Beyond the Surface: Endocytosis of Mosquito-Borne Flaviviruses

Advances in antiviral strategies targeting mosquito-borne viruses: cellular, viral, and immune-related approaches

Mosquito-borne viruses (MBVs) are a major global health threat, causing significant morbidity and mortality. MBVs belong to several distinct viral families, each with unique characteristics. The primary families include Flaviviridae (e.g., Dengue, Zika, West Nile, Yellow Fever, Japanese Encephalitis), transmitted predominantly by Aedes and Culex mosquitoes; Togaviridae, which consists of the genus Alphavirus (e.g., Chikungunya, Eastern and Western Equine Encephalitis viruses), also transmitted by Aedes and Culex; Bunyaviridae (recently reorganized), containing viruses like Rift Valley Fever and Oropouche virus, transmitted by mosquitoes and sometimes sandflies; and Reoviridae, which includes the genus Orbivirus (e.g., West Nile and Bluetongue viruses), primarily affecting animals and transmitted by mosquitoes and sandflies. Despite extensive research, effective antiviral treatments for MBVs remain scarce, and current therapies mainly provide symptomatic relief and supportive care. This review examines the viral components and cellular and immune factors involved in the life cycle of MBVs. It also highlights recent advances in antiviral strategies targeting host factors such as lipid metabolism, ion channels, and proteasomes, as well as viral targets like NS2B-NS3 proteases and nonstructural proteins. Additionally, it explores immunomodulatory therapies to enhance antiviral responses and emphasizes the potential of drug repurposing, bioinformatics, artificial intelligence, and deep learning in identifying novel antiviral candidates. Continued research is crucial in mitigating MBVs' impact and preventing future outbreaks.

Comparison of Three Chimeric Zika Vaccine Prototypes Developed on the Genetic Background of the Clinically Proven Live-Attenuated Japanese Encephalitis Vaccine SA14-14-2

Zika virus (ZIKV) is a medically important mosquito-borne orthoflavivirus, but no vaccines are currently available to prevent ZIKV-associated disease. In this study, we compared three recombinant chimeric viruses developed as candidate vaccine prototypes (rJEV/ZIKVMR-766, rJEV/ZIKVP6-740, and rJEV/ZIKVPRVABC-59), in which the two neutralizing antibody-inducing prM and E genes from each of three genetically distinct ZIKV strains were used to replace the corresponding genes of the clinically proven live-attenuated Japanese encephalitis virus vaccine SA14-14-2 (rJEV). In WHO-certified Vero cells (a cell line suitable for vaccine production), rJEV/ZIKVP6-740 exhibited the slowest viral growth, formed the smallest plaques, and displayed a unique protein expression profile with the highest ratio of prM to cleaved M when compared to the other two chimeric viruses, rJEV/ZIKVMR-766 and rJEV/ZIKVPRVABC-59, as well as their vector, rJEV. In IFNAR-/- mice, an animal model of ZIKV infection, subcutaneous inoculation of rJEV/ZIKVP6-740 caused a low-level localized infection limited to the spleen, with no clinical signs of infection, weight loss, or mortality; in contrast, the other two chimeric viruses and their vector caused high-level systemic infections involving multiple organs, consistently leading to clear clinical signs of infection, rapid weight loss, and 100% mortality. Subsequently, subcutaneous immunization with rJEV/ZIKVP6-740 proved highly effective, offering complete protection against a lethal intramuscular ZIKV challenge 28 days after a single-dose immunization. This protection was specific to ZIKV prM/E and likely mediated by neutralizing antibodies targeting ZIKV prM/E. Therefore, our data indicate that the chimeric virus rJEV/ZIKVP6-740 is a highly promising vaccine prototype for developing a safe and effective vaccine for inducing neutralizing antibody-mediated protective immunity against ZIKV.

Insights into mosquito-borne arbovirus receptors

The increasing global prevalence of mosquito-borne viruses has emerged as a significant threat to human health and life. Identifying receptors for these viruses is crucial for improving our knowledge of viral pathogenesis and developing effective antiviral strategies. The widespread application of CRISPR-Cas9 screening have led to the discovery of many mosquito-borne virus receptors. The revealed structures of virus-receptor complexes also provide important information for understanding their interaction mechanisms. This review provides a comprehensive summary of both conventional and novel approaches for identifying new viral receptors and the putative entry factors of the most prevalent mosquito-borne viruses within the Flaviviridae, Togaviridae, and Bunyavirales. At the same time, we emphasize the common receptors utilized by these viruses for entry into both vertebrate hosts and mosquito vectors. We discuss promising avenues for developing anti-mosquito-borne viral strategies that target these receptors. Notably, targeting universal receptors of specific mosquito-borne viruses in both vertebrates and mosquitoes offers dual benefits for disease prevention. Additionally, the widespread use of AI-based machine learning and protein structure prediction will accelerate the identification of new viral receptors and provide new avenues for antiviral drug discovery.

