Mutations conferring a noncytotoxic phenotype on chikungunya virus replicons compromise enzymatic properties of nonstructural protein 2

4'-Fluorouridine inhibits alphavirus replication and infection in vitro and in vivo

Chikungunya virus (CHIKV) is an enveloped, positive-sense RNA virus that has re-emerged to cause millions of human infections worldwide. In humans, acute CHIKV infection causes fever and severe muscle and joint pain. Chronic and debilitating arthritis and joint pain can persist for months to years. To date, there are no approved antivirals against CHIKV. Recently, the ribonucleoside analog 4'-fluorouridine (4'-FlU) was reported as a highly potent orally available inhibitor of SARS-CoV-2, respiratory syncytial virus, and influenza virus replication. In this study, we assessed 4'-FlU's potency and breadth of inhibition against a panel of alphaviruses including CHIKV, and found that it broadly suppressed alphavirus production in cell culture. 4'-FlU acted on the viral RNA replication step, and the first 4 hours post-infection were the critical time for its antiviral effect. In vitro replication assays identified nsP4 as the target of inhibition. In vivo, treatment with 4'-FlU reduced disease signs, inflammatory responses, and viral tissue burden in mouse models of CHIKV and Mayaro virus infection. Treatment initiated at 2 hours post-infection was most effective; however, treatment initiated as late as 24-48 hours post-infection produced measurable antiviral effects in the CHIKV mouse model. 4'-FlU showed effective oral delivery in our mouse model and resulted in the accumulation of both 4'-FlU and its bioactive triphosphate form in tissues relevant to arthritogenic alphavirus pathogenesis. Together, our data indicate that 4'-FlU inhibits CHIKV infection in vitro and in vivo and is a promising oral therapeutic candidate against CHIKV infection.IMPORTANCEAlphaviruses including chikungunya virus (CHIKV) are mosquito-borne positive-strand RNA viruses that can cause various diseases in humans. Although compounds that inhibit CHIKV and other alphaviruses have been identified in vitro, there are no licensed antivirals against CHIKV. Here, we investigated a ribonucleoside analog, 4'-fluorouridine (4'-FlU), and demonstrated that it inhibited infectious virus production by several alphaviruses in vitro and reduced virus burden in mouse models of CHIKV and Mayaro virus infection. Our studies also indicated that 4'-FlU treatment reduced CHIKV-induced footpad swelling and reduced the production of pro-inflammatory cytokines. Inhibition in the mouse model correlated with effective oral delivery of 4'-FlU and accumulation of both 4'-FlU and its bioactive form in relevant tissues. In summary, 4'-FlU exhibits potential as a novel anti-alphavirus agent targeting the replication of viral RNA.

ADP-Ribosylation in Antiviral Innate Immune Response

Adenosine diphosphate (ADP)-ribosylation is a reversible post-translational modification catalyzed by ADP-ribosyltransferases (ARTs). ARTs transfer one or more ADP-ribose from nicotinamide adenine dinucleotide (NAD⁺) to the target substrate and release the nicotinamide (Nam). Accordingly, it comes in two forms: mono-ADP-ribosylation (MARylation) and poly-ADP-ribosylation (PARylation). ADP-ribosylation plays important roles in many biological processes, such as DNA damage repair, gene regulation, and energy metabolism. Emerging evidence demonstrates that ADP-ribosylation is implicated in host antiviral immune activity. Here, we summarize and discuss ADP-ribosylation modifications that occur on both host and viral proteins and their roles in host antiviral response.

Rapid microfluidic platform for screening and enrichment of cells secreting virus neutralizing antibodies

As part of the body's immune response, antibodies (Abs) have the ability to neutralize pathogenic viruses to prevent infection. To screen for neutralizing Abs (nAbs) from the immune repertoire, multiple screening techniques have been developed. However, conventional methods have a trade-off between screening throughput and the ability to screen for nAbs via their functional efficacy. Although droplet microfluidic platforms have the ability to bridge this disparity, the majority of such reported platforms still rely on Ab-binding assays as a proxy for function, which results in irrelevant hits. Herein, we report the multi-module Droplet-based Platform for Effective Antibody RetrievaL (DROP-PEARL) platform, which can achieve high-throughput enrichment of Ab-secreting cells (ASCs) based on the neutralizing activity of secreted nAbs against the a target virus. In this study, in-droplet Chikungunya virus (CHIKV) infection of host cells and neutralization was demonstrated via sequential delivery of viruses and host cells via picoinjection. In addition, we demonstrate the ability of the sorting system to accurately discriminate and isolate uninfected droplets from a mixed population of droplets at a rate of 150 000 cells per hour. As a proof of concept, a single-cell neutralization assay was performed on two populations of cells (nAb-producing and non-Ab producing cells), and up to 2.75-fold enrichment of ASCs was demonstrated. Finally, we demonstrated that DROP-PEARL is able to achieve similar enrichment for low frequency (∼2%) functional nAb-producing cells in a background of excess cells secreting irrelevant antibodies, highlighting its potential prospect as a first round enrichment platform for functional ASCs. We envision that the DROP-PEARL platform could potentially be used to accelerate the discovery of nAbs against other pathogenic viral targets, and we believe it will be a useful in the ongoing fight against biological threats.

Expression of Alphavirus Nonstructural Protein 2 (nsP2) in Mosquito Cells Inhibits Viral RNA Replication in Both a Protease Activity-Dependent and -Independent Manner

Alphaviruses are positive-strand RNA viruses, mostly being mosquito-transmitted. Cells infected by an alphavirus become resistant to superinfection due to a block that occurs at the level of RNA replication. Alphavirus replication proteins, called nsP1-4, are produced from nonstructural polyprotein precursors, processed by the protease activity of nsP2. Trans-replicase systems and replicon vectors were used to study effects of nsP2 of chikungunya virus and Sindbis virus on alphavirus RNA replication in mosquito cells. Co-expressed wild-type nsP2 reduced RNA replicase activity of homologous virus; this effect was reduced but typically not abolished by mutation in the protease active site of nsP2. Mutations in the replicase polyprotein that blocked its cleavage by nsP2 reduced the negative effect of nsP2 co-expression, confirming that nsP2-mediated inhibition of RNA replicase activity is largely due to nsP2-mediated processing of the nonstructural polyprotein. Co-expression of nsP2 also suppressed the activity of replicases of heterologous alphaviruses. Thus, the presence of nsP2 inhibits formation and activity of alphavirus RNA replicase in protease activity-dependent and -independent manners. This knowledge improves our understanding about mechanisms of superinfection exclusion for alphaviruses and may aid the development of anti-alphavirus approaches.

nsP4 Is a Major Determinant of Alphavirus Replicase Activity and Template Selectivity

Alphaviruses have positive-strand RNA genomes containing two open reading frames (ORFs). The first ORF encodes the nonstructural (ns) polyproteins P123 and P1234 that act as precursors for the subunits of the viral RNA replicase (nsP1 to nsP4). Processing of P1234 leads to the formation of a negative-strand replicase consisting of nsP4 (RNA polymerase) and P123 components. Subsequent processing of P123 results in a positive-strand replicase. The second ORF encoding the structural proteins is expressed via the synthesis of a subgenomic RNA. Alphavirus replicase is capable of using template RNAs that contain essential cis-active sequences. Here, we demonstrate that the replicases of nine alphaviruses, expressed in the form of separate P123 and nsP4 components, are active. Their activity depends on the abundance of nsP4. The match of nsP4 to its template strongly influences efficient subgenomic RNA synthesis. nsP4 of Barmah Forest virus (BFV) formed a functional replicase only with matching P123, while nsP4s of other alphaviruses were compatible also with several heterologous P123s. The P123 components of Venezuelan equine encephalitis virus and Sindbis virus (SINV) required matching nsP4s, while P123 of other viruses could form active replicases with different nsP4s. Chimeras of Semliki Forest virus, harboring the nsP4 of chikungunya virus, Ross River virus, BFV, or SINV were viable. In contrast, chimeras of SINV, harboring an nsP4 from different alphaviruses, exhibited a temperature-sensitive phenotype. These findings highlight the possibility for formation of new alphaviruses via recombination events and provide a novel approach for the development of attenuated chimeric viruses for vaccination strategies. IMPORTANCE A key element of every virus with an RNA genome is the RNA replicase. Understanding the principles of RNA replicase formation and functioning is therefore crucial for understanding and responding to the emergence of new viruses. Reconstruction of the replicases of nine alphaviruses from nsP4 and P123 polyproteins revealed that the nsP4 of the majority of alphaviruses, including the mosquito-specific Eilat virus, could form a functional replicase with P123 originating from a different virus, and the corresponding chimeric viruses were replication-competent. nsP4 also had an evident role in determining the template RNA preference and the efficiency of RNA synthesis. The revealed broad picture of the compatibility of the replicase components of alphaviruses is important for understanding the formation and functioning of the alphavirus RNA replicase and highlights the possibilities for recombination between different alphavirus species.

Semliki Forest Virus Chimeras with Functional Replicase Modules from Related Alphaviruses Survive by Adaptive Mutations in Functionally Important Hot Spots

Alphaviruses (family Togaviridae) include both human pathogens such as chikungunya virus (CHIKV) and Sindbis virus (SINV) and model viruses such as Semliki Forest virus (SFV). The alphavirus positive-strand RNA genome is translated into nonstructural (ns) polyprotein(s) that are precursors for four nonstructural proteins (nsPs). The three-dimensional structures of nsP2 and the N-terminal 2/3 of nsP3 reveal that these proteins consist of several domains. Cleavage of the ns-polyprotein is performed by the strictly regulated protease activity of the nsP2 region. Processing results in the formation of a replicase complex that can be considered a network of functional modules. These modules work cooperatively and should perform the same task for each alphavirus. To investigate functional interactions between replicase components, we generated chimeras using the SFV genome as a backbone. The functional modules corresponding to different parts of nsP2 and nsP3 were swapped with their counterparts from CHIKV and SINV. Although some chimeras were nonfunctional, viruses harboring the CHIKV N-terminal domain of nsP2 or any domain of nsP3 were viable. Viruses harboring the protease part of nsP2, the full-length nsP2 of CHIKV, or the nsP3 macrodomain of SINV required adaptive mutations for functionality. Seven mutations that considerably improved the infectivity of the corresponding chimeric genomes affected functionally important hot spots recurrently highlighted in previous alphavirus studies. These data indicate that alphaviruses utilize a rather limited set of strategies to survive and adapt. Furthermore, functional analysis revealed that the disturbance of processing was the main defect resulting from chimeric alterations within the ns-polyprotein. IMPORTANCE Alphaviruses cause debilitating symptoms and have caused massive outbreaks. There are currently no approved antivirals or vaccines for treating these infections. Understanding the functions of alphavirus replicase proteins (nsPs) provides valuable information for both antiviral drug and vaccine development. The nsPs of all alphaviruses consist of similar functional modules; however, to what extent these are independent in functionality and thus interchangeable among homologous viruses is largely unknown. Homologous domain swapping was used to study the functioning of modules from nsP2 and nsP3 of other alphaviruses in the context of Semliki Forest virus. Most of the introduced substitutions resulted in defects in the processing of replicase precursors that were typically compensated by adaptive mutations that mapped to determinants of polyprotein processing. Understanding the principles of virus survival strategies and identifying hot spot mutations that permit virus adaptation highlight a route to the rapid development of attenuated viruses as potential live vaccine candidates.

