The antiviral compound enviroxime targets the 3A coding region of rhinovirus and poliovirus

Targeting PI4KB and Src/Abl host kinases as broad-spectrum antiviral strategy: Myth or real opportunity?

Viruses pose a continuous threat to human health. Limited treatment options exist for current viruses, and the risk of infections with newly emerging or re-emerging viruses is increasing. In a pandemic scenario, having a broad-spectrum antiviral to limit viral spread while developing specific antivirals and vaccines is crucial. Targeting host kinases represents a valuable strategy due to the higher barrier to resistance and the broad-spectrum activity it offers. While cells have redundant kinases for the same biological function, viruses rely on specific kinases for their replication cycle, enabling targeted antiviral action with limited toxicity. This review focuses on two extensively studied kinase targets: the lipid kinase phosphatidylinositol 4-kinase IIIβ (PI4KB) and the tyrosine kinase proteins Src and Abl. Compounds active against these targets are reviewed in terms of the viruses they inhibit, their mechanisms of action and their stage of development. While PI4KB inhibitors have reached clinical trials, those targeting Src and Abl remain largely in the preclinical phase. Nevertheless, opportunities exist to improve potency and further understand the specific roles of these kinases in the life cycle of multiple viruses.

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.

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.

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.

Direct-Acting Antivirals and Host-Targeting Approaches against Enterovirus B Infections: Recent Advances

Enterovirus B (EV-B)-related diseases, which can be life threatening in high-risk populations, have been recognized as a serious health problem, but their clinical treatment is largely supportive, and no selective antivirals are available on the market. As their clinical relevance has become more serious, efforts in the field of anti-EV-B inhibitors have greatly increased and many potential antivirals with very high selectivity indexes and promising in vitro activities have been discovered. The scope of this review encompasses recent advances in the discovery of new compounds with anti-viral activity against EV-B, as well as further progress in repurposing drugs to treat these infections. Current progress and future perspectives in drug discovery against EV-Bs are briefly discussed and existing gaps are spotlighted.

Functions of Viroporins in the Viral Life Cycle and Their Regulation of Host Cell Responses

Viroporins are virally encoded transmembrane proteins that are essential for viral pathogenicity and can participate in various stages of the viral life cycle, thereby promoting viral proliferation. Viroporins have multifaceted effects on host cell biological functions, including altering cell membrane permeability, triggering inflammasome formation, inducing apoptosis and autophagy, and evading immune responses, thereby ensuring that the virus completes its life cycle. Viroporins are also virulence factors, and their complete or partial deletion often reduces virion release and reduces viral pathogenicity, highlighting the important role of these proteins in the viral life cycle. Thus, viroporins represent a common drug-protein target for inhibiting drugs and the development of antiviral therapies. This article reviews current studies on the functions of viroporins in the viral life cycle and their regulation of host cell responses, with the aim of improving the understanding of this growing family of viral proteins.

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.

Enteroviral replication inhibition by N-Alkyl triazolopyrimidinone derivatives through a non-capsid binding mode

Small-molecule inhibitors exhibiting broad-spectrum enteroviral inhibition by targeting viral replication proteins are highly desirable in antiviral drug discovery. We used the previously identified antiviral compound 1 as the starting material to develop a novel compound series with high efficacy against human rhinovirus (hRV). Further optimization of N-substituted triazolopyrimidinone derivatives revealed that the N-alkyl triazolopyrimidinone derivatives (2) had more potent antiviral activity against hRVs than compound 1. The new compounds showed improved selectivity index values, and compound 2c (KR-25210) displayed broad anti-hRV activity, with half-maximal effective concentration values ≤ 2 µM against all tested hRVs. In addition, 2c showed notable activity against other enteroviruses. Drug-likeness elucidation showed that 2c exhibited reasonable human and rat liver microsomal phase-I stability and safe CYP inhibition. Replication studies revealed that 2c is not a capsid inhibitor, and a time-of-addition assay indicated that 2c targets the virus replication stages.

Enterovirus A71 antivirals: Past, present, and future

Enterovirus A71 (EV-A71) is a significant human pathogen, especially in children. EV-A71 infection is one of the leading causes of hand, foot, and mouth diseases (HFMD), and can lead to neurological complications such as acute flaccid myelitis (AFM) in severe cases. Although three EV-A71 vaccines are available in China, they are not broadly protective and have reduced efficacy against emerging strains. There is currently no approved antiviral for EV-A71. Significant progress has been made in developing antivirals against EV-A71 by targeting both viral proteins and host factors. However, viral capsid inhibitors and protease inhibitors failed in clinical trials of human rhinovirus infection due to limited efficacy or side effects. This review discusses major discoveries in EV-A71 antiviral development, analyzes the advantages and limitations of each drug target, and highlights the knowledge gaps that need to be addressed to advance the field forward.

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.