Appraising the life-course impact of Epstein-Barr virus exposure and its genetic signature on periodontitis

BACKGROUND

Periodontitis arises from a multifaceted interplay of environmental variables and genetic susceptibility, where microbial infection plays an indispensable part. Epstein-Barr virus (EBV) exposure has long been considered associated with periodontitis activity; however, the causal relationship and genetic connection between them remain unknown.

METHODS

Within a life-course context, our study employed comprehensive Mendelian randomization (MR) methods, including univariable, multivariable, Bayesian model averaging, and reverse MR, to investigate the causal association between EBV exposure and periodontitis. Additionally, linkage disequilibrium score regression and colocalization analysis were utilized to assess the cross-trait genetic correlations, followed by transcriptome-wide association and enrichment analysis to discern the genetic-phenotypic biological profiles.

RESULTS

Heightened levels of EBV antibodies, particularly early antigen diffuses (which serve as indicators of early infection or reactivation), are associated with an increased risk of periodontitis (odds ratio [OR]: 1.27 [1.09-1.47], p = 6.05 × 10-3) and demonstrate a significant genetic correlation (p = 4.11 × 10-3). This pathogenesis may involve the high-confidence causal gene RNASEK located in 17p13.1. Genetically predicted early-life anti-EBV immunoglobulin G (IgG) levels are correlated to a reduced periodontitis risk (OR: 0.89 [0.82-0.97], p = 1.76 × 10-3).

CONCLUSIONS

The present study highlights the impact of life-course EBV exposure and its genetic hallmark on periodontitis, providing novel perspectives into the underlying pathogenesis and management strategies for EBV-related periodontitis. These findings underscore diverse clinical and public health implications, encompassing antiviral therapies, viral vaccination strategies, and tailored interventions for individualized periodontitis management. Further research is required to validate and expand upon our findings.

PLAIN LANGUAGE SUMMARY

Periodontitis is a chronic inflammatory disease driven by interactions between microbial pathogens and the host immune system. While bacteria have traditionally been the focus of research, recent studies highlight the significance of virus-bacteria interactions, particularly the role of Epstein-Barr virus (EBV)-a herpesvirus infecting over 90% of the global population-in the development of periodontitis. However, the underlying causal and genetic mechanisms remain unclear. Our study employed genome-wide multi-omics approaches to investigate the link between EBV exposure and periodontitis. We found that recent EBV infection or reactivation increases the risk of periodontitis, whereas early-life exposure, possibly enabling immune resistance, may reduce it. Essential genes were identified as potential mediators, including CRTC3-AS1, HLA-DQA1, and RNASEK. These findings provide novel insights into the EBV-periodontitis connection. For example, viral testing and control could benefit patients unresponsive to standard bacterial treatments, and early viral exposure via vaccination might reduce the risk of periodontitis. Further clinical studies are required to elucidate these underlying mechanisms and the contribution of virus-bacteria interactions.

Effects of the oxoaporphine alkaloid hernandonine on dengue virus. Evidence for its mechanisms of action

BACKGROUND

Dengue, caused by the dengue virus (Orthoflavivirus dengue, encompassing DENV types 1-4), is a member of the Flaviviridae family. The symptoms of dengue range from subclinical or mild manifestations to potentially fatal complications. The management of severe dengue is exceptionally challenging due to the absence of effective antiviral medications. In this context, natural products, whether in the form of pure compounds or standardized plant extracts, have emerged as a promising source for the development of novel antiviral therapeutics. Hernandonine, an oxoaporphine alkaloid found in Hernandia nymphaeifolia (C. Presl) Kubitzki. serves both as a metabolite and an inhibitor of human immunodeficiency virus type 1 (HIV-1) integrase.

PURPOSE

This study investigated the ability of hernandonine to inhibit DENV infection and explored its potential mechanisms.

