A conserved arginine in NS5 binds genomic 3' stem-loop RNA for primer-independent initiation of flavivirus RNA replication.

The commitment to replicate the RNA genome of flaviviruses without a primer involves RNA-protein interactions that have been shown to include the recognition of the stem-loop A (SLA) in the 5' untranslated region (UTR) by the nonstructural protein NS5. We show that DENV2 NS5 arginine 888, located within the carboxy-terminal 18 residues, is completely conserved in all flaviviruses and interacts specifically with the top-loop of 3'SL in the 3'UTR which contains the pentanucleotide 5'-CACAG-3' previously shown to be critical for flavivirus RNA replication. We present virological and biochemical data showing the importance of this Arg 888 in virus viability and de novo initiation of RNA polymerase activity in vitro. Based on our binding studies, we hypothesize that ternary complex formation of NS5 with 3'SL, followed by dimerization, leads to the formation of the de novo initiation complex that could be regulated by the reversible zipping and unzipping of cis-acting RNA elements.

Mutations in the cytoplasmic domain of dengue virus NS4A affect virus fitness and interactions with other non-structural proteins.

The dengue virus (DENV) replication complex is made up of its non-structural (NS) proteins and yet-to-be identified host proteins, but the molecular interactions between these proteins are not fully elucidated. In this work, we sought to uncover the interactions between DENV NS1 and its fellow NS proteins using a yeast two-hybrid (Y2H) approach, and found that domain II of NS1 binds to an N-terminal cytoplasmic fragment of NS4A. Mutations in amino acid residues 41 and 43 in this cytoplasmic region of NS4A disrupted the interaction between NS1 and the NS4A-2K-4B precursor protein. When the NS4A Y41F mutation was introduced into the context of the virus via a DENV2 infectious clone, this mutant virus exhibited impaired viral fitness and decreased infectious virus production. The NS4A Y41F mutant virus triggered a significantly muted transcriptional activation of interferon-stimulated genes compared to wild-type virus that is independent of NS4A's ability to antagonize type I interferon signalling. Taken together, we have identified a link between DENV NS1 and the cytoplasmic domain in NS4A that is important for its cellular and viral functions.

Dengue Virus and Zika Virus Serological Cross-reactivity and Their Impact on Pathogenesis in Mice.

Preexisting immunity to Zika virus (ZIKV) or dengue virus (DENV) may alter the course of their infection, and here we use robust mouse models to examine pathological outcomes following passive immunization, sequential cross-infection, or vaccination with inactivated virus. DENV infection was enhanced (through antibody-dependent enhancement [ADE]) or was suppressed by both DENV and ZIKV immunity. Notably, inactivated ZIKV vaccination enhanced dengue disease severity, although it was highly protective against ZIKV infection. On the other hand, ADE was not observed upon ZIKV infection in mice that were passively immunized or preinfected with DENV. Surprisingly, however, we found that vaccination with inactivated DENV enhanced ZIKV infection, mainly in the mesenteric lymph node, indicating the potential for DENV immunity to cause ADE in vivo. Collectively, our data call for greater attention to detail in the design of ZIKV or DENV vaccines.

Dengue Virus Selectively Annexes Endoplasmic Reticulum-Associated Translation Machinery as a Strategy for Co-opting Host Cell Protein Synthesis.

A primary question in dengue virus (DENV) biology is the molecular strategy for recruitment of host cell protein synthesis machinery. Here, we combined cell fractionation, ribosome profiling, and transcriptome sequencing (RNA-seq) to investigate the subcellular organization of viral genome translation and replication as well as host cell translation and its response to DENV infection. We report that throughout the viral life cycle, DENV plus- and minus-strand RNAs were highly partitioned to the endoplasmic reticulum (ER), identifying the ER as the primary site of DENV translation. DENV infection was accompanied by an ER compartment-specific remodeling of translation, where ER translation capacity was subverted from host transcripts to DENV plus-strand RNA, particularly at late stages of infection. Remarkably, translation levels and patterns in the cytosol compartment were only modestly affected throughout the experimental time course of infection. Comparisons of ribosome footprinting densities of the DENV plus-strand RNA and host mRNAs indicated that DENV plus-strand RNA was only sparsely loaded with ribosomes. Combined, these observations suggest a mechanism where ER-localized translation and translational control mechanisms, likely cis encoded, are used to repurpose the ER for DENV virion production. Consistent with this view, we found ER-linked cellular stress response pathways commonly associated with viral infection, namely, the interferon response and unfolded protein response, to be only modestly activated during DENV infection. These data support a model where DENV reprograms the ER protein synthesis and processing environment to promote viral survival and replication while minimizing the activation of antiviral and proteostatic stress response pathways.IMPORTANCE DENV, a prominent human health threat with no broadly effective or specific treatment, depends on host cell translation machinery for viral replication, immune evasion, and virion biogenesis. The molecular mechanism by which DENV commandeers the host cell protein synthesis machinery and the subcellular organization of DENV replication and viral protein synthesis is poorly understood. Here, we report that DENV has an almost exclusively ER-localized life cycle, with viral replication and translation largely restricted to the ER. Surprisingly, DENV infection largely affects only ER-associated translation, with relatively modest effects on host cell translation in the cytosol. DENV RNA translation is very inefficient, likely representing a strategy to minimize disruption of ER proteostasis. Overall these findings demonstrate that DENV has evolved an ER-compartmentalized life cycle; thus, targeting the molecular signatures and regulation of the DENV-ER interaction landscape may reveal strategies for therapeutic intervention.

