Sulfated Escherichia coli K5 polysaccharide derivatives inhibit dengue virus infection of human microvascular endothelial cells by interacting with the viral envelope protein E domain III

Research Progress on Antiviral Activity of Heparin

Heparin, as a glycosaminoglycan, is known for its anticoagulant and antithrombotic properties for several decades. Heparin is a life-saving drug and is widely used for anticoagulation in medical practice. In recent years, there have been extensive studies that heparin plays an important role in non-anticoagulant diseases, such as anti-inflammatory, anti-viral, anti-angiogenesis, anti-neoplastic, anti-metastatic effects, and so on. Clinical observation and in vitro experiments indicate that heparin displays a potential multitarget effect. In this brief review, we will summarize heparin and its derivative's recently studied progress for the treatment of various viral infections. The aim is to maximize the benefits of drugs through medically targeted development, to meet the unmet clinical needs of serious viral diseases.

Antiviral strategies against human metapneumovirus: Targeting the fusion protein

Human metapneumoviruses have emerged in the past decades as an important global pathogen that causes severe upper and lower respiratory tract infections. Children under the age of 2, the elderly and immunocompromised individuals are more susceptible to HMPV infection than the general population due to their suboptimal immune system. Despite the recent discovery of HMPV as a novel important respiratory virus, reports have rapidly described its epidemiology, biology, and pathogenesis. However, progress is still to be made in the development of vaccines and drugs against HMPV infection as none are currently available. Herein, we discuss the importance of HMPV and review the reported strategies for anti-HMPV drug candidates. We also present the fusion protein as a promising antiviral drug target due to its multiple roles in the HMPV lifecycle. This key viral protein has previously been targeted by a range of inhibitors, which will be discussed as they represent opportunities for future drug design.

Flavivirus Entry Inhibitors

Flaviviruses, including Dengue virus, Zika virus, Yellow fever virus, Japanese encephalitis virus, West Nile virus, cause thousands of deaths and millions of illnesses each year. The large outbreak of ZIKV in 2016 reminds us that flaviviruses can pose a serious threat to human safety and public health as emerging and re-emerging viruses. However, there are no specific drugs approved for the treatment of flavivirus infections. Due to no need to enter the cells, viral entry inhibitors have the unique advantage in suppressing viral infections. Flaviviruses bind to receptors and attach to the cell surface, then enter the endosome in a clathrin-dependent manner and finalizes the viral entry process after fusion with the cell membrane in a low pH environment. Small molecules, antibodies or peptides can inhibit flavivirus entry by targeting the above processes. Here, we focus on flavivirus entry inhibitors with well-defined target and antiviral activity. We hope that our review will provide a theoretical basis for flavivirus treatment and drug research and help to accelerate the clinical application of flavivirus entry inhibitors.

Infection of Endothelial Cells by Dengue Virus Induces ROS Production by Different Sources Affecting Virus Replication, Cellular Activation, Death and Vascular Permeability

Exacerbated inflammatory response and altered vascular function are hallmarks of dengue disease. Reactive oxygen species (ROS) production has been associated to endothelial barrier disturbance and microvascular alteration in distinct pathological conditions. Increased ROS has been reported in in vitro models of dengue virus (DENV) infection, but its impact for endothelial cell physiology had not been fully investigated. Our group had previously demonstrated that infection of human brain microvascular endothelial cells (HBMEC) with DENV results in the activation of RNA sensors and production of proinflammatory cytokines, which culminate in cell death and endothelial permeability. Here, we evaluated the role of mitochondrial function and NADPH oxidase (NOX) activation for ROS generation in HBMEC infected by DENV and investigated whether altered cellular physiology could be a consequence of virus-induced oxidative stress. DENV-infected HBMECs showed a decrease in the maximal respiratory capacity and altered membrane potential, indicating functional mitochondrial alteration, what might be related to mtROS production. Indeed, mtROS was detected at later time points after infection. Specific inhibition of mtROS diminished virus replication, cell death, and endothelial permeability, but did not affect cytokine production. On the other hand, inhibition of NOX-associated ROS production decreased virus replication and cell death, as well as the secretion of inflammatory cytokines, including IL-6, IL-8, and CCL5. These results demonstrated that DENV replication in endothelial cells induces ROS production by different pathways, which impacts biological functions that might be relevant for dengue pathogenesis. Those data also indicate oxidative stress events as relevant therapeutical targets to avoid vascular permeability, inflammation, and neuroinvasion during DENV infection.