STUDY DESIGN

To assess the in vitro anti-DENV activity, cells or induced pluripotent stem cell (iPSC)-derived cerebral organoids were exposed to hernandonine before or after infection with DENV. Along with hernandonine, the endocytosis modulators, genistein, wortmannin, Methyl-β-cyclodextrin (MβCD) and lovastatin, were used in the assays.

METHODS

The DENV infectivity and virion production in cells or cerebral organoids treated with compounds were determined. Various methods, including cell and cerebral organoids imaging, protein and gene detection were conducted to explore their antiviral mechanisms.

RESULTS

The results revealed notable antiviral properties of hernandonine, particularly in inhibiting DENV during the early stages of infection. Mechanistic analysis demonstrated that, akin to genistein, wortmannin, methyl-β-cyclodextrin (MβCD), and lovastatin, hernandonine exerted an influence on cholesterol-rich lipid rafts. It also restrained the pseudopodial movement ability of cells, potentially through the downregulation of cytoskeleton and endocytosis regulatory genes or protein expression. Moreover, hernandonine's virucidal activity was demonstrated. Hernandonine's inhibition of DENV infection was further validated in a disease-relevant iPSC-derived cerebral organoids model, a novel DENV-2 infection system worthy of further application.

CONCLUSION

This study evidenced the potential of hernandonine as a novel candidate in the fight against DENV infection.

Anti-influenza activity of CPAVM1 protease secreted by Bacillus subtilis LjM2

Bacillus spp. has been considered a promising source for identifying new antimicrobial substances, including anti-viral candidates. Here, we successfully isolated a number of bacteria strains from aged dry citrus peel (Chenpi). Of note, the culture supernatant of a new isolate named Bacillus subtilis LjM2 demonstrated strong inhibition of influenza A virus (IAV) infection in multiple experimental systems in vitro and in vivo. In addition, the anti-viral effect of LjM2 was attributed to its direct lysis of viral particles. Further analysis showed that a protease which we named CPAVM1 isolated from the culture supernatant of LjM2 was the key component responsible for its anti-viral function. Importantly, the therapeutic effect of CPAVM1 was still significant when applied 12 hours after IAV infection of experimental mice. Moreover, we found that the CPAVM1 protease cleaved multiple IAV proteins via targeting basic amino acid Arg or Lys. Furthermore, this study reveals the molecular structure and catalytic mechanism of CPAVM1 protease. During catalysis, Tyr75, Tyr77, and Tyr102 are important active sites. Therefore, the present work identified a special protease CPAVM1 secreted by a new strain of Bacillus subtilis LjM2 against influenza A virus infection via direct cleavage of critical viral proteins, thus facilitates future biotechnological applications of Bacillus subtilis LjM2 and the protease CPAVM1.

The Role of TIM-1 and CD300a in Zika Virus Infection Investigated with Cell-Based Electrical Impedance

Orthoflaviviruses cause a major threat to global public health, and no antiviral treatment is available yet. Zika virus (ZIKV) entry, together with many other viruses, is known to be enhanced by phosphatidylserine (PS) receptors such as T-cell immunoglobulin mucin domain protein 1 (TIM-1). In this study, we demonstrate for the first time, using cell-based electrical impedance (CEI) biosensing, that ZIKV entry is also enhanced by expression of CD300a, another PS receptor. Furthermore, inhibiting CD300a in immature monocyte-derived dendritic cells partially but significantly inhibits ZIKV replication. As we have previously demonstrated that CEI is a useful tool to study Orthoflavivirus infection in real time, we now use this technology to determine how these PS receptors influence the kinetics of in vitro ZIKV infection. Results show that ZIKV entry is highly sensitive to minor changes in TIM-1 expression, both after overexpression of TIM-1 in infection-resistant HEK293T cells, as well as after partial knockout of TIM-1 in susceptible A549 cells. These results are confirmed by quantification of viral copy number and viral infectivity, demonstrating that CEI is highly suited to study and compare virus-host interactions. Overall, the results presented here demonstrate the potential of targeting this universal viral entry pathway.

