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.

SUMMARY Itraconazole (ITZ) is a well-known antifungal agent that also has anti-cancer activity. In this study, we identified ITZ as a broad-spectrum inhibitor of enteroviruses (e.g. poliovirus, coxsackievirus, enterovirus-71, rhinovirus). We demonstrate that ITZ inhibits viral RNA replication by targeting oxysterol-binding protein (OSBP) and OSBP-related protein 4 (ORP4). Consistently, OSW-1, a specific OSBP/ORP4 antagonist, also inhibits enterovirus replication. Knockdown of OSBP inhibits virus replication whereas overexpression of OSBP or ORP4 counteracts the antiviral effects of ITZ and OSW-1. ITZ binds OSBP and inhibits its function, i.e. shuttling of cholesterol and phosphatidylinositol-4-phosphate between membranes, thereby likely perturbing the virus-induced membrane alterations essential for viral replication organelle formation. ITZ also inhibits hepatitis C virus replication, which also relies on OSBP. Together, these data implicate OSBP/ORP4 as novel molecular targets of ITZ and point to an essential role of OSBP/ORP4-mediated lipid exchange in virus replication that can be targeted by antiviral drugs.

The secretory pathway is of vital importance for eukaryotic cells and has a pivotal role in the synthesis, sorting, processing and secretion of a large variety of bioactive molecules involved in intercellular communication. One of the key processes in the secretory pathway concerns the transport of cargo proteins from the ER (endoplasmic reticulum) to the Golgi. Type-I transmembrane proteins of ∼24 kDa are abundantly present in the membranes of the early secretory pathway, and bind the COPI and COPII coat complexes that cover vesicles travelling between the membranes. These p24 proteins are thought to play an important role in the selective transport processes at the ER-Golgi interface, although their exact functioning is still obscure. One model proposes that p24 proteins couple cargo selection in the lumen with vesicle coat recruitment in the cytosol. Alternatively, p24 proteins may furnish subcompartments of the secretory pathway with the correct subsets of machinery proteins. Here we review the current knowledge of the p24 proteins and the various roles proposed for the p24 family members.

RNA viruses can rapidly mutate and acquire resistance to drugs that directly target viral enzymes, which poses serious problems in a clinical context. Therefore, there is a growing interest in the development of antiviral drugs that target host factors critical for viral replication, since they are unlikely to mutate in response to therapy. We recently demonstrated that phosphatidylinositol-4-kinase IIIβ (PI4KIIIβ) and its product phosphatidylinositol-4-phosphate (PI4P) are essential for replication of enteroviruses, a group of medically important RNA viruses including poliovirus (PV), coxsackievirus, rhinovirus, and enterovirus 71. Here, we show that enviroxime and GW5074 decreased PI4P levels at the Golgi complex by directly inhibiting PI4KIIIβ. Coxsackievirus mutants resistant to these inhibitors harbor single point mutations in the non-structural protein 3A. These 3A mutations did not confer compound-resistance by restoring the activity of PI4KIIIβ in the presence of the compounds. Instead, replication of the mutant viruses no longer depended on PI4KIIIβ, since their replication was insensitive to siRNA-mediated depletion of PI4KIIIβ. The mutant viruses also did not rely on other isoforms of PI4K. Consistently, no high level of PI4P could be detected at the replication sites induced by the mutant viruses in the presence of the compounds. Collectively, these findings indicate that through specific single point mutations in 3A, CVB3 can bypass an essential host factor and lipid for its propagation, which is a new example of RNA viruses acquiring resistance against antiviral compounds, even when they directly target host factors.

