid receptors to biologically evaluate the receptor tropism for influenza A viruses. Using this system, we observed unique receptor tropism in several virus strains that was undetectable using conventional solid-phase binding assays that measure physical binding between the virus and artificially synthesized sialylglycans. This study contributes to elucidation of the relationship between the physical binding of virus and receptor and viral infectivity. Furthermore, the system using sialic acid knockout cells could provide a useful tool to explore the sialic acid-independent entry mechanism. In addition, our system could be safely used to identify mutations that could acquire human-type receptor tropism.

TIM-1 Augments Cellular Entry of Ebola Virus Species and Mutants, Which Is Blocked by Recombinant TIM-1 Protein

Ebola virus, a member of the Filoviridae family, utilizes the attachment factors on host cells to support its entry and cause severe tissue damage. TIM-1 has been identified as a predominant attachment factor via interaction with phosphatidylserine (PS) localized on the viral envelope and glycoprotein (GP). In this study, we give the first demonstration that TIM-1 enhances the cellular entry of three species of Ebola virus, as well as those harboring GP mutations (A82V, T544I, and A82V T544I). Furthermore, two TIM-1 variants (i.e., TIM-1-359aa and TIM-1-364aa) had comparable effects on promoting Zaire Ebola virus (EBOV) attachment, internalization, and infection. Importantly, recombinant TIM-1 ectodomain (ECD) protein could decrease the infectivity of Ebola virus and display synergistic inhibitory effects with ADI-15946, a monoclonal antibody with broad neutralizing activity to Ebola virus. Of note, EBOV strains harboring GP mutations (K510E and D552N), which were refractory to antibody treatment, were still sensitive to TIM-1 protein-mediated impairment of infectivity, indicating that TIM-1 protein may represent an alternative therapeutic regimen when antibody evasion occurs. IMPORTANCE The viral genome has acquired numerous mutations with the potential to increase transmission during the 2013-to-2016 outbreak of Ebola virus. EBOV strains harboring GP mutations (A82V, T544I, and A82V T544I), which have been identified to increase viral infectivity in humans, have attracted our attention. Herein, we give the first report that polymorphic TIM-1 enhances the infectivity of three species of Ebola virus, as well as those harboring GP mutations (A82V, T544I, and A82V T544I). We show that recombinant TIM-1 ECD protein could decrease the infectivity of Ebola virus with or without a point mutation and displays synergistic inhibitory effects with ADI-15946. Furthermore, TIM-1 protein potently blocked cell entry of antibody-evading Ebola virus species. These findings highlight the role of TIM-1 in Ebola virus infection and indicate that TIM-1 protein represents a potential therapeutic avenue for Ebola virus and its mutated species.

The role of phosphatidylserine on the membrane in immunity and blood coagulation

The negatively charged aminophospholipid, phosphatidylserine (PtdSer), is located in the inner leaflet of the plasma membrane in normal cells, and may be exposed to the outer leaflet under some immune and blood coagulation processes. Meanwhile, Ptdser exposed to apoptotic cells can be recognized and eliminated by various immune cells, whereas on the surface of activated platelets Ptdser interacts with coagulation factors prompting enhanced production of thrombin which significantly facilitates blood coagulation. In the case where PtdSer fails in exposure or mistakenly occurs, there are occurrences of certain immunological and haematological diseases, such as the Scott syndrome and Systemic lupus erythematosus. Besides, viruses (e.g., Human Immunodeficiency Virus (HIV), Ebola virus (EBOV)) can invade host cells through binding the exposed PtdSer. Most recently, the Corona Virus Disease 2019 (COVID-19) has been similarly linked to PtdSer or its receptors. Therefore, it is essential to comprehensively understand PtdSer and its functional characteristics. Therefore, this review summarizes Ptdser, its eversion mechanism; interaction mechanism, particularly with its immune receptors and coagulation factors; recognition sites; and its function in immune and blood processes. This review illustrates the potential aspects for the underlying pathogenic mechanism of PtdSer-related diseases, and the discovery of new therapeutic strategies as well.

The Methanolic Extract of Perilla frutescens Robustly Restricts Ebola Virus Glycoprotein-Mediated Entry

Ebola virus (EBOV), one of the most infectious human viruses and a leading cause of viral hemorrhagic fever, imposes a potential public health threat with several recent outbreaks. Despite the difficulties associated with working with this pathogen in biosafety level-4 containment, a protective vaccine and antiviral therapeutic were recently approved. However, the high mortality rate of EBOV infection underscores the necessity to continuously identify novel antiviral strategies to help expand the scope of prophylaxis/therapeutic management against future outbreaks. This includes identifying antiviral agents that target EBOV entry, which could improve the management of EBOV infection. Herein, using EBOV glycoprotein (GP)-pseudotyped particles, we screened a panel of natural medicinal extracts, and identified the methanolic extract of Perilla frutescens (PFME) as a robust inhibitor of EBOV entry. We show that PFME dose-dependently impeded EBOV GP-mediated infection at non-cytotoxic concentrations, and exerted the most significant antiviral activity when both the extract and the pseudoparticles are concurrently present on the host cells. Specifically, we demonstrate that PFME could block viral attachment and neutralize the cell-free viral particles. Our results, therefore, identified PFME as a potent inhibitor of EBOV entry, which merits further evaluation for development as a therapeutic strategy against EBOV infection.

