Determination of the cleavage site of enterovirus 71 VP0 and the effect of this cleavage on viral infectivity and assembly

A single amino acid mutation in VP1 of coxsackievirus A6 determining efficiency of VP0 cleavage and proliferation

Coxsackievirus A6 (CV-A6) has emerged as a major pathogen associated with hand, foot, and mouth disease (HFMD), capable of infecting both children and adults. However, currently, there is no effective vaccine to prevent HFMD caused by non-EV-A71 enteroviruses. In this study, a pair of CV-A6 strains was selected from a rhabdomyosarcoma (RD)-isolated and Vero-adapted stock with a difference of 7 nucleotides in their genomes, resulting in three amino acid mutations in the structural proteins. Distinct differences in propagation, virulence in cells, and plaque size were observed. A series of single-site mutants was constructed, and a single mutation in VP1-143 was mapped to associate with phenotype changes. The mutation from glycine to arginine at VP1-143 dramatically increased infectivity but decreased virulence, growth rate, and plaque size. Furthermore, the experiments using both purified whole virus and full particle (FP) demonstrated that glycine-to-arginine mutation increased VP0 cleavage efficiency because of decreased VP0/VP2 ratio. The decrease in VP0 cleavage efficiency led to the accumulation of non-infectious provirion. The efficiency of virus transmission between cells determined the rates of viral RNA (vRNA) and protein synthesis and was related to fast-slow growth and virulence phenotypes. In addition, the data indicated that the mutation did not affect the encapsidation of the genomic RNA, and the ratio of empty and full particles was unchanged. The results are important for understanding the mechanism of VP0 cleavage regulation and are relevant to developing vaccines and therapeutic reagents against CV-A6 infection and diseases.

IMPORTANCE

CV-A6 is a major pathogen in the context of HFMD. The cost of treatment and hospitalization of children with HFMD may have a considerable financial impact on the families of patients. CV-A6 is a member of picornaviruses and forms infectious virion through maturation cleavage of VP0 into VP4 and VP2. Although it is well accepted that the autocatalytic process involves viral RNA, the detailed mechanism remains unclear. In this study, residues in VP1-143 were demonstrated to regulate the efficiency of VP0 cleavage and affect the ratio of provirion and virion. Glycine-to-arginine mutation was tolerant, not abolished, but affected the efficiency of VP0 cleavage. The results support a theory that residue mutations on a structural protein of a serotype/genotype within enteroviruses, not well-conserved across picornaviruses and far away from the VP0 cleavage site on the outside surface, regulate the efficiency of VP0 cleavage and render phenotypically different strains.

Insight into the Life Cycle of Enterovirus-A71

Human enterovirus 71 (EV-A71), a member of the Picornaviridae family, is predominantly associated with hand, foot, and mouth disease in infants and young children. Additionally, EV-A71 can cause severe neurological complications, including aseptic meningitis, brainstem encephalitis, and fatalities. The molecular mechanisms underlying these symptoms are complex and involve the viral tissue tropism, evasion from the host immune responses, induction of the programmed cell death, and cytokine storms. This review article delves into the EV-A71 life cycle, with a particular emphasis on recent advancements in understanding the virion structure, tissue tropism, and the interplay between the virus and host regulatory networks during replication. The comprehensive review is expected to contribute to our understanding of EV-A71 pathogenesis and inform the development of antiviral therapies and vaccines.

Efficient Production of Enterovirus 71 (EV71) Virus-like Particles by Controlling Promoter Strength in Insect Cells

This study was conducted to efficiently produce virus-like particles (VLPs) of enterovirus 71 (EV71), a causative virus of hand, foot, and mouth disease (HFMD). The expression level of the P1 precursor, a structural protein of EV71, was modified to increase VLP production, and the optimal expression level and duration of the 3CD protein for P1 cleavage were determined. The expression level and duration of 3CD were controlled by the p10 promoter, which was weakened by repeated burst sequence (BS) applications, as well as the OpIE2 promoter, which was weakened by the insertion of random untranslated region sequences of various lengths. The cleavage and production efficiency of the P1 precursor were compared based on the expression time and level of 3CD, revealing that the p10-BS5 promoter with four repeated BSs was the most effective. When P1 and 3CD were expressed using the hyperexpression vector and the p10-BS5 promoter, high levels of structural protein production and normal HFMD-VLP formation were observed, respectively. This study suggests that the production efficiency of HFMD-VLPs can be significantly enhanced by increasing the expression of the P1 precursor and controlling the amount and duration of 3CD expression.

Quantitation of Enterovirus A71 Empty and Full Particles by Sedimentation Velocity Analytical Ultracentrifugation

The enterovirus A71 (EV71) inactivated vaccine is an effective intervention to control the spread of the virus and prevent EV71-associated hand, foot, and mouth disease (HFMD). It is widely administered to infants and children in China. The empty particles (EPs) and full particles (FPs) generated during production have different antigenic and immunogenic properties. However, the antigen detection methods currently used were established without considering the differences in antigenicity between EPs and FPs. There is also a lack of other effective analytical methods for detecting the different particle forms, which hinders the consistency between batches of products. In this study, we analyzed the application of sedimentation velocity analytical ultracentrifugation (SV-AUC) in characterizing the EPs and FPs of EV71. Our results showed that the proportions of the two forms could be quantified simultaneously by SV-AUC. We also determined the repeatability and accuracy of this method and found that both parameters were satisfactory. We assessed SV-AUC for bulk vaccine quality control, and our findings indicated that SV-AUC can be used effectively to analyze the percentage of EPs and FPs and monitor the consistency of the process to ensure the quality of the vaccine.

Analysis of full and empty ratio of EV71 virus by using capillary zone electrophoresis

Hand, foot, and mouth disease is a serious public health problem, and the main pathogen is enterovirus 71 (EV71). Its capsid assembly mechanism including capsid protein processing has been widely studied. Full and empty capsids have different immunological efficacy. Therefore, tracking full/empty capsid ratio throughout the EV71 production process is important to ensure consistent product quality and proper dosing response. The analysis of full/empty capsid ratio of intact virus has been widely reported as well. A variety of techniques have been employed to evaluate the full/empty capsid ratios. However, there has not been a rapid, reproducible, and robust assay to determine the full/empty capsid ratios of final and in-process products. In this study, a novel assay based on capillary zone electrophoresis was established. The separation of full and empty species could be achieved within 10 min and the ratio of peak areas was used to calculate the full/empty capsid ratio directly. The results showed good reproducibility and linearity for the determination of full/empty capsid ratios.

Update on enteroviral protease 2A: Structure, function, and host factor interaction

Enteroviruses (EVs) are human pathogens commonly observed in children aged 0-5 years and adults. EV infections usually cause the common cold and hand-foot-and-mouth disease; however, more severe infections can result in multiorgan complications, such as polio, aseptic meningitis, and myocarditis. The molecular mechanisms by which enteroviruses cause these diseases are still poorly understood, but accumulating evidence points to two enterovirus proteases, 2Apro and 3Cpro, as the key players in pathogenesis. The 2Apro performs post-translational proteolytic processing of viral polyproteins and cleaves several host factors to evade antiviral immune responses and promote viral replication. It was also discovered that coxsackievirus-induced cardiomyopathy was caused by 2Apro-mediated cleavage of dystrophin in cardiomyocytes, indicating that cellular protein proteolysis may play a key role in enterovirus-associated diseases. Therefore, studies of 2Apro could reveal additional substrates that may be associated with specific diseases. Here, we discuss the genetic and structural properties of 2Apro and review how the protease antagonizes innate immune responses to promote viral replication, as well as novel substrates and mechanisms for 2Apro. We also summarize the current approaches for identifying the substrates of 2Apro to discover novel mechanisms relating to certain diseases.

Vemurafenib Inhibits Enterovirus A71 Genome Replication and Virus Assembly

Enterovirus A71 (EV-A71) infection is a major cause of hand, foot, and mouth disease (HFMD), which may be occasionally associated with severe neurological complications. There is currently a lack of treatment options for EV-A71 infection. The Raf-MEK-ERK signaling pathway, in addition to its critical importance in the regulation of cell growth, differentiation, and survival, has been shown to be essential for virus replication. In this study, we investigated the anti-EV-A71 activity of vemurafenib, a clinically approved B-Raf inhibitor used in the treatment of late-stage melanoma. Vemurafenib exhibits potent anti-EV-A71 effect in cytopathic effect inhibition and viral load reduction assays, with half maximal effective concentration (EC50) at nanomolar concentrations. Mechanistically, vemurafenib interrupts both EV-A71 genome replication and assembly. These findings expand the list of potential antiviral candidates of anti-EV-A71 therapeutics.

Host neuronal PRSS3 interacts with enterovirus A71 3A protein and its role in viral replication

Enterovirus A71 (EV-A71) causes hand, foot, and mouth disease associated with neurological complications in young children. Currently, there is no specific treatment for EV-A71 infection due to the inadequate information on viral biology and neuropathogenesis. Among enteroviruses, nonstructural 3A protein mediates the formation of replication organelles which plays a major role in viral RNA synthesis and assembly. Although enteroviral 3A proteins have been intensively studied, the data on EV-A71 3A, especially in neuronal cells, are still limited. In this study, PRSS3 (mesotrypsinogen, also known as brain trypsinogen) was identified as EV-A71 3A-interacting counterpart from the transfected human neuroblastoma SH-SY5Y cells by pull-down assay and liquid chromatography tandem mass spectrometry. It was confirmed that PRSS3 variant 3 derived from human SH-SY5Y cells had the physical interaction with EV-A71 3A. Importantly, the role of PRSS3 in EV-A71 replication was verified by overexpression and siRNA-mediated gene silencing approaches. The detailed mechanism of the PRSS3 involved in EV-A71 replication and neuropathogenesis warrants further experimental elucidation. In conclusion, this study has discovered a novel EV-A71 3A interacting protein that offers the opportunity to study the neuropathogenesis of the infection which paves the way for developing a specific and effective treatment for the disease.

Human Antibodies to VP4 Inhibit Replication of Enteroviruses Across Subgenotypes and Serotypes, and Enhance Host Innate Immunity

Hand, foot, and mouth disease (HFMD) is a highly contagious disease that usually affects infants and young children (<5 years). HFMD outbreaks occur frequently in the Asia-Pacific region, and these outbreaks are associated with enormous healthcare and socioeconomic burden. There is currently no specific antiviral agent to treat HFMD and/or the severe complications that are frequently associated with the enterovirus of serotype EV71. Therefore, the development of a broadly effective and safe anti-enterovirus agent is an existential necessity. In this study, human single-chain antibodies (HuscFvs) specific to the EV71-internal capsid protein (VP4) were generated using phage display technology. VP4 specific-HuscFvs were linked to cell penetrating peptides to make them cell penetrable HuscFvs (transbodies), and readily accessible to the intracellular target. The transbodies, as well as the original HuscFvs that were tested, entered the enterovirus-infected cells, bound to intracellular VP4, and inhibited replication of EV71 across subgenotypes A, B, and C, and coxsackieviruses CVA16 and CVA6. The antibodies also enhanced the antiviral response of the virus-infected cells. Computerized simulation, indirect and competitive ELISAs, and experiments on cells infected with EV71 particles to which the VP4 and VP1-N-terminus were surface-exposed (i.e., A-particles that don't require receptor binding for infection) indicated that the VP4 specific-antibodies inhibit virus replication by interfering with the VP4-N-terminus, which is important for membrane pore formation and virus genome release leading to less production of virus proteins, less infectious virions, and restoration of host innate immunity. The antibodies may inhibit polyprotein/intermediate protein processing and cause sterically strained configurations of the capsid pentamers, which impairs virus morphogenesis. These antibodies should be further investigated for application as a safe and broadly effective HFMD therapy.

Myristoylation of EV71 VP4 is Essential for Infectivity and Interaction with Membrane Structure

The Enterovirus 71 (EV71) VP4 is co-translationally linked to myristic acid at its amino-terminal glycine residue. However, the role of this myristoylation in the EV71 life cycle remains largely unknown. To investigate this issue, we developed a myristoylation-deficient virus and reporter (luciferase) pseudovirus with a Gly-to-Ala mutation (G2A) on EV71 VP4. When transfecting the EV71-G2A genome encoding plasmid in cells, the loss of myristoylation on VP4 did not affect the expression of viral proteins and the virus morphology, however, it did significantly influence viral infectivity. Further, in myristoylation-deficient reporter pseudovirus-infected cells, the luciferase activity and viral genome RNA decreased significantly as compared to that of wild type virus; however, cytopathic effect and viral capsid proteins were not detected in myristoylation-deficient virus-infected cells. Also, although myristoylation-deficient viral RNA and proteins were detected in the second blind passage of infection, they were much fewer in number compared to that of the wild type virus. The replication of genomic RNA and negative-strand viral RNA were both blocked in myristoylation-deficient viruses, suggesting that myristoylation affects viral genome RNA release from capsid to cytoplasm. Besides, loss of myristoylation on VP4 altered the distribution of VP4-green fluorescent protein protein, which disappeared from the membrane structure fraction. Finally, a liposome leakage assay showed that EV71 myristoylation mediates the permeability of the model membrane. Hence, the amino-terminal myristoylation of VP4 is pivotal to EV71 infection and capsid-membrane structure interaction. This study provides novel molecular mechanisms regarding EV71 infection and potential molecular targets for antiviral drug design.

Sequence analysis of the first B5 subgenogroup strain of enterovirus 71 isolated in Korea

Enterovirus A71 (EV71), the main etiological agent of handfoot- mouth disease (HFMD), circulates in many areas of the world and has caused large epidemics since 1997, especially in the Asia-Pacific region. In this study, we determined the full-genome sequence of CMC718, a newly isolated EV71 strain in Korea. The CMC718 genome was 7,415 nucleotides in length and was confirmed by whole-genome phylogenetic analysis to belong to the B5 genotype. In particular, CMC718 demonstrated maximum identity with strain M988 of the B5 genotype and numerous amino acid variants were detected in the 3D domain of the viral protein P3, which is consistent with the mutation pattern of a B5 strain isolated in 2012–2013. Comparison of the CMC718 sequence with other EV71 reference strains confirmed the relationship and genetic variation of CMC718. Our study was a full-genome sequence analysis of the first EV71 strain of the B5 genotype isolated in South Korea. This information will be a valuable reference for the development of methods for the detection of recombinant viruses, the tracking of infections, and the diagnosis of EV71.

Multifunctionality of structural proteins in the enterovirus life cycle

Members of the genus Enterovirus have a significant effect on human health, especially in infants and children. Since the viral genome has limited coding capacity, Enteroviruses subvert a range of cellular processes for viral infection via the interaction of viral proteins and numerous cellular factors. Intriguingly, the capsid-receptor interaction plays a crucial role in viral entry and has significant implications in viral pathogenesis. Moreover, interactions between structural proteins and host factors occur directly or indirectly in multiple steps of viral replication. In this review, we focus on the current understanding of the multifunctionality of structural proteins in the viral life cycle, which may constitute valuable targets for antiviral and therapeutic interventions.