A crystal structure of the Dengue virus NS5 protein reveals a novel inter-domain interface essential for protein flexibility and virus replication.

Flavivirus RNA replication occurs within a replication complex (RC) that assembles on ER membranes and comprises both non-structural (NS) viral proteins and host cofactors. As the largest protein component within the flavivirus RC, NS5 plays key enzymatic roles through its N-terminal methyltransferase (MTase) and C-terminal RNA-dependent-RNA polymerase (RdRp) domains, and constitutes a major target for antivirals. We determined a crystal structure of the full-length NS5 protein from Dengue virus serotype 3 (DENV3) at a resolution of 2.3 Å in the presence of bound SAH and GTP. Although the overall molecular shape of NS5 from DENV3 resembles that of NS5 from Japanese Encephalitis Virus (JEV), the relative orientation between the MTase and RdRp domains differs between the two structures, providing direct evidence for the existence of a set of discrete stable molecular conformations that may be required for its function. While the inter-domain region is mostly disordered in NS5 from JEV, the NS5 structure from DENV3 reveals a well-ordered linker region comprising a short 310 helix that may act as a swivel. Solution Hydrogen/Deuterium Exchange Mass Spectrometry (HDX-MS) analysis reveals an increased mobility of the thumb subdomain of RdRp in the context of the full length NS5 protein which correlates well with the analysis of the crystallographic temperature factors. Site-directed mutagenesis targeting the mostly polar interface between the MTase and RdRp domains identified several evolutionarily conserved residues that are important for viral replication, suggesting that inter-domain cross-talk in NS5 regulates virus replication. Collectively, a picture for the molecular origin of NS5 flexibility is emerging with profound implications for flavivirus replication and for the development of therapeutics targeting NS5.

Dengue Virus Infection with Highly Neutralizing Levels of Cross-Reactive Antibodies Causes Acute Lethal Small Intestinal Pathology without a High Level of Viremia in Mice.

UNLABELLED: Severe dengue virus (DENV)-associated diseases can occur in patients who have preexisting DENV antibodies (Abs) through antibody-dependent enhancement (ADE) of infection. It is well established that during ADE, DENV-antibody immune complexes (ICs) infect Fcγ receptor-bearing cells and increase the systemic viral burden that can be measured in the blood. For protection against infection with DENV serotypes 1 to 4, strongly neutralizing Abs must be elicited to overcome the effect of ADE. Clinical observations in infants who have maternal DENV Abs or recent phase II/III clinical trials with a leading tetravalent dengue vaccine suggested a lack of correlation between Ab neutralization and in vivo disease prevention. In addressing this gap in knowledge, we found that inoculation of ICs formed with serotype cross-reactive Abs that are more than 98% neutralized in vitro promotes high mortality in AG129 mice even though peak viremia was lower than that in direct virus infection. This suggests that the serum viremia level is not always correlated with disease severity. We further demonstrated that infection with the ICs resulted in increased vascular permeability, specifically in the small intestine, accompanied with increased tissue viral load and cytokine production, which can be suppressed by anti-tumor necrosis factor alpha (anti-TNF-α) Abs. Flow cytometric analysis identified increased infection in CD11b(int) CD11c(int/hi) CD103(-) antigen-presenting cells by IC inoculation, suggesting that these infected cells may be responsible for the increase in TNF-α production and vascular permeability in the small intestine that lead to mortality in mice. Our findings may have important implications for the development of dengue therapeutics. IMPORTANCE: We examined the relationship between the neutralizing level of Abs at the time of infection and subsequent disease progression in a mouse model in order to understand why patients who are shown to have a neutralizing quantity of Abs still allow sufficient DENV replication to induce severe dengue manifestations, which sometimes do not correlate with viremia level. Strikingly, we found that high mortality was induced in AG129 mice by the increase in TNF-α-induced vascular permeability accompanied by an increased viral load, specifically in the small intestine, even when the initial infection level is suppressed to less than 5% and the peak viremia level is not enhanced. This suggests that ADE overcomes the protective efficacy of Abs in a tissue-dependent manner that leads to severe small intestinal pathology. Our findings may serve to address the pathogenic role of Abs on severe dengue disease and also help to develop safe Ab-based therapeutic strategies.

