N6-Methyladenosine in Flaviviridae Viral RNA Genomes Regulates Infection

SARS-CoV-2 and HCoV-OC43 regulate host m6A modification via activation of the mTORC1 signalling pathway to facilitate viral replication

N6-methyladenosine (m6A) is the most prevalent post-transcriptional modification in eukaryotic RNA and is also present in various viral RNAs, where it plays a crucial role in regulating the viral life cycle. However, the molecular mechanisms through which viruses regulate host RNA m6A methylation are not fully understood. In this study, we reveal that SARS-CoV-2 and HCoV-OC43 infection enhance host m6A modification by activating the mTORC1 signalling pathway. Specifically, the viral non-structural protein nsp14 upregulates the expression of S-adenosylmethionine synthase MAT2A in an mTORC1-dependent manner. This mTORC1-MAT2A axis subsequently stimulates the synthesis of S-adenosylmethionine (SAM). The increase of SAM then enhances the m6A methylation of host RNA and facilitates viral replication. Our findings uncover a molecular mechanism by which viruses regulate host m6A methylation and provide insights into how SARS-CoV-2 hijacks host cellular epitranscriptomic modifications to promote its replication.

N6-methyladenosine modification of host Hsc70 attenuates nucleopolyhedrovirus infection in the lepidopteran model insect Bombyx mori

N6-methyladenosine (m6A) is the most prevalent internal modification on mRNA and plays critical roles in various biological processes including virus infection. It has been shown that m6A methylation is able to regulate virus proliferation and host innate immunity in mammals and plants, however, this antiviral defense in insects is largely unknown. Here we investigated function of m6A and its associated methyltransferases in nucleopolyhedrovirus (BmNPV) infection in silkworm. We reported significant changes of m6A methyltransferases METTL3 and METTL14 upon BmNPV treatment. Knockdown of METTL3 and METTL14 enhanced BmNPV infection and promoted viral proliferation, whereas overexpression of these enzymes could prevent viral replication. Further study revealed that host heat shock cognate 70 (Hsc70) as a target gene of m6A would contribute to BmNPV proliferation. CRISPR-dCas9-targeted methylation of Hsc70 by METTL3/METTL14 decreased its expression and further attenuated BmNPV infection. Consistently, knockout of METTL3 in silkworm individuals by CRISPR-Cas9 reduced overall m6A levels, which led to rapid death of silkworms and increase of BmNPV upon virus infection likely due to upregulated expression of Hsc70. Collectively, these findings provided a novel insight into antiviral activity of m6A and demonstrated a distinct immune response via attenuating host Hsc70 expression to counteract BmNPV replication in lepidopteran silkworm.

N6-methyladenosine modification of the subgroup J avian leukosis viral RNAs attenuates host innate immunity via MDA5 signaling

Subgroup J avian leukosis virus (ALV-J), a retrovirus, elicits immunosuppression and persistent infections in chickens. Although it is widely acknowledged that ALV-J can evade the host's innate immune defenses, the mechanisms behind this immune evasion remain elusive. N6-methyladenosine (m6A), the most prevalent internal RNA modification, plays a role in innate immune evasion. Our research identified ALV-J as an inefficient stimulator of innate immunity in vitro and in vivo, with its genomic RNA featuring m6A modifications predominantly in the envelope protein (Env) region and 3' untranslated region (3'UTR). To elucidate the functional consequences of m6A modification, we subsequently generated m6A-deficient ALV-J through its culturing in the DF-1 overexpressing fat mass and obesity-associated protein (FTO) cells. The m6A-deficient ALV-J virus, or its RNAs significantly enhanced IFN-β production compared to the wild-type (wt) ALV-J, suggesting a pivotal regulatory function of m6A modifications in modulating innate immune response. Mechanistically, the m6A modification of the ALV-J genomic RNA directly impacted its recognition by MDA5, weakening its binding and ubiquitination and attenuating IFN-β activation. Moreover, m6A-deficient ALV-J, created by inducing mutations in m6A sites within Env and 3'UTR, exhibited reduced replication capacity and elevated IFN-β expression in host cells. Importantly, this phenomenon was abolished in MDA5-knockout DF-1 cells, further demonstrating the core role of MDA5. These data demonstrate that m6A modification of ALV-J genomic RNA dampens the host's innate immune response through MDA5 signaling pathway.

Wolbachia elevates host methyltransferase expression and alters the m6A methylation landscape in Aedes aegypti mosquito cells

Wolbachia pipientis is an intracellular endosymbiotic bacterium that blocks the replication of several arboviruses in transinfected Aedes aegypti mosquitoes, yet its antiviral mechanism remains unknown. For the first time, we employed Nanopore direct RNA sequencing technology to investigate the impact of wAlbB strain of Wolbachia on the host's N6-methyladenosine (m6A) machinery and post-transcriptional modification landscape. Our study revealed that Wolbachia infection elevates the expression of genes involved in the mosquito's m6A methyltransferase complex. However, knocking down these m6A-related genes did not affect Wolbachia density. Nanopore sequencing identified 1,392 differentially modified m6A DRACH motifs on mosquito transcripts, with 776 showing increased and 616 showing decreased m6A levels due to Wolbachia. These m6A sites were predominantly enriched in coding sequences and 3'-untranslated regions. Gene Ontology analysis revealed that genes with reduced m6A levels were over-represented in functional GO terms associated with purine nucleotide binding functions critical in the post-transcriptional modification process of m6A. Differential gene expression analysis of the Nanopore data uncovered that a total of 643 protein-coding genes were significantly differentially expressed, 427 were downregulated, and 216 were upregulated. Several classical and non-classical immune-related genes were amongst the downregulated DEGs. Notably, it revealed a critical host factor, transmembrane protein 41B (TMEM41B), which is required for flavivirus infection, was upregulated and methylated in the presence of Wolbachia. Indeed, there is a strong correlation between gene expression being upregulated in genes with both increased and decreased levels of m6A modification, respectively. Our findings underscore Wolbachia's ability to modulate many intracellular aspects of its mosquito host by influencing post-transcriptional m6A modifications and gene expression, and it unveils a potential link behind its antiviral properties.

Direct RNA sequencing of the Escherichia coli epitranscriptome uncovers alterations under heat stress

Modifications of RNA, known as the epitranscriptome, affect gene expression, translation, and splicing in eukaryotes, with implications for developmental processes, cancer, and viral infections. In prokaryotes, regulation at the level of the epitranscriptome is still poorly understood. Here, we used nanopore direct RNA sequencing of Escherichia coli to study RNA modifications and their changes under heat stress. With a single sequencing reaction, we detected most known modification types in ribosomal RNA (rRNA), transfer RNA (tRNA), and messenger RNA (mRNA). RNA sequencing was complemented by a multifaceted approach that included mass spectrometry, deletion mutants, single-nucleotide polymerase chain reaction, and in vitro methylation. Known 5-methylcytidine (m5C) and N6-methyladenosine (m6A) sites in the rRNA were confirmed, but these types of modifications could not be localized in the mRNA. In response to heat stress, levels of m5C, m6A, and N6,N6-dimethyladenosine increased in the 16S rRNA. Sequencing and mass spectrometry data demonstrated a decrease in tRNA modification abundance in the anticodon loop at 45°C. In general, mRNA modifications at 37°C were enriched in the coding regions of genes associated with general metabolism and RNA processing, which shifted to genes involved in cell wall synthesis and membrane transport under heat stress. This study provides new insights into the complexity of post-transcriptional regulation in bacteria.