LINC08148 promotes the caveola-mediated endocytosis of Zika virus through upregulating transcription of Src

Long non-coding RNAs (lncRNAs) represent a new group of host factors involved in viral infection. Current study identified an intergenic lncRNA, LINC08148, as a proviral factor of Zika virus (ZIKV) and Dengue virus 2 (DENV2). Knockout (KO) or silencing of LINC08148 decreases the replication of ZIKV and DENV2. LINC08148 mainly acts at the endocytosis step of ZIKV but at a later stage of DENV2. RNA-seq analysis reveals that LINC08148 knockout downregulates the transcription levels of five endocytosis-related genes including AP2B1, CHMP4C, DNM1, FCHO1, and Src. Among them, loss of Src significantly decreases the uptake of ZIKV. Trans-complementation of Src in the LINC08148KO cells largely restores the caveola-mediated endocytosis of ZIKV, indicating that the proviral effect of LINC08148 is exerted through Src. Finally, LINC08148 upregulates the Src transcription through associating with its transcription factor SP1. This work establishes an essential role of LINC08148 in the ZIKV entry, underscoring a significance of lncRNAs in the viral infection.

IMPORTANCE

Long non-coding RNAs (lncRNAs), like proteins, participate in viral infection. However, functions of most lncRNAs remain unknown. In this study, we performed a functional screen based on microarray data and identified a new proviral lncRNA, LINC08148. Then, we uncovered that LINC08148 is involved in the caveola-mediated endocytosis of ZIKV, rather than the classical clathrin-mediated endocytosis. Mechanistically, LINC08148 upregulates the transcription of Src, an initiator of caveola-mediated endocytosis, through binding to its transcription factor SP1. This study identifies a new lncRNA involved in the ZIKV infection, suggesting lncRNAs and cellular proteins are closely linked and cooperate to regulate viral infection.

Roles of NS1 Protein in Flavivirus Pathogenesis

Flaviviruses such as dengue, Zika, and West Nile viruses are highly concerning pathogens that pose significant risks to public health. The NS1 protein is conserved among flaviviruses and is synthesized as a part of the flavivirus polyprotein. It plays a critical role in viral replication, disease progression, and immune evasion. Post-translational modifications influence NS1's stability, secretion, antigenicity, and interactions with host factors. NS1 protein forms extensive interactions with host cellular proteins allowing it to affect vital processes such as RNA processing, gene expression regulation, and cellular homeostasis, which in turn influence viral replication, disease pathogenesis, and immune responses. NS1 acts as an immune evasion factor by delaying complement-dependent lysis of infected cells and contributes to disease pathogenesis by inducing endothelial cell damage and vascular leakage and triggering autoimmune responses. Anti-NS1 antibodies have been shown to cross-react with host endothelial cells and platelets, causing autoimmune destruction that is hypothesized to contribute to disease pathogenesis. However, in contrast, immunization of animal models with the NS1 protein confers protection against lethal challenges from flaviviruses such as dengue and Zika viruses. Understanding the multifaceted roles of NS1 in flavivirus pathogenesis is crucial for effective disease management and control. Therefore, further research into NS1 biology, including its host protein interactions and additional roles in disease pathology, is imperative for the development of strategies and therapeutics to combat flavivirus infections successfully. This Review provides an in-depth exploration of the current available knowledge on the multifaceted roles of the NS1 protein in the pathogenesis of flaviviruses.

Dronabinol as an answer to flavivirus infections: an in-silico investigation

Flavivirus infections are common in several parts of the world. Two major types of flaviviruses are dengue and zika viruses. Both these two viral infections have caused many fatalities around the world. There is an absence of a vaccine and an effective medication against these viruses. In this study, we analyzed the ability of dronabinol to act as a potential cure against these viral infections. We performed the docking of dronabinol with several viral proteins followed by molecular dynamics simulation, MM/PBSA and PCA analysis. We checked the ability of the polyphenol dronabinol to interfere with the binding of viral helicases to their cellular targets. We performed 2 D-QSAR studies, drug likeliness, ADMET and target prediction studies. From our study, we observed that dronabinol had the best docking ability against the helicase proteins of dengue and zika. Molecular dynamics simulation and MM/PBSA investigation confirmed the stability of the binding while PCA investigation showed a lowering of molecular motions in response to dronabinol docking to the helicases. Dronabinol interfered in the binding of the helicases to RNA. 2 D QSAR studies revealed a low IC50 value for dronabinol. Dronabinol showed favorable drug-likeness, ADMET properties and target prediction results. Thus we propose dronabinol be further investigated in-vitro as a cure against dengue and zika virus infections.Communicated by Ramaswamy H. Sarma.