Flexibility of NS5 Methyltransferase-Polymerase Linker Region Is Essential for Dengue Virus Replication.

We examined the function of the conserved Val/Ile residue within the dengue virus NS5 interdomain linker (residues 263 to 272) by site-directed mutagenesis. Gly substitution or Gly/Pro insertion after the conserved residue increased the linker flexibility and created slightly attenuated viruses. In contrast, Pro substitution abolished virus replication by imposing rigidity in the linker and restricting NS5's conformational plasticity. Our biochemical and reverse genetics experiments demonstrate that NS5 utilizes conformational regulation to achieve optimum viral replication.

Secreted dengue virus NS1 from infection is predominantly dimeric and in complex with high-density lipoprotein.

Severe dengue infections are characterized by endothelial dysfunction shown to be associated with the secreted nonstructural protein 1 (sNS1), making it an attractive vaccine antigen and biotherapeutic target. To uncover the biologically relevant structure of sNS1, we obtained infection-derived sNS1 (isNS1) from dengue virus (DENV)-infected Vero cells through immunoaffinity purification instead of recombinant sNS1 (rsNS1) overexpressed in insect or mammalian cell lines. We found that isNS1 appeared as an approximately 250 kDa complex of NS1 and ApoA1 and further determined the cryoEM structures of isNS1 and its complex with a monoclonal antibody/Fab. Indeed, we found that the major species of isNS1 is a complex of the NS1 dimer partially embedded in a high-density lipoprotein (HDL) particle. Crosslinking mass spectrometry studies confirmed that the isNS1 interacts with the major HDL component ApoA1 through interactions that map to the NS1 wing and hydrophobic domains. Furthermore, our studies demonstrated that the sNS1 in sera from DENV-infected mice and a human patient form a similar complex as isNS1. Our results report the molecular architecture of a biological form of sNS1, which may have implications for the molecular pathogenesis of dengue.

Serotype-Specific Regulation of Dengue Virus NS5 Protein Subcellular Localization.

Dengue virus (DENV) nonstructural protein 5 (NS5), consisting of methyltransferase and RNA-dependent RNA polymerase (RdRp) domains, is critical for viral RNA synthesis within endoplasmic reticulum-derived replication complexes in the cytoplasm. However, a significant proportion of NS5 is localized to the nucleus of infected cells for DENV2, 3, and 4, whereas DENV1 NS5 is localized diffusely in the cytoplasm. We still have an incomplete understanding of how the DENV NS5 subcellular localization is regulated. Within NS5, two putative nuclear localization signal (NLS) sequences have been identified: NLSCentral residing in the palm of the RdRp domain as well as the recently discovered NLSC-term residing in the flexible region at the C-terminal of the RdRp domain. We have previously shown that DENV2 NS5 nuclear localization can be significantly reduced by single-point mutations to the NLSC-term. Here, we present biochemical, virological, and structural data demonstrating that the relative importance of either NLS in NS5 nuclear localization is unique to each of the four DENV serotypes. DENV1 NS5's cytoplasmic localization appears to be due to a functionally weak interaction between its NLSCentral and importin-α (IMPα), while DENV2 NS5 is almost exclusively nuclear through its NLSC-term's strong interaction with IMPα. Both NLSs of DENV3 NS5 appear to contribute to directing its nuclear localization. Lastly, in the case of DENV4, the regulation of its NS5 nuclear localization remains an enigma but appears to be associated with its NLSC-term.

Tissue-specific expansion of Zika virus isogenic variants drive disease pathogenesis.