The importance of viral and cellular factors on flavivirus entry

The flavivirus are emerging and re-emerging arthropod-borne pathogens responsible for significant mortality and morbidity worldwide. The genus comprises more than 70 viruses, and despite genomic and structural similarities, infections by different flaviviruses result in different clinical presentations. In the absence of a safe and effective vaccine against these infections, the search for new strategies to inhibit viral infection is necessary. The life cycle of arboviruses begins with the entry process composed of multiple steps: attachment, internalization, endosomal escape and capsid uncoating. This mini-review describes factors and mechanisms involved in the viral entry as events required to take over the cellular machinery and host factors and cellular pathways commonly used by flaviviruses as possible approaches for developing broad-spectrum antiviral drugs.

Hepatitis C Virus Glycan-Dependent Interactions and the Potential for Novel Preventative Strategies

Chronic hepatitis C virus (HCV) infections continue to be a major contributor to liver disease worldwide. HCV treatment has become highly effective, yet there are still no vaccines or prophylactic strategies available to prevent infection and allow effective management of the global HCV burden. Glycan-dependent interactions are crucial to many aspects of the highly complex HCV entry process, and also modulate immune evasion. This review provides an overview of the roles of viral and cellular glycans in HCV infection and highlights glycan-focused advances in the development of entry inhibitors and vaccines to effectively prevent HCV infection.

Dengue structural proteins as antiviral drug targets: Current status in the drug discovery & development

Dengue virus belongs to the class of RNA viruses and subclass of enveloped single-stranded positive-sense RNA virus. It causes dengue fever (DF), dengue hemorrhagic fever (DHF), or dengue shock syndrome (DSS), where DHF and DSS are life-threatening. Even though dengue is an age-old disease, it is still a mystery and continues to be a global threat. Numerous attempts have been carried out in the past few decades to eradicate the virus through vaccine and antiviral drugs, but still battle continues. In this review, the possible drug targets for discovery and development of potential antiviral drugs against structural proteins of dengue virus, the current development status of the antiviral drugs against dengue around the world, and challenges that need to be addressed to overcome the shortcomings in the process of drug discovery have been discussed.

Recent advances in antiviral drug development towards dengue virus

Despite the high disease burden of dengue virus, there is no approved antiviral treatment or broadly applicable vaccine to treat or prevent dengue virus infection. In the last decade, many antiviral compounds have been identified but only few have been further evaluated in pre-clinical or clinical trials. This review will give an overview of the direct-acting and host-directed antivirals identified to date. Furthermore, important parameters for further development that is, drug properties including efficacy, specificity and stability, pre-clinical animal testing, and combinational drug therapy will be discussed.

Amino acid starvation enhances vaccine efficacy by augmenting neutralizing antibody production

Specific reduction in the intake of proteins or amino acids (AAs) offers enormous health benefits, including increased life span, protection against age-associated disorders, and improved metabolic fitness and immunity. Cells respond to conditions of AA starvation by activating the amino acid starvation response (AAR). Here, we showed that mimicking AAR with halofuginone (HF) enhanced the magnitude and affinity of neutralizing, antigen-specific antibody responses in mice immunized with dengue virus envelope domain III protein (DENVrEDIII), a potent vaccine candidate against DENV. HF enhanced the formation of germinal centers (GCs) and increased the production of the cytokine IL-10 in the secondary lymphoid organs of vaccinated mice. Furthermore, HF promoted the transcription of genes associated with memory B cell formation and maintenance and maturation of GCs in the draining lymph nodes of vaccinated mice. The increased abundance of IL-10 in HF-preconditioned mice correlated with enhanced GC responses and may promote the establishment of long-lived plasma cells that secrete antigen-specific, high-affinity antibodies. Thus, these data suggest that mimetics of AA starvation could provide an alternative strategy to augment the efficacy of vaccines against dengue and other infectious diseases.

A novel flavivirus entry inhibitor, BP34610, discovered through high-throughput screening with dengue reporter viruses

Dengue virus (DENV) is a global health problem that affects approximately 3.9 billion people worldwide. Since safety concerns were raised for the only licensed vaccine, Dengvaxia, and since the present treatment is only supportive care, the development of more effective therapeutic anti-DENV agents is urgently needed. In this report, we identified a potential small-molecule inhibitor, BP34610, via cell-based high-throughput screening (HTS) of 12,000 compounds using DENV-2 reporter viruses. BP34610 reduced the virus yields of type 2 DENV-infected cells with a 50% effective concentration (EC₅₀) and selectivity index value of 0.48 ± 0.06 μM and 197, respectively. Without detectable cytotoxicity, the compound inhibited not only all four serotypes of DENV but also Japanese encephalitis virus (JEV). Time-of-addition experiments suggested that BP34610 may act at an early stage of DENV virus infection. Sequencing analyses of several individual clones derived from BP34610-resistant viruses revealed a consensus amino acid substitution (S397P) in the N-terminal stem region of the E protein. Introduction of S397P into the DENV reporter viruses conferred an over 14.8-fold EC₉₀ shift for BP34610. Importantly, the combination of BP34610 with a viral replication inhibitor, ribavirin, displayed synergistic enhancement of anti-DENV-2 activity. Our results identify an effective small-molecule inhibitor, BP34610, which likely targets the DENV E protein. BP34610 could be developed as an anti-flavivirus agent in the future.

