Mechanism of Poliovirus Resistance to Host Phosphatidylinositol-4 Kinase III β Inhibitor

Evolution of drug resistance against antiviral agents that target cellular factors

Antiviral drugs have classically been developed by directly disrupting the functions of viral proteins. However, this strategy has been largely unsuccessful due to the rapid generation of viral escape mutants. It has been well established that as compared to the virus-centric approach, the strategy of developing antiviral drugs by targeting host-dependency factors (HDFs) minimizes drug resistance. However, recent reports have indicated that drug resistance against some of the host-targeting antiviral agents can in fact occur under some circumstances. Long-term selection pressure of a host-targeting antiviral agent may induce the virus to use an alternate cellular factor or alters its affinity towards the target that confers resistance. Alternatively, virus may synchronize its life cycle with the patterns of drug therapy. In addition, virus may subvert host's immune system to perpetuate under the limiting conditions of the targeted cellular factor. This review describes novel potential mechanisms that may account for the acquiring resistance against agents that target HDFs.

Antiviral Development for the Polio Endgame: Current Progress and Future Directions

As the world is approaching the eradication of wild poliovirus serotype 1, the last of the three wild types, the question of how to maintain a polio-free world becomes imminent. To mitigate the risk of sporadic vaccine-associated paralytic polio (VAPP) caused by oral polio vaccines (OPVs) that are routinely used in global immunization programs, the Polio Antivirals Initiative (PAI) was established in 2006. The primary goal of the PAI is to facilitate the discovery and development of antiviral drugs to stop the excretion of immunodeficiency-associated vaccine-derived poliovirus (iVDPV) in B cell-deficient individuals. This review summarizes the major progress that has been made in the development of safe and effective poliovirus antivirals and highlights the candidates that have shown promising results in vitro, in vivo, and in clinical trials.

Bithiazole inhibitors of PI4KB show broad-spectrum antiviral activity against different viral families

Broad-spectrum antivirals can be extremely important for pandemic preparedness. Targeting host factors dispensable for the host but indispensable for the virus can result in high barrier to resistance and a large range of viruses targeted. PI4KB is a lipid kinase involved in the replication of several RNA viruses, but common inhibitors of this target are mainly active against members of the Picornaviridae family. Herein we describe the optimization of bithiazole PI4KB inhibitors as broad-spectrum antivirals (BSAs) active against different members of the Picornaviridae, Coronaviridae, Flaviviridae and Poxviridae families. Since some of these viruses are transmitted via respiratory route, the efficacy of one of the most promising compounds was evaluated in an airway model. The molecule showed complete viral inhibition and absence of toxicity. These results pave the road for the development of new BSAs.

An efficient trans complementation system for in vivo replication of defective poliovirus mutants

The picornavirus genome encodes a large, single polyprotein that is processed by viral proteases to form an active replication complex. The replication complex is formed with the viral genome, host proteins, and viral proteins that are produced/translated directly from each of the viral genomes (viral proteins provided in cis). Efficient complementation in vivo of replication complex formation by viral proteins provided in trans, thus exogenous or ectopically expressed viral proteins, remains to be demonstrated. Here, we report an efficient trans complementation system for the replication of defective poliovirus (PV) mutants by a viral polyprotein precursor in HEK293 cells. Viral 3AB in the polyprotein, but not 2BC, was processed exclusively in cis. Replication of a defective PV replicon mutant, with a disrupted cleavage site for viral 3Cpro protease between 3Cpro and 3Dpol (3C/D[A/G] mutant) could be rescued by a viral polyprotein provided in trans. Only a defect of 3Dpol activity of the replicon could be rescued in trans; inactivating mutations in 2CATPase/hel, 3B, and 3Cpro of the replicon completely abrogated the trans-rescued replication. An intact N-terminus of the 3Cpro domain of the 3CDpro provided in trans was essential for the trans-active function. By using this trans complementation system, a high-titer defective PV pseudovirus (PVpv) (>107 infectious units per mL) could be produced with the defective mutants, whose replication was completely dependent on trans complementation. This work reveals potential roles of exogenous viral proteins in PV replication and offers insights into protein/protein interaction during picornavirus infection.

IMPORTANCE

Viral polyprotein processing is an elaborately controlled step by viral proteases encoded in the polyprotein; fully processed proteins and processing intermediates need to be correctly produced for replication, which can be detrimentally affected even by a small modification of the polyprotein. Purified/isolated viral proteins can retain their enzymatic activities required for viral replication, such as protease, helicase, polymerase, etc. However, when these proteins of picornavirus are exogenously provided (provided in trans) to the viral replication complex with a defective viral genome, replication is generally not rescued/complemented, suggesting the importance of viral proteins endogenously provided (provided in cis) to the replication complex. In this study, I discovered that only the viral polymerase activity of poliovirus (PV) (the typical member of picornavirus family) could be efficiently rescued by exogenously expressed viral proteins. The current study reveals potential roles for exogenous viral proteins in viral replication and offers insights into interactions during picornavirus infection.

Picornavirus 2C proteins: structure-function relationships and interactions with host factors

Picornaviruses, which are positive-stranded, non-enveloped RNA viruses, are known to infect people and animals with a broad spectrum of diseases. Among the nonstructural proteins in picornaviruses, 2C proteins are highly conserved and exhibit multiple structural domains, including amphipathic α-helices, an ATPase structural domain, and a zinc finger structural domain. This review offers a comprehensive overview of the functional structures of picornaviruses' 2C protein. We summarize the mechanisms by which the 2C protein enhances viral replication. 2C protein interacts with various host factors to form the replication complex, ultimately promoting viral replication. We review the mechanisms through which picornaviruses' 2C proteins interact with the NF-κB, RIG-I, MDA5, NOD2, and IFN pathways, contributing to the evasion of the antiviral innate immune response. Additionally, we provide an overview of broad-spectrum antiviral drugs for treating various enterovirus infections, such as guanidine hydrochloride, fluoxetine, and dibucaine derivatives. These drugs may exert their inhibitory effects on viral infections by targeting interactions with 2C proteins. The review underscores the need for further research to elucidate the precise mechanisms of action of 2C proteins and to identify additional host factors for potential therapeutic intervention. Overall, this review contributes to a deeper understanding of picornaviruses and offers insights into the antiviral strategies against these significant viral pathogens.