Chinese sacbrood virus mediates m6A modification to target and suppress the expression of hemolymph maintenance gene AF9, exacerbating bee infections

The Chinese sacbrood virus (CSBV) severely threatens the beekeeping industry, wherein 3- to 5-day-old larvae in the critical differentiation stage are highly susceptible to low levels of CSBV exposure. Once infected, larvae cannot undergo normal pupation, but the pathogenic mechanism remains unclear. Previous studies have shown that m6A modification plays an important regulatory role in larval development during the critical differentiation stage. However, it is unknown whether CSBV infection affects the pupation of honeybee larvae by altering m6A modification. Here, a novel immunoregulatory factor, AF9, was identified in honeybee larvae through combined methylated RNA immunoprecipitation sequencing (MeRIP-seq) and RNA-seq analysis following CSBV infection. Enzyme-linked immunosorbent assay (ELISA) quantification of m6A methylation in total RNA and MeRIP-qPCR further revealed that CSBV infection of honeybee larvae inhibits the expression of AF9 via m6A modification, thereby hindering the host innate immune response and promoting CSBV replication. MeRIP-qPCR was then used to demonstrate that AcMETTL3 targets and modifies AF9 mRNA, thereby inhibiting AF9 expression. Homology and functional analysis of human-derived AF9 (MLLT3) suggested that AF9 exerted a similar effect as MLLT3 on honeybee hemolymph functioning. dsRNA was then fed to silence genes, followed by RNA extraction and expression analysis from hemolymph. Downregulation of AF9 expression led to decreased numbers of live cells in the hemolymph of honeybee larvae and a reduction in phenoloxidase activity, thereby inhibiting the host immune response. Finally, an Apis mellifera pupation infection model was constructed to further explore the antiviral activities associated with AmAF9. AmAF9 exerted a similarly significant antiviral effect against deformed wing virus (DWV) and acute bee paralysis virus (ABPV) infections in Apis mellifera pupae. These results indicate that CSBV infection promotes overall m6A modification in the host and inhibits the expression of AF9 through AcMETTL3 targeting, leading to host immunosuppression and exacerbating honeybee infection. Similarly, AF9 is stably expressed in Apis mellifera and exhibits the same antiviral effect, making it a broad-spectrum target in honeybee viruses.

IMPORTANCE

The Chinese sacbrood virus (CSBV) poses a serious threat to the health of Apis cerana colonies, yet its specific pathogenic mechanism remains unclear. This study shows that infection with CSBV can enhance overall m6A modification levels in Apis cerana larvae and suppress the expression of AF9 by promoting targeting of AcMETTL3, thereby inhibiting the innate immune response and exacerbating CSBV infection. Further analyses indicated that AF9 functions similarly as the mammalian homologous gene MLLT3 by maintaining normal functions of hemolymph. Moreover, AF9 can also significantly inhibit infections by common Apis mellifera viruses. In summary, a new mechanism is detailed here by which CSBV escapes the host's innate immune response by enhancing m6A modification to target and suppress the immune response gene AF9. This study also provides new insights into the mechanisms by which bee viruses inhibit host immune responses and suggests that AF9 may serve as a potential new broad-spectrum antiviral target in bees.

The pseudorabies virus UL13 protein kinase triggers phosphorylation of the RNA demethylase FTO, which is associated with FTO-dependent suppression of interferon-stimulated gene expression

Alpha-ketoglutarate-dependent dioxygenase, also known as fat mass and obesity-associated protein (FTO), is an RNA demethylase that mediates the demethylation of N6,2-O-dimethyladenosine (m6Am) and N6-methyladenosine (m6A). Both m6Am and m6A are prevalent modifications in mRNA and affect different aspects of transcript biology, including splicing, nuclear export, translation efficiency, and degradation. The role of FTO during (herpes) virus infection remains largely unexplored. In this study, we show that the UL13 protein kinase of the alphaherpesvirus pseudorabies virus (PRV) triggers phosphorylation of FTO. In primary epithelial cells, depletion of FTO leads to increased expression of antiviral interferon-stimulated genes (ISGs) and UL13 triggers FTO-dependent suppression of ISG expression. Although PRV infection suppresses m6Am levels in host small nuclear RNA, this is independent of UL13. The current data highlight FTO as an important regulator of antiviral ISG expression and suggest that UL13-mediated phosphorylation of FTO may serve as a previously unrecognized viral strategy to suppress the antiviral interferon response.IMPORTANCERNA modification pathways play important roles in diverse cellular processes and virus life cycles. Although previous studies have demonstrated that alphaherpesviruses can substantially influence cellular RNA modifications, such as m6A, the impact on the m6Am epitranscriptome machinery remains largely unexplored. The present work reports that the UL13 protein kinase of pseudorabies virus (PRV), an alphaherpesvirus, mediates phosphorylation of the m6Am/m6A eraser FTO and that this correlates with a UL13- and FTO-dependent suppression of antiviral interferon-stimulated gene (ISG) expression. Furthermore, PRV infection leads to a pronounced reduction in m6Am levels in host snRNA and also induces phosphorylation of the m6Am writer PCIF1. These data highlight FTO as an important regulator of ISG expression and reveal that viral manipulation of FTO, such as UL13-induced phosphorylation of FTO, may serve as a previously unrecognized interferon evasion strategy.

Targeting eEF1A reprograms translation and uncovers broad-spectrum antivirals against cap or m6A protein synthesis routes

Plitidepsin is an antitumoral compound safe for treating COVID-19 that targets the translation elongation factor eEF1A. Here we detect that plitidepsin decreases de novo cap-dependent translation of SARS-CoV-2 and non-viral RNAs but affects less than 13% of the host proteome, thus preserving cellular viability. In response to plitidepsin, cells upregulate EIF2AK3 and proteins that reduce translation, but also proteins that support proteostasis via ribosome synthesis and cap-independent translation by eIF4G2 and IGF2BP2. While plitidepsin inhibits cap- or internal ribosome entry sites (IRES)-mediated translation, its impact on N6-methyladenosine (m6A) translation is limited. In agreement, plitidepsin blocks members of Coronaviridae, Flaviviridae, Pneumoviridae and Herpesviridae families. Yet, it fails to inhibit retroviruses that exploit m6A synthesis routes and are blocked by drugs targeting IGF2BP2 m6A reader. By deciphering the molecular fingerprint of cells treated with therapies targeting translation we identify a rational approach to select broad-spectrum antivirals with potential to counteract future pandemic viruses.

N6-methyladenosine in inflammatory diseases: Important actors and regulatory targets

N6-methyladenosine (m6A) is one of the most prevalent epigenetic modifications in eukaryotic cells. It regulates RNA function and stability by modifying RNA methylation through writers, erasers, and readers. As a result, m6A plays a critical role in a wide range of biological processes. Inflammation is a common and fundamental pathological process. Numerous studies have investigated the role of m6A modifications in inflammatory diseases. This review highlights the mechanisms by which m6A contributes to inflammation, focusing on pathogen-induced infectious diseases, autoimmune disorders, allergic conditions, and metabolic disorder-related inflammatory diseases.

N6-methyladenosine (m6A) modification: Emerging regulators in plant-virus interactions

N6-methyladenosine (m6A), a reversible epigenetic modification, is widely present on both cellular and viral RNAs. This modification undergoes catalysis by methyltransferases (writers), removal by demethylases (erasers), and recognition by m6A-binding proteins (readers), ultimately influencing the fate and function of modified RNA molecules. With recent advances in sequencing technologies, the genome-wide mapping of m6A has become possible, enabling a deeper exploration of its roles during viral infections. So far, while the significance of m6A in regulating virus-host interactions has been well-established in animal viruses, research on its involvement in plant viruses remains in its early stages. In this review, we summarize the current knowledge regarding the functions and molecular mechanisms of m6A in plant-virus interactions. A better understanding of these complex interactions may provide valuable insights for developing novel antiviral strategies, potentially leading to more effective control of plant viral diseases in the field.