Contemporary Strategies and Current Trends in Designing Antiviral Drugs against Dengue Fever via Targeting Host-Based Approaches

Dengue virus (DENV) is an arboviral human pathogen transmitted through mosquito bite that infects an estimated ~400 million humans (~5% of the global population) annually. To date, no specific therapeutics have been developed that can prevent or treat infections resulting from this pathogen. DENV utilizes numerous host molecules and factors for transcribing the single-stranded ~11 kb positive-sense RNA genome. For example, the glycosylation machinery of the host is required for viral particles to assemble in the endoplasmic reticulum. Since a variety of host factors seem to be utilized by the pathogens, targeting these factors may result in DENV inhibitors, and will play an important role in attenuating the rapid emergence of other flaviviruses. Many experimental studies have yielded findings indicating that host factors facilitate infection, indicating that the focus should be given to targeting the processes contributing to pathogenesis along with many other immune responses. Here, we provide an extensive literature review in order to elucidate the progress made in the development of host-based approaches for DENV viral infections, focusing on host cellular mechanisms and factors responsible for viral replication, aiming to aid the potential development of host-dependent antiviral therapeutics.

Heparan Sulfate Proteoglycans and Viral Attachment: True Receptors or Adaptation Bias?

Heparan sulfate proteoglycans (HSPG) are composed of unbranched, negatively charged heparan sulfate (HS) polysaccharides attached to a variety of cell surface or extracellular matrix proteins. Widely expressed, they mediate many biological activities, including angiogenesis, blood coagulation, developmental processes, and cell homeostasis. HSPG are highly sulfated and broadly used by a range of pathogens, especially viruses, to attach to the cell surface.

Aminopeptidase secreted by Chromobacterium sp. Panama inhibits dengue virus infection by degrading the E protein

Dengue virus (DENV) is the most prevalent and burdensome arbovirus transmitted by Aedes mosquitoes, against which there is only a limited licensed vaccine and no approved drug treatment. A Chromobacterium species, C. sp. Panama, isolated from the midgut of A. aegypti is able to inhibit DENV replication within the mosquito and in vitro. Here we show that C. sp. Panama mediates its anti-DENV activity through secreted factors that are proteinous in nature. The inhibitory effect occurs prior to virus attachment to cells, and is attributed to a factor that destabilizes the virion by promoting the degradation of the viral envelope protein. Bioassay-guided fractionation, coupled with mass spectrometry, allowed for the identification of a C. sp. Panama-secreted neutral protease and an aminopeptidase that are co-expressed and appear to act synergistically to degrade the viral envelope (E) protein and thus prevent viral attachment and subsequent infection of cells. This is the first study characterizing the anti-DENV activity of a common soil and mosquito-associated bacterium, thereby contributing towards understanding how such bacteria may limit disease transmission, and providing new tools for dengue prevention and therapeutics.

An Overview of Current Approaches Toward the Treatment and Prevention of West Nile Virus Infection

The persistence of West Nile virus (WNV) infections throughout the USA since its inception in 1999 and its continuous spread throughout the globe calls for an urgent need of effective treatments and prevention measures. Although the licensing of several WNV vaccines for veterinary use provides a proof of concept, similar efforts on the development of an effective vaccine for humans remain still unsuccessful. Increased understanding of biology and pathogenesis of WNV together with recent technological advancements have raised hope that an effective WNV vaccine may be available in the near future. In addition, rapid progress in the structural and functional characterization of WNV and other flaviviral proteins have provided a solid base for the design and development of several classes of inhibitors as potential WNV therapeutics. Moreover, the therapeutic monoclonal antibodies demonstrate an excellent efficacy against WNV in animal models and represent a promising class of WNV therapeutics. However, there are some challenges as to the design and development of a safe and efficient WNV vaccine or therapeutic. In this chapter, we discuss the current approaches, progress, and challenges toward the development of WNV vaccines, therapeutic antibodies, and antiviral drugs.

