Cleavage of protein prM is necessary for infection of BHK-21 cells by tick-borne encephalitis virus

Multi-omics analysis of antiviral interactions of Elizabethkingia anophelis and Zika virus

The microbial communities residing in the mosquito midgut play a key role in determining the outcome of mosquito pathogen infection. Elizabethkingia anophelis, originally isolated from the midgut of Anopheles gambiae possess a broad-spectrum antiviral phenotype, yet a gap in knowledge regarding the mechanistic basis of its interaction with viruses exists. The current study aims to identify pathways and genetic factors linked to E. anophelis antiviral activity. The understanding of E. anophelis antiviral mechanism could lead to novel transmission barrier tools to prevent arboviral outbreaks. We utilized a non-targeted multi-omics approach, analyzing extracellular lipids, proteins, metabolites of culture supernatants coinfected with ZIKV and E. anophelis. We observed a significant decrease in arginine and phenylalanine levels, metabolites that are essential for viral replication and progression of viral infection. This study provides insights into the molecular basis of E. anophelis antiviral phenotype. The findings lay a foundation for in-depth mechanistic studies.

The structure of immature tick-borne encephalitis virus supports the collapse model of flavivirus maturation

We present structures of three immature tick-borne encephalitis virus (TBEV) isolates. Our atomic models of the major viral components, the E and prM proteins, indicate that the pr domains of prM have a critical role in holding the heterohexameric prM3E3 spikes in a metastable conformation. Destabilization of the prM furin-sensitive loop at acidic pH facilitates its processing. The prM topology and domain assignment in TBEV is similar to the mosquito-borne Binjari virus, but is in contrast to other immature flavivirus models. These results support that prM cleavage, the collapse of E protein ectodomains onto the virion surface, the large movement of the membrane domains of both E and M, and the release of the pr fragment from the particle render the virus mature and infectious. Our work favors the collapse model of flavivirus maturation warranting further studies of immature flaviviruses to determine the sequence of events and mechanistic details driving flavivirus maturation.

N-linked glycosylation of flavivirus E protein contributes to viral particle formation

In the case of the Japanese encephalitis virus (JEV), the envelope protein (E), a major component of viral particles, contains a highly conserved N-linked glycosylation site (E: N154). Glycosylation of the E protein is thought to play an important role in the ability of the virus to attach to target cells during transmission; however, its role in viral particle formation and release remains poorly understood. In this study, we investigated the role of N-glycosylation of flaviviral structural proteins in viral particle formation and secretion by introducing mutations in viral structural proteins or cellular factors involved in glycoprotein transport and processing. The number of secreted subviral particles (SVPs) was significantly reduced in N154A, a glycosylation-null mutant, but increased in D67N, a mutant containing additional glycosylation sites, indicating that the amount of E glycosylation regulates the release of SVPs. SVP secretion was reduced in cells deficient in galactose, sialic acid, and N-acetylglucosamine modifications in the Golgi apparatus; however, these reductions were not significant, suggesting that glycosylation mainly plays a role in pre-Golgi transport. Fluorescent labeling of SVPs using a split green fluorescent protein (GFP) system and time-lapse imaging by retention using selective hooks (RUSH) system revealed that the glycosylation-deficient mutant was arrested before endoplasmic reticulum (ER)- Golgi transport. However, the absence of ERGIC-53 and ERGIC-L, ER-Golgi transport cargo receptors that recognize sugar chains on cargo proteins, does not impair SVP secretion. In contrast, the solubility of the N154A mutant of E or the N15A/T17A mutant of prM in cells was markedly lower than that of the wild type, and proteasome-mediated rapid degradation of these mutants was observed, indicating the significance of glycosylation of both prM and E in proper protein folding and assembly of viral particles in the ER.

Tick-Borne Encephalitis Virus: A Comprehensive Review of Transmission, Pathogenesis, Epidemiology, Clinical Manifestations, Diagnosis, and Prevention

Tick-borne encephalitis virus (TBEV), a member of the Flaviviridae family, can cause serious infection of the central nervous system in humans, resulting in potential neurological complications and fatal outcomes. TBEV is primarily transmitted to humans through infected tick bites, and the viral agent circulates between ticks and animals, such as deer and small mammals. The occurrence of the infection aligns with the seasonal activity of ticks. As no specific antiviral therapy exists for TBEV infection, treatment approaches primarily focus on symptomatic relief and support. Active immunization is highly effective, especially for individuals in endemic areas. The burden of TBEV infections is increasing, posing a growing health concern. Reported incidence rates rose from 0.4 to 0.9 cases per 100,000 people between 2015 and 2020. The Baltic and Central European countries have the highest incidence, but TBE is endemic across a wide geographic area. Various factors, including social and environmental aspects, improved medical awareness, and advanced diagnostics, have contributed to the observed increase. Diagnosing TBEV infection can be challenging due to the non-specific nature of the initial symptoms and potential co-infections. Accurate diagnosis is crucial for appropriate management, prevention of complications, and effective control measures. In this comprehensive review, we summarize the molecular structure of TBEV, its transmission and circulation in natural environments, the pathogenesis of TBEV infection, the epidemiology and global distribution of the virus, associated risk factors, clinical manifestations, and diagnostic approaches. By improving understanding of these aspects, we aim to enhance knowledge and promote strategies for timely and accurate diagnosis, appropriate management, and the implementation of effective control measures against TBEV infections.

Key Residue in the Precursor Region of M Protein Contributes to the Neurovirulence and Neuroinvasiveness of the African Lineage of Zika Virus

The Zika virus (ZIKV) represents an important global health threat due to its unusual association with congenital Zika syndrome. ZIKV strains are phylogenetically grouped into the African and Asian lineages. However, the viral determinants underlying the phenotypic differences between the lineages remain unknown. Here, multiple sequence alignment revealed a highly conserved residue at position 21 of the premembrane (prM) protein, which is glutamic acid and lysine in the Asian and African lineages, respectively. Using reverse genetics, we generated a recombinant virus carrying an E21K mutation based on the genomic backbone of the Asian lineage strain FSS13025 (termed E21K). The E21K mutation significantly increased viral replication in multiple neural cell lines with a higher ratio of M to prM production. Animal studies showed E21K exhibited increased neurovirulence in suckling mice, leading to more severe defects in mouse brains by causing more neural cell death and destruction of hippocampus integrity. Moreover, the E21K substitution enhanced neuroinvasiveness in interferon alpha/beta (IFN-α/β) receptor knockout mice, as indicated by the increased mortality, and enhanced replication in mouse brains. The global transcriptional analysis showed E21K infection profoundly altered neuron development networks and induced stronger antiviral immune response than wild type (WT) in both neural cells and mouse brains. More importantly, the reverse K21E mutation based on the genomic backbone of the African strain MR766 caused less mouse neurovirulence. Overall, our findings support the 21st residue of prM functions as a determinant for neurovirulence and neuroinvasiveness of the African lineage of ZIKV. IMPORTANCE The suspected link of Zika virus (ZIKV) to birth defects led the World Health Organization to declare ZIKV a Public Health Emergency of International Concern. ZIKV has been identified to have two dominant phylogenetic lineages, African and Asian. Significant differences exist between the two lineages in terms of neurovirulence and neuroinvasiveness in mice. However, the viral determinants underlying the phenotypic differences are still unknown. Here, combining reverse genetics, animal studies, and global transcriptional analysis, we provide evidence that a single E21K mutation of prM confers to the Asian lineage strain FSS130125 significantly enhanced replication in neural cell lines and more neurovirulent and neuroinvasiveness phenotypes in mice. Our findings support that the highly conserved residue at position 21 of prM functions as a determinant of neurovirulence and neuroinvasiveness of the African lineage of ZIKV in mice.