Lassa Fever Virus Binds Matriglycan-A Polymer of Alternating Xylose and Glucuronate-On α-Dystroglycan

Lassa fever virus (LASV) can cause life-threatening hemorrhagic fevers for which there are currently no vaccines or targeted treatments. The late Prof. Stefan Kunz, along with others, showed that the high-affinity host receptor for LASV, and other Old World and clade-C New World mammarenaviruses, is matriglycan-a linear repeating disaccharide of alternating xylose and glucuronic acid that is polymerized uniquely on α-dystroglycan by like-acetylglucosaminyltransferase-1 (LARGE1). Although α-dystroglycan is ubiquitously expressed, LASV preferentially infects vascular endothelia and professional phagocytic cells, which suggests that viral entry requires additional cell-specific factors. In this review, we highlight the work of Stefan Kunz detailing the molecular mechanism of LASV binding and discuss the requirements of receptors, such as tyrosine kinases, for internalization through apoptotic mimicry.

Structure-Based Design of Prefusion-Stabilized Filovirus Glycoprotein Trimers

Ebola virus causes severe hemorrhagic fever, often leading to death in humans. The trimeric fusion glycoprotein (GP) is the sole target for neutralizing antibodies and is the major focus of vaccine development. Soluble GP ectodomains are unstable and mostly monomeric when not fused to a heterologous trimerization domain. Here, we report structure-based designs of Ebola and Marburg GP trimers based on a stabilizing mutation in the hinge loop in refolding region 1 and substitution of a partially buried charge at the interface of the GP1 and GP2 subunits. The combined substitutions (T577P and K588F) substantially increased trimer expression for Ebola GP proteins. We determined the crystal structure of stabilized GP from the Makona Zaire ebolavirus strain without a trimerization domain or complexed ligand. The structure reveals that the stabilized GP adopts the same trimeric prefusion conformation, provides insight into triggering of GP conformational changes, and should inform future filovirus vaccine development.

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Filovirus GP expression increases by stabilizing mutations in hinge loop and base helix

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Charged lysine in base helix and GP1 N terminus are trapped in metastable conformation

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Crystal structure of stabilized Makona Δmucin GP confirms successful stabilization

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These findings may be useful for understanding fusion mechanisms and vaccine design

Drug repurposing screens reveal cell-type-specific entry pathways and FDA-approved drugs active against SARS-Cov-2

There is an urgent need for antivirals to treat the newly emerged severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). To identify new candidates, we screen a repurposing library of ∼3,000 drugs. Screening in Vero cells finds few antivirals, while screening in human Huh7.5 cells validates 23 diverse antiviral drugs. Extending our studies to lung epithelial cells, we find that there are major differences in drug sensitivity and entry pathways used by SARS-CoV-2 in these cells. Entry in lung epithelial Calu-3 cells is pH independent and requires TMPRSS2, while entry in Vero and Huh7.5 cells requires low pH and triggering by acid-dependent endosomal proteases. Moreover, we find nine drugs are antiviral in respiratory cells, seven of which have been used in humans, and three are US Food and Drug Administration (FDA) approved, including cyclosporine. We find that the antiviral activity of cyclosporine is targeting Cyclophilin rather than calcineurin, revealing essential host targets that have the potential for rapid clinical implementation.

AXL, an Important Host Factor for DENV and ZIKV Replication

Flaviviruses, as critically important pathogens, are still major public health problems all over the world. For instance, the evolution of ZIKV led to large-scale outbreaks in the Yap island in 2007. DENV was considered by the World Health Organization (WHO) as one of the 10 threats to global health in 2019. Enveloped viruses hijack a variety of host factors to complete its replication cycle. Phosphatidylserine (PS) receptor, AXL, is considered to be a candidate receptor for flavivirus invasion. In this review, we discuss the molecular structure of ZIKV and DENV, and how they interact with AXL to successfully invade host cells. A more comprehensive understanding of the molecular mechanisms of flavivirus-AXL interaction will provide crucial insights into the virus infection process and the development of anti-flavivirus therapeutics.

Immunotherapeutic strategies to target vulnerabilities in the Ebolavirus glycoprotein

The 2014 Ebola virus disease (EVD) outbreak in West Africa and the subsequent outbreaks of 2018-2020 in Equator and North Kivu provinces of the Democratic Republic of the Congo illustrate the public health challenges of emerging and reemerging viruses. EVD has a high case fatality rate with a rapidly progressing syndrome of fever, rash, vomiting, diarrhea, and bleeding diathesis. Recently, two monoclonal-antibody-based therapies received United States Food and Drug Administration (FDA) approval, and there are several other passive immunotherapies that hold promise as therapeutics against other species of Ebolavirus. Here, we review concepts needed to understand mechanisms of action, present an expanded schema to define additional sites of vulnerability on the viral glycoprotein, and review current antibody-based therapeutics. The concepts described are used to gain insights into the key characteristics that represent functional targets for immunotherapies against Zaire Ebolavirus and other emerging viruses within the Ebolavirus genus.

