Sensing of double-stranded RNA in human cells: molecular mechanisms and cellular consequences.

Double-stranded RNA (dsRNA) is a universal indicator of viral replication and dysregulated RNA metabolism. Detection of dsRNA triggers some of the most powerful innate immune responses in human cells. Although these molecules differ in origin and structure, viral dsRNAs share the defining geometric and electrostatic features of the A-form helix, enabling their sequence-independent recognition by multiple sensor systems. Cytosolic receptors, like retinoic acid-inducible gene I (RIG-I), melanoma differentiation associated gene 5 (MDA5), and protein kinase R (PKR), as well as the oligoadenylate synthase (OAS)/RNase L pathway, convert dsRNA binding into interferon induction, translational arrest, and widespread RNA decay, while endosomal Toll-like receptor 3 (TLR3) and the inflammasome sensor NLR family pyrin domain containing 1 (NLRP1) expand surveillance to internalised or structurally disruptive RNAs. Counterbalancing these pathways, the RNA-editing enzyme adenosine deaminase acting on RNA 1 (ADAR1) marks endogenous dsRNA through A-to-I conversion, preventing inadvertent activation of innate immune response and maintaining self versus non-self discrimination. Although all of these sensors recognise the A-form helix, each extracts distinct structural and chemical information from dsRNA and converts it into a specific response: RIG-I detects short duplexes with 5'-triphosphorylated ends; MDA5 assembles cooperatively along long uninterrupted helices; PKR integrates duplex length with translