Japanese encephalitis virus infects porcine kidney epithelial PK15 cells via clathrin- and cholesterol-dependent endocytosis

CD4 is an important host factor for Japanese encephalitis virus entry and replication in PK-15 cells

Japanese encephalitis virus (JEV) is a flavivirus that is spread through mosquito bites and is the leading cause of viral encephalitis in Asia. JEV can infect a variety of cell types; however, crucial receptor molecules remain unclear. The purpose of this study was to determine whether porcine CD4 protein is a receptor protein that impacts JEV entry into PK15 cells and subsequent viral replication. We confirmed the interaction between the JEV E protein and the CD4 protein through Co-IP, virus binding and internalization, antibody blocking, and overexpression and created a PK-15 cell line with CD4 gene knockdown by CRISPR/Cas9. The results show that CD4 interacts with JEV E and that CD4 knockdown cells altered virus adsorption and internalization, drastically reducing virus attachment. The level of viral transcription in CD4 antibody-blocked cells, vs. control cells, was decreased by 49.1%. Based on these results, we believe that CD4 is a receptor protein for JEVs. Furthermore, most viral receptors appear to be associated with lipid rafts, and colocalization studies demonstrate the presence of CD4 protein on lipid rafts. RT‒qPCR and WB results show that virus replication was suppressed in PK-15-CD4KD cells. The difference in viral titer between KD and WT PK-15 cells peaked at 24 h, and the viral titer in WT PK-15 cells was 5.6 × 106, whereas in PK-15-CD4KD cells, it was only 1.8 × 106, a 64% drop, demonstrating that CD4 deficiency has an effect on the process of viral replication. These findings suggest that JEV enters porcine kidney cells via lipid raft-colocalized CD4, and the proliferation process is positively correlated with CD4.

Japanese Encephalitis Virus-Infected Cells

RNA virus infections have been a leading cause of pandemics. Aided by global warming and increased connectivity, their threat is likely to increase over time. The flaviviruses are one such RNA virus family, and its prototypes such as the Japanese encephalitis virus (JEV), Dengue virus, Zika virus, West Nile virus, etc., pose a significant health burden on several endemic countries. All viruses start off their life cycle with an infected cell, wherein a series of events are set in motion as the virus and host battle for autonomy. With their remarkable capacity to hijack cellular systems and, subvert/escape defence pathways, viruses are able to establish infection and disseminate in the body, causing disease. Using this strategy, JEV replicates and spreads through several cell types such as epithelial cells, fibroblasts, monocytes and macrophages, and ultimately breaches the blood-brain barrier to infect neurons and microglia. The neurotropic nature of JEV, its high burden on the paediatric population, and its lack of any specific antivirals/treatment strategies emphasise the need for biomedical research-driven solutions. Here, we highlight the latest research developments on Japanese encephalitis virus-infected cells and discuss how these can aid in the development of future therapies.

The Antiviral Effect of Novel Steroidal Derivatives on Flaviviruses

Flaviviruses are the major emerging arthropod-borne pathogens globally. However, there is still no practical anti-flavivirus approach. Therefore, existing and emerging flaviviruses desperately need active broad-spectrum drugs. In the present study, the antiviral effect of steroidal dehydroepiandrosterone (DHEA) and 23 synthetic derivatives against flaviviruses such as Japanese encephalitis virus (JEV), Zika virus (ZIKV), and Dengue virus (DENV) were appraised by examining the characteristics of virus infection both in vitro and in vivo. Our results revealed that AV1003, AV1004 and AV1017 were the most potent inhibitors of flavivirus propagation in cells. They mainly suppress the viral infection in the post-invasion stage in a dose-dependent manner. Furthermore, orally administered compound AV1004 protected mice from lethal JEV infection by increasing the survival rate and reducing the viral load in the brain of infected mice. These results indicate that the compound AV1004 might be a potential therapeutic drug against JEV infection. These DHEA derivatives may provide lead scaffolds for further design and synthesis of potential anti-flavivirus potential drugs.

Porcine deltacoronavirus enters porcine IPI-2I intestinal epithelial cells via macropinocytosis and clathrin-mediated endocytosis dependent on pH and dynamin

Porcine deltacoronavirus (PDCoV), an emerging enteropathogenic coronavirus, causes serious diarrhoea in suckling piglets and has the potential for cross-species transmission. Although extensive studies have been reported on the biology and pathogenesis of PDCoV, the mechanisms by which PDCoV enters cells are not well characterized. In this study, we investigated how PDCoV enters IPI-2I cells, a line of porcine intestinal epithelial cells derived from pig ileum. Immunofluorescence assays, siRNA interference, specific pharmacological inhibitors and dominant-negative mutation results revealed that PDCoV entry into IPI-2I cells depended on clathrin, dynamin and a low-pH environment, but was independent of caveolae. Specific inhibition of phosphatidylinositol 3-kinase (PI3K) and the Na⁺/H⁺ exchanger (NHE) revealed that PDCoV entry involves macropinocytosis and depends on NHE rather than on PI3K. Additionally, Rab5 and Rab7, but not Rab11, regulated PDCoV endocytosis. This is the first study to demonstrate that PDCoV uses clathrin-mediated endocytosis and macropinocytosis as alternative endocytic pathways to enter porcine intestinal epithelial cells. We also discussed the entry pathways of PDCoV into other porcine cell lines. Our findings reveal the entry mechanisms of PDCoV and provide new insight into the PDCoV life cycle. IMPORTANCE An emerging enteropathogenic coronavirus, PDCoV has the potential for cross-species transmission, attracting extensive attenuation. Characterizing the detailed process of PDCoV entry into cells will deepen our understanding of the viral infection and pathogenesis, and provide the clues for therapeutic intervention against PDCoV. With the objective, we used complementary approaches to dissect the process in PDCoV-infected IPI-2I cells, a line of more physiologically relevant intestinal epithelial cells to PDCoV infection in vivo. Here, we demonstrate that PDCoV enters IPI-2I cells via macropinocytosis that does not require a specific receptor and clathrin-mediated endocytosis that requires a low-pH environment and dynamin, while a caveola-mediated endocytic pathway is used by PDCoV to enter swine testicular (ST) cells and porcine kidney (LLC-PK1) cells. These findings provide a molecular detail of the cellular entry pathways of PDCoV and may direct us toward novel antiviral drug development.

Pathobiology of Japanese encephalitis virus infection

Japanese encephalitis virus (JEV) is a flavivirus, spread by the bite of carrier Culex mosquitoes. The subsequent disease caused is Japanese encephalitis (JE), which is the leading global cause of virus-induced encephalitis. The disease is predominant in the entire Asia-Pacific region with the potential of global spread. JEV is highly neuroinvasive with symptoms ranging from mild fever to severe encephalitis and death. One-third of JE infections are fatal, and half of the survivors develop permanent neurological sequelae. Disease prognosis is determined by a series of complex and intertwined signaling events dictated both by the virus and the host. All flaviviruses, including JEV replicate in close association with ER derived membranes by channelizing the protein and lipid components of the ER. This leads to activation of acute stress responses in the infected cell-oxidative stress, ER stress, and autophagy. The host innate immune and inflammatory responses also enter the fray, the components of which are inextricably linked to the cellular stress responses. These are especially crucial in the periphery for dendritic cell maturation and establishment of adaptive immunity. The pathogenesis of JEV is a combination of direct virus induced neuronal cell death and an uncontrolled neuroinflammatory response. Here we provide a comprehensive review of the JEV life cycle and how the cellular stress responses dictate the pathobiology and resulting immune response. We also deliberate on how modulation of these stress pathways could be a potential strategy to develop therapeutic interventions, and define the persisting challenges.

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.

Membrane Rafts: Portals for Viral Entry

Membrane rafts are dynamic, small (10-200 nm) domains enriched with cholesterol and sphingolipids that compartmentalize cellular processes. Rafts participate in roles essential to the lifecycle of different viral families including virus entry, assembly and/or budding events. Rafts seem to participate in virus attachment and recruitment to the cell surface, as well as the endocytic and non-endocytic mechanisms some viruses use to enter host cells. In this review, we will introduce the specific role of rafts in viral entry and define cellular factors implied in the choice of one entry pathway over the others. Finally, we will summarize the most relevant information about raft participation in the entry process of enveloped and non-enveloped viruses.

Beyond the Surface: Endocytosis of Mosquito-Borne Flaviviruses

Flaviviruses are a group of positive-sense RNA viruses that are primarily transmitted through arthropod vectors and are capable of causing a broad spectrum of diseases. Many of the flaviviruses that are pathogenic in humans are transmitted specifically through mosquito vectors. Over the past century, many mosquito-borne flavivirus infections have emerged and re-emerged, and are of global importance with hundreds of millions of infections occurring yearly. There is a need for novel, effective, and accessible vaccines and antivirals capable of inhibiting flavivirus infection and ameliorating disease. The development of therapeutics targeting viral entry has long been a goal of antiviral research, but most efforts are hindered by the lack of broad-spectrum potency or toxicities associated with on-target effects, since many host proteins necessary for viral entry are also essential for host cell biology. Mosquito-borne flaviviruses generally enter cells by clathrin-mediated endocytosis (CME), and recent studies suggest that a subset of these viruses can be internalized through a specialized form of CME that has additional dependencies distinct from canonical CME pathways, and antivirals targeting this pathway have been discovered. In this review, we discuss the role and contribution of endocytosis to mosquito-borne flavivirus entry as well as consider past and future efforts to target endocytosis for therapeutic interventions.