Short linear motif candidates in the cell entry system used by SARS-CoV-2 and their potential therapeutic implications

The first reported receptor for SARS-CoV-2 on host cells was the angiotensin-converting enzyme 2 (ACE2). However, the viral spike protein also has an RGD motif, suggesting that cell surface integrins may be co-receptors. We examined the sequences of ACE2 and integrins with the Eukaryotic Linear Motif (ELM) resource and identified candidate short linear motifs (SLiMs) in their short, unstructured, cytosolic tails with potential roles in endocytosis, membrane dynamics, autophagy, cytoskeleton, and cell signaling. These SLiM candidates are highly conserved in vertebrates and may interact with the μ2 subunit of the endocytosis-associated AP2 adaptor complex, as well as with various protein domains (namely, I-BAR, LC3, PDZ, PTB, and SH2) found in human signaling and regulatory proteins. Several motifs overlap in the tail sequences, suggesting that they may act as molecular switches, such as in response to tyrosine phosphorylation status. Candidate LC3-interacting region (LIR) motifs are present in the tails of integrin β3 and ACE2, suggesting that these proteins could directly recruit autophagy components. Our findings identify several molecular links and testable hypotheses that could uncover mechanisms of SARS-CoV-2 attachment, entry, and replication against which it may be possible to develop host-directed therapies that dampen viral infection and disease progression. Several of these SLiMs have now been validated to mediate the predicted peptide interactions.

HER2-mediated enhancement of Ebola virus entry

Multiple cell surface molecules including TAM receptors (TYRO3, AXL, and MERTK), a family of tyrosine kinase receptors, can serve as attachment receptors for Ebola virus (EBOV) entry into cells. The interaction of these receptors with EBOV particles is believed to trigger the initial internalization events that lead to macropinocytosis. However, the details of how these interactions lead to EBOV internalization have yet to be elucidated. Here, we screened receptor tyrosine kinase (RTK) inhibitors for anti-EBOV activity by using our previously established biologically contained Ebola virus that lacks the VP30 gene (EBOVΔVP30) and identified several RTKs, including human epidermal growth factor receptor 2 (HER2), as potential targets of anti-EBOV inhibitors and as novel host factors that have a role in EBOV infection. Of these identified RTKs, it was only HER2 whose knockdown by siRNAs impaired EBOVΔVP30-induced AKT1 phosphorylation, an event that is required for AKT1 activation and subsequent macropinocytosis. Stable expression of HER2 resulted in constitutive activation of AKT1, resulting in the enhancement of EBOVΔVP30 growth, EBOV GP-mediated entry, and macropinocytosis. Moreover, we found that HER2 interacts with the TAM receptors, and in particular forms a complex with TYRO3 and EBOVΔVP30 particles on the cell surface. Interestingly, HER2 was required for EBOVΔVP30-induced TYRO3 and AKT1 activation, but the other TAM receptors (TYRO3 and MERTK) were not essential for EBOVΔVP30-induced HER2 and AKT1 activation. Our findings demonstrate that HER2 plays an important role in EBOV entry and provide novel insights for the development of therapeutics against the virus.

Therapeutic aspects of the Axl/Gas6 molecular system

Axl receptor tyrosine kinase (RTK) and its ligand, growth arrest-specific protein 6 (Gas6), are involved in several biological functions and participate in the development and progression of a range of malignancies and autoimmune disorders. In this review, we present this molecular system from a drug discovery perspective, highlighting its therapeutic implications and challenges that need to be addressed. We provide an update on Axl/Gas6 axis biology, exploring its role in fields ranging from angiogenesis, cancer development and metastasis, immune response and inflammation to viral infection. Finally, we summarize the molecules that have been developed to date to target the Axl/Gas6 molecular system for therapeutic and diagnostic applications.

Single virus tracking of Ebola virus entry through lipid rafts in living host cells

Ebola virus (EBOV) is one of the most pathogenic viruses in humans which can cause a lethal hemorrhagic fever. Understanding the cellular entry mechanisms of EBOV can promote the development of new therapeutic strategies to control virus replication and spread. It has been known that EBOV virions bind to factors expressed at the host cell surface. Subsequently, the virions are internalized by a macropinocytosis-like process, followed by being trafficked through early and late endosomes. Recent researches indicate that the entry of EBOV into cells requires integrated and functional lipid rafts. Whilst lipid rafts have been hypothesized to play a role in virus entry, there is a current lack of supporting data. One major technical hurdle is the lack of effective approaches for observing viral entry. To provide evidence on the involvement of lipid rafts in the entry process of EBOV, we generated the fluorescently labeled Ebola virus like particles (VLPs), and utilized single-particle tracking (SPT) to visualize the entry of fluorescent Ebola VLPs in live cells and the interaction of Ebola VLPs with lipid rafts. In this study, we demonstrate the compartmentalization of Ebola VLPs in lipid rafts during entry process, and inform the essential function of lipid rafts for the entry of Ebola virus. As such, our study provides evidence to show that the raft integrity is critical for Ebola virus pathogenesis and that lipid rafts can serve as potential targets for the development of novel therapeutic strategies.