BACKGROUND: The Asian lineage Zika virus (ZIKV) emerged as a public health emergency in 2016 causing severe neurological pathologies with no apparent historical correlate to the mild, disease-causing innocuous member of the mosquito-borne flavivirus genus that was discovered in Africa in 1947. Replication error rate of RNA viruses combined with viral protein/RNA structural plasticity can lead to evolution of virus-induced pathogenicity that is critical to identify and validate. METHODS: Infection studies in cells and A129 interferon alpha/beta receptor deficient mice with ZIKV French Polynesian H/PF/2013 clinical isolate, plaque-purified isogenic clone derivatives as well as infectious cDNA clone derived wild-type and site-specific mutant viruses, were employed together with Next-Generation Sequencing (NGS) to pin-point the contributions of specific viral variants in neurovirulence recapitulated in our ZIKV mouse model. FINDINGS: NGS analysis of the low-passage inoculum virus as well as mouse serum, brain and testis derived virus, revealed specific enrichment in the mouse brain that were not found in the other tissues. Specifically, non-structural (NS) protein 2A variant at position 117 along with changes in NS1 and NS4B were uniquely associated with the mouse brain isolate. Mutational analysis of these variants in cDNA infectious clones identified the NS2A A117V as the lethal pathogenic determinant with potential epistatic contribution of NS1 and NS4B variants in ZIKV brain penetrance. INTERPRETATION: Our findings confirm that viral subpopulations drive ZIKV neuropathogenicity and identify specific sequence variants that expand in the mouse brain that associates with this phenotype which can serve as predictors of severe epidemics. FUNDING: Duke-NUS Khoo Post-doctoral Fellowship Award 2020 (KWKC) and National Medical Research Council of Singapore grants MOH-000524 (OFIRG) (SW) and MOH-OFIRG20nov-0002 (SGV).

Therapeutics for flaviviral infections.

Flaviviruses are vector-borne pathogens capable of causing devastating human diseases. The re-emergence of Zika in 2016 notoriously led to a widescale epidemic in the Americas. New daunting evidence suggests that a single mutation in Zika virus genome may increase transmission and pathogenesis, further highlighting the need to be prepared for flavivirus outbreaks. Dengue, in particular infects about 400 million people each year, leading to reoccurring local outbreaks. Public health efforts to mitigate flavivirus transmission is largely dependent on vector control strategies, as only a limited number of flavivirus vaccines have been developed thus far. There are currently no commercially available antivirals for flaviviruses, leaving supportive care as the primary treatment option. In this review, we will briefly paint a broad picture of the flavivirus landscape in terms of therapeutics, with particular focus on viral targets, promising novel compounds entering the drug discovery pipeline, as well as model systems for evaluating drug efficacy.

Functionalized sulfonyl anthranilic acid derivatives inhibit replication of all the four dengue serotypes.

Dengue virus (DENV), a mosquito-borne flavivirus, continues to be a major public health threat in many countries and no approved antiviral therapeutics are available yet. In this work, we designed and synthesized a series of sulfonyl anthranilic acid (SAA) derivatives using a ligand-based scaffold morphing approach of the 2,1-benzothiazine 2,2-dioxide core, previously used by us to develop DENV polymerase inhibitors resulting devoid of any cell-based antiviral activity. Several derivatives based on the new SAA chemotype exhibited potent inhibition against DENV infection in the cell-based assay but did not inhibit DENV NS5 polymerase activity in the in vitro de novo initiation and elongation assays. Notably, best compounds 26 and 39 showed EC50 values in the range of 0.54-1.36 µM against cells infected with the four dengue serotypes (DENV-1-4). Time-of-drug-addition assay revealed that analogue 26 is a post-entry replication inhibitor that appears to be specific for cells of primate origin, implicating a host target with a high barrier to resistance. In conclusion, SAA derivatives offer a valuable starting point for developing effective Dengue antiviral therapeutics.

A cellular screening platform, stably expressing DENV2 NS5, defines a novel anti-DENV mechanism of action of Apigenin based on STAT2 activation.

Type I interferon (IFN-I) evasion by Dengue virus (DENV) is key in DENV pathogenesis. The non-structural protein 5 (NS5) antagonizes IFN-I response through the degradation of the signal transducer and activator of transcription 2 (STAT2). We developed a K562 cell-based platform, for high throughput screening of compounds potentially counteracting the NS5-mediated antagonism of IFN-I signaling. Upon a screening with a library of 1220 approved drugs, 3 compounds previously linked to DENV inhibition (Apigenin, Chrysin, and Luteolin) were identified. Luteolin and Apigenin determined a significant inhibition of DENV2 replication in Huh7 cells and the restoration of STAT2 phosphorylation in both cell systems. Apigenin and Luteolin were able to stimulate STAT2 even in the absence of infection. Despite the "promiscuous" and "pan-assay-interfering" nature of Luteolin, Apigenin promotes STAT2 Tyr 689 phosphorylation and activation, highlighting the importance of screening for compounds able to interact with host factors, to counteract viral proteins capable of dampening innate immune responses.