Caveolae as Potential Hijackable Gates in Cell Communication

Caveolae are membrane microdomains described in many cell types involved in endocytocis, transcytosis, cell signaling, mechanotransduction, and aging. They are found at the interface with the extracellular environment and are structured by caveolin and cavin proteins. Caveolae and caveolins mediate transduction of chemical messages via signaling pathways, as well as non-chemical messages, such as stretching or shear stress. Various pathogens or signals can hijack these gates, leading to infectious, oncogenic and even caveolin-related diseases named caveolinopathies. By contrast, preclinical and clinical research have fallen behind in their attempts to hijack caveolae and caveolins for therapeutic purposes. Caveolae involvement in human disease is not yet fully explored or understood and, of all their scaffold proteins, only caveolin-1 is being considered in clinical trials as a possible biomarker of disease. This review briefly summarizes current knowledge about caveolae cell signaling and raises the hypothesis whether these microdomains could serve as hijackable “gatekeepers” or “gateways” in cell communication. Furthermore, because cell signaling is one of the most dynamic domains in translating data from basic to clinical research, we pay special attention to translation of caveolae, caveolin, and cavin research into clinical practice.

CRISPR screening of porcine sgRNA library identifies host factors associated with Japanese encephalitis virus replication

Japanese encephalitis virus (JEV) is a mosquito-borne zoonotic flavivirus that causes encephalitis and reproductive disorders in mammalian species. However, the host factors critical for its entry, replication, and assembly are poorly understood. Here, we design a porcine genome-scale CRISPR/Cas9 knockout (PigGeCKO) library containing 85,674 single guide RNAs targeting 17,743 protein-coding genes, 11,053 long ncRNAs, and 551 microRNAs. Subsequently, we use the PigGeCKO library to identify key host factors facilitating JEV infection in porcine cells. Several previously unreported genes required for JEV infection are highly enriched post-JEV selection. We conduct follow-up studies to verify the dependency of JEV on these genes, and identify functional contributions for six of the many candidate JEV-related host genes, including EMC3 and CALR. Additionally, we identify that four genes associated with heparan sulfate proteoglycans (HSPGs) metabolism, specifically those responsible for HSPGs sulfurylation, facilitate JEV entry into porcine cells. Thus, beyond our development of the largest CRISPR-based functional genomic screening platform for pig research to date, this study identifies multiple potentially vulnerable targets for the development of medical and breeding technologies to treat and prevent diseases caused by JEV.

Mechanism and Complex Roles of HSC70 in Viral Infections

Heat shock cognate 71-kDa protein (HSC70), a constitutively expressed molecular chaperon within the heat shock protein 70 family, plays crucial roles in maintaining cellular environmental homeostasis through implicating in a wide variety of physiological processes, such as ATP metabolism, protein folding and transporting, antigen processing and presentation, endocytosis, and autophagy. Notably, HSC70 also participates in multiple non-communicable diseases and some pathogen-caused infectious diseases. It is known that virus is an obligatory intracellular parasite and heavily relies on host machineries to self-replication. Undoubtedly, HSC70 is a striking target manipulated by virus to ensure the successful propagation. In this review, we summarize the recent advances of the regulatory mechanisms of HSC70 during viral infections, which will be conducive to further study viral pathogenesis.

Multifaceted Functions of Host Cell Caveolae/Caveolin-1 in Virus Infections

Virus infection has drawn extensive attention since it causes serious or even deadly diseases, consequently inducing a series of social and public health problems. Caveolin-1 is the most important structural protein of caveolae, a membrane invagination widely known for its role in endocytosis and subsequent cytoplasmic transportation. Caveolae/caveolin-1 is tightly associated with a wide range of biological processes, including cholesterol homeostasis, cell mechano-sensing, tumorigenesis, and signal transduction. Intriguingly, the versatile roles of caveolae/caveolin-1 in virus infections have increasingly been appreciated. Over the past few decades, more and more viruses have been identified to invade host cells via caveolae-mediated endocytosis, although other known pathways have been explored. The subsequent post-entry events, including trafficking, replication, assembly, and egress of a large number of viruses, are caveolae/caveolin-1-dependent. Deprivation of caveolae/caveolin-1 by drug application or gene editing leads to abnormalities in viral uptake, viral protein expression, or virion release, whereas the underlying mechanisms remain elusive and must be explored holistically to provide potential novel antiviral targets and strategies. This review recapitulates our current knowledge on how caveolae/caveolin-1 functions in every step of the viral infection cycle and various relevant signaling pathways, hoping to provide a new perspective for future viral cell biology research.

Characterization of Zika Virus Endocytic Pathways in Human Glioblastoma Cells

Zika virus (ZIKV) infections can cause microcephaly and neurological disorders. However, the early infection events of ZIKV in neural cells remain to be characterized. Here, by using a combination of pharmacological and molecular approaches and the human glioblastoma cell T98G as a model, we first observed that ZIKV infection was inhibited by chloroquine and NH₄Cl, indicating a requirement of low intracellular pH. We further showed that dynamin is required as the ZIKV entry was affected by the specific inhibitor dynasore, small interfering RNA (siRNA) knockdown of dynamin, or by expressing the dominant-negative K44A mutant. Moreover, the ZIKV entry was significantly inhibited by chlorpromazine, pitstop2, or siRNA knockdown of clathrin heavy chain, indicating an involvement of clathrin-mediated endocytosis. In addition, genistein treatment, siRNA knockdown of caveolin-1, or overexpression of a dominant-negative caveolin mutant impacted the ZIKV entry, with ZIKV particles being observed to colocalize with caveolin-1, implying that caveola endocytosis can also be involved. Furthermore, we found that the endocytosis of ZIKV is dependent on membrane cholesterol, microtubules, and actin cytoskeleton. Importantly, ZIKV infection was inhibited by silencing of Rab5 and Rab7, while confocal microscopy showed that ZIKV particles localized in Rab5- and Rab7-postive endosomes. These results indicated that, after internalization, ZIKV likely moves to Rab5-positive early endosome and Rab7-positive late endosomes before delivering its RNA into the cytoplasm. Taken together, our study, for the first time, described the early infection events of ZIKV in human glioblastoma cell T98G.