Picornavirus 3C Proteins Intervene in Host Cell Processes through Proteolysis and Interactions with RNA

The Picornaviridae family comprises a large group of non-enveloped viruses with enormous impact on human and animal health. The picornaviral genome contains one open reading frame encoding a single polyprotein that can be processed by viral proteases. The picornaviral 3C proteases share similar three-dimensional structures and play a significant role in the viral life cycle and virus-host interactions. Picornaviral 3C proteins also have conserved RNA-binding activities that contribute to the assembly of the viral RNA replication complex. The 3C protease is important for regulating the host cell response through the cleavage of critical host cell proteins, acting to selectively 'hijack' host factors involved in gene expression, promoting picornavirus replication, and inactivating key factors in innate immunity signaling pathways. The protease and RNA-binding activities of 3C are involved in viral polyprotein processing and the initiation of viral RNA synthesis. Most importantly, 3C modifies critical molecules in host organelles and maintains virus infection by subtly subverting host cell death through the blocking of transcription, translation, and nucleocytoplasmic trafficking to modulate cell physiology for viral replication. Here, we discuss the molecular mechanisms through which 3C mediates physiological processes involved in promoting virus infection, replication, and release.

CUR-N399, a PI4KB inhibitor, for the treatment of Enterovirus A71 infection

Over the years, the hand, foot and mouth disease (HFMD) has sparked epidemics across many countries which mainly affected young children. While symptoms are usually mild, severe complications may arise, and some even lead to death. Such concerns, coupled with the lack of approved vaccines and antivirals to date, create an urgency in the identification of safe therapeutics against HFMD. The disease is mainly transmitted by enteroviruses like enterovirus A71 (EV-A71). Essential for enterovirus replication is the host protein, PI4KB. In this study, we investigate the antiviral efficacy of a novel PI4KB inhibitor, CUR-N399. We found that CUR-N399 displayed broad-spectrum antiviral activity against picornaviruses in cell culture models. Using a suckling mouse model of lethal EV-A71 infection, CUR-N399 was found to be well-tolerated, promote survival and reduce viral titre in mice organs. Together, these support the discovery of CUR-N399 as an antiviral against EV-A71 and potentially other closely related viruses.

Vemurafenib Inhibits Acute and Chronic Enterovirus Infection by Affecting Cellular Kinase Phosphatidylinositol 4-Kinase Type IIIβ

Enteroviruses are one of the most abundant viruses causing mild to serious acute infections in humans and also contributing to chronic diseases like type 1 diabetes. Presently, there are no approved antiviral drugs against enteroviruses. Here, we studied the potency of vemurafenib, an FDA-approved RAF kinase inhibitor for treating BRAFV600E mutant-related melanoma, as an antiviral against enteroviruses. We showed that vemurafenib prevented enterovirus translation and replication at low micromolar dosage in an RAF/MEK/ERK-independent manner. Vemurafenib was effective against group A, B, and C enteroviruses, as well as rhinovirus, but not parechovirus or more remote viruses such as Semliki Forest virus, adenovirus, and respiratory syncytial virus. The inhibitory effect was related to a cellular phosphatidylinositol 4-kinase type IIIβ (PI4KB), which has been shown to be important in the formation of enteroviral replication organelles. Vemurafenib prevented infection efficiently in acute cell models, eradicated infection in a chronic cell model, and lowered virus amounts in pancreas and heart in an acute mouse model. Altogether, instead of acting through the RAF/MEK/ERK pathway, vemurafenib affects the cellular PI4KB and, hence, enterovirus replication, opening new possibilities to evaluate further the potential of vemurafenib as a repurposed drug in clinical care. IMPORTANCE Despite the prevalence and medical threat of enteroviruses, presently, there are no antivirals against them. Here, we show that vemurafenib, an FDA-approved RAF kinase inhibitor for treating BRAFV600E mutant-related melanoma, prevents enterovirus translation and replication. Vemurafenib shows efficacy against group A, B, and C enteroviruses, as well as rhinovirus, but not parechovirus or more remote viruses such as Semliki Forest virus, adenovirus, and respiratory syncytial virus. The inhibitory effect acts through cellular phosphatidylinositol 4-kinase type IIIβ (PI4KB), which has been shown to be important in the formation of enteroviral replication organelles. Vemurafenib prevents infection efficiently in acute cell models, eradicates infection in a chronic cell model, and lowers virus amounts in pancreas and heart in an acute mouse model. Our findings open new possibilities to develop drugs against enteroviruses and give hope for repurposing vemurafenib as an antiviral drug against enteroviruses.

Characterization of Anti-Poliovirus Compounds Isolated from Edible Plants

Poliovirus (PV) is the causative agent of poliomyelitis and is a target of the global eradication programs of the World Health Organization (WHO). After eradication of type 2 and 3 wild-type PVs, vaccine-derived PV remains a substantial threat against the eradication as well as type 1 wild-type PV. Antivirals could serve as an effective means to suppress the outbreak; however, no anti-PV drugs have been approved at present. Here, we screened for effective anti-PV compounds in a library of edible plant extracts (a total of 6032 extracts). We found anti-PV activity in the extracts of seven different plant species. We isolated chrysophanol and vanicoside B (VCB) as the identities of the anti-PV activities of the extracts of Rheum rhaponticum and Fallopia sachalinensis, respectively. VCB targeted the host PI4KB/OSBP pathway for its anti-PV activity (EC₅₀ = 9.2 μM) with an inhibitory effect on in vitro PI4KB activity (IC₅₀ = 5.0 μM). This work offers new insights into the anti-PV activity in edible plants that may serve as potent antivirals for PV infection.

