Stress Granule Formation is One of the Early Antiviral Mechanisms for Host Cells Against Coxsackievirus B Infection

Pro-Viral and Anti-Viral Roles of the RNA-Binding Protein G3BP1

Viruses depend on host cellular resources to replicate. Interaction between viral and host proteins is essential for the pathogens to ward off immune responses as well as for virus propagation within the infected cells. While different viruses employ unique strategies to interact with diverse sets of host proteins, the multifunctional RNA-binding protein G3BP1 is one of the common targets for many viruses. G3BP1 controls several key cellular processes, including mRNA stability, translation, and immune responses. G3BP1 also serves as the central hub for the protein-protein and protein-RNA interactions within a class of biomolecular condensates called stress granules (SGs) during stress conditions, including viral infection. Increasing evidence suggests that viruses utilize distinct strategies to modulate G3BP1 function-either by degradation, sequestration, or redistribution-and control the viral life cycle positively and negatively. In this review, we summarize the pro-viral and anti-viral roles of G3BP1 during infection among different viral families.

The PERK/PKR-eIF2α pathway negatively regulates porcine hemagglutinating encephalomyelitis virus replication by attenuating global protein translation and facilitating stress granule formation

The replication of coronaviruses, including severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV), and the recently emerged severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is closely associated with the endoplasmic reticulum (ER) of infected cells. The unfolded protein response (UPR), which is mediated by ER stress (ERS), is a typical outcome in coronavirus-infected cells and is closely associated with the characteristics of coronaviruses. However, the interaction between virus-induced ERS and coronavirus replication is poorly understood. Here, we demonstrate that infection with the betacoronavirus porcine hemagglutinating encephalomyelitis virus (PHEV) induced ERS and triggered all three branches of the UPR signaling pathway both in vitro and in vivo. In addition, ERS suppressed PHEV replication in mouse neuro-2a (N2a) cells primarily by activating the protein kinase R-like ER kinase (PERK)–eukaryotic initiation factor 2α (eIF2α) axis of the UPR. Moreover, another eIF2α phosphorylation kinase, interferon (IFN)-induced double-stranded RNA-dependent protein kinase (PKR), was also activated and acted cooperatively with PERK to decrease PHEV replication. Furthermore, we demonstrate that the PERK/PKR-eIF2α pathways negatively regulated PHEV replication by attenuating global protein translation. Phosphorylated eIF2α also promoted the formation of stress granules (SGs), which in turn repressed PHEV replication. In summary, our study presents a vital aspect of the host innate response to invading pathogens and reveals attractive host targets (e.g., PERK, PKR, and eIF2α) for antiviral drugs.

IMPORTANCE Coronavirus diseases are caused by different coronaviruses of importance in humans and animals, and specific treatments are extremely limited. ERS, which can activate the UPR to modulate viral replication and the host innate response, is a frequent occurrence in coronavirus-infected cells. PHEV, a neurotropic betacoronavirus, causes nerve cell damage, which accounts for the high mortality rates in suckling piglets. However, it remains incompletely understood whether the highly developed ER in nerve cells plays an antiviral role in ERS and how ERS regulates viral proliferation. In this study, we found that PHEV infection induced ERS and activated the UPR both in vitro and in vivo and that the activated PERK/PKR-eIF2α axis inhibited PHEV replication through attenuating global protein translation and promoting SG formation. A better understanding of coronavirus-induced ERS and UPR activation may reveal the pathogenic mechanism of coronavirus and facilitate the development of new treatment strategies for these diseases.

The role of host eIF2α in viral infection

Background

eIF2α is a regulatory node that controls protein synthesis initiation by its phosphorylation or dephosphorylation. General control nonderepressible-2 (GCN2), protein kinase R-like endoplasmic reticulum kinase (PERK), double-stranded RNA (dsRNA)-dependent protein kinase (PKR) and heme-regulated inhibitor (HRI) are four kinases that regulate eIF2α phosphorylation.