New Insights into Chikungunya Virus Infection and Pathogenesis

Chikungunya virus (CHIKV) is a re-emerging mosquito-borne alphavirus responsible for major outbreaks of disease since 2004 in the Indian Ocean islands, South east Asia, and the Americas. CHIKV causes debilitating musculoskeletal disorders in humans that are characterized by fever, rash, polyarthralgia, and myalgia. The disease is often self-limiting and nonlethal; however, some patients experience atypical or severe clinical manifestations, as well as a chronic rheumatic syndrome. Unfortunately, no efficient antivirals against CHIKV infection are available so far, highlighting the importance of deepening our knowledge of CHIKV host cell interactions and viral replication strategies. In this review, we discuss recent breakthroughs in the molecular mechanisms that regulate CHIKV infection and lay down the foundations to understand viral pathogenesis. We describe the role of the recently identified host factors co-opted by the virus for infection and pathogenesis, and emphasize the importance of CHIKV nonstructural proteins in both replication complex assembly and host immune response evasion. Expected final online publication date for the Annual Review of Virology, Volume 8 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The Putative Roles and Functions of Indel, Repetition and Duplication Events in Alphavirus Non-Structural Protein 3 Hypervariable Domain (nsP3 HVD) in Evolution, Viability and Re-Emergence

Alphavirus non-structural proteins 1–4 (nsP1, nsP2, nsP3, and nsP4) are known to be crucial for alphavirus RNA replication and translation. To date, nsP3 has been demonstrated to mediate many virus–host protein–protein interactions in several fundamental alphavirus mechanisms, particularly during the early stages of replication. However, the molecular pathways and proteins networks underlying these mechanisms remain poorly described. This is due to the low genetic sequence homology of the nsP3 protein among the alphavirus species, especially at its 3′ C-terminal domain, the hypervariable domain (HVD). Moreover, the nsP3 HVD is almost or completely intrinsically disordered and has a poor ability to form secondary structures. Evolution in the nsP3 HVD region allows the alphavirus to adapt to vertebrate and insect hosts. This review focuses on the putative roles and functions of indel, repetition, and duplication events that have occurred in the alphavirus nsP3 HVD, including characterization of the differences and their implications for specificity in the context of virus–host interactions in fundamental alphavirus mechanisms, which have thus directly facilitated the evolution, adaptation, viability, and re-emergence of these viruses.

Novel Analogues of the Chikungunya Virus Protease Inhibitor: Molecular Design, Synthesis, and Biological Evaluation

The Chikungunya virus (CHIKV) is an arbovirus belonging to the genus Alphavirus of the Togaviridae family. CHIKV is transmitted by the mosquitoes and causes Chikungunya fever. CHIKV outbreaks have occurred in Africa, Asia, Europe, and the countries of Indian and Pacific Oceans. In 2013, CHIKV cases were registered for the first time in the Americas on the Caribbean islands. There is currently no vaccine to prevent or medicines to treat CHIKV infection. The CHIKV nonstructural protease (nsP2) is a promising potential target for the development of drugs against CHIKV infection because this protein is one of the key components of the viral replication complex and is involved in multiple steps of virus infection. In this work, novel analogues of the potential CHIKV nsP2 protease inhibitor, first reported by Das et al. in 2016, were identified using molecular modeling methods, synthesized, and evaluated in vitro. The optimization of the structure of the inhibitor allowed to increase the antiviral activity of the compound 2-10 times. The possible mechanism of action of the identified potential inhibitors of the CHIKV nsP2 protease was studied in detail using molecular dynamics (MD) simulations. According to the MD results, the most probable mechanism of action is the blocking of conformational changes in the nsP2 protease required for substrate recognition and binding.

Phosphorylation Sites in the Hypervariable Domain in Chikungunya Virus nsP3 Are Crucial for Viral Replication

Chikungunya virus (CHIKV, family Togaviridae) is a mosquito-transmitted alphavirus. The positive-sense RNA genome of CHIKV encodes four nonstructural proteins (nsP1 to nsP4) that are virus-specific subunits of the RNA replicase. Among nsP functions, those of nsP3 are the least understood. The C-terminal hypervariable domain (HVD) in nsP3 is disordered and serves as a platform for interactions with multiple host proteins. For Sindbis virus (SINV) and Semliki Forest virus (SFV), the nsP3 HVD has been shown to be phosphorylated. Deletion of phosphorylated regions has a mild effect on the growth of SFV and SINV in vertebrate cells. Using radiolabeling, we demonstrated that nsP3 in CHIKV and o'nyong-nyong virus is also phosphorylated. We showed that the phosphorylated residues in CHIKV nsP3 are not clustered at the beginning of the HVD. The substitution of 20 Ser/Thr residues located in the N-terminal half of the HVD or 26 Ser/Thr residues located in its C-terminal half with Ala residues reduced the activity of the CHIKV replicase and the infectivity of CHIKV in mammalian cells. Furthermore, the substitution of all 46 potentially phosphorylated residues resulted in the complete loss of viral RNA synthesis and infectivity. The mutations did not affect the interaction of the HVD in nsP3 with the host G3BP1 protein; interactions with CD2AP, BIN1, and FHL1 proteins were significantly reduced but not abolished. Thus, CHIKV differs from SFV and SINV both in the location of the phosphorylated residues in the HVD in nsP3 and, significantly, in their effect on replicase activity and virus infectivity.IMPORTANCE CHIKV outbreaks have affected millions of people, creating a need for the development of antiviral approaches. nsP3 is a component of the CHIKV RNA replicase and is involved in interactions with host proteins and signaling cascades. Phosphorylation of the HVD in nsP3 is important for the virulent alphavirus phenotype. Here, we demonstrate that nsP3 in CHIKV is phosphorylated and that the phosphorylation sites in the HVD are distributed in a unique pattern. Furthermore, the abrogation of some of the phosphorylation sites results in the attenuation of CHIKV, while abolishing all the phosphorylation sites completely blocked its replicase activity. Thus, the phosphorylation of nsP3 and/or the phosphorylation sites in nsP3 have a major impact on CHIKV infectivity. Therefore, they represent promising targets for antiviral compounds and CHIKV attenuation. In addition, this new information offers valuable insight into the vast network of virus-host interactions.

1,3-Thiazolbenzamide Derivatives as Chikungunya Virus nsP2 Protease Inhibitors

Chikungunya fever results from an infection with Chikungunya virus (CHIKV, genus Alphavirus) that is prevalent in tropical regions and is spreading fast to temperate climates with documented outbreaks in Europe and the Americas. Currently, there are no available vaccines or antiviral drugs for prevention or treatment of Chikungunya fever. The nonstructural proteins (nsPs) of CHIKV responsible for virus replication are promising targets for the development of new antivirals. This study was attempted to find out new potential inhibitors of CHIKV nsP2 protease using the ligand-based drug design. Two compounds 10 and 10c, identified by molecular docking, showed antiviral activity against CHIKV with IC₅₀ of 13.1 and 8.3 μM, respectively. Both compounds demonstrated the ability to inhibit the activity of nsP2 in a cell-free assay, and the impact of compound 10 on virus replication was confirmed by western blot. The molecular dynamics study of the interactions of compounds 10 and 10c with CHIKV nsP2 showed that a possible mechanism of action of these compounds is the blocking of the active site and the catalytic dyad of nsP2.

Interdomain Flexibility of Chikungunya Virus nsP2 Helicase-Protease Differentially Influences Viral RNA Replication and Infectivity

Chikungunya virus (CHIKV) is a mosquito-borne alphavirus responsible for chikungunya fever. Nonstructural protein 2 (nsP2), a multifunctional protein essential for viral replication, has an N-terminal helicase region (nsP2h), which has both nucleotide triphosphatase and RNA triphosphatase activities, as well as a C-terminal cysteine protease region (nsP2p), which is responsible for nonstructural polyprotein processing. The two functional units are connected through a linker of 14 residues. Although crystal structures of the helicase and protease regions of CHIKV nsP2 have been solved separately, the conformational arrangement of the full-length nsP2 and the biological role of the linker remain elusive. Using the small-angle X-ray scattering (SAXS) method, we demonstrated that the full-length nsP2 is elongated and partially folded in solution. The reconstructed model of the structure of nsP2 contains a flexible interdomain linker, and there is no direct interaction between the two structured regions. To examine the function of the interdomain linker, we constructed and characterized a set of CHIKV mutants. The deletion of three or five amino acid residues in the linker region resulted in a modest defect in viral RNA replication and transcription but completely abolished viral infectivity. In contrast, increasing the flexibility of nsP2 by lengthening the interdomain linker increased both genomic RNA replication and viral infectivity. The enzymatic activities of the corresponding mutant proteins were largely unaffected. This work suggests that increasing the interdomain flexibility of nsP2 could facilitate the assembly of the replication complex (RC) with increased efficiency and promote virus production.IMPORTANCE CHIKV nsP2 plays multiple roles in viral RNA replication and virus-host interactions. The helicase and protease regions of nsP2 are connected through a short linker. Here, we determined that the conformation of full-length CHIKV nsP2 is elongated and that the protein is flexible in solution. We also highlight the importance of the flexibility of the interdomain of nsP2 on viral RNA synthesis and infectivity. CHIKV mutants harboring shortened linkers fail to produce infectious virus particles despite showing only relatively mild defects in genomic and subgenomic RNA synthesis. Mutations increasing the length of the interdomain linker have only mild and generally beneficial impacts on virus replication. Thus, our findings link interdomain flexibility with the regulation of viral RNA replication and infectivity of the viral genome.

