Internalization and Propagation of the Dengue Virus in Human Hepatoma (HepG2) Cells

αVβ3 Integrin Expression Is Essential for Replication of Mosquito and Tick-Borne Flaviviruses in Murine Fibroblast Cells

The Flavivirus genus includes a number of important viruses that are pathogenic to humans and animals and are responsible for outbreaks across the globe. Integrins, a family of heterodimeric transmembrane molecules expressed in all nucleated cells mediate critical functions of cell physiology and cell cycle. Integrins were previously postulated to be involved in flavivirus entry and to modulate flavivirus replication efficiency. In the present study, mouse embryonic fibroblasts (MEF), lacking the expression of αVβ3 integrin (MEF-αVβ3^−/−), were infected with four different flaviviruses, namely yellow fever virus (YFV), West Nile virus (WNV), Usutu virus (USUV) and Langat virus (LGTV). The effects of the αVβ3 integrin absence in double-knockout MEF-αVβ3^−/− on flavivirus binding, internalization and replication were compared to the respective wild-type cells. Binding to the cell surface for all four flaviviruses was not affected by the ablation of αVβ3 integrin, whereas internalization of USUV and WNV was slightly affected by the loss of αVβ3 integrin expression. Most interestingly, the deletion of αVβ3 integrin strongly impaired replication of all flaviviruses with a reduction of up to 99% on virus yields and a strong reduction on flavivirus anti-genome RNA synthesis. In conclusion, our results demonstrate that αVβ3 integrin expression in flavivirus-susceptible cell lines enhances the flavivirus replication.

Pathogen-induced inflammation is attenuated by the iminosugar MON-DNJ via modulation of the unfolded protein response

Sepsis is a life-threatening condition involving a dysregulated immune response to infectious agents that cause injury to host tissues and organs. Current treatments are limited to early administration of antibiotics and supportive care. While appealing, the strategy of targeted inhibition of individual molecules in the inflammatory cascade has not proved beneficial. Non-targeted, systemic immunosuppression with steroids has shown limited efficacy and raises concern for secondary infection. Iminosugars are a class of small molecule glycomimetics with distinct inhibition profiles for glycan processing enzymes based on stereochemistry. Inhibition of host endoplasmic reticulum resident glycoprotein processing enzymes has demonstrated efficacy as a broad-spectrum antiviral strategy, but limited consideration has been given to the effects on host glycoprotein production and consequent disruption of signalling cascades. This work demonstrates that iminosugars inhibit dengue virus, bacterial lipopolysaccharide and fungal antigen-stimulated cytokine responses in human macrophages. In spite of decreased inflammatory mediator production, viral replication is suppressed in the presence of iminosugar. Transcriptome analysis reveals the key interaction of pathogen-induced endoplasmic reticulum stress, the resulting unfolded protein response and inflammation. Our work shows that iminosugars modulate these interactions. Based on these findings, we propose a new therapeutic role for iminosugars as treatment for sepsis-related inflammatory disorders associated with excess cytokine secretion.

Analysis of the virus propagation profile of 14 dengue virus isolates in Aedes albopictus C6/36 cells

OBJECTIVE

The mosquito transmitted RNA virus dengue virus (DENV) shows significant variation as a consequence of the lack of proofreading activity of the RNA-dependent RNA polymerase that synthesizes new virus genomes. How this variation affects DENV replication, and how this in turn impacts drug development remains largely unknown. Given the technical limitations in working with large numbers of isolates few studies have sought to investigate this area. This study used a panel of 14 DENV isolates of different serotypes and origins to determine how much virus replication in Aedes albopictus C6/36 cells was affected by DENV variability.

RESULTS

The results showed that there was considerable variation, with peak titers ranging from 6Log10 to 8Log10, and maximum titer being reached from day 3 to day 9 post infection. While strains from DENV 1 and 4 serotypes showed considerable uniformity, DENV 2 and 3 strains showed much greater variation. Overall, these results show that serotype specific strain variation can have a significant impact on DENV replication, suggesting that studies either investigating DENV pathogenesis or developing drug therapeutics should consider the contribution of DENV variability.