Charting the epitranscriptomic landscape across RNA biotypes using native RNA nanopore sequencing

RNA modifications are conserved chemical features found in all domains of life and across diverse RNA biotypes, shaping gene expression profiles and enabling rapid responses to environmental changes. Their broad chemical diversity and dynamic nature pose significant challenges for studying them comprehensively. These limitations can now be addressed through direct RNA nanopore sequencing (DRS), which allows simultaneous identification of diverse RNA modification types at single-molecule and single-nucleotide resolution. Here, we review recent efforts pioneering the use of DRS to better understand the epitranscriptomic landscape. We highlight how DRS can be applied to investigate different RNA biotypes, emphasizing the use of specialized library preparation protocols and downstream bioinformatic workflows to detect both natural and synthetic RNA modifications. Finally, we provide a perspective on the future role of DRS in epitranscriptomic research, highlighting remaining challenges and emerging opportunities from improved sequencing yields and accuracy enabled by the latest DRS chemistry.

Transcriptome-wide dynamics of m6A methylation in ISKNV and Siniperca chuatsi cells infected with ISKNV

Infectious spleen and kidney necrosis virus (ISKNV) is a highly virulent and rapidly transmissible fish virus that poses threats to the aquaculture of a wide variety of freshwater and marine fish. N6-methyladenosine (m6A), recognized as a common epigenetic modification of RNA, plays an important regulatory role during viral infection. However, the impact of m6A RNA methylation on the pathogenicity of ISKNV remains unexplored. Here, methylated RNA immunoprecipitation sequencing (MeRIP-seq) coupled with RNA sequencing (RNA-seq) was used to systematically detect variations in m6A methylation and gene expression between ISKNV-infected and noninfected MFF-1 cells, followed by functional enrichment and co-expression joint analysis. The findings revealed that the m6A methylation peaks were located mainly in coding sequences (CDSs), with more than 90% of the transcripts containing 1-5 m6A peaks. Through MeRIP-seq, 4361 differentially m6A-methylated mRNAs were identified. Gene enrichment analysis revealed that m6A-related genes were enriched in biological processes and pathways such as gene expression, cellular structure, immune responses, and cell death. Co-expression analysis revealed that the genes differentially expressed at both the mRNA and m6A modification levels were enriched in pathways such as the hippo, ErbB, and JAK-STAT pathways. The m6A modification at the genome-wide transcription level of ISKNV was subsequently shown to be pronounced in several pivotal genes, such as putative vascular endothelial growth factor, ribonucleotide reductase small subunit, and E3 ubiquitin ligase. This study comprehensively describes the m6A expression profile in ISKNV- and ISKNV-infected MFF-1 cells, providing a basis for investigating the role of m6A modification during ISKNV infection.

Epigenetic Mechanisms in Osteoporosis: Exploring the Power of m6A RNA Modification

Osteoporosis, recognised as a metabolic disorder, has emerged as a significant burden on global health. Although available treatments have made considerable advancements, they remain inadequately addressed. In recent years, the role of epigenetic mechanisms in skeletal disorders has garnered substantial attention, particularly concerning m6A RNA modification. m6A is the most prevalent dynamic and reversible modification in eukaryotes, mediating various metabolic processes of mRNAs, including splicing, structural conversion, translation, translocation and degradation and serves as a crucial component of epigenetic modification. Research has increasingly validated that m6A plays a vital role in the proliferation, differentiation, migration, invasion,and repair of bone marrow mesenchymal stem cells (BMSCs), osteoblasts and osteoclasts, all of which impact the whole process of osteoporosis pathogenesis. Continuous efforts have been made to target m6A regulators and natural products derived from traditional medicine, which exhibit multiple biological activities such as anti-inflammatory and anticancer effects, have emerged as a valuable resources for m6A drug discovery. This paper elaborates on m6A methylation and its regulatory role in osteoporosis, emphasising its implications for diagnosis and treatment, thereby providing theoretical references.

Zika virus remodels and hijacks IGF2BP2 ribonucleoprotein complex to promote viral replication organelle biogenesis

Zika virus (ZIKV) infection causes significant human disease that, with no approved treatment or vaccine, constitutes a major public health concern. Its life cycle entirely relies on the cytoplasmic fate of the viral RNA genome (vRNA) through a fine-tuned equilibrium between vRNA translation, replication, and packaging into new virions, all within virus-induced replication organelles (vROs). In this study, with an RNA interference (RNAi) mini-screening and subsequent functional characterization, we have identified insulin-like growth factor 2 mRNA-binding protein 2 (IGF2BP2) as a new host dependency factor that regulates vRNA synthesis. In infected cells, IGF2BP2 associates with viral NS5 polymerase and redistributes to the perinuclear viral replication compartment. Combined fluorescence in situ hybridization-based confocal imaging, in vitro binding assays, and immunoprecipitation coupled to RT-qPCR showed that IGF2BP2 directly interacts with ZIKV vRNA 3' nontranslated region. Using ZIKV sub-genomic replicons and a replication-independent vRO induction system, we demonstrated that IGF2BP2 knockdown impairs de novo vRO biogenesis and, consistently, vRNA synthesis. Finally, the analysis of immunopurified IGF2BP2 complex using quantitative mass spectrometry and RT-qPCR revealed that ZIKV infection alters the protein and RNA interactomes of IGF2BP2. Altogether, our data support that ZIKV hijacks and remodels the IGF2BP2 ribonucleoprotein complex to regulate vRO biogenesis and vRNA neosynthesis.

RNA Modifications in Pathogenic Viruses: Existence, Mechanism, and Impacts

RNA modification is a key posttranscriptional process playing various biological roles, and one which has been reported to exist extensively in cellular RNAs. Interestingly, recent studies have shown that viral RNAs also contain a variety of RNA modifications, which are regulated dynamically by host modification machinery and play critical roles in different stages of the viral life cycle. In this review, we summarize the reports of four typical modifications reported on viral RNAs, including N6-methyladenosine (m6A), 5-methylcytosine (m5C), N4-acetylcytosine (ac4C), and N1-methyladenosine (m1A), describe the molecular mechanisms of these modification processes, and illustrate their impacts on viral replication, pathogenicity, and innate immune responses. Notably, we find that RNA modifications in different viruses share some common features and mechanisms in their generation, regulation, and function, highlighting the potential for viral RNA modifications and the related host machinery to serve as the targets or bases for the development of antiviral therapeutics and vaccines.

N6-methyladenosine RNA modification promotes Severe Fever with Thrombocytopenia Syndrome Virus infection

Severe Fever with Thrombocytopenia Syndrome Virus (SFTSV), a novel bunyavirus primarily transmitted by Haemaphysalis longicornis, induces severe disease with a high mortality rate. N6-methyladenosine (m6A) is a prevalent internal chemical modification in eukaryotic mRNA that has been reported to regulate viral infection. However, the role of m6A modification during SFTSV infection remains elusive. We here reported that SFTSV RNAs bear m6A modification during infection. Manipulating the expressions or activities of host m6A regulators significantly impacted SFTSV infection. Mechanistically, SFTSV recruited m6A regulators through the nucleoprotein to modulate the m6A modification of viral RNA, eventually resulting in enhanced infection by promoting viral mRNA translation efficiency and/or genome RNA stability. m6A mutations in the S genome diminished virus particle production, while m6A mutations in the G transcript impaired the replication of recombinant vesicular stomatitis virus (rVSV) expressing G protein in vitro and in vivo. Interestingly, m6A modification was evolutionarily conserved and facilitated SFTSV infection in primary tick cells. These findings may open an avenue for the development of m6A-targeted anti-SFTSV vaccines, drugs, and innovative strategies for the prevention and control of tick-borne disease.