Essential Domains of Oxysterol-Binding Protein Required for Poliovirus Replication

Oxysterol-binding protein (OSBP) is a host factor required for enterovirus (EV) replication. OSBP locates at membrane contact site and acts as a lipid exchanger of cholesterol and phosphatidylinositol 4-phosphate (PI4P) between cellular organelles; however, the essential domains required for the viral replication remain unknown. In this study, we define essential domains of OSBP for poliovirus (PV) replication by a functional dominance assay with a series of deletion variants of OSBP. We show that the pleckstrin homology domain (PHD) and the ligand-binding domain, but not the N-terminal intrinsically disordered domain, coiled-coil region, or the FFAT motif, are essential for PV replication. The PHD serves as the primary determinant of OSBP targeting to the replication organelle in the infected cells. These results suggest that not all the domains that support important biological functions of OSBP are essential for the viral replication.

Ligand Recognition by the Lipid Transfer Domain of Human OSBP Is Important for Enterovirus Replication

Oxysterol-binding protein (OSBP), which transports cholesterol and phosphatidylinositol 4-monophosphate (PtdIns[4]P) between different organelles, serves as a conserved host factor for the replication of various viruses, and OSBP inhibitors exhibit antiviral effects. Here, we determined the crystal structure of the lipid transfer domain of human OSBP in complex with endogenous cholesterol. The hydrocarbon tail and tetracyclic ring of cholesterol interact with the hydrophobic tunnel of OSBP, and the hydroxyl group of cholesterol forms a hydrogen bond network at the bottom of the tunnel. Systematic mutagenesis of the ligand-binding region revealed that M446W and L590W substitutions confer functional tolerance to an OSBP inhibitor, T-00127-HEV2. Employing the M446W variant as a functional replacement for the endogenous OSBP in the presence of T-00127-HEV2, we have identified previously unappreciated amino acid residues required for viral replication. The combined use of the inhibitor and the OSBP variant will be useful in elucidating the enigmatic in vivo functions of OSBP.

High-Order Epistasis and Functional Coupling of Infection Steps Drive Virus Evolution toward Independence from a Host Pathway

The phosphatidylinositol-4 kinase IIIβ (PI4KB)/oxysterol-binding protein (OSBP) family I pathway serves as an essential host pathway for the formation of viral replication complex for viral plus-strand RNA synthesis; however, poliovirus (PV) could evolve toward substantial independence from this host pathway with four mutations. Recessive epistasis of the two mutations (3A-R54W and 2B-F17L) is essential for viral RNA replication. Quantitative analysis of effects of the other two mutations (2B-Q20H and 2C-M187V) on each step of infection reveals functional couplings between viral replication, growth, and spread conferred by the 2B-Q20H mutation, while no enhancing effect was conferred by the 2C-M187V mutation. The effects of the 2B-Q20H mutation occur only via another recessive epistasis between the 3A-R54W/2B-F17L mutations. These mutations confer enhanced replication in PI4KB/OSBP-independent infection concomitantly with an increased ratio of viral plus-strand RNA to the minus-strand RNA. This work reveals the essential roles of the functional coupling and high-order, multi-tiered recessive epistasis in viral evolution toward independence from an obligatory host pathway.

IMPORTANCE Each virus has a different strategy for its replication, which requires different host factors. Enterovirus, a model RNA virus, requires host factors PI4KB and OSBP, which form an obligatory functional axis to support viral replication. In an experimental evolution system in vitro, virus mutants that do not depend on these host factors could arise only with four mutations. The two mutations (3A-R54W and 2B-F17L) are required for the replication but are not sufficient to support efficient infection. Another mutation (2B-Q20H) is essential for efficient spread of the virus. The order of introduction of the mutations in the viral genome is essential (known as “epistasis”), and functional couplings of infection steps (i.e., viral replication, growth, and spread) have substantial roles to show the effects of the 2B-Q20H mutation. These observations would provide novel insights into an evolutionary pathway of the virus to require host factors for infection.

Research progress of phosphatidylinositol 4-kinase and its inhibitors in inflammatory diseases

Phosphatidylinositol 4-kinase (PI4K) is a lipid kinase that can catalyze the transfer of phosphate group from ATP to the inositol ring of phosphatidylinositol (PtdIns) resulting in the phosphorylation of PtdIns at 4-OH sites, to generate phosphatidylinositol 4-phosphate (PI4P). Studies on biological functions reveal that PI4K is closely related to the occurrence and development of various inflammatory diseases such as obesity, cancer, viral infections, malaria, Alzheimer's disease, etc. PI4K-related inhibitors have been found to have the effects of inhibiting virus replication, anti-cancer, treating malaria and reducing rejection in organ transplants, among which MMV390048, an anti-malaria drug, has entered phase II clinical trial. This review discusses the classification, structure, distribution and related inhibitors of PI4K and their role in the progression of cancer, viral replication, and other inflammation induced diseases to explore their potential as therapeutic targets.

Novel capsid binder and PI4KIIIbeta inhibitors for EV-A71 replication inhibition

The Hand, Foot and Mouth Disease (HFMD) is a highly contagious viral illness generally manifests as a mild disease in young children and immunocompromised adults. It has however emerged as a significant public health threat in recent years as outbreaks have been occurring regularly, especially in the Asia–Pacific. The disease can result from infections by a wide variety of human enteroviruses, particularly, Enterovirus A71 (EV-A71) has garnered more attention due to its association with severe disease in infected patients. Despite the potential to result severe neurological complications or even fatality, there is currently no effective antiviral for treatment of EV-A71 infections and the only vaccines available are restricted to distribution in China. In this study, we report the in vitro and in vivo evaluation of two candidate antiviral compounds active against EV-A71, a viral capsid inhibitor (G197) and a novel host-targeting phosphatidylinositol 4-kinase III beta inhibitor (N373) which, especially when used in combination, can significantly improve the survival and pathology of infected mice.

