Recent advances from studies on the role of structural proteins in enterovirus infection

Molecular Detection and Genetic Characterization of Vertically Transmitted Viruses in Ducks

To investigate the distribution and genetic variation in four vertically transmitted duck pathogens, including duck hepatitis B virus (DHBV), duck circovirus (DuCV), duck hepatitis A virus 3 (DHAV-3), and avian reoviruses (ARV), we conducted an epidemiology study using PCR and RT-PCR assays on a duck population. We found that DHBV was the most prevalent virus (69.74%), followed by DuCV (39.48%), and then ARV (19.92%) and DHAV-3 (8.49%). Among the 271 duck samples, two, three or four viruses were detected in the same samples, indicating that the coinfection of vertical transmission agents is common in ducks. The genetic analysis results showed that all four identified DuCV strains belonged to genotype 1, the DHAV-3 strain was closely clustered with previously identified strains from China, and the ARV stain was clustered under genotype 1. These indicate that different viral strains are circulating among the ducks. Our findings will improve the knowledge of the evolution of DuCV, DHAV-3, and ARV, and help choose suitable strains for vaccination.

The DHAV-1 protein VP1 interacts with PI3KC3 to induce autophagy through the PI3KC3 complex

Duck hepatitis A virus type 1 (DHAV-1) is one of the main pathogens responsible for death in ducklings. Autophagy is a catabolic process that maintains cellular homeostasis, and the PI3KC3 protein plays an important role in the initiation of autophagy. DHAV-1 infection induces autophagy in duck embryo fibroblasts (DEFs) but the molecular mechanism between it and autophagy has not been reported. First, we determined that DHAV-1 infection induces autophagy in DEFs and that autophagy induction is dependent on the integrity of viral proteins by infecting DEFs with UV-inactivated or heat-inactivated DHAV-1. Then, in experiments using the pharmacological autophagy inducer rapamycin and the autophagy inhibitor chloroquine, autophagy inhibition was shown to reduce intracellular and extracellular DHAV-1 genome copies and viral titres. These results suggest that autophagy activated by DHAV-1 infection in DEFs affects DHAV-1 proliferation and extracellular release. Next, we screened the autophagy-inducing effects of the DHAV-1 structural proteins VP0, VP3, and VP1 and found that all DHAV-1 structural proteins could induce autophagy in DEFs but not the full autophagic flux. Finally, we found that VP1 promotes protein expression of PI3KC3 and Beclin1 by western blot experiments and that VP1 interacts with PI3KC3 by co-immunoprecipitation experiments; moreover, 3-MA-induced knockdown of PI3KC3 inhibited VP1 protein-induced autophagy in DEFs. In conclusion, the DHAV-1 structural protein VP1 regulates the PI3KC3 complex by interacting with PI3KC3 to induce autophagy in DEFs.

First Report of Duck Hepatitis A Virus 3 from Duckling Flocks of Egypt

Duck hepatitis A viruses (DHAV-1, DHAV-2, and DHAV-3) are the predominant causes of duck virus hepatitis (DVH), a disease of ducklings that leads to massive morbidities, mortalities, and economic losses. As a duck-producing country, Egypt suffered lately from several attacks of DVH, despite the regular vaccination of birds. Between Spring 2016 and Summer 2018, 54 duckling flocks in the Sharkia province of Egypt were tested using the reverse-transcription PCR (RT-PCR) based on the DHAV-3D targeting primers. Of them, 27.8% (15/54) were positive. Upon retesting of positive samples using RT-PCR and duck hepatitis A virus (DHAV)-3 VP1-based primers, 33.3% (5/15) contained DHAV-3 RNA. For further analysis at the molecular level, the VP1 and the 3D genes were sequenced using the same primer sets used earlier. The phylogenetic trees confirmed that study sequences belonged to DHAV-3. However, they were displayed as a separate cluster following a geographically dependent distribution. They were also completely unrelated to the Egyptian DHAV-1-based vaccine. This was further confirmed by low nucleotide and amino acid identities in relation to this vaccine. In addition, the VP1 and 3D genes had the same phylogenetic topography. The study VP1 sequences had three unique amino acid substitutions (L59, V208 only in one strain, and C219). As far as we know, this is the first report on DHAV-3 outside Asia, particularly in Egypt. Accordingly, the vaccination strategy against DHAV should be quickly updated to avoid further dissemination of the virus. The epidemiology, pathogenicity, and evolution of DHAV-3 should be carefully monitored in Egypt.