Cardioviruses, including encephalomyocarditis virus (EMCV) and the human Saffold virus, are small non-enveloped viruses belonging to the Picornaviridae, a large family of positive-sense RNA [(+)RNA] viruses. All (+)RNA viruses remodel intracellular membranes into unique structures for viral genome replication. Accumulating evidence suggests that picornaviruses from different genera use different strategies to generate viral replication organelles (ROs). For instance, enteroviruses (e.g. poliovirus, coxsackievirus, rhinovirus) rely on the Golgi-localized phosphatidylinositol 4-kinase III beta (PI4KB), while cardioviruses replicate independently of the kinase. By which mechanisms cardioviruses develop their ROs is currently unknown. Here we show that cardioviruses manipulate another PI4K, namely the ER-localized phosphatidylinositol 4-kinase III alpha (PI4KA), to generate PI4P-enriched ROs. By siRNA-mediated knockdown and pharmacological inhibition, we demonstrate that PI4KA is an essential host factor for EMCV genome replication. We reveal that the EMCV nonstructural protein 3A interacts with and is responsible for PI4KA recruitment to viral ROs. The ensuing phosphatidylinositol 4-phosphate (PI4P) proved important for the recruitment of oxysterol-binding protein (OSBP), which delivers cholesterol to EMCV ROs in a PI4P-dependent manner. PI4P lipids and cholesterol are shown to be required for the global organization of the ROs and for viral genome replication. Consistently, inhibition of OSBP expression or function efficiently blocked EMCV RNA replication. In conclusion, we describe for the first time a cellular pathway involved in the biogenesis of cardiovirus ROs. Remarkably, the same pathway was reported to promote formation of the replication sites of hepatitis C virus, a member of the Flaviviridae family, but not other picornaviruses or flaviviruses. Thus, our results highlight the convergent recruitment by distantly related (+)RNA viruses of a host lipid-modifying pathway underlying formation of viral replication sites.

Members of the Enterovirus (poliovirus [PV], coxsackieviruses, and human rhinoviruses) and Kobuvirus (Aichi virus) genera in the Picornaviridae family rely on PI4KIII␤ (phosphatidylinositol-4-kinase III␤) for efficient replication. The small membraneanchored enteroviral protein 3A recruits PI4KIII␤ to replication organelles, yet the underlying mechanism has remained elusive. Recently, it was shown that kobuviruses recruit PI4KIII␤ through interaction with ACBD3 (acyl coenzyme A [acyl-CoA]-binding protein domain 3), a novel interaction partner of PI4KIII␤. Therefore, we investigated a possible role for ACBD3 in recruiting PI4KIII␤ to enterovirus replication organelles. Although ACBD3 interacted directly with coxsackievirus B3 (CVB3) 3A, its depletion from cells by RNA interference did not affect PI4KIII␤ recruitment to replication organelles and did not impair CVB3 RNA replication. Enterovirus 3A was previously also proposed to recruit PI4KIII␤ via GBF1/Arf1, based on the known interaction of 3A with GBF1, an important regulator of secretory pathway transport and a guanine nucleotide exchange factor (GEF) of Arf1. However, our results demonstrate that inhibition of GBF1 or Arf1 either by pharmacological inhibition or depletion with small interfering RNA (siRNA) treatment did not affect the ability of 3A to recruit PI4KIII␤. Furthermore, we show that a 3A mutant that no longer binds GBF1 was capable of recruiting PI4KIII␤, even in ACBD3-depleted cells. Together, our findings indicate that unlike originally envisaged, coxsackievirus recruits PI4KIII␤ to replication organelles independently of ACBD3 and GBF1/Arf1. IMPORTANCEA hallmark of enteroviral infection is the generation of new membranous structures to support viral RNA replication. The functionality of these "replication organelles" depends on the concerted actions of both viral nonstructural proteins and co-opted host factors. It is thus essential to understand how these structures are formed and which cellular components are key players in this process. GBF1/Arf1 and ACBD3 have been proposed to contribute to the recruitment of the essential lipid-modifying enzyme PI4KIII␤ to enterovirus replication organelles. Here we show that the enterovirus CVB3 recruits PI4KIII␤ by a mechanism independent of both GBF1/Arf1 and ACBD3. This study shows that the strategy employed by coxsackievirus to recruit PI4KIII␤ to replication organelles is far more complex than initially anticipated.

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Positive-strand RNA (+RNA) viruses (e.g. poliovirus, hepatitis C virus, dengue virus, SARS-coronavirus) remodel cellular membranes to form so-called viral replication compartments (VRCs), which are the sites where viral RNA genome replication takes place. To induce VRC formation, these viruses extensively rewire lipid metabolism. Disparate viruses have many commonalities as well as disparities in their interactions with the host lipidome and accumulate specific sets of lipids (sterols, glycerophospholipids, sphingolipids) at their VRCs. Recent years have seen an upsurge in studies investigating the role of lipids in +RNA virus replication, in particular of sterols, and uncovered that membrane contact sites and lipid transfer proteins are hijacked by viruses and play pivotal roles in VRC formation.