The 3C protease activity of enterovirus 71 induces human neural cell apoptosis

CRISPR/Cas12a Protein Switch Powered Label-Free Electrochemical Biosensor for Sensitive Viral Protease Detection

Viral proteases are critical molecular targets in viral pathogenesis, representing pivotal biomarkers for understanding viral infection mechanisms and developing antiviral therapeutics. This study introduces a label-free electrochemical biosensor that enables sensitive viral protease detection by integrating protease-responsive CRISPR/Cas protein switches (CasPSs) with a hemin aptamer-functionalized electrochemical interface. The biosensor's mechanism relies on viral protease-mediated proteolysis, which leads to the release of active Cas12a proteins from CasPSs and generates amplified electrochemical responses through continuous cleavage of immobilized redox-active hemin/aptamer complexes. This biosensor achieved specific hepatitis C virus NS3/4A protease sensing with femtomolar sensitivity and could be readily expanded to other viral proteases by replacing the CasPS module. The feasibility of this biosensor was demonstrated by monitoring enterovirus 71 3C protease activities in virus-infected cell samples with different viral loads and postinfection times. This study provides a promising strategy for integrating CRISPR biosensing with electrochemical platforms, offering a helpful analytical tool for viral infection monitoring and antiviral drug screening.

Ferroptosis, from the virus point of view: opportunities and challenges

Ferroptosis is a new type of cell death, which is mainly dependent on the formation and accumulation of reactive oxygen species and lipid peroxides mediated by iron. It is distinct from other forms of regulation of cell death in morphology, immunology, biochemistry, and molecular biology. Various cell death mechanisms have been observed in many viral infections, and virus-induced cell death has long been considered as a double-edged sword that can inhibit or aggravate viral infections. However, understanding of the role of ferroptosis in various viral infections is limited. Special attention will be paid to the mechanisms of ferroptosis in mediating viral infection and antiviral treatment associated with ferroptosis. In this paper, we outlined the mechanism of ferroptosis. Additionally, this paper also review research on ferroptosis from the perspective of the virus, discussed the research status of ferroptosis in virus infection and classified and summarized research on the interaction between viral infections and ferroptosis.

Non-Polio Enterovirus Inhibitors: Scaffolds, Targets, and Potency─What's New?

Enterovirus (EV) is a genus that includes a large diversity of viruses spread around the world. They are the main cause of numerous diseases with seasonal clusters, like hand-foot-mouth disease (HFMD). A vaccine is marketed in China for the prevention of HFMD caused by EV-A71. Despite the need, no antiviral is marketed to date. Therefore, several compounds have been currently evaluated to inhibit non-polio Enterovirus (NPEV), namely direct antiviral agents and host target inhibitor. We propose to make a review of the latest molecules evaluated as NPEV inhibitors and to summarize structure-activity relationships between these inhibitors and their target. We provide access to all recent information on Enterovirus inhibitors, regardless of the species, to facilitate the design of future broad-spectrum drugs.

Cleavage of Stau2 by 3C protease promotes EV-A71 replication

BACKGROUND

Enterovirus A71 (EV-A71), as a neurotropic virus, mainly affects infants and young children under the age of 5. EV-A71 infection causes hand-foot-mouth disease and herpetic angina, and even life-threatening neurological complications. However, the molecular mechanism by which EV-A71 induces nervous system damage remains elusive. The viral protease 3C plays an important role during EV-A71 infection and is also a key intersection of virus-host interactions. Previously, we used yeast two-hybrid to screen out the host protein Double-stranded RNA-binding protein Staufen homolog 2 (Stau2), an important member involved in neuronal mRNA transport, potentially interacts with 3C.

METHODS

We used coimmunoprecipitation (Co-IP) and immunofluorescence assay (IFA) to confirm that EV-A71 3C interacts with Stau2. By constructing the mutant of Stau2, we found the specific site where the 3C protease cleaves Stau2. Detection of VP1 protein using Western blotting characterized EV-A71 viral replication, and overexpression or knockdown of Stau2 exhibited effects on EV-A71 replication. The effect of different cleavage products on EV-A71 replication was demonstrated by constructing Stau2 truncates.

RESULTS

In this study, we found that EV-A71 3C interacts with Stau2. Stau2 is cleaved by 3C at the Q507-G508 site. Overexpression of Stau2 promotes EV-A71 VP1 protein expression, whereas depletion of Stau2 by small interfering RNA inhibits EV-A71 replication. Stau2 is essential for EV-A71 replication, and the product of Stau2 cleavage by 3C, 508-570 aa, has activity that promotes EV-A71 replication. In addition, we found that mouse Stau2 is also cleaved by EV-A71 3C at the same site.

CONCLUSIONS

Our research provides an example for EV-A71-host interaction, enriching key targets of host factors that contribute to viral replication.

Temporal regulation of MDA5 inactivation by Caspase-3 dependent cleavage of 14-3-3η

The kinetics of type I interferon (IFN) induction versus the virus replication compete, and the result of the competition determines the outcome of the infection. Chaperone proteins that involved in promoting the activation kinetics of PRRs rapidly trigger antiviral innate immunity. We have previously shown that prior to the interaction with MAVS to induce type I IFN, 14-3-3η facilitates the oligomerization and intracellular redistribution of activated MDA5. Here we report that the cleavage of 14-3-3η upon MDA5 activation, and we identified Caspase-3 activated by MDA5-dependent signaling was essential to produce sub-14-3-3η lacking the C-terminal helix (αI) and tail. The cleaved form of 14-3-3η (sub-14-3-3η) could strongly interact with MDA5 but could not support MDA5-dependent type I IFN induction, indicating the opposite functions between the full-length 14-3-3η and sub-14-3-3η. During human coronavirus or enterovirus infections, the accumulation of sub-14-3-3η was observed along with the activation of Caspase-3, suggesting that RNA viruses may antagonize 14-3-3η by promoting the formation of sub-14-3-3η to impair antiviral innate immunity. In conclusion, sub-14-3-3η, which could not promote MDA5 activation, may serve as a negative feedback to return to homeostasis to prevent excessive type I IFN production and unnecessary inflammation.

Scorpion Venom Antimicrobial Peptide Derivative BmKn2-T5 Inhibits Enterovirus 71 in the Early Stages of the Viral Life Cycle In Vitro

Enterovirus 71 (EV71), a typical representative of unenveloped RNA viruses, is the main pathogenic factor responsible for hand, foot, and mouth disease (HFMD) in infants. This disease seriously threatens the health and lives of humans worldwide, especially in the Asia-Pacific region. Numerous animal antimicrobial peptides have been found with protective functions against viruses, bacteria, fungi, parasites, and other pathogens, but there are few studies on the use of scorpion-derived antimicrobial peptides against unenveloped viruses. Here, we investigated the antiviral activities of scorpion venom antimicrobial peptide BmKn2 and five derivatives, finding that BmKn2 and its derivative BmKn2-T5 exhibit a significant inhibitory effect on EV71. Although both peptides exhibit characteristics typical of amphiphilic α-helices in terms of their secondary structure, BmKn2-T5 displayed lower cellular cytotoxicity than BmKn2. BmKn2-T5 was further found to inhibit EV71 in a dose-dependent manner in vitro. Moreover, time-of-drug-addition experiments showed that BmKn2-T5 mainly restricts EV71, but not its virion or replication, at the early stages of the viral cycle. Interestingly, BmKn2-T5 was also found to suppress the replication of the enveloped viruses DENV, ZIKV, and HSV-1 in the early stages of the viral cycle, which suggests they may share a common early infection step with EV71. Together, the results of our study identified that the scorpion-derived antimicrobial peptide BmKn2-T5 showed valuable antiviral properties against EV71 in vitro, but also against other enveloped viruses, making it a potential new candidate therapeutic molecule.