A viroporin-like 2B protein of duck hepatitis A virus 1 that induces incomplete autophagy in DEF cells

Duck hepatitis A virus 1 (DHAV-1) can cause high morbidity and fatal acute infectious hepatitis in ducklings, which seriously endangers animal husbandry. Viroporin is a small molecular weight hydrophobic transmembrane protein encoded by the virus, that has been suggested to induce autophagy in host cells by increasing the membrane permeability through disturbing the ion balance. In this study, we aimed to investigate whether the DHAV-1 2B protein can induce autophagy in DEF cells with a viroporin-like function. Bioinformatics analysis has indicated that the 2B protein is characterized by a viroporin domain, which is consistent with the type IA viroporin transmembrane protein. We experimentally confirmed that the 2B protein disturbed the Ca2+ balance of infected cells by elevating the intracellular Ca2+ concentration. Eukaryotic expression of the 2B protein upregulates the expression of microtubule-associated protein 1 light chain 3 II (LC3-II) and the number of autophagosomes in the cell. Interestingly, the Western Blot (WB) results showed that 2B protein expression induced less protein degradation of the autophagic substrate sequestosome 1 (SQSTM1/p62) than the positive control, while microscopy observations showed that the autophagosomes did not colocalize with the lysosomes. In summary, 2B protein expression induced autophagy in host cells, but the autophagic flow was incomplete. The results of this experiment are expected to provide reference scientific data for elucidating the infective and pathogenic mechanism of DHAV-1.

Enterovirus Replication Organelles and Inhibitors of Their Formation

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

Bioorthogonal Metabolic Labeling Utilizing Protein Biosynthesis for Dynamic Visualization of Nonenveloped Enterovirus 71 Infection

Bioorthogonal metabolic labeling through the endogenous cellular metabolic pathways (e.g., phospholipid and sugar) is a promising approach for effectively labeling live viruses. However, it remains a big challenge to label nonenveloped viruses due to lack of host-derived envelopes. Herein, a novel bioorthogonal labeling strategy is developed utilizing protein synthesis pathway to label and trace nonenveloped viruses. The results show that l-azidohomoalanine (Aha), an azido derivative of methionine, is more effective than azido sugars to introduce azido motifs into viral capsid proteins by substituting methionine residues during viral protein biosynthesis and assembly. The azide-modified EV71 (N₃-EV71) particles are then effectively labeled with dibenzocyclooctyl (DBCO)-functionalized fluorescence probes through an in situ bioorthogonal reaction with well-preserved viral infectivity. Dual-labeled imaging clearly clarifies that EV71 virions primarily bind to scavenger receptors and are internalized through clathrin-mediated endocytosis. The viral particles are then transported into early and late endosomes where viral RNA is released in a low-pH dependent manner at about 70 min postinfection. These results first reveal viral trafficking and uncoating mechanisms, which may shed light on the pathogenesis of EV71 infection and contribute to antiviral drug discovery.

The VP3 protein of duck hepatitis A virus mediates host cell adsorption and apoptosis

Duck hepatitis A virus (DHAV) causes an infectious disease that mainly affects 1- to 4-week-old ducklings, resulting in considerable loss to the duck industry. Although there have been many studies on DHAV in recent years, the effects on host infection and pathogenesis of DHAV-1 remain largely unknown. This study investigated the effects of the DHAV-1 structural protein VP3 on DHAV-1 virus adsorption and apoptosis to explore the role of VP3 in the viral life cycle. The effects of DHAV-1 VP3 and an antibody against the protein on virion adsorption was analyzed by qRT-PCR. The results showed that the virus copy number for the rabbit anti-VP3 IgG-treated group was significantly lower than that for the negative control group but higher than that for the rabbit anti-DHAV-1 IgG-treated group. This result indicates that VP3 mediates DHAV-1 virus adsorption but that it is not the only protein that involved in this process. In addition, a eukaryotic recombinant plasmid, pCAGGS/VP3, was transfected into duck embryo fibroblasts (DEFs), and the apoptotic rate was determined by DAPI staining, the TUNEL assay and flow cytometry. DAPI staining showed nucleus fragmentation and nuclear edge shifting. TUNEL assay results revealed yellow nuclei, and flow cytometry indicated a significant increase in the apoptotic rate. In addition, qRT-PCR revealed increased in the transcriptional levels of the apoptotic caspase-3, −8 and −9, with the largest increase for caspase-3, followed by caspase-9 and caspase-8. Enzyme activity analysis confirmed these results. Furthermore, the VP3 protein decreased the mitochondrial membrane potential, and the transcriptional levels of the proapoptotic factors Bak, Cyt c and Apaf-1 in the mitochondrial apoptotic pathway were significantly upregulated. These data suggest that expression of VP3 in DEFs induces apoptosis and may primarily activate caspase-3-induced apoptosis through mitochondrion-mediated intrinsic pathways. The findings provide scientific data to clarify DHAV-1 infection and pathogenesis.

