The innate immune system is a broad collection of critical intra‐ and extra‐cellular processes that limit the infectivity of diverse pathogens. The 2′‐5′‐oligoadenylate synthetase (OAS) family of enzymes are important sensors of cytosolic double‐stranded RNA (dsRNA) that play a critical role in limiting viral infection by activating the latent ribonuclease (RNase L) to halt viral replication and establish an antiviral state. Attesting to the importance of the OAS/RNase L pathway, diverse viruses have developed numerous distinct strategies to evade the effects of OAS activation. How OAS proteins are regulated by viral or cellular RNAs is not fully understood but several recent studies have provided important new insights into the molecular mechanisms of OAS activation by dsRNA. Other studies have revealed unanticipated features of RNA sequence and structure that strongly enhance activation of at least one OAS family member. While these discoveries represent important advances, they also underscore the fact that much remains to be learned about RNA‐mediated regulation of the OAS/RNase L pathway. In particular, defining the full complement of RNA molecular signatures that activate OAS is essential to our understanding of how these proteins maximize their protective role against pathogens while still accurately discriminating host molecules to avoid inadvertent activation by cellular RNAs. A more complete knowledge of OAS regulation may also serve as a foundation for the development of novel antiviral therapeutic strategies and lead the way to a deeper understanding of currently unappreciated cellular functions of the OAS/RNase L pathway in the absence of infection. This article is categorized under: RNA in Disease and Development > RNA in Disease RNA Interactions with Proteins and Other Molecules > Protein–RNA Interactions: Functional Implications Translation > Translation Regulation

Background: Adenoviruses use the short non-coding transcript VA RNA I to inhibit host antiviral kinase PKR. Results: VA RNA I contains a pH-and Mg 2ϩ -sensitive tertiary structure that, unexpectedly, is not required for PKR inhibition. Conclusion: Structural requirements for an RNA inhibitor of PKR are simpler than appreciated previously. Significance: These findings explain how non-coding RNAs of varied sequence and structure can efficiently inhibit PKR.

Analysis of autoinflammatory and immunodeficiency disorders elucidates human immunity and fosters the development of targeted therapies. Oligoadenylate synthetase 1 is a type I interferon–induced, intracellular double-stranded RNA (dsRNA) sensor that generates 2′-5′-oligoadenylate to activate ribonuclease L (RNase L) as a means of antiviral defense. We identified four de novo heterozygous OAS1 gain-of-function variants in six patients with a polymorphic autoinflammatory immunodeficiency characterized by recurrent fever, dermatitis, inflammatory bowel disease, pulmonary alveolar proteinosis, and hypogammaglobulinemia. To establish causality, we applied genetic, molecular dynamics simulation, biochemical, and cellular functional analyses in heterologous, autologous, and inducible pluripotent stem cell–derived macrophages and/or monocytes and B cells. We found that upon interferon-induced expression, OAS1 variant proteins displayed dsRNA-independent activity, which resulted in RNase L–mediated RNA cleavage, transcriptomic alteration, translational arrest, and dysfunction and apoptosis of monocytes, macrophages, and B cells. RNase L inhibition with curcumin modulated and allogeneic hematopoietic cell transplantation cured the disorder. Together, these data suggest that human OAS1 is a regulator of interferon-induced hyperinflammatory monocyte, macrophage, and B cell pathophysiology.

2′-5′-Oligoadenylate synthetases (OAS) are innate immune sensors of cytosolic double-stranded RNA (dsRNA) and play a critical role in limiting viral infection. dsRNA binding induces allosteric structural changes in OAS1 that reorganize its catalytic center to promote synthesis of 2′-5′-oligoadenylate and thus activation of endoribonuclease L. Specific RNA sequences and structural motifs can also enhance activation of OAS1 through currently undefined mechanisms. To better understand these drivers of OAS activation, we tested the impact of defined sequence changes within a short dsRNA that strongly activates OAS1. Both in vitro and in human A549 cells, appending a 3′-end single-stranded pyrimidine (3′-ssPy) can strongly enhance OAS1 activation or have no effect depending on its location, suggesting that other dsRNA features are necessary for correct presentation of the motif to OAS1. Consistent with this idea, we also find that the dsRNA binding position is dictated by an established consensus sequence (WWN9WG). Unexpectedly, however, not all sequences fitting this consensus activate OAS1 equivalently, with strong dependence on the identity of both partially conserved (W) and non-conserved (N9) residues. A picture thus emerges in which both specific RNA features and the context in which they are presented dictate the ability of short dsRNAs to activate OAS1.

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