Research progress on pathogenic and therapeutic mechanisms of Enterovirus A71

In recent years, enterovirus A71 (EV-A71) infection has become a major global public health problem, especially for infants and young children. The results of epidemiological research show that EV-A71 infection can cause acute hand, foot, and mouth disease (HFMD) and complications of the nervous system in severe cases, including aseptic pediatric meningoencephalitis, acute flaccid paralysis, and even death. Many studies have demonstrated that EV-A71 infection may trigger a variety of intercellular and intracellular signaling pathways, which are interconnected to form a network that leads to the innate immune response, immune escape, inflammation, and apoptosis in the host. This article aims to provide an overview of the possible mechanisms underlying infection, signaling pathway activation, the immune response, immune evasion, apoptosis, and the inflammatory response caused by EV-A71 infection and an overview of potential therapeutic strategies against EV-A71 infection to better understand the pathogenesis of EV-A71 and to aid in the development of antiviral drugs and vaccines.

Current status of hand-foot-and-mouth disease

Hand-foot-and-mouth disease (HFMD) is a viral illness commonly seen in young children under 5 years of age, characterized by typical manifestations such as oral herpes and rashes on the hands and feet. These symptoms typically resolve spontaneously within a few days without complications. Over the past two decades, our understanding of HFMD has greatly improved and it has received significant attention. A variety of research studies, including epidemiological, animal, and in vitro studies, suggest that the disease may be associated with potentially fatal neurological complications. These findings reveal clinical, epidemiological, pathological, and etiological characteristics that are quite different from initial understandings of the illness. It is important to note that HFMD has been linked to severe cardiopulmonary complications, as well as severe neurological sequelae that can be observed during follow-up. At present, there is no specific pharmaceutical intervention for HFMD. An inactivated Enterovirus A71 (EV-A71) vaccine that has been approved by the China Food and Drug Administration (CFDA) has been shown to provide a high level of protection against EV-A71-related HFMD. However, the simultaneous circulation of multiple pathogens and the evolution of the molecular epidemiology of infectious agents make interventions based solely on a single agent comparatively inadequate. Enteroviruses are highly contagious and have a predilection for the nervous system, particularly in child populations, which contributes to the ongoing outbreak. Given the substantial impact of HFMD around the world, this Review synthesizes the current knowledge of the virology, epidemiology, pathogenesis, therapy, sequelae, and vaccine development of HFMD to improve clinical practices and public health efforts.

Antiviral effects of the petroleum ether extract of Tournefortia sibirica L. against enterovirus 71 infection in vitro and in vivo

Enterovirus 71 (EV71) is the major cause of severe hand, foot, and mouth disease (HFMD). Compared to other HFMD pathogens, like coxsackievirus A16 (CVA16), EV71 can invade the central nervous system and cause permanent damage. At present, there are no available antivirals against EV71 for clinical treatment. Herein, multiple Chinese botanical drugs were collected, and 47 types of botanical extracts were extracted using aqueous solutions and organic solvents. Based on the cytopathic effect inhibition assay, petroleum ether extract of Tournefortia sibirica L. (PE-TS) demonstrated 97.25% and 94.75% inhibition rates for EV71 infection (at 250 μg/ml) and CVA16 infection (at 125 μg/ml), respectively, with low cytotoxicity. Preliminary mechanistic studies showed that PE-TS inhibits replication of EV71 genomic RNA and synthesis of the EV71 protein. The released extracellular EV71 progeny virus titer decreased by 3.75 lg under PE-TS treatment. Furthermore, using a newborn mouse model, PE-TS treatment protected 70% and 66.7% of mice from lethal dose EV71 intracranial challenge via administration of intraperitoneal injection at 0.4 mg/g and direct lavage at 0.8 mg/g, respectively. The chemical constituents of the PE-TS were analyzed by Gas Chromatography-Mass Spectrometer (GC-MS), and a total of 60 compounds were identified. Compound-target network analysis and molecular docking implied potential bioactive compounds and their protein targets against EV71 associated pathology. The present study identified antiviral effects of PE-TS against EV71/CVA16 infection in vitro and EV71 infection in vivo, providing a potential antiviral botanical drug extract candidate for HFMD drug development.

Activation of Host Cellular Signaling and Mechanism of Enterovirus 71 Viral Proteins Associated with Hand, Foot and Mouth Disease

Enteroviruses are members of the Picornaviridae family consisting of human enterovirus groups A, B, C, and D as well as nonhuman enteroviruses. Human enterovirus type 71 (EV71) has emerged as a major cause of viral encephalitis, known as hand, foot, and mouth disease (HFMD), in children worldwide, especially in the Asia-Pacific region. EV71 and coxsackievirus A16 are the two viruses responsible for HFMD which are members of group A enteroviruses. The identified EV71 receptors provide useful information for understanding viral replication and tissue tropism. Host factors interact with the internal ribosome entry site (IRES) of EV71 to regulate viral translation. However, the specific molecular features of the respective viral genome that determine virulence remain unclear. Although a vaccine is currently approved, there is no effective therapy for treating EV71-infected patients. Therefore, understanding the host-pathogen interaction could provide knowledge in viral pathogenesis and further benefits to anti-viral therapy development. The aim of this study was to investigate the latest findings about the interaction of viral ligands with the host receptors as well as the activation of immunerelated signaling pathways for innate immunity and the involvement of different cytokines and chemokines during host-pathogen interaction. The study also examined the roles of viral proteins, mainly 2A and 3C protease, interferons production and their inhibitory effects.

The SARS-CoV-2 main protease doesn't induce cell death in human cells in vitro

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the cause of coronavirus disease 2019 (COVID-19) which has extremely rapidly spread worldwide. In order to develop the effective antiviral therapies, it is required to understand the molecular mechanisms of the SARS-CoV-2 pathogenesis. The main protease, or 3C-like protease (3CLpro), plays the essential role in the coronavirus replication that makes the enzyme a promising therapeutic target. Viral enzymes are known to be multifunctional. Particularly, 3CLpro of SARS-CoV was shown to induce apoptosis in addition to its main function. In the present study we analyzed the cytotoxicity of active SARS-CoV-2 3CLpro and its inactivated form upon their individual expression in four human cell lines. For this purpose, we constructed a protein biosensor which allows to detect the proteolytic activity of SARS-CoV-2 3CLpro and confirmed the expression of the active protease in all cell lines used. We studied viability and morphology of the cells and found that both active and inactivated enzyme variants induce no cell death in contrast to the homologous 3CL protease of SARS-CoV. These results indicate that SARS-CoV-2 3CLpro is unlikely contribute to the cytopathic effect observed during viral infection directly.

Gasdermin E is required for induction of pyroptosis and severe disease during enterovirus 71 infection

Pyroptosis is an inflammatory form of programmed cell death that is executed by the gasdermin (GSDM)-N domain of GSDM family proteins, which form pores in the plasma membrane. Although pyroptosis acts as a host defense against invasive pathogen infection, its role in the pathogenesis of enterovirus 71 (EV71) infection is unclear. In the current study, we found that EV71 infection induces cleavage of GSDM E (GSDME) by using western blotting analysis, an essential step in the switch from caspase-3-mediated apoptosis to pyroptosis. We show that this cleavage is independent of the 3C and 2A proteases of EV71. However, caspase-3 activation is essential for this cleavage, as GSDME could not be cleaved in caspase-3-KO cells upon EV71 infection. Further analyses showed that EV71 infection induced pyroptosis in WT cells but not in caspase-3/GSDME double-KO cells. Importantly, GSDME is required to induce severe disease during EV71 infection, as GSDME deficiency in mice was shown to alleviate pathological symptoms. In conclusion, our results reveal that GSDME is important for the pathogenesis of EV71 via mediating initiation of pyroptosis.