Dengue Virus

Dengue virus (DENV) belongs to the family Flaviviridae, genus Flavivirus. It is a single-stranded positive-sense ribonucleic acid virus with 10,700 bases. The genus Flavivirus includes other arthropod borne viruses such as yellow fever virus, West Nile virus, Zika virus, tick-borne encephalitis virus. It infects ~50–200 million people annually, putting over 3.6 billion people living in tropical regions at risk and causing ~20,000 deaths annually. The expansion of dengue is attributed to factors such as the modern dynamics of climate change, globalization, travel, trade, socioeconomics, settlement, and also viral evolution. There are four antigenically different serotypes of DENV based on the differences in their viral structural and nonstructural proteins. DENV infection causes a spectrum of illness ranging from asymptomatic to dengue fever to severe dengue shock syndrome. Infection with one serotype confers lifelong immunity against that serotype, but heterologus infection leads to severe dengue hemorrhagic fever due to antibody-dependent enhancement. Diagnosis of dengue infections is based mainly on serological detection of either antigen in acute cases or antibodies in both acute and chronic infection. Viral detection and real-time PCR detection though helpful is not feasible in resource poor setup. Treatment of dengue depends on symptomatic management along with fluid resuscitation and may require platelet transfusion. Although vaccine development is in late stages of development, developing a single vaccine against four serotypes often causes serious challenges to researchers; hence, the main stay of prevention is vector control and management.

Drug repurposing of N-acetyl cysteine as antiviral against dengue virus infection

Liver injury is one of the hallmark features of severe dengue virus (DENV) infection since DENV can replicate in the liver and induce hepatocytes to undergo apoptosis. N-acetyl cysteine (NAC), which is a clinically-used drug for treating acetaminophen toxicity, was found to benefit patients with DENV-induced liver injury; however, its mechanism of action remains unclear. Accordingly, our aim was to repurpose NAC in the preclinical studies to investigate its mechanism of action. Time of addition experiments in HepG2 cells elucidated effectiveness of NAC to reduce infectious virion at pre-, during- and post infection. In DENV-infected mice, NAC improved DENV-associated clinical manifestations, including leucopenia and thrombocytopenia, and reduced liver injury and hepatocyte apoptosis. Interestingly, we discovered that NAC significantly reduced DENV production in HepG2 cells and in liver of DENV-infected mice by induction of antiviral responses via interferon signaling. NAC treatment in DENV-infected mice helped to maintain antioxidant enzymes and redox balance in the liver. Therefore, NAC reduces DENV production and oxidative damage to ameliorate DENV-induced liver injury. Taken together, these findings suggest the novel therapeutic potential of NAC in DENV-induced liver injury and recommend evaluating its efficacy and safety in humans with DENV-induced liver injury.

Inhibition of N-myristoyltransferase1 affects dengue virus replication

Dengue virus (DENV) causes dengue fever, a self‐limiting disease that could be fatal due to serious complications. No specific treatment is currently available and the preventative vaccine is only partially protective. To develop a potential drug target for dengue fever, we need to understand its biology and pathogenesis thoroughly. N‐myristoyltransferase (NMT) is an N‐terminal protein lipidation enzyme that catalyzes the covalent cotranslational attachment of fatty acids to the amino‐terminal glycine residue of a number of proteins, leading to the modulation of various signaling molecules. In this study, we investigated the interaction of dengue viral proteins with host NMT and its subsequent effect on DENV. Our bioinformatics, molecular docking, and far‐western blotting analyses demonstrated the interaction of viral envelope protein (E) with NMT. The gene expression of NMT was strongly elevated in a dependent manner during the viral replication phase in dendritic cells. Moreover, NMT gene silencing significantly inhibited DENV replication in dendritic cells. Further studies investigating the target cell types of other host factors are suggested.

The Role of Host Cholesterol During Flavivirus Infection

In recent years the emergence and resurgence of arboviruses have generated a global health alert. Among arboviruses, Dengue (DENV), Zika (ZIKV), Yellow Fever (YFV), and West Nile (WNV) virus, belong to the genus Flavivirus, cause high viremia and occasionally fatal clinical disease in humans. Given the genetic austerity of the virus, they depend on cellular factors and organelles to complete its replication. One of the cellular components required for flavivirus infection is cholesterol. Cholesterol is an abundant lipid in biomembranes of eukaryotes cells and is necessary to maintain the cellular homeostasis. Recently, it has been reported, that cholesterol is fundamental during flavivirus infection in both mammal and insect vector models. During infection with DENV, ZIKV, YFV, and WNV the modulation of levels of host-cholesterol facilitates viral entry, replicative complexes formation, assembly, egress, and control of the interferon type I response. This modulation involves changes in cholesterol uptake with the concomitant regulation of cholesterol receptors as well as changes in cholesterol synthesis related to important modifications in cellular metabolism pathways. In view of the flavivirus dependence of cholesterol and the lack of an effective anti-flaviviral treatment, this cellular lipid has been proposed as a therapeutic target to treat infection using FDA-approved cholesterol-lowering drugs. This review aims to address the dependence of cholesterol by flaviviruses as well as the basis for anti flaviviral therapy using drugs which target is cholesterol synthesis or uptake.