Mutations in VP0 and 2C Proteins of Duck Hepatitis A Virus Type 3 Attenuate Viral Infection and Virulence

Duck hepatitis A virus (DHAV) is prevalent worldwide and has caused significant economic losses. As the predominant serotype in China, DHAV-3 has become a major challenge to the local duck industry. Here the genetics and pathogenesis of a virulent DHAV-3 strain and its embryo-passaged strain were assessed. There were only two amino acid substitutions (Y164N in VP0 protein and L71I in 2C protein) introduced during the adaptation process. The pathogenicity of these strains was further evaluated in vivo. Clinical signs, gross pathology, and histopathological analysis showed that the embryo-passaged strain was attenuated. Meanwhile, the viral RNA loads were significantly lower in the liver tissues of the ducklings infected with the attenuated strain. As expected, infection with the virulent and attenuated strains led to the activation of different innate immune genes. We suspected that the loss of replication efficiency in ducklings was responsible for the attenuation phenotype of the embryo-passaged strain. In addition, different innate immune responses in the liver of ducklings were at least partly responsible for the differential infectivity phenotype. These findings provide new insights into the genetics and pathogenesis of DHAV-3, which may aid the development of new vaccines and the implementation of immunization strategies.

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.

Biochemical characterization of recombinant Avihepatovirus 3C protease and its localization

BACKGROUND

The picornaviral 3C protease mediates viral polyprotein maturation and multiple cleavages of host proteins to modulate viral translation and transcription. The 3C protease has been regarded as a valid target due to its structural similarity among different picornaviruses and minimal sequence similarity with host proteins; therefore, the development of potent inhibitors against the 3C protease as an antiviral drug is ongoing. Duck hepatitis A virus (DHAV) belongs to the Picornavidea family and is a major threat to the poultry industry. To date, little is known about the roles of the DHAV 3C protease plays during infection.

METHODS

In this study, we compared the full-length DHAV 3C protein sequence with other 3C sequences to obtain an alignment for the construction of a phylogenetic tree. Then, we expressed and purified recombinant DHAV 3C protease in the BL21 expression system using nickel-NTA affinity chromatography. The optimization of the cleavage assay conditions and the kinetic analysis for DHAV 3C protease were done by in vitro cleavage assays with a fluorogenic peptide respectively. The inhibitory activity of rupintrivir against the DHAV 3C protease was further evaluated. The localization of the 3C protease in infected and transfected cells was determined using immunofluorescence and confocal microscopy.

RESULTS

Under different expression conditions, the 3C protease was found to be highly expressed after induction with 1 mM IPTG at 16 °C for 10 h. We synthesized a fluorogenic peptide derived from the cleavage site of the DHAV polyprotein and evaluated the protease activity of the DHAV 3C protease for the first time. We used fluorimetric based kinetic analysis to determine kinetic parameters, and Vmax and Km values were determined to be 16.52 nmol/min and 50.78 μM, respectively. Rupintrivir was found to exhibit inhibitory activity against the DHAV 3C protease. Using polyclonal antibody and an indirect immunofluorescence microscopy assay (IFA), it was determined that the DHAV 3C protease was found in the nucleus during infection. In addition, the DHAV 3C protease can enter into the nucleus without the cooperation of viral proteins.

CONCLUSIONS

This is the first study to examine the activity of the DHAV 3C protease, and the activity of the DHAV 3C protease is temperature-, pH- and NaCl concentration- dependent. The DHAV 3C protease localizes throughout DHAV-infected cells and can enter into the nucleus in the absence of other viral proteins. The kinetic analysis was calculated, and the Vmax and Km values were 16.52 nmol/min and 50.78 μM, respectively, using the Lineweaver-Burk plot.