Pharmacological perturbation of CXCL1 signaling alleviates neuropathogenesis in a model of HEVA71 infection

Hand, foot and mouth disease (HFMD) caused by Human Enterovirus A71 (HEVA71) infection is typically a benign infection. However, in minority of cases, children can develop severe neuropathology that culminate in fatality. Approximately 36.9% of HEVA71-related hospitalizations develop neurological complications, of which 10.5% are fatal. Yet, the mechanism by which HEVA71 induces these neurological deficits remain unclear. Here, we show that HEVA71-infected astrocytes release CXCL1 which supports viral replication in neurons by activating the CXCR2 receptor-associated ERK1/2 signaling pathway. Elevated CXCL1 levels correlates with disease severity in a HEVA71-infected mice model. In humans infected with HEVA71, high CXCL1 levels are only present in patients presenting neurological complications. CXCL1 release is specifically triggered by VP4 synthesis in HEVA71-infected astrocytes, which then acts via its receptor CXCR2 to enhance viral replication in neurons. Perturbing CXCL1 signaling or VP4 myristylation strongly attenuates viral replication. Treatment with AZD5069, a CXCL1-specific competitor, improves survival and lessens disease severity in infected animals. Collectively, these results highlight the CXCL1-CXCR2 signaling pathway as a potential target against HFMD neuropathogenesis.

4-Iminooxazolidin-2-One as a Bioisostere of Cyanohydrin Suppresses EV71 Proliferation by Targeting 3Cpro

The fatal pathogen enterovirus 71 (EV71) is a major cause of hand-foot-and-mouth disease (HFMD), which leads to serious neurological syndromes. While there are no effective clinical agents available for EV71 treatment thus far, EV71 3C protease (3Cpro), a cysteine protease encoded by the virus, has become a promising drug target for discovery of antiviral drugs, given that it plays a crucial role in virus proliferation and interferes with host cell function. Here, we report two inhibitors of EV71 3Cpro, FOPMC and FIOMC, that were developed from previously reported cyanohydrin derivative (R)-1 by replacing the acyl cyanohydrin group with 4-iminooxazolidin-2-one. FOPMC and FIOMC have potent antiviral activity and dramatically improved metabolic stability. These two inhibitors demonstrated broad anti-EV effects on various cell lines and five epidemic viral strains. We further illuminated the binding models between 3Cpro and FOPMC/FIOMC through molecular docking and molecular dynamics simulations. The substitution of an acyl cyanohydrin group with 4-iminooxazolidin-2-one does make FOPMC and FIOMC potent anti-EV71 drug candidates as universal nonclassical bioisosteres with a cyanohydrin moiety. IMPORTANCE EV71 is one of the most epidemic agents of HFMD. Thus far, there are no antiviral drugs available for clinical usage. The conserved EV71 3Cpro plays pivotal roles in virus proliferation and defense host immunity, as well as having no homology in host cells, making it a most promising antiviral target. In this work, we identified that propyl- and isopropyl-substituted 4-iminooxazolidin-2-one moieties (FOPMC and FIOMC) effectively inhibited five epidemic viral strains in rhabdomyosarcoma (RD), HEK-293T, and VeroE6 cell lines. The inhibition mechanism was also illustrated with molecular docking and molecular dynamics (MD) simulations. The successful replacement of the labile cyanohydrin greatly improved the stability and pharmacokinetic properties of (R)-1, making 4-iminooxazolidin-2-one a nonclassical bioisosteric moiety of cyanohydrin. This discovery addressed a critical issue of the primitive structural scaffold of these promising anti-EV71 inhibitors and could lead to their development as broad-spectrum anti-EV agents.

3C protease of enterovirus 71 cleaves promyelocytic leukemia protein and impairs PML-NBs production

BACKGROUND

Enterovirus 71 (EV71) usually infects infants causing hand-foot-mouth disease (HFMD), even fatal neurological disease like aseptic meningitis. Effective drug for preventing and treating EV71 infection is unavailable currently. EV71 3C mediated the cleavage of many proteins and played an important role in viral inhibiting host innate immunity. Promyelocytic leukemia (PML) protein, the primary organizer of PML nuclear bodies (PML-NBs), can be induced by interferon and is involved in antiviral activity. PML inhibits EV71 replication, and EV71 infection reduces PML expression, but the molecular mechanism is unclear.

METHODS

The cleavage of PMLIII and IV was confirmed by co-transfection of EV71 3C protease and PML. The detailed cleavage sites were evaluated further by constructing the Q to A mutant of PML. PML knockout cells were infected with EV71 to identify the effect of cleavage on EV71 replication. Immunofluorescence analysis to examine the interference of EV71 3C on the formation of PML-NBs.

RESULTS

EV71 3C directly cleaved PMLIII and IV. Furthermore, 3C cleaved PMLIV at the sites of Q430-A431 and Q444-S445 through its protease activity. Overexpression of PMLIV Q430A/Q444A variant exhibited stronger antiviral potential than the wild type. PMLIV Q430A/Q444A formed normal nuclear bodies that were not affected by 3C, suggesting that 3C may impair PML-NBs production via PMLIV cleavage and counter its antiviral activities. PML, especially PMLIV, which sequesters viral proteins in PML-NBs and inhibits viral production, is a novel target of EV71 3C cleavage.

CONCLUSIONS

EV71 3C cleaves PMLIV at Q430-A431 and Q444-S445. Cleavage reduces the antiviral function of PML and decomposes the formation of PML-NBs, which is conducive to virus replication.

Ectopic Expression of TRIM25 Restores RIG-I Expression and IFN Production Reduced by Multiple Enteroviruses 3Cpro

Enteroviruses (EVs) 3C proteins suppress type I interferon (IFN) responses mediated by retinoid acid-inducible gene I (RIG-I), while an E3 ubiquitin ligase, tripartite motif protein 25 (TRIM25)-mediated RIG-I ubiquitination is essential for RIG-I antiviral activity. Therefore, whether the effect of EVs 3C on RIG-I is associated with TRIM25 expression is worth to be further investigated. Here, we demonstrate that 3C proteins of EV71 and coxsackievirus B3 (CVB3) reduced not only RIG-I expression but also TRIM25 expression through protease cleavage activity, while overexpression of TRIM25 restored RIG-I expression and IFN-β production reduced by 3C proteins. Further investigation confirmed that the two amino acids and functional domains in TRIM25 required for RIG-I ubiquitination and TRIM25 structural conformation were essential for the recovery of RIG-I expression. Moreover, we also observed that TRIM25 could rescue RIG-I expression reduced by 3C proteins of CVA6 and EV-D68 but not CVA16. Our findings provide an insightful interpretation of 3C-mediated host innate immune suppression and support TRIM25 as an attractive target against multiple EVs infection.

Degradation of Host Proteins and Apoptosis Induced by Foot-and-Mouth Disease Virus 3C Protease

Foot-and-mouth disease (FMD), induced by the foot-and-mouth disease virus (FMDV), is a highly contagious disease of cloven-hoofed animals. Previous studies have reported that FMDV 3C protease could degrade multiple host proteins; however, the degradation mechanism mediated by FMDV 3C is still unclear. Here, we found that transient expression of FMDV 3C degraded various molecules in NF-κB signaling in a dose-dependent manner, and the proteolytic activity of FMDV 3C is important for inducing degradation. Additionally, 3C-overexpression was associated with the induction of apoptosis. In this study, we showed that an apoptosis inhibitor CrmA abolished the ability of 3C to degrade molecules in NF-κB signaling. Further experiments using specific caspase inhibitors confirmed the irrelevance of caspase3, caspase8, and caspase9 activity for degradation induced by 3C. Altogether, these results suggest that FMDV 3C induces the widespread degradation of host proteins through its proteolytic activity and that the apoptosis pathway might be an important strategy to mediate this process. Further exploration of the relationship between apoptosis and degradation induced by 3C could provide novel insights into the pathogenic mechanisms of FMDV.