Nitrogen-Based Heterocyclic Compounds: A Promising Class of Antiviral Agents against Chikungunya Virus

Arboviruses, in general, are a global threat due to their morbidity and mortality, which results in an important social and economic impact. Chikungunya virus (CHIKV), one of the most relevant arbovirus currently known, is a re-emergent virus that causes a disease named chikungunya fever, characterized by a severe arthralgia (joint pains) that can persist for several months or years in some individuals. Until now, no vaccine or specific antiviral drug is commercially available. Nitrogen heterocyclic scaffolds are found in medications, such as aristeromycin, favipiravir, fluorouracil, 6-azauridine, thioguanine, pyrimethamine, among others. New families of natural and synthetic nitrogen analogous compounds are reported to have significant anti-CHIKV effects. In the present work, we focus on these nitrogen-based heterocyclic compounds as an important class with CHIKV antiviral activity. We summarize the present understanding on this class of compounds against CHIKV and also present their possible mechanism of action.

Small-Molecule Inhibitors of Chikungunya Virus: Mechanisms of Action and Antiviral Drug Resistance

Chikungunya virus (CHIKV) is a mosquito-transmitted alphavirus that has spread to more than 60 countries worldwide. CHIKV infection leads to a febrile illness known as chikungunya fever (CHIKF), which is characterized by long-lasting and debilitating joint and muscle pain. CHIKV can cause large-scale epidemics with high attack rates, which substantiates the need for development of effective therapeutics suitable for outbreak containment. In this review, we highlight the different strategies used for developing CHIKV small-molecule inhibitors, ranging from high-throughput cell-based screening to in silico screens and enzymatic assays with purified viral proteins.

Chikungunya virus (CHIKV) is a mosquito-transmitted alphavirus that has spread to more than 60 countries worldwide. CHIKV infection leads to a febrile illness known as chikungunya fever (CHIKF), which is characterized by long-lasting and debilitating joint and muscle pain. CHIKV can cause large-scale epidemics with high attack rates, which substantiates the need for development of effective therapeutics suitable for outbreak containment. In this review, we highlight the different strategies used for developing CHIKV small-molecule inhibitors, ranging from high-throughput cell-based screening to in silico screens and enzymatic assays with purified viral proteins. We further discuss the current status of the most promising molecules, including in vitro and in vivo findings. In particular, we focus on describing host and/or viral targets, mode of action, and mechanisms of antiviral drug resistance and associated mutations. Knowledge of the key molecular determinants of drug resistance will aid selection of the most promising antiviral agent(s) for clinical use. For these reasons, we also summarize the available information about drug-resistant phenotypes in Aedes mosquito vectors. From this review, it is evident that more of the active molecules need to be evaluated in preclinical and clinical models to address the current lack of antiviral treatment for CHIKF.

Cross-utilisation of template RNAs by alphavirus replicases

Most alphaviruses (family Togaviridae) including Sindbis virus (SINV) and other human pathogens, are transmitted by arthropods. The first open reading frame in their positive strand RNA genome encodes for the non-structural polyprotein, a precursor to four separate subunits of the replicase. The replicase interacts with cis-acting elements located near the intergenic region and at the ends of the viral RNA genome. A trans-replication assay was developed and used to analyse the template requirements for nine alphavirus replicases. Replicases of alphaviruses of the Semliki Forest virus complex were able to cross-utilize each other’s templates as well as those of outgroup alphaviruses. Templates of outgroup alphaviruses, including SINV and the mosquito-specific Eilat virus, were promiscuous; in contrast, their replicases displayed a limited capacity to use heterologous templates, especially in mosquito cells. The determinants important for efficient replication of template RNA were mapped to the 5' region of the genome. For SINV these include the extreme 5'- end of the genome and sequences corresponding to the first stem-loop structure in the 5' untranslated region. Mutations introduced in these elements drastically reduced infectivity of recombinant SINV genomes. The trans-replicase tools and approaches developed here can be instrumental in studying alphavirus recombination and evolution, but can also be applied to study other viruses such as picornaviruses, flaviviruses and coronaviruses.

Alphaviruses are positive-strand RNA viruses, most of which use mosquitoes to spread between vertebrate hosts; many are human pathogens with potentially severe medical consequences. Some alphavirus species are believed to have resulted from the recombination between different members of the genus and there is evidence of movement of alphaviruses between continents. Here, a novel assay uncoupling viral replicase and template RNA production was developed and used to analyse cross-utilization of alphavirus template RNAs. We observed that replicases of closely related alphaviruses belonging to the Semliki Forest virus complex can generally use each other’s template RNAs as well as those of distantly related outgroup viruses. In contrast, replicases of outgroup viruses clearly preferred homologous template RNAs. These trends were observed in both mammalian and mosquito cells, with template preferences generally more pronounced in mosquito cells. Interestingly, the template RNA of the mosquito-specific Eilat virus was efficiently used by other alphavirus replicases while Eilat replicase could not use heterologous templates. Determinants for template selectivity were mapped to the beginning of the RNA genome and template recognition was more likely based on the recognition of RNA sequences than recognition of structural elements formed by the RNAs.

Identification of Natural Molecular Determinants of Ross River Virus Type I Interferon Modulation

Ross River virus (RRV) belongs to the genus Alphavirus and is prevalent in Australia. RRV infection can cause arthritic symptoms in patients and may include rash, fever, arthralgia, and myalgia. Type I interferons (IFN) are the primary antiviral cytokines and trigger activation of the host innate immune system to suppress the replication of invading viruses. Alphaviruses are able to subvert the type I IFN system, but the mechanisms used are ill defined. In this study, seven RRV field strains were analyzed for induction of and sensitivity to type I IFN. The sensitivities of these strains to human IFN-β varied significantly and were highest for the RRV 2548 strain. Compared to prototype laboratory strain RRV-T48, RRV 2548 also induced higher type I IFN levels both in vitro and in vivo and caused milder disease. To identify the determinants involved in type I IFN modulation, the region encoding the nonstructural proteins (nsPs) of RRV 2548 was sequenced, and 42 amino acid differences from RRV-T48 were identified. Using fragment swapping and site-directed mutagenesis, we discovered that substitutions E402A and R522Q in nsP1 as well as Q619R in nsP2 were responsible for increased sensitivity of RRV 2548 to type I IFN. In contrast, substitutions A31T, N219T, S580L, and Q619R in nsP2 led to induction of higher levels of type I IFN. With exception of E402A, all these variations are common for naturally occurring RRV strains. However, they are different from all known determinants of type I IFN modulation reported previously in nsPs of alphaviruses.IMPORTANCE By identifying natural Ross River virus (RRV) amino acid determinants for type I interferon (IFN) modulation, this study gives further insight into the mechanism of type I IFN modulation by alphaviruses. Here, the crucial role of type I IFN in the early stages of RRV disease pathogenesis is further demonstrated. This study also provides a comparison of the roles of different parts of the RRV nonstructural region in type I IFN modulation, highlighting the importance of nonstructural protein 1 (nsP1) and nsP2 in this process. Three substitutions in nsP1 and nsP2 were found to be independently associated with enhanced type I IFN sensitivity, and four independent substitutions in nsP2 were important in elevated type I IFN induction. Such evidence has clear implications for RRV immunobiology, persistence, and pathology. The identification of viral proteins that modulate type I IFN may also have importance for the pathogenesis of other alphaviruses.

Development of encoded Broccoli RNA aptamers for live cell imaging of alphavirus genomic and subgenomic RNAs

Sindbis virus (SINV) can infect neurons and cause encephalomyelitis in mice. Nonstructural proteins are translated from genomic RNA and structural proteins from subgenomic RNA. While visualization of viral proteins in living cells is well developed, imaging of viral RNAs has been challenging. RNA aptamers that bind and activate conditional fluorophores provide a tool for RNA visualization. We incorporated cassettes of two F30-scaffolded dimers of the Broccoli aptamer into a SINV cDNA clone using sites in nsP3 (genomic RNA), the 3′UTR (genomic and subgenomic RNAs) and after a second subgenomic promoter resulting in 4–28 Broccoli copies. After addition of the cell-permeable 3,5-difluoro-4-hydroxybenzylidene imidazolinone (DFHBI-1T) conditional fluorophore and laser excitation, infected cells emitted green fluorescence that correlated with Broccoli copy numbers. All recombinant viruses replicated well in BHK and undifferentiated neural cells but viruses with 14 or more Broccoli copies were attenuated in differentiated neurons and mice. The signal survived fixation and allowed visualization of viral RNAs in differentiated neurons and mouse brain, as well as BHK cells. Subgenomic RNA was diffusely distributed in the cytoplasm with genomic RNA also in perinuclear vesicle-like structures near envelope glycoproteins or mitochondria. Broccoli aptamer-tagging provides a valuable tool for live cell imaging of viral RNA.