Picornaviruses: A View from 3A

Picornaviruses are comprised of a positive-sense RNA genome surrounded by a protein shell (or capsid). They are ubiquitous in vertebrates and cause a wide range of important human and animal diseases. The genome encodes a single large polyprotein that is processed to structural (capsid) and non-structural proteins. The non-structural proteins have key functions within the viral replication complex. Some, such as 3D^pol (the RNA dependent RNA polymerase) have conserved functions and participate directly in replicating the viral genome, whereas others, such as 3A, have accessory roles. The 3A proteins are highly divergent across the Picornaviridae and have specific roles both within and outside of the replication complex, which differ between the different genera. These roles include subverting host proteins to generate replication organelles and inhibition of cellular functions (such as protein secretion) to influence virus replication efficiency and the host response to infection. In addition, 3A proteins are associated with the determination of host range. However, recent observations have challenged some of the roles assigned to 3A and suggest that other viral proteins may carry them out. In this review, we revisit the roles of 3A in the picornavirus life cycle. The 3AB precursor and mature 3A have distinct functions during viral replication and, therefore, we have also included discussion of some of the roles assigned to 3AB.

Effect of Consecutive Alternating Administration of a Triple Combination of Anti-Enteroviral Compounds in Mice Infected with Coxsackievirus B3

A novel approach for treatment of enterovirus infections was characterized. Application of treatment course of consecutive alternating administration (CAA) of triple combination of enterovirus replication inhibitors in experimental infections (20 MLD50) with coxsackievirus B3 (CVB3) strains in newborn mice is presented. It was established that in infection with cardiotropic Woodruff strain the combination of pleconaril, МDL-860 and oxoglaucine (PMO) subjected to the CAA scheme, a significant protective effect was observed. Monotherapeutic courses as well as simultaneously daily applied PMO were without effect. Analogous data were observed at experimental infection with the neurotriopic Nancy strain of CVB3. Following IC50 values of virus samples taken every day from target organs of infected animals during the whole period of study, a drug-resistance was established in monotherapy with compounds-partners in the PMO combination. At courses by the treatment scheme CAA of PMO development of drug-resistance was not established, but an increased susceptibility to the effect of the inhibitor-components in the combination was proven. Toxicity of PMO applied via the CAA scheme and in the monotherapeutic courses in both healthy and CVB3 infected animals was recorded. All data obtained prove the potential of the CAA treatment scheme for development of effective chemotherapy of enterovirus infections.

Enterovirus Replication Organelles and Inhibitors of Their Formation

Enteroviral replication reorganizes the cellular membrane. Upon infection, viral proteins and hijacked host factors generate unique structures called replication organelles (ROs) to replicate their viral genomes. ROs promote efficient viral genome replication, coordinate the steps of the viral replication cycle, and protect viral RNA from host immune responses. More recent researches have focused on the ultrastructure structures, formation mechanism, and functions in the virus life cycle of ROs. Dynamic model of enterovirus ROs structure is proposed, and the secretory pathway, the autophagy pathway, and lipid metabolism are found to be associated in the formation of ROs. With deeper understanding of ROs, some compounds have been found to show inhibitory effects on viral replication by targeting key proteins in the process of ROs formation. Here, we review the recent findings concerning the role, morphology, biogenesis, formation mechanism, and inhibitors of enterovirus ROs.

Host-Directed Antiviral Therapy

Antiviral drugs have traditionally been developed by directly targeting essential viral components. However, this strategy often fails due to the rapid generation of drug-resistant viruses. Recent genome-wide approaches, such as those employing small interfering RNA (siRNA) or clustered regularly interspaced short palindromic repeats (CRISPR) or those using small molecule chemical inhibitors targeting the cellular "kinome," have been used successfully to identify cellular factors that can support virus replication. Since some of these cellular factors are critical for virus replication, but are dispensable for the host, they can serve as novel targets for antiviral drug development. In addition, potentiation of immune responses, regulation of cytokine storms, and modulation of epigenetic changes upon virus infections are also feasible approaches to control infections. Because it is less likely that viruses will mutate to replace missing cellular functions, the chance of generating drug-resistant mutants with host-targeted inhibitor approaches is minimized. However, drug resistance against some host-directed agents can, in fact, occur under certain circumstances, such as long-term selection pressure of a host-directed antiviral agent that can allow the virus the opportunity to adapt to use an alternate host factor or to alter its affinity toward the target that confers resistance. This review describes novel approaches for antiviral drug development with a focus on host-directed therapies and the potential mechanisms that may account for the acquisition of antiviral drug resistance against host-directed agents.

Emerging Role for Acyl-CoA Binding Domain Containing 3 at Membrane Contact Sites During Viral Infection

Acyl-coenzyme A binding domain containing 3 (ACBD3) is a multifunctional protein residing in the Golgi apparatus and is involved in several signaling pathways. The current knowledge on ACBD3 has been extended to virology. ACBD3 has recently emerged as a key factor subverted by viruses, including kobuvirus, enterovirus, and hepatitis C virus. The ACBD3-PI4KB complex is critical for the role of ACBD3 in viral replication. In most cases, ACBD3 plays a positive role in viral infection. ACBD3 associates with viral 3A proteins from a variety of Picornaviridae family members at membrane contact sites (MCSs), which are used by diverse viruses to ensure lipid transfer to replication organelles (ROs). In this review, we discuss the mechanisms underlying the involvement of ACBD3 in viral infection at MCSs. Our review will highlight the current research and reveal potential avenues for future research.

Surfeit 4 Contributes to the Replication of Hepatitis C Virus Using Double-Membrane Vesicles