Individual Expression of Hepatitis A Virus 3C Protease Induces Ferroptosis in Human Cells In Vitro

Regulated cell death (RCD) is a fundamental process common to nearly all living beings and essential for the development and tissue homeostasis in animals and humans. A wide range of molecules can induce RCD, including a number of viral proteolytic enzymes. To date, numerous data indicate that picornaviral 3C proteases can induce RCD. In most reported cases, these proteases induce classical caspase-dependent apoptosis. In contrast, the human hepatitis A virus 3C protease (3Cpro) has recently been shown to cause caspase-independent cell death accompanied by previously undescribed features. Here, we expressed 3Cpro in HEK293, HeLa, and A549 human cell lines to characterize 3Cpro-induced cell death morphologically and biochemically using flow cytometry and fluorescence microscopy. We found that dead cells demonstrated necrosis-like morphological changes including permeabilization of the plasma membrane, loss of mitochondrial potential, as well as mitochondria and nuclei swelling. Additionally, we showed that 3Cpro-induced cell death was efficiently blocked by ferroptosis inhibitors and was accompanied by intense lipid peroxidation. Taken together, these results indicate that 3Cpro induces ferroptosis upon its individual expression in human cells. This is the first demonstration that a proteolytic enzyme can induce ferroptosis, the recently discovered and actively studied type of RCD.

Inhibitors of Coronavirus 3CL Proteases Protect Cells from Protease-Mediated Cytotoxicity

We describe a mammalian cell-based assay to identify coronavirus 3CL protease (3CLpro) inhibitors. This assay is based on rescuing protease-mediated cytotoxicity and does not require live virus. By enabling the facile testing of compounds across a range of 15 distantly related coronavirus 3CLpro enzymes, we identified compounds with broad 3CLpro-inhibitory activity. We also adapted the assay for use in compound screening and in doing so uncovered additional severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) 3CLpro inhibitors. We observed strong concordance between data emerging from this assay and those obtained from live-virus testing. The reported approach democratizes the testing of 3CLpro inhibitors by developing a simplified method for identifying coronavirus 3CLpro inhibitors that can be used by the majority of laboratories, rather than the few with extensive biosafety infrastructure. We identified two lead compounds, GC376 and compound 4, with broad activity against all 3CL proteases tested, including 3CLpro enzymes from understudied zoonotic coronaviruses. IMPORTANCE Multiple coronavirus pandemics have occurred over the last 2 decades. This has highlighted a need to be proactive in the development of therapeutics that can be readily deployed in the case of future coronavirus pandemics. We developed and validated a simplified cell-based assay for the identification of chemical inhibitors of 3CL proteases encoded by a wide range of coronaviruses. This assay is reporter free, does not require specialized biocontainment, and is optimized for performance in high-throughput screening. By testing reported 3CL protease inhibitors against a large collection of 3CL proteases with variable sequence similarity, we identified compounds with broad activity against 3CL proteases and uncovered structural insights into features that contribute to their broad activity. Furthermore, we demonstrated that this assay is suitable for identifying chemical inhibitors of proteases from families other than 3CL proteases.

Picornavirus 3C - a protease ensuring virus replication and subverting host responses

The protease 3C is encoded by all known picornaviruses, and the structural features related to its protease and RNA-binding activities are conserved; these contribute to the cleavage of viral polyproteins and the assembly of the viral RNA replication complex during virus replication. Furthermore, 3C performs functions in the host cell through its interaction with host proteins. For instance, 3C has been shown to selectively 'hijack' host factors involved in gene expression, promoting picornavirus replication, and to inactivate key factors in innate immunity signaling pathways, inhibiting the production of interferon and inflammatory cytokines. Importantly, 3C maintains virus infection by subtly subverting host cell death and modifying critical molecules in host organelles. This Review focuses on the molecular mechanisms through which 3C mediates physiological processes involved in virus-host interaction, thus highlighting the picornavirus-mediated pathogenesis caused by 3C.

Egress of non-enveloped enteric RNA viruses

A long-standing paradigm in virology was that non-enveloped viruses induce cell lysis to release progeny virions. However, emerging evidence indicates that some non-enveloped viruses exit cells without inducing cell lysis, while others engage both lytic and non-lytic egress mechanisms. Enteric viruses are transmitted via the faecal-oral route and are important causes of a wide range of human infections, both gastrointestinal and extra-intestinal. Virus cellular egress, when fully understood, may be a relevant target for antiviral therapies, which could minimize the public health impact of these infections. In this review, we outline lytic and non-lytic cell egress mechanisms of non-enveloped enteric RNA viruses belonging to five families: Picornaviridae, Reoviridae, Caliciviridae, Astroviridae and Hepeviridae. We discuss factors that contribute to egress mechanisms and the relevance of these mechanisms to virion stability, infectivity and transmission. Since most data were obtained in traditional two-dimensional cell cultures, we will further attempt to place them into the context of polarized cultures and in vivo pathogenesis. Throughout the review, we highlight numerous knowledge gaps to stimulate future research into the egress mechanisms of these highly prevalent but largely understudied viruses.

Enterovirus 71 induces neural cell apoptosis and autophagy through promoting ACOX1 downregulation and ROS generation

Enterovirus 71 (EV71) infection causes hand, foot, and mouth disease (HFMD), and even fatal neurological complications. However, the mechanisms underlying EV71 neurological pathogeneses are largely unknown. This study reveals a distinct mechanism by which EV71 induces apoptosis and autophagy in neural cells. EV71 non-structure protein 3D (also known as RNA-dependent RNA polymerase, RdRp) interacts with the peroxisomal protein acyl-CoA oxidase 1 (ACOX1), and contributes to ACOX1 downregulation. Further studies demonstrate that EV71 reduces peroxisome numbers. Additionally, knockdown of ACOX1 or peroxin 19 (PEX19) induces apoptosis and autophagy in neural cells including human neuroblastoma (SK-N-SH) cells and human astrocytoma (U251) cells, and EV71 infection induces neural cell death through attenuating ACOX1 production. Moreover, EV71 infection and ACOX1 knockdown facilitate reactive oxygen species (ROS) production and attenuate the cytoprotective protein deglycase (DJ-1)/Nuclear factor erythroid 2-related factor 2 (NRF2)/Heme oxygenase 1 (HO-1) pathway (DJ-1/NRF2/HO-1), which collectively result in ROS accumulation in neural cells. In conclusion, EV71 downregulates ACOX1 protein expression, reduces peroxisome numbers, enhances ROS generation, and attenuates the DJ-1/NRF2/HO-1 pathway, thereby inducing apoptosis and autophagy in neural cells. These findings provide new insights into the mechanism underlying EV71-induced neural pathogenesis, and suggest potential treatments for EV71-associated diseases.

Intrinsic apoptosis and cytokine induction regulated in human tonsillar epithelial cells infected with enterovirus A71

Enterovirus A71 (EV-A71) has emerged as a clinically important neurotropic virus following poliovirus eradication. Recent studies have shown that human tonsillar epithelial cell lines (UT-SCC-60A and UT-SCC-60B) were susceptible to EV-A71, suggesting that human tonsillar crypt epithelium could be important in EV-A71 pathogenesis. However, the mechanism about how EV-A71 infects the upper oro-digestive tract remains largely unclear. In this study, we demonstrated that the human tonsillar epithelial cells infected with EV-A71 underwent apoptotic, in which cytochrome c was released from the mitochondria to the cytosol and caspase-9 was activated, while caspase-2 and -8 were not cleaved or activated during the infection. A selective inhibitor of caspase-9, Z-LEHD-FMK, inhibited the cleavage of the executioner caspase-3 and -7, indicating that only mitochondria-mediated intrinsic apoptotic pathway was activated in EV-A71-infected tonsillar epithelial cells. No evidence of pyroptosis or necroptosis was involved in the cell death. EV-A71 infection induced interferon, pro-inflammatory cytokines and chemokines, including IFN-β, IL-6, CCL5, and TNF-α in tonsillar epithelial cells, which may play a critical role in EV-A71-caused herpangina. Our data indicated that the induction of the cytokines was partially regulated by the mitogen-activated protein kinases (MAPKs) signaling pathway. The findings unveiled the host response to EV-A71 and its regulation mechanism, and will further our understanding the significance about the tonsillar crypt epithelium as the initial and primary portal in viral pathogenesis for EV-A71 infection.