Engineered Livers for Infectious Diseases

Engineered liver systems come in a variety of platform models, from 2-dimensional cocultures of primary human hepatocytes and stem cell-derived progeny, to 3-dimensional organoids and humanized mice. Because of the species-specificity of many human hepatropic pathogens, these engineered systems have been essential tools for biologic discovery and therapeutic agent development in the context of liver-dependent infectious diseases. Although improvement of existing models is always beneficial, and the addition of a robust immune component is a particular need, at present, considerable progress has been made using this combination of research platforms. We highlight advances in the study of hepatitis B and C viruses and malaria-causing Plasmodium falciparum and Plasmodium vivax parasites, and underscore the importance of pairing the most appropriate model system and readout modality with the particular experimental question at hand, without always requiring a platform that recapitulates human physiology in its entirety.

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.

The significance of Hsp70 subnetwork for Dengue virus lifecycle

Viruses hijack host machineries for replicating themselves efficiently. Host protein quality control machineries (QC) not only assist protein folding to form bona fide proteins with active functions but also get rid of un/misfolded proteins via degradation to maintain the protein homeostasis. Previous studies have reported that viruses utilize QC at various steps for their lifecycles. Recently we defined Hsp70s and their cochaperones, DnaJs functions on Dengue lifecycle. Here we summarize the significance of QC on Dengue virus.

miR-21 promotes dengue virus serotype 2 replication in HepG2 cells

Infection with the mosquito transmitted dengue virus (DENV) remains a significant worldwide public health problem. While the majority of infections are asymptomatic, infection can result in a range of symptoms. MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression through repression or degradation of mRNAs. To understand the contribution of miRNAs to DENV 2 replication, we screened a number of candidate miRNAs for variations in expression levels during DENV 2 infection of HepG2 (liver) cells. Seven miRNAs were identified as differentially expressed, and one, miR-21, was differentially expressed at all time points examined. Interestingly, miR-21 was also differentially regulated in DENV 2 infection under conditions of antibody dependent enhancement of infection, and in direct Zika virus infection, but not in DENV 4 infection. The role of miR-21 during DENV infection was further examined by treating HepG2 cells with an anti-miR-21 (AMO-21) before DENV infection. The results showed a significant reduction in DENV 2 production, clearly suggesting that miR-21 plays a key role in DENV 2 replication. To further confirm the role of miR-21 in DENV infection, a peptide nucleic acid-21 (PNA-21) construct with a nucleotide sequence complementary to AMO-21, was co-administered with AMO-21 as an AMO-21/PNA-21 complex followed by DENV 2 infection. The results showed that AMO-21 significantly reduced DENV 2 titer, PNA-21 significantly increased DENV 2 titer and the combined AMO-21/PNA-21 showed no difference from non-treated infection controls. Taken together, the results show that miR-21 promotes DENV 2 replication, and this mechanism could serve as a possible therapeutic intervention point.

[Three-dimensional morphology of C6/36 cells infected by dengue virus: a study based on digital holographic microscopy]

OBJECTIVE

To monitor the 3-dimensional (3D) morphological changes of C6/36 cells during dengue virus (DENV) infection using a live-cell imaging technique based on digital holographic microscopy and provide clues for better understanding the mechanisms of DENV infection.

METHODS

C6/36 cells were seeded in 6-well plates to determine the optimal imaging density under a holographic cell imager, and the morphological changes of the cells were recorded in response to a culture temperature change from 28 degrees celsius; to 37 degrees celsius; C6/36 cells were infected with 4 DENV strains with different serotypes at 28 degrees celsius; and incubated at 37 degrees celsius; for 24 h, and the 3D holograms and relevant morphological parameters were recorded at different time points using HoloMonitor M4 holographic cell imaging and analysis system.

RESULTS

The holograms of C6/36 cells inoculated at the optimal density for imaging (4×10⁵ per well) showed unified 3D morphologies of the single cells with minimal dispersions in the cell area, thickness and volume (P<0.05), which did not undergo obvious changes when the cells were incubated at 37 degrees celsius; for 24 h (P>0.05). The cell area and volume of the cells infected with the 4 DENV strains all increased and the cell thickness was reduced during incubation. Among the 4 strains, DENV-1 and DENV-2 caused reduced cell thickness while DENV-3 and DENV-4 increased the cell thickness, and the pattern and degree of such changes differ among the 4 strains.

CONCLUSIONS

Digital holographic microscopy allows monitoring of the complex morphological changes of cells during DENV infection. The 4 DENV strains with different serotypes causes characteristic cell damages during infection.