DHAV-1 Inhibits Type I Interferon Signaling to Assist Viral Adaption by Increasing the Expression of SOCS3

Duck hepatitis A virus type 1 (DHAV-1) is one of the most lethal pathogens in the duck industry. The attenuated vaccine (the CH60 strain) is cultivated through serial passage in chicken embryos and is widely used for the prevention and control of the disease. However, the specific mechanism underlying its adaptation in chicken embryos has not been fully elucidated. In this study, we first infected chicken embryo fibroblasts (CEFs) with the DHAV-1 CH60 strain. The peak of viral proliferation occurred within 36–48 h post-infection. The different DHAV-1 strains significantly induced the expression of IFNα, IFNγ, and Suppressor of cytokine signaling 3 (SOCS3) in CEFs, and we found that SOCS3 overexpression significantly promoted viral replication. Furthermore, SOCS3 overexpression significantly inhibited the expression of IFNα but promoted the expression of IFNγ. In addition, SOCS3 overexpression clearly decreased the mRNA levels of STAT1 and STAT3 in the Janus kinase (JAK)-STAT signaling pathway and inhibited the expression of the antiviral proteins MX1 and OASL. Immune-precipitation assays indicated that SOCS3 and IFNα do not physically interact. Subcellular localization of SOCS3 and IFNα revealed that SOCS3 was mainly located in the nucleus and cytoplasm, while IFNα was located only in the cytoplasm. Co-localization of these two proteins was not observed in the cytoplasm. In conclusion, the DHAV-1 CH60 strain may inhibit the expression of IFNα by increasing the SOCS3 protein and SOCS3 can in turn decrease STAT1 and STAT3 mRNA levels, thereby inhibiting the antiviral protein MX1 and ultimately promoting viral proliferation, indirectly assisting in viral adaptation in chicken embryos.

Development and evaluation of an indirect ELISA based on recombinant nonstructural protein 3A to detect antibodies to duck hepatitis A virus type 1

To develop an indirect enzyme-linked immunosorbent assay(I-ELISA) method based on 3A protein of duck hepatitis A virus type 1(DHAV-1) for detection of DHAV-1 antibody, the recombinant protein 3A of DHAV-1 was expressed in E.coli and detected by Western blotting with DHAV-1 infected duck serum. A 3A-ELISA method using the expressed 3A protein as coating antigen for the detection of antibodies to DHAV-1 was developed. The optimal antigen, serum and enzyme-labeled antibody dilutions were 1:200(6.185 μg/ml), 1:20 and 1:2000, respectively. The optimal blocking buffer was 5% BSA. The cutoff value was determined to be 0.274, and the analytical sensitivity was 1:1280. There was no cross reaction between DHAV-1 infected duck serum and other common pathogenic duck serum, indicating that I-ELISA could be used to detect DHAV-1 infected duck serum. The coefficients of variation(CVs) were lower than 10%. The concordance between the I-ELISA based on the 3A subunit of DHAV-1 and that based on the whole DHAV-1 particle was 92.7%. Taken together, the 3A-ELISA method is a highly sensitive and specific test that could be used for screening for DHAV-1 infection and monitoring DHAV-1 antibody.

DHAV-1 2A1 Peptide - A Newly Discovered Co-expression Tool That Mediates the Ribosomal "Skipping" Function

Duck hepatitis A virus 1 (DHAV-1) belongs to the genus Avihepatovirus in the family Picornaviridae. Little research has been carried out on the non-structural proteins of this virus. This study reports that 2A1 protein, the first non-structural protein on the DHAV-1 genome, has a ribosomal “skipping” function mediated by a “-GxExNPGP-” motif. In addition, we prove that when the sequence is extended 10aa to VP1 from the N-terminal of 2A1, the ribosome “skips” completely. However, as the N-terminus of 2A is shortened, the efficiency of ribosomal “skipping” reduces. When 2A1 is shortened to 10aa, it does not function. In addition, we demonstrate that N¹⁸, P¹⁹ G²⁰, and P²¹ have vital roles in this function. We find that the expression of upstream and downstream proteins linked by 2A1 is different, and the expression of the upstream protein is much greater than that of the downstream protein. In addition, we demonstrate that it is the nature of 2A1 that is responsible for the expression imbalance. We also shows that the protein “cleavage” is not due to RNA “cleavage” or RNA transcription abnormalities, and the expressed protein level is independent of RNA transcriptional level. This study provides a systematic analysis of the activity of the DHAV-1 2A1 sequence and, therefore, adds to the “tool-box” that can be deployed for the co-expression applications. It provides a reference for how to apply 2A1 as a co-expression tool.

