N⁶-methyladenosine (m⁶A) is an abundant internal RNA modification, influencing transcript fate and function in uninfected and virus-infected cells. Installation of m⁶A by the nuclear RNA methyltransferase METTL3 occurs cotranscriptionally; however, the genomes of some cytoplasmic RNA viruses are also m⁶A-modified. How the cellular m⁶A modification machinery impacts coronavirus replication, which occurs exclusively in the cytoplasm, is unknown. Here we show that replication of SARS-CoV-2, the agent responsible for the COVID-19 pandemic, and a seasonal human β-coronavirus HCoV-OC43, can be suppressed by depletion of METTL3 or cytoplasmic m⁶A reader proteins YTHDF1 and YTHDF3 and by a highly specific small molecule METTL3 inhibitor. Reduction of infectious titer correlates with decreased synthesis of viral RNAs and the essential nucleocapsid (N) protein. Sites of m⁶A modification on genomic and subgenomic RNAs of both viruses were mapped by methylated RNA immunoprecipitation sequencing (meRIP-seq). Levels of host factors involved in m⁶A installation, removal, and recognition were unchanged by HCoV-OC43 infection; however, nuclear localization of METTL3 and cytoplasmic m⁶A readers YTHDF1 and YTHDF2 increased. This establishes that coronavirus RNAs are m⁶A-modified and host m⁶A pathway components control β-coronavirus replication. Moreover, it illustrates the therapeutic potential of targeting the m⁶A pathway to restrict coronavirus reproduction.

Platelet-derived MRP8/14 amplifies endotheliopathy in COVID-19.

Poly(A)-binding protein 1 (PABP1) has a fundamental role in the regulation of mRNA translation and stability, both of which are crucial for a wide variety of cellular processes. Although generally a diffuse cytoplasmic protein, it can be found in discrete foci such as stress and neuronal granules. Mammals encode several additional cytoplasmic PABPs that remain poorly characterised, and with the exception of PABP4, appear to be restricted in their expression to a small number of cell types. We have found that PABP4, similarly to PABP1, is a diffusely cytoplasmic protein that can be localised to stress granules. However, UV exposure unexpectedly relocalised both proteins to the nucleus. Nuclear relocalisation of PABPs was accompanied by a reduction in protein synthesis but was not linked to apoptosis. In examining the mechanism of PABP relocalisation, we found that it was related to a change in the distribution of poly(A) RNA within cells. Further investigation revealed that this change in RNA distribution was not affected by PABP knockdown but that perturbations that block mRNA export recapitulate PABP relocalisation. Our results support a model in which nuclear export of PABPs is dependent on ongoing mRNA export, and that a block in this process following UV exposure leads to accumulation of cytoplasmic PABPs in the nucleus. These data also provide mechanistic insight into reports that transcriptional inhibitors and expression of certain viral proteins cause relocation of PABP to the nucleus.

As genetic information is transmitted through successive generations, it passes between pluripotent cells in the early embryo and germ cells in the developing foetus and adult animal. Tex19.1 encodes a protein of unknown function, whose expression is restricted to germ cells and pluripotent cells. During male spermatogenesis, Tex19.1 expression is highest in mitotic spermatogonia and diminishes as these cells differentiate and progress through meiosis. In pluripotent stem cells, Tex19.1 expression is also downregulated upon differentiation. However, it is not clear whether Tex19.1 has an essential function in germ cells or pluripotent stem cells, or what that function might be. To analyse the potential role of Tex19.1 in pluripotency or germ cell function we have generated Tex19.1^−/− knockout mice and analysed the Tex19.1^−/− mutant phenotype. Adult Tex19.1^−/− knockout males exhibit impaired spermatogenesis. Immunostaining and histological analysis revealed defects in meiotic chromosome synapsis, the persistence of DNA double-strand breaks during meiosis, and a loss of post-meiotic germ cells in the testis. Furthermore, expression of a class of endogenous retroviruses is upregulated during meiosis in the Tex19.1^−/− testes. Increased transposition of endogenous retroviruses in the germline of Tex19.1^−/− mutant mice, and the concomitant increase in DNA damage, may be sufficient to disrupt the normal processes of recombination and chromosome synapsis during meiosis and cause defects in spermatogenesis. Our results suggest that Tex19.1 is part of a specialised mechanism that operates in the germline to repress transposable genetic elements and maintain genomic stability through successive generations. The germ cells—eggs in females and sperm in males—are responsible for passing genetic information from one generation to the next. As any genetic changes that arise in the germ cells can be transmitted to the next generation, germ cells are a prime target for the activity of mobile genetic elements. Mobile genetic elements make up around 40% of a mammalian genome, and many of these elements are derived from retroviruses that have infected germ cells, or early embryonic precursors to germ cells, and have integrated into the genome. Here, we characterise the function of Tex19.1, a gene whose expression is restricted to germ cells and the pluripotent cells that are early embryonic precursors to germ cells. We show that when Tex19.1 is deleted from mice, germ cells have problems progressing through meiosis, and sperm production is impaired. Furthermore, we show that, in the absence of Tex19.1, endogenous retroviruses are activated in male germ cells attempting to go through meiosis. Our results suggest that Tex19.1 is part of a specialised mechanism that guards against mutagenic endogenous retrovirus activity in germ cells and pluripotent cells and thus helps to maintain the integrity and stability of the genome through successive generations.

The regulation of translation has emerged as a major determinant of gene expression and is critical for both normal cellular function and the development of disease. Numerous studies have highlighted the diverse, and sometimes related, mechanisms which underlie the regulation of global translation rates and the translational control of specific mRNAs. In the present paper, we discuss the emerging roles of the basal translation factor PABP [poly(A)-binding protein] in mRNA-specific translational control in metazoa which suggest that PABP function is more complex than first recognized.