Sensitivity of Alphaviruses to G3BP Deletion Correlates with Efficiency of Replicase Polyprotein Processing

We present a comprehensive overview of the dependency of several Old World alphaviruses for the host protein G3BP. Based on their replication ability in G3BP-deleted cells, Old World alphaviruses can be categorized into two groups, being either resistant or sensitive to G3BP deletion. We observed that all sensitive viruses have an Arg residue at the P4 position of the cleavage site between the nonstructural protein P1 (nsP1) and nsP2 regions of the replicase precursor polyprotein (1/2 site), while a different residue is found at this site in viruses resistant to G3BP deletion. Swapping this residue between resistant and sensitive viruses also switches the G3BP deletion sensitivity. In the absence of G3BP, chikungunya virus (CHIKV) replication is at the limit of detection. The P4 Arg-to-His substitution partially rescues this defect. The P4 residue of the 1/2 site is known to play a regulatory role during processing at this site, and we found that if processing is blocked, the influence of the P4 residue on the sensitivity to G3BP deletion is abolished. Immunofluorescence experiments with CHIKV replicase with manipulated processing indicate that the synthesis of double-stranded RNA is defective in the absence of G3BP and suggest a role of G3BP during negative-strand RNA synthesis. This study provides a functional link between the host protein G3BP and the P4 residue of the 1/2 site for viral RNA replication of Old World alphaviruses. While this suggests a link between G3BP proteins and viral replicase polyprotein processing, we propose that G3BP proteins do not have a regulatory role during polyprotein processing.IMPORTANCE Old World alphaviruses comprise several medically relevant viruses, including chikungunya virus and Ross River virus. Recurrent outbreaks and the lack of antivirals and vaccines demand ongoing research to fight the emergence of these infectious diseases. In this context, a thorough investigation of virus-host interactions is critical. Here, we highlight the importance of the host protein G3BP for several Old World alphaviruses. Our data strongly suggest that G3BP plays a crucial role for the activity of the viral replicase and, thus, the amplification of the viral RNA genome. To our knowledge, the present work is the first to provide a functional link between the regulation of viral polyprotein processing and RNA replication and a host factor for alphaviruses. Moreover, the results of this study raise several questions about the fundamental regulatory mechanisms that dictate the activity of the viral replicase, thereby paving the way for future studies.

Typical Stress Granule Proteins Interact with the 3' Untranslated Region of Enterovirus D68 To Inhibit Viral Replication

Stress granules (SGs) are formed in the cytoplasm under environmental stress, including viral infection. Human enterovirus D68 (EV-D68) is a highly pathogenic virus which can cause serious respiratory and neurological diseases. At present, there is no effective drug or vaccine against EV-D68 infection, and the relationship between EV-D68 infection and SGs is poorly understood. This study revealed the biological function of SGs in EV-D68 infection. Our results suggest that EV-D68 infection induced the accumulation of SG marker proteins Ras GTPase-activated protein-binding protein 1 (G3BP1), T cell intracellular antigen 1 (TIA1), and human antigen R (HUR) in the cytoplasm of infected host cells during early infection but inhibited their accumulation during the late stage. Simultaneously, we revealed that EV-D68 infection induces HUR, TIA1, and G3BP1 colocalization, which marks the formation of typical SGs dependent on protein kinase R (PKR) and eIF2α phosphorylation. In addition, we found that TIA1, HUR, and G3BP1 were capable of targeting the 3' untranslated regions (UTRs) of EV-D68 RNA to inhibit viral replication. However, the formation of SGs in response to arsenite (Ars) gradually decreased as the infection progressed, and G3BP1 was cleaved in the late stage as a strategy to antagonize SGs. Our findings have important implications in understanding the mechanism of interaction between EV-D68 and the host while providing a potential target for the development of antiviral drugs.IMPORTANCE EV-D68 is a serious threat to human health, and there are currently no effective treatments or vaccines. SGs play an important role in cellular innate immunity as a target with antiviral effects. This manuscript describes the formation of SGs induced by EV-D68 early infection but inhibited during the late stage of infection. Moreover, TIA1, HUR, and G3BP1 can chelate a specific site of the 3' UTR of EV-D68 to inhibit viral replication, and this interaction is sequence and complex dependent. However, this inhibition can be antagonized by overexpression of the minireplicon. These findings increase our understanding of EV-D68 infection and may help identify new antiviral targets that can inhibit viral replication and limit the pathogenesis of EV-D68.

Antiviral drug discovery against arthritogenic alphaviruses: Tools and molecular targets

Alphaviruses are (mainly) arthropod-borne viruses that belong to the family of the Togaviridae. Based on the disease they cause, alphaviruses are divided into an arthritogenic and an encephalitic group. Arthritogenic alphaviruses such as the chikungunya virus (CHIKV), the Ross River virus (RRV) and the Mayaro virus (MAYV) have become a serious public health concern in recent years. Epidemics are associated with high morbidity and the infections cause in many patients debilitating joint pain that can persist for months to years. The recent (2013-2014) introduction of CHIKV in the Americas resulted in millions of infected persons. Massive outbreaks of CHIKV and other arthritogenic alphaviruses are likely to occur in the future. Despite the worldwide (re-)emergence of these viruses, there are no antivirals or vaccines available for the treatment or prevention of infections with alphaviruses. It is therefore of utmost importance to develop antiviral strategies against these viruses. We here review the possible molecular targets in the replication cycle of these viruses for the development of antivirals. In addition, we provide an overview of the currently available in vitro systems and mouse infection models that can be used to assess the potential antiviral effect against these viruses.

VCP/p97 Is a Proviral Host Factor for Replication of Chikungunya Virus and Other Alphaviruses

The evolutionarily conserved AAA+ ATPase valosin-containing protein (VCP) was previously shown to be a proviral host factor for several viruses from different viral families such as Flaviviridae, Picornaviridae, and Herpesviridae. VCP was shown to affect trafficking of Sindbis virus receptor and functions as a component of Semliki Forest virus (SFV) replicase compartment. However, the role of this cellular protein was not evaluated during replication of alphaviruses including chikungunya virus (CHIKV). Using siRNA, chemical inhibitors, and trans-replication assays, we show here that VCP is a proviral factor involved in the replication of CHIKV. Immunofluorescence assays confirmed that VCP co-localized with non-structural replicase proteins but not with dsRNA foci possibly due to VCP epitope unavailability. VCP pro-viral role is also observed with other alphaviruses such as o’nyong’nyong virus (ONNV) and SFV in different human cell lines. VCP proviral roles on several viral families now extend to replication of alphaviruses CHIKV and ONNV, emphasizing the pivotal role of VCP in virus–host interaction biology.

Design and Use of Chikungunya Virus Replication Templates Utilizing Mammalian and Mosquito RNA Polymerase I-Mediated Transcription

Chikungunya virus (CHIKV) is a mosquito-borne alphavirus. It has a positive-sense RNA genome that also serves as the mRNA for four nonstructural proteins (nsPs) representing subunits of the viral replicase. Coupling of nsP and RNA synthesis complicates analysis of viral RNA replication. We developed trans-replication systems, where production of replication-competent RNA and expression of viral replicase are uncoupled. Mammalian and mosquito RNA polymerase I promoters were used to produce noncapped RNA templates, which are poorly translated relative to CHIKV replicase-generated capped RNAs. It was found that, in human cells, constructs driven by RNA polymerase I promoters of human and Chinese hamster origin performed equally well. In contrast, RNA polymerase I promoters from Aedes mosquitoes exhibited strong species specificity. In both mammalian and mosquito cells, novel trans-replicase assays had exceptional sensitivity, with up to 10⁵-fold higher reporter expression in the presence of replicase relative to background. Using this highly sensitive assay to analyze CHIKV nsP1 functionality, several mutations that severely reduced, but did not completely block, CHIKV replicase activity were identified: (i) nsP1 tagged at its N terminus with enhanced green fluorescent protein; (ii) mutations D63A and Y248A, blocking the RNA capping; and (iii) mutation R252E, affecting nsP1 membrane anchoring. In contrast, a mutation in the nsP1 palmitoylation site completely inactivated CHIKV replicase in both human and mosquito cells and was lethal for the virus. Our data confirm that this novel system provides a valuable tool to study CHIKV replicase, RNA replication, and virus-host interactions.IMPORTANCE Chikungunya virus (CHIKV) is a medically important pathogen responsible for recent large-scale epidemics. The development of efficient therapies against CHIKV has been hampered by gaps in our understanding of how nonstructural proteins (nsPs) function to form the viral replicase and replicate virus RNA. Here we describe an extremely sensitive assay to analyze the effects of mutations on the virus RNA synthesis machinery in cells of both mammalian (host) and mosquito (vector) origin. Using this system, several lethal mutations in CHIKV nsP1 were shown to reduce but not completely block the ability of its replicase to synthesize viral RNAs. However, in contrast to related alphaviruses, CHIKV replicase was completely inactivated by mutations preventing palmitoylation of nsP1. These data can be used to develop novel, virus-specific antiviral treatments.

Lack of nsP2-specific nuclear functions attenuates chikungunya virus replication both in vitro and in vivo

Chikungunya virus (CHIKV) is an important arthritogenic human pathogen that is already circulating in both hemispheres. In the present study, we substituted VLoop, located on the surface of nsP2, by other amino acid sequences. These modifications had deleterious effects on viral nuclear functions and made CHIKV incapable of interfering with the induction of type I interferon and the antiviral response in both mouse and human cells. Importantly, the identified mutations have no significant effects on the synthesis of virus-specific RNAs and viral structural proteins. The designed mutants induced a few orders of magnitude lower viremia but remained highly immunogenic in mice. Thus, the proposed modifications of nsP2 can additionally improve the safety of the attenuated strain CHIKV 181/25. Furthermore, defined mutations in the macro domain of another nonstructural protein, nsP3, additionally reduce cytopathogenicity of nsP2 mutants in human cells, and can be potentially applied for CHIKV attenuation.

Structural insights into RNA recognition by the Chikungunya virus nsP2 helicase

Chikungunya virus (CHIKV) is transmitted to humans through mosquitoes and causes Chikungunya fever. Nonstructural protein 2 (nsP2) exhibits the protease and RNA helicase activities that are required for viral RNA replication and transcription. Unlike for the C-terminal protease, the structure of the N-terminal RNA helicase (nsP2h) has not been determined. Here, we report the crystal structure of the nsP2h bound to the conserved 3'-end 14 nucleotides of the CHIKV genome and the nonhydrolyzable transition-state nucleotide analog ADP-AlF₄ Overall, the structural analysis revealed that nsP2h adopts a uniquely folded N-terminal domain followed by a superfamily 1 RNA helicase fold. The conserved helicase motifs establish polar contacts with the RNA backbone. There are three hydrophobic residues (Y161, F164, and F287) which form stacking interactions with RNA bases and thereby bend the RNA backbone. An F287A substitution that disrupted these stacking interactions increased the basal ATPase activity but decreased the RNA binding affinity. Furthermore, the F287A substitution reduced viral infectivity by attenuating subgenomic RNA synthesis. Replication of the mutant virus was restored by pseudoreversion (A287V) or adaptive mutations in the RecA2 helicase domain (T358S or V410I). Y161A and/or F164A substitutions, which were designed to disrupt the interactions with the RNA molecule, did not affect the ATPase activity but completely abolished the replication and transcription of viral RNA and the infectivity of CHIKV. Our study sheds light on the roles of the RNA helicase region in viral replication and provides insights that might be applicable to alphaviruses and other RNA viruses in general.