Molecular Responses of Human Retinal Cells to Infection with Dengue Virus

Recent clinical reports indicate that infection with dengue virus (DENV) commonly has ocular manifestations. The most serious threat to vision is dengue retinopathy, including retinal vasculopathy and macular edema. Mechanisms of retinopathy are unstudied, but observations in patients implicate retinal pigment epithelial cells and retinal endothelial cells. Human retinal cells were inoculated with DENV-2 and monitored for up to 72 hours. Epithelial and endothelial cells supported DENV replication and release, but epithelial cells alone demonstrated clear cytopathic effect, and infection was more productive in those cells. Infection induced type I interferon responses from both cells, but this was stronger in epithelial cells. Endothelial cells increased expression of adhesion molecules, with sustained overexpression of vascular adhesion molecule-1. Transcellular impedance decreased for epithelial monolayers, but not endothelial monolayers, coinciding with cytopathic effect. This reduction was accompanied by disorganization of intracellular filamentous-actin and decreased expression of junctional molecules, zonula occludens 1, and catenin-β1. Changes in endothelial expression of adhesion molecules are consistent with the retinal vasculopathy seen in patients infected with DENV; decreases in epithelial junctional protein expression, paralleling loss of integrity of the epithelium, provide a molecular basis for DENV-associated macular edema. These molecular processes present potential therapeutic targets for vision-threatening dengue retinopathy.

Peptides as Therapeutic Agents for Dengue Virus

Dengue is an important global threat caused by dengue virus (DENV) that records an estimated 390 million infections annually. Despite the availability of CYD-TDV as a commercial vaccine, its long-term efficacy against all four dengue virus serotypes remains unsatisfactory. There is therefore an urgent need for the development of antiviral drugs for the treatment of dengue. Peptide was once a neglected choice of medical treatment but it has lately regained interest from the pharmaceutical industry following pioneering advancements in technology. In this review, the design of peptide drugs, antiviral activities and mechanisms of peptides and peptidomimetics (modified peptides) action against dengue virus are discussed. The development of peptides as inhibitors for viral entry, replication and translation is also described, with a focus on the three main targets, namely, the host cell receptors, viral structural proteins and viral non-structural proteins. The antiviral peptides designed based on these approaches may lead to the discovery of novel anti-DENV therapeutics that can treat dengue patients.

Pathogenesis and Inhibition of Flaviviruses from a Carbohydrate Perspective

Flaviviruses are enveloped, positive single stranded ribonucleic acid (RNA) viruses with various routes of transmission. While the type and severity of symptoms caused by pathogenic flaviviruses vary from hemorrhagic fever to fetal abnormalities, their general mechanism of host cell entry is similar. All pathogenic flaviviruses, such as dengue virus, yellow fever virus, West Nile virus, Japanese encephalitis virus, and Zika virus, bind to glycosaminglycans (GAGs) through the putative GAG binding sites within their envelope proteins to gain access to the surface of host cells. GAGs are long, linear, anionic polysaccharides with a repeating disaccharide unit and are involved in many biological processes, such as cellular signaling, cell adhesion, and pathogenesis. Flavivirus envelope proteins are N-glycosylated surface proteins, which interact with C-type lectins, dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN) through their glycans. In this review, we discuss both host and viral surface receptors that have the carbohydrate components, focusing on the surface interactions in the early stage of flavivirus entry. GAG-flavivirus envelope protein interactions as well as interactions between flavivirus envelope proteins and DC-SIGN are discussed in detail. This review also examines natural and synthetic inhibitors of flaviviruses that are carbohydrate-based or carbohydrate-targeting. Both advantages and drawbacks of these inhibitors are explored, as are potential strategies to improve their efficacy to ultimately help eradicate flavivirus infections.

Sulfated Glycans and Related Digestive Enzymes in the Zika Virus Infectivity: Potential Mechanisms of Virus-Host Interaction and Perspectives in Drug Discovery

As broadly reported, there is an ongoing Zika virus (ZIKV) outbreak in countries of Latin America. Recent findings have demonstrated that ZIKV causes severe defects on the neural development in fetuses in utero and newborns. Very little is known about the molecular mechanisms involved in the ZIKV infectivity. Potential therapeutic agents are also under investigation. In this report, the possible mechanisms of action played by glycosaminoglycans (GAGs) displayed at the surface proteoglycans of host cells, and likely in charge of interactions with surface proteins of the ZIKV, are highlighted. As is common for the most viruses, these sulfated glycans serve as receptors for virus attachment onto the host cells and consequential entry during infection. The applications of (1) exogenous sulfated glycans of different origins and chemical structures capable of competing with the virus attachment receptors (supposedly GAGs) and (2) GAG-degrading enzymes able to digest the virus attachment receptors on the cells may be therapeutically beneficial as anti-ZIKV. This communication attempts, therefore, to offer some guidance for the future research programs aimed to unveil the molecular mechanisms underlying the ZIKV infectivity and to develop therapeutics capable of decreasing the devastating consequences caused by ZIKV outbreak in the Americas.