Rhinovirus Inhibitors: Including a New Target, the Viral RNA

Rhinoviruses (RVs) are the main cause of recurrent infections with rather mild symptoms characteristic of the common cold. Nevertheless, RVs give rise to enormous numbers of absences from work and school and may become life-threatening in particular settings. Vaccination is jeopardised by the large number of serotypes eliciting only poorly cross-neutralising antibodies. Conversely, antivirals developed over the years failed FDA approval because of a low efficacy and/or side effects. RV species A, B, and C are now included in the fifteen species of the genus Enteroviruses based upon the high similarity of their genome sequences. As a result of their comparably low pathogenicity, RVs have become a handy model for other, more dangerous members of this genus, e.g., poliovirus and enterovirus 71. We provide a short overview of viral proteins that are considered potential drug targets and their corresponding drug candidates. We briefly mention more recently identified cellular enzymes whose inhibition impacts on RVs and comment novel approaches to interfere with infection via aggregation, virus trapping, or preventing viral access to the cell receptor. Finally, we devote a large part of this article to adding the viral RNA genome to the list of potential drug targets by dwelling on its structure, folding, and the still debated way of its exit from the capsid. Finally, we discuss the recent finding that G-quadruplex stabilising compounds impact on RNA egress possibly via obfuscating the unravelling of stable secondary structural elements.

Benzimidazole based hybrids against complex diseases: A catalogue of the SAR profile

The fused heterocyclic ring system has been recognized as a privileged structure that is used as a template in medicinal chemistry for drug discovery. Benzimidazole is one of the common scaffolds found in several natural products such as histidine, purines, and an integral part of vitamin B12. This hetero-aromatic bicyclic ring system acts as a pharmacophore in various drugs of therapeutic interest and has a broad spectrum of activity. Literature reports suggest that diversely substituted benzimidazoles possess distinct pharmacological profiles with multi-targeting potential, thereby, an indispensable anchor for the development of novel therapeutic agents against complex diseases such as cancer, malaria, inflammatory disorders, microbial diseases, hypertension, etc. Thus, lots of efforts have been diverted towards exploring the therapeutic potential of benzimidazoles. Despite great efforts made by the research community, still, some multi-factorial diseases continue to progress due to their complex pathophysiology. Under these sets of circumstances, there is a need to explore this nucleus for hybrid designing with multi-targeting potential against complex diseases. Benzimidazole-based hybrids have been reported to treat multifactorial diseases, making it a scaffold of interest for various pharmaceutical companies and research groups. In this write-up, we shed light on the recent pharmacological profiles, various designing strategies, and structure-activity relationships (SAR) of different benzimidazole-based hybrids.

Regulation of the proteostasis network during enterovirus infection: A feedforward mechanism for EV-A71 and EV-D68

The proteostasis network guarantees successful protein synthesis, folding, transportation, and degradation. Mounting evidence has revealed that this network maintains proteome integrity and is linked to cellular physiology, pathology, and virus infection. Human enterovirus A71 (EV-A71) and EV-D68 are suspected causative agents of acute flaccid myelitis, a severe poliomyelitis-like neurologic syndrome with no known cure. In this context, further clarification of the molecular mechanisms underlying EV-A71 and EV-D68 infection is paramount. Here, we summarize the components of the proteostasis network that are intercepted by EV-A71 and EV-D68, as well as antivirals that target this network and may help develop improved antiviral drugs.

Antivirals blocking entry of enteroviruses and therapeutic potential

Viruses from the genus Enterovirus (EV) of the Picornaviridae family are known to cause diseases such as hand foot and mouth disease (HFMD), respiratory diseases, encephalitis and myocarditis. The capsid of EV is an attractive target for the development of direct-acting small molecules that can interfere with viral entry. Some of the capsid binders have been evaluated in clinical trials but the majority have failed due to insufficient efficacy or unacceptable off-target effects. Furthermore, most of the capsid binders exhibited a low barrier to resistance. Alternatively, host-targeting inhibitors such as peptides derived from the capsid of EV that can recognize cellular receptors have been identified. However, the majority of these peptides displayed low anti-EV potency (µM range) as compared to the potency of small molecule compounds (nM range). Nonetheless, the development of anti-EV peptides is warranted as they may complement the small-molecules in a drug combination strategy to treat EVs. Lastly, structure-based approach to design antiviral peptides should be utilized to unearth potent anti-EV peptides.

Convergent evolution in the mechanisms of ACBD3 recruitment to picornavirus replication sites

Enteroviruses, members of the family of picornaviruses, are the most common viral infectious agents in humans causing a broad spectrum of diseases ranging from mild respiratory illnesses to life-threatening infections. To efficiently replicate within the host cell, enteroviruses hijack several host factors, such as ACBD3. ACBD3 facilitates replication of various enterovirus species, however, structural determinants of ACBD3 recruitment to the viral replication sites are poorly understood. Here, we present a structural characterization of the interaction between ACBD3 and the non-structural 3A proteins of four representative enteroviruses (poliovirus, enterovirus A71, enterovirus D68, and rhinovirus B14). In addition, we describe the details of the 3A-3A interaction causing the assembly of the ACBD3-3A heterotetramers and the interaction between the ACBD3-3A complex and the lipid bilayer. Using structure-guided identification of the point mutations disrupting these interactions, we demonstrate their roles in the intracellular localization of these proteins, recruitment of downstream effectors of ACBD3, and facilitation of enterovirus replication. These structures uncovered a striking convergence in the mechanisms of how enteroviruses and kobuviruses, members of a distinct group of picornaviruses that also rely on ACBD3, recruit ACBD3 and its downstream effectors to the sites of viral replication.

Enteroviruses are the most common viruses infecting humans. They cause a broad spectrum of diseases ranging from common cold to life-threatening diseases, such as poliomyelitis. To date, no effective antiviral therapy for enteroviruses has been approved yet. To ensure efficient replication, enteroviruses hijack several host factors, recruit them to the sites of virus replication, and use their physiological functions for their own purposes. Here, we characterize the complexes composed of the host protein ACBD3 and the ACBD3-binding viral proteins (called 3A) of four representative enteroviruses. Our study reveals the atomic details of these complexes and identifies the amino acid residues important for the interaction. We found out that the 3A proteins of enteroviruses bind to the same regions of ACBD3 as the 3A proteins of kobuviruses, a distinct group of viruses that also rely on ACBD3, but are oriented in the opposite directions. This observation reveals a striking case of convergent evolutionary pathways that have evolved to allow enteroviruses and kobuviruses (which are two distinct groups of the Picornaviridae family) to recruit a common host target, ACBD3, and its downstream effectors to the sites of viral replication.

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.

Rhinoviruses and Their Receptors

Human rhinoviruses (RVs) are picornaviruses that can cause a variety of upper and lower respiratory tract illnesses, including the common cold, bronchitis, pneumonia, and exacerbations of chronic respiratory diseases such as asthma. There are currently > 160 known types of RVs classified into three species (A, B, and C) that use three different cellular membrane glycoproteins expressed in the respiratory epithelium to enter the host cell. These viral receptors are intercellular adhesion molecule 1 (used by the majority of RV-A and all RV-B types), low-density lipoprotein receptor family members (used by 12 RV-A types), and cadherin-related family member 3 (CDHR3; used by RV-C). RV-A and RV-B interactions with intercellular adhesion molecule 1 and low-density lipoprotein receptor glycoproteins are well defined and their cellular functions have been described, whereas the mechanisms of the RV-C interaction with CDHR3 and its cellular functions are being studied. A single nucleotide polymorphism (rs6967330) in CDHR3 increases cell surface expression of this protein and, as a result, also promotes RV-C infections and illnesses. There are currently no approved vaccines or antiviral therapies available to treat or prevent RV infections, which is a major unmet medical need. Understanding interactions between RV and cellular receptors could lead to new insights into the pathogenesis of respiratory illnesses as well as lead to new approaches to control respiratory illnesses caused by RV infections.

A Single Point Mutation in the Rhinovirus 2B Protein Reduces the Requirement for Phosphatidylinositol 4-Kinase Class III Beta in Viral Replication

Rhinoviruses (RVs) replicate on cytoplasmic membranes derived from the Golgi apparatus. They encode membrane-targeted proteins 2B, 2C, and 3A, which control trafficking and lipid composition of the replication membrane. The virus recruits host factors for replication, such as phosphatidylinositol 4 (PI4)-kinase 3beta (PI4K3b), which boosts PI4-phosphate (PI4P) levels and drives lipid countercurrent exchange of PI4P against cholesterol at endoplasmic reticulum-Golgi membrane contact sites through the lipid shuttling protein oxysterol binding protein 1 (OSBP1). We identified a PI4K3b inhibitor-resistant RV-A16 variant with a single point mutation in the conserved 2B protein near the cytosolic carboxy terminus, isoleucine 92 to threonine (termed 2B[I92T]). The mutation did not confer resistance to cholesterol-sequestering compounds or OSBP1 inhibition, suggesting invariant dependency on the PI4P/cholesterol lipid countercurrents. In the presence of PI4K3b inhibitor, Golgi reorganization and PI4P lipid induction occurred in RV-A16 2B[I92] but not in wild-type infection. The knockout of PI4K3b abolished the replication of both the 2B[I92T] mutant and the wild type. Doxycycline-inducible expression of PI4K3b in PI4K3b knockout cells efficiently rescued the 2B[I92T] mutant and, less effectively, wild-type virus infection. Ectopic expression of 2B[I92T] or 2B was less efficient than that of 3A in recruiting PI4K3b to perinuclear membranes, suggesting a supportive rather than decisive role of 2B in recruiting PI4K3b. The data suggest that 2B tunes the recruitment of PI4K3b to the replication membrane and allows the virus to adapt to cells with low levels of PI4K3b while still maintaining the PI4P/cholesterol countercurrent for establishing Golgi-derived RV replication membranes.IMPORTANCE Human rhinoviruses (RVs) are the major cause of the common cold worldwide. They cause asthmatic exacerbations and chronic obstructive pulmonary disease. Despite recent advances, the development of antivirals and vaccines has proven difficult due to the high number and variability of RV types. The identification of critical host factors and their interactions with viral proteins and membrane lipids for the establishment of viral replication is a basis for drug development strategies. Our findings here shed new light on the interactions between nonstructural viral membrane proteins and class III phosphatidylinositol 4 kinases from the host and highlight the importance of phosphatidylinositol 4 phosphate for RV replication.