On-demand Labeling of SNAP-tagged Viral Protein for Pulse-Chase Imaging, Quench-Pulse-Chase Imaging, and Nanoscopy-based Inspection of Cell Lysates

Advanced labeling technologies allow researchers to study protein turnover inside intact cells and to track the labeled protein in downstream applications. In the context of a viral infection, the combination of imaging and fluorescent labeling of viral proteins sheds light on their biological activity and interaction with the host cell. Initial approaches have fused fluorescent proteins such as green fluorescent protein (GFP) to the viral protein-of-interest. In contrast, self-labeling enzyme tags such as the commercial SNAP-tag, a modified version of human O⁶-alkylguanine-DNA-alkyltransferase, covalently link synthetic ligands, which users can add on demand. The first two protocols presented here build on previously published protocols for fluorescent labeling in pulse-chase and quench-pulse-chase experiments; the combination of fluorescent labeling with advanced light microscopy visualizes the dynamic turnover of the SNAP-tagged viral protein in intact mammalian cells. A third protocol also outlines how to inspect cellular lysates microscopically for detergent-resistant assemblies of the labeled viral protein. These protocols showcase the flexibility of the SNAP-based labeling system for tracking a viral protein-of-interest in live cells, intact fixed cells, and cell lysates. Moreover, the protocols employ recently developed commercial microscopes (e.g., Airyscan microscopy) that balance resolution, speed, phototoxicity, photobleaching, and ease-of-use.

Novel Mutations in nsP2 Abolish Chikungunya Virus-Induced Transcriptional Shutoff and Make the Virus Less Cytopathic without Affecting Its Replication Rates

Alphavirus infections are characterized by global inhibition of cellular transcription and rapid induction of a cytopathic effect (CPE) in cells of vertebrate origin. Transcriptional shutoff impedes the cellular response to alphavirus replication and prevents establishment of an antiviral state. Chikungunya virus (CHIKV) is a highly pathogenic alphavirus representative, and its nonstructural protein 2 (nsP2) plays critical roles in both inhibition of transcription and CPE development. Previously, we have identified a small peptide in Sindbis virus (SINV) nsP2 (VLoop) that determined the protein's transcriptional inhibition function. It is located in the surface-exposed loop of the carboxy-terminal domain of nsP2 and exhibits high variability between members of different alphavirus serocomplexes. In this study, we found that SINV-specific mutations could not be directly applied to CHIKV. However, by using a new selection approach, we identified a variety of new VLoop variants that made CHIKV and its replicons incapable of inhibiting cellular transcription and dramatically less cytopathic. Importantly, the mutations had no negative effect on RNA and viral replication rates. In contrast to parental CHIKV, the developed VLoop mutants were unable to block induction of type I interferon. Consequently, they were cleared from interferon (IFN)-competent cells without CPE development. Alternatively, in murine cells that have defects in type I IFN production or signaling, the VLoop mutants established persistent, noncytopathic replication. The mutations in nsP2 VLoop may be used for development of new vaccine candidates against alphavirus infections and vectors for expression of heterologous proteins.IMPORTANCE Chikungunya virus is an important human pathogen which now circulates in both the Old and New Worlds. As in the case of other Old World alphaviruses, CHIKV nsP2 not only has enzymatic functions in viral RNA replication but also is a critical inhibitor of the antiviral response and one of the determinants of CHIKV pathogenesis. In this study, we have applied a new strategy to select a variety of CHIKV nsP2 mutants that no longer exhibited transcription-inhibitory functions. The designed CHIKV variants became potent type I interferon inducers and acquired a less cytopathic phenotype. Importantly, they demonstrated the same replication rates as the parental CHIKV. Mutations in the same identified peptide of nsP2 proteins derived from other Old World alphaviruses also abolished their nuclear functions. Such mutations can be further exploited for development of new attenuated alphaviruses.

Understanding Molecular Pathogenesis with Chikungunya Virus Research Tools

Since its re-emergence in 2006, Chikungunya has been a major health concern in endemic areas. Transmitted by Aedes mosquitoes to mammalian hosts, Chikungunya leads to persistent debilitating symptoms in a high proportion of symptomatic human cases. In this review, we present several tools on the mosquito vector side as well as on the mammalian side that have been used to advance research on Chikungunya transmission and immunopathogenesis. These tools lead to key understandings of viral replication in both hosts, and innate and adaptive responses mediating virus clearance and pathology in mammals. This comprehension of viral mechanisms has allowed the development of promising treatment avenues in animal models that will need to be further explored. However, research efforts need to continue in order to develop better and unbiased tools to assess antiviral and treatment strategies as well as further understand immune mechanisms at play in human pathologies.

Sindbis Virus Infection Causes Cell Death by nsP2-Induced Transcriptional Shutoff or by nsP3-Dependent Translational Shutoff

Sindbis virus (SINV) is a representative member of the Alphavirus genus in the Togaviridae family. The hallmark of SINV replication in vertebrate cells is a rapid development of the cytopathic effect (CPE), which usually occurs within 24 h postinfection. Mechanistic understanding of CPE might lead to development of new prophylactic vaccines and therapeutic means against alphavirus infections. However, development of noncytopathic SINV variants and those of other Old World alphaviruses was always highly inefficient and usually resulted in selection of mutants demonstrating poor replication of the viral genome and transcription of subgenomic RNA. This likely caused a nonspecific negative effect on the rates of CPE development. The results of this study demonstrate that CPE induced by SINV and likely by other Old World alphaviruses is a multicomponent process, in which transcriptional and translational shutoffs are the key contributors. Inhibition of cellular transcription and translation is determined by SINV nsP2 and nsP3 proteins, respectively. Defined mutations in the nsP2-specific peptide between amino acids (aa) 674 and 688 prevent virus-induced degradation of the catalytic subunit of cellular-DNA-dependent RNA polymerase II and transcription inhibition and make SINV a strong type I interferon (IFN) inducer without affecting its replication rates. Mutations in the nsP3 macrodomain, which were demonstrated to inhibit its mono-ADP-ribosylhydrolase activity, downregulate the second component of CPE development, inhibition of cellular translation, and also have no effect on virus replication rates. Only the combination of nsP2- and nsP3-specific mutations in the SINV genome has a dramatic negative effect on the ability of virus to induce CPE.IMPORTANCE Alphaviruses are a group of important human and animal pathogens with worldwide distribution. Their characteristic feature is a highly cytopathic phenotype in cells of vertebrate origin. The molecular mechanism of CPE remains poorly understood. In this study, by using Sindbis virus (SINV) as a model of the Old World alphaviruses, we demonstrated that SINV-specific CPE is redundantly determined by viral nsP2 and nsP3 proteins. NsP2 induces the global transcriptional shutoff, and this nuclear function can be abolished by the mutations of the small, surface-exposed peptide in the nsP2 protease domain. NsP3, in turn, determines the development of translational shutoff, and this activity depends on nsP3 macrodomain-associated mono-ADP-ribosylhydrolase activity. A combination of defined mutations in nsP2 and nsP3, which abolish SINV-induced transcription and translation inhibition, in the same viral genome does not affect SINV replication rates but makes it noncytopathic and a potent inducer of type I interferon.

ADP-ribosyl-binding and hydrolase activities of the alphavirus nsP3 macrodomain are critical for initiation of virus replication

Alphaviruses are plus-strand RNA viruses that cause encephalitis, rash, and arthritis. The nonstructural protein (nsP) precursor polyprotein is translated from genomic RNA and processed into four nsPs. nsP3 has a highly conserved macrodomain (MD) that binds ADP-ribose (ADPr), which can be conjugated to protein as a posttranslational modification involving transfer of ADPr from NAD⁺ by poly ADPr polymerases (PARPs). The nsP3^MD also removes ADPr from mono ADP-ribosylated (MARylated) substrates. To determine which aspects of alphavirus replication require nsP3^MD ADPr-binding and/or hydrolysis function, we studied NSC34 neuronal cells infected with chikungunya virus (CHIKV). Infection induced ADP-ribosylation of cellular proteins without increasing PARP expression, and inhibition of MARylation decreased virus replication. CHIKV with a G32S mutation that reduced ADPr-binding and hydrolase activities was less efficient than WT CHIKV in establishing infection and in producing nsPs, dsRNA, viral RNA, and infectious virus. CHIKV with a Y114A mutation that increased ADPr binding but reduced hydrolase activity, established infection like WT CHIKV, rapidly induced nsP translation, and shut off host protein synthesis with reduced amplification of dsRNA. To assess replicase function independent of virus infection, a transreplicase system was used. Mutant nsP3^MDs D10A, G32E, and G112E with no binding or hydrolase activity had no replicase activity, G32S had little, and Y114A was intermediate to WT. Therefore, ADP ribosylation of proteins and nsP3^MD ADPr binding are necessary for initiation of alphavirus replication, while hydrolase activity facilitates amplification of replication complexes. These observations are consistent with observed nsP3^MD conservation and limited tolerance for mutation.