Recent advances of enterovirus 71 3 C p r o targeting Inhibitors

With CA16, enterovirus-71 is the causative agent of hand foot and mouth disease (HFMD) which occurs mostly in children under 5 years-old and responsible of several outbreaks since a decade. Most of the time, HFMD is a mild disease but can progress to severe complications such as meningitis, brain stem encephalitis, acute flaccid paralysis (AFP) and even death; EV71 has been identified in all severe cases. Therefore, it is actually one of the most public health issues that threatens children's life.3 C p r o is a protease which plays important functions in EV71 infection. To date, a lot of3 C p r o inhibitors have been tested but none of them has been approved yet. Therefore, a drug screening is still an utmost importance in order to treat and/or prevent EV71 infections. This work highlights the EV71 life cycle,3 C p r o functions and3 C p r o inhibitors recently screened. It permits to well understand all mechanisms about3 C p r o and consequently allow further development of drugs targeting3 C p r o . Thus, this review is helpful for screening of more new3 C p r o inhibitors or for designing analogues of well known3 C p r o inhibitors in order to improve its antiviral activity.

The Function and Mechanism of Enterovirus 71 (EV71) 3C Protease

Enterovirus 71 (EV71) is the main pathogen of the hand, foot, and mouth disease. It was firstly isolated from sputum specimens of infants with central nervous system diseases in California in 1969, and has been repeatedly reported in various parts of the world, especially in the Asia-Pacific region. EV71 3C protein is a 183 amino acid cysteine protease that can cleave most structural and non-structural proteins of EV71. Based on the analysis and understanding of EV71 3C protease, it is helpful to study and treat diseases caused by EV71 virus infection. The EV71 3C protease promotes virus replication by cleaving EV71 synthesis or host proteins. Moreover, EV71 3C protease inhibits the innate immune system and causes apoptosis. At present, in order to deal with the damage caused by the EV71, it is urgent to develop antiviral drugs targeting 3C protease. This review will focus on the structure, function, and mechanism of EV71 3C protease.

A simplified cell-based assay to identify coronavirus 3CL protease inhibitors

We describe a mammalian cell-based assay capable of identifying coronavirus 3CL protease (3CLpro) inhibitors without requiring the use of live virus. By enabling the facile testing of compounds across a range of coronavirus 3CLpro enzymes, including the one from SARS-CoV-2, we are able to quickly identify compounds with broad or narrow spectra of activity. We further demonstrate the utility of our approach by performing a curated compound screen along with structure-activity profiling of a series of small molecules to identify compounds with antiviral activity. Throughout these studies, we observed concordance between data emerging from this assay and from live virus assays. By democratizing the testing of 3CL inhibitors to enable screening in the majority of laboratories rather than the few with extensive biosafety infrastructure, we hope to expedite the search for coronavirus 3CL protease inhibitors, to address the current epidemic and future ones that will inevitably arise.

Regulation of Apoptosis by Enteroviruses

Enterovirus infection can cause a variety of diseases and severely impair the health of humans, animals, poultry, and other organisms. To resist viral infection, host organisms clear infected cells and viruses via apoptosis. However, throughout their long-term competition with host cells, enteroviruses have evolved a series of mechanisms to regulate the balance of apoptosis in order to replicate and proliferate. In the early stage of infection, enteroviruses mainly inhibit apoptosis by regulating the PI3K/Akt pathway and the autophagy pathway and by impairing cell sensors, thereby delaying viral replication. In the late stage of infection, enteroviruses mainly regulate apoptotic pathways and the host translation process via various viral proteins, ultimately inducing apoptosis. This paper discusses the means by which these two phenomena are balanced in enteroviruses to produce virus-favoring conditions – in a temporal sequence or through competition with each other. This information is important for further elucidation of the relevant mechanisms of acute infection by enteroviruses and other members of the picornavirus family.

Structural Biology of the Enterovirus Replication-Linked 5'-Cloverleaf RNA and Associated Virus Proteins

Although enteroviruses are associated with a wide variety of diseases and conditions, their mode of replication is well conserved. Their genome is carried as a single, positive-sense RNA strand. At the 5' end of the strand is an approximately 90-nucleotide self-complementary region called the 5' cloverleaf, or the oriL. This noncoding region serves as a platform upon which host and virus proteins, including the 3B, 3C, and 3D virus proteins, assemble in order to initiate replication of a negative-sense RNA strand. The negative strand in turn serves as a template for synthesis of multiple positive-sense RNA strands. Building on structural studies of individual RNA stem-loops, the structure of the intact 5' cloverleaf from rhinovirus has recently been determined via nuclear magnetic resonance/small-angle X-ray scattering (NMR/SAXS)-based methods, while structures have also been determined for enterovirus 3A, 3B, 3C, and 3D proteins. Analysis of these structures, together with structural and modeling studies of interactions between host and virus proteins and RNA, has begun to provide insight into the enterovirus replication mechanism and the potential to inhibit replication by blocking these interactions.

A nonstructural 2B protein of enterovirus A71 increases cytosolic Ca2+ and induces apoptosis in human neuroblastoma SH-SY5Y cells

Enterovirus A71 (EV-A71) is one of the causative agents causing the hand-foot-mouth disease which associated with fatal neurological complications. Several sporadic outbreaks of EV-A71 infections have been recently reported from Asia-Pacific regions and potentially established endemicity in the area. Currently, there is no effective vaccine or antiviral drug for EV-A71 available. This may be attributable to the limited information about its pathogenesis. In this study, the recombinant nonstructural 2B protein of EV-A71 was successfully produced in human neuroblastoma SH-SY5Y cells and evaluated for its effects on induction of the cell apoptosis and the pathway involved. The EV-A71 2B-transfected SH-SY5Y cells showed significantly higher difference in the cell growth inhibition than the mock and the irrelevant protein controls. The transfected SH-SY5Y cells underwent apoptosis and showed the significant upregulation of caspase-9 (CASP9) and caspase-12 (CASP12) genes at 3- and 24-h post-transfection, respectively. Interestingly, the level of cytosolic Ca²⁺ was significantly elevated in the transfected SH-SY5Y cells at 6- and 12-h post-transfection. The caspase-9 is activated by mitochondrial signaling pathway while the caspase-12 is activated by ER signaling pathway. The results suggested that EV-A71 2B protein triggered transient increase of the cytosolic Ca²⁺ level and associated with ER-mitochondrial interactions that drive the caspase-dependent apoptosis pathways. The detailed mechanisms warrant further studies for understanding the implication of EV-A71 infection in neuropathogenesis. The gained knowledge is essential for the development of the effective therapeutics and antiviral drugs.

EV71 3C protease induces apoptosis by cleavage of hnRNP A1 to promote apaf-1 translation

Enterovirus 71 (EV71) induces apoptosis to promote viral particle release. Earlier work showed that EV71 utilizes its 3C protease to induce apoptosis in a caspase-3-dependent pathway, though the mechanism is unknown. However, work from Vagner, Holcik and colleagues showed that host protein heterogeneous ribonucleoprotein A1 (hnRNP A1) binds the IRES of cellular apoptotic peptidase activating factor 1 (apaf-1) mRNA to repress its translation. In this work, we show that apaf-1 expression is essential for EV71-induced apoptosis. EV71 infection or ectopic expression of 3C protease cleaves hnRNP A1, which abolishes its binding to the apaf-1 IRES. This allows IRES-dependent synthesis of apaf-1, activation of caspase-3, and apoptosis. Thus, we reveal a novel mechanism that EV71 utilizes for virus release via a 3C protease-hnRNP A1-apaf-1-caspase-3-apoptosis axis.