Dengue virus infection - a review of pathogenesis, vaccines, diagnosis and therapy

The transmission of dengue virus (DENV) from an infected Aedes mosquito to a human, causes illness ranging from mild dengue fever to fatal dengue shock syndrome. The similar conserved structure and sequence among distinct DENV serotypes or different flaviviruses has resulted in the occurrence of cross reaction followed by antibody-dependent enhancement (ADE). Thus far, the vaccine which can provide effective protection against infection by different DENV serotypes remains the biggest hurdle to overcome. Therefore, deep investigation is crucial for the potent and effective therapeutic drugs development. In addition, the cross-reactivity of flaviviruses that leads to false diagnosis in clinical settings could result to delay proper intervention management. Thus, the accurate diagnostic with high specificity and sensitivity is highly required to provide prompt diagnosis in respect to render early treatment for DENV infected individuals. In this review, the recent development of neutralizing antibodies, antiviral agents, and vaccine candidates in therapeutic platform for DENV infection will be discussed. Moreover, the discovery of antigenic cryptic epitopes, principle of molecular mimicry, and application of single-chain or single-domain antibodies towards DENV will also be presented.

Molecular pathogenesis of Japanese encephalitis and possible therapeutic strategies

Japanese encephalitis virus (JEV), a single-stranded, enveloped RNA virus, is a health concern across Asian countries, associated with severe neurological disorders, especially in children. Primarily, pigs, bats, and birds are the natural hosts for JEV, but humans are infected incidentally. JEV requires a few host proteins for its entry and replication inside the mammalian host cell. The endoplasmic reticulum (ER) plays a significant role in JEV genome replication and assembly. During this process, the ER undergoes stress due to its remodelling and accumulation of viral particles and unfolded proteins, leading to an unfolded protein response (UPR). Here, we review the overall strategy used by JEV to infect the host cell and various cytopathic effects caused by JEV infection. We also highlight the role of JEV structural proteins (SPs) and non-structural proteins (NSPs) at various stages of the JEV life cycle that are involved in up- and downregulation of different host proteins and are potentially relevant for developing efficient therapeutic drugs.

Increased Cleavage of Japanese Encephalitis Virus prM Protein Promotes Viral Replication but Attenuates Virulence

In flavivirus, the furin-mediated cleavage of prM is mandatory to produce infectious particles, and the immature particles containing uncleaved prM cannot undergo membrane fusion and release to the extracellular environment. However, the detailed relationship between viral replication or pathogenicity and furin in Japanese encephalitis virus (JEV) hasn't been clarified. Here, JEV with the mutations in furin cleavage sites and its nearby were constructed. Compared with WT virus, the mutant virus showed enhanced cleavage efficiency of prM protein and increased replication ability. Furthermore, we found that the mutations mainly promote genomic replication and assembly of JEV. However, the mutant formed smaller plaques than WT virus in plaque forming assay, indicating the lower cytopathogenicity of mutant virus. To assess the virulence of JEV mutant, an in vivo assay was performed using a mouse model. A higher survival rate and attenuated neuroinflammation were observed in JEV mutant-infected mice than those of WT-infected mice, suggesting the cleavage of prM by furin was closely related to viral virulence. These findings will provide new understanding on JEV pathogenesis and contribute to the development of novel JEV vaccines. IMPORTANCE Japanese encephalitis virus (JEV) is the leading cause of viral encephalitis epidemics in Southeast Asia, affecting mostly children, with high morbidity and mortality. During the viral maturation process, prM is cleaved into M by the cellular endoprotease furin in the acidic secretory system. After cleavage of the prM protein, mature virions are exocytosed. Here, the mutant in furin cleavage sites and its nearby was constructed, and the results showed that the mutant virus with enhanced replication mainly occurred in the process of genomic replication and assembly. Meanwhile, the mutant showed an attenuated virulence than WT virus in vivo. Our study contributes to understanding the function of prM and M proteins and provides new clues for live vaccine designation for JEV.

Conserved motifs in the flavivirus NS3 RNA helicase enzyme

Flaviviruses are a major health concern because over half of the world population is at risk of infection and there are very few antiviral therapeutics to treat diseases resulting from infection. Replication is an essential part of the flavivirus survival. One of the viral proteins, NS3 helicase, is critical for unwinding the double stranded RNA intermediate during flaviviral replication. The helicase performs the unwinding of the viral RNA intermediate structure in an ATP-dependent manner. NS3 helicase is a member of the Viral/DEAH-like subfamily of the superfamily 2 helicase containing eight highly conserved structural motifs (I, Ia, II, III, IV, IVa, V, and VI) localized between the ATP-binding and RNA-binding pockets. Of these structural motifs only three are well characterized for function in flaviviruses (I, II, and VI). The roles of the other structural motifs are not well understood for NS3 helicase function, but comparison of NS3 with other superfamily 2 helicases within the viral/DEAH-like, DEAH/RHA, and DEAD-box subfamilies can be used to elucidate the roles of these structural motifs in the flavivirus NS3 helicase. This review aims to summarize the role of each conserved structural motif within flavivirus NS3 in RNA helicase function. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications RNA in Disease and Development > RNA in Disease.

Replication of Dengue Virus in K562-Megakaryocytes Induces Suppression in the Accumulation of Reactive Oxygen Species

Dengue virus can infect human megakaryocytes leading to decreased platelet biogenesis. In this article, we report a study of Dengue replication in human K562 cells undergoing PMA-induced differentiation into megakaryocytes. PMA-induced differentiation in these cells recapitulates steps of megakaryopoiesis including gene activation, expression of CD41/61 and CD61 platelet surface markers and accumulation of intracellular reactive oxygen species (ROS). Our results show differentiating megakaryocyte cells to support higher viral replication without any apparent increase in virus entry. Further, Dengue replication suppresses the accumulation of ROS in differentiating cells, probably by only augmenting the activity of the transcription factor NFE2L2 without influencing the expression of the coding gene. Interestingly pharmacological modulation of NFE2L2 activity showed a simultaneous but opposite effect on intracellular ROS and virus replication suggesting the former to have an inhibitory effect on the later. Also cells that differentiated while supporting intracellular virus replication showed reduced level of surface markers compared to uninfected differentiated cells.