Direct and Indirect Effects on Viral Translation and RNA Replication Are Required for AUF1 Restriction of Enterovirus Infections in Human Cells

Picornaviruses primarily infect the gastrointestinal or upper respiratory tracts of humans and animals and may disseminate to tissues of the central nervous system, heart, skin, liver, or pancreas. Many common human pathogens belong to the Picornaviridae family, which includes viruses known to cause paralytic poliomyelitis (poliovirus); myocarditis (coxsackievirus B3 [CVB3]); the common cold (human rhinovirus [HRV]); and hand, foot, and mouth disease (enterovirus 71 [EV71]), among other illnesses. There are no specific treatments for infection, and vaccines exist for only two picornaviruses: poliovirus and hepatitis A virus. Given the worldwide distribution and prevalence of picornaviruses, it is important to gain insight into the host mechanisms used to restrict infection. Other than proteins involved in the innate immune response, few host factors have been identified that restrict picornavirus replication. The work presented here seeks to define the mechanism of action for the host restriction factor AUF1 during infection by poliovirus and CVB3.

The cellular mRNA decay protein AUF1 acts as a restriction factor during infection by picornaviruses, including poliovirus, coxsackievirus, and human rhinovirus. AUF1 relocalizes from the nucleus to the cytoplasm during infection by these viruses due to the disruption of nucleocytoplasmic trafficking by viral proteinases. Previous studies have demonstrated that AUF1 binds to poliovirus and coxsackievirus B3 (CVB3) RNA during infection, with binding shown to occur within the internal ribosome entry site (IRES) of the 5′ noncoding region (NCR) or the 3′ NCR, respectively. Binding to different sites within the viral RNA suggests that AUF1 may negatively regulate infection by these viruses using different mechanisms. The work presented here addresses the mechanism of AUF1 inhibition of the replication of poliovirus and CVB3. We demonstrate that AUF1 knockdown in human cells results in increased viral translation, RNA synthesis, and virus production. AUF1 is shown to negatively regulate translation of a poliovirus and CVB3 IRES reporter RNA during infection but not in uninfected cells. We found that this inhibitory activity is not mediated through destabilization of viral genomic RNA; however, it does require virus-induced relocalization of AUF1 from the nucleus to the cytoplasm during the early phases of infection. Our findings suggest that AUF1 restriction of poliovirus and CVB3 replication uses a common mechanism through the viral IRES, which is distinct from the canonical role that AUF1 plays in regulated mRNA decay in uninfected host cells.

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.

Enterovirus inhibitory activity of C-8-tert-butyl substituted 4-aryl-6,7,8,9-tetrahydrobenzo[4,5]thieno[3,2-e][1,2,4]triazolo[4,3-a]pyrimidin-5(4H)-ones

Members of a series of 4-aryl-6,7,8,9-tetrahydrobenzo[4,5]thieno[3,2-e][1,2,4]triazolo[4,3-a]pyrimidin-5(4H)-ones (1, Fig. 2) were prepared and tested against representative enteroviruses including Human Coxsackievirus B1 (Cox B1), Human Coxsackievirus B3 (Cox B3), human Poliovirus 3 (PV3), human Rhinovirus 14 (HRV14), human Rhinovirus 21 (HRV 21) and human Rhinovirus 71 (HRV 71). The C-8-tert-butyl group on the tetrahydrobenzene ring in these substances was found to be crucial for their enterovirus activity. One member of this group, 1e, showed single digit micromolar activities (1.6-8.85μM) against a spectrum of viruses screened, and the highest selectivity index (SI) values for Cox B1 (>11.2), for Cox B3 (>11.5), and for PV3 (>51.2), respectively. In contrast, 1p, was the most active analog against the selected HRVs (1.8-2.6μM), and showed the highest selectivity indices among the group of compounds tested. The SI values for 1p were 11.5 for HRV14, 8.4 for HRV21, and 12.1 for HRV71, respectively.

Identifying novel antiviral targets against enterovirus 71: where are we?

Human enterovirus 71 (HEV71) has been considered as an essential human pathogen, which causes hand, foot and mouth disease in young children. Several HEV71 outbreaks have been observed in many Asia-Pacific countries for the past two decades with significant fatalities. However, there are no competent vaccines or antivirals against HEV71 infection to date. Thus, it is of critical priority to delve into the search for anti-HEV71 agents. Prior to this, there is a need to gain knowledge about the distinct targets of HEV71 that are available and that have been exploited for antiviral therapy. This review aims to provide a better understanding of HEV71 virology and feature potential antivirals for progressive clinical development with respect to their elucidated mechanistic actions.

Mutations in VP1 and 3A proteins improve binding and replication of rhinovirus C15 in HeLa-E8 cells

Viruses in the rhinovirus C species (RV-C) can cause severe respiratory illnesses in children including pneumonia and asthma exacerbations. A transduced cell line (HeLa-E8) stably expressing the CDHR3-Y529 receptor variant, supports propagation of RV-C after infection. C15 clinical or recombinant isolates replicate in HeLa-E8, however progeny yields are lower than those of related strains of RV-A and RV-B. Serial passaging of C15 in HeLa-E8 resulted in stronger cytopathic effects and increased (≥10-fold) virus binding to cells and progeny yields. The adaptation was acquired by two mutations which increased binding (VP1 T125K) and replication (3A E41K), respectively. A similar 3A mutation engineered into C2 and C41 cDNAs also improved viral replication (2-8 fold) in HeLa but the heparan sulfate mediated cell-binding enhancement by the VP1 change was C15-specific. The findings now enable large-scale cost-effective C15 production by infection and the testing of RV-C infectivity by plaque assay.