Enterovirus 71 VP1 promotes mouse Schwann cell autophagy via ER stress‑mediated PMP22 upregulation

Enterovirus 71 (EV71) accounts for the majority of hand, foot and mouth disease‑related deaths due to fatal neurological complications. EV71 structural viral protein 1 (VP1) promotes viral replication by inducing autophagy in neuron cells, but the effect of VP1 on myelin cells is unclear. The present study aimed to investigate the role and mechanism of VP1 in autophagy of mouse Schwann cells. An EV71 VP1‑expressing vector (pEGFP‑C3‑VP1) was generated and transfected into mouse Schwann cells. Transmission electron microscopy and western blot analysis for microtubule‑associated protein 1 light chain 3 α (LC3) II (an autophagy marker) were used to assess autophagy. Reverse transcription‑quantitative PCR and immunofluorescence were performed to determine the expression of peripheral myelin protein 22 (PMP22). Small interfering RNA against PMP22 was used to investigate the role of PMP22 in mouse Schwann cell autophagy. Salubrinal [a selective endoplasmic reticulum (ER) stress inhibitor] was used to determine whether PMP22 expression was affected by ER stress. The present results indicated that VP1 promoted mouse Schwann cell autophagy. Overexpression of VP1 upregulated PMP22. PMP22 deficiency downregulated LC3II and thus inhibited autophagy. Furthermore, PMP22 expression was significantly suppressed by salubrinal. In conclusion, VP1 promoted mouse Schwann cell autophagy through upregulation of ER stress‑mediated PMP22 expression. Therefore, the VP1/ER stress/PMP22 autophagy axis may be a potential therapeutic target for EV71 infection‑induced fatal neuronal damage.

The Saffold Virus-Penang 2B and 3C Proteins, but not the L Protein, Induce Apoptosis in HEp-2 and Vero Cells

Our previous work has shown that Saffold virus (SAFV) induced several rodent and primate cell lines to undergo apoptosis (Xu et al. in Emerg Microb Infect 3:1-8, 2014), but the essential viral proteins of SAFV involved in apoptotic activity lack study. In this study, we individually transfected the viral proteins of SAFV into HEp-2 and Vero cells to assess their ability to induce apoptosis, and found that the 2B and 3C proteins are proapoptotic. Further investigation indicated the transmembrane domain of the 2B protein is essential for the apoptotic activity and tetramer formation of the 2B protein. Our research provides clues for the possible mechanisms of apoptosis induced by SAFV in different cell lines. It also opens up new directions to study viral proteins (the 2B, 3C protein), and sets the stage for future exploration of any possible link between SAFV, inclusive of its related uncultivable genotypes, and multiple sclerosis.

Senecavirus A 3C Protease Mediates Host Cell Apoptosis Late in Infection

Senecavirus A (SVA), an oncolytic picornavirus used for cancer treatment in humans, has recently emerged as a vesicular disease (VD)-causing agent in swine worldwide. Notably, SVA-induced VD is indistinguishable from foot-and-mouth disease (FMD) and other high-consequence VDs of pigs. Here we investigated the role of apoptosis on infection and replication of SVA. Given the critical role of the nuclear factor-kappa B (NF-κB) signaling pathway on modulation of cell death, we first assessed activation of NF-κB during SVA infection. Results here show that while early during infection SVA induces activation of NF-κB, as evidenced by nuclear translocation of NF-κB-p65 and NF-κB-mediated transcription, late in infection a cleaved product corresponding to the C-terminus of NF-κB-p65 is detected in infected cells, resulting in lower NF-κB transcriptional activity. Additionally, we assessed the potential role of SVA 3C protease (3Cpro) in SVA-induced host-cell apoptosis and cleavage of NF-κB-p65. Transient expression of SVA 3Cpro was associated with cleavage of NF-κB-p65 and Poly (ADP-ribose) polymerase (PARP), suggesting its involvement in virus-induced apoptosis. Most importantly, we showed that while cleavage of NF-κB-p65 is secondary to caspase activation, the proteolytic activity of SVA 3Cpro is essential for induction of apoptosis. Experiments using the pan-caspase inhibitor Z-VAD-FMK confirmed the relevance of late apoptosis for SVA infection, indicating that SVA induces apoptosis, presumably, as a mechanism to facilitate virus release and/or spread from infected cells. Together, these results suggest an important role of apoptosis for SVA infection biology.

Rabbit Hemorrhagic Disease Virus Non-structural Protein 6 Induces Apoptosis in Rabbit Kidney Cells

Rabbit hemorrhagic disease (RHD) is a highly contagious disease caused by rabbit hemorrhagic disease virus (RHDV). Previous research has shown that RHDV induces apoptosis in numerous cell types, although the molecular mechanisms underlying the apoptosis induced by RHDV are not well understood. One possible factor is non-structural protein 6 (NSP6), a 3C-like protease that plays an important role in processing viral polyprotein precursors into mature non-structural proteins. To fully establish a role for NSP6, the present study examined the effects of ectopic expression of the protein in rabbit (RK13) and human (HeLa and HepG2) cells. We found that NSP6 suppressed cell viability and promoted apoptosis in all three cell types in a dose-dependent manner. We also identified increased caspase-3, -8, and -9 activities in RK13 cell, and an increased Bax to Bcl2 mRNA ratio. Mechanistically, the ability of NSP6 to induce apoptosis was impaired by mutation of the catalytic His27 residue. Our study has shown that RHDV NSP6 can induce apoptosis in host cells and is likely an important contributor to RHDV-induced apoptosis and pathogenesis.

An update on enterovirus 71 infection and interferon type I response

Enteroviruses are members of Pichornaviridae family consisting of human enterovirus group A, B, C, and D as well as nonhuman enteroviruses. Hand, foot, and mouth disease (HFMD) is a serious disease which is usually seen in the Asia-Pacific region in children. Enterovirus 71 and coxsackievirus A16 are two important viruses responsible for HFMD which are members of group A enterovirus. IFN α and β are two cytokines, which have a major activity in the innate immune system against viral infections. Most of the viruses have some weapons against these cytokines. EV71 has two main proteases called 2A and 3C, which are important for polyprotein processing and virus maturation. Several studies have indicated that they have a significant effect on different cellular pathways such as interferon production and signaling pathway. The aim of this study was to investigate the latest findings about the interaction of 2A and 3C protease of EV71 and IFN production/signaling pathway and their inhibitory effects on this pathway.

Recent Progress on Functional Genomics Research of Enterovirus 71

Enterovirus 71 (EV71) is one of the main pathogens that causes hand-foot-and-mouth disease (HFMD). HFMD caused by EV71 infection is mostly self-limited; however, some infections can cause severe neurological diseases, such as aseptic meningitis, brain stem encephalitis, and even death. There are still no effective clinical drugs used for the prevention and treatment of HFMD. Studying EV71 protein function is essential for elucidating the EV71 replication process and developing anti-EV71 drugs and vaccines. In this review, we summarized the recent progress in the studies of EV71 non-coding regions (5' UTR and 3' UTR) and all structural and nonstructural proteins, especially the key motifs involving in viral infection, replication, and immune regulation. This review will promote our understanding of EV71 virus replication and pathogenesis, and will facilitate the development of novel drugs or vaccines to treat EV71.

Toll-Like Receptor 3 Is Involved in Detection of Enterovirus A71 Infection and Targeted by Viral 2A Protease

Enterovirus A71 (EV-A71) has emerged as a major pathogen causing hand, foot, and mouth disease, as well as neurological disorders. The host immune response affects the outcomes of EV-A71 infection, leading to either resolution or disease progression. However, the mechanisms of how the mammalian innate immune system detects EV-A71 infection to elicit antiviral immunity remain elusive. Here, we report that the Toll-like receptor 3 (TLR3) is a key viral RNA sensor for sensing EV-A71 infection to trigger antiviral immunity. Expression of TLR3 in HEK293 cells enabled the cells to sense EV-A71 infection, leading to type I, IFN-mediated antiviral immunity. Viral double-stranded RNA derived from EV-A71 infection was a key ligand for TLR3 detection. Silencing of TLR3 in mouse and human primary immune cells impaired the activation of IFN-β upon EV-A71 infection, thus reinforcing the importance of the TLR3 pathway in defending against EV-A71 infection. Our results further demonstrated that TLR3 was a target of EV-A71 infection. EV-A71 protease 2A was implicated in the downregulation of TLR3. Together, our results not only demonstrate the importance of the TLR3 pathway in response to EV-A71 infection, but also reveal the involvement of EV-A71 protease 2A in subverting TLR3-mediated antiviral defenses.