Receptors and routes of dengue virus entry into the host cells

Dengue is the most prevalent arthropod-borne viral disease, caused by dengue virus, a member of the Flaviviridae family. Its worldwide incidence is now a major health problem, with 2.5 billion people living in risk areas. In this review, we integrate the structural rearrangements of each viral protein and their functions in all the steps of virus entry into the host cells. We describe in detail the putative receptors and attachment factors in mammalian and mosquito cells, and the recognition of viral immunocomplexes via Fcγ receptor in immune cells. We also discuss that virus internalization might occur through distinct entry pathways, including clathrin-mediated or non-classical clathrin-independent endocytosis, depending on the host cell and virus serotype or strain. The implications of viral maturation in virus entry are also explored. Finally, we discuss the mechanisms of viral genome access to the cytoplasm. This includes the role of low pH-induced conformational changes in the envelope protein that mediate membrane fusion, and original insights raised by our recent work that supports the hypothesis that capsid protein would also be an active player in this process, acting on viral genome translocation into the cytoplasm.

Modulation of α-enolase post-translational modifications by dengue virus: increased secretion of the basic isoforms in infected hepatic cells

Hepatic cells are major sites of dengue virus (DENV) replication and liver injury constitutes a characteristic of severe forms of dengue. The role of hepatic cells in dengue pathogenesis is not well established, but since hepatocytes are the major source of plasma proteins, changes in protein secretion by these cells during infection might contribute to disease progression. Previously, we showed that DENV infection alters the secretion pattern of hepatic HepG2 cells, with α-enolase appearing as one of the major proteins secreted in higher levels by infected cells. ELISA analysis demonstrated that DENV infection modulates α-enolase secretion in HepG2 cells in a dose-dependent manner, but has no effect on its gene expression and on the intracellular content of the protein as assessed by PCR and western blot analyses, respectively. Two-dimensional western blots showed that both intracellular and secreted forms of α-enolase appear as five spots, revealing α-enolase isoforms with similar molecular weights but distinct isoeletric points. Remarkably, quantification of each spot content revealed that DENV infection shifts the isoform distribution pattern of secreted α-enolase towards the basic isoforms, whereas the intracellular protein remains unaltered, suggesting that post-translational modifications might be involved in α-enolase secretion by infected cells. These findings provide new insights into the mechanisms underlying α-enolase secretion by hepatic cells and its relationship with the role of liver in dengue pathogenesis. In addition, preliminary results obtained with plasma samples from DENV-infected patients suggest an association between plasma levels of α-enolase and disease severity. Since α-enolase binds plasminogen and modulates its activation, it is plausible to speculate the association of the increase in α-enolase secretion by infected hepatic cells with the haemostatic dysfunction observed in dengue patients including the promotion of fibrinolysis and vascular permeability alterations.

Revisiting dengue virus-host cell interaction: new insights into molecular and cellular virology

Dengue virus (DENV) is an emerging mosquito-borne human pathogen that affects millions of individuals each year by causing severe and potentially fatal syndromes. Despite intense research efforts, no approved vaccine or antiviral therapy is yet available. Overcoming this limitation requires detailed understanding of the intimate relationship between the virus and its host cell, providing the basis to devise optimal prophylactic and therapeutic treatment options. With the advent of novel high-throughput technologies including functional genomics, transcriptomics, proteomics, and lipidomics, new important insights into the DENV replication cycle and the interaction of this virus with its host cell have been obtained. In this chapter, we provide a comprehensive overview on the current status of the DENV research field, covering every step of the viral replication cycle with a particular focus on virus-host cell interaction. We will also review specific chemical inhibitors targeting cellular factors and processes of relevance for the DENV replication cycle and their possible exploitation for the development of next generation antivirals.

Dengue 2 infection of HepG2 liver cells results in endoplasmic reticulum stress and induction of multiple pathways of cell death

BACKGROUND

A number of studies have implicated the direct involvement of the liver in dengue virus (DENV) infection, and it has been widely shown that liver cells subsequently undergo apoptosis. The mechanism by which liver cells undergo apoptosis in response to DENV infection remains unclear. To provide further information on the mechanism of apoptosis in DENV infected liver cells, HepG2 cells were infected with DENV 2 and analyzed for the induction of ER stress, apoptosis and autophagy.

RESULTS

In response to DENV infection, HepG2 cells showed the induction of both the ER resident unfolded protein response as well as the Noxa/PUMA stress response pathways. Proteolytic activation of caspases 4, 7, 8 and 9 was observed as well as changes in mitochondrial transmembrane potential. Increased monodansylcadaverine staining was observed in DENV infected cells, consistent with the previously reported induction of autophagy.

CONCLUSIONS

These results are consistent with a model in which the induction of multiple ER stress pathways is coupled with the induction of multiple cell death pathways as a mechanism to ensure the removal of infected liver cells from the system.