Experimental evidence for a high rate of maternal-fetal transmission of dengue virus in the presence of antibodies in immunocompromised mice.

BACKGROUND: Congenital disorders associated with prenatal vertical transmission of Zika virus (ZIKV) is well established since the 2016 outbreak in the Americas. However, despite clinical reports of similar mode of transmission for other flaviviruses such as dengue virus (DENV), the phenomenon has not been experimentally explored. METHODS: Pregnant AG129 mice were infected with DENV1 in the presence or absence of enhancing antibodies at different gestational time points. ZIKV was used for comparison. We quantified viral load in fetus and placentas and performed comprehensive gene expression profiling in the maternal (decidua) and fetal portion of placenta separately. FINDINGS: We demonstrate in a laboratory experimental setting that DENV can be transmitted vertically in a gestation stage-dependent manner similar to ZIKV, and this incidence drastically increases in the presence of enhancing antibodies. Interestingly, a high rate of DENV fetal infection occurs even though the placental viral load is significantly lower than that found in ZIKV-infected dams. Comprehensive gene expression profiling revealed DENV infection modulates a variety of inflammation-associated genes comparable to ZIKV in decidua and fetal placenta in early pregnancy. INTERPRETATION: Our findings suggest that the virus-induced modulation of host gene expression may facilitate DENV to cross the placental barrier in spite of lower viral burden compared to ZIKV. This mouse model may serve to identify the host determinants required for the vertical transmission of flaviviruses and develop appropriate countermeasures. FUNDING: National Medical Research Council/Open Fund Individual Research Grant MOH-000524 (SW), MOH-000086 and OFIRG20nov-0017 (SGV).

Amidoxime prodrugs convert to potent cell-active multimodal inhibitors of the dengue virus protease.

The flavivirus genus of the Flaviviridae family comprises Dengue, Zika and West-Nile viruses which constitute unmet medical needs as neither appropriate antivirals nor safe vaccines are available. The dengue NS2BNS3 protease is one of the most promising validated targets for developing a dengue treatment however reported protease inhibitors suffer from toxicity and cellular inefficacy. Here we report SAR on our previously reported Zika-active carbazole scaffold, culminating prodrug compound SP-471P (EC50 1.10 µM, CC50 > 100 µM) that generates SP-471; one of the most potent, non-cytotoxic and cell-active protease inhibitors described in the dengue literature. In cell-based assays, SP-471P leads to inhibition of viral RNA replication and complete abolishment of infective viral particle production even when administered 6 h post-infection. Mechanistically, SP-471 appears to inhibit both normal intermolecular protease processes and intramolecular cleavage events at the NS2BNS3 junction, as well as at NS3 internal sites, all critical for virus replication. These render SP-471 a unique to date multimodal inhibitor of the dengue protease.

Optimal flexibility of the linker region of Zika virus NS5 methyltransferase-polymerase is critical for virus replication.

The flavivirus NS5 protein contains an N-terminal methyl-transferase (MTase) connected through a flexible linker with a C-terminal RNA-dependent RNA-polymerase (RdRp) domain, that work cooperatively to replicate and methylate the viral genome. In this study we probed the importance of an evolutionary-conserved hydrophobic residue (Val266) located at the start of the ten-residue interdomain linker of Zika virus (ZIKV) NS5. In flavivirus NS5 crystal structures, the start of the linker forms a 310 helix when NS5 adopts a compact conformation, but becomes disordered or extended in open conformations. Using reverse genetics system, we either introduced rigidity in the linker through mutation to a proline or flexibility through a glycine mutation at position 266. ZIKV NS5 Val 266 to Pro mutation was lethal for viral RNA replication while the Gly mutation was severely attenuated. Serial passaging of cell culture supernatant derived from C6/36 mosquito cells transfected with mutant ZIKV RNA showed that the attenuation can be rescued. Next generation deep sequencing revealed four single nucleotide polymorphisms that occur with an allele frequency >98%. The single non-synonymous NS5 mutation Glu419 to Lys is adjacent to RdRp motif G at the tip of the fingers subdomain, while the remaining three are synonymous variants at nucleotide positions 1403, 4403 and 6653 in the genome. Reverse engineering the changes into the ZIKV NS5/Val266Gly background followed by serial passaging revealed that residue 266 is under strong positive selection to revert back to Val. The interaction of the specific conformation of the NS5 linker with Val at position 266 and the RNA binding motif G region may present a potential strategy for allosteric antiviral drug development.