Avian Flavivirus Enters BHK-21 Cells by a Low pH-Dependent Endosomal Pathway

Duck Tembusu virus (DTMUV), a pathogenic member of the Flavivirus family, was first discovered in the coastal provinces of South-Eastern China in 2010. Many previous reports have clearly shown that some Flaviviruses utilize several endocytic pathways to enter the host cells, however, the detailed mechanism of DTMUV entry into BHK-21 cells, which is usually employed to produce commercial veterinary vaccines for DTMUV, as well as of other Flaviviruses by serial passages, is still unknown. In this study, DTMUV entry into BHK-21 cells was found to be inhibited by noncytotoxic concentrations of the agents chloroquine, NH₄Cl, and Bafilomycin A1, which blocked the acidification of the endosomes. Inactivation of virions by acid pretreatment is a hallmark of viruses that utilize a low-pH-mediated entry pathway. Exposure of DTMUV virions to pH 5.0 in the absence of host cell membranes decreased entry into cells by 65%. Furthermore, DTMUV infection was significantly decreased by chlorpromazine treatment, or by knockdown of the clathrin heavy chain (CHC) through RNA interference, which suggested that DTMUV entry depends on clathrin. Taken together, these findings highlight that a low endosomal pH is an important route of entry for DTMUV.

Zika virus: mapping and reprogramming the entry

BACKGROUND

The flaviviridae family comprises single-stranded RNA viruses that enter cells via clathrin-mediated pH-dependent endocytosis. Although the initial events of the virus entry have been already identified, data regarding intracellular virus trafficking and delivery to the replication site are limited. The purpose of this study was to map the transport route of Zika virus and to identify the fusion site within the endosomal compartment.

METHODS

Tracking of viral particles in the cell was carried out with confocal microscopy. Immunostaining of two structural proteins of Zika virus enabled precise mapping of the route of the ribonucleocapsid and the envelope and, consequently, mapping the fusion site in the endosomal compartment. The results were verified using RNAi silencing and chemical inhibitors.

RESULTS

After endocytic internalization, Zika virus is trafficked through the endosomal compartment to fuse in late endosomes. Inhibition of endosome acidification using bafilomycin A1 hampers the infection, as the fusion is inhibited; instead, the virus is transported to late compartments where it undergoes proteolytic degradation. The degradation products are ejected from the cell via slow recycling vesicles. Surprisingly, NH₄Cl, which is also believed to block endosome acidification, shows a very different mode of action. In the presence of this basic compound, the endocytic hub is reprogrammed. Zika virus-containing vesicles never reach the late stage, but are rapidly trafficked to the plasma membrane via a fast recycling pathway after the clathrin-mediated endocytosis. Further, we also noted that, similarly as other members of the flaviviridae family, Zika virus undergoes furin- or furin-like-dependent activation during late steps of infection, while serine or cysteine proteases are not required for Zika virus maturation or entry.

CONCLUSIONS

Zika virus fusion occurs in late endosomes and is pH-dependent. These results broaden our understanding of Zika virus intracellular trafficking and may in future allow for development of novel treatment strategies. Further, we identified a novel mode of action for agents commonly used in studies of virus entry. Schematic representation of differences in ZIKV trafficking in the presence of Baf A1 and NH₄Cl.

Comparative analysis of viral entry for Asian and African lineages of Zika virus

Zika virus (ZIKV) is an emerging pathogen with global health and economic impacts. ZIKV circulates as two major lineages, Asian or African. The Asian lineage has recently been associated with significant disease in humans. Numerous studies have revealed differences between African and Asian ZIKV strains with respect to cellular infectivity, pathogenesis, and immune activation. Less is known about the mechanism of ZIKV entry and whether viral entry differs between strains. Here, we characterized ZIKV entry with two Asian and two African strains. All viruses exhibited a requirement for clathrin-mediated endocytosis and Rab5a function. Additionally, all ZIKV strains tested were sensitive to pH in the range of 6.5-6.1 and were reliant on endosomal acidification for infection. Finally, we provide direct evidence that ZIKV primarily fuses with late endosomes. These findings contribute new insight into the ZIKV entry process and suggest that divergent ZIKV strains enter cells in a highly conserved manner.