Coxsackievirus B3-Induced m6A Modification of RNA Enhances Viral Replication via Suppression of YTHDF-Mediated Stress Granule Formation

N6-methyladenosine (m6A) is the most prevalent internal RNA modification. Here, we demonstrate that coxsackievirus B3 (CVB3), a common causative agent of viral myocarditis, induces m6A modification primarily at the stop codon and 3' untranslated regions of its genome. As a positive-sense single-stranded RNA virus, CVB3 replicates exclusively in the cytoplasm through a cap-independent translation initiation mechanism. Our study shows that CVB3 modulates the expression and nucleo-cytoplasmic transport of the m6A machinery components-METTL3, ALKBH5 and YTHDFs-resulting in increased m6A modifications that enhance viral replication. Mechanistically, this enhancement is mediated through YTHDF-driven stress granule (SG) formation. We observed that YTHDF proteins co-localize with human antigen R (HuR), a protein facilitating cap-independent translation, in SGs during early infection. Later in infection, YTHDFs are cleaved, suppressing SG formation. Notably, for the first time, we identified that during early infection CVB3's RNA-dependent RNA polymerase (3D) and double-stranded RNA (dsRNA) are stored in SGs, co-localizing with HuR. This early-stage sequestration likely protects viral components for use in late-phase replication, when SGs are disrupted due to YTHDF cleavage. In summary, our findings reveal that CVB3-induced m6A modifications enhance viral replication by regulating YTHDF-mediated SG dynamics. This study provides a potential therapeutic strategy for CVB3-induced myocarditis.

Lactylation of RNA m6A demethylase ALKBH5 promotes innate immune response to DNA herpesviruses and mpox virus

RNA N6-methyladenosine (m6A) demethylase AlkB homolog 5 (ALKBH5) plays a crucial role in regulating innate immunity. Lysine acylation, a widespread protein modification, influences protein function, but its impact on ALKBH5 during viral infections has not been well characterized. This study investigates the presence and regulatory mechanisms of a previously unidentified lysine acylation in ALKBH5 and its role in mediating m6A modifications to activate antiviral innate immune responses. We demonstrate that ALKBH5 undergoes lactylation, which is essential for an effective innate immune response against DNA herpesviruses, including herpes simplex virus type 1 (HSV-1), Kaposi's sarcoma-associated herpesvirus (KSHV), and mpox virus (MPXV). This lactylation attenuates viral replication. Mechanistically, viral infections enhance ALKBH5 lactylation by increasing its interaction with acetyltransferase ESCO2 and decreasing its interaction with deacetyltransferase SIRT6. Lactylated ALKBH5 binds interferon-beta (IFN-β) messenger RNA (mRNA), leading to demethylation of its m6A modifications and promoting IFN-β mRNA biogenesis. Overexpression of ESCO2 or depletion of SIRT6 further enhances ALKBH5 lactylation to strengthen IFN-β mRNA biogenesis. Our results identify a posttranslational modification of ALKBH5 and its role in regulating antiviral innate immune responses through m6A modification. The finding provides an understanding of innate immunity and offers a potential therapeutic target for HSV-1, KSHV, and MPXV infections.

HSV-1-induced N6-methyladenosine reprogramming via ICP0-mediated suppression of METTL14 potentiates oncolytic activity in glioma

Upon infection with herpes simplex virus 1 (HSV-1), the virus deploys multiple strategies to evade the host's innate immune response. However, the mechanisms governing this phenomenon remain elusive. Here, we find that HSV-1 leads to a decrease in overall m6A levels by selectively reducing METTL14 protein during early infection in glioma cells. Specifically, the HSV-1-encoded immediate-early protein ICP0 interacts with METTL14 within ND10 bodies and serves as an E3 ubiquitin protein ligase, targeting and ubiquitinating METTL14 at the lysine 156 and 162 sites. Subsequently, METTL14 undergoes proteasomal degradation. Furthermore, METTL14 stabilizes ISG15 mRNA mediated by IGF2BP3 to promote antiviral effects. Notably, METTL14 suppression significantly enhances the anti-tumor effect of oncolytic HSV-1 (oHSV-1) in mice bearing glioma xenografts. Collectively, these findings establish that ICP0-guided m6A modification controls the antiviral immune response and suggest that targeting METTL14/ISG15 represents a potential strategy to enhance the oncolytic activity of oHSV-1 in glioma treatment.

Transcriptome-wide N6-methyladenosine profiling reveals growth-defense trade-offs in the response of rice to brown planthopper (Nilaparvata lugens) infestation

BACKGROUND

N6-Methyladenosine (m6A) is a common messenger RNA (mRNA) modification that affects various physiological processes in stress responses. However, the role of m6A modifications in plants responses to herbivore stress remains unclear.

RESULTS

Here, we found that an infestation of brown planthopper (Nilaparvata lugens) female adults enhanced the resistance of rice to N. lugens. The m6A methylome analysis of N. lugens-infested and uninfested rice samples was performed to explore the interaction between rice and N. lugens. The m6A methylation mainly occurred in genes that were actively expressed in rice following N. lugens infestation, while an analysis of the whole-genomic mRNA distribution of m6A showed that N. lugens infestation caused an overall decrease in the number of m6A methylation sites across the chromosomes. The m6A methylation of genes involved in the m6A modification machinery and several defense-related phytohormones (jasmonic acid and salicylic acid) pathways was increased in N. lugens-infested rice compared to that in uninfested rice. In contrast, m6A modification levels of growth-related phytohormone (auxin and gibberellin) biosynthesis-related genes were significantly attenuated during N. lugens infestation, accompanied by the down-regulated expression of these transcripts, indicating that rice growth was restricted during N. lugens attack to rapidly optimize resource allocation for plant defense. Integrative analysis of the differential patterns of m6A methylation and the corresponding transcripts showed a positive correlation between m6A methylation and transcriptional regulation.

CONCLUSION

The m6A modification is an important strategy for regulating the expression of genes involved in rice defense and growth during rice-N. lugens interactions. These findings provide new ideas for formulating strategies to control herbivorous pests. © 2024 Society of Chemical Industry.

Differing Transcriptomic Responses in High Titer versus Low Titer Aedes aegypti Mosquitoes after Oral Infection with Sindbis Virus

Oral infection of mosquitoes by arboviruses often results in a large degree of variation in the amount of infectious virus between individual mosquitoes, even when the mosquitoes are from inbred laboratory strains. This variability in arbovirus load has been shown to affect virus transmissibility. Previously, our group described population genetic and specific infectivity differences between the virus populations found in high and low titer Aedes aegypti mosquitoes that had been orally infected with Sindbis virus (SINV). In this study, we sought to investigate whether there were also differences in transcriptomic response between these high and low titer mosquitoes. Results from the transcriptomic data analysis showed that more genes involved in antiviral activity, endopeptidase activity, and methyltransferase activity were upregulated in low titer mosquitoes than in high titer mosquitoes, relative to blood-fed controls. Meanwhile, genes involved in ion transport, energy metabolism, acetylation, glycosylation, lipid metabolism, and transport tended to be upregulated in high titer mosquitoes more than in low titer mosquitoes, relative to blood-fed mosquitoes. Overall, genes involved in antiviral activities tended to be upregulated in low titer mosquitoes while genes involved in proviral activities were mostly upregulated in high titer mosquitoes. This study has identified a number of candidate mosquito genes that are putatively associated with SINV titer variability after oral infection of Ae. aegypti, and these can now be investigated in order to ascertain their roles in virus replication and their contributions to determining vector competence.

Methyl-Transferase-Like Protein 16 (METTL16): The Intriguing Journey of a Key Epitranscriptomic Player Becoming an Emerging Biological Target

Key epitranscriptomic players have been increasingly characterized for their structural features and their involvement in several diseases. Accordingly, the design and synthesis of novel epitranscriptomic modulators have started opening a glimmer for drug discovery. m6A is a reversible modification occurring on a specific site and is catalyzed by three sets of proteins responsible for opposite functions. Writers (e.g., methyl-transferase-like protein (METTL) 3/METTL14 complex and METTL16) introduce the methyl group on adenosine N-6, by transferring the methyl group from the methyl donor S-adenosyl-methionine (SAM) to the substrate. Despite the rapidly advancing drug discovery progress on METTL3/METTL14, the METTL16 m6A writer has been marginally explored so far. We herein provide the first comprehensive overview of structural and biological features of METTL16, highlighting the state of the art in the field of its biological and structural characterization. We also showcase initial efforts in the identification of structural templates and preliminary structure-activity relationships for METTL16 modulators.