Dengue Virus Serotype 1 Conformational Dynamics Confers Virus Strain-Dependent Patterns of Neutralization by Polyclonal Sera

Dengue virus cocirculates globally as four serotypes (DENV1 to -4) that vary up to 40% at the amino acid level. Viral strains within a serotype further cluster into multiple genotypes. Eliciting a protective tetravalent neutralizing antibody response is a major goal of vaccine design, and efforts to characterize epitopes targeted by polyclonal mixtures of antibodies are ongoing. Previously, we identified two E protein residues (126 and 157) that defined the serotype-specific antibody response to DENV1 genotype 4 strain West Pac-74. DENV1 and DENV2 human vaccine sera neutralized DENV1 viruses incorporating these substitutions equivalently. In this study, we explored the contribution of these residues to the neutralization of DENV1 strains representing distinct genotypes. While neutralization of the genotype 1 strain TVP2130 was similarly impacted by mutation at E residues 126 and 157, mutation of these residues in the genotype 2 strain 16007 did not markedly change neutralization sensitivity, indicating the existence of additional DENV1 type-specific antibody targets. The accessibility of antibody epitopes can be strongly influenced by the conformational dynamics of virions and modified allosterically by amino acid variation. We found that changes at E domain II residue 204, shown previously to impact access to a poorly accessible E domain III epitope, impacted sensitivity of DENV1 16007 to neutralization by vaccine immune sera. Our data identify a role for minor sequence variation in changes to the antigenic structure that impacts antibody recognition by polyclonal immune sera. Understanding how the many structures sampled by flaviviruses influence antibody recognition will inform the design and evaluation of DENV immunogens. IMPORTANCE Dengue virus (DENV) is an important human pathogen that cocirculates globally as four serotypes. Because sequential infection by different DENV serotypes is associated with more severe disease, eliciting a protective neutralizing antibody response against all four serotypes is a major goal of vaccine efforts. Here, we report that neutralization of DENV serotype 1 by polyclonal antibody is impacted by minor sequence variation among virus strains. Our data suggest that mechanisms that control neutralization sensitivity extend beyond variation within antibody epitopes but also include the influence of single amino acids on the ensemble of structural states sampled by structurally dynamic virions. A more detailed understanding of the antibody targets of DENV-specific polyclonal sera and factors that govern their access to antibody has important implications for flavivirus antigen design and evaluation.

Morphological Diversity and Dynamics of Dengue Virus Affecting Antigenicity

The four serotypes of the mature dengue virus can display different morphologies, including the compact spherical, the bumpy spherical and the non-spherical clubshape morphologies. In addition, the maturation process of dengue virus is inefficient and therefore some partially immature dengue virus particles have been observed and they are infectious. All these viral particles have different antigenicity profiles and thus may affect the type of the elicited antibodies during an immune response. Understanding the molecular determinants and environmental conditions (e.g., temperature) in inducing morphological changes in the virus and how potent antibodies interact with these particles is important for designing effective therapeutics or vaccines. Several techniques, including cryoEM, site-directed mutagenesis, hydrogen-deuterium exchange mass spectrometry, time-resolve fluorescence resonance energy transfer, and molecular dynamic simulation, have been performed to investigate the structural changes. This review describes all known morphological variants of DENV discovered thus far, their surface protein dynamics and the key residues or interactions that play important roles in the structural changes.

Neuroinvasiveness of the MR766 strain of Zika virus in IFNAR-/- mice maps to prM residues conserved amongst African genotype viruses

Zika virus (ZIKV) strains are classified into the African and Asian genotypes. The higher virulence of the African MR766 strain, which has been used extensively in ZIKV research, in adult IFNα/β receptor knockout (IFNAR-/-) mice is widely viewed as an artifact associated with mouse adaptation due to at least 146 passages in wild-type suckling mouse brains. To gain insights into the molecular determinants of MR766's virulence, a series of genes from MR766 were swapped with those from the Asian genotype PRVABC59 isolate, which is less virulent in IFNAR-/- mice. MR766 causes 100% lethal infection in IFNAR-/- mice, but when the prM gene of MR766 was replaced with that of PRVABC59, the chimera MR/PR(prM) showed 0% lethal infection. The reduced virulence was associated with reduced neuroinvasiveness, with MR766 brain titers ≈3 logs higher than those of MR/PR(prM) after subcutaneous infection, but was not significantly different in brain titers of MR766 and MR/PR(prM) after intracranial inoculation. MR/PR(prM) also showed reduced transcytosis when compared with MR766 in vitro. The high neuroinvasiveness of MR766 in IFNAR-/- mice could be linked to the 10 amino acids that differ between the prM proteins of MR766 and PRVABC59, with 5 of these changes affecting positive charge and hydrophobicity on the exposed surface of the prM protein. These 10 amino acids are highly conserved amongst African ZIKV isolates, irrespective of suckling mouse passage, arguing that the high virulence of MR766 in adult IFNAR-/- mice is not the result of mouse adaptation.

The Antiviral and Virucidal Activities of Voacangine and Structural Analogs Extracted from Tabernaemontana cymosa Depend on the Dengue Virus Strain

Currently, no specific licensed antiviral exists for treating the illness caused by dengue virus (DENV). Therefore, the search for compounds of natural origin with antiviral activity is an important area of research. In the present study, three compounds were isolated and identified from seeds of Tabernaemontana cymosa plants. The in vitro antiviral effect of those compounds and voacangine against different DENV strains was assessed using different experimental approaches: compounds added before the infection (Pre), at the same time with the virus (Trans), after the infection (Post) or compounds present in all moments of the experiment (Pre-Trans-Post, Combined treatment). In silico studies (docking and molecular dynamics) were also performed to explain the possible antiviral mechanisms. The identified compounds were three structural analogs of voacangine (voacangine-7-hydroxyindolenine, rupicoline and 3-oxo-voacangine). In the Pre-treatment, only voacangine-7-hydroxyindolenine and rupicoline inhibited the infection caused by the DENV-2/NG strain (16.4% and 29.6% infection, respectively). In the Trans-treatment approach, voacangine, voacangine-7-hydroxyindolenine and rupicoline inhibited the infection in both DENV-2/NG (11.2%, 80.4% and 75.7% infection, respectively) and DENV-2/16681 infection models (73.7%, 74.0% and 75.3% infection, respectively). The latter strain was also inhibited by 3-oxo-voacangine (82.8% infection). Moreover, voacangine (most effective virucidal agent) was also effective against one strain of DENV-1 (DENV-1/WestPac/74) and against the third strain of DENV-2 (DENV-2/S16803) (48.5% and 32.4% infection, respectively). Conversely, no inhibition was observed in the post-treatment approach. The last approach (combined) showed that voacangine, voacangine-7-hydroxyindolenine and rupicoline inhibited over 90% of infections (3.5%, 6.9% and 3.5% infection, respectively) of both strains (DENV-2/NG and DENV-2/16681). The free energy of binding obtained with an in silico approach was favorable for the E protein and compounds, which ranged between −5.1 and −6.3 kcal/mol. Finally, the complex formed between DENV-2 E protein and the best virucidal compound was stable for 50 ns. Our results show that the antiviral effect of indole alkaloids derived from T. cymose depends on the serotype and the virus strain.