A number of positive-strand RNA viruses, such as hepatitis C virus (HCV) and poliovirus, use double-membrane vesicles (DMVs) as replication sites. However, the role of cellular proteins in DMV formation during virus replication is poorly understood. HCV NS4B protein induces the formation of a "membranous web" structure that provides a platform for the assembly of viral replication complexes. Our previous screen of NS4B-associated host membrane proteins by dual-affinity purification, liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS), and small interfering RNA (siRNA) methods revealed that the Surfeit 4 (Surf4) gene, which encodes an integral membrane protein, is involved in the replication of the JFH1 subgenomic replicon. Here, we investigated in detail the effect of Surf4 on HCV replication. Surf4 affects HCV replication in a genotype-independent manner, whereas HCV replication does not alter Surf4 expression. The influence of Surf4 on HCV replication indicates that while Surf4 regulates replication, it has no effect on entry, translation, assembly, or release. Analysis of the underlying mechanism showed that Surf4 is recruited into HCV RNA replication complexes by NS4B and is involved in the formation of DMVs and the structural integrity of RNA replication complexes. Surf4 also participates in the replication of poliovirus, which uses DMVs as replication sites, but it has no effect on the replication of dengue virus, which uses invaginated/sphere-type vesicles as replication sites. These findings clearly show that Surf4 is a novel cofactor that is involved in the replication of positive-strand RNA viruses using DMVs as RNA replication sites, which provides valuable clues for DMV formation during positive-strand RNA virus replication.IMPORTANCE Hepatitis C virus (HCV) NS4B protein induces the formation of a membranous web (MW) structure that provides a platform for the assembly of viral replication complexes. The main constituents of the MW are double-membrane vesicles (DMVs). Here, we found that the cellular protein Surf4, which maintains endoplasmic reticulum (ER)-Golgi intermediate compartments and the Golgi compartment, is recruited into HCV RNA replication complexes by NS4B and is involved in the formation of DMVs. Moreover, Surf4 participates in the replication of poliovirus, which uses DMVs as replication sites, but has no effect on the replication of dengue virus, which uses invaginated vesicles as replication sites. These results indicate that the cellular protein Surf4 is involved in the replication of positive-strand RNA viruses that use DMVs as RNA replication sites, providing new insights into DMV formation during virus replication and potential targets for the diagnosis and treatment of positive-strand RNA viruses.

The life cycle of non-polio enteroviruses and how to target it

The genus Enterovirus (EV) of the family Picornaviridae includes poliovirus, coxsackieviruses, echoviruses, numbered enteroviruses and rhinoviruses. These diverse viruses cause a variety of diseases, including non-specific febrile illness, hand-foot-and-mouth disease, neonatal sepsis-like disease, encephalitis, paralysis and respiratory diseases. In recent years, several non-polio enteroviruses (NPEVs) have emerged as serious public health concerns. These include EV-A71, which has caused epidemics of hand-foot-and-mouth disease in Southeast Asia, and EV-D68, which recently caused a large outbreak of severe lower respiratory tract disease in North America. Infections with these viruses are associated with severe neurological complications. For decades, most research has focused on poliovirus, but in recent years, our knowledge of NPEVs has increased considerably. In this Review, we summarize recent insights from enterovirus research with a special emphasis on NPEVs. We discuss virion structures, host-receptor interactions, viral uncoating and the recent discovery of a universal enterovirus host factor that is involved in viral genome release. Moreover, we briefly explain the mechanisms of viral genome replication, virion assembly and virion release, and describe potential targets for antiviral therapy. We reflect on how these recent discoveries may help the development of antiviral therapies and vaccines.

Essential domains of phosphatidylinositol-4 kinase III β required for enterovirus replication

Phosphatidylinositol-4 kinase III β (PI4KB) is a host factor that is required for enterovirus (EV) replication. In this study, the importance of host proteins that interact with PI4KB in EV replication was analyzed by trans complementation with PI4KB mutants in a PI4KB-knockout cell line. Ectopically expressed PI4KB mutants, which lack binding regions for ACBD3, RAB11, and 14-3-3 proteins, rescued replication of poliovirus and enterovirus 71. These findings suggest that interaction of PI4KB with these host proteins is not essential for EV replication once PI4KB has been expressed and that PI4KB is functionally independent from these host proteins regarding EV replication.

Poliovirus Evolution toward Independence from the Phosphatidylinositol-4 Kinase III β/Oxysterol-Binding Protein Family I Pathway

Phosphatidylinositol-4 kinase III β (PI4KB) and oxysterol-binding protein (OSBP) family I provide a conserved host pathway required for enterovirus replication. Here, we analyze the role and essentiality of this pathway in enterovirus replication. Phosphatidylinositol 4-phosphate (PI4P) production and cholesterol accumulation in the replication organelle (RO) are severely suppressed in cells infected with a poliovirus (PV) mutant isolated from a PI4KB-knockout cell line (RD[Δ PI4KB]). Major determinants of the mutant for infectivity in RD(Δ PI4KB) cells map to the A5270U(3A-R54W) and U3881C(2B-F17L) mutations. The 3A mutation is required for PI4KB-independent development of RO. The 2B mutation rather sensitizes PV to PI4KB/OSBP inhibitors by itself but confers substantially complete resistance to the inhibitors with the 3A mutation. The 2B mutation also confers hypersensitivity to interferon alpha treatment on PV. These suggest that the PI4KB/OSBP pathway is not necessarily essential for enterovirus replication in vitro. This work supports a two-step resistance model of enterovirus to PI4KB/OSBP inhibitors involving unique recessive epistasis of 3A and 2B and offers insights into a potential evolutionary pathway of enterovirus toward independence from the PI4KB/OSBP pathway.

Escaping Host Factor PI4KB Inhibition: Enterovirus Genomic RNA Replication in the Absence of Replication Organelles

Enteroviruses reorganize cellular endomembranes into replication organelles (ROs) for genome replication. Although enterovirus replication depends on phosphatidylinositol 4-kinase type IIIβ (PI4KB), its role, and that of its product, phosphatidylinositol 4-phosphate (PI4P), is only partially understood. Exploiting a mutant coxsackievirus resistant to PI4KB inhibition, we show that PI4KB activity has distinct functions both in proteolytic processing of the viral polyprotein and in RO biogenesis. The escape mutation rectifies a proteolytic processing defect imposed by PI4KB inhibition, pointing to a possible escape mechanism. Remarkably, under PI4KB inhibition, the mutant virus could replicate its genome in the absence of ROs, using instead the Golgi apparatus. This impaired RO biogenesis provided an opportunity to investigate the proposed role of ROs in shielding enteroviral RNA from cellular sensors. Neither accelerated sensing of viral RNA nor enhanced innate immune responses was observed. Together, our findings challenge the notion that ROs are indispensable for enterovirus genome replication and immune evasion.

