YTHDF2 Is Downregulated in Response to Host Shutoff Induced by DNA Virus Infection and Regulates Interferon-Stimulated Gene Expression

m6A Methylation in Regulation of Antiviral Innate Immunity

The epitranscriptomic modification m6A is a prevalent RNA modification that plays a crucial role in the regulation of various aspects of RNA metabolism. It has been found to be involved in a wide range of physiological processes and disease states. Of particular interest is the role of m6A machinery and modifications in viral infections, serving as an evolutionary marker for distinguishing between self and non-self entities. In this review article, we present a comprehensive overview of the epitranscriptomic modification m6A and its implications for the interplay between viruses and their host, focusing on immune responses and viral replication. We outline future research directions that highlight the role of m6A in viral nucleic acid recognition, initiation of antiviral immune responses, and modulation of antiviral signaling pathways. Additionally, we discuss the potential of m6A as a prognostic biomarker and a target for therapeutic interventions in viral infections.

SUMOylation of the m6A reader YTHDF2 by PIAS1 promotes viral RNA decay to restrict EBV replication

YTH N6-methyladenosine RNA-binding protein F2 (YTHDF2) is a member of the YTH protein family that binds to N6-methyladenosine (m6A)-modified RNA, regulating RNA stability and restricting viral replication, including Epstein-Barr virus (EBV). PIAS1 is an E3 small ubiquitin-like modifier (SUMO) ligase known as an EBV restriction factor, but its role in YTHDF2 SUMOylation remains unclear. In this study, we investigated the functional regulation of YTHDF2 by PIAS1. We found that PIAS1 promotes the SUMOylation of YTHDF2 at three specific lysine residues (K281, K571, and K572). Importantly, PIAS1 synergizes with wild-type YTHDF2, but not a SUMOylation-deficient mutant, to limit EBV lytic replication. Mechanistically, YTHDF2 lacking SUMOylation exhibits reduced binding to EBV transcripts, leading to increased viral mRNA stability. Furthermore, PIAS1 mediates SUMOylation of YTHDF2's paralogs, YTHDF1 and YTHDF3, to restrict EBV replication. These results collectively uncover a unique mechanism whereby YTHDF family proteins control EBV replication through PIAS1-mediated SUMOylation, highlighting the significance of SUMOylation in regulating viral mRNA stability and EBV replication.IMPORTANCEm6A RNA modification pathway plays important roles in diverse cellular processes and viral life cycle. Here, we investigated the relationship between PIAS1 and the m6A reader protein YTHDF2, which is involved in regulating RNA stability by binding to m6A-modified RNA. We found that both the N-terminal and C-terminal regions of YTHDF2 interact with PIAS1. We showed that PIAS1 promotes the SUMOylation of YTHDF2 at three specific lysine residues. We also demonstrated that PIAS1 enhances the anti-EBV activity of YTHDF2. We further revealed that PIAS1 mediates the SUMOylation of other YTHDF family members, namely, YTHDF1 and YTHDF3, to limit EBV replication. These findings together illuminate an important regulatory mechanism of YTHDF proteins in controlling viral RNA decay and EBV replication through PIAS1-mediated SUMOylation.

Modifying the antiviral innate immune response by selective writing, erasing, and reading of m6A on viral and cellular RNA

Viral infection and the antiviral innate immune response are regulated by the RNA modification m6A. m6A directs nearly all aspects of RNA metabolism by recruiting RNA-binding proteins that mediate the fate of m6A-containing RNA. m6A controls the antiviral innate immune response in diverse ways, including shielding viral RNA from detection by antiviral sensors and influencing the expression of cellular mRNAs encoding antiviral signaling proteins, cytokines, and effector proteins. While m6A and the m6A machinery are important for the antiviral response, the precise mechanisms that determine how the m6A machinery selects specific viral or cellular RNA molecules for modification during infection are not fully understood. In this review, we highlight recent findings that shed light on how viral infection redirects the m6A machinery during the antiviral response. A better understanding of m6A targeting during viral infection could lead to new immunomodulatory and therapeutic strategies against viral infection.