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.

The structure of an infectious immature flavivirus redefines viral architecture and maturation

Flaviviruses are the cause of severe human diseases transmitted by mosquitoes and ticks. These viruses use a potent fusion machinery to enter target cells that needs to be restrained during viral assembly and egress. A molecular chaperone, premembrane (prM) maintains the virus particles in an immature, fusion-incompetent state until they exit the cell. Taking advantage of an insect virus that produces particles that are both immature and infectious, we determined the structure of the first immature flavivirus with a complete spike by cryo-electron microscopy. Unexpectedly, the prM chaperone forms a supporting pillar that maintains the immature spike in an asymmetric and upright state, primed for large rearrangements upon acidification. The collapse of the spike along a path defined by the prM chaperone is required, and its inhibition by a multivalent immunoglobulin M blocks infection. The revised architecture and collapse model are likely to be conserved across flaviviruses.

Structural and biochemical insights into flavivirus proteins

Flaviviruses are the fastest spreading arthropod-borne viruses that cause severe symptoms such as hepatitis, hemorrhagic fever, encephalitis, and congenital deformities. Nearly 40 % of the entire human population is at risk of flavivirus epidemics. Yet, effective vaccination is restricted only to a few flaviviruses such as yellow fever and Japanese encephalitis viruses, and most recently for select cases of dengue virus infections. Despite the global spread of dengue virus, and emergence of new threats such as Zika virus and a new genotype of Japanese encephalitis virus, insights into flavivirus targets for potentially broad-spectrum vaccination are limited. In this review article, we highlight biochemical and structural differences in flavivirus proteins critical for virus assembly and host interactions. A comparative sequence analysis of pH-responsive properties of viral structural proteins identifies trends in conservation of complementary acidic-basic character between interacting viral structural proteins. This is highly relevant to the understanding of pH-sensitive differences in virus assembly in organelles such as neutral ER and acidic Golgi. Surface residues in viral interfaces identified by structural approaches are shown to demonstrate partial conservation, further reinforcing virus-specificity in assembly and interactions with host proteins. A comparative analysis of epitope conservation in emerging flaviviruses identifies therapeutic antibody candidates that have potential as broad spectrum anti-virals, thus providing a path towards development of vaccines.

Structure-guided paradigm shifts in flavivirus assembly and maturation mechanisms

The flavivirus genus encompasses more than 75 unique viruses, including dengue virus which accounts for almost 390 million global infections annually. Flavivirus infection can result in a myriad of symptoms ranging from mild rash and flu-like symptoms, to severe encephalitis and even hemorrhagic fever. Efforts to combat the impact of these viruses have been hindered due to limited antiviral drug and vaccine development. However, the advancement of knowledge in the structural biology of flaviviruses over the last 25 years has produced unique perspectives for the identification of potential therapeutic targets. With particular emphasis on the assembly and maturation stages of the flavivirus life cycle, it is the goal of this review to comparatively analyze the structural similarities between flaviviruses to provide avenues for new research and innovation.

Castanospermine reduces Zika virus infection-associated seizure by inhibiting both the viral load and inflammation in mouse models

Zika virus (ZIKV) outbreaks have been reported worldwide, including a recent occurrence in Brazil where it spread rapidly, and an association with increased cases of microcephaly was observed in addition to neurological issues such as GBS that were reported during previous outbreaks. Following infection of neuronal tissues, ZIKV can cause inflammation, which may lead to neuronal abnormalities, including seizures and paralysis. Therefore, a drug containing both anti-viral and immunosuppressive properties would be of great importance in combating ZIKV related neurological abnormalities. Castanospermine (CST) is potentially a right candidate drug as it reduced viral load and brain inflammation with the resulting appearance of delayed neuronal disorders, including seizures and paralysis in an Ifnar1^−/− mouse.

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Anti-ZIKV activity of castanospermine (CST) In vivo and in vitro.

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CST reduces ZIKV induced inflammation of brain.

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CST delays the ZIKV induced seizure and improves neuronal disorders such as motor function.

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CST gives marginal improvement in survivability in Ifnar1−/− mice.

N-glycosylation in the Pre-Membrane Protein Is Essential for the Zika Virus Life Cycle

Asparagine (N)-linked protein glycosylation plays an important role in protein synthesis and modification. Two Zika virus (ZIKV) structural proteins, the pre-membrane (prM) and envelope (E) protein are N-glycosylated. The prM protein of all ZIKV strains contains a single N-linked glycosylation site, while not all strains contain an N-linked site in the E protein. Our aim was to examine the impact of prM and E N-linked glycosylation on ZIKV infectivity and cell trafficking. Using a ZIKV infectious clone, we found that when the N-glycan sites were removed, the prM- and the prM/E-double mutants did not produce an infectious virus in the supernatant. Further, by using ZIKV prME constructs, we found that N-glycosylation was necessary for effective secretion of ZIKV virions. The absence of the N-glycan on prM or E caused protein aggregation in the rough endoplasmatic reticulum (ER) compartment. The aggregation was more pronounced for the prM-mutation, and the mutant virus lost the ER-Golgi intermediate compartment (ERGIC) localization. In addition, lack of the N-glycan on prM induced nuclear translocation of CCAAT-enhancer-binding protein homologous protein (CHOP), an ER stress marker. To conclude, we show that the prM N-glycan is essential for the ZIKV infectious cycle, and plays an important role in viral protein trafficking, protein folding, and virion assembly.

Tick-Borne Encephalitis Virus: A Quest for Better Vaccines against a Virus on the Rise

Tick-borne encephalitis virus (TBEV), a member of the family Flaviviridae, is one of the most important tick-transmitted viruses in Europe and Asia. Being a neurotropic virus, TBEV causes infection of the central nervous system, leading to various (permanent) neurological disorders summarized as tick-borne encephalitis (TBE). The incidence of TBE cases has increased due to the expansion of TBEV and its vectors. Since antiviral treatment is lacking, vaccination against TBEV is the most important protective measure. However, vaccination coverage is relatively low and immunogenicity of the currently available vaccines is limited, which may account for the vaccine failures that are observed. Understanding the TBEV-specific correlates of protection is of pivotal importance for developing novel and improved TBEV vaccines. For affording robust protection against infection and development of TBE, vaccines should induce both humoral and cellular immunity. In this review, the adaptive immunity induced upon TBEV infection and vaccination as well as novel approaches to produce improved TBEV vaccines are discussed.