Comparison of endocytosis pathways of Duck Tembusu virus in BHK-21 and duck embryo fibroblasts

The Duck Tembusu virus (DTMUV) is a zoonotic flavivirus characterized by nonsuppurative encephalitis and decreasing egg production that has adversely affected the poultry industry. While the way of invasion of DTMUV into different host cells, especially primary cells, remains elusive. In the present study, the ultrastructural pathological characteristics showed that DTMUV underwent a typical maturation and replication process: progeny virus particles gathered in rough endoplasmic reticulum (RER) cisternae, reached the cell membrane via Golgi body's endocrine channel, then were released in the infected baby hamster kidney-21 (BHK-21) and duck embryo fibroblast (DEF). Endoplasmic reticulum vesicles in BHK-21 were short rods and densely arranged like honeycombs, whereas vesicles in DEF were round and dispersed. Further study showed that the virus replication peak in mammalian BHK-21 cells was at 48 hpi, whereas in avian DEF cells was at 24 hpi. DTMUV entry into BHK-21 and DEF cells was blocked by clathrin inhibitor, chlorpromazine (CPZ), indicating that the flavivirus DTMUV enters BHK-21 and DEF both via a clathrin-mediated endocytosis (CME) pathway rather than caveola-mediated endocytosis or micropinocytosis. The endocytic difference in DTMUV entry into BHK-21 and DEF cells might provide insight into understanding the underlying virulence difference between passaged cells and cultured cells.

Impact of structural dynamics on biological functions of flaviviruses

Flaviviruses comprise a number of mosquito- or tick-transmitted human pathogens of global public health importance. Advances in structural biology techniques have contributed substantially to our current understanding of the life cycle of these small enveloped RNA viruses and led to deep insights into details of virus assembly, maturation and cell entry. In addition to large-scale conformational changes and oligomeric rearrangements of envelope proteins during these processes, there is increasing evidence that smaller-scale protein dynamics (referred to as virus "breathing") can confer extra flexibility to these viruses for the fine-tuning of their interactions with the immune system and possibly with cellular factors they encounter in their complex ecological cycles in arthropod and vertebrate hosts. In this review, we discuss how work with tick-borne encephalitis virus has extended our view on flavivirus breathing, leading to the identification of a novel mechanism of antibody-mediated infection enhancement and demonstrating breathing intermediates of the envelope protein in the process of membrane fusion. These data are discussed in the context of other flaviviruses and the perspective of a potential role of virus breathing to cope with the requirements of adaptation and replication in evolutionarily very different hosts.

Exploration of novel hexahydropyrrolo[1,2-e]imidazol-1-one derivatives as antiviral agents against ZIKV and USUV

Zika virus (ZIKV) and Usutu virus (USUV) are two emerging flaviviruses mostly transmitted by mosquitos. ZIKV is associated with microcephaly in newborns and the less-known USUV, with its reported neurotropism and its extensive spread in Europe, represents a growing concern for human health. There is still no approved vaccine or specific antiviral against ZIKV and USUV infections. The main goal of this study is to investigate the anti-ZIKV and anti-USUV activity of a new library of compounds and to preliminarily investigate the mechanism of action of the selected hit compounds in vitro. Two potent anti-ZIKV and anti-USUV agents, namely ZDL-115 and ZDL-116, were discovered, both presenting low cytotoxicity, cell-line independent antiviral activity in the low micromolar range and ability of reducing viral progeny production. The analysis of the structure-activity relationship (SAR) revealed that introduction of 2-deoxyribose to 3-arene was fundamental to enhance the solubility and improve the antiviral action. Additionally, we demonstrated that ZDL-115 and ZDL-116 are significantly active against both viruses when added on cells for at least 24 h prior to viral inoculation or immediately post-infection. The docking analysis showed that ZDL-116 could target the host vitamin D receptor (VDR) and viral proteins. Future experiments will be focused on compound modification to discover analogues that are more potent and on the clarification of the mechanism of action and the specific drug target. The discovery and the development of a novel anti-flavivirus drug will have a significant impact in a context where there are no fully effective antiviral drugs or vaccines for most flaviviruses.

Mosquito-Borne Flaviviruses and Current Therapeutic Advances

Mosquito-borne flavivirus infections affect approximately 400 million people worldwide each year and are global threats to public health. The common diseases caused by such flaviviruses include West Nile, yellow fever, dengue, Zika infection and Japanese encephalitis, which may result in severe symptoms and disorders of multiple organs or even fatal outcomes. Till now, no specific antiviral agents are commercially available for the treatment of the diseases. Numerous strategies have been adopted to develop novel and promising inhibitors against mosquito-borne flaviviruses, including drugs targeting the critical viral components or essential host factors during infection. Research advances in antiflaviviral therapy might optimize and widen the treatment options for flavivirus infection. This review summarizes the current developmental progresses and involved molecular mechanisms of antiviral agents against mosquito-borne flaviviruses.

Signaling events evoked by domain III of envelop glycoprotein of tick-borne encephalitis virus and West Nile virus in human brain microvascular endothelial cells

Tick-borne encephalitis virus and West Nile virus can cross the blood–brain barrier via hematogenous route. The attachment of a virion to the cells of a neurovascular unit, which is mediated by domain III of glycoprotein E, initiates a series of events that may aid viral entry. Thus, we sought to uncover the post-attachment biological events elicited in brain microvascular endothelial cells by domain III. RNA sequencing of cells treated with DIII of TBEV and WNV showed significant alteration in the expression of 309 and 1076 genes, respectively. Pathway analysis revealed activation of the TAM receptor pathway. Several genes that regulate tight-junction integrity were also activated, including pro-inflammatory cytokines and chemokines, cell-adhesion molecules, claudins, and matrix metalloprotease (mainly ADAM17). Results also indicate activation of a pro-apoptotic pathway. TLR2 was upregulated in both cases, but MyD88 was not. In the case of TBEV DIII, a MyD88 independent pathway was activated. Furthermore, both cases showed dramatic dysregulation of IFN and IFN-induced genes. Results strongly suggest that the virus contact to the cell surface emanates a series of events namely viral attachment and diffusion, breakdown of tight junctions, induction of virus uptake, apoptosis, reorganization of the extracellular-matrix, and activation of the innate immune system.

Development of antiviral carbon quantum dots that target the Japanese encephalitis virus envelope protein

Japanese encephalitis is a mosquito-borne disease caused by the Japanese encephalitis virus (JEV) that is prevalent in Asia and the Western Pacific. Currently, there is no effective treatment for Japanese encephalitis. Curcumin (Cur) is a compound extracted from the roots of Curcuma longa, and many studies have reported its antiviral and anti-inflammatory activities. However, the high cytotoxicity and very low solubility of Cur limit its biomedical applications. In this study, Cur carbon quantum dots (Cur-CQDs) were synthesized by mild pyrolysis-induced polymerization and carbonization, leading to higher water solubility and lower cytotoxicity, as well as superior antiviral activity against JEV infection. We found that Cur-CQDs effectively bound to the E protein of JEV, preventing viral entry into the host cells. In addition, after continued treatment of JEV with Cur-CQDs, a mutant strain of JEV was evolved that did not support binding of Cur-CQDs to the JEV envelope. Using transmission electron microscopy, biolayer interferometry, and molecular docking analysis, we revealed that the S123R and K312R mutations in the E protein play a key role in binding Cur-CQDs. The S123 and K312 residues are located in structural domains II and III of the E protein, respectively, and are responsible for binding to receptors on and fusing with the cell membrane. Taken together, our results suggest that the E protein of flaviviruses represents a potential target for the development of CQD-based inhibitors to prevent or treat viral infections.

Structural Basis of Zika Virus Helicase in RNA Unwinding and ATP Hydrolysis

The flavivirus nonstructural protein 3 helicase (NS3hel) is a multifunctional domain protein that is associated with DNA/RNA helicase, nucleoside triphosphatase (NTPase), and RNA 5'-triphosphatase (RTPase) activities. As an NTPase-dependent superfamily 2 (SF2) member, NS3hel employs an NTP-driven motor force to unwind double-stranded RNA while translocating along single-stranded RNA and is extensively involved in the viral replication process. Although the structures of SF2 helicases are widely investigated as promising drug targets, the mechanism of energy transduction between NTP hydrolysis and the RNA binding sites in ZIKV NS3hel remains elusive. Here, we report the crystal structure of ZIKV NS3hel in complex with its natural substrates ATP-Mn²⁺ and ssRNA. Distinct from other members of the Flavivirus genus, ssRNA binding to ZIKV NS3hel induces relocation of the active water molecules and ATP-associated metal ions in the NTP hydrolysis active site, which promotes the hydrolysis of ATP and the production of AMP. Our findings highlight the importance of the allosteric role of ssRNA on the modulation of ATP hydrolysis and energy utilization.

Potential Role of Flavivirus NS2B-NS3 Proteases in Viral Pathogenesis and Anti-flavivirus Drug Discovery Employing Animal Cells and Models: A Review

Flaviviruses are known to cause a variety of diseases in humans in different parts of the world. There are very limited numbers of antivirals to combat flavivirus infection, and therefore new drug targets must be explored. The flavivirus NS2B-NS3 proteases are responsible for the cleavage of the flavivirus polyprotein, which is necessary for productive viral infection and for causing clinical infections; therefore, they are a promising drug target for devising novel drugs against different flaviviruses. This review highlights the structural details of the NS2B-NS3 proteases of different flaviviruses, and also describes potential antiviral drugs that can interfere with the viral protease activity, as determined by various studies. Moreover, optimized in vitro reaction conditions for studying the NS2B-NS3 proteases of different flaviviruses may vary and have been incorporated in this review. The increasing availability of the in silico and crystallographic/structural details of flavivirus NS2B-NS3 proteases in free and drug-bound states can pave the path for the development of promising antiflavivirus drugs to be used in clinics. However, there is a paucity of information available on using animal cells and models for studying flavivirus NS2B-NS3 proteases, as well as on the testing of the antiviral drug efficacy against NS2B-NS3 proteases. Therefore, on the basis of recent studies, an effort has also been made to propose potential cellular and animal models for the study of flavivirus NS2B-NS3 proteases for the purposes of exploring flavivirus pathogenesis and for testing the efficacy of possible drugs targets, in vitro and in vivo.

Structurally conserved domains between flavivirus and alphavirus fusion glycoproteins contribute to replication and infectious virion production

Alphaviruses and flaviviruses have class II fusion glycoproteins that are essential for virion assembly and infectivity. Importantly, the tip of domain II is structurally conserved between the alphavirus and flavivirus fusion proteins, yet whether these structural similarities between virus families translate to functional similarities is unclear. Using in vivo evolution of Zika virus (ZIKV), we identified several novel emerging variants including an envelope glycoprotein variant in β-strand c (V114M) of domain II. We have previously shown that the analogous β-strand c and the ij loop, located in the tip of domain II of the alphavirus E1 glycoprotein, are important for infectivity. This led us to hypothesize that flavivirus E β-strand c also contributes to flavivirus infection. We generated this ZIKV glycoprotein variant and found that while it had little impact on infection in mosquitoes, it reduced replication in human cells and mice, and increased virus sensitivity to ammonium chloride, as seen for alphaviruses. In light of these results and given our alphavirus ij loop studies, we mutated a conserved alanine at the tip of the flavivirus ij loop to valine to test its effect on ZIKV infectivity. Interestingly, this mutation inhibited infectious virion production of ZIKV and yellow fever virus, but not West Nile virus. Together, these studies show that shared domains of the alphavirus and flavivirus class II fusion glycoproteins harbor structurally analogous residues that are functionally important and contribute to virus infection in vivo. Importance Arboviruses are a significant global public health threat, yet there are no antivirals targeting these viruses. This problem is in part due to our lack of knowledge on the molecular mechanisms involved in the arbovirus life cycle. In particular, virus entry and assembly are essential processes in the virus life cycle and steps that can be targeted for the development of antiviral therapies. Therefore, understanding common, fundamental mechanisms used by different arboviruses for entry and assembly is essential. In this study, we show that flavivirus and alphavirus residues located in structurally conserved and analogous regions of the class II fusion proteins contribute to common mechanisms of entry, dissemination, and infectious virion production. These studies highlight how class II fusion proteins function and provide novel targets for development of antivirals.

Dengue Virus Infection: A Tale of Viral Exploitations and Host Responses

Dengue is a mosquito-borne viral disease (arboviral) caused by the Dengue virus. It is one of the prominent public health problems in tropical and subtropical regions with no effective vaccines. Every year around 400 million people get infected by the Dengue virus, with a mortality rate of about 20% among the patients with severe dengue. The Dengue virus belongs to the Flaviviridae family, and it is an enveloped virus with positive-sense single-stranded RNA as the genetic material. Studies of the infection cycle of this virus revealed potential host targets important for the virus replication cycle. Here in this review article, we will be discussing different stages of the Dengue virus infection cycle inside mammalian host cells and how host proteins are exploited by the virus in the course of infection as well as how the host counteracts the virus by eliciting different antiviral responses.

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.