T-cell immunoglobulin and mucin (TIM) contributes to the infection of human airway epithelial cells by pseudotype viruses containing Hantaan virus glycoproteins

Hantaviruses are rodent-borne hemorrhagic fever viruses leading to serious diseases. Viral attachment and entry represent the first steps in virus transmission and are promising targets for antiviral therapeutic intervention. Here we investigated receptor use in human airway epithelium of the Old and New World hantaviruses Hantaan virus (HTNV) and Andes virus (ANDV). Using a biocontained recombinant vesicular stomatitis virus pseudotype platform, we provide first evidence for a role of the cellular phosphatidylserine (PS) receptors of the T-cell immunoglobulin and mucin (TIM) protein family in HTNV and ANDV infection. In line with previous studies, HTNV, but not ANDV, was able to use glycosaminoglycan heparan sulfate and α_vβ₃ integrin as co-receptors. In sum, our studies demonstrate for the first time that hantaviruses make use of apoptotic mimicry for infection of human airway epithelium, which may explain why these viruses can easily break the species barrier.

Ebola Virus Entry: From Molecular Characterization to Drug Discovery

Ebola Virus Disease (EVD) is one of the most lethal transmissible infections, characterized by a high fatality rate, and caused by a member of the Filoviridae family. The recent large outbreak of EVD in Western Africa (2013–2016) highlighted the worldwide threat represented by the disease and its impact on global public health and the economy. The development of highly needed anti-Ebola virus antivirals has been so far hampered by the shortage of tools to study their life cycle in vitro, allowing to screen for potential active compounds outside a biosafety level-4 (BSL-4) containment. Importantly, the development of surrogate models to study Ebola virus entry in a BSL-2 setting, such as viral pseudotypes and Ebola virus-like particles, tremendously boosted both our knowledge of the viral life cycle and the identification of promising antiviral compounds interfering with viral entry. In this context, the combination of such surrogate systems with large-scale small molecule compounds and haploid genetic screenings, as well as rational drug design and drug repurposing approaches will prove priceless in our quest for the development of a treatment for EVD.

Loss of the TAM Receptor Axl Ameliorates Severe Zika Virus Pathogenesis and Reduces Apoptosis in Microglia

The TAM receptor, Axl, has been implicated as a candidate entry receptor for Zika virus (ZIKV) infection but has been shown as inessential for virus infection in mice. To probe the role of Axl in murine ZIKV infection, we developed a mouse model lacking the Axl receptor and the interferon alpha/beta receptor (Ifnar^−/−Axl^−/−), conferring susceptibility to ZIKV. This model validated that Axl is not required for murine ZIKV infection and that mice lacking Axl are resistant to ZIKV pathogenesis. This resistance correlates to lower pro-interleukin-1β production and less apoptosis in microglia of ZIKV-infected mice. This apoptosis occurs through both intrinsic (caspase 9) and extrinsic (caspase 8) manners, and is age dependent, as younger Axl-deficient mice are susceptible to ZIKV pathogenesis. These findings suggest that Axl plays an important role in pathogenesis in the brain during ZIKV infection and indicates a potential role for Axl inhibitors as therapeutics during viral infection.

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IFNAR^−/−Axl^−/− mice show Axl unnecessary for Zika virus replication in mice

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Mice lacking Axl receptor are significantly resistant to Zika virus neuropathogenesis

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IFNAR^−/−Axl^−/− mice have less ZIKV-driven caspase-dependent apoptosis in brain

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Axl deficient mice have fewer apoptotic microglia after ZIKV infection

Characterization of a novel species of adenovirus from Japanese microbat and role of CXADR as its entry factor

Recently, bat adenoviruses (BtAdVs) of genus Mastadenovirus have been isolated from various bat species, some of them displaying a wide host range in cell culture. In this study, we isolated two BtAdVs from Japanese wild microbats. While one isolate was classified as Bat mastadenovirus A, the other was phylogenetically independent of other BtAdVs. It was rather related to, but serologically different from, canine adenoviruses. We propose that the latter, isolated from Asian parti-colored bat, should be assigned to a novel species of Bat mastadenovirus. Both isolates replicated in various mammalian cell lines, implying their wide cell tropism. To gain insight into cell tropism of these BtAdVs, we investigated the coxsackievirus and adenovirus receptor (CXADR) for virus entry to the cells. We prepared CXADR-knockout canine kidney cells and found that replication of BtAdVs was significantly hampered in these cells. For confirmation, their replication in canine CXADR-addback cells was rescued to the levels with the original cells. We also found that viral replication was corrected in human or bat CXADR-transduced cells to similar levels as in canine CXADR-addback cells. These results suggest that BtAdVs were able to use several mammalian-derived CXADRs as entry factors.

Molecular Mechanism of Externalization of Phosphatidylserine on the Surface of Ebola Virus Particles

Ebola virus (EBOV) is an enveloped filamentous virus that causes severe hemorrhagic fever in humans and nonhuman primates with up to 90% fatality. Accumulating evidence indicates that various viruses, including EBOV, exploit the host apoptotic clearance machinery to enhance their entry into host cells by externalizing phosphatidylserine (PS) in the viral envelope. PS is typically distributed in the inner layer of the plasma membrane (PM) in normal cells. Progeny EBOV virions bud from the PM of infected cells, suggesting that PS is likely flipped to the outer leaflet of the envelope of Ebola virions. Currently, the intracellular dynamics of PS during EBOV infection are poorly understood. This review summarizes recent progress in determining the molecular mechanism of externalization of PS in the envelope of EBOV particles. We also discuss future directions and how viral apoptotic mimicry could be targeted for therapeutics.