The continued threat of emerging flaviviruses

Flaviviruses are vector-borne RNA viruses that can emerge unexpectedly in human populations and cause a spectrum of potentially severe diseases including hepatitis, vascular shock syndrome, encephalitis, acute flaccid paralysis, congenital abnormalities and fetal death. This epidemiological pattern has occurred numerous times during the last 70 years, including epidemics of dengue virus and West Nile virus, and the most recent explosive epidemic of Zika virus in the Americas. Flaviviruses are now globally distributed and infect up to 400 million people annually. Of significant concern, outbreaks of other less well-characterized flaviviruses have been reported in humans and animals in different regions of the world. The potential for these viruses to sustain epidemic transmission among humans is poorly understood. In this Review, we discuss the basic biology of flaviviruses, their infectious cycles, the diseases they cause and underlying host immune responses to infection. We describe flaviviruses that represent an established ongoing threat to global health and those that have recently emerged in new populations to cause significant disease. We also provide examples of lesser-known flaviviruses that circulate in restricted areas of the world but have the potential to emerge more broadly in human populations. Finally, we discuss how an understanding of the epidemiology, biology, structure and immunity of flaviviruses can inform the rapid development of countermeasures to treat or prevent human infections as they emerge.

Yellow Fever: Integrating Current Knowledge with Technological Innovations to Identify Strategies for Controlling a Re-Emerging Virus

Yellow fever virus (YFV) represents a re-emerging zoonotic pathogen, transmitted by mosquito vectors to humans from primate reservoirs. Sporadic outbreaks of YFV occur in endemic tropical regions, causing a viral hemorrhagic fever (VHF) associated with high mortality rates. Despite a highly effective vaccine, no antiviral treatments currently exist. Therefore, YFV represents a neglected tropical disease and is chronically understudied, with many aspects of YFV biology incompletely defined including host range, host–virus interactions and correlates of host immunity and pathogenicity. In this article, we review the current state of YFV research, focusing on the viral lifecycle, host responses to infection, species tropism and the success and associated limitations of the YFV-17D vaccine. In addition, we highlight the current lack of available treatments and use publicly available sequence and structural data to assess global patterns of YFV sequence diversity and identify potential drug targets. Finally, we discuss how technological advances, including real-time epidemiological monitoring of outbreaks using next-generation sequencing and CRISPR/Cas9 modification of vector species, could be utilized in future battles against this re-emerging pathogen which continues to cause devastating disease.

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

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

Furin-mediated protein processing in infectious diseases and cancer

Proteolytic cleavage regulates numerous processes in health and disease. One key player is the ubiquitously expressed serine protease furin, which cleaves a plethora of proteins at polybasic recognition motifs. Mammalian substrates of furin include cytokines, hormones, growth factors and receptors. Thus, it is not surprising that aberrant furin activity is associated with a variety of disorders including cancer. Furthermore, the enzymatic activity of furin is exploited by numerous viral and bacterial pathogens, thereby enhancing their virulence and spread. In this review, we describe the physiological and pathophysiological substrates of furin and discuss how dysregulation of a simple proteolytic cleavage event may promote infectious diseases and cancer. One major focus is the role of furin in viral glycoprotein maturation and pathogenicity. We also outline cellular mechanisms regulating the expression and activation of furin and summarise current approaches that target this protease for therapeutic intervention.

Tick-borne encephalitis virus: molecular determinants of neuropathogenesis of an emerging pathogen

Tick-borne encephalitis virus (TBEV) is a zoonotic agent causing severe encephalitis. The transmission cycle involves the virus, the Ixodes tick vector, and a vertebrate reservoir, such as small mammals (rodents, or shrews). Humans are accidentally involved in this transmission cycle. Tick-borne encephalitis (TBE) has been a growing public health problem in Europe and Asia over the past 30 years. The mechanisms involved in the development of TBE are very complex and likely multifactorial, involving both host and viral factors. The purpose of this review is to provide an overview of the current literature on TBE neuropathogenesis in the human host and to demonstrate the emergence of common themes in the molecular pathogenesis of TBE in humans. We discuss and review data on experimental study models and on both viral (molecular genetics of TBEV) and host (immune response, and genetic background) factors involved in TBE neuropathogenesis in the context of human infection.

Neutralization of Acidic Intracellular Vesicles by Niclosamide Inhibits Multiple Steps of the Dengue Virus Life Cycle In Vitro

Dengue fever is one of the most important mosquito-borne viral infections in large parts of tropical and subtropical countries and is a significant public health concern and socioeconomic burden. There is an urgent need to develop antivirals that can effectively reduce dengue virus (DENV) replication and decrease viral load. Niclosamide, an antiparasitic drug approved for human use, has been recently identified as an effective antiviral agent against a number of pH-dependent viruses, including flaviviruses. Here, we reveal that neutralization of low-pH intracellular compartments by niclosamide affects multiple steps of the DENV infectious cycle. Specifically, niclosamide-induced endosomal neutralization not only prevents viral RNA replication but also affects the maturation of DENV particles, rendering them non-infectious. We found that niclosamide-induced endosomal neutralization prevented E glycoprotein conformational changes on the virion surface of flaviviruses, resulting in the release of non-infectious immature virus particles with uncleaved pr peptide from host cells. Collectively, our findings support the potential application of niclosamide as an antiviral agent against flavivirus infection and highlight a previously uncharacterized mechanism of action of the drug.

The Many Faces of a Dynamic Virion: Implications of Viral Breathing on Flavivirus Biology and Immunogenicity

Flaviviruses are arthropod-borne RNA viruses that are a significant threat to global health due to their widespread distribution, ability to cause severe disease in humans, and capacity for explosive spread following introduction into new regions. Members of this genus include dengue, tick-borne encephalitis, yellow fever, and Zika viruses. Vaccination has been a highly successful means to control flaviviruses, and neutralizing antibodies are an important component of a protective immune response. High-resolution structures of flavivirus structural proteins and virions, alone and in complex with antibodies, provide a detailed understanding of viral fusion mechanisms and virus-antibody interactions. However, mounting evidence suggests these structures provide only a snapshot of an otherwise structurally dynamic virus particle. The contribution of the structural ensemble arising from viral breathing to the biology, antigenicity, and immunity of flaviviruses is discussed, including implications for the development and evaluation of flavivirus vaccines.

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.

The Roles of prM-E Proteins in Historical and Epidemic Zika Virus-mediated Infection and Neurocytotoxicity

The Zika virus (ZIKV) was first isolated in Africa in 1947. It was shown to be a mild virus that had limited threat to humans. However, the resurgence of the ZIKV in the most recent Brazil outbreak surprised us because it causes severe human congenital and neurologic disorders including microcephaly in newborns and Guillain-Barré syndrome in adults. Studies showed that the epidemic ZIKV strains are phenotypically different from the historic strains, suggesting that the epidemic ZIKV has acquired mutations associated with the altered viral pathogenicity. However, what genetic changes are responsible for the changed viral pathogenicity remains largely unknown. One of our early studies suggested that the ZIKV structural proteins contribute in part to the observed virologic differences. The objectives of this study were to compare the historic African MR766 ZIKV strain with two epidemic Brazilian strains (BR15 and ICD) for their abilities to initiate viral infection and to confer neurocytopathic effects in the human brain’s SNB-19 glial cells, and further to determine which part of the ZIKV structural proteins are responsible for the observed differences. Our results show that the historic African (MR766) and epidemic Brazilian (BR15 and ICD) ZIKV strains are different in viral attachment to host neuronal cells, viral permissiveness and replication, as well as in the induction of cytopathic effects. The analysis of chimeric viruses, generated between the MR766 and BR15 molecular clones, suggests that the ZIKV E protein correlates with the viral attachment, and the C-prM region contributes to the permissiveness and ZIKV-induced cytopathic effects. The expression of adenoviruses, expressing prM and its processed protein products, shows that the prM protein and its cleaved Pr product, but not the mature M protein, induces apoptotic cell death in the SNB-19 cells. We found that the Pr region, which resides on the N-terminal side of prM protein, is responsible for prM-induced apoptotic cell death. Mutational analysis further identified four amino-acid residues that have an impact on the ability of prM to induce apoptosis. Together, the results of this study show that the difference of ZIKV-mediated viral pathogenicity, between the historic and epidemic strains, contributed in part the functions of the structural prM-E proteins.