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PI4KB activity expedites the formation of coxsackievirus replication organelles (ROs)

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PI4KB inhibition impairs polyprotein processing, which is rescued by a 3A mutation

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Upon PI4KB inhibition, this mutant replicates at the Golgi in the absence of ROs

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Innate immune responses are not enhanced when RO biogenesis is delayed

Hijacking of multiple phospholipid biosynthetic pathways and induction of membrane biogenesis by a picornaviral 3CD protein

RNA viruses induce specialized membranous structures for use in genome replication. These structures are often referred to as replication organelles (ROs). ROs exhibit distinct lipid composition relative to other cellular membranes. In many picornaviruses, phosphatidylinositol-4-phosphate (PI4P) is a marker of the RO. Studies to date indicate that the viral 3A protein hijacks a PI4 kinase to induce PI4P by a mechanism unrelated to the cellular pathway, which requires Golgi-specific brefeldin A-resistance guanine nucleotide exchange factor 1, GBF1, and ADP ribosylation factor 1, Arf1. Here we show that a picornaviral 3CD protein is sufficient to induce synthesis of not only PI4P but also phosphatidylinositol-4,5-bisphosphate (PIP2) and phosphatidylcholine (PC). Synthesis of PI4P requires GBF1 and Arf1. We identified 3CD derivatives: 3CD^m and 3C^mD, that we used to show that distinct domains of 3CD function upstream of GBF1 and downstream of Arf1 activation. These same 3CD derivatives still supported induction of PIP2 and PC, suggesting that pathways and corresponding mechanisms used to induce these phospholipids are distinct. Phospholipid induction by 3CD is localized to the perinuclear region of the cell, the outcome of which is the proliferation of membranes in this area of the cell. We conclude that a single viral protein can serve as a master regulator of cellular phospholipid and membrane biogenesis, likely by commandeering normal cellular pathways.

Picornaviruses replicate their genomes in association with host membranes. Early during infection, existing membranes are used but remodeled to contain a repertoire of lipids best suited for virus multiplication. Later, new membrane synthesis occurs, which requires biosynthesis of phosphatidylcholine in addition to the other more specialized lipids. We have learned that a single picornaviral protein is able to induce membrane biogenesis and decorate these membranes with some of the specialized lipids induced by the virus. A detailed mechanism of induction has been elucidated for one of these lipids. The ability of a single viral protein to commandeer host pathways that lead to membrane biogenesis was unexpected. This discovery reveals a new target for antiviral therapy with the potential to completely derail all aspects of the viral lifecycle requiring membrane biogenesis.

The Origin, Dynamic Morphology, and PI4P-Independent Formation of Encephalomyocarditis Virus Replication Organelles

Picornaviruses induce dramatic rearrangements of endomembranes in the cells that they infect to produce dedicated platforms for viral replication. These structures, termed replication organelles (ROs), have been well characterized for the Enterovirus genus of the Picornaviridae However, it is unknown whether the diverse RO morphologies associated with enterovirus infection are conserved among other picornaviruses. Here, we use serial electron tomography at different stages of infection to assess the three-dimensional architecture of ROs induced by encephalomyocarditis virus (EMCV), a member of the Cardiovirus genus of the family of picornaviruses that is distantly related. Ultrastructural analyses revealed connections between early single-membrane EMCV ROs and the endoplasmic reticulum (ER), establishing the ER as a likely donor organelle for their formation. These early single-membrane ROs appear to transform into double-membrane vesicles (DMVs) as infection progresses. Both single- and double-membrane structures were found to support viral RNA synthesis, and progeny viruses accumulated in close proximity, suggesting a spatial association between RNA synthesis and virus assembly. Further, we explored the role of phosphatidylinositol 4-phosphate (PI4P), a critical host factor for both enterovirus and cardiovirus replication that has been recently found to expedite enterovirus RO formation rather than being strictly required. By exploiting an EMCV escape mutant, we found that low-PI4P conditions could also be overcome for the formation of cardiovirus ROs. Collectively, our data show that despite differences in the membrane source, there are striking similarities in the biogenesis, morphology, and transformation of cardiovirus and enterovirus ROs, which may well extend to other picornaviruses.IMPORTANCE Like all positive-sense RNA viruses, picornaviruses induce the rearrangement of host cell membranes to form unique structures, or replication organelles (ROs), that support viral RNA synthesis. Here, we investigate the architecture and biogenesis of cardiovirus ROs and compare them with those induced by enteroviruses, members of the well-characterized picornavirus genus Enterovirus The origins and dynamic morphologies of cardiovirus ROs are revealed using electron tomography, which points to the endoplasmic reticulum as the donor organelle usurped to produce single-membrane tubules and vesicles that transform into double-membrane vesicles. We show that PI4P, a critical lipid for cardiovirus and enterovirus replication, is not strictly required for the formation of cardiovirus ROs, as functional ROs with typical morphologies are formed under phosphatidylinositol 4-kinase type III alpha (PI4KA) inhibition in cells infected with an escape mutant. Our data show that the transformation from single-membrane structures to double-membrane vesicles is a conserved feature of cardiovirus and enterovirus infections that likely extends to other picornavirus genera.

Negative charge and membrane-tethered viral 3B cooperate to recruit viral RNA dependent RNA polymerase 3D pol

Most single stranded plus RNA viruses hijack phosphatidylinositol 4-kinases (PI4Ks) to generate membranes highly enriched in phosphatidylinositol 4-phosphate (PI4P). These membranous compartments known as webs, replication factories or replication organelles are essential for viral replication because they provide protection from the innate intracellular immune response while serving as platforms for viral replication. Using purified recombinant proteins and biomimetic model membranes we show that the nonstructural viral 3A protein is sufficient to promote membrane hyper-phosphorylation given the proper intracellular cofactors (PI4KB and ACBD3). However, our bio-mimetic in vitro reconstitution assay revealed that rather than the presence of PI4P specifically, negative charge alone is sufficient for the recruitment of 3D^pol enzymes to the surface of the lipid bilayer. Additionally, we show that membrane tethered viral 3B protein (also known as Vpg) works in combination with the negative charge to increase the efficiency of membrane recruitment of 3D^pol.

