MAVS-Based Reporter Systems for Real-Time Imaging of EV71 Infection and Antiviral Testing

The class III phosphatidylinositol 3-kinase VPS34 supports EV71 replication by promoting viral replication organelle formation

Enterovirus 71 (EV71) belongs to the family of Picornaviridae; it could cause a variety of illnesses and pose a great threat to public health worldwide. Currently, there is no specific drug treatment for this virus, and a better understanding of virus-host interaction is crucial for novel antiviral development. Here, we find that the class III phosphatidylinositol 3-kinase, VPS34, is an essential host factor for EV71 infection. VPS34 inhibition with either shRNA or specific chemical inhibitor significantly reduces EV71 infection. Meanwhile, EV71 infection upregulates phosphatidylinositol 3-phosphate (PI3P) production in viral replication organelles (ROs), while the depletion of PI3P by phosphatase overexpression inhibits EV71 infection. In addition, the PI3P-binding protein, double FYVE-containing protein 1 (DFCP1), is also required for an efficient replication of EV71. DFCP1 could interact with viral 2C protein and facilitate viral association with lipid droplets (LDs), which are important lipid sources for viral RO biogenesis. Taken together, these results indicate that EV71 virus exploits the VPS34-PI3P-DFCP1-LDs pathway to promote viral RO formation and viral infection, and they also illuminate novel targets for antiviral development.IMPORTANCEEnterovirus 71 (EV71) is a major pathogen that causes hand-foot-and-mouth disease (HFMD) and other serious complications, which are big threats to children under 5 years old. Unravelling the interactions between virus and the host cells will open new avenues in antiviral research. Here, we found the class III phosphatidylinositol 3-kinase, VPS34, and its effector, double FYVE-containing protein 1 (DFCP1), were essential for EV71 infection, both of which could support EV71 viral replication by enhancing the biogenesis of viral replication organelles (ROs). As DFCP1 localizes to lipid droplets, hijacking of these host factors will enable viral utilization of lipids from LDs for the generation of membrane structures during RO biogenesis. In addition, the VPS34 kinase inhibitor was found to be potent against EV71 infection; therefore, this study also brings up a novel target for future anti-EV71 drug development.

Receptors and Host Factors for Enterovirus Infection: Implications for Cancer Therapy

Enteroviruses, with their diverse clinical manifestations ranging from mild or asymptomatic infections to severe diseases such as poliomyelitis and viral myocarditis, present a public health threat. However, they can also be used as oncolytic agents. This review shows the intricate relationship between enteroviruses and host cell factors. Enteroviruses utilize specific receptors and coreceptors for cell entry that are critical for infection and subsequent viral replication. These receptors, many of which are glycoproteins, facilitate virus binding, capsid destabilization, and internalization into cells, and their expression defines virus tropism towards various types of cells. Since enteroviruses can exploit different receptors, they have high oncolytic potential for personalized cancer therapy, as exemplified by the antitumor activity of certain enterovirus strains including the bioselected non-pathogenic Echovirus type 7/Rigvir, approved for melanoma treatment. Dissecting the roles of individual receptors in the entry of enteroviruses can provide valuable insights into their potential in cancer therapy. This review discusses the application of gene-targeting techniques such as CRISPR/Cas9 technology to investigate the impact of the loss of a particular receptor on the attachment of the virus and its subsequent internalization. It also summarizes the data on their expression in various types of cancer. By understanding how enteroviruses interact with specific cellular receptors, researchers can develop more effective regimens of treatment, offering hope for more targeted and efficient therapeutic strategies.

Natural-Target-Mimicking Translocation-Based Fluorescent Sensor for Detection of SARS-CoV-2 PLpro Protease Activity and Virus Infection in Living Cells

Papain-like protease PLpro, a domain within a large polyfunctional protein, nsp3, plays key roles in the life cycle of SARS-CoV-2, being responsible for the first events of cleavage of a polyprotein into individual proteins (nsp1-4) as well as for the suppression of cellular immunity. Here, we developed a new genetically encoded fluorescent sensor, named PLpro-ERNuc, for detection of PLpro activity in living cells using a translocation-based readout. The sensor was designed as follows. A fragment of nsp3 protein was used to direct the sensor on the cytoplasmic surface of the endoplasmic reticulum (ER) membrane, thus closely mimicking the natural target of PLpro. The fluorescent part included two bright fluorescent proteins-red mScarlet I and green mNeonGreen-separated by a linker with the PLpro cleavage site. A nuclear localization signal (NLS) was attached to ensure accumulation of mNeonGreen into the nucleus upon cleavage. We tested PLpro-ERNuc in a model of recombinant PLpro expressed in HeLa cells. The sensor demonstrated the expected cytoplasmic reticular network in the red and green channels in the absence of protease, and efficient translocation of the green signal into nuclei in the PLpro-expressing cells (14-fold increase in the nucleus/cytoplasm ratio). Then, we used PLpro-ERNuc in a model of Huh7.5 cells infected with the SARS-CoV-2 virus, where it showed robust ER-to-nucleus translocation of the green signal in the infected cells 24 h post infection. We believe that PLpro-ERNuc represents a useful tool for screening PLpro inhibitors as well as for monitoring virus spread in a culture.

N-terminal acetyltransferase 6 facilitates enterovirus 71 replication by regulating PI4KB expression and replication organelle biogenesis

Enterovirus 71 (EV71) is one of the major pathogens causing hand, foot, and mouth disease in children under 5 years old, which can result in severe neurological complications and even death. Due to limited treatments for EV71 infection, the identification of novel host factors and elucidation of mechanisms involved will help to counter this viral infection. N-terminal acetyltransferase 6 (NAT6) was identified as an essential host factor for EV71 infection with genome-wide CRISPR/Cas9 screening. NAT6 facilitates EV71 viral replication depending on its acetyltransferase activity but has little effect on viral release. In addition, NAT6 is also required for Echovirus 7 and coxsackievirus B5 infection, suggesting it might be a pan-enterovirus host factor. We further demonstrated that NAT6 is required for Golgi integrity and viral replication organelle (RO) biogenesis. NAT6 knockout significantly inhibited phosphatidylinositol 4-kinase IIIβ (PI4KB) expression and PI4P production, both of which are key host factors for enterovirus infection and RO biogenesis. Further mechanism studies confirmed that NAT6 formed a complex with its substrate actin and one of the PI4KB recruiters-acyl-coenzyme A binding domain containing 3 (ACBD3). Through modulating actin dynamics, NAT6 maintained the integrity of the Golgi and the stability of ACBD3, thereby enhancing EV71 infection. Collectively, these results uncovered a novel mechanism of N-acetyltransferase supporting EV71 infection.IMPORTANCEEnterovirus 71 (EV71) is an important pathogen for children under the age of five, and currently, no effective treatment is available. Elucidating the mechanism of novel host factors supporting viral infection will reveal potential antiviral targets and aid antiviral development. Here, we demonstrated that a novel N-acetyltransferase, NAT6, is an essential host factor for EV71 replication. NAT6 could promote viral replication organelle (RO) formation to enhance viral replication. The formation of enterovirus ROs requires numerous host factors, including acyl-coenzyme A binding domain containing 3 (ACBD3) and phosphatidylinositol 4-kinase IIIβ (PI4KB). NAT6 could stabilize the PI4KB recruiter, ACBD3, by inhibiting the autophagy degradation pathway. This study provides a fresh insight into the relationship between N-acetyltransferase and viral infection.