Brefeldin A and Cytochalasin B reduce dengue virus replication in cell cultures but do not protect mice against viral challenge

BACKGROUND

Dengue is an emerging arboviral disease caused by dengue virus (DENV). DENV belongs to the family Flaviviridae and genus Flavivirus. No specific anti-DENV drugs are currently available.

METHODS

We investigated the antiviral activity of Brefeldin A (BFA) and Cytochalasin B (CB) against this infection. The drugs BFA and CB were used in the in vitro treatment of dengue-2 virus (DENV-2) infections in Vero cell cultures and in protection from lethality by post-challenge administration in Swiss mice. Viral load was quantified by qRT-PCR and plaque assay in Vero cell cultures, post-infection, treated or not with the drugs. Post-challenge drug levels were evaluated by survival analysis.

RESULTS

Our results indicate that doses of 5 µg ml^-1 of BFA and 10 µg ml^-1 of CB are not toxic to the cells and induce a statistically significant inhibition of DENV-2 replication in Vero cells when compared to control. No BFA- or CB-treated mice survived the challenge with DENV-2.

CONCLUSION

These data suggest that BFA and CB have an antiviral action against DENV-2 replication in Vero cell culture, but do not alter infected mice mortality.

What Does the Future Hold for Yellow Fever Virus? (II)

As revealed by the recent resurgence of yellow fever virus (YFV) activity in the tropical regions of Africa and South America, YFV control measures need urgent rethinking. Over the last decade, most reported outbreaks occurred in, or eventually reached, areas with low vaccination coverage but that are suitable for virus transmission, with an unprecedented risk of expansion to densely populated territories in Africa, South America and Asia. As reflected in the World Health Organization's initiative launched in 2017, it is high time to strengthen epidemiological surveillance to monitor accurately viral dissemination, and redefine vaccination recommendation areas. Vector-control and immunisation measures need to be adapted and vaccine manufacturing must be reconciled with an increasing demand. We will have to face more yellow fever (YF) cases in the upcoming years. Hence, improving disease management through the development of efficient treatments will prove most beneficial. Undoubtedly, these developments will require in-depth descriptions of YFV biology at molecular, physiological and ecological levels. This second section of a two-part review describes the current state of knowledge and gaps regarding the molecular biology of YFV, along with an overview of the tools that can be used to manage the disease at the individual, local and global levels.

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.

An Immunopharmacoinformatics Approach in Development of Vaccine and Drug Candidates for West Nile Virus

An outbreak of West Nile Virus (WNV) like the recent Ebola can be more epidemic and fatal to public health throughout the world. WNV possesses utmost threat as no vaccine or drug is currently available for its treatment except mosquito control. The current study applied the combined approach of immunoinformatics and pharmacoinformatics to design potential epitope-based vaccines and drug candidates against WNV. By analyzing the whole proteome of 2994 proteins, the WNV envelope glycoprotein was selected as a therapeutic target based on its highest antigenicity. After proper assessment “KSFLVHREW” and “ITPSAPSYT” were found to be the most potential T and B-cell epitopes, respectively. Besides, we have designed and validated four novel drugs from a known WNV inhibitor, AP30451 by adopting computational approaches. Toxicity assessment and drug score confirmed the effectiveness of these drug candidates. This in silico research might greatly facilitate the wet lab experiments to develop vaccine and drug against WNV.

Tick-Borne Encephalitis Virus: A Structural View

Tick-borne encephalitis virus (TBEV) is a growing health concern. It causes a severe disease that can lead to permanent neurological complications or death and the incidence of TBEV infections is constantly rising. Our understanding of TBEV’s structure lags behind that of other flaviviruses, but has advanced recently with the publication of a high-resolution structure of the TBEV virion. The gaps in our knowledge include: aspects of receptor binding, replication and virus assembly. Furthermore, TBEV has mostly been studied in mammalian systems, even though the virus’ interaction with its tick hosts is a central part of its life cycle. Elucidating these aspects of TBEV biology are crucial for the development of TBEV antivirals, as well as the improvement of diagnostics. In this review, we summarise the current structural knowledge on TBEV, bringing attention to the current gaps in our understanding, and propose further research that is needed to truly understand the structural-functional relationship of the virus and its hosts.

Tick-Borne Flaviviruses and the Type I Interferon Response

Flaviviruses are globally distributed pathogens causing millions of human infections every year. Flaviviruses are arthropod-borne viruses and are mainly transmitted by either ticks or mosquitoes. Mosquito-borne flaviviruses and their interactions with the innate immune response have been well-studied and reviewed extensively, thus this review will discuss tick-borne flaviviruses and their interactions with the host innate immune response.

Probing Molecular Insights into Zika Virus⁻Host Interactions

The recent Zika virus (ZIKV) outbreak in the Americas surprised all of us because of its rapid spread and association with neurologic disorders including fetal microcephaly, brain and ocular anomalies, and Guillain–Barré syndrome. In response to this global health crisis, unprecedented and world-wide efforts are taking place to study the ZIKV-related human diseases. Much has been learned about this virus in the areas of epidemiology, genetic diversity, protein structures, and clinical manifestations, such as consequences of ZIKV infection on fetal brain development. However, progress on understanding the molecular mechanism underlying ZIKV-associated neurologic disorders remains elusive. To date, we still lack a good understanding of; (1) what virologic factors are involved in the ZIKV-associated human diseases; (2) which ZIKV protein(s) contributes to the enhanced viral pathogenicity; and (3) how do the newly adapted and pandemic ZIKV strains alter their interactions with the host cells leading to neurologic defects? The goal of this review is to explore the molecular insights into the ZIKV–host interactions with an emphasis on host cell receptor usage for viral entry, cell innate immunity to ZIKV, and the ability of ZIKV to subvert antiviral responses and to cause cytopathic effects. We hope this literature review will inspire additional molecular studies focusing on ZIKV–host Interactions.