Illustrating and homology modeling the proteins of the Zika virus

The Zika virus (ZIKV) is a flavivirus of the family Flaviviridae, which is similar to dengue virus, yellow fever and West Nile virus. Recent outbreaks in South America, Latin America, the Caribbean and in particular Brazil have led to concern for the spread of the disease and potential to cause Guillain-Barré syndrome and microcephaly. Although ZIKV has been known of for over 60 years there is very little in the way of knowledge of the virus with few publications and no crystal structures. No antivirals have been tested against it either in vitro or in vivo. ZIKV therefore epitomizes a neglected disease. Several suggested steps have been proposed which could be taken to initiate ZIKV antiviral drug discovery using both high throughput screens as well as structure-based design based on homology models for the key proteins. We now describe preliminary homology models created for NS5, FtsJ, NS4B, NS4A, HELICc, DEXDc, peptidase S7, NS2B, NS2A, NS1, E stem, glycoprotein M, propeptide, capsid and glycoprotein E using SWISS-MODEL. Eleven out of 15 models pass our model quality criteria for their further use. While a ZIKV glycoprotein E homology model was initially described in the immature conformation as a trimer, we now describe the mature dimer conformer which allowed the construction of an illustration of the complete virion. By comparing illustrations of ZIKV based on this new homology model and the dengue virus crystal structure we propose potential differences that could be exploited for antiviral and vaccine design. The prediction of sites for glycosylation on this protein may also be useful in this regard. While we await a cryo-EM structure of ZIKV and eventual crystal structures of the individual proteins, these homology models provide the community with a starting point for structure-based design of drugs and vaccines as well as a for computational virtual screening.

Inhibition of Human Metapneumovirus Binding to Heparan Sulfate Blocks Infection in Human Lung Cells and Airway Tissues

Human metapneumovirus (HMPV), a recently discovered paramyxovirus, infects nearly 100% of the world population and causes severe respiratory disease in infants, the elderly, and immunocompromised patients. We previously showed that HMPV binds heparan sulfate proteoglycans (HSPGs) and that HMPV binding requires only the viral fusion (F) protein. To characterize the features of this interaction critical for HMPV binding and the role of this interaction in infection in relevant models, we utilized sulfated polysaccharides, heparan sulfate mimetics, and occluding compounds. Iota-carrageenan demonstrated potent anti-HMPV activity by inhibiting binding to lung cells mediated by the F protein. Furthermore, analysis of a minilibrary of variably sulfated derivatives of Escherichia coli K5 polysaccharide mimicking the HS structure revealed that the highly O-sulfated K5 polysaccharides inhibited HMPV infection, identifying a potential feature of HS critical for HMPV binding. The peptide dendrimer SB105-A10, which binds HS, reduced binding and infection in an F-dependent manner, suggesting that occlusion of HS at the target cell surface is sufficient to prevent infection. HMPV infection was also inhibited by these compounds during apical infection of polarized airway tissues, suggesting that these interactions take place during HMPV infection in a physiologically relevant model. These results reveal key features of the interaction between HMPV and HS, supporting the hypothesis that apical HS in the airway serves as a binding factor during infection, and HS modulating compounds may serve as a platform for potential antiviral development.

IMPORTANCE

Human metapneumovirus (HMPV) is a paramyxovirus that causes respiratory disease worldwide. It has been previously shown that HMPV requires binding to heparan sulfate on the surfaces of target cells for attachment and infection. In this study, we characterize the key features of this binding interaction using heparan sulfate mimetics, identify an important sulfate modification, and demonstrate that these interactions occur at the apical surface of polarized airway tissues. These findings provide insights into the initial binding step of HMPV infection that has potential for antiviral development.

Pharmacology of Heparin and Related Drugs

Heparin has been recognized as a valuable anticoagulant and antithrombotic for several decades and is still widely used in clinical practice for a variety of indications. The anticoagulant activity of heparin is mainly attributable to the action of a specific pentasaccharide sequence that acts in concert with antithrombin, a plasma coagulation factor inhibitor. This observation has led to the development of synthetic heparin mimetics for clinical use. However, it is increasingly recognized that heparin has many other pharmacological properties, including but not limited to antiviral, anti-inflammatory, and antimetastatic actions. Many of these activities are independent of its anticoagulant activity, although the mechanisms of these other activities are currently less well defined. Nonetheless, heparin is being exploited for clinical uses beyond anticoagulation and developed for a wide range of clinical disorders. This article provides a "state of the art" review of our current understanding of the pharmacology of heparin and related drugs and an overview of the status of development of such drugs.

New developments in flavivirus drug discovery

INTRODUCTION

Flaviviruses are major causes of infectious disease. The vast global, social and economic impact due to morbidity and mortality associated with diseases caused by these viruses urgently demands effective therapeutic interventions. There is currently no specific antiviral therapy available for the effective clinical treatment of infections by any of the flaviviridae. Development of more effective vaccines and antiviral agents for the prevention and treatment of most flavivirus infections remains a clear public health priority in the 21st century.