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.

Antipicornavirus Drugs: Current Status

Considerable efforts have been made over the past several years to discover a broad-spectrum antipicornavirus agent. The X-ray crystal structure of several rhinovirus serotypes, as well as a coxsackievirus, has provided valuable information with respect to the virus structure as well as the location of the binding site of several capsid-binding compounds. This has aided in the design of broad-spectrum compounds. Several potential drug candidates have reached clinical status and some progress has been made in achieving efficacy. However, none of these compounds has as yet become a marketable drug. This review summarizes the current status of efforts in this area.

Antiviral activity of micafungin against enterovirus 71

Background

Enterovirus 71 (EV71) is a major causative agent of hand-foot-mouth disease (HFMD) and also causes severe neurological complications, leading to fatality in young children. However, no effective therapy is currently available for the treatment of this infection.

Methods

We identified small-molecule inhibitors of EV71 from a screen of 968 Food and Drug Administration (FDA)-approved drugs, with which clinical application for EV71-associated diseases would be more feasible, using EV71 subgenomic replicon system. Primary hits were extensively evaluated for their antiviral activities in EV71-infected cells.

Results

We identified micafungin, an echinocandin antifungal drug, as a novel inhibitor of EV71. Micafungin potently inhibits the proliferation of EV71 as well as the replication of EV71 replicon in cells with a low micromolar IC₅₀ (~5 μM). The strong antiviral effect of micafungin on EV71 replicon and the result from time-of-addition experiment demonstrated a targeting of micafungin on virion-independent intracellular process(es) during EV71 infection. Moreover, an extensive analysis excluded the involvement of 2C and 3A proteins, IRES-dependent translation, and also that of polyprotein processing in the antiviral effect of micafungin.

Conclusions

Our research revealed a new indication of micafungin as an effective inhibitor of EV71, which is the first case reporting antiviral activity of micafungin, an antifungal drug.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-016-0557-8) contains supplementary material, which is available to authorized users.

Susceptibilities of enterovirus D68, enterovirus 71, and rhinovirus 87 strains to various antiviral compounds

Compounds were evaluated for antiviral activity in rhabdomyosarcoma (RD) cells against a recent 2014 clinical isolate of enterovirus D68 (EV-D68), a 1962 strain of EV-68D, rhinovirus 87 (RV-87, serologically the same as EV-D68), and enterovirus 71 (EV-71). Test substances included known-active antipicornavirus agents (enviroxime, guanidine HCl, pirodavir, pleconaril, and rupintrivir), nucleobase/nucleoside analogs (3-deazaguanine and ribavirin), and three novel epidithiodiketopiperazines (KCN-2,2'-epi-19, KCN-19, and KCN-21). Of these, rupintrivir was the most potent, with 50% inhibition of viral cytopathic effect (EC50) and 90% inhibition (EC90) of virus yield at 0.0022-0.0053 μM against EV-D68. Enviroxime, pleconaril and the KCN compounds showed efficacy at 0.01-0.3 μM; 3-deazaguanine and pirodavir inhibited EV-D68 at 7-13 μM, and guanidine HCl and ribavirin were inhibitory at 80-135 μM. Pirodavir was active against EV-71 (EC50 of 0.78 μM) but not against RV-87 or EV-D68, and all other compounds were less effective against EV-71 than against RV-87 and EV-D68. The most promising compound inhibiting both virus infections at low concentrations was rupintrivir. Antiviral activity was confirmed for the ten compounds in virus yield reduction (VYR) assays in RD cells, and for enviroxime, guanidine HCl, and pirodavir by cytopathic effect (CPE) assays in A549, HeLa-Ohio-1, and RD cells. These studies may serve as a basis for further pre-clinical discovery of anti-enterovirus inhibitors. Furthermore, the antiviral profiles and growth characteristics observed herein support the assertion that EV-D68 should be classified together with RV-87.

Phosphatidylinositol 4-kinase III beta is the target of oxoglaucine and pachypodol (Ro 09-0179) for their anti-poliovirus activities, and is located at upstream of the target step of brefeldin A

In recent years, phosphatidylinositol 4-kinase III beta (PI4KB) has emerged as a conserved target of anti-picornavirus compounds. In the present study, PI4KB was identified as the direct target of the plant-derived anti-picornavirus compounds, oxoglaucine and pachypodol (also known as Ro 09-0179). PI4KB was also identified as the target via which pachypodol interferes with brefeldin A (BFA)-induced Golgi disassembly in non-infected cells. Oxysterol-binding protein (OSBP) inhibitor also has interfering activity against BFA. It seems that this interference is not essential for the anti-poliovirus (PV) activities of BFA and PI4KB/OSBP inhibitors. BFA inhibited early to late phase PV replication (0 to 6 hr postinfection) as well as PI4KB inhibitor, but with some delay compared to guanidine hydrochloride treatment. In contrast with PI4KB/OSBP inhibitors, BFA inhibited viral nascent RNA synthesis, suggesting that BFA targets some step of viral RNA synthesis located downstream of the PI4KB/OSBP pathway in PV replication. Our results suggest that PI4KB is a major target of anti-picornavirus compounds identified in vitro for their anti-picornavirus activities and for some uncharacterized biological phenomena caused by these compounds, and that BFA and PI4KB/OSBP inhibitors synergistically repress PV replication by targeting distinct steps in viral RNA replication.