Persistent Replication of a Chikungunya Virus Replicon in Human Cells Is Associated with Presence of Stable Cytoplasmic Granules Containing Nonstructural Protein 3

Chikungunya virus (CHIKV), a mosquito-borne human pathogen, causes a disabling disease characterized by severe joint pain that can persist for weeks, months, or even years in patients. The nonstructural protein 3 (nsP3) plays essential roles during acute infection, but little is known about the function of nsP3 during chronic disease. Here, we used subdiffraction multicolor microscopy for spatial and temporal analysis of CHIKV nsP3 within human cells that persistently replicate replicon RNA. Round cytoplasmic granules of various sizes (i) contained nsP3 and stress granule assembly factors 1 and 2 (G3BP1/2), (ii) were next to double-stranded RNA foci and nsP1-positive structures, and (iii) were close to the nuclear membrane and the nuclear pore complex protein Nup98. Analysis of protein turnover and mobility by live-cell microscopy revealed that the granules could persist for hours to days, accumulated newly synthesized protein, and moved through the cytoplasm at various speeds. The granules also had a static internal architecture and were stable in cell lysates. Refractory cells that had cleared the noncytotoxic replicon regained the ability to respond to arsenite-induced stress. In summary, nsP3 can form uniquely stable granular structures that persist long-term within the host cell. This continued presence of viral and cellular protein complexes has implications for the study of the pathogenic consequences of lingering CHIKV infection and the development of strategies to mitigate the burden of chronic musculoskeletal disease brought about by a medically important arthropod-borne virus (arbovirus).IMPORTANCE Chikungunya virus (CHIKV) is a reemerging alphavirus transmitted by mosquitos and causes transient sickness but also chronic disease affecting muscles and joints. No approved vaccines or antivirals are available. Thus, a better understanding of the viral life cycle and the role of viral proteins can aid in identifying new therapeutic targets. Advances in microscopy and development of noncytotoxic replicons (A. Utt, P. K. Das, M. Varjak, V. Lulla, A. Lulla, A. Merits, J Virol 89:3145-3162, 2015, https://doi.org/10.1128/JVI.03213-14) have allowed researchers to study viral proteins within controlled laboratory environments over extended durations. Here we established human cells that stably replicate replicon RNA and express tagged nonstructural protein 3 (nsP3). The ability to track nsP3 within the host cell and during persistent replication can benefit fundamental research efforts to better understand long-term consequences of the persistence of viral protein complexes and thereby provide the foundation for new therapeutic targets to control CHIKV infection and treat chronic disease symptoms.

Timeliness of Proteolytic Events Is Prerequisite for Efficient Functioning of the Alphaviral Replicase

Polyprotein processing has an important regulatory role in the life cycle of positive-strand RNA viruses. In the case of alphaviruses, sequential cleavage of the nonstructural polyprotein (ns-polyprotein) at three sites eventually yields four mature nonstructural proteins (nsPs) that continue working in complex to replicate viral genomic RNA and transcribe subgenomic RNA. Recognition of cleavage sites by viral nsP2 protease is guided by short sequences upstream of the scissile bond and, more importantly, by the spatial organization of the replication complex. In this study, we analyzed the consequences of the artificially accelerated processing of the Semliki Forest virus ns-polyprotein. It was found that in mammalian cells, not only the order but also the correct timing of the cleavage events is essential for the success of viral replication. Analysis of the effects of compensatory mutations in rescued viruses as well as in vitro translation and trans-replicase assays corroborated our findings and revealed the importance of the V515 residue in nsP2 for recognizing the P4 position in the nsP1/nsP2 cleavage site. We also extended our conclusions to Sindbis virus by analyzing the properties of the hyperprocessive variant carrying the N614D mutation in nsP2. We conclude that the sequence of the nsP1/nsP2 site in alphaviruses is under selective pressure to avoid the presence of sequences that are recognized too efficiently and would otherwise lead to premature cleavage at this site before completion of essential tasks of RNA synthesis or virus-induced replication complex formation. Even subtle changes in the ns-polyprotein processing pattern appear to lead to virus attenuation.IMPORTANCE The polyprotein expression strategy is a cornerstone of alphavirus replication. Three sites within the ns-polyprotein are recognized by the viral nsP2 protease and cleaved in a defined order. Specific substrate targeting is achieved by the recognition of the short sequence upstream of the scissile bond and a correct macromolecular assembly of ns-polyprotein. Here, we highlighted the importance of the timeliness of proteolytic events, as an additional layer of regulation of efficient virus replication. We conclude that, somewhat counterintuitively, the cleavage site sequences at the nsP1/nsP2 and nsP2/nsP3 junctions are evolutionarily selected to be recognized by protease inefficiently, to avoid premature cleavages that would be detrimental for the assembly and functionality of the replication complex. Understanding the causes and consequences of viral polyprotein processing events is important for predicting the properties of mutant viruses and should be helpful for the development of better vaccine candidates and understanding potential mechanisms of resistance to protease inhibitors.

A Chikungunya Virus trans-Replicase System Reveals the Importance of Delayed Nonstructural Polyprotein Processing for Efficient Replication Complex Formation in Mosquito Cells

Chikungunya virus (CHIKV) is a medically important alphavirus that is transmitted by Aedes aegypti and Aedes albopictus mosquitoes. The viral replicase complex consists of four nonstructural proteins (nsPs) expressed as a polyprotein precursor and encompasses all enzymatic activities required for viral RNA replication. nsPs interact with host components of which most are still poorly understood, especially in mosquitos. A CHIKV trans-replicase system that allows the uncoupling of RNA replication and nsP expression was adapted to mosquito cells and subsequently used for analysis of universal and host-specific effects of 17 different nonstructural polyprotein (ns-polyprotein) mutations. It was found that mutations blocking nsP enzymatic activities as well as insertions of enhanced green fluorescent protein (EGFP) into different nsPs had similar effects on trans-replicase activity regardless of the host (i.e., mammalian or mosquito). Mutations that slow down or accelerate ns-polyprotein processing generally had no effect or reduced trans-replicase activity in mammalian cells, while in mosquito cells most of them increased trans-replicase activity prominently. Increased RNA replication in mosquito cells was counteracted by an antiviral RNA interference (RNAi) response. Substitution of the W258 residue in the membrane binding peptide of nsP1 resulted in a temperature-sensitive defect, in the context of both the trans-replicase and infectious CHIKV. The defect was compensated for by secondary mutations selected during passaging of mutant CHIKV. These findings demonstrate the value of alphavirus trans-replicase systems for studies of viral RNA replication and virus-host interactions.IMPORTANCE Chikungunya virus is an important mosquito-transmitted human pathogen. This virus actively replicates in mosquitoes, but the underlying molecular mechanisms and interactions of viral and host components are poorly understood. This is partly due to the lack of reliable systems for functional analysis of viral nonstructural polyproteins (ns-polyproteins) and nonstructural proteins (nsPs) in mosquito cells. Adaption of a CHIKV trans-replicase system allowed study of the effects of mutations in the ns-polyprotein on RNA replication in cells derived from mammalian and mosquito hosts. We found that a slowdown of ns-polyprotein processing facilitates replication complex formation and/or functioning in mosquito cells and that this process is antagonized by the natural RNAi defense system present in mosquito cells. The mosquito-adapted CHIKV trans-replicase system represents a valuable tool to study alphavirus-mosquito interactions at the molecular level and to develop advanced antiviral strategies.

Chikungunya fever: a threat to global public health

Chikungunya fever is an emerging arbovirus infection, representing a serious public health problem. Its etiological agent is the Chikungunya virus (CHIKV). Transmission of this virus is mainly vector by mosquitoes of the genus Aedes, although transmission by blood transfusions and vertical transmission has also been reported. The disease presents high morbidity caused mainly by the arthralgia and arthritis generated. Cardiovascular and neurological manifestations have also been reported. The severity of the infection seems to be directly associated with the action of the virus, but also with the decompensation of preexisting comorbidities. Currently, there are no therapeutic products neither vaccines licensed to the infection CHIKV control, although several vaccine candidates are being evaluated and human polyvalent immunoglobulins anti-CHIKV had been tested. Antibodies can protect against the infection, but in sub-neutralizing concentrations can augment virus infection and exacerbate disease severity. So, the prevention still depends on the use of personal protection measures and vector control, which are only minimally effective.

Molecular Virology of Chikungunya Virus

Chikungunya virus (CHIKV) was discovered more than six decades ago, but has remained poorly investigated. However, after a recent outbreak of CHIK fever in both hemispheres and viral adaptation to new species of mosquitoes, it has attracted a lot of attention. The currently available experimental data suggest that molecular mechanisms of CHIKV replication in vertebrate and mosquito cells are similar to those of other New and Old World alphaviruses. However, this virus exhibits a number of unique characteristics that distinguish it from the other, better studied members of the alphavirus genus. This review is an attempt to summarize the data accumulated thus far regarding the molecular mechanisms of alphavirus RNA replication and interaction with host cells. Emphasis was placed on demonstrating the distinct features of CHIKV in utilizing host factors to build replication complexes and modify the intracellular environment for efficient viral replication and inhibition of the innate immune response. The available data suggest that our knowledge about alphavirus replication contains numerous gaps that potentially hamper the development of new therapeutic means against CHIKV and other pathogenic alphaviruses.

Reverse genetic system, genetically stable reporter viruses and packaged subgenomic replicon based on a Brazilian Zika virus isolate

Zika virus (ZIKV, genus Flavivirus) has emerged as a major mosquito-transmitted human pathogen, with recent outbreaks associated with an increased incidence of neurological complications, particularly microcephaly and the Guillain-Barré syndrome. Because the virus has only very recently emerged as an important pathogen, research is being hampered by a lack of reliable molecular tools. Here we report an infectious cDNA (icDNA) clone for ZIKV isolate BeH819015 from Brazil, which was selected as representative of South American ZIKV isolated at early stages of the outbreak. icDNA clones were assembled from synthetic DNA fragments corresponding to the consensus sequence of the BeH819015 isolate. Virus rescued from the icDNA clone had properties identical to a natural ZIKV isolate from South America. Variants of the clone-derived virus, expressing nanoluciferase, enhanced green fluorescent or mCherry marker proteins in both mammalian and insect cells and being genetically stable for multiple in vitro passages, were obtained. A ZIKV subgenomic replicon, lacking a prM- and E glycoprotein encoding region and expressing a Gaussia luciferase marker, was constructed and shown to replicate both in mammalian and insect cells. In the presence of the Semliki Forest virus replicon, expressing ZIKV structural proteins, the ZIKV replicon was packaged into virus-replicon particles. Efficient reverse genetic systems, genetically stable marker viruses and packaged replicons offer significant improvements for biological studies of ZIKV infection and disease, as well as for the development of antiviral approaches.