AREAS COVERED

This review describes some of the recent discoveries in the field of flavivirus inhibitor development, with a particular focus on targeting viral proteins. Emphasis is placed on the advances published during the 2012-2015 period.

EXPERT OPINION

The field of drug discovery targeting viral proteins has progressed slowly in recent years. New information, particularly on structures, location and mechanisms of action of established protein targets have been reported. There have also been studies on repurposing known drugs as templates for targeting flavivirus proteins and these hits could be promising templates for developing new more potent inhibitors. Further research should be conducted to improve in vitro assays that better reflect the conditions found in cellular environments.

Illustrating and homology modeling the proteins of the Zika virus

The Zika virus (ZIKV) is a flavivirus of the family Flaviviridae, which is similar to dengue virus, yellow fever and West Nile virus. Recent outbreaks in South America, Latin America, the Caribbean and in particular Brazil have led to concern for the spread of the disease and potential to cause Guillain-Barré syndrome and microcephaly. Although ZIKV has been known of for over 60 years there is very little in the way of knowledge of the virus with few publications and no crystal structures. No antivirals have been tested against it either in vitro or in vivo. ZIKV therefore epitomizes a neglected disease. Several suggested steps have been proposed which could be taken to initiate ZIKV antiviral drug discovery using both high throughput screens as well as structure-based design based on homology models for the key proteins. We now describe preliminary homology models created for NS5, FtsJ, NS4B, NS4A, HELICc, DEXDc, peptidase S7, NS2B, NS2A, NS1, E stem, glycoprotein M, propeptide, capsid and glycoprotein E using SWISS-MODEL. Eleven out of 15 models pass our model quality criteria for their further use. While a ZIKV glycoprotein E homology model was initially described in the immature conformation as a trimer, we now describe the mature dimer conformer which allowed the construction of an illustration of the complete virion. By comparing illustrations of ZIKV based on this new homology model and the dengue virus crystal structure we propose potential differences that could be exploited for antiviral and vaccine design. The prediction of sites for glycosylation on this protein may also be useful in this regard. While we await a cryo-EM structure of ZIKV and eventual crystal structures of the individual proteins, these homology models provide the community with a starting point for structure-based design of drugs and vaccines as well as a for computational virtual screening.

Basic chemokine-derived glycosaminoglycan binding peptides exert antiviral properties against dengue virus serotype 2, herpes simplex virus-1 and respiratory syncytial virus

Chemokines attract leukocytes to sites of infection in a G protein-coupled receptor (GPCR) and glycosaminoglycan (GAG) dependent manner. Therefore, chemokines are crucial molecules for proper functioning of our antimicrobial defense mechanisms. In addition, some chemokines have GPCR-independent defensin-like antimicrobial activities against bacteria and fungi. Recently, high affinity for GAGs has been reported for the positively charged COOH-terminal region of the chemokine CXCL9. In addition to CXCL9, also CXCL12γ has such a positively charged COOH-terminal region with about 50% positively charged amino acids. In this report, we compared the affinity of COOH-terminal peptides of CXCL9 and CXCL12γ for GAGs and KD values in the low nM range were detected. Several enveloped viruses such as herpesviruses, hepatitis viruses, human immunodeficiency virus (HIV), dengue virus (DENV), etc. are known to bind to GAGs such as the negatively charged heparan sulfate (HS). In this way GAGs are important for the initial contacts between viruses and host cells and for the infection of the cell. Thus, inhibiting the virus-cell interactions, by blocking GAG-binding sites on the host cell, might be a way to target multiple virus families and resistant strains. This article reports that the COOH-terminal peptides of CXCL9 and CXCL12γ have antiviral activity against DENV serotype 2, clinical and laboratory strains of herpes simplex virus (HSV)-1 and respiratory syncytial virus (RSV). Moreover, we show that CXCL9(74-103) competes with DENV envelope protein domain III for binding to heparin. These short chemokine-derived peptides may be lead molecules for the development of novel antiviral agents.

Flavivirus Entry Inhibitors

Many flaviviruses are significant human pathogens that are transmitted by mosquitoes and ticks. Although effective vaccines are available for yellow fever virus, Japanese encephalitic virus, and tick-borne encephalitis virus, these and other flaviviruses still cause thousands of human deaths and millions of illnesses each year. No clinically approved antiviral therapy is available for flavivirus treatment. To meet this unmet medical need, industry and academia have taken multiple approaches to develop antiflavivirus therapy, among which targeting viral entry has been actively pursued in the past decade. Here we review the current knowledge of flavivirus entry and its use for small molecule drug discovery. Inhibitors of two major steps of flaviviral entry have been reported: (i) molecules that block virus-receptor interaction; (ii) compounds that prevent conformational change of viral envelope protein during virus-host membrane fusion. We also discuss the advantages and disadvantages of targeting viral entry for treatment of flavivirus infection as compared to targeting viral replication proteins.