N6-Methyladenosine Positively Regulates Coxsackievirus B3 Replication

Enteroviruses such as coxsackievirus B3 are identified as a common cause of viral myocarditis, but the potential mechanism of its replication and pathogenesis are largely unknown. The genomes of a variety of viruses contain N6-methyladenosine (m6A), which plays important roles in virus replication. Here, by using the online bioinformatics tools SRAMP and indirect immunofluorescence assay (IFA), we predict that the CVB3 genome contains m6A sites and found that CVB3 infection could alter the expression and cellular localization of m6A-related proteins. Moreover, we found that 3-deazaadenosine (3-DAA), an m6A modification inhibitor, significantly decreased CVB3 replication. We also observed that the m6A methyltransferases methyltransferase-like protein 3 (METTL3) and METTL14 play positive roles in CVB3 replication, whereas m6A demethylases fat mass and obesity-associated protein (FTO) or AlkB homolog 5 (ALKBH5) have opposite effects. Knockdown of the m6A binding proteins YTH domain family protein 1 (YTHDF1), YTHDF2 and YTHDF3 strikingly decreased CVB3 replication. Finally, the m6A site mutation in the CVB3 genome decreased the replication of CVB3 compared with that in the CVB3 wild-type (WT) strain. Taken together, our results demonstrated that CVB3 could exploit m6A modification to promote viral replication, which provides new insights into the mechanism of the interaction between CVB3 and the host.

Discovery of a new inhibitor for YTH domain-containing m6A RNA readers

N 6-methyladenosine (m6A) is an abundant modification in mammalian mRNAs and plays important regulatory roles in gene expression, primarily mediated through specific recognition by "reader" proteins. YTH family proteins are one major family of known m6A readers, which specifically recognize m6A-modified transcripts via the YTH domains. Despite the significant relevance of YTH-m6A recognition in biology and diseases, few small molecule inhibitors are available for specifically perturbing this interaction. Here we report the discovery of a new inhibitor ("N-7") for YTH-m6A RNA recognition, from the screening of a nucleoside analogue library against the YTH domain of the YTHDF1 protein. N-7 is characterized to be a pan-inhibitor in vitro against five YTH domains from human YTHDF1, YTHDF2, YTHDF3, YTHDC1, and YTHDC2 proteins, with IC50 values in the range of 30-48 μM measured using a fluorescence polarization competition assay. We demonstrated that N-7 directly interacts with the YTH domain proteins via a thermal shift assay. N-7 expands the chemical structure landscape of the m6A YTH domain-containing reader inhibitors and potentiates future inhibitor development for reader functional studies and therapeutic efforts in targeting the epitranscriptome.

RNA modifications in insects

More than 100 RNA chemical modifications to cellular RNA have been identified. N 6-methyladenosine (m6A) is the most prevalent modification of mRNA. RNA modifications have recently attracted significant attention due to their critical role in regulating mRNA processing and metabolism. tRNA and rRNA rank among the most heavily modified RNAs, and their modifications are essential for maintaining their structure and function. With our advanced understanding of RNA modifications, increasing evidence suggests RNA modifications are important in regulating various aspects of insect life. In this review, we will summarize recent studies investigating the impact of RNA modifications in insects, particularly highlighting the role of m6A in insect development, reproduction, and adaptation to the environment.

The extensive m5C epitranscriptome of Thermococcus kodakarensis is generated by a suite of RNA methyltransferases that support thermophily

RNAs are often modified to invoke new activities. While many modifications are limited in frequency, restricted to non-coding RNAs, or present only in select organisms, 5-methylcytidine (m5C) is abundant across diverse RNAs and fitness-relevant across Domains of life, but the synthesis and impacts of m5C have yet to be fully investigated. Here, we map m5C in the model hyperthermophile, Thermococcus kodakarensis. We demonstrate that m5C is ~25x more abundant in T. kodakarensis than human cells, and the m5C epitranscriptome includes ~10% of unique transcripts. T. kodakarensis rRNAs harbor tenfold more m5C compared to Eukarya or Bacteria. We identify at least five RNA m5C methyltransferases (R5CMTs), and strains deleted for individual R5CMTs lack site-specific m5C modifications that limit hyperthermophilic growth. We show that m5C is likely generated through partial redundancy in target sites among R5CMTs. The complexity of the m5C epitranscriptome in T. kodakarensis argues that m5C supports life in the extremes.

Epitranscriptomic m5C methylation of SARS-CoV-2 RNA regulates viral replication and the virulence of progeny viruses in the new infection

While the significance of N6-methyladenosine (m6A) in viral regulation has been extensively studied, the functions of 5-methylcytosine (m5C) modification in viral biology remain largely unexplored. In this study, we demonstrate that m5C is more abundant than m6A in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and provide a comprehensive profile of the m5C landscape of SARS-CoV-2 RNA. Knockout of NSUN2 reduces m5C levels in SARS-CoV-2 virion RNA and enhances viral replication. Nsun2 deficiency mice exhibited higher viral burden and more severe lung tissue damages. Combined RNA-Bis-seq and m5C-MeRIP-seq identified the NSUN2-dependent m5C-methylated cytosines across the positive-sense genomic RNA of SARS-CoV-2, and the mutations of these cytosines enhance RNA stability. The progeny SARS-CoV-2 virions from Nsun2 deficiency mice with low levels of m5C modification exhibited a stronger replication ability. Overall, our findings uncover the vital role played by NSUN2-mediated m5C modification during SARS-CoV-2 replication and propose a host antiviral strategy via epitranscriptomic addition of m5C methylation to SARS-CoV-2 RNA.

The nucleolar phase of signal recognition particle assembly

The signal recognition particle is essential for targeting transmembrane and secreted proteins to the endoplasmic reticulum. Remarkably, because they work together in the cytoplasm, the SRP and ribosomes are assembled in the same biomolecular condensate: the nucleolus. How important is the nucleolus for SRP assembly is not known. Using quantitative proteomics, we have investigated the interactomes of SRP components. We reveal that SRP proteins are associated with scores of nucleolar proteins important for ribosome biogenesis and nucleolar structure. Having monitored the subcellular distribution of SRP proteins upon controlled nucleolar disruption, we conclude that an intact organelle is required for their proper localization. Lastly, we have detected two SRP proteins in Cajal bodies, which indicates that previously undocumented steps of SRP assembly may occur in these bodies. This work highlights the importance of a structurally and functionally intact nucleolus for efficient SRP production and suggests that the biogenesis of SRP and ribosomes may be coordinated in the nucleolus by common assembly factors.

Recent insights into N6-methyladenosine during viral infection

The RNA modification of N6-methyladenosine (m6A) controls many aspects of RNA function that impact biological processes, including viral infection. In this review, we highlight recent work that shapes our current understanding of the diverse mechanisms by which m6A can regulate viral infection by acting on viral or cellular mRNA molecules. We focus on emerging concepts and understanding, including how viral infection alters the localization and function of m6A machinery proteins, how m6A regulates antiviral innate immunity, and the multiple roles of m6A in regulating specific viral infections. We also summarize the recent studies on m6A during SARS-CoV-2 infection, focusing on points of convergence and divergence. Ultimately, this review provides a snapshot of the latest research on m6A during viral infection.