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

Phosphatidylinositol-4 kinase III β (PI4KB) and oxysterol-binding protein (OSBP) family I have been identified as the major targets of anti-enterovirus drug candidates. Resistance mutations in poliovirus (PV) to these inhibitors have been identified in viral 3A protein, represented by a G5318A (3A-Ala70Thr) mutation, but the mechanism of viral resistance to host PI4KB/OSBP inhibitors remained unknown. In this study, we found that a G5318A mutation enhances the basal levels of phosphatidylinositol 4-phosphate (PI4P) and of the 3A protein and decreases the levels of the 3AB protein during PV replication. The 3A protein acted as a major effector responsible for the resistance to PI4KB inhibitor, but did not enhance the PI4KB activity in vitro in contrast to the 2C, 2BC, 3AB, and 3D proteins. The 3AB protein acted as the primary target of a G5318A mutation and also as an effector. We identified novel resistance mutations to a PI4KB inhibitor [C5151U (3A-T14M) and C5366U (3A-H86Y) mutations] and found that there is a positive correlation between the extent of the resistance phenotype and the levels of the 3A proteins. These results suggested that the 3A protein overproduced by enhanced processing of the 3AB protein with the resistance mutations overcomes the inhibitory effect of PI4KB inhibitor on PV replication independently of the hyperactivation of the PI4KB/OSBP pathway.

The Autophagic Machinery in Enterovirus Infection

The Enterovirus genus of the Picornaviridae family comprises many important human pathogens, including polioviruses, rhinovirus, enterovirus A71, and enterovirus D68. They cause a wide variety of diseases, ranging from mild to severe life-threatening diseases. Currently, no effective vaccine is available against enteroviruses except for poliovirus. Enteroviruses subvert the autophagic machinery to benefit their assembly, maturation, and exit from host. Some enteroviruses spread between cells via a process described as autophagosome-mediated exit without lysis (AWOL). The early and late phases of autophagy are regulated through various lipids and their metabolizing enzymes. Some of these lipids and enzymes are specifically regulated by enteroviruses. In the present review, we summarize the current understanding of the regulation of autophagic machinery by enteroviruses, and provide updates on recent developments in this field.

Synergistic antiviral activity of gemcitabine and ribavirin against enteroviruses

Enteroviruses are major causative agents of various human diseases, and some of them are currently considered to be an enormous threat to public health. However, no effective therapy is currently available for the treatment of these infections. We identified gemcitabine, a nucleoside-analog drug used for cancer treatment, from a screen of bioactive chemicals as a novel inhibitor of coxsackievirus B3 (CVB3) and enterovirus 71 (EV71). Gemcitabine potently inhibited the proliferation of CVB3 and EV71, as well as the replication of CVB3 and EV71 replicons, in cells with a low micromolar IC50 (1-5 μM). Its strong inhibitory effect was also observed in cells infected with human rhinoviruses, demonstrating broad-spectrum antiviral effects on enteroviruses. Mechanistically, an extensive analysis excluded the involvement of 2C, 3A, IRES-dependent translation, and also that of polyprotein processing in the antiviral effects of gemcitabine. Importantly, gemcitabine in combination with ribavirin, an antiviral drug currently being used against a few RNA viruses, exhibited a synergistic antiviral effect on the replication of CVB3 and EV71 replicons. Consequently, our results clearly demonstrate a new indication for gemcitabine as an effective broad-spectrum inhibitor of enteroviruses and strongly suggest a new therapeutic strategy using gemcitabine alone or in combination with ribavirin for the treatment of various diseases associated with enterovirus infection.

Replication and Inhibitors of Enteroviruses and Parechoviruses

The Enterovirus (EV) and Parechovirus genera of the picornavirus family include many important human pathogens, including poliovirus, rhinovirus, EV-A71, EV-D68, and human parechoviruses (HPeV). They cause a wide variety of diseases, ranging from a simple common cold to life-threatening diseases such as encephalitis and myocarditis. At the moment, no antiviral therapy is available against these viruses and it is not feasible to develop vaccines against all EVs and HPeVs due to the great number of serotypes. Therefore, a lot of effort is being invested in the development of antiviral drugs. Both viral proteins and host proteins essential for virus replication can be used as targets for virus inhibitors. As such, a good understanding of the complex process of virus replication is pivotal in the design of antiviral strategies goes hand in hand with a good understanding of the complex process of virus replication. In this review, we will give an overview of the current state of knowledge of EV and HPeV replication and how this can be inhibited by small-molecule inhibitors.