Early Events in Japanese Encephalitis Virus Infection: Viral Entry

Japanese encephalitis virus (JEV), a mosquito-borne zoonotic flavivirus, is an enveloped positive-strand RNA virus that can cause a spectrum of clinical manifestations, ranging from mild febrile illness to severe neuroinvasive disease. Today, several killed and live vaccines are available in different parts of the globe for use in humans to prevent JEV-induced diseases, yet no antivirals are available to treat JEV-associated diseases. Despite the progress made in vaccine research and development, JEV is still a major public health problem in southern, eastern, and southeastern Asia, as well as northern Oceania, with the potential to become an emerging global pathogen. In viral replication, the entry of JEV into the cell is the first step in a cascade of complex interactions between the virus and target cells that is required for the initiation, dissemination, and maintenance of infection. Because this step determines cell/tissue tropism and pathogenesis, it is a promising target for antiviral therapy. JEV entry is mediated by the viral glycoprotein E, which binds virions to the cell surface (attachment), delivers them to endosomes (endocytosis), and catalyzes the fusion between the viral and endosomal membranes (membrane fusion), followed by the release of the viral genome into the cytoplasm (uncoating). In this multistep process, a collection of host factors are involved. In this review, we summarize the current knowledge on the viral and cellular components involved in JEV entry into host cells, with an emphasis on the initial virus-host cell interactions on the cell surface.

Rab5, Rab7, and Rab11 Are Required for Caveola-Dependent Endocytosis of Classical Swine Fever Virus in Porcine Alveolar Macrophages

The members of Flaviviridae utilize several endocytic pathways to enter a variety of host cells. Our previous work showed that classical swine fever virus (CSFV) enters porcine kidney (PK-15) cells through a clathrin-dependent pathway that requires Rab5 and Rab7. The entry mechanism for CSFV into other cell lines remains unclear, for instance, porcine alveolar macrophages (3D4/21 cells). More importantly, the trafficking of CSFV within endosomes controlled by Rab GTPases is unknown in 3D4/21 cells. In this study, entry and postinternalization of CSFV were analyzed using chemical inhibitors, RNA interference, and dominant-negative (DN) mutants. Our data demonstrated that CSFV entry into 3D4/21 cells depends on caveolae, dynamin, and cholesterol but not clathrin or macropinocytosis. The effects of DN mutants and knockdown of four Rab proteins that regulate endosomal trafficking were examined on CSFV infection, respectively. The results showed that Rab5, Rab7, and Rab11, but not Rab9, regulate CSFV endocytosis. Confocal microscopy showed that virus particles colocalize with Rab5, Rab7, or Rab11 within 30 min after virus entry and further with lysosomes, suggesting that after internalization CSFV moves to early, late, and recycling endosomes and then into lysosomes before the release of the viral genome. Our findings provide insights into the life cycle of pestiviruses in macrophages.IMPORTANCE Classical swine fever, is caused by classical swine fever virus (CSFV). The disease is notifiable to World Organisation for Animal Health (OIE) in most countries and causes significant financial losses to the pig industry globally. Understanding the processes of CSFV endocytosis and postinternalization will advance our knowledge of the disease and provide potential novel drug targets against CSFV. With this objective, we used systematic approaches to dissect these processes in CSFV-infected 3D4/21 cells. The data presented here demonstrate for the first time to our knowledge that CSFV is able to enter cells via caveola-mediated endocytosis that requires Rab5, Rab7 and Rab11, in addition to the previously described classical clathrin-dependent pathway that requires Rab5 and Rab7. The characterization of CSFV entry will further promote our current understanding of Pestivirus cellular entry pathways and provide novel targets for antiviral drug development.

Rab5 and Rab11 Are Required for Clathrin-Dependent Endocytosis of Japanese Encephalitis Virus in BHK-21 Cells

During infection Japanese encephalitis virus (JEV) generally enters host cells via receptor-mediated clathrin-dependent endocytosis. The trafficking of JEV within endosomes is controlled by Rab GTPases, but which Rab proteins are involved in JEV entry into BHK-21 cells is unknown. In this study, entry and postinternalization of JEV were analyzed using biochemical inhibitors, RNA interference, and dominant negative (DN) mutants. Our data demonstrate that JEV entry into BHK-21 cells depends on clathrin, dynamin, and cholesterol but not on caveolae or macropinocytosis. The effect on JEV infection of dominant negative (DN) mutants of four Rab proteins that regulate endosomal trafficking was examined. Expression of DN Rab5 and DN Rab11, but not DN Rab7 and DN Rab9, significantly inhibited JEV replication. These results were further tested by silencing Rab5 or Rab11 expression before viral infection. Confocal microscopy showed that virus particles colocalized with Rab5 or Rab11 within 15 min after virus entry, suggesting that after internalization JEV moves to early and recycling endosomes before the release of the viral genome. Our findings demonstrate the roles of Rab5 and Rab11 on JEV infection of BHK-21 cells through the endocytic pathway, providing new insights into the life cycle of flaviviruses.IMPORTANCE Although Japanese encephalitis virus (JEV) utilizes different endocytic pathways depending on the cell type being infected, the detailed mechanism of its entry into BHK-21 cells is unknown. Understanding the process of JEV endocytosis and postinternalization will advance our knowledge of JEV infection and pathogenesis as well as provide potential novel drug targets for antiviral intervention. With this objective, we used systematic approaches to dissect this process. The results show that entry of JEV into BHK-21 cells requires a low-pH environment and that the process occurs through dynamin-, actin-, and cholesterol-dependent clathrin-mediated endocytosis that requires Rab5 and Rab11. Our work provides a detailed picture of the entry of JEV into BHK-21 cells and the cellular events that follow.