The feline calicivirus leader of the capsid protein causes survivin and XIAP downregulation and apoptosis

Calicivirus infection causes intrinsic apoptosis, leading to viral propagation in the host. During murine norovirus infection, a reduction in the anti-apoptotic protein survivin has been documented. Here we report that in feline calicivirus infection, a downregulation of the anti-apoptotic proteins survivin and XIAP occur, which correlates with the translocation of the pro-apoptotic protein Smac/DIABLO from the mitochondria to the cytoplasm and the activation of caspase-3. Inhibition of survivin degradation by lactacystin treatment caused a delay in apoptosis progression, reducing virus release, without affecting virus production. However, the overexpression of survivin caused a negative effect in viral progeny production. Overexpression of the leader of the capsid protein (LC), but not of the protease-polymerase NS6/7, results in the downregulation of survivin and XIAP, caspase activation and mitochondrial damage. These results indicate that LC is responsible for the induction of apoptosis in transfected cells and most probably in FCV infection.

4-Iminooxazolidin-2-one as a Bioisostere of the Cyanohydrin Moiety: Inhibitors of Enterovirus 71 3C Protease

A recently reported potent inhibitor of enterovirus 71 3C protease, ( R)-1, was found to have stability and potential toxicity issues due to the presence of a cyanohydrin moiety. Modifying the labile cyanohydrin moiety, by serendipity, led to the discovery of 4-iminooxazolidin-2-one-based inhibitors 4e and 4g with potent inhibitory activity and significantly improved stability. In vivo pharmacokinetic studies of 4e also demonstrated high plasma exposure and moderate half-life. These compounds have shown potential of becoming anti-EV71 drug candidates.

Inhibition of enterovirus 71 replication and viral 3C protease by quercetin

BACKGROUND

Enterovirus 71 (EV71) is one of the major causative agents of hand, foot, and mouth disease (HFMD), which is sometimes associated with severe central nervous system disease in children. There is currently no specific medication for EV71 infection. Quercetin, one of the most widely distributed flavonoids in plants, has been demonstrated to inhibit various viral infections. However, investigation of the anti-EV71 mechanism has not been reported to date.

METHODS

The anti-EV71 activity of quercetin was evaluated by phenotype screening, determining the cytopathic effect (CPE) and EV71-induced cells apoptosis. The effects on EV71 replication were evaluated further by determining virus yield, viral RNA synthesis and protein expression, respectively. The mechanism of action against EV71 was determined from the effective stage and time-of-addition assays. The possible inhibitory functions of quercetin via viral 2Apro, 3Cpro or 3Dpol were tested. The interaction between EV71 3Cpro and quercetin was predicted and calculated by molecular docking.

RESULTS

Quercetin inhibited EV71-mediated cytopathogenic effects, reduced EV71 progeny yields, and prevented EV71-induced apoptosis with low cytotoxicity. Investigation of the underlying mechanism of action revealed that quercetin exhibited a preventive effect against EV71 infection and inhibited viral adsorption. Moreover, quercetin mediated its powerful therapeutic effects primarily by blocking the early post-attachment stage of viral infection. Further experiments demonstrated that quercetin potently inhibited the activity of the EV71 protease, 3Cpro, blocking viral replication, but not the activity of the protease, 2Apro, or the RNA polymerase, 3Dpol. Modeling of the molecular binding of the 3Cpro-quercetin complex revealed that quercetin was predicted to insert into the substrate-binding pocket of EV71 3Cpro, blocking substrate recognition and thereby inhibiting EV71 3Cpro activity.

CONCLUSIONS

Quercetin can effectively prevent EV71-induced cell injury with low toxicity to host cells. Quercetin may act in more than one way to deter viral infection, exhibiting some preventive and a powerful therapeutic effect against EV71. Further, quercetin potently inhibits EV71 3Cpro activity, thereby blocking EV71 replication.

Caspase-3 Inhibition Attenuates the Cytopathic Effects of EV71 Infection

Previous studies demonstrate that human enterovirus 71 (EV71), a primary causative agent for hand, foot, and mouth disease, activates caspase-3 through the non-structural viral 3C protein to induce host cell apoptosis; however, until now it was unclear how 3C activates caspase-3 and how caspase-3 activation affects viral production. Our results demonstrate that 3C binds caspase-8 and caspase-9 but does not directly bind caspase-3 to activate them, and that the proteolytic activity of 3C is required by the activation of caspase-8, caspase-9, and caspase-3. Inhibition of caspase-3 activity attenuates apoptosis in 3C-transfected cells. Furthermore, caspase-3 inhibitor protects host cells from the cytopathic effect of EV71 infection and prevents cell cycle arrest, which is known to be favored for EV71 viral replication. Inhibition of caspase-3 activity decreases EV71 viral protein expression and viral production, but has no effect on viral entry, replication, even polyprotein translation. Therefore, caspase-3 is exploited functionally by EV71 to facilitate its production, which suggests a novel therapeutic approach for the treatment and prevention of hand, foot, and mouth disease.

Enterovirus A71 Proteins: Structure and Function

Enterovirus A71 (EV-A71) infection has grown to become a serious threat to global public health. It is one of the major causes of hand, foot, and mouth disease (HFMD) in infants and young children. EV-A71 can also infect the central nervous system (CNS) and induce diverse neurological complications, such as brainstem encephalitis, aseptic meningitis, and acute flaccid paralysis, or even death. Viral proteins play a crucial role in EV-A71 infection. Many recent studies have discussed the structure and function of EV-A71 proteins, and the findings reported will definitely aid the development of vaccines and therapeutic approaches. This article reviews the progress in the research on the structure and function of EV-A71 proteins. Available literature can provide a basis for studying the pathogenesis of EV-A71 infection in detail.

A novel role of ER stress signal transducer ATF6 in regulating enterovirus A71 viral protein stability

Background

Due to limited coding capacity of viral genome, enterovirus A71 (EV-A71) co-opts host nuclear proteins for its replication. Upon ER stress, the ER-localized 90 kDa activating transcription factor 6 (p90ATF6) is proteolytically cleaved to produce the transcriptionally active amino-terminal 50 kDa (p50ATF6) product where it enters the nucleus to activate a subset of unfolded protein response and ER-associated degradation (also known as ERAD) genes. During EV-A71 infection, however, this p50ATF6 product was not detected in the nucleus, and its downstream target genes were not activated.

Methods

We examined the role of ATF6 during EV-A71 infection, including its cleavage process and its role in viral life cycle by silencing or overexpressing ATF6.

Results

We showed that a potential cleavage in the middle of p90ATF6 produced an amino-terminal ~ 45 kDa fragment in a viral protease-independent but EV-A71-dependent manner. The disappearance of ATF6 was not restricted to a specific strain of EV-A71 or cell type, and was not simply caused by picornavirus-mediated global translational shutoff. This cleavage of ATF6, which was most likely mediated by the host response, was nevertheless independent of both cellular caspases and XBP1-associated proteasomes. The silencing of ATF6 expression by small interfering RNA suppressed viral titers due to reduced viral protein stability. This effect was markedly restored by the ectopic expression of p90ATF6.

Conclusion

Our findings indicate that ATF6 plays a distinct role in viral protein stability and that the host uses different cleavage strategies, rather than conventional cleavage by generating p50ATF6, to combat viral infection.

Electronic supplementary material

The online version of this article (10.1186/s12929-018-0412-x) contains supplementary material, which is available to authorized users.