The YTH domain-containing protein family: Emerging players in immunomodulation and tumour immunotherapy targets

BACKGROUND

The modification of N6-methyladenosine (m6A) plays a pivotal role in tumor by altering both innate and adaptive immune systems through various pathways, including the regulation of messenger RNA. The YTH domain protein family, acting as "readers" of m6A modifications, affects RNA splicing, stability, and immunogenicity, thereby playing essential roles in immune regulation and antitumor immunity. Despite their significance, the impact of the YTH domain protein family on tumor initiation and progression, as well as their involvement in tumor immune regulation and therapy, remains underexplored and lacks comprehensive review.

CONCLUSION

This review introduces the molecular characteristics of the YTH domain protein family and their physiological and pathological roles in biological behavior, emphasizing their mechanisms in regulating immune responses and antitumor immunity. Additionally, the review discusses the roles of the YTH domain protein family in immune-related diseases and tumor resistance, highlighting that abnormal expression or dysfunction of YTH proteins is closely linked to tumor resistance.

KEY POINTS

This review provides an in-depth understanding of the YTH domain protein family in immune regulation and antitumor immunity, suggesting new strategies and directions for immunotherapy of related diseases. These insights not only deepen our comprehension of m6A modifications and YTH protein functions but also pave the way for future research and clinical applications.

Transcriptomic Signatures of Zika Virus Infection in Patients and a Cell Culture Model

Zika virus (ZIKV), a re-emerging flavivirus, is associated with devasting developmental and neurological disease outcomes particularly in infants infected in utero. Towards understanding the molecular underpinnings of the unique ZIKV disease pathologies, numerous transcriptome-wide studies have been undertaken. Notably, these studies have overlooked the assimilation of RNA-seq analysis from ZIKV-infected patients with cell culture model systems. In this study we find that ZIKV-infection of human lung adenocarcinoma A549 cells, mirrored both the transcriptional and alternative splicing profiles from previously published RNA-seq data of peripheral blood mononuclear cells collected from pediatric patients during early acute, late acute, and convalescent phases of ZIKV infection. Our analyses show that ZIKV infection in cultured cells correlates with transcriptional changes in patients, while the overlap in alternative splicing profiles was not as extensive. Overall, our data indicate that cell culture model systems support dissection of select molecular changes detected in patients and establishes the groundwork for future studies elucidating the biological implications of alternative splicing during ZIKV infection.

HCV 5-Methylcytosine Enhances Viral RNA Replication through Interaction with m5C Reader YBX1

Hepatitis C virus (HCV) is a positive-stranded RNA virus that mainly causes chronic hepatitis, cirrhosis and hepatocellular carcinoma. Recently we confirmed m5C modifications within NS5A gene of HCV RNA genome. However, the roles of the m5C modification and its interaction with host proteins in regulating HCV's life cycle, remain unexplored. Here, we demonstrate that HCV infection enhances the expression of the host m5C reader YBX1 through the transcription factor MAX. YBX1 acts as an m5C reader, recognizing the m5C-modified NS5A C7525 site in the HCV RNA genome and significantly enhancing HCV RNA stability. This m5C-modification is also required for YBX1 colocalization with lipid droplets and HCV Core protein. Moreover, YBX1 facilitates HCV RNA replication, as well as viral assembly/budding. The tryptophan residue at position 65 (W65) of YBX1 is critical for these functions. Knockout of YBX1 or the application of YBX1 inhibitor SU056 suppresses HCV RNA replication and viral protein translation. To our knowledge, this is the first report demonstrating that the interaction between host m5C reader YBX1 and HCV RNA m5C methylation facilitates viral replication. Therefore, hepatic-YBX1 knockdown holds promise as a potential host-directed strategy for HCV therapy.

Antibiotic-Induced Gut Microbiota Dysbiosis Modulates Host Transcriptome and m6A Epitranscriptome via Bile Acid Metabolism

Gut microbiota can influence host gene expression and physiology through metabolites. Besides, the presence or absence of gut microbiome can reprogram host transcriptome and epitranscriptome as represented by N6-methyladenosine (m6A), the most abundant mammalian mRNA modification. However, which and how gut microbiota-derived metabolites reprogram host transcriptome and m6A epitranscriptome remain poorly understood. Here, investigation is conducted into how gut microbiota-derived metabolites impact host transcriptome and m6A epitranscriptome using multiple mouse models and multi-omics approaches. Various antibiotics-induced dysbiotic mice are established, followed by fecal microbiota transplantation (FMT) into germ-free mice, and the results show that bile acid metabolism is significantly altered along with the abundance change in bile acid-producing microbiota. Unbalanced gut microbiota and bile acids drastically change the host transcriptome and the m6A epitranscriptome in multiple tissues. Mechanistically, the expression of m6A writer proteins is regulated in animals treated with antibiotics and in cultured cells treated with bile acids, indicating a direct link between bile acid metabolism and m6A biology. Collectively, these results demonstrate that antibiotic-induced gut dysbiosis regulates the landscape of host transcriptome and m6A epitranscriptome via bile acid metabolism pathway. This work provides novel insights into the interplay between microbial metabolites and host gene expression.

RNA N6-methyladenosine methylation in influenza A virus infection

Influenza A virus (IAV) is a negative-sense single-stranded RNA virus that causes acute lung injury and acute respiratory distress syndrome, posing a serious threat to both animal and human health. N6-methyladenosine (m6A), a prevalent and abundant post-transcriptional methylation of RNA in eukaryotes, plays a crucial regulatory role in IAV infection by altering viral RNA and cellular transcripts to affect viral infection and the host immune response. This review focuses on the molecular mechanisms underlying m6A modification and its regulatory function in the context of IAV infection and the host immune response. This will provide a better understanding of virus-host interactions and offer insights into potential anti-IAV strategies.

Elucidation of the Epitranscriptomic RNA Modification Landscape of Chikungunya Virus

The genomes of positive-sense (+) single-stranded RNA (ssRNA) viruses are believed to be subjected to a wide range of RNA modifications. In this study, we focused on the chikungunya virus (CHIKV) as a model (+) ssRNA virus to study the landscape of viral RNA modification in infected human cells. Among the 32 distinct RNA modifications analysed by mass spectrometry, inosine was found enriched in the genomic CHIKV RNA. However, orthogonal validation by Illumina RNA-seq analyses did not identify any inosine modification along the CHIKV RNA genome. Moreover, CHIKV infection did not alter the expression of ADAR1 isoforms, the enzymes that catalyse the adenosine to inosine conversion. Together, this study highlights the importance of a multidisciplinary approach to assess the presence of RNA modifications in viral RNA genomes.

The Functions of N-methyladenosine (m6A) Modification on HIV-1 mRNA

In recent years, there has been a growing interest in the study of RNA modifications, with some researchers focusing specifically on the connection between these modifications and viruses, as well as the impact they have on viral mRNA and its functionality. The most common type of RNA chemical modification is m6A, which involves the addition of a methyl group covalently to the N6 position of adenosine. It is a widely observed and evolutionarily conserved RNA modification. The regulation of m6A modification primarily involves methyltransferases (writers) and demethylases (erasers) and is mediated by m6A-binding proteins (readers). In HIV-1, m6A sites are predominantly located in the 5' untranslated region (5'UTR) and 3' untranslated region (3'UTR). Additionally, m6A modifications are also present in the RRE RNA of HIV-1. This review provides a detailed account of the effects of these m6A modifications on HIV-1 functionality.