Effects of Soluble Tumor Necrosis Factor (TNF) on Antibody-Dependent Cellular Cytotoxicity of Therapeutic anti-TNF-α Antibody

Anti-TNF antibodies are major therapeutics for rheumatoid arthritis and have been approved for marketing in many countries. Antibody-dependent cellular cytotoxicity (ADCC) is considered to be a potential mechanism of action of anti-TNF antibodies, since some anti-TNF antibodies have been confirmed to induce cytotoxic effects on TNF-producing cells via ADCC and complement-dependent cytotoxicity (CDC) in in vitro experiments. In this study, we established a new stable effector cell line expressing human FcγRIIIa, CD16:KHYG-1, and compared the performance of this cell line with that of peripheral blood mononuclear cells (PBMCs) in ADCC assays against CHO-derived target cells expressing protease-sensitive pro-TNF. Although an inhibitory effect of soluble TNF released from pro-TNF expressing cells on ADCC activity was seen, clear dose-responsive ADCC activities were observed even in the presence or absence of TNF-α converting enzyme (TACE) inhibitor. However, significant differences in the ADCC activities in the presence or absence of TACE inhibitor were only noted when CD16:KHYG-1 cells were used as the effector cells. Our findings indicate that soluble TNF may influence ADCC activity of anti-TNF antibody. Moreover, the fact that the influence was able to be detected only in the case using stable effector cell also suggests that the stable effector cell established this time enable highly accurate ADCC measurement.

Cellular Targets and Receptor of Sexual Transmission of Zika Virus

STUDY QUESTION

What is the mechanism of sexual transmission of Zika virus (ZIKV)?

SUMMARY ANSWER

By utilizing exquisite reverse transcriptase-initiated in situ polymerase chain reaction (RT-in situ PCR), which enables an improved visualization of spermatozoa's subcellular compartment, we precisely localized the mid-piece of sperm that carry receptors for ZIKV.

WHAT IS ALREADY KNOWN

ZIKV is transmitted sexually and recent studies have verified ZIKV presence in semen of previously Zika-infected patients for >6-month postinfection when ZIKV had disappeared from blood, saliva, and urine. Strong serial analyses of various body fluids suggest that ZIKV can be transmitted between sexual partners. Currently, there is limited information on the association of the virus with human semen cell types that may carry the virus.

STUDY DESIGN, SIZE, DURATION

Analyses were carried out to localize ZIKV for subcellular localization of ZIKV on cell types. The Tyro3 receptor for ZIKV was colocalized by dual immunocytochemistry with specific monoclonal antibodies.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Three semen specimens were purchased from a commercial sperm bank. Motile sperm was separated from nonmotile cells by the "swim-up" technique. Each of the semen fractions was infected with ZIKV at the multiplicity of infection of 0.1.0 and 1.0 and evaluated for the primary targets of ZIKV in the semen cells by RT-in situ PCR and confirmed by real-time RT-PCR.

MAIN RESULTS AND THE ROLE OF CHANCE

ZIKV was present primarily at the mid-piece of mature spermatozoa in about 30% of the sperm. In addition, we determined that Tyro3 receptors, primarily expressed on mid-piece of human spermatozoa, play a role in ZIKV-binding and entry into spermatozoa. Our data strongly suggest a potential sexual/horizontal route of transmission for ZIKV primarily via infected sperms; most likely ZIKV enters the sperm via the Tyro3 receptor found at the mid-piece of the mature spermatozoa.

LIMITATIONS, REASONS FOR CAUTION

We are uncertain as to what phase of spermatogenesis, that in human takes about 120 days, sperms are permissive to ZIKV. If permissiveness was very early during spermatogenesis males could be infectious for ∼120 days after the disappearance of viremia in an infected man.

WIDER IMPLICATIONS OF THE FINDINGS

Our findings bring a new focus on the current affords to develop ZIKV vaccine. Why in the presence of anti-ZIKV antibodies infected men are still able to transmit the virus sexually? We suggest that only certain subclass of immunoglobulin (Ig)G (ie, IgG4) can cross the blood-Sertoli barrier therefore, a successful vaccine must provoke a subclass of IgG can quell ZIKV inside the seminiferous tubules.

LY6E mediates an evolutionarily conserved enhancement of virus infection by targeting a late entry step

Interferons (IFNs) contribute to cell-intrinsic antiviral immunity by inducing hundreds of interferon-stimulated genes (ISGs). In a screen to identify antiviral ISGs, we unexpectedly found that LY6E, a member of the LY6/uPAR family, enhanced viral infection. Here, we show that viral enhancement by ectopically expressed LY6E extends to several cellular backgrounds and affects multiple RNA viruses. LY6E does not impair IFN antiviral activity or signaling, but rather promotes viral entry. Using influenza A virus as a model, we narrow the enhancing effect of LY6E to uncoating after endosomal escape. Diverse mammalian orthologs of LY6E also enhance viral infectivity, indicating evolutionary conservation of function. By structure-function analyses, we identify a single amino acid in a predicted loop region that is essential for viral enhancement. Our study suggests that LY6E belongs to a class of IFN-inducible host factors that enhance viral infectivity without suppressing IFN antiviral activity.