The 3C protease of enterovirus A71 counteracts the activity of host zinc-finger antiviral protein (ZAP)

Enterovirus A71 (EV-A71) is a positive-strand RNA virus that causes hand-foot-mouth disease and neurological complications in children and infants. Although the underlying mechanisms remain to be further defined, impaired immunity is thought to play an important role. The host zinc-finger antiviral protein (ZAP), an IFN-stimulated gene product, has been reported to specifically inhibit the replication of certain viruses. However, whether ZAP restricts the infection of enteroviruses remains unknown. Here, we report that EV-A71 infection upregulates ZAP mRNA in RD and HeLa cells. Moreover, ZAP overexpression rendered 293 T cells resistant to EV-A71 infection, whereas siRNA-mediated depletion of endogenous ZAP enhanced EV-A71 infection. The EV-A71 infection stimulated site-specific proteolysis of two ZAP isoforms, leading to the accumulation of a 40 kDa N-terminal ZAP fragment in virus-infected cells. We further revealed that the 3C protease (3Cpro) of EV-A71 mediates ZAP cleavage, which requires protease activity. Furthermore, ZAP variants with single amino acid substitutions at Gln-369 were resistant to 3Cpro cleavage, implying that Gln-369 is the sole cleavage site in ZAP. Moreover, although ZAP overexpression inhibited EV-A71 replication, the cleaved fragments did not show this effect. Our results indicate that an equilibrium between ZAP and enterovirus 3Cpro controls viral infection. The findings in this study suggest that viral 3Cpro mediated ZAP cleavage may represent a mechanism to escape host antiviral responses.

Neurotropism In Vitro and Mouse Models of Severe and Mild Infection with Clinical Strains of Enterovirus 71

Enterovirus 71 (EV71) is a common etiological agent of hand, foot, and mouth disease and fatal neurological diseases in children. The neuropathogenicity of severe EV71 infection has been documented, but studies comparing mouse models of severe and mild EV71 infection are lacking. The aim of the study was to investigate the neurovirulence of EV71 strains and the differences in serum cytokine and chemokine levels in mouse models of severe and mild EV71 infection. Nine EV71 isolates belonging to the C4 subgenogroup (proposed as genotype D) displayed infectivity in human neuroblastoma SK-N-SH cells; moreover, ultrastructural observation confirmed viral particle replication. The survival rate of the severe model was 71.43% (5/7), and 60% (3/5) of the surviving severe model mice displayed sequelae of paralysis, whereas the only symptom in mild model mice was ruffled fur. Dynamic detection of serum cytokine and chemokine levels demonstrated that interleukin (IL)-5, IL-13, IL-6, monocyte chemotactic protein 1 (MCP-1), and chemokine (C-C motif) ligand 5 (also called Regulated upon Activation, Normal T-cell Expressed, and Secreted (CCL5/RANTES) were significantly up-regulated at the early period of infection, indicating that these factors might herald a severe outcome. Our findings suggest that elevated cytokines and chemokines may have potential value as prognostic markers in mouse models.

Development of a luciferase-based biosensor to assess enterovirus 71 3C protease activity in living cells

Enterovirus 71 (EV71) is a major pathogen of hand, foot, and mouth disease (HFMD). To date, no antiviral drug has been approved to treat EV71 infection. Due to the essential role that EV71 3 C protease (3Cpro) plays in the viral life cycle, it is generally considered as a highly appealing target for antiviral drug development. In this study, we present a transgene-encoded biosensor that can accurately, sensitively and quantitatively report the proteolytic activity of EV71 3Cpro. This biosensor is based on the catalyzed activity of a pro-interleukin (IL)-1β-enterovirus 3Cpro cleavage site-Gaussia Luciferase (GLuc) fusion protein that we named i-3CS-GLuc. GLuc enzyme is inactive in the fusion protein because of aggregation caused by pro-IL-1β. However, the 3Cpro of EV71 and other enteroviruses, such as coxsackievirus A9 (CVA9), coxsackievirus B3 (CVB3), and poliovirus can recognize and process the canonical enterovirus 3Cpro cleavage site between pro-IL-1β and GLuc, thereby releasing and activating GLuc and resulting in increased luciferase activity. The high sensitivity, ease of use, and applicability as a transgene in cell-based assays of i-3CS-GLuc biosensor make it a powerful tool for studying viral protease proteolytic events in living cells and for achieving high-throughput screening of antiviral agents.

Enterovirus 71 Inhibits Pyroptosis through Cleavage of Gasdermin D

Enterovirus 71 (EV71) can cause hand-foot-and-mouth disease (HFMD) in young children. Severe infection with EV71 can lead to neurological complications and even death. However, the molecular basis of viral pathogenesis remains poorly understood. Here, we report that EV71 induces degradation of gasdermin D (GSDMD), an essential component of pyroptosis. Remarkably, the viral protease 3C directly targets GSDMD and induces its cleavage, which is dependent on the protease activity. Further analyses show that the Q193-G194 pair within GSDMD is the cleavage site of 3C. This cleavage produces a shorter N-terminal fragment spanning amino acids 1 to 193 (GSDMD_1-193). However, unlike the N-terminal fragment produced by caspase-1 cleavage, this fragment fails to trigger cell death or inhibit EV71 replication. Importantly, a T239D or F240D substitution abrogates the activity of GSDMD consisting of amino acids 1 to 275 (GSDMD_1-275). This is correlated with the lack of pyroptosis or inhibition of viral replication. These results reveal a previously unrecognized strategy for EV71 to evade the antiviral response.IMPORTANCE Recently, it has been reported that GSDMD plays a critical role in regulating lipopolysaccharide and NLRP3-mediated interleukin-1β (IL-1β) secretion. In this process, the N-terminal domain of p30 released from GSDMD acts as an effector in cell pyroptosis. We show that EV71 infection downregulates GSDMD. EV71 3C cleaves GSDMD at the Q193-G194 pair, resulting in a truncated N-terminal fragment disrupted for inducing cell pyroptosis. Notably, GSDMD_1-275 (p30) inhibits EV71 replication whereas GSDMD_1-193 does not. These results reveal a new strategy for EV71 to evade the antiviral response.

Disruption of MDA5-Mediated Innate Immune Responses by the 3C Proteins of Coxsackievirus A16, Coxsackievirus A6, and Enterovirus D68

Coxsackievirus A16 (CV-A16), CV-A6, and enterovirus D68 (EV-D68) belong to the Picornaviridae family and are major causes of hand, foot, and mouth disease (HFMD) and pediatric respiratory disease worldwide. The biological characteristics of these viruses, especially their interplay with the host innate immune system, have not been well investigated. In this study, we discovered that the 3Cpro proteins from CV-A16, CV-A6, and EV-D68 bind melanoma differentiation-associated gene 5 (MDA5) and inhibit its interaction with MAVS. Consequently, MDA5-triggered type I interferon (IFN) signaling in the retinoic acid-inducible gene I-like receptor (RLR) pathway was blocked by the CV-A16, CV-A6, and EV-D68 3Cpro proteins. Furthermore, the CV-A16, CV-A6, and EV-D68 3Cpro proteins all cleave transforming growth factor β-activated kinase 1 (TAK1), resulting in the inhibition of NF-κB activation, a host response also critical for Toll-like receptor (TLR)-mediated signaling. Thus, our data demonstrate that circulating HFMD-associated CV-A16 and CV-A6, as well as severe respiratory disease-associated EV-D68, have developed novel mechanisms to subvert host innate immune responses by targeting key factors in the RLR and TLR pathways. Blocking the ability of 3Cpro proteins from diverse enteroviruses and coxsackieviruses to interfere with type I IFN induction should restore IFN antiviral function, offering a potential novel antiviral strategy.IMPORTANCE CV-A16, CV-A6, and EV-D68 are emerging pathogens associated with hand, foot, and mouth disease and pediatric respiratory disease worldwide. The pathogenic mechanisms of these viruses are largely unknown. Here we demonstrate that the CV-A16, CV-A6, and EV-D68 3Cpro proteins block MDA5-triggered type I IFN induction. The 3Cpro proteins of these viruses bind MDA5 and inhibit its interaction with MAVS. In addition, the CV-A16, CV-A6, and EV-D68 3Cpro proteins cleave TAK1 to inhibit the NF-κB response. Thus, our data demonstrate that circulating HFMD-associated CV-A16 and CV-A6, as well as severe respiratory disease-associated EV-D68, have developed a mechanism to subvert host innate immune responses by simultaneously targeting key factors in the RLR and TLR pathways. These findings indicate the potential merit of targeting the CV-A16, CV-A6, and EV-D68 3Cpro proteins as an antiviral strategy.