Integrin αvβ3 promotes infection by Japanese encephalitis virus

Japanese encephalitis virus (JEV) is a mosquito-borne flavivirus that is one of the major causes of viral encephalitis diseases worldwide. The JEV envelope protein facilitates viral entry, and its domain III contains an Arg-Gly-Asp (RGD) motif, that may modulate JEV entry through the RGD-binding integrin. In this study, the roles of integrin αv and β3 on the infection of JEV were evaluated. Reduced expression of integrin αv/β3 by special shRNA confers 2 to 4-fold inhibition of JEV replication in BHK-21 cells. Meanwhile, antibodies specific for integrin αv/β3 displayed ~58% and ~33% inhibition of JEV infectivity and RGD-specific peptides produced ~36% of inhibition. Expression of E protein and JEV RNA loads were clearly increased in CHO cells transfected with cDNA encoding human integrin β3. Moreover, integrin αv mediates JEV infection in viral binding stage of life cycle. Therefore, our study suggested that integrin αv and β3 serve as a host factor associated with JEV entry into the target cells.

Sphingomyelin generated by sphingomyelin synthase 1 is involved in attachment and infection with Japanese encephalitis virus

Japanese encephalitis virus (JEV) is a mosquito-borne RNA virus which infects target cells via the envelope protein JEV-E. However, its cellular targets are largely unknown. To investigate the role of sphingomyelin (SM) in JEV infection, we utilized SM-deficient immortalized mouse embryonic fibroblasts (tMEF) established from SM synthase 1 (SMS1)/SMS2 double knockout mice. SMS deficiency significantly reduced both intracellular and extracellular JEV levels at 48 h after infection. Furthermore, after 15 min treatment with JEV, the early steps of JEV infection such as attachment and cell entry were also diminished in SMS-deficient tMEFs. The inhibition of JEV attachment and infection were recovered by overexpression of SMS1 but not SMS2, suggesting SMS1 contributes to SM production for JEV attachment and infection. Finally, intraperitoneal injection of JEV into SMS1-deficient mice showed an obvious decrease of JEV infection and its associated pathologies, such as meningitis, lymphocyte infiltration, and elevation of interleukin 6, compared with wild type mice. These results suggest that SMS1-generated SM on the plasma membrane is related in JEV attachment and subsequent infection, and may be a target for inhibition of JEV infection.

Entry of Classical Swine Fever Virus into PK-15 Cells via a pH-, Dynamin-, and Cholesterol-Dependent, Clathrin-Mediated Endocytic Pathway That Requires Rab5 and Rab7

Classical swine fever virus (CSFV), a member of the genus Pestivirus within the family Flaviviridae, is a small, enveloped, positive-strand RNA virus. Due to its economic importance to the pig industry, the biology and pathogenesis of CSFV have been investigated extensively. However, the mechanisms of CSFV entry into cells are not well characterized. In this study, we used systematic approaches to dissect CSFV cell entry. We first observed that CSFV infection was inhibited by chloroquine and NH4Cl, suggesting that viral entry required a low-pH environment. By using the specific inhibitor dynasore, or by expressing the dominant negative (DN) K44A mutant, we verified that dynamin is required for CSFV entry. CSFV particles were observed to colocalize with clathrin at 5 min postinternalization, and CSFV infection was significantly reduced by chlorpromazine treatment, overexpression of a dominant negative form of the EPS15 protein, or knockdown of the clathrin heavy chain by RNA interference. These results suggested that CSFV entry depends on clathrin. Additionally, we found that endocytosis of CSFV was dependent on membrane cholesterol, while neither the overexpression of a dominant negative caveolin mutant nor the knockdown of caveolin had an effect. These results further suggested that CSFV entry required cholesterol and not caveolae. Importantly, the effect of DN mutants of three Rab proteins that regulate endosomal traffic on CSFV infection was examined. Expression of DN Rab5 and Rab7 mutants, but not the DN Rab11 mutant, significantly inhibited CSFV replication. These results were confirmed by silencing of Rab5 and Rab7. Confocal microscopy showed that virus particles colocalized with Rab5 or Rab7 during the early phase of infection within 45 min after virus entry. These results indicated that after internalization, CSFV moved to early and late endosomes before releasing its RNA. Taken together, our findings demonstrate for the first time that CSFV enters cells through the endocytic pathway, providing new insights into the life cycle of pestiviruses.