The interferon-induced gene LY6E increases virus infection, but the underlying mechanism is poorly understood. Here, Mar et al. show that LY6E enhances uncoating of influenza A virus after endosomal escape and that viral enhancement by LY6E is conserved across evolution.

Ebola virus requires a host scramblase for externalization of phosphatidylserine on the surface of viral particles

Cell surface receptors for phosphatidylserine contribute to the entry of Ebola virus (EBOV) particles, indicating that the presence of phosphatidylserine in the envelope of EBOV is important for the internalization of EBOV particles. Phosphatidylserine is typically distributed in the inner layer of the plasma membrane in normal cells. Progeny virions bud from the plasma membrane of infected cells, suggesting that phosphatidylserine is likely flipped to the outer leaflet of the plasma membrane in infected cells for EBOV virions to acquire it. Currently, the intracellular dynamics of phosphatidylserine during EBOV infection are poorly understood. Here, we explored the role of XK-related protein (Xkr) 8, which is a scramblase responsible for exposure of phosphatidylserine in the plasma membrane of apoptotic cells, to understand its significance in phosphatidylserine-dependent entry of EBOV. We found that Xkr8 and transiently expressed EBOV glycoprotein GP often co-localized in intracellular vesicles and the plasma membrane. We also found that co-expression of GP and viral major matrix protein VP40 promoted incorporation of Xkr8 into ebolavirus-like particles (VLPs) and exposure of phosphatidylserine on their surface, although only a limited amount of phosphatidylserine was exposed on the surface of the cells expressing GP and/or VP40. Downregulating Xkr8 or blocking caspase-mediated Xkr8 activation did not affect VLP production, but they reduced the amount of phosphatidylserine on the VLPs and their uptake in recipient cells. Taken together, our findings indicate that Xkr8 is trafficked to budding sites via GP-containing vesicles, is incorporated into VLPs, and then promote the entry of the released EBOV to cells in a phosphatidylserine-dependent manner.

Although Ebola virus causes severe hemorrhagic fever with a high mortality rate, there are no approved therapeutics. The viral entry process is one of the targets for antiviral development. Previous studies suggest that binding of phosphatidylserine, a component of the viral envelop, to the receptors promotes the entry of Ebola virus. Ebola virus is released from the surface membrane of infected cells. However, phosphatidylserine normally distributes in the inner layer of the cell surface membrane, suggesting that phosphatidylserine is likely flipped to the outer leaflet of the membrane in infected cells for Ebola virus to acquire it. Because the mechanism by which phosphatidylserine changes its orientation in Ebola virus-infected cells is poorly understood, we studied and identified a cellular enzyme, XK-related protein 8 (Xkr8), as a responsible factor involved in this process. We demonstrated that the Ebola virus glycoprotein promoted the incorporation of Xkr8 in viral particles, which flips phosphatidylserine on their surface, enhancing their entry to cells. Our findings provide new insights into the mechanism of Ebola virus infection, which may be exploited for the development of therapeutics against Ebola virus infection.

Toxoplasma gondii RON4 binds to heparan sulfate on the host cell surface

Toxoplasma gondii rhoptry neck protein 4 (TgRON4) is a component of the moving junction, a key structure for host cell invasion. We previously showed that host cellular β-tubulin is a binding partner of TgRON4 in the invasion process. Here, to identify other binding partners of TgRON4 in the host cell, we examined the binding of TgRON4 to components of the host cell surface. TgRON4 binds to various mammalian cells, but this binding disappeared in glycosaminoglycan- and heparan sulfate-deficient CHO cells and after heparitinase treatment of mammalian cells. The C-terminal half of TgRON4 showed relatively strong binding to cells and heparin agarose. A glycoarray assay indicated that TgRON4 binds to heparin and modified heparin derivatives. Immunoprecipitation of T. gondii-infected CHO cell lysates showed that TgRON4 interacts with glypican 1 during Toxoplasma invasion. This interaction suggests a role for heparan sulfate in parasite invasion.

Heparan Sulfate Proteoglycan Is an Important Attachment Factor for Cell Entry of Akabane and Schmallenberg Viruses

Akabane virus (AKAV) and Schmallenberg virus (SBV) are members of the genus Orthobunyavirus, which are transmitted by arthropod vectors with a broad cellular tropism in vitro as well as in vivo Both AKAV and SBV cause arthrogryposis-hydranencephaly syndrome in ruminants. The main cellular receptor and attachment factor for entry of these orthobunyaviruses are unknown. Here, we found that AKAV and SBV infections were inhibited by the addition of heparin or enzymatic removal of cell surface heparan sulfates. To confirm this finding, we prepared heparan sulfate proteoglycan (HSPG)-knockout (KO) cells by using a clustered regularly interspaced short palindromic repeat (CRISPR)-Cas9 system and measured the quantities of binding of these viruses to cell surfaces. We observed a substantial reduction in AKAV and SBV binding to cells, limiting the infections by these viruses. These data demonstrate that HSPGs are important cellular attachment factors for AKAV and SBV, at least in vitro, to promote virus replication in susceptible cells.IMPORTANCE AKAV and SBV are the etiological agents of arthrogryposis-hydranencephaly syndrome in ruminants, which causes considerable economic losses in the livestock industry. Here, we identified heparan sulfate proteoglycan as a major cellular attachment factor for the entry of AKAV and SBV. Moreover, we found that heparin is a strong inhibitor of AKAV and SBV infections. Revealing the molecular mechanisms of virus-host interactions is critical in order to understand virus biology and develop novel live attenuated vaccines.