RNA Binding by the m6A Methyltransferases METTL16 and METTL3

Methyltransferases are a wide-ranging, yet well-conserved, class of molecules that have been found to modify a wide variety of substrates. Interest in RNA methylation has surged in recent years with the identification of the major eukaryotic mRNA m6A methyltransferase METTL3. METTL16 has also been identified as an RNA m6A methyltransferase; however, much less is known about its targets and actions. Interestingly, in addition to their catalytic activities, both METTL3 and METTL16 also have "methylation-independent" functions, including translational regulation, which have been discovered. However, evidence suggests that METTL16's role as an RNA-binding protein may be more significant than is currently recognized. In this review, we will introduce RNA methylation, specifically m6A, and the enzymes responsible for its deposition. We will discuss the varying roles that these enzymes perform and delve deeper into their RNA targets and possible roles as methylation-independent RNA binding proteins. Finally, we will touch upon the many open questions still remaining.

m6A Reader YTHDC1 Impairs Respiratory Syncytial Virus Infection by Downregulating Membrane CX3CR1 Expression

Respiratory syncytial virus (RSV) is the most prevalent cause of acute lower respiratory infection in young children. Currently, the first RSV vaccines are approved by the FDA. Recently, N6-methyladenosine (m6A) RNA methylation has been implicated in the regulation of the viral life cycle and replication of many viruses, including RSV. m6A methylation of RSV RNA has been demonstrated to promote replication and prevent anti-viral immune responses by the host. Whether m6A is also involved in viral entry and whether m6A can also affect RSV infection via different mechanisms than methylation of viral RNA is poorly understood. Here, we identify m6A reader YTH domain-containing protein 1 (YTHDC1) as a novel negative regulator of RSV infection. We demonstrate that YTHDC1 abrogates RSV infection by reducing the expression of RSV entry receptor CX3C motif chemokine receptor 1 (CX3CR1) on the cell surface of lung epithelial cells. Altogether, these data reveal a novel role for m6A methylation and YTHDC1 in the viral entry of RSV. These findings may contribute to the development of novel treatment options to control RSV infection.

Dengue virus pathogenesis and host molecular machineries

Dengue viruses (DENV) are positive-stranded RNA viruses belonging to the Flaviviridae family. DENV is the causative agent of dengue, the most rapidly spreading viral disease transmitted by mosquitoes. Each year, millions of people contract the virus through bites from infected female mosquitoes of the Aedes species. In the majority of individuals, the infection is asymptomatic, and the immune system successfully manages to control virus replication within a few days. Symptomatic individuals may present with a mild fever (Dengue fever or DF) that may or may not progress to a more critical disease termed Dengue hemorrhagic fever (DHF) or the fatal Dengue shock syndrome (DSS). In the absence of a universally accepted prophylactic vaccine or therapeutic drug, treatment is mostly restricted to supportive measures. Similar to many other viruses that induce acute illness, DENV has developed several ways to modulate host metabolism to create an environment conducive to genome replication and the dissemination of viral progeny. To search for new therapeutic options, understanding the underlying host-virus regulatory system involved in various biological processes of the viral life cycle is essential. This review aims to summarize the complex interaction between DENV and the host cellular machinery, comprising regulatory mechanisms at various molecular levels such as epigenetic modulation of the host genome, transcription of host genes, translation of viral and host mRNAs, post-transcriptional regulation of the host transcriptome, post-translational regulation of viral proteins, and pathways involved in protein degradation.

Challenges to mapping and defining m6A function in viral RNA

Viral RNA molecules contain multiple layers of regulatory information. This includes features beyond the primary sequence, such as RNA structures and RNA modifications, including N6-methyladenosine (m6A). Many recent studies have identified the presence and location of m6A in viral RNA and have found diverse regulatory roles for this modification during viral infection. However, to date, viral m6A mapping strategies have limitations that prevent a complete understanding of the function of m6A on individual viral RNA molecules. While m6A sites have been profiled on bulk RNA from many viruses, the resulting m6A maps of viral RNAs described to date present a composite picture of m6A across viral RNA molecules in the infected cell. Thus, for most viruses, it is unknown if unique viral m6A profiles exist throughout infection, nor if they regulate specific viral life cycle stages. Here, we describe several challenges to defining the function of m6A in viral RNA molecules and provide a framework for future studies to help in the understanding of how m6A regulates viral infection.

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.

N6-methyladenosine modification is not a general trait of viral RNA genomes

Despite the nuclear localization of the m6A machinery, the genomes of multiple exclusively-cytoplasmic RNA viruses, such as chikungunya (CHIKV) and dengue (DENV), are reported to be extensively m6A-modified. However, these findings are mostly based on m6A-Seq, an antibody-dependent technique with a high rate of false positives. Here, we address the presence of m6A in CHIKV and DENV RNAs. For this, we combine m6A-Seq and the antibody-independent SELECT and nanopore direct RNA sequencing techniques with functional, molecular, and mutagenesis studies. Following this comprehensive analysis, we find no evidence of m6A modification in CHIKV or DENV transcripts. Furthermore, depletion of key components of the host m6A machinery does not affect CHIKV or DENV infection. Moreover, CHIKV or DENV infection has no effect on the m6A machinery's localization. Our results challenge the prevailing notion that m6A modification is a general feature of cytoplasmic RNA viruses and underscore the importance of validating RNA modifications with orthogonal approaches.

Decoding m6A mRNA methylation by reader proteins in liver diseases

N6-methyladenosine (m6A) is a dynamic and reversible epigenetic regulation. As the most prevalent internal post-transcriptional modification in eukaryotic RNA, it participates in the regulation of gene expression through various mechanisms, such as mRNA splicing, nuclear export, localization, translation efficiency, mRNA stability, and structural transformation. The involvement of m6A in the regulation of gene expression depends on the specific recognition of m6A-modified RNA by reader proteins. In the pathogenesis and treatment of liver disease, studies have found that the expression levels of key genes that promote or inhibit the development of liver disease are regulated by m6A modification, in which abnormal expression of reader proteins determines the fate of these gene transcripts. In this review, we introduce m6A readers, summarize the recognition and regulatory mechanisms of m6A readers on mRNA, and focus on the biological functions and mechanisms of m6A readers in liver cancer, viral hepatitis, non-alcoholic fatty liver disease (NAFLD), hepatic fibrosis (HF), acute liver injury (ALI), and other liver diseases. This information is expected to be of high value to researchers deciphering the links between m6A readers and human liver diseases.

The Significance of N6-Methyladenosine RNA Methylation in Regulating the Hepatitis B Virus Life Cycle

N6-methyladenosine (m6A) RNA methylation has recently emerged as a significant co-transcriptional modification involved in regulating various RNA functions. It plays a vital function in numerous biological processes. Enzymes referred to as m6A methyltransferases, such as the methyltransferaselike (METTL) 3-METTL14-Wilms tumor 1 (WT1)-associated protein (WTAP) complex, are responsible for adding m6A modifications, while m6A demethylases, including fat mass and obesity-associated protein (FTO) and alkB homolog 5 (ALKBH5), can remove m6A methylation. The functions of m6A-methylated RNA are regulated through the recognition and interaction of m6A reader proteins. Recent research has shown that m6A methylation takes place at multiple sites within hepatitis B virus (HBV) RNAs, and the location of these modifications can differentially impact the HBV infection. The addition of m6A modifications to HBV RNA can influence its stability and translation, thereby affecting viral replication and pathogenesis. Furthermore, HBV infection can also alter the m6A modification pattern of host RNA, indicating the virus's ability to manipulate host cellular processes, including m6A modification. This manipulation aids in establishing chronic infection, promoting liver disease, and contributing to pathogenesis. A comprehensive understanding of the functional roles of m6A modification during HBV infection is crucial for developing innovative approaches to combat HBV-mediated liver disease. In this review, we explore the functions of m6A modification in HBV replication and its impact on the development of liver disease.

N6-methyladenosine modification of the mRNA for a key gene in purine nucleotide metabolism regulates virus proliferation in an insect vector

The titer of viruses that persist and propagate in their insect vector must be high enough for transmission yet not harm the insect, but the mechanism of this dynamic balance is unclear. Here, expression of inosine monophosphate dehydrogenase (LsIMPDH), a rate-limiting enzyme for guanosine triphosphate (GTP) synthesis, is shown to be downregulated by increased levels of N6-methyladenosine (m6A) on LsIMPDH mRNA in rice stripe virus (RSV)-infected small brown planthoppers (SBPHs; Laodelphax striatellus), the RSV vector, which decreases GTP content, thus limiting viral proliferation. Moreover, planthopper methyltransferase-like protein 3 (LsMETTL3) and m6A reader protein LsYTHDF3 are found to catalyze and recognize the m6A on LsIMPDH mRNA, respectively, and cooperate in destabilizing LsIMPDH transcripts. Co-silencing assays show that negative regulation of viral proliferation by both LsMETTL3 and LsYTHDF3 is partially dependent on LsIMPDH. This distinct mechanism limits virus replication in an insect vector, providing a potential gene target to block viral transmission.