Recognition of heparan sulfate by clinical strains of dengue virus serotype 1 using recombinant subviral particles

Dengue is the most important arthropod-borne viral disease in humans, with an estimated 3.6 billion people at risk for infection and more than 200 million infections per year. Identification of the cellular receptors for dengue virus (DV), the causative agent of dengue, is important toward understanding the pathogenesis of the disease. Heparan sulfate (HS) has been characterized as a DV receptor in multiple model systems, however the physiological relevance of these findings has been questioned by observations that flaviviruses, including DV, can undergo cell culture adaptation changes resulting in increased binding to HS. It thus remains unclear whether HS is utilized by clinical, non-cell culture-adapted strains of DV. To address this question, herein we describe a set of methodologies using recombinant subviral particles (RSPs) to determine the utilization of HS by clinical strains of DV serotype 1 (DV1). RSPs of clinically isolated strains with low cell culture passage histories were used to study HS interaction. Biochemically characterized RSPs showed dose-dependent binding to immobilized heparin, which could be competed by heparin and HS but not structurally related glycosaminoglycans chondroitin sulfate A and hyaluronic acid. The relevance of heparin and HS biochemical interactions was demonstrated by competition of RSP and DV binding to cells with soluble heparin and HS. Our results demonstrate that clinical strains of DV1 can specifically interact with heparin and HS. Together, these data support the possibility that HS on cell surfaces is utilized in the DV-human infection process.

Recent advances in DENV receptors

Dengue is an old disease caused by the mosquito-borne dengue viruses (DENVs), which have four antigenically distinct serotypes (DENV1-4). Infection by any of them can cause dengue fever (DF) and/or a more serious disease, that is, dengue hemorrhagic fever (DHF) or dengue shock syndrome (DSS). In recent decades, incidence of dengue disease has increased 30-fold, putting a third to half of the world's population living in dengue-endemic areas at high infection risk. However, the pathogenesis of the disease is still poorly understood. The virus binding with its host cell is not only a first and critical step in their replication cycle but also a key factor for the pathogenicity. In recent years, there have been significant advances in understanding interactions of DENVs with their target cells such as dendritic cells (DC), macrophages, endothelial cells, and hepatocytes. Although DENVs reportedly attach to a variety of receptors on these cells, consensus DENV receptors have not been defined. In this review, we summarize receptors for DENVs on different cells identified in recent years.

Biological and phylogenetic characteristics of yellow fever virus lineages from West Africa

The yellow fever virus (YFV), the first proven human-pathogenic virus, although isolated in 1927, is still a major public health problem, especially in West Africa where it causes outbreaks every year. Nevertheless, little is known about its genetic diversity and evolutionary dynamics, mainly due to a limited number of genomic sequences from wild virus isolates. In this study, we analyzed the phylogenetic relationships of 24 full-length genomes from YFV strains isolated between 1973 and 2005 in a sylvatic context of West Africa, including 14 isolates that had previously not been sequenced. By this, we confirmed genetic variability within one genotype by the identification of various YF lineages circulating in West Africa. Further analyses of the biological properties of these lineages revealed differential growth behavior in human liver and insect cells, correlating with the source of isolation and suggesting host adaptation. For one lineage, repeatedly isolated in a context of vertical transmission, specific characteristics in the growth behavior and unique mutations of the viral genome were observed and deserve further investigation to gain insight into mechanisms involved in YFV emergence and maintenance in nature.

Dengue virus entry as target for antiviral therapy

Dengue virus (DENV) infections are expanding worldwide and, because of the lack of a vaccine, the search for antiviral products is imperative. Four serotypes of DENV are described and they all cause a similar disease outcome. It would be interesting to develop an antiviral product that can interact with all four serotypes, prevent host cell infection and subsequent immune activation. DENV entry is thus an interesting target for antiviral therapy. DENV enters the host cell through receptor-mediated endocytosis. Several cellular receptors have been proposed, and DC-SIGN, present on dendritic cells, is considered as the most important DENV receptor until now. Because DENV entry is a target for antiviral therapy, various classes of compounds have been investigated to inhibit this process. In this paper, an overview is given of all the putative DENV receptors, and the most promising DENV entry inhibitors are discussed.

An update on mosquito cell expressed dengue virus receptor proteins

Dengue is the most important mosquito transmitted viral disease of humans worldwide. Despite intensive study over several decades, many of the fine details of the dengue virus (DENV) replication cycle remain unknown, although generally more is known about the phase of the replication cycle in mammalian cells as compared to the phase in mosquito cells. This results from a combination of less research emphasis on the mosquito stage, as well as fewer tools such as specific antibodies against mosquito proteins and insect informatics databases. The binding of a virus to a host cell is a first and critical stage in the infectious process and the mechanism and identity of cellular proteins involved in this process remains largely unknown. This short review aims to provide an update on our current understanding of the proteins expressed by mosquito cells that mediate DENV binding as a prerequisite to DENV entry and replication.