Structure and function of Zika virus NS5 protein: perspectives for drug design

Zika virus (ZIKV) belongs to the positive-sense single-stranded RNA-containing Flaviviridae family. Its recent outbreak and association with human diseases (e.g. neurological disorders) have raised global health concerns, and an urgency to develop a therapeutic strategy against ZIKV infection. However, there is no currently approved antiviral against ZIKV. Here we present a comprehensive overview on recent progress in structure-function investigation of ZIKV NS5 protein, the largest non-structural protein of ZIKV, which is responsible for replication of the viral genome, RNA capping and suppression of host interferon responses. Structural comparison of the N-terminal methyltransferase domain and C-terminal RNA-dependent RNA polymerase domain of ZIKV NS5 with their counterparts from related viruses provides mechanistic insights into ZIKV NS5-mediated RNA replication, and identifies residues critical for its enzymatic activities. Finally, a collection of recently identified small molecule inhibitors against ZIKV NS5 or its closely related flavivirus homologues are also discussed.

The structural proteins of epidemic and historical strains of Zika virus differ in their ability to initiate viral infection in human host cells

Mosquito-borne Zika virus (ZIKV) recently emerged in South Pacific islands and Americas where large epidemics were documented. In the present study, we investigated the contribution of the structural proteins C, prM and E in the permissiveness of human host cells to epidemic strains of ZIKV. To this end, we evaluated the capacity of the epidemic strain BeH819015 to infect epithelial A549 and neuronal SH-SY5Y cells in comparison to the African historical MR766 strain. For that purpose, we generated a molecular clone of BeH819015 and a chimeric clone of MR766 which contains the BeH819015 structural protein region. We showed that ZIKV containing BeH819015 structural proteins was much less efficient in cell-attachment leading to a reduced susceptibility of A549 and SH-SY5Y cells to viral infection. Our data illustrate a previously underrated role for C, prM, and E in ZIKV epidemic strain ability to initiate viral infection in human host cells.

Structure of tick-borne encephalitis virus and its neutralization by a monoclonal antibody

Tick-borne encephalitis virus (TBEV) causes 13,000 cases of human meningitis and encephalitis annually. However, the structure of the TBEV virion and its interactions with antibodies are unknown. Here, we present cryo-EM structures of the native TBEV virion and its complex with Fab fragments of neutralizing antibody 19/1786. Flavivirus genome delivery depends on membrane fusion that is triggered at low pH. The virion structure indicates that the repulsive interactions of histidine side chains, which become protonated at low pH, may contribute to the disruption of heterotetramers of the TBEV envelope and membrane proteins and induce detachment of the envelope protein ectodomains from the virus membrane. The Fab fragments bind to 120 out of the 180 envelope glycoproteins of the TBEV virion. Unlike most of the previously studied flavivirus-neutralizing antibodies, the Fab fragments do not lock the E-proteins in the native-like arrangement, but interfere with the process of virus-induced membrane fusion.

The tick-borne encephalitis virus (TBEV) causes thousands of cases of meningitis and encephalitis annually. Here, the authors describe a cryo-EM structure of the TBEV virion bound by Fab fragments of the neutralizing antibody 19/1786, revealing a mechanism whereby this antibody prevents virus membrane fusion.

The Development of Peptide-based Antimicrobial Agents against Dengue Virus

Dengue fever has become an imminent threat to international public health because of global warming and climate change. The World Health Organization proclaimed that more than 50% of the world's population is at risk of dengue virus (DENV) infection. Therefore, developing a clinically approved vaccine and effective therapeutic remedy for treating dengue fever is imperative. Peptide drug development has become a novel pharmaceutical research field. This article reviews various peptidesbased antimicrobial agents targeting three pathways involved in the DENV lifecycle. Specifically, they are peptide vaccines from immunomodulation, peptide drugs that inhibit virus entry, and peptide drugs that interfere with viral replication. Many antiviral peptide studies against DENV have been conducted in animal model trials, and progression to clinical trials for these promising peptide drugs is anticipated.

[Tick-borne encephalitis]

Tick-borne encephalitis virus (TBEV) belongs to the Flaviviridae family and Flavivirus genus. TBEV is maintained in transmission cycles between Ixodid ticks and wild mammalian hosts, particularly rodents. A wide range of animal species are also infected with TBEV by the bite of infected ticks, and TBEV infection causes fatal encephalitis in humans. TBEV is endemic widely in the Eurasian continent, and more than 10,000 cases of the disease are reported annually. In Japan, the 1st confirmed case of TBE was reported in the southern area of Hokkaido in 1993, and after 20 years, the 2nd to 4th cases were reported in Hokkaido in 2016 and 2017. Our sero-epizootiological survey indicated endemic foci of TBEV are widely distributed in Hokkaido and that those of TBEV or tick-borne flavivirus outside Hokkaido. In this review, I introduced recent topics of TBEV including newly developed diagnostic methods, epidemiology and pathogenesis of TBEV.

Investigating Tick-borne Flaviviral-like Particles as a Delivery System for Gene Therapy

Background

Research on the biogenesis of tick-borne encephalitis virus (TBEV) would benefit gene therapy. Due to specific arrangements of genes along the TBEV genome, its viral-like particles (VLPs) could be exploited as shuttles to deliver their replicon, which carries therapeutic genes, to immune system cells.

Objective

To develop a flaviviral vector for gene delivery as a part of gene therapy research that can be expressed in secretable VLP suicidal shuttles and provide abundant unique molecular and structural data supporting this gene therapy concept.

Method

TBEV structural gene constructs of a Swedish Torö strain were cloned into plasmids driven by the promoters CAG and CMV and then transfected into various cell lines, including COS-1 and BHK-21. Time-course sampling of the cells, culture fluid, cell lysate supernatant, and pellet specimens were performed. Western blotting and electron microscopy analyses of collected specimens were used to investigate molecular and structural processing of TBEV structural proteins.

Results

Western blotting analysis showed differences between promoters in directing the gene expression of the VLPs constructs. The premature flaviviral polypeptides as well as mature VLPs could be traced. Using electron microscopy, the premature and mature VLP accumulation in cellular compartments—and also endoplasmic reticulum proliferation as a virus factory platform—were observed in addition to secreted VLPs.

Conclusions

The abundant virologic and cellular findings in this study show the natural processing and safety of inserting flaviviral structural genes into suicidal VLP shuttles. Thus, we propose that these VLPs are a suitable gene delivering system model in gene therapy.

Zika virus: An emerging flavivirus

Zika virus (ZIKV) is a previously little-known flavivirus closely related to Japanese encephalitis, West Nile, dengue, and yellow fever viruses, all of which are primarily transmitted by blood-sucking mosquitoes. Since its discovery in Uganda in 1947, ZIKV has continued to expand its geographic range, from equatorial Africa and Asia to the Pacific Islands, then further afield to South and Central America and the Caribbean. Currently, ZIKV is actively circulating not only in much of Latin America and its neighbors but also in parts of the Pacific Islands and Southeast Asia. Although ZIKV infection generally causes only mild symptoms in some infected individuals, it is associated with a range of neuroimmunological disorders, including Guillain-Barré syndrome, meningoencephalitis, and myelitis. Recently, maternal ZIKV infection during pregnancy has been linked to neonatal malformations, resulting in various degrees of congenital abnormalities, microcephaly, and even abortion. Despite its emergence as an important public health problem, however, little is known about ZIKV biology, and neither vaccine nor drug is available to control ZIKV infection. This article provides a brief introduction to ZIKV with a major emphasis on its molecular virology, in order to help facilitate the development of diagnostics, therapeutics, and vaccines.