TAM Receptors Are Not Required for Zika Virus Infection in Mice

Tyro3, Axl, and Mertk (TAM) receptors are candidate entry receptors for infection with the Zika virus (ZIKV), an emerging flavivirus of global public health concern. To investigate the requirement of TAM receptors for ZIKV infection, we used several routes of viral inoculation and compared viral replication in wild-type versus Axl^-/-, Mertk^-/-, Axl^-/-Mertk^-/-, and Axl^-/-Tyro3^-/- mice in various organs. Pregnant and non-pregnant mice treated with interferon-α-receptor (IFNAR)-blocking (MAR1-5A3) antibody and infected subcutaneously with ZIKV showed no reliance on TAMs for infection. In the absence of IFNAR-blocking antibody, adult female mice challenged intravaginally with ZIKV showed no difference in mucosal viral titers. Similarly, in young mice that were infected with ZIKV intracranially or intraperitoneally, ZIKV replication occurred in the absence of TAM receptors, and no differences in cell tropism were observed. These findings indicate that, in mice, TAM receptors are not required for ZIKV entry and infection.

Virus–Platelet Associations

Virus–platelet interplay is complex. Diverse virus types have been shown to associate with numerous distinct platelet receptors. This association can benefit the virus or the host, and thus the platelet is somewhat of a renegade. Evidence is accumulating to suggest that viruses are capable of entering platelets. For at least one type of RNA virus (dengue virus), the platelet has the necessary post-translational and packaging machinery required for production of replicative viral progeny. As a facilitator of immunity, the platelet also participates in eradicating the virus by direct and indirect mechanisms involving presentation of the pathogen to the innate and adaptive immune systems, thus enhancing inflammation by release of cytokines and other agonists. Virus-induced thrombocytopenia is caused by tangential imbalance of thrombopoeisis, autoimmunity, and loss of platelet function and integrity.

Mechanisms of Filovirus Entry

Filovirus entry into cells is complex, perhaps as complex as any viral entry mechanism identified to date. However, over the past 10 years, the important events required for filoviruses to enter into the endosomal compartment and fuse with vesicular membranes have been elucidated (Fig. 1). Here, we highlight the important steps that are required for productive entry of filoviruses into mammalian cells.

Antiviral Screening of Multiple Compounds against Ebola Virus

In light of the recent outbreak of Ebola virus (EBOV) disease in West Africa, there have been renewed efforts to search for effective antiviral countermeasures. A range of compounds currently available with broad antimicrobial activity have been tested for activity against EBOV. Using live EBOV, eighteen candidate compounds were screened for antiviral activity in vitro. The compounds were selected on a rational basis because their mechanisms of action suggested that they had the potential to disrupt EBOV entry, replication or exit from cells or because they had displayed some antiviral activity against EBOV in previous tests. Nine compounds caused no reduction in viral replication despite cells remaining healthy, so they were excluded from further analysis (zidovudine; didanosine; stavudine; abacavir sulphate; entecavir; JB1a; Aimspro; celgosivir; and castanospermine). A second screen of the remaining compounds and the feasibility of appropriateness for in vivo testing removed six further compounds (ouabain; omeprazole; esomeprazole; Gleevec; D-LANA-14; and Tasigna). The three most promising compounds (17-DMAG; BGB324; and NCK-8) were further screened for in vivo activity in the guinea pig model of EBOV disease. Two of the compounds, BGB324 and NCK-8, showed some effect against lethal infection in vivo at the concentrations tested, which warrants further investigation. Further, these data add to the body of knowledge on the antiviral activities of multiple compounds against EBOV and indicate that the scientific community should invest more effort into the development of novel and specific antiviral compounds to treat Ebola virus disease.

AXL-Mediated Productive Infection of Human Endothelial Cells by Zika Virus

RATIONALE

The mosquito-borne Zika virus (ZIKV) is now recognized as a blood-borne pathogen, raising an important question about how the virus gets into human bloodstream. The imminent threat of the ZIKV epidemic to the global blood supply also demands novel therapeutics to stop virus transmission though transfusion.

OBJECTIVE

We intend to characterize ZIKV tropism for human endothelial cells (ECs) and provide potential targets for intervention.

METHODS AND RESULTS

We conducted immunostaining, plaque assay, and quantitative reverse transcription-polymerase chain reaction of ZIKV RNA to evaluate the possible infection of ECs by ZIKV. Both the African and the South American ZIKV strains readily infect human umbilical vein endothelial cells and human ECs derived from aortic and coronary artery, as well as the saphenous vein. Infected ECs released infectious progeny virus. Compared with the African strains, South American ZIKV isolates replicate faster in ECs and are partially cytopathic, suggesting enhanced virulence of these isolates. Flow cytometric analyses showed that the susceptibility of ECs positively correlated with the cell surface levels of tyrosine-protein kinase receptor UFO (AXL) receptor tyrosine kinase. Gain- and loss-of-function studies further revealed that AXL is required for ZIKV entry at a postbinding step. Finally, small-molecule inhibitors of the AXL kinase significantly reduced ZIKA infection of ECs.