Dengue virus strain DEN2 16681 utilizes a specific glycochain of syndecan-2 proteoglycan as a receptor

Dengue virus (DENV) causes fever and severe haemorrhagic symptoms in humans. The DEN2 16681 strain, derived from a dengue haemorrhagic fever patient, has been widely used in studies related to DENV pathogenesis, such as mouse and non-human primate haemorrhagic models and human vascular endothelial-cell permeability. To clarify the entry mechanism of the 16681 strain, we characterized a novel cell receptor for this strain. Our two major findings were as follows: firstly, the SDC2 membrane protein was an effective DEN2 16681 receptor in a cloned K562 cell line. Secondly, a heparan sulfate (HS) glycochain (of four glycochains in SDC2) is the specific binding site of DENV and seems to be involved in tissue-culture adaptation. Our findings present an entry mechanism that could be implicated for DENV adaptation and HS-mediated DENV infection.

Activation of Toll-like receptor 3 impairs the dengue virus serotype 2 replication through induction of IFN-β in cultured hepatoma cells

Toll-like receptors (TLRs) play an important role in innate immunity against invading pathogens. Although TLR signaling has been indicated to protect cells from infection of several viruses, the role of TLRs in Dengue virus (DENV) replication is still unclear. In the present study, we examined the replication of DENV serotype 2 (DENV2) by challenging hepatoma cells HepG2 with different TLR ligands. Activation of TLR3 showed an antiviral effect, while pretreatment of other TLR ligands (including TLR1/2, TLR2/6, TLR4, TLR5 or TLR7/8) did not show a significant effect. TLR3 ligand poly(I∶C) treatment prior to viral infection or simultaneously, but not post-treatment, significantly down-regulated virus replication. Pretreatment with poly(I∶C) reduced viral mRNA expression and viral staining positive cells, accompanying an induction of the type I interferon (IFN-β) and type III IFN (IL-28A/B). Intriguingly, neutralization of IFN-β alone successfully restored the poly(I∶C)-inhibited replication of DENV2. The poly(I∶C)-mediated effects, including IFN induction and DENV2 suppression, were significantly reversed by IKK inhibitor, further suggesting that IFN-β is the dominant factor involved in the poly(I∶C) mediated antiviral effect. Our study presented the first evidence to show that activation of TLR3 is effective in blocking DENV2 replication via IFN-β, providing an experimental clue that poly(I∶C) may be a promising immunomodulatory agent against DENV infection and might be applicable for clinical prevention.

A role for autophagolysosomes in dengue virus 3 production in HepG2 cells

We have recently proposed that amphisomes act as a site for translation and replication of dengue virus (DENV)-2 and that DENV-2 entry and replication are linked through an ongoing association with membranes of an endosomal-autophagosomal lineage. In this report, we present the results of an investigation into the interaction between DENV-3 and the autophagy machinery. Critically, treatment with the lysosomal fusion inhibitor l-asparagine differentiated the interaction of DENV-3 from that of DENV-2. Inhibition of fusion of autophagosomes and amphisomes with lysosomes resulted in decreased DENV-3 production, implying a role for the autophagolysosome in the DENV-3 life cycle. Evidence based upon the co-localization of LC3 and cathepsin D with double stranded RNA and NS1 protein, as assessed by confocal microscopy, support a model in which DENV-3 interacts with both amphisomes and autophagolysosomes. These results demonstrate that the interactions between DENV and the host cell autophagy machinery are complex and may be determined in part by virus-encoded factors.

Characterization of dengue virus entry into HepG2 cells

Background

Despite infections by the dengue virus being a significant problem in tropical and sub-tropical countries, the mechanism by which the dengue virus enters into mammalian cells remains poorly described.

Methods

A combination of biochemical inhibition, dominant negative transfection of Eps15 and siRNA mediated gene silencing was used to explore the entry mechanism of dengue into HepG2 cells.

Results

Results were consistent with entry via multiple pathways, specifically via clathrin coated pit mediated endocytosis and macropinocytosis, with clathrin mediated endocytosis being the predominant pathway.

Conclusion

We propose that entry of the dengue virus to mammalian cells can occur by multiple pathways, and this opens the possibility of the virus being directed to multiple cellular compartments. This would have significant implications in understanding the interaction of the dengue virus with the host cell machinery.