Characterization of cytopathic factors through genome-wide analysis of the Zika viral proteins in fission yeast

The Zika virus (ZIKV) causes microcephaly and the Guillain-Barré syndrome. Little is known about how ZIKV causes these conditions or which ZIKV viral protein(s) is responsible for the associated ZIKV-induced cytopathic effects, including cell hypertrophy, growth restriction, cell-cycle dysregulation, and cell death. We used fission yeast for the rapid, global functional analysis of the ZIKV genome. All 14 proteins or small peptides were produced under an inducible promoter, and we measured the intracellular localization and the specific effects on ZIKV-associated cytopathic activities of each protein. The subcellular localization of each ZIKV protein was in overall agreement with its predicted protein structure. Five structural and two nonstructural ZIKV proteins showed various levels of cytopathic effects. The expression of these ZIKV proteins restricted cell proliferation, induced hypertrophy, or triggered cellular oxidative stress leading to cell death. The expression of premembrane protein (prM) resulted in cell-cycle G1 accumulation, whereas membrane-anchored capsid (anaC), membrane protein (M), envelope protein (E), and nonstructural protein 4A (NS4A) caused cell-cycle G2/M accumulation. A mechanistic study revealed that NS4A-induced cellular hypertrophy and growth restriction were mediated specifically through the target of rapamycin (TOR) cellular stress pathway involving Tor1 and type 2A phosphatase activator Tip41. These findings should provide a reference for future research on the prevention and treatment of ZIKV diseases.

Deconstructing the Antiviral Neutralizing-Antibody Response: Implications for Vaccine Development and Immunity

The antibody response plays a key role in protection against viral infections. While antiviral antibodies may reduce the viral burden via several mechanisms, the ability to directly inhibit (neutralize) infection of cells has been extensively studied. Eliciting a neutralizing-antibody response is a goal of many vaccine development programs and commonly correlates with protection from disease. Considerable insights into the mechanisms of neutralization have been gained from studies of monoclonal antibodies, yet the individual contributions and dynamics of the repertoire of circulating antibody specificities elicited by infection and vaccination are poorly understood on the functional and molecular levels. Neutralizing antibodies with the most protective functionalities may be a rare component of a polyclonal, pathogen-specific antibody response, further complicating efforts to identify the elements of a protective immune response. This review discusses advances in deconstructing polyclonal antibody responses to flavivirus infection or vaccination. Our discussions draw comparisons to HIV-1, a virus with a distinct structure and replication cycle for which the antibody response has been extensively investigated. Progress toward deconstructing and understanding the components of polyclonal antibody responses identifies new targets and challenges for vaccination strategies.

Enhancing dengue virus maturation using a stable furin over-expressing cell line

Flaviviruses are positive-stranded RNA viruses that incorporate envelope (E) and premembrane (prM) proteins into the virion. Furin-mediated cleavage of prM defines a required maturation step in the flavivirus lifecycle. Inefficient prM cleavage results in structurally heterogeneous virions with unique antigenic and functional characteristics. Recent studies with dengue virus suggest that viruses produced in tissue culture cells are less mature than those produced in primary cells. In this study, we describe a Vero cell line that ectopically expresses high levels of human furin (Vero-furin) for use in the production of more homogenous mature flavivirus populations. Laboratory-adapted and clinical dengue virus isolates grow efficiently in Vero-furin cells. Biochemical and structural techniques demonstrate efficient prM cleavage in Vero-furin derived virus preparations. These virions also were less sensitive to neutralization by antibodies that bind efficiently to immature virions. This furin-expressing cell line will be of considerable utility for flavivirus neutralization and structural studies.

Kissing-loop interaction between 5' and 3' ends of tick-borne Langat virus genome 'bridges the gap' between mosquito- and tick-borne flaviviruses in mechanisms of viral RNA cyclization: applications for virus attenuation and vaccine development

Insertion of microRNA target sequences into the flavivirus genome results in selective tissue-specific attenuation and host-range restriction of live attenuated vaccine viruses. However, previous strategies for miRNA-targeting did not incorporate a mechanism to prevent target elimination under miRNA-mediated selective pressure, restricting their use in vaccine development. To overcome this limitation, we developed a new approach for miRNA-targeting of tick-borne flavivirus (Langat virus, LGTV) in the duplicated capsid gene region (DCGR). Genetic stability of viruses with DCGR was ensured by the presence of multiple cis-acting elements within the N-terminal capsid coding region, including the stem-loop structure (5′SL6) at the 3′ end of the promoter. We found that the 5′SL6 functions as a structural scaffold for the conserved hexanucleotide motif at its tip and engages in a complementary interaction with the region present in the 3′ NCR to enhance viral RNA replication. The resulting kissing-loop interaction, common in tick-borne flaviviruses, supports a single pair of cyclization elements (CYC) and functions as a homolog of the second pair of CYC that is present in the majority of mosquito-borne flaviviruses. Placing miRNA targets into the DCGR results in superior attenuation of LGTV in the CNS and does not interfere with development of protective immunity in immunized mice.

A Single Amino Acid Substitution in the M Protein Attenuates Japanese Encephalitis Virus in Mammalian Hosts

Japanese encephalitis virus (JEV) membrane (M) protein plays important structural roles in the processes of fusion and maturation of progeny virus during cellular infection. The M protein is anchored in the viral membrane, and its ectodomain is composed of a flexible N-terminal loop and a perimembrane helix. In this study, we performed site-directed mutagenesis on residue 36 of JEV M protein and showed that the resulting mutation had little or no effect on the entry process but greatly affected virus assembly in mammalian cells. Interestingly, this mutant virus had a host-dependent phenotype and could develop a wild-type infection in insect cells. Experiments performed on infectious virus as well as in a virus-like particle (VLP) system indicate that the JEV mutant expresses structural proteins but fails to form infectious particles in mammalian cells. Using a mouse model for JEV pathogenesis, we showed that the mutation conferred complete attenuation in vivo. The production of JEV neutralizing antibodies in challenged mice was indicative of the immunogenicity of the mutant virus in vivo. Together, our results indicate that the introduction of a single mutation in the M protein, while being tolerated in insect cells, strongly impacts JEV infection in mammalian hosts.

IMPORTANCE

JEV is a mosquito-transmitted flavivirus and is a medically important pathogen in Asia. The M protein is thought to be important for accommodating the structural rearrangements undergone by the virion during viral assembly and may play additional roles in the JEV infectious cycle. In the present study, we show that a sole mutation in the M protein impairs the JEV infection cycle in mammalian hosts but not in mosquito cells. This finding highlights differences in flavivirus assembly pathways among hosts. Moreover, infection of mice indicated that the mutant was completely attenuated and triggered a strong immune response to JEV, thus providing new insights for further development of JEV vaccines.