Dengue virus entry and trafficking: perspectives as antiviral target for prevention and therapy

ABSTRACT 

Dengue virus (DENV) is the etiological agent of the most important human viral infection transmitted by mosquitoes in the world. In spite of the serious health threat that dengue represents, at present there are no vaccine or antiviral agents available and treatment of patients consists of supportive therapy. This review will focus on the process of DENV entry into the host cell as a potential target for antiviral therapy. The recent advances in the knowledge of viral and cellular molecules and mechanisms involved in binding, internalization and trafficking of DENV into the host cell until virion uncoating are discussed, together with an overview of the strategies and compounds evaluated for development of antiviral agents targeted to DENV entry.

Dengue Virus Infection of Primary Endothelial Cells Induces Innate Immune Responses, Changes in Endothelial Cells Function and Is Restricted by Interferon-Stimulated Responses

Although endothelial cell (EC) infection is not widespread during dengue virus (DENV) infection in vivo, the endothelium is the site of the pathogenic effects seen in severe DENV disease. In this study, we investigated DENV infection of primary EC and defined factors that influence infection in this cell type. Consistent with in vivo findings where EC infection is infrequent, only 3%-15% of EC became productively DENV-2-infected in vitro. This low level infection could not be attributed to inhibition by heparin, EC donor variation, heterogeneity, or biological source. DENV-infection of EC was associated with induction of innate immune responses, including increased STAT1 protein, STAT1- phosphorylation, interferon (IFN)-β, OAS-1, IFIT-1/ISG56, and viperin mRNA. Antibody blocking of IFN-β inhibited the induction of OAS1, IFIT1/ISG56, and viperin while shRNA knockdown of viperin enhanced DENV-infection in EC. DENV-infection of EC resulted in increased activity of sphingosine kinase 1, a factor important in maintaining vascular integrity, and altered basal and stimulated changes in barrier integrity of DENV-infected EC monolayers. Thus, DENV productively infects only a small percentage of primary EC but this has a major influence on induction of IFN-β driven innate immune responses that can restrict infection while the EC themselves are functionally altered. These changes may have important consequences for the endothelium and are reflective of pathogenic changes associated with vascular leakage, as seen in DENV disease.

Endothelial dysfunction in dengue virus pathology

Dengue virus (DENV) is a leading cause of illness and death, mainly in the (sub)tropics, where it causes dengue fever and/or the more serious diseases dengue hemorrhagic fever and dengue shock syndrome that are associated with changes in vascular permeability. Despite extensive research, the pathogenesis of DENV is still poorly understood and, although endothelial cells represent the primary fluid barrier of the blood vessels, the extent to which these cells contribute to DENV pathology is still under debate. The primary target cells for DENV are dendritic cells and monocytes/macrophages that release various chemokines and cytokines upon infection, which can activate the endothelium and are thought to play a major role in DENV-induced vascular permeability. However, recent studies indicate that DENV also replicates in endothelial cells and that DENV-infected endothelial cells may directly contribute to viremia, immune activation, vascular permeability and immune targeting of the endothelium. Also, the viral non-structural protein-1 and antibodies directed against this secreted protein have been reported to be involved in endothelial cell dysfunction. This review provides an extensive overview of the effects of DENV infection on endothelial cell physiology and barrier function.

Galectin-1 exerts inhibitory effects during DENV-1 infection

Dengue virus (DENV) is an enveloped RNA virus that is mosquito-transmitted and can infect a variety of immune and non-immune cells. Response to infection ranges from asymptomatic disease to a severe disorder known as dengue hemorrhagic fever. Despite efforts to control the disease, there are no effective treatments or vaccines. In our search for new antiviral compounds to combat infection by dengue virus type 1 (DENV-1), we investigated the role of galectin-1, a widely-expressed mammalian lectin with functions in cell-pathogen interactions and immunoregulatory properties. We found that DENV-1 infection of cells in vitro exhibited caused decreased expression of Gal-1 in several different human cell lines, suggesting that loss of Gal-1 is associated with virus production. In test of this hypothesis we found that exogenous addition of human recombinant Gal-1 (hrGal-1) inhibits the virus production in the three different cell types. This inhibitory effect was dependent on hrGal-1 dimerization and required its carbohydrate recognition domain. Importantly, the inhibition was specific for hrGal-1, since no effect was observed using recombinant human galectin-3. Interestingly, we found that hrGal-1 directly binds to dengue virus and acts, at least in part, during the early stages of DENV-1 infection, by inhibiting viral adsorption and its internalization to target cells. To test the in vivo role of Gal-1 in DENV infection, Gal-1-deficient-mice were used to demonstrate that the expression of endogenous Galectin-1 contributes to resistance of macrophages to in vitro-infection with DENV-1 and it is also important to physiological susceptibility of mice to in vivo infection with DENV-1. These results provide novel insights into the functions of Gal-1 in resistance to DENV infection and suggest that Gal-1 should be explored as a potential antiviral compound.