Fulminant liver failure secondary to haemorrhagic dengue in an international traveller

Dengue infections are caused by a single-stranded RNA virus, which has four serotypes (DEN 1-4); mosquitoes of the genus Aedes serve as vectors of transmission. Risk factors for dengue infection are related to both the host and virus. Age, gender, immune status, and genetic background of the host all contribute to the severity of dengue infection. Recently, international travel to endemic areas has also been identified as a major risk factor for both primary and secondary dengue infection. Dengue remains a diagnostic challenge, given its protean nature, ranging from mild febrile illness to profound shock. The most severe manifestation of dengue infection is dengue shock syndrome, which has an estimated mortality rate close to 50%. Dengue shock syndrome typically presents with increased anion gap metabolic acidosis, disseminated intravascular coagulation, severe hypotension, and jaundice. Liver involvement appears to occur more frequently when infections involve DEN-3 and DEN-4 serotypes. While hepatocellular damage has been reported previously in dengue infection, acute liver failure is an extremely rare occurrence in adults. We report a patient with dengue shock syndrome who presented with acute liver failure and hepatic encephalopathy after recent travel to an endemic area.

cDNA-AFLP analysis of differential gene expression in human hepatoma cells (HepG2) upon dengue virus infection

In infectious diseases, the disease pathogenesis is the outcome of the interaction between the genome of the host and the genome of the pathogen. Despite the wide distribution of dengue infections in the world, and the large number of annual infections, few studies have investigated how the dengue genome alters the global transcriptional profile of the host cell. To investigate alterations in the liver cell transcriptome in response to dengue virus infection, liver cells (HepG2) were infected with dengue serotype 2 at MOI 5 and at 3 days post-infection RNA extracted and analyzed by cDNA-AFLP in parallel with mock-infected cells. From 73 primer combinations over 5,000 transcription-derived fragments (TDFs) were observed, of which approximately 10% were regulated differentially in response to infection. Sixty-five TDFs were subsequently cloned and sequenced and 27 unique gene transcripts identified. Semi-quantitative reverse transcription (RT)-PCR was used to validate the expression of 12 of these genes and 10 transcripts (CK2, KIAA509, HSP70, AK3L, NIPA, PHIP, RiboS4, JEM-1, MALT1, and HSI12044) were confirmed to be differentially regulated, with four transcripts (HSP70, NIPA, RiboS4, and JEM-1) showing a greater than twofold regulation. These results suggest that the expression of a large number of genes is altered in response to dengue virus infection of liver cells, and that cDNA-AFLP is a useful tool for obtaining information on both characterized and as yet uncharacterized transcripts whose expression is altered during the infection process.

Dengue virus entry into liver (HepG2) cells is independent of hsp90 and hsp70

Recently, several stress-related proteins including GRP78, hsp70, and hsp90 have been implicated as dengue virus receptors in various cell types, with hsp90/70 being implicated as a receptor complex in monocytes and macrophages, while GRP78 has been implicated as a liver cell expressed dengue virus receptor. To assess whether the hsp90/70 complex plays a role in the internalization of the dengue viruses into liver cells, we undertook infection inhibition studies with lipopolysaccharide and antibodies directed against both hsp70 and hsp90, individually and in combination. No inhibition of any dengue serotype was seen in the presence of lipopolysaccharide or antibodies directed against either hsp70 or hsp90 either singly or in combination. A moderate inhibition of dengue virus serotype 2 entry into liver cells was observed in the presence of antibodies directed against GRP78. These results confirm a proposed role for GRP78 as a dengue virus serotype 2 receptor protein and suggest that the recently identified hsp90/70 complex does not play a role in dengue virus internalization into liver cells.

Entry into and production of the Japanese encephalitis virus from C6/36 cells

OBJECTIVES

The mosquito-borne Japanese encephalitis virus is a leading cause of encephalitis worldwide. However, few studies have investigated the kinetics of Japanese encephalitis virus internalization and production in mosquito cells, and fewer still have investigated the nature of the molecules involved in the binding of the virus to mosquito cells.

METHODS

Using the Aedes albopictus/Stegomyia albopicta-derived C6/36 cell line, Japanese encephalitis virus internalization and production were assayed by standard plaque assay, and virus binding was investigated through preinfection enzymatic treatment of cells and virus overlay protein-binding assay of membrane fractions in native and denaturing gels.

RESULTS

The internalization of the virus was nonlinear, and intracellular infective viruses were detected 8 h after infection and exported to the medium 10 h after infection. The internalization of the virus was primarily mediated by protein elements, and several bands were observed after overlay assay to membrane proteins, although mass spectroscopy was unable to identify candidate proteins. Soluble laminin produced a marginal, but dose-dependent inhibition of infection.

CONCLUSIONS

These results suggest that the mechanism of Japanese encephalitis entry, production, attachment and receptor usage are distinct from those employed by the dengue viruses.