SNAP-tagged Chikungunya Virus Replicons Improve Visualisation of Non-Structural Protein 3 by Fluorescence Microscopy

Chikungunya virus (CHIKV), a mosquito-borne alphavirus, causes febrile disease, muscle and joint pain, which can become chronic in some individuals. The non-structural protein 3 (nsP3) plays essential roles during infection, but a complete understanding of its function is lacking. Here we used a microscopy-based approach to image CHIKV nsP3 inside human cells. The SNAP system consists of a self-labelling enzyme tag, which catalyses the covalent linking of exogenously supplemented synthetic ligands. Genetic insertion of this tag resulted in viable replicons and specific labelling while preserving the effect of nsP3 on stress granule responses and co-localisation with GTPase Activating Protein (SH3 domain) Binding Proteins (G3BPs). With sub-diffraction, three-dimensional, optical imaging, we visualised nsP3-positive structures with variable density and morphology, including high-density rod-like structures, large spherical granules, and small, low-density structures. Next, we confirmed the utility of the SNAP-tag for studying protein turnover by pulse-chase labelling. We also revealed an association of nsP3 with cellular lipid droplets and examined the spatial relationships between nsP3 and the non-structural protein 1 (nsP1). Together, our study provides a sensitive, specific, and versatile system for fundamental research into the individual functions of a viral non-structural protein during infection with a medically important arthropod-borne virus (arbovirus).

Imipramine Inhibits Chikungunya Virus Replication in Human Skin Fibroblasts through Interference with Intracellular Cholesterol Trafficking

Chikungunya virus (CHIKV) is an emerging arbovirus of the Togaviridae family that poses a present worldwide threat to human in the absence of any licensed vaccine or antiviral treatment to control viral infection. Here, we show that compounds interfering with intracellular cholesterol transport have the capacity to inhibit CHIKV replication in human skin fibroblasts, a major viral entry site in the human host. Pretreatment of these cells with the class II cationic amphiphilic compound U18666A, or treatment with the FDA-approved antidepressant drug imipramine resulted in a near total inhibition of viral replication and production at the highest concentration used without any cytotoxic effects. Imipramine was found to affect both the fusion and replication steps of the viral life cycle. The key contribution of cholesterol availability to the CHIKV life cycle was validated further by the use of fibroblasts from Niemann-Pick type C (NPC) patients in which the virus was unable to replicate. Interestingly, imipramine also strongly inhibited the replication of several Flaviviridae family members, including Zika, West Nile and Dengue virus. Together, these data show that this compound is a potential drug candidate for anti-arboviral treatment.

Chikungunya virus: an update on the biology and pathogenesis of this emerging pathogen

Re-emergence of chikungunya virus, a mosquito-transmitted pathogen, is of serious public health concern. In the past 15 years, after decades of infrequent, sporadic outbreaks, the virus has caused major epidemic outbreaks in Africa, Asia, the Indian Ocean, and more recently the Caribbean and the Americas. Chikungunya virus is mainly transmitted by Aedes aegypti mosquitoes in tropical and subtropical regions, but the potential exists for further spread because of genetic adaptation of the virus to Aedes albopictus, a species that thrives in temperate regions. Chikungunya virus represents a substantial health burden to affected populations, with symptoms that include severe joint and muscle pain, rashes, and fever, as well as prolonged periods of disability in some patients. The inflammatory response coincides with raised levels of immune mediators and infiltration of immune cells into infected joints and surrounding tissues. Animal models have provided insights into disease pathology and immune responses. Although host innate and adaptive responses have a role in viral clearance and protection, they can also contribute to virus-induced immune pathology. Understanding the mechanisms of host immune responses is essential for the development of treatments and vaccines. Inhibitory compounds targeting key inflammatory pathways, as well as attenuated virus vaccines, have shown some success in animal models, including an attenuated vaccine strain based on an isolate from La Reunion incorporating an internal ribosome entry sequence that prevents the virus from infecting mosquitoes and a vaccine based on virus-like particles expressing envelope proteins. However, immune correlates of protection, as well as the safety of prophylactic and therapeutic candidates, are important to consider for their application in chikungunya infections. In this Review, we provide an update on chikungunya virus with regard to its epidemiology, molecular virology, virus-host interactions, immunological responses, animal models, and potential antiviral therapies and vaccines.

Design and Validation of Novel Chikungunya Virus Protease Inhibitors

Chikungunya virus (CHIKV; genus Alphavirus) is the causative agent of chikungunya fever. CHIKV replication can be inhibited by some broad-spectrum antiviral compounds; in contrast, there is very little information about compounds specifically inhibiting the enzymatic activities of CHIKV replication proteins. These proteins are translated in the form of a nonstructural (ns) P1234 polyprotein precursor from the CHIKV positive-strand RNA genome. Active forms of replicase enzymes are generated using the autoproteolytic activity of nsP2. The available three-dimensional (3D) structure of nsP2 protease has made it a target for in silico drug design; however, there is thus far little evidence that the designed compounds indeed inhibit the protease activity of nsP2 and/or suppress CHIKV replication. In this study, a set of 12 compounds, predicted to interact with the active center of nsP2 protease, was designed using target-based modeling. The majority of these compounds were shown to inhibit the ability of nsP2 to process recombinant protein and synthetic peptide substrates. Furthermore, all compounds found to be active in these cell-free assays also suppressed CHIKV replication in cell culture, the 50% effective concentration (EC₅₀) of the most potent inhibitor being ∼1.5 μM. Analysis of stereoisomers of one compound revealed that inhibition of both the nsP2 protease activity and CHIKV replication depended on the conformation of the inhibitor. Combining the data obtained from different assays also indicates that some of the analyzed compounds may suppress CHIKV replication using more than one mechanism.

Chikungunya virus infectivity, RNA replication and non-structural polyprotein processing depend on the nsP2 protease's active site cysteine residue

Chikungunya virus (CHIKV), genus Alphavirus, family Togaviridae, has a positive-stand RNA genome approximately 12 kb in length. In infected cells, the genome is translated into non-structural polyprotein P1234, an inactive precursor of the viral replicase, which is activated by cleavages carried out by the non-structural protease, nsP2. We have characterized CHIKV nsP2 using both cell-free and cell-based assays. First, we show that Cys478 residue in the active site of CHIKV nsP2 is indispensable for P1234 processing. Second, the substrate requirements of CHIKV nsP2 are quite similar to those of nsP2 of related Semliki Forest virus (SFV). Third, substitution of Ser482 residue, recently reported to contribute to the protease activity of nsP2, with Ala has almost no negative effect on the protease activity of CHIKV nsP2. Fourth, Cys478 to Ala as well as Trp479 to Ala mutations in nsP2 completely abolished RNA replication in CHIKV and SFV trans-replication systems. In contrast, trans-replicases with Ser482 to Ala mutation were similar to wild type counterparts. Fifth, Cys478 to Ala as well as Trp479 to Ala mutations in nsP2 abolished the rescue of infectious virus from CHIKV RNA transcripts while Ser482 to Ala mutation had no effect. Thus, CHIKV nsP2 is a cysteine protease.

Versatile Trans-Replication Systems for Chikungunya Virus Allow Functional Analysis and Tagging of Every Replicase Protein

Chikungunya virus (CHIKV; genus Alphavirus, family Togaviridae) has recently caused several major outbreaks affecting millions of people. There are no licensed vaccines or antivirals, and the knowledge of the molecular biology of CHIKV, crucial for development of efficient antiviral strategies, remains fragmentary. CHIKV has a 12 kb positive-strand RNA genome, which is translated to yield a nonstructural (ns) or replicase polyprotein. CHIKV structural proteins are expressed from a subgenomic RNA synthesized in infected cells. Here we have developed CHIKV trans-replication systems, where replicase expression and RNA replication are uncoupled. Bacteriophage T7 RNA polymerase or cellular RNA polymerase II were used for production of mRNAs for CHIKV ns polyprotein and template RNAs, which are recognized by CHIKV replicase and encode for reporter proteins. CHIKV replicase efficiently amplified such RNA templates and synthesized large amounts of subgenomic RNA in several cell lines. This system was used to create tagged versions of ns proteins including nsP1 fused with enhanced green fluorescent protein and nsP4 with an immunological tag. Analysis of these constructs and a matching set of replicon vectors revealed that the replicases containing tagged ns proteins were functional and maintained their subcellular localizations. When cells were co-transfected with constructs expressing template RNA and wild type or tagged versions of CHIKV replicases, formation of characteristic replicase complexes (spherules) was observed. Analysis of mutations associated with noncytotoxic phenotype in CHIKV replicons showed that a low level of RNA replication is not a pre-requisite for reduced cytotoxicity. The CHIKV trans-replicase does not suffer from genetic instability and represents an efficient, sensitive and reliable tool for studies of different aspects of CHIKV RNA replication process.

Mefenamic acid in combination with ribavirin shows significant effects in reducing chikungunya virus infection in vitro and in vivo

Chikungunya virus (CHIKV) infection is a persistent problem worldwide due to efficient adaptation of the viral vectors, Aedes aegypti and Aedes albopictus mosquitoes. Therefore, the absence of effective anti-CHIKV drugs to combat chikungunya outbreaks often leads to a significant impact on public health care. In this study, we investigated the antiviral activity of drugs that are used to alleviate infection symptoms, namely, the non-steroidal anti-inflammatory drugs (NSAIDs), on the premise that active compounds with potential antiviral and anti-inflammatory activities could be directly subjected for human use to treat CHIKV infections. Amongst the various NSAID compounds, Mefenamic acid (MEFE) and Meclofenamic acid (MECLO) showed considerable antiviral activity against viral replication individually or in combination with the common antiviral drug, Ribavirin (RIBA). The 50% effective concentration (EC50) was estimated to be 13 μM for MEFE, 18 μM for MECLO and 10 μM for RIBA, while MEFE + RIBA (1:1) exhibited an EC50 of 3 μM, and MECLO + RIBA (1:1) was 5 μM. Because MEFE is commercially available and its synthesis is easier compared with MECLO, MEFE was selected for further in vivo antiviral activity analysis. Treatment with MEFE + RIBA resulted in a significant reduction of hypertrophic effects by CHIKV on the mouse liver and spleen. Viral titre quantification in the blood of CHIKV-infected mice through the plaque formation assay revealed that treatment with MEFE + RIBA exhibited a 6.5-fold reduction compared with untreated controls. In conclusion, our study demonstrated that MEFE in combination with RIBA exhibited significant anti-CHIKV activity by impairing viral replication in vitro and in vivo. Indeed, this finding may lead to an even broader application of these combinatorial treatments against other viral infections.