Modulation of proteolytic polyprotein processing by coxsackievirus mutants resistant to inhibitors targeting phosphatidylinositol-4-kinase IIIβ or oxysterol binding protein

Enteroviruses (e.g. poliovirus, coxsackievirus, and rhinovirus) require several host factors for genome replication. Among these host factors are phosphatidylinositol-4-kinase IIIβ (PI4KB) and oxysterol binding protein (OSBP). Enterovirus mutants resistant to inhibitors of PI4KB and OSBP were previously isolated, which demonstrated a role of single substitutions in the non-structural 3A protein in conferring resistance. Besides the 3A substitutions (i.e., 3A-I54F and 3A-H57Y) in coxsackievirus B3 (CVB3), substitution N2D in 2C was identified in each of the PI4KB-inhibitor resistant CVB3 pools, but its possible benefit has not been investigated yet. In this study, we set out to investigate the possible role of 2C-N2D in the resistance to PI4KB and OSBP inhibition. We show that 2C-N2D by itself did not confer any resistance to inhibitors of PI4KB and OSBP. However, the double mutant (i.e., 2C-N2D/3A-H57Y) showed better replication than the 3A-H57Y single mutant in the presence of inhibitors. Growing evidence suggests that alterations in lipid homeostasis affect the proteolytic processing of the poliovirus polyprotein. Therefore, we studied the effect of PI4KB or OSBP inhibition on proteolytic processing of the CVB3 polyprotein during infection as well as in a replication-independent system. We show that both PI4KB and OSBP inhibitors specifically affected the cleavage at the 3A-3B junction, and that mutation 3A-H57Y recovered impaired proteolytic processing at this junction. Although 2C-N2D enhanced replication of the 3A-H57Y single mutant, we did not detect additional effects of this substitution on polyprotein processing, which leaves the mechanism of how 2C-N2D contributes to the resistance to be revealed.

Allosteric Regulation of Phosphatidylinositol 4-Kinase III Beta by an Antipicornavirus Compound MDL-860

MDL-860 is a broad-spectrum antipicornavirus compound discovered in 1982 and one of the few promising candidates effective in in vivo virus infection. Despite the effectiveness, the target and the mechanism of action of MDL-860 remain unknown. Here, we have characterized antipoliovirus activity of MDL-860 and identified host phosphatidylinositol-4 kinase III beta (PI4KB) as the target. MDL-860 treatment caused covalent modification and irreversible inactivation of PI4KB. A cysteine residue at amino acid 646 of PI4KB, which locates at the bottom of a surface pocket apart from the active site, was identified as the target site of MDL-860. This work reveals the mechanism of action of this class of PI4KB inhibitors and offers insights into novel allosteric regulation of PI4KB activity.

Inhibition of herpes simplex virus 1 (HSV-1) and poliovirus (PV-1) by bacteriocins from lactococcus lactis subsp. lactis and enterococcus durans strains isolated from goat milk

Bacteriocins have unusual inhibitory activity, including antiviral properties, and this can be exploited to give alternative applications. Semi-purified bacteriocins of six lactic acid bacteria (LAB) strains isolated from goat milk (two Lactococcus lactis: GLc03 and GLc05, and four Enterococcus durans: GEn09, GEn12, GEn14 and GEn17) were tested for cytotoxicity in Vero cells (50% Cytotoxicity Concentration: CC₅₀), and for their antiviral activities against herpes simplex virus 1 (HVS-1) and poliovirus (PV-1). Semi-purified bacteriocins presented low cytotoxicity, with CC₅₀ varying from 256.2 µg/mL (GLc05) to 1084.5 µg/mL (GEn14). CC₁₀ was determined for all isolates (GLc03: 36.9 µg/mL; GLc05: 51.2 µg/mL; GEn09: 88.1 µg/mL; GEn12: 99.9 µg/mL; GEn14: 275 µg/mL; and GEn17: 62.2 µg/mL) and considered for antiviral activity assays. Antiviral activity before virus adsorption was recorded against PV-1 for GLc05 (4.9%), GEn09 (3.4%), GEn12 (24.7%) and GEn17 (23.5%), and against HSV-1 for GEn12 (27.9%), GEn14 (58.7%) and GEn17 (39.2%). Antiviral activity after virus adsorption was identified against PV-1 for GLc05 (32.7%), GEn09 (91.0%), GEn12 (93.7%) and GEn17 (57.2%), and against HSV-1 for GEn17 (71.6%). The results obtained indicate the potential of some bacteriocins, particularly those produced by E. durans strains investigated in the present study, in viral inhibition and their application as new antiviral agents.

Direct-acting antivirals and host-targeting strategies to combat enterovirus infections

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Enteroviruses cause many human diseases, yet no antiviral drugs are available.

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Capsids and viral enzymes are promising targets for direct-acting antiviral therapy.

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Fundamental research has unveiled host factors for broad-spectrum drug development.

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Drug repurposing screens have yielded new promising enterovirus inhibitors.

Enteroviruses (e.g., poliovirus, enterovirus-A71, coxsackievirus, enterovirus-D68, rhinovirus) include many human pathogens causative of various mild and more severe diseases, especially in young children. Unfortunately, antiviral drugs to treat enterovirus infections have not been approved yet. Over the past decades, several direct-acting inhibitors have been developed, including capsid binders, which block virus entry, and inhibitors of viral enzymes required for genome replication. Capsid binders and protease inhibitors have been clinically evaluated, but failed due to limited efficacy or toxicity issues. As an alternative approach, host-targeting inhibitors with potential broad-spectrum activity have been identified. Furthermore, drug repurposing screens have recently uncovered promising new inhibitors with disparate viral and host targets. Together, these findings raise hope for the development of (broad-range) anti-enteroviral drugs.