Host-cell interaction of attenuated and wild-type strains of yellow fever virus can be differentiated at early stages of hepatocyte infection

Yellow fever (YF) virus is currently found in tropical Africa and South America, and is responsible for a febrile to severe illness characterized by organ failure and shock. The attenuated YF 17D strain, used in YF vaccine, was derived from the wild-type strain Asibi. Although studies have been done on genetic markers of YF virulence, differentiation of the two strains in terms of host-cell interaction during infection remains elusive. As YF wild-type strains are hepatotropic, we chose a hepatic cell line (HepG2) to study YF virus-host cell interaction. HepG2 cells rapidly produced high titres of infectious viral particles for 17D and Asibi YF strains. However, HepG2 cells were more susceptible to the attenuated 17D virus infection, and only this virus strain induced early apoptosis in these cells. Molecular markers specific for the 17D virus were identified by microarray analysis and confirmed by quantitative RT-PCR analysis. As early as 1h postinfection, three genes, (IEX-1, IRF-1, DEC-1) all implicated in apoptosis pathways, were upregulated. Later in infection (48 h) two other genes (HSP70-1A and 1B), expressed in cases of cellular stress, were highly upregulated in 17D-infected HepG2 cells. The early specific upregulation of these cellular genes in HepG2 cells may be considered markers of the 17D virus. This study on the YF attenuated strain gives a new approach to the analysis of the factors involved in virus attenuation.

Growth and production of the dengue virus in C6/36 cells and identification of a laminin-binding protein as a candidate serotype 3 and 4 receptor protein

OBJECTIVES

Although dengue is one of the most common mosquito-borne viral diseases, few studies have investigated the relationship between the dengue virus and mosquito cells, and this study sought to describe the binding and propagation of the dengue viruses in C6/36 cells.

METHODS

The internalization and production of the dengue virus was assayed by standard plaque assay methodologies, while dengue virus receptor proteins were examined by a virus overlay protein-binding assay and candidate gene analysis coupled with virus inhibition studies.

RESULTS

All four serotypes were internalized linearly, and de novo virus production occurred 14-19 h postinfection. Virus overlay protein-binding assay identified a band of 50 kDa for dengue serotypes 2, 3 and 4 which comigrated with a protein that reacts with antibodies directed against the human 37/67-kDa high-affinity laminin receptor. Both antibodies directed against the human 37/67-kDa high-affinity laminin receptor protein and soluble laminin inhibited the binding and internalization of serotypes 3 and 4, but not serotypes 1 and 2.

CONCLUSIONS

The results suggest that multiple receptors may be used by the dengue virus to enter into insect cells, and that one of these proteins may be a laminin-binding protein.

Pathogenesis of liver involvement during dengue viral infections

The dengue virus can infect many cell types and cause diverse clinical and pathological effects. We describe clinical and experimental observations that suggest that liver involvement occurs during dengue infections, and we outline the possible role played by host immune responses in this process.

Serotype-specific entry of dengue virus into liver cells: identification of the 37-kilodalton/67-kilodalton high-affinity laminin receptor as a dengue virus serotype 1 receptor

Dengue virus, the causative agent of dengue fever, dengue shock syndrome, and dengue hemorrhagic fever, infects susceptible cells by initially binding to a receptor(s) located on the host cell surface. Evidence to date suggests that receptor usage may be cell and serotype specific, and this study sought to identify dengue virus serotype 1 binding proteins on the surface of liver cells, a known target organ. By using a virus overlay protein binding assay (VOPBA), in both nondenaturing and denaturing gel systems, a putative dengue virus serotype 1 binding protein of approximately 37 kDa expressed on the surface of liver (HepG2) cells was identified. Mass spectrometry analysis identified a candidate protein, the 37/67-kDa high-affinity laminin receptor. Entry of the dengue virus serotype 1 was significantly inhibited in a dose-dependent manner by both antibodies directed against the 37/67-kDa high-affinity laminin receptor and soluble laminin. No inhibition of virus entry was seen with dengue virus serotypes 2, 3, or 4, demonstrating that the 37/67-kDa high-affinity laminin receptor is a serotype-specific receptor for dengue virus entry into liver cells.

Internalization of the dengue virus is cell cycle modulated in HepG2, but not Vero cells

While many studies have investigated the relationship between cell type and dengue virus infection, no study to date has examined the effect of cell physiology on permissiveness to infection. Unsynchronized and artificially synchronized cell populations at different stages of the cell cycle of two cell types (Vero and HepG2) were examined for permissiveness to infection by two dengue virus serotypes (serotypes 2 and 3) by determining both the levels of virus produced as well as the percentage of cells infected. Vero cells showed no significant differences between either viral production or percentage of cells infected as compared to unsynchronized cells for any of the phases investigated, although production of virus (for both serotypes 2 and 3) was somewhat lower for cells infected during S phase. In contrast, HepG2 cells were significantly more permissive for both infection and virus production in the G(2) phase as compared to other phases examined and serotype differences in permissiveness to infection were noted with cells in the M phase of the cell cycle. These results suggest that the cell cycle may be a mediator of cell permissiveness in some cell types.