Main body

In the viral infection process, dsRNA or viral proteins produced by viral proliferation activate different eIF2α kinases, resulting in eIF2α phosphorylation, which hinders ternary tRNA^Met-GTP-eIF2 complex formation and inhibits host or viral protein synthesis. The stalled messenger ribonucleoprotein (mRNP) complex aggregates under viral infection stress to form stress granules (SGs), which encapsulate viral RNA and transcription- and translation-related proteins, thereby limiting virus proliferation. However, many viruses have evolved a corresponding escape mechanism to synthesize their own proteins in the event of host protein synthesis shutdown and SG formation caused by eIF2α phosphorylation, and viruses can block the cell replication cycle through the PERK-eIF2α pathway, providing a favorable environment for their own replication. Subsequently, viruses can induce host cell autophagy or apoptosis through the eIF2α-ATF4-CHOP pathway.

Conclusions

This review summarizes the role of eIF2α in viral infection to provide a reference for studying the interactions between viruses and hosts.

The 3A protein of coxsackievirus B3 acts as a viral suppressor of RNA interference

RNA interference (RNAi) is a potent antiviral defence mechanism in eukaryotes, and numerous viruses have been found to encode viral suppressors of RNAi (VSRs). Coxsackievirus B3 (CVB3) belongs to the genus Enterovirus in the family Picornaviridae, and has been reported to be a major causative pathogen for viral myocarditis. Despite the importance of CVB3, it is unclear whether CVB3 can also encode proteins that suppress RNAi. Here, we showed that the CVB3 nonstructural protein 3A suppressed RNAi triggered by either small hairpin RNAs (shRNAs) or small interfering RNAs (siRNAs) in mammalian cells. We further uncovered that CVB3 3A interacted directly with double-stranded RNAs (dsRNAs) and siRNAs in vitro. Through mutational analysis, we found that the VSR activity of CVB3 3A was significantly reduced by mutations of D24A/L25A/L26A, Y37A/C38A and R60A in conserved residues. In addition, the 3A protein encoded by coxsackievirus B5 (CVB5), another member of Enterovirus, also showed VSR activity. Taken together, our findings showed that CVB3 3A has in vitro VSR activity, thereby providing insights into the pathogenesis of CVB3 and other enteroviruses.

Strategies for Success. Viral Infections and Membraneless Organelles

Regulation of RNA homeostasis or “RNAstasis” is a central step in eukaryotic gene expression. From transcription to decay, cellular messenger RNAs (mRNAs) associate with specific proteins in order to regulate their entire cycle, including mRNA localization, translation and degradation, among others. The best characterized of such RNA-protein complexes, today named membraneless organelles, are Stress Granules (SGs) and Processing Bodies (PBs) which are involved in RNA storage and RNA decay/storage, respectively. Given that SGs and PBs are generally associated with repression of gene expression, viruses have evolved different mechanisms to counteract their assembly or to use them in their favor to successfully replicate within the host environment. In this review we summarize the current knowledge about the viral regulation of SGs and PBs, which could be a potential novel target for the development of broad-spectrum antiviral therapies.

Expression Profile and Function Analysis of Long Non-coding RNAs in the Infection of Coxsackievirus B3

The roles of lncRNAs in the infection of enteroviruses have been barely demonstrated. In this study, we used coxsackievirus B3 (CVB3), a typical enterovirus, as a model to investigate the expression profiles and functional roles of lncRNAs in enterovirus infection. We profiled lncRNAs and mRNA expression in CVB3-infected HeLa cells by lncRNA-mRNA integrated microarrays. As a result, 700 differentially expressed lncRNAs (431 up-regulated and 269 down-regulated) and 665 differentially expressed mRNAs (299 up-regulated and 366 down-regulated) were identified in CVB3 infection. Then we performed lncRNA-mRNA integrated pathway analysis to identify potential functional impacts of the differentially expressed mRNAs, in which lncRNA-mRNA correlation network was built. According to lncRNA-mRNA correlation, we found that XLOC-001188, an lncRNA down-regulated in CVB3 infection, was negatively correlated with NFAT5 mRNA, an anti-CVB3 gene reported previously. This interaction was supported by qPCR detection following siRNA-mediated knockdown of XLOC-001188, which showed an increase of NFAT5 mRNA and a reduction of CVB3 genomic RNA. In addition, we observed that four most significantly altered lncRNAs, SNHG11, RP11-145F16.2, RP11-1023L17.1 and RP11-1021N1.2 share several common correlated genes critical for CVB3 infection, such as BRE and IRF2BP1. In all, our studies reveal the alteration of lncRNA expression in CVB3 infection and its potential influence on CVB3 replication, providing useful information for future studies of enterovirus infection.