Progress in the identification of dengue virus entry/fusion inhibitors

Dengue fever, a reemerging disease, is putting nearly 2.5 billion people at risk worldwide. The number of infections and the geographic extension of dengue fever infection have increased in the past decade. The disease is caused by the dengue virus, a flavivirus that uses mosquitos Aedes sp. as vectors. The disease has several clinical manifestations, from the mild cold-like illness to the more serious hemorrhagic dengue fever and dengue shock syndrome. Currently, there is no approved drug for the treatment of dengue disease or an effective vaccine to fight the virus. Therefore, the search for antivirals against dengue virus is an active field of research. As new possible receptors and biological pathways of the virus biology are discovered, new strategies are being undertaken to identify possible antiviral molecules. Several groups of researchers have targeted the initial step in the infection as a potential approach to interfere with the virus. The viral entry process is mediated by viral proteins and cellular receptor molecules that end up in the endocytosis of the virion, the fusion of both membranes, and the release of viral RNA in the cytoplasm. This review provides an overview of the targets and progress that has been made in the quest for dengue virus entry inhibitors.

Dengue type four viruses with E-Glu345Lys adaptive mutation from MRC-5 cells induce low viremia but elicit potent neutralizing antibodies in rhesus monkeys

Knowledge of virulence and immunogenicity is important for development of live-attenuated dengue vaccines. We previously reported that an infectious clone-derived dengue type 4 virus (DENV-4) passaged in MRC-5 cells acquired a Glu₃₄₅Lys (E-E₃₄₅K) substitution in the E protein domain III (E-DIII). The same cloned DENV-4 was found to yield a single E-Glu₃₂₇Gly (E-E₃₂₇G) mutation after passage in FRhL cells and cause the loss of immunogenicity in rhesus monkeys. Here, we used site-directed mutagenesis to generate the E-E₃₄₅K and E-E₃₂₇G mutants from DENV-4 and DENV-4Δ30 infectious clones and propagated in Vero or MRC-5 cells. The E-E₃₄₅K mutations were consistently presented in viruses recovered from MRC-5 cells, but not Vero cells. Recombinant E-DIII proteins of E₃₄₅K and E₃₂₇G increased heparin binding correlated with the reduced infectivity by heparin treatment in cell cultures. Different from the E-E₃₂₇G mutant viruses to lose the immunogencity in rhesus monkeys, the E-E₃₄₅K mutant viruses were able to induce neutralizing antibodies in rhesus monkeys with an almost a 10-fold lower level of viremia as compared to the wild type virus. Monkeys immunized with the E-E₃₄₅K mutant virus were completely protected with no detectable viremia after live virus challenges with the wild type DENV-4. These results suggest that the E-E₃₄₅K mutant virus propagated in MRC-5 cells may have potential for the use in live-attenuated DENV vaccine development.

Highly sulfated K5 Escherichia coli polysaccharide derivatives inhibit respiratory syncytial virus infectivity in cell lines and human tracheal-bronchial histocultures

Respiratory syncytial virus (RSV) exploits cell surface heparan sulfate proteoglycans (HSPGs) as attachment receptors. The interaction between RSV and HSPGs thus presents an attractive target for the development of novel inhibitors of RSV infection. In this study, selective chemical modification of the Escherichia coli K5 capsular polysaccharide was used to generate a collection of sulfated K5 derivatives with a backbone structure that mimics the heparin/heparan sulfate biosynthetic precursor. The screening of a series of N-sulfated (K5-NS), O-sulfated (K5-OS), and N,O-sulfated (K5-N,OS) derivatives with different degrees of sulfation revealed the highly sulfated K5 derivatives K5-N,OS(H) and K5-OS(H) to be inhibitors of RSV. Their 50% inhibitory concentrations were between 1.07 nM and 3.81 nM in two different cell lines, and no evidence of cytotoxicity was observed. Inhibition of RSV infection was maintained in binding and attachment assays but not in preattachment assays. Moreover, antiviral activity was also evident when the K5 derivatives were added postinfection, both in cell-to-cell spread and viral yield reduction assays. Finally, both K5-N,OS(H) and K5-OS(H) prevented RSV infection in human-derived tracheal/bronchial epithelial cells cultured to form a pseudostratified, highly differentiated model of the epithelial tissue of the human respiratory tract. Together, these features put K5-N,OS(H) and K5-OS(H) forward as attractive candidates for further development as RSV inhibitors.