IMPORTANCE

Bovine viral diarrhea virus (BVDV), a single-stranded, positive-sense pestivirus within the family Flaviviridae, is internalized by clathrin-dependent receptor-mediated endocytosis. However, the detailed mechanism of cell entry is unknown for other pestiviruses, such as classical swine fever (CSF) virus (CSFV). CSFV is the etiological agent of CSF, a highly contagious disease of swine that causes numerous deaths in pigs and enormous economic losses in China. Understanding the entry pathway of CSFV will not only advance our knowledge of CSFV infection and pathogenesis but also provide novel drug targets for antiviral intervention. Based on this objective, we used systematic approaches to dissect the pathway of entry of CSFV into PK-15 cells. This is the first report to show that the entry of CSFV into PK-15 cells requires a low-pH environment and involves dynamin- and cholesterol-dependent, clathrin-mediated endocytosis that requires Rab5 and Rab7.

miR-124 attenuates Japanese encephalitis virus replication by targeting DNM2

Background

Japanese encephalitis virus (JEV) is a mosquito-borne flavivirus that causes acute viral encephalitis in humans. Pigs are important amplifier hosts of JEV. Emerging evidence indicates that host microRNAs (miRNAs) play key roles in modulating viral infection and pathogenesis. However, mechanistic studies delineating the roles of miRNAs in regulating host-JEV interactions remain scarce.

Results

In this study, we demonstrated that miR-124 inhibited JEV replication in porcine kidney epithelial PK15 cells. Furthermore, using bioinformatics tools, we identified dynamin2 (DNM2), a GTPase responsible for vesicle scission, as a target of miR-124. Small interfering RNA (siRNA) depletion studies inicated that dynamin2 was required for efficient JEV replication. We also demonstrated that upregulation of miR-124 expression corresponded to decreased expression of its target, DNM2, in the JEV-infected PK15 cells.

Conclusions

Overall, these results suggest the importance of miR-124 in modulating JEV replication and provide a scientific basis for using cellular miRNAs in anti-JEV therapies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-016-0562-y) contains supplementary material, which is available to authorized users.

Hepatitis E virus enters liver cells through a dynamin-2, clathrin and membrane cholesterol-dependent pathway

The hepatitis E virus (HEV) causes large outbreaks and sporadic cases of acute viral hepatitis in developing countries. In the developed world, HEV occurrence has increased as a result of zoonotic transmission from swine. The cellular aspects of HEV infection, especially the determinants of entry, are poorly understood. In the absence of a robust in vitro culture system for HEV, it is not possible to produce high titre infectious virus that can be labeled for tracking its internalization. We have therefore used an Escherichia coli expressed HEV-like particle (HEV-LP) to study HEV entry. Following internalization, the HEV-LP initially trafficks to Rab5-positive compartments en route to acidic lysosomal compartments where it is degraded. Using pharmacological inhibitors, dominant negative and constitutively active mutants, and siRNA-mediated perturbations, we show that HEV entry requires dynamin-2, clathrin, membrane cholesterol and actin, but is independent of factors associated with macropinocytosis. The HEV-LP results were further validated through infection of liver cells with virus from the stool of an infected patient. The comparative analysis also showed involvement of the phosphatidylinositol-3-kinase/Akt pathway in an early post-entry step of viral replication. This report provides a detailed description of endocytic processes associated with HEV infection.

Differential Expression Profile of Chicken Embryo Fibroblast DF-1 Cells Infected with Cell-Adapted Infectious Bursal Disease Virus

RNA-Seq was used to unveil the transcriptional profile of DF-1 cells at the early stage of caIBDV infection. Total RNAs were extracted from virus-infected cells at 0, 6 and 12 hpi. RNA-Seq datasets of respective samples mapped to 56.5–57.6% of isoforms in the reference genome Galgal4.73. At 6 hpi, 23 isoforms underwent an elevated expression, while 128 isoforms were up-regulated and 5 were down-regulated at 12 hpi in the virus-infected group. Besides, 10 isoforms were exclusively expressed in the virus-infected cells. Though no significant change was detected in cytokine and interferon expression levels at the first 12 hours of infection, modulations of the upstream regulators were observed. In addition to the reported regulatory factors including EIF2AK2, MX, OAS*A, GBP7 and IFIT, IBDV infection also triggered a IFIT5-IRF1/3-RSAD5 pathway in the DF-1 cells which potentially restricted the viral replication cycle in the early infection stage. Over-expression of LIPA and CH25H, together with the suppression of STARD4, LSS and AACS genes implied a modulation of membrane fluidity and lipid raft arrangement in the infected cells. Alternative splicing of the EFR3 homolog A gene was also through to be involved in the lipid membrane regulation, and these cumulative responses projected an inhibition of viral endocytosis. Recognition of viral RNA genomes and intermediates was presumably enhanced by the elevated levels of IFIH1, DHX58 and TRIM25 genes which possess properties on detecting viral dsRNA. On the other hand, the caIBDV arrested the host's apoptotic process by inducing the expression of apoptosis inhibitors including NFKBIA/Z, TNFAIP2/3 and ITA at the first 12 hours of infection. In conclusion, the differential expression landscape demonstrated with RNA-Seq provides a comprehensive picture on the molecular interactions between host cells and virus at the early stage of infection.