Transcriptomic Characterization of a Chicken Embryo Model Infected With Duck Hepatitis A Virus Type 1

Duck hepatitis A virus type 1 (DHAV-1) is one of the most common and lethal pathogens in young ducklings. Live-attenuated DHAV vaccine (CH60 strain) developed by passaging in chicken embryos provided effective immune protection for ducklings. However, the accurate mechanism for such adaption in chicken embryos is not fully revealed. Here, we utilize RNA-sequencing to perform global transcriptional analysis of DHAV-1-innoculated embryonated livers along with histopathological and ultrastructural analysis. This study revealed that infection with DHAV-1 strain CH60 is associated with enhanced type I and II interferon responses, activated innate immune responses, elevated levels of suppressor of cytokine signaling 1 and 3 (SOCS1 and SOCS3) accompanied with abnormalities in multiple metabolic pathways. Excessive inflammatory and innate immune responses induced by the CH60 strain are related to severe liver damage. Our study presents a comprehensive characterization of the transcriptome of chicken embryos infected with DHAV-CH60 and provides insight for in-depth exploration of viral adaption and virus–host interactions.

Incompatible Translation Drives a Convergent Evolution and Viral Attenuation During the Development of Live Attenuated Vaccine

Live attenuated vaccines are widely used to protect humans or animals from pathogen infections. We have previously developed a chicken embryo-attenuated Duck Hepatitis A Virus genotype 1 (DHAV-1) vaccine (CH60 strain). This study aims to understand the mechanisms that drive a virulent strain to an attenuated virus. Here, we systematically compared five DHAV-1 chicken embryo attenuated strains and 68 virulent strains. Phylogenetic analysis indicated that duck virulent strains isolated from different geographic regions of China undergo a convergent evolution in the chicken embryos. Comparative analysis indicated that the codon usage bias of the attenuated strains were shaped by chicken codons usage bias, which essentially contributed to viral adaption in the unsuitable host driven by incompatible translation. Of note, the missense mutations in coding region and mutations in untranslated regions may also contribute to viral attenuation of DHAV-1 to some extent. Importantly, we have experimentally confirmed that the expression levels of four viral proteins (2A3pro, 2A3pro, 3C^pro, and 3D^pro) in the liver and kidney of ducks infected with an attenuated strain are significantly lower than that infected with a virulent strain, despite with similar virus load. Thus, the key mechanisms of viral attenuation revealed by this study may lead to innovative and easy approaches in designing live attenuated vaccines.

Cytokine storms are primarily responsible for the rapid death of ducklings infected with duck hepatitis A virus type 1

Duck hepatitis A virus type 1 (DHAV-1) is one of the most harmful pathogens in the duck industry. The infection of adult ducks with DHAV-1 was previously shown to result in transient cytokine storms in their kidneys. To understand how DHAV-1 infection impacts the host liver, we conducted animal experiments with the virulent CH DHAV-1 strain and the attenuated CH60 commercial vaccine strain. Visual observation and standard hematoxylin and eosin staining were performed to detect pathological damage in the liver, and viral copy numbers and cytokine expression in the liver were evaluated by quantitative PCR. The CH strain (10^8.4 copies/mg) had higher viral titers than the CH60 strain (10^4.9 copies/mg) in the liver and caused ecchymotic hemorrhaging on the liver surface. Additionally, livers from ducklings inoculated with the CH strain were significantly infiltrated by numerous red blood cells, accompanied by severe cytokine storms, but similar signs were not observed in the livers of ducklings inoculated with the CH60 strain. In conclusion, the severe cytokine storm caused by the CH strain apparently induces hemorrhagic lesions in the liver, which might be a key factor in the rapid death of ducklings.

Genotyping of human rhinovirus in adult patients with acute respiratory infections identified predominant infections of genotype A21

Human rhinovirus (HRV) is an important causative agent of acute respiratory tract infections (ARTIs). The roles of specific HRV genotypes in patients suffering from ARTIs have not been well established. We recruited 147 adult inpatients with community-acquired pneumonia (CAP) and 291 adult outpatients with upper ARTIs (URTIs). Respiratory pathogens were screened via PCR assays. HRV was detected in 42 patients, with 35 species A, five B and two C. Seventeen genotypes were identified, and HRV-A21 ranked the highest (9/42, 21.4%). The HRV-A21-positive infections were detected in four patients with CAP and in five with URTIs, all without co-infections. The HRV-A21 genome sequenced in this study contained 12 novel coding polymorphisms in viral protein (VP) 1, VP2 EF loop, VP3 knob and 3D regions. The infections of HRV-A21 virus obtained in this study could not be neutralized by antiserum of HRV-A21 prototype strain (VR-1131), indicating remarkable antigenic variation. Metagenomic analysis showed the HRV-A21 reads were dominant in bronchoalveolar lavage fluid of the three HRV-A21-positive patients with severe CAP, in which two dead. Our results highlight an unexpected infection of genotype HRV-A21 in the clinic, indicating the necessity of precise genotyping and surveillance of HRVs to improve the clinical management of ARTIs.