CONCLUSIONS

We identified EC as a key cell type for ZIKV infection. These data support the view of hematogenous dissemination of ZIKV and implicate AXL as a new target for antiviral therapy.

Normalization of TAM post-receptor signaling reveals a cell invasive signature for Axl tyrosine kinase

Background

Tyro3, Axl, and Mertk (TAMs) are a family of three conserved receptor tyrosine kinases that have pleiotropic roles in innate immunity and homeostasis and when overexpressed in cancer cells can drive tumorigenesis.

Methods

In the present study, we engineered EGFR/TAM chimeric receptors (EGFR/Tyro3, EGFR/Axl, and EGF/Mertk) with the goals to interrogate post-receptor functions of TAMs, and query whether TAMs have unique or overlapping post-receptor activation profiles. Stable expression of EGFR/TAMs in EGFR-deficient CHO cells afforded robust EGF inducible TAM receptor phosphorylation and activation of downstream signaling.

Results

Using a series of unbiased screening approaches, that include kinome-view analysis, phosphor-arrays, RNAseq/GSEA analysis, as well as cell biological and in vivo readouts, we provide evidence that each TAM has unique post-receptor signaling platforms and identify an intrinsic role for Axl that impinges on cell motility and invasion compared to Tyro3 and Mertk.

Conclusion

These studies demonstrate that TAM show unique post-receptor signatures that impinge on distinct gene expression profiles and tumorigenic outcomes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12964-016-0142-1) contains supplementary material, which is available to authorized users.

Identification and characterization of the role of c-terminal Src kinase in dengue virus replication

We screened a siRNA library targeting human tyrosine kinases in Huh-7 cells and identified c-terminal Src kinase (Csk) as one of the kinases involved in dengue virus replication. Knock-down of Csk expression by siRNAs or inhibition of Csk by an inhibitor reduced dengue virus RNA levels but did not affect viral entry. Csk partially colocalized with viral replication compartments. Dengue infection was drastically reduced in cells lacking the three ubiquitous src family kinases, Src, Fyn and Yes. Csk knock-down in these cells failed to block dengue virus replication suggesting that the effect of Csk is via regulation of Src family kinases. Csk was found to be hyper-phosphorylated during dengue infection and inhibition of protein kinase A led to a block in Csk phosphorylation and dengue virus replication. Overexpression studies suggest an important role for the kinase and SH3 domains in this process. Our results identified a novel role for Csk as a host tyrosine kinase involved in dengue virus replication and provide further insights into the role of host factors in dengue replication.

AXL receptor tyrosine kinase is required for T cell priming and antiviral immunity

The receptor tyrosine kinase (RTK) AXL is induced in response to type I interferons (IFNs) and limits their production through a negative feedback loop. Enhanced production of type I IFNs in Axl^-/-dendritic cells (DCs) in vitro have led to speculation that inhibition of AXL would promote antiviral responses. Notwithstanding, type I IFNs also exert potent immunosuppressive functions. Here we demonstrate that ablation of AXL enhances the susceptibility to infection by influenza A virus and West Nile virus. The increased type I IFN response in Axl^-/- mice was associated with diminished DC maturation, reduced production of IL-1β, and defective antiviral T cell immunity. Blockade of type I IFN receptor or administration of IL-1β to Axl^-/- mice restored the antiviral adaptive response and control of infection. Our results demonstrate that AXL is essential for limiting the immunosuppressive effects of type I IFNs and enabling the induction of protective antiviral adaptive immunity.

DOI:http://dx.doi.org/10.7554/eLife.12414.001

The immune system must be ever vigilant to ward off infections. Immune cells called T-cells can identify and eliminate microbes, but if they are too aggressive, they can damage the body. To prevent this, the body has systems that control immune responses. For example, another type of immune cell called a dendritic cell produces proteins known as type 1 interferons, which help to fight viral infections while limiting other immune responses.

An enzyme called AXL blocks the production of type 1 interferons. Many scientists believe that this activity reduces the ability of individual cells in the body to defend themselves against attacking viruses. In fact, experiments with cells grown in the laboratory have shown that some viruses activate the AXL enzyme to help them infect. Similar studies have also shown that inhibiting AXL and related enzymes can make cells more able to fight off certain types of viral infection. These and other studies suggested that some drugs that block AXL might be useful treatments for viral infections, however it was not clear if this was the case for all viruses.

Now, Schmid et al. show that the loss of AXL actually makes mice more prone to infections by the influenza virus and West Nile Virus. In the experiments, mice genetically engineered to lack AXL were more likely than normal mice to become ill after exposure to one of the viruses. Furthermore, fewer T cells matured to the stage where they could attack the virus in these mice.

Next, Schmid et al. show that blocking the production of type 1 interferons in the mice that lack AXL restores their ability to fight off these viral infections. This is because type 1 interferons limit the production of a protein that helps the dendritic cells to mature. Therefore, Schmid et al.’s findings show that AXL is vital for mice to fight off viral infections because it helps to balance the antiviral and immune suppressing activities of type 1 interferons. The findings also suggest that using drugs that block AXL to treat infections with certain viruses, including influenza and West Nile Virus, might do more harm than good.

DOI:http://dx.doi.org/10.7554/eLife.12414.002