The Molecular Basis of Aichi Virus 3A Protein Activation of Phosphatidylinositol 4 Kinase IIIβ, PI4KB, through ACBD3

Phosphatidylinositol 4-kinase III beta (PI4KIIIβ) is an essential enzyme in mediating membrane transport, and plays key roles in facilitating viral infection. Many pathogenic positive-sense single-stranded RNA viruses activate PI4KIIIβ to generate phosphatidylinositol 4-phosphate (PI4P)-enriched organelles for viral replication. The molecular basis for PI4KIIIβ activation during viral infection has remained largely unclear. We describe the biochemical reconstitution and characterization of the complex of PI4KIIIβ with the Golgi protein Acyl-coenzyme A binding domain containing protein 3 (ACBD3) and Aichi virus 3A protein on membranes. We find that 3A directly activates PI4KIIIβ, and this activation is sensitized by ACBD3. The interfaces between PI4KIIIβ-ACBD3 and ACBD3-3A were mapped with hydrogen-deuterium exchange mass spectrometry (HDX-MS). Determination of the crystal structure of the ACBD3 GOLD domain revealed a unique N terminus that mediates the interaction with 3A. Rationally designed complex-disrupting mutations in both ACBD3 and PI4KIIIβ completely abrogated the sensitization of 3A activation by ACBD3.

Poliovirus Studies during the Endgame of the Polio Eradication Program

Since the beginning of Global Polio Eradication Initiative in 1988, poliomyelitis cases caused by wild poliovirus (PV) have been drastically reduced, with only 74 cases reported in 2 endemic countries in 2015. The current limited PV transmission suggests that we are in the endgame of the polio eradication program. However, specific challenges have emerged in the endgame, including tight budget, switching of the vaccines, and changes in biorisk management of PV. To overcome these challenges, several PV studies have been implemented in the eradication program. Some of the responses to the emerging challenges in the polio endgame might be valuable in other infectious diseases eradication programs. Here, I will review challenges that confront the polio eradication program and current research to address these challenges.

Mutations in Encephalomyocarditis Virus 3A Protein Uncouple the Dependency of Genome Replication on Host Factors Phosphatidylinositol 4-Kinase IIIα and Oxysterol-Binding Protein

Positive-strand RNA viruses modulate lipid homeostasis to generate unique, membranous “replication organelles” (ROs) where viral genome replication takes place. Hepatitis C virus, encephalomyocarditis virus (EMCV), and enteroviruses have convergently evolved to hijack host phosphatidylinositol 4-kinases (PI4Ks), which produce PI4P lipids, to recruit oxysterol-binding protein (OSBP), a PI4P-binding protein that shuttles cholesterol to ROs. Consistent with the proposed coupling between PI4K and OSBP, enterovirus mutants resistant to PI4KB inhibitors are also resistant to OSBP inhibitors. Here, we show that EMCV can replicate without accumulating PI4P/cholesterol at ROs, by acquiring point mutations in nonstructural protein 3A. Remarkably, the mutations conferred resistance to PI4K but not OSBP inhibitors, thereby uncoupling the levels of dependency of EMCV RNA replication on PI4K and OSBP. This work may contribute to a deeper understanding of the roles of PI4K/PI4P and OSBP/cholesterol in membrane modifications induced by positive-strand RNA viruses.

Positive-strand RNA [(+)RNA] viruses are true masters of reprogramming host lipid trafficking and synthesis to support virus genome replication. Via their membrane-associated 3A protein, picornaviruses of the genus Enterovirus (e.g., poliovirus, coxsackievirus, and rhinovirus) subvert Golgi complex-localized phosphatidylinositol 4-kinase IIIβ (PI4KB) to generate “replication organelles” (ROs) enriched in phosphatidylinositol 4-phosphate (PI4P). PI4P lipids serve to accumulate oxysterol-binding protein (OSBP), which subsequently transfers cholesterol to the ROs in a PI4P-dependent manner. Single-point mutations in 3A render enteroviruses resistant to both PI4KB and OSBP inhibition, indicating coupled dependency on these host factors. Recently, we showed that encephalomyocarditis virus (EMCV), a picornavirus that belongs to the Cardiovirus genus, also builds PI4P/cholesterol-enriched ROs. Like the hepatitis C virus (HCV) of the Flaviviridae family, it does so by hijacking the endoplasmic reticulum (ER)-localized phosphatidylinositol 4-kinase IIIα (PI4KA). Here we provide genetic evidence for the critical involvement of EMCV protein 3A in this process. Using a genetic screening approach, we selected EMCV mutants with single amino acid substitutions in 3A, which rescued RNA virus replication upon small interfering RNA (siRNA) knockdown or pharmacological inhibition of PI4KA. In the presence of PI4KA inhibitors, the mutants no longer induced PI4P, OSBP, or cholesterol accumulation at ROs, which aggregated into large cytoplasmic clusters. In contrast to the enterovirus escape mutants, we observed little if any cross-resistance of EMCV mutants to OSBP inhibitors, indicating an uncoupled level of dependency of their RNA replication on PI4KA and OSBP activities. This report may contribute to a better understanding of the roles of PI4KA and OSBP in membrane modifications induced by (+)RNA viruses.

IMPORTANCE Positive-strand RNA viruses modulate lipid homeostasis to generate unique, membranous “replication organelles” (ROs) where viral genome replication takes place. Hepatitis C virus, encephalomyocarditis virus (EMCV), and enteroviruses have convergently evolved to hijack host phosphatidylinositol 4-kinases (PI4Ks), which produce PI4P lipids, to recruit oxysterol-binding protein (OSBP), a PI4P-binding protein that shuttles cholesterol to ROs. Consistent with the proposed coupling between PI4K and OSBP, enterovirus mutants resistant to PI4KB inhibitors are also resistant to OSBP inhibitors. Here, we show that EMCV can replicate without accumulating PI4P/cholesterol at ROs, by acquiring point mutations in nonstructural protein 3A. Remarkably, the mutations conferred resistance to PI4K but not OSBP inhibitors, thereby uncoupling the levels of dependency of EMCV RNA replication on PI4K and OSBP. This work may contribute to a deeper understanding of the roles of PI4K/PI4P and OSBP/cholesterol in membrane modifications induced by positive-strand RNA viruses.