Discovery of itraconazole with broad-spectrum in vitro antienterovirus activity that targets nonstructural protein 3A

There is currently no approved antiviral therapy for the prophylaxis or treatment of enterovirus infections, which remain a substantial threat to public health. To discover inhibitors that can be immediately repurposed for treatment of enterovirus infections, we developed a high-throughput screening assay that measures the cytopathic effect induced by enterovirus 71 (EV71) to screen an FDA-approved drug library. Itraconazole (ITZ), a triazole antifungal agent, was identified as an effective inhibitor of EV71 replication in the low-micromolar range (50% effective concentrations [EC50s], 1.15 μM). Besides EV71, the compound also inhibited other enteroviruses, including coxsackievirus A16, coxsackievirus B3, poliovirus 1, and enterovirus 68. Study of the mechanism of action by time-of-addition assay and transient-replicon assay revealed that ITZ targeted a step involved in RNA replication or polyprotein processing. We found that the mutations (G5213U and U5286C) conferring the resistance to the compound were in nonstructural protein 3A, and we confirmed the target amino acid substitutions (3A V51L and 3A V75A) using a reverse genetic approach. Interestingly, posaconazole, a new oral azole with a molecular structure similar to that of ITZ, also exhibited anti-EV71 activity. Moreover, ITZ-resistant viruses do not exhibit cross-resistance to posaconazole or the enviroxime-like compound GW5074, which also targets the 3A region, indicating that they may target a specific site(s) in viral genome. Although the protective activity of ITZ or posaconazole (alone or in combination with other antivirals) remains to be assessed in animal models, our findings may represent an opportunity to develop therapeutic interventions for enterovirus infection.

Phosphatidylinositol-4 kinase III beta and oxysterol-binding protein accumulate unesterified cholesterol on poliovirus-induced membrane structure

Studies on anti-picornavirus compounds have revealed an essential role of a novel cellular pathway via host phosphatidylinositol-4 kinase III beta (PI4KB) and oxysterol-binding protein (OSBP) family I in poliovirus (PV) replication. However, the molecular role for this pathway in PV replication has yet to be determined. Here, viral and host proteins modulating production of phosphatidylinositol 4-phosphate (PI4P) and accumulation of unesterified cholesterol (UC) in cells were analyzed and the role of the PI4KB/OSBP pathway in PV replication characterized. Virus protein 2BC was identified as a novel interactant of PI4KB. PI4KB and VCP/p97 bind to a partially overlapped region of 2BC with different sensitivity to a 2C inhibitor. Production of PI4P and accumulation of UC were enhanced by virus protein 2BC, but suppressed by virus proteins 3A and 3AB. In PV-infected cells, a PI4KB inhibitor suppressed production of PI4P, and both a PI4KB inhibitor and an OSBP ligand suppressed accumulation of UC on virus-induced membrane structure. Inhibition of PI4KB activity caused dissociation of OSBP from virus-induced membrane structure in PV-infected cells. Synthesis of viral nascent RNA in PV-infected cells was not affected in the presence of PI4KB inhibitor and OSBP ligand; however, transient pre-treatment of PV-infected cells with these inhibitors suppressed viral RNA synthesis. These results suggest that virus proteins modulate PI4KB activity and provide PI4P for recruitment of OSBP to accumulate UC on virus-induced membrane structure for formation of a virus replication complex.

In vitro and in vivo protection against enterovirus 71 by an antisense phosphorothioate oligonucleotide

Enterovirus 71 (EV71) is a highly infectious virus that is a major cause of hand, foot, and mouth disease (HFMD), which can lead to severe neurological complications. Currently, there is no effective therapy against EV71. Five antisense oligodeoxynucleotides targeting the 5'-terminal conserved domain of the viral genome were designed using a method based on multiple predicted target mRNA structures. They were then screened for anti-EV71 activity in vitro based on their ability to inhibit an EV71-induced cytopathic effect (CPE). A novel antisense oligonucleotide (EV5) was tested both in rhabdomyosarcoma (RD) cells and in vivo using a mouse model, with a random oligonucleotide (EV5R) of EV5 as a control. EV5 was identified as having significant anti-EV71 activity in vitro and in vivo without significant cytotoxicity. Treatment of RD and Vero cells with antisense oligodeoxynucleotide EV5 significantly and specifically alleviated the cytopathic effect of EV71 in vitro. The inhibitory effect was dose dependent and specific, with a corresponding decrease in viral RNA and viral protein levels. In vivo, EV5 was specifically effective against EV71 virus in preventing death, decreasing weight reduction and reducing the viral RNA copy number and the level of viral proteins in the lungs, intestines and muscles. These results demonstrate the potential and feasibility of using antisense oligodeoxynucleotides specific for the 5'-terminal conserved domain of the viral genome as an antiviral therapy for EV71 disease.

Fitness and virulence of a coxsackievirus mutant that can circumnavigate the need for phosphatidylinositol 4-kinase class III beta

Coxsackieviruses require phosphatidylinositol-4-kinase IIIβ (PI4KIIIβ) for replication but can bypass this need by an H57Y mutation in protein 3A (3A-H57Y). We show that mutant coxsackievirus is not outcompeted by wild-type virus during 10 passages in vitro. In mice, the mutant virus proved as virulent as wild-type virus, even when mice were treated with a PI4KIIIβ inhibitor. Our data suggest that upon emergence, the 3A-H57Y mutant has the fitness to establish a resistant population with a virulence similar to that of wild-type virus.