Epigenetic Restriction Factors (eRFs) in Virus Infection

The ongoing arms race between viruses and their hosts is constantly evolving. One of the ways in which cells defend themselves against invading viruses is by using restriction factors (RFs), which are cell-intrinsic antiviral mechanisms that block viral replication and transcription. Recent research has identified a specific group of RFs that belong to the cellular epigenetic machinery and are able to restrict the gene expression of certain viruses. These RFs can be referred to as epigenetic restriction factors or eRFs. In this review, eRFs have been classified into two categories. The first category includes eRFs that target viral chromatin. So far, the identified eRFs in this category include the PML-NBs, the KRAB/KAP1 complex, IFI16, and the HUSH complex. The second category includes eRFs that target viral RNA or, more specifically, the viral epitranscriptome. These epitranscriptomic eRFs have been further classified into two types: those that edit RNA bases-adenosine deaminase acting on RNA (ADAR) and pseudouridine synthases (PUS), and those that covalently modify viral RNA-the N6-methyladenosine (m6A) writers, readers, and erasers. We delve into the molecular machinery of eRFs, their role in limiting various viruses, and the mechanisms by which viruses have evolved to counteract them. We also examine the crosstalk between different eRFs, including the common effectors that connect them. Finally, we explore the potential for new discoveries in the realm of epigenetic networks that restrict viral gene expression, as well as the future research directions in this area.

N-acetyltransferase 10 regulates alphavirus replication via N4-acetylcytidine (ac4C) modification of the lymphocyte antigen six family member E (LY6E) mRNA

Epitranscriptomic RNA modifications can regulate the stability of mRNA and affect cellular and viral RNA functions. The N4-acetylcytidine (ac4C) modification in the RNA viral genome was recently found to promote viral replication; however, the mechanism by which RNA acetylation in the host mRNA regulates viral replication remains unclear. To help elucidate this mechanism, the roles of N-acetyltransferase 10 (NAT10) and ac4C during the infection and replication processes of the alphavirus, Sindbis virus (SINV), were investigated. Cellular NAT10 was upregulated, and ac4C modifications were promoted after alphavirus infection, while the loss of NAT10 or inhibition of its N-acetyltransferase activity reduced alphavirus replication. The NAT10 enhanced alphavirus replication as it helped to maintain the stability of lymphocyte antigen six family member E mRNA, which is a multifunctional interferon-stimulated gene that promotes alphavirus replication. The ac4C modification was thus found to have a non-conventional role in the virus life cycle through regulating host mRNA stability instead of viral mRNA, and its inhibition could be a potential target in the development of new alphavirus antivirals.IMPORTANCEThe role of N4-acetylcytidine (ac4C) modification in host mRNA and virus replication is not yet fully understood. In this study, the role of ac4C in the regulation of Sindbis virus (SINV), a prototype alphavirus infection, was investigated. SINV infection results in increased levels of N-acetyltransferase 10 (NAT10) and increases the ac4C modification level of cellular RNA. The NAT10 was found to positively regulate SINV infection in an N-acetyltransferase activity-dependent manner. Mechanistically, the NAT10 modifies lymphocyte antigen six family member E (LY6E) mRNA-the ac4C modification site within the 3'-untranslated region (UTR) of LY6E mRNA, which is essential for its translation and stability. The findings of this study demonstrate that NAT10 regulated mRNA stability and translation efficiency not only through the 5'-UTR or coding sequence but also via the 3'-UTR region. The ac4C modification of host mRNA stability instead of viral mRNA impacting the viral life cycle was thus identified, indicating that the inhibition of ac4C could be a potential target when developing alphavirus antivirals.

N6-methyladenosine modification positively regulate Japanese encephalitis virus replication

N6-methyladenosine (m6A) is present in diverse viral RNA and plays important regulatory roles in virus replication and host antiviral innate immunity. However, the role of m6A in regulating JEV replication has not been investigated. Here, we show that the JEV genome contains m6A modification upon infection of mouse neuroblast cells (neuro2a). JEV infection results in a decrease in the expression of m6A writer METTL3 in mouse brain tissue. METTL3 knockdown by siRNA leads to a substantial decrease in JEV replication and the production of progeny viruses at 48 hpi. Mechanically, JEV triggered a considerable increase in the innate immune response of METTL3 knockdown neuro2a cells compared to the control cells. Our study has revealed the distinctive m6A signatures of both the virus and host in neuro2a cells infected with JEV, illustrating the positive role of m6A modification in JEV infection. Our study further enhances understanding of the role of m6A modification in Flaviviridae viruses.

A bibliometric analysis of m6A methylation in viral infection from 2000 to 2022

BACKGROUND

N6-methyladenosine (m6A) methylation has become an active research area in viral infection, while little bibliometric analysis has been performed. In this study, we aim to visualize hotspots and trends using bibliometric analysis to provide a comprehensive and objective overview of the current research dynamics in this field.

METHODS

The data related to m6A methylation in viral infection were obtained through the Web of Science Core Collection form 2000 to 2022. To reduce bias, the literature search was conducted on December 1, 2022. Bibliometric and visual analyzes were performed using CiteSpace and Bibliometrix package. After screening, 319 qualified records were retrieved.

RESULTS

These publications mainly came from 28 countries led by China and the United States (the US), with the US ranking highest in terms of total link strength.The most common keywords were m6A, COVID-19, epitranscriptomics, METTL3, hepatitis B virus, innate immunity and human immunodeficiency virus 1. The thematic map showed that METTL3, plant viruses, cancer progression and type I interferon (IFN-I) reflected a good development trend and might become a research hotspot in the future, while post-transcriptional modification, as an emerging or declining theme, might not develop well.

CONCLUSIONS

In conclusion, m6A methylation in viral infection is an increasingly important topic in articles. METTL3, plant viruses, cancer progression and IFN-I may still be research hotspots and trends in the future.

Transcriptomic evaluation of N6-methyladenosine modification can be used to identify differentially gene and immune-related biological processes in TX mice with liver fibrosis

BACKGROUND

N6-methyladenosine (m6A) modification controls the stability, splicing, and translation of mRNA, which is important in the development of illnesses. Wilson's disease (WD) is an autosomal recessive liver copper metabolic disorder that causes liver fibrosis. The role of m6A methylation in WD-induced liver fibrosis development is still unclear. Thus, the goal of this study was to examine the scope of m6A methylation and further explore the potential targets related to WD-induced liver fibrosis.

RESULTS

A total of 1930 significantly different m6A peaks were found on 1737 mRNAs, of which 993 were hypermethylated and 744 were hypomethylated when comparing normal and WD-induced liver fibrosis mice (n = 3). In parallel, 1261 differentially expressed mRNAs, comprising 557 upregulated and 704 downregulated mRNAs, were found. Overall, 114 mRNAs with significant changes in m6A levels and RNA expression were identified via joint analysis. Then, through PPI network construction and functional enrichment analysis, 12 hub genes were identified, these genes were mainly enriched in the inflammatory response and immunomodulation, and they are associated with immune cell infiltration.

CONCLUSIONS

The significant difference in the amount of mRNA m6A modifications indicates that m6A modification is involved in the progression of WD-induced liver fibrosis, and theidentified hub genes are involved in inflammation and immune infiltration. These results may provide insights for subsequent studies on potential regulatory mechanisms.