The poxvirus F17 protein counteracts mitochondrially orchestrated antiviral responses

Poxviruses are unusual DNA viruses that replicate in the cytoplasm. To do so, they encode approximately 100 immunomodulatory proteins that counteract cytosolic nucleic acid sensors such as cGAMP synthase (cGAS) along with several other antiviral response pathways. Yet most of these immunomodulators are expressed very early in infection while many are variable host range determinants, and significant gaps remain in our understanding of poxvirus sensing and evasion strategies. Here, we show that after infection is established, subsequent progression of the viral lifecycle is sensed through specific changes to mitochondria that coordinate distinct aspects of the antiviral response. Unlike other viruses that cause extensive mitochondrial damage, poxviruses sustain key mitochondrial functions including membrane potential and respiration while reducing reactive oxygen species that drive inflammation. However, poxvirus replication induces mitochondrial hyperfusion that independently controls the release of mitochondrial DNA (mtDNA) to prime nucleic acid sensors and enables an increase in glycolysis that is necessary to support interferon stimulated gene (ISG) production. To counter this, the poxvirus F17 protein localizes to mitochondria and dysregulates mTOR to simultaneously destabilize cGAS and block increases in glycolysis. Our findings reveal how the poxvirus F17 protein disarms specific mitochondrially orchestrated responses to later stages of poxvirus replication.

The role of post-transcriptional regulation in SARS-CoV-2 infection and pathogenicity

The COVID-19 pandemic caused by the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) virus has had a significant impact on global social and economic stability. To combat this, researchers have turned to omics approaches, particularly epitranscriptomics, to limit infection and develop effective therapeutic strategies. Multi-omics can provide the host response dynamics during multiple disease phases to reveal the molecular and cellular landscapes. Epitranscriptomics focuses on the mechanisms of gene transcription in cells and tissues and the relationship between genetic material and epigenetic regulation. This review highlights the role of post-transcriptional regulation in SARS-CoV-2, which affect various processes such as virus infection, replication, immunogenicity, and pathogenicity. The review also explains the formation mechanism of post-transcriptional modifications and how they can be regulated to combat viral infection and pathogenicity.

Alphaherpesvirus-mediated remodeling of the cellular transcriptome results in depletion of m6A-containing transcripts

The mechanisms by which viruses regulate host mRNAs during infection are still poorly understood. Several host transcripts that encode proteins that contribute to the anti-viral response contain the N6-methyladenosine nucleotide (m6A). In this study, we investigated if and how viruses from different (sub) families specifically affect m6A-containing host transcripts. Systematic analysis of host transcriptomes after infection with diverse types of viruses showed that m6A-methylated transcripts are selectively downregulated during infection with Sendai virus, African swine fever virus and the alphaherpesviruses herpes simplex virus 1 (HSV-1) and pseudorabies virus (PRV). Focusing on PRV and HSV-1, we found that downregulation of m6A-methylated transcripts depends on the YTHDF family of m6A-binding proteins, and correlates with localization of these proteins to enlarged P-bodies. Knockdown of YTHDF proteins in primary cells reduced PRV protein expression and increased expression of antiviral interferon-stimulated genes, suggesting that virus-induced depletion of host m6A-containing transcripts constitutes an immune evasion strategy.

SUMOylation of the m6A reader YTHDF2 by PIAS1 promotes viral RNA decay to restrict EBV replication

YTHDF2 is a member of the YTH protein family that binds to N6-methyladenosine (m6A)-modified RNA, regulating RNA stability and restricting viral replication, including Epstein-Barr virus (EBV). PIAS1 is an E3 SUMO ligase known as an EBV restriction factor, but its role in YTHDF2 SUMOylation remains unclear. In this study, we investigated the functional regulation of YTHDF2 by PIAS1. We found that PIAS1 promotes the SUMOylation of YTHDF2 at three specific lysine residues (K281, K571, and K572). Importantly, PIAS1 enhances the antiviral activity of YTHDF2, and SUMOylation-deficient YTHDF2 shows reduced anti-EBV activity. Mechanistically, YTHDF2 lacking SUMOylation exhibits reduced binding to EBV transcripts, leading to increased viral mRNA stability. Furthermore, PIAS1 mediates SUMOylation of YTHDF2's paralogs, YTHDF1 and YTHDF3. These results collectively uncover a unique mechanism whereby YTHDF2 controls EBV replication through PIAS1-mediated SUMOylation, highlighting the significance of SUMOylation in regulating viral mRNA stability and EBV replication.