Identification and analysis of differentially-expressed microRNAs in Japanese encephalitis virus-infected PK-15 cells with deep sequencing

Japanese encephalitis virus (JEV), a mosquito-borne Flavivirus, causes acute viral encephalitis with high morbidity and mortality in humans and animals. MicroRNAs (miRNAs) are small noncoding RNAs that are important modulators of the intricate host-pathogen interaction networks. However, our knowledge of the changes that occur in miRNAs in host cells after JEV infection is still limited. To understand the molecular pathogenesis of JEV at the level of posttranscriptional regulation, we used Illumina deep sequencing to sequence two small RNA libraries prepared from PK-15 cells before and after JEV infection. We identified 522 and 427 miRNAs in the infected and uninfected cells, respectively. Overall, 132 miRNAs were expressed significantly differently after challenge with JEV: 78 were upregulated and 54 downregulated. The sequencing results for selected miRNAs were confirmed with RT-qPCR. GO analysis of the host target genes revealed that these dysregulated miRNAs are involved in complex cellular pathways, including the metabolic pathway, inflammatory response and immune response. To our knowledge, this is the first report of the comparative expression of miRNAs in PK-15 cells after JEV infection. Our findings will underpin further studies of miRNAs' roles in JEV replication and identify potential candidates for antiviral therapies against JEV.

Heat shock cognate protein 70 isoform D is required for clathrin-dependent endocytosis of Japanese encephalitis virus in C6/36 cells

Japanese encephalitis virus (JEV), one of encephalitic flaviviruses, is naturally transmitted by mosquitoes. During infection, JEV generally enters host cells via receptor-mediated clathrin-dependent endocytosis that requires the 70 kDa heat-shock protein (Hsp70). Heat-shock cognate protein 70 (Hsc70) is one member of the Hsp70 family and is constitutively expressed; thus, it may be expressed under physiological conditions. In C6/36 cells, Hsc70 is upregulated in response to JEV infection. Since Hsc70 shows no relationship with viruses attaching to the cell surface, it probably does not serve as the receptor according to our results in the present study. In contrast, Hsc70 is evidently associated with virus penetration into the cell and resultant acidification of intracellular vesicles. It suggests that Hsc70 is highly involved in clathrin-mediated endocytosis, particularly at the late stage of viral entry into host cells. Furthermore, we found that Hsc70 is composed of at least three isoforms, including B, C and D; of these, isoform D helps JEV to penetrate C6/36 cells via clathrin-mediated endocytosis. This study provides relevant evidence that sheds light on the regulatory mechanisms of JEV infection in host cells, especially on the process of clathrin-mediated endocytosis.

Japanese encephalitis virus upregulates the expression of SOCS3 in mouse brain and Raw264.7 Cells

Japanese encephalitis virus (JEV) is one of the pathogens that can invade the central nervous system, causing acute infection and inflammation of brain. SOCS3 protein plays a vital role in immune processes and inflammation of the central nervous system. In this study, Raw264.7 cells and suckling mice were infected with JEV, and SOCS3 expression was analyzed by the gene expression profile, semiquantitative RT-PCR, qRT-PCR, immunohistochemistry (IHC) and Western blot. Results indicated that 520 genes were found to be differentially expressed (fold change ≥ 2.0, p < 0.05) in total. The differentially regulated genes were involved in biological processes, such as stimulus response, biological regulation and immune system processes. JEV early infection could induce SOCS3 expression, upregulating both the mRNA and protein levels in Raw264.7 cells in a time-dependent manner. The SOCS3 expression was much lower in Raw264.7 cells infected with inactivated JEV than wild-type JEV. In vivo, SOCS3 protein was also found to upregulate the expression of mRNA and protein in JEV-infected mouse brain. Taken together, our data showed that JEV early infection could induce the upregulation of SOCS3 expression, both in vitro and in vivo, providing the basic theoretical foundation for future research on the invasion mechanism of JEV.

Voltage dependent anion channel is redistributed during Japanese encephalitis virus infection of insect cells

Despite the availability of an effective vaccine, Japanese encephalitis remains a significant cause of morbidity and mortality in many parts of Asia. Japanese encephalitis is caused by the Japanese encephalitis virus (JEV), a mosquito transmitted flavivirus. Many of the details of the virus replication cycle in mosquito cells remain unknown. This study sought to determine whether GRP78, a well-characterized flavivirus E protein interacting protein, interacted with JEV E protein in insect cells, and whether this interaction was mediated at the cell surface. GRP78 was shown to interact with JEV E protein by coimmunoprecipitation, and was additionally shown to interact with voltage dependent anion protein (VDAC) through the same methodology. Antibody inhibition experiments showed that neither GRP78 nor VDAC played a role in JEV internalization to insect cells. Interestingly, VDAC was shown to be significantly relocalized in response to JEV infection, and significant levels of colocalization between VDAC and GRP78 and VDAC and ribosomal L28 protein were seen in JEV infected but not uninfected cells. This is the first report of relocalization of VDAC in response to JEV infection and suggests that this may be a part of the JEV replication strategy in insect cells.