18F-fluorodeoxyglucose positron emission tomography as a window into human dengue pathophysiology.

In mouse models of dengue virus (DENV) infection, 18F-FDG PET is able to sensitively detect tissue-specific sites of inflammation and disease activity, as well as track therapeutic response to anti- DENV agents. However, the use of 18F-FDG PET to study the pathogenesis of inflammation and disease activity in DENV infection in humans, has not been clinically validated. Here we report the 18F-FDG PET imaging results of two patients during the febrile phase of acute DENV infection, paired with serial serum viral load, NS1 and proinflammatory cytokine measurements. Our findings demonstrate that 18F-FDG PET is able to sensitively detect and quantify organ-specific inflammation in the lymph nodes and spleen, in classic acute dengue fever. This raises the potential for 18F-FDG PET to be used as a research tool that may provide further insights into disease pathogenesis.

Zika virus nonstructural protein 5 residue R681 is critical for dimer formation and enzymatic activity.

Zika virus (ZIKV) relies on its nonstructural protein 5 (NS5) for capping and synthesis of the viral RNA. Recent small-angle X-ray scattering (SAXS) data of recombinant ZIKV NS5 protein showed that it is dimeric in solution. Here, we present insights into the critical residues responsible for its dimer formation. SAXS studies of the engineered ZIKV NS5 mutants revealed that R681A mutation on NS5 (NS5R681A ) disrupts the dimer formation and affects its RNA-dependent RNA polymerase activity as well as the subcellular localization of NS5R681A in mammalian cells. The critical residues involved in the dimer arrangement of ZIKV NS5 are discussed, and the data provide further insights into the diversity of flaviviral NS5 proteins in terms of their propensity for oligomerization.

Assessing the utility of antivirals for preventing maternal-fetal transmission of zika virus in pregnant mice.

Zika virus (ZIKV) infection during pregnancy has been associated with adverse outcomes and birth defects such as microcephaly in newborn children. Congenital malformations associated with ZIKV are believed to occur via direct infection of the fetus. Unfortunately, there are no licensed therapeutic or preventative tools to block maternal-fetal transmission of ZIKV. In this study, we developed a mouse model of ZIKV infection that specifically establishes vertical maternal-fetal transmission of ZIKV in 40-60% of fetuses when the dams acquire ZIKV infection during pregnancy. This mouse model somewhat mirrors the experience in humans at the peak of the epidemic in the Americas. Using this model, we demonstrate that a well-documented directly acting antiviral (DAA) compound that targets flaviviral RNA synthesis can completely prevent fetal infection when the treatment is started at the time of infection. Notably, we show that the treatment commenced at the time of peak viremia is still able to reduce the risk of fetal infection concomitant with significant reduction in placental viral load. Our results show for the first time the potential for clinical development of antiviral drugs for preventing vertical maternal-fetal transmission of ZIKV.

The Potential Role of the ZIKV NS5 Nuclear Spherical-Shell Structures in Cell Type-Specific Host Immune Modulation during ZIKV Infection.

The Zika virus (ZIKV) non-structural protein 5 (NS5) plays multiple viral and cellular roles during infection, with its primary role in virus RNA replication taking place in the cytoplasm. However, immunofluorescence assay studies have detected the presence of ZIKV NS5 in unique spherical shell-like structures in the nuclei of infected cells, suggesting potentially important cellular roles of ZIKV NS5 in the nucleus. Hence ZIKV NS5's subcellular distribution and localization must be tightly regulated during ZIKV infection. Both ZIKV NS5 expression or ZIKV infection antagonizes type I interferon signaling, and induces a pro-inflammatory transcriptional response in a cell type-specific manner, but the mechanisms involved and the role of nuclear ZIKV NS5 in these cellular functions has not been elucidated. Intriguingly, these cells originate from the brain and placenta, which are also organs that exhibit a pro-inflammatory signature and are known sites of pathogenesis during ZIKV infection in animal models and humans. Here, we discuss the regulation of the subcellular localization of the ZIKV NS5 protein, and its putative role in the induction of an inflammatory response and the occurrence of pathology in specific organs during ZIKV infection.