Functions of Chikungunya Virus Nonstructural Proteins

The nonstructural proteins (nsPs) of chikungunya virus (CHIKV) are expressed as one or two polyprotein precursors, which are translated directly from the viral genomic RNA. Mature nsPs are generated by precise processing of these polyproteins. Both the precursors and mature nsPs are essential for CHIKV replication. Similar to other alphaviruses, CHIKV nsPs not only perform virus RNA replication but are also crucial for other activities essential for virus infection and pathogenesis. Thus far the best-studied CHIKV ns-protein is nsP2, for which protease, NTPase, RNA triphosphatase, and RNA helicase activities have been demonstrated. In addition, nsP2 is crucial for shut-off of host cell transcription and translation and it counteracts cellular antiviral responses. Compared to their homologues from the well-studied Sindbis and Semliki Forest viruses, CHIKV nsP1, nsP3, and nsP4 have been subjected to only few studies. Nevertheless, there are strong indirect pieces of evidence indicating that these CHIKV proteins have the same enzymatic activities as their counterparts in the other alphaviruses. Information concerning the specific interaction of CHIKV nsPs with host components is beginning to emerge. All the nsPs are involved in the functioning of membrane-bound replication complexes also called spherules, but the finer details of the structure and assembly of these complexes are currently poorly understood.

Evaluation of chikungunya virus infection in children from India during 2009-2010: A cross sectional observational study

Chikungunya virus, a small (about 60-70 nm diameter), spherical, enveloped, positive, single stranded RNA virus is transmitted by Aedes mosquitoes. After a short period of incubation (3-5 days) symptoms like fever with joint pains and others start appearing. After a gap of 20 years, this virus re-emerged during 2006-2008 in India causing a major outbreak of CHIKV in India. This study was conducted subsequent to the major outbreak in order to evaluate the proportion of chikungunya virus infection in children with suggestive symptoms at three geographical locations of India. Lineage of circulating strains and changes in the E1 structural polypeptide were also determined. Blood samples were collected (in Sodium citrate vacutainer tubes) during 1st June 2009 to 31st May 2010 from children (age 0 ≤ 18 years) suspected to have chikungunya infection, that is, those who presented with sudden onset of fever and/or joint pain, myalgia, and headache from three regions of India, All India Institute of Medical Sciences (AIIMS) in New Delhi, Karnataka Institute of Medical Sciences (KIMS) in Hubli and Sawai Mansingh Medical College (SMS) in Jaipur. Detection of CHIKV antibodies in all acute-phase patient plasma samples was done by IgM ELISA and for samples within ≤5 days of fever, a one-step RT-PCR was carried out on a block thermo-cycler targeting 294 bp region of E1 gene that codes for the viral envelope protein. Comparison of nucleotide and amino acid sequences from few positive samples of two regions was done with African S-27 reference strain using BioEdit. A phylogenetic tree was constructed using MEGA 6 by using the Maximum Likelihood method based on the Kimura 2-parameter model. Out of the 723 acute phase samples tested from three geographical locations of India, Chikungunya virus infection was detected in 249/723 (34.44%) subjects by either IgM Elisa (180/723) or RT-PCR (69/412). RT-PCR was employed in samples collected from children with ≤5 days of fever. Maximum positive cases were from KIMS center, Hubli. Seasonally, positivity varied with number of enrolled cases at KIMS and SMS. Joint pain was significantly associated with CHIKV positivity (P = 0.0156). Presence/absence of certain clinical features varied with age (P < 0.05). Sequence analysis revealed four amino acid changes. Phylogenetic analysis with partial sequences of E1 gene from KIMS (n = 12) and SMS (n = 5) showed that the study isolates clustered with Indian Ocean Lineage strains (IOL) of East, Central and South African (ECSA) type. Evaluation of chikungunya virus infection in children from India during 2009-2010 showed high proportion of CHIKV infection in Southern region of India compared to Northern region. The circulating CHIKV strains were of Indian Ocean Lineage (IOL) group within the East, Central, and South African (ECSA) genotype. However few amino acid changes were observed in E1 polypeptide with reference to African strain S-27 (AF369024). Further studies are needed to know the implications of these changes in vector-pathogen compatibility and host-pathogen interactivity. As a whole, this study highlighted the proportion of CHIKV cases, lineage of causative strain and evolutionary pattern of circulating strain in terms of amino acid changes in the structural protein.

Chikungunya nsP2 protease is not a papain-like cysteine protease and the catalytic dyad cysteine is interchangeable with a proximal serine

Chikungunya virus is the pathogenic alphavirus that causes chikungunya fever in humans. In the last decade millions of cases have been reported around the world from Africa to Asia to the Americas. The alphavirus nsP2 protein is multifunctional and is considered to be pivotal to viral replication, as the nsP2 protease activity is critical for proteolytic processing of the viral polyprotein during replication. Classically the alphavirus nsP2 protease is thought to be papain-like with the enzyme reaction proceeding through a cysteine/histidine catalytic dyad. We performed structure-function studies on the chikungunya nsP2 protease and show that the enzyme is not papain-like. Characterization of the catalytic dyad cysteine residue enabled us to identify a nearby serine that is catalytically interchangeable with the dyad cysteine residue. The enzyme retains activity upon alanine replacement of either residue but a replacement of both cysteine and serine residues results in no detectable activity. Protein dynamics appears to allow the use of either the cysteine or the serine residue in catalysis. This switchable dyad residue has not been previously reported for alphavirus nsP2 proteases and would have a major impact on the nsP2 protease as an anti-viral target.

Differential Phosphatidylinositol-3-Kinase-Akt-mTOR Activation by Semliki Forest and Chikungunya Viruses Is Dependent on nsP3 and Connected to Replication Complex Internalization

Many viruses affect or exploit the phosphatidylinositol-3-kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR) pathway, a crucial prosurvival signaling cascade. We report that this pathway was strongly activated in cells upon infection with the Old World alphavirus Semliki Forest virus (SFV), even under conditions of complete nutrient starvation. We mapped this activation to the hyperphosphorylated/acidic domain in the C-terminal tail of SFV nonstructural protein nsP3. Viruses with a deletion of this domain (SFV-Δ50) but not of other regions in nsP3 displayed a clearly delayed and reduced capacity of Akt stimulation. Ectopic expression of the nsP3 of SFV wild type (nsP3-wt), but not nsP3-Δ50, equipped with a membrane anchor was sufficient to activate Akt. We linked PI3K-Akt-mTOR stimulation to the intracellular dynamics of viral replication complexes, which are formed at the plasma membrane and subsequently internalized in a process blocked by the PI3K inhibitor wortmannin. Replication complex internalization was observed upon infection of cells with SFV-wt and SFV mutants with deletions in nsP3 but not with SFV-Δ50, where replication complexes were typically accumulated at the cell periphery. In cells infected with the closely related chikungunya virus (CHIKV), the PI3K-Akt-mTOR pathway was only moderately activated. Replication complexes of CHIKV were predominantly located at the cell periphery. Exchanging the hypervariable C-terminal tail of nsP3 between SFV and CHIKV induced the phenotype of strong PI3K-Akt-mTOR activation and replication complex internalization in CHIKV. In conclusion, infection with SFV but not CHIKV boosts PI3K-Akt-mTOR through the hyperphosphorylated/acidic domain of nsP3 to drive replication complex internalization.

IMPORTANCE

SFV and CHIKV are very similar in terms of molecular and cell biology, e.g., regarding replication and molecular interactions, but are strikingly different regarding pathology: CHIKV is a relevant human pathogen, causing high fever and joint pain, while SFV is a low-pathogenic model virus, albeit neuropathogenic in mice. We show that both SFV and CHIKV activate the prosurvival PI3K-Akt-mTOR pathway in cells but greatly differ in their capacities to do so: Akt is strongly and persistently activated by SFV infection but only moderately activated by CHIKV. We mapped this activation capacity to a region in nonstructural protein 3 (nsP3) of SFV and could functionally transfer this region to CHIKV. Akt activation is linked to the subcellular dynamics of replication complexes, which are efficiently internalized from the cell periphery for SFV but not CHIKV. This difference in signal pathway stimulation and replication complex localization may have implications for pathology.

Viral Polymerase-Helicase Complexes Regulate Replication Fidelity To Overcome Intracellular Nucleotide Depletion

To date, the majority of work on RNA virus replication fidelity has focused on the viral RNA polymerase, while the potential role of other viral replicase proteins in this process is poorly understood. Previous studies used resistance to broad-spectrum RNA mutagens, such as ribavirin, to identify polymerases with increased fidelity that avoid misincorporation of such base analogues. We identified a novel variant in the alphavirus viral helicase/protease, nonstructural protein 2 (nsP2) that operates in concert with the viral polymerase nsP4 to further alter replication complex fidelity, a functional linkage that was conserved among the alphavirus genus. Purified chikungunya virus nsP2 presented delayed helicase activity of the high-fidelity enzyme, and yet purified replication complexes manifested stronger RNA polymerization kinetics. Because mutagenic nucleoside analogs such as ribavirin also affect intracellular nucleotide pools, we addressed the link between nucleotide depletion and replication fidelity by using purine and pyrimidine biosynthesis inhibitors. High-fidelity viruses were more resistant to these conditions, and viral growth could be rescued by the addition of exogenous nucleosides, suggesting that mutagenesis by base analogues requires nucleotide pool depletion. This study describes a novel function for nsP2, highlighting the role of other components of the replication complex in regulating viral replication fidelity, and suggests that viruses can alter their replication complex fidelity to overcome intracellular nucleotide-depleting conditions.

IMPORTANCE

Previous studies using the RNA mutagen ribavirin to select for drug-resistant variants have highlighted the essential role of the viral RNA-dependent RNA polymerase in regulating replication fidelity. However, the role of other viral replicase components in replication fidelity has not been studied in detail. We identified here an RNA mutagen-resistant variant of the nsP2 helicase/protease that conferred increased fidelity and yet could not operate in the same manner as high-fidelity polymerases. We show that the alphavirus helicase is a key component of the fidelity-regulating machinery. Our data show that the RNA mutagenic activity of compounds such as ribavirin is coupled to and potentiated by nucleotide depletion and that RNA viruses can fine-tune their replication fidelity when faced with an intracellular environment depleted of nucleotides.