Phosphatidylinositol 4-kinase III beta is essential for replication of human rhinovirus and its inhibition causes a lethal phenotype in vivo

Human rhinovirus (HRV) is the predominant cause of the common cold, but more importantly, infection may have serious repercussions in asthmatics and chronic obstructive pulmonary disorder (COPD) patients. A cell-based antiviral screen against HRV was performed with a subset of our proprietary compound collection, and an aminothiazole series with pan-HRV species and enteroviral activity was identified. The series was found to act at the level of replication in the HRV infectious cycle. In vitro selection and sequencing of aminothiazole series-resistant HRV variants revealed a single-nucleotide mutation leading to the amino acid change I42V in the essential HRV 3A protein. This same mutation has been previously implicated in resistance to enviroxime, a former clinical-stage antipicornavirus agent. Enviroxime-like compounds have recently been shown to target the lipid kinase phosphatidylinositol 4-kinase III beta (PI4KIIIβ). A good correlation between PI4KIIIβ activity and HRV antiviral potency was found when analyzing the data over 80 compounds of the aminothiazole series, covering a 750-fold potency range. The mechanism of action through PI4KIIIβ inhibition was further demonstrated by small interfering RNA (siRNA) knockdown of PI4KB, which reduced HRV replication and also increased the potency of the PI4KIIIβ inhibitors. Inhibitors from two different structural classes with promising pharmacokinetic profiles and with very good selectivity for PI4KIIIβ were used to dissociate compound-related toxicity from target-related toxicity. Mortality was seen in all dosing groups of mice treated with either compound, therefore suggesting that short-term inhibition of PI4KIIIβ is deleterious.

Oxysterol-binding protein family I is the target of minor enviroxime-like compounds

Enviroxime is an antipicornavirus compound that targets host phosphatidylinositol 4-kinase III beta (PI4KB) activity for its antipicornavirus activity. To date, several antipoliovirus (PV) compounds similar to enviroxime that are associated with a common resistance mutation in viral protein 3A (a G5318A [3A-Ala70Thr] mutation in PV) have been identified. Most of these compounds have a direct inhibitory effect on PI4KB activity, as well as enviroxime (designated major enviroxime-like compounds). However, one of the compounds, AN-12-H5, showed no inhibitory effect on PI4KB and was considered to belong to another group of enviroxime-like compounds (designated minor enviroxime-like compounds). In the present study, we performed a small interfering RNA (siRNA) sensitization assay targeting PI4KB-related genes and identified oxysterol-binding protein (OSBP) as a target of minor enviroxime-like compounds. Knockdown of OSBP and OSBP2 increased the anti-PV activities of AN-12-H5 and a newly identified minor enviroxime-like compound, T-00127-HEV2, and also to T-00127-HEV1 to a minor extent, in the cells. A ligand of OSBP, 25-hydroxycholesterol (25-HC), acted as a minor enviroxime-like compound. Minor enviroxime-like compounds induced relocalization of OSBP to the Golgi apparatus in cells. Treatment of the cells with major or minor enviroxime-like compounds suppressed the expression of genes (HMGCS1 and SQLE) in the SREBP/SCAP regulatory pathway and diminished endogenous phosphatidylinositol 4-phosphate (PI4P) at the Golgi apparatus. Our results suggested that minor enviroxime-like compounds are phenotypically identical to 25-HC and that major and minor enviroxime-like compounds suppress the production and/or accumulation of PI4P in PV-infected cells by targeting PI4KB and OSBP family I activities, respectively.

Strategies to develop antivirals against enterovirus 71

Enterovirus 71 (EV71) is an important human pathogen which may cause severe neurological complications and death in children. The virus caused several outbreaks in the Asia-Pacific region during the past two decades and has been considered a significant public health problem in the post-poliovirus eradication era. Unlike poliovirus, there is no effective vaccine or approved antivirals against EV71. To explore anti-EV71 agents therefore is of vital importance. Several strategies have been employed to develop antivirals based on the molecular characteristics of the virus. Among these, some small molecules that were developed against human rhinoviruses and poliovirus are under evaluation. In this review, we discuss the recent development of such small molecules against EV71, known drug resistance and possible solutions to it, and animal models for evaluating the efficacy of these antivirals. Although further investigation is required for clinical applications of the existing candidates, the molecular mechanisms revealed for the inhibition of EV71 replication can be used for designing new molecules against this virus in the future.

[Exploration for anti-enterovirus compounds and analysis on the mechanism of its inhibitory effect on virus infection]

Poliovirus (PV) is a small non-enveloped virus belonging to the family Picornaviridae, and is the causative agent of poliomyelitis. With established vaccines, the global eradication program for poliomyelitis is ongoing by the World Health Organization since 1988. In the eradication program, antivirals are anticipated to have some roles in the endgame and post-eradication era of PV. During our search for potent anti-PV compounds, we identified candidate compounds that are associated with a common resistance mutation in viral protein 3A similar to enviroxime (designated as enviroxime-like compounds). Recently, PIK93, an inhibitor of host phosphatidylinositol 4-kinase III beta (PI4KB), was identified as a potent anti-enterovirus compound (Hsu et al., Cell 141:799-811). We found that PIK93 is an enviroxime-like compound, and showed that T-00127-HEV1, which is a novel enviroxime-like compound identified in high-throughput screening, is a specific PI4KB inhibitor. We also showed that PI4KB is an enterovirus-specific host factor required for its viral RNA replication. Analysis of anti-enterovirus compounds would unravel novel host factors that could serve as promising antiviral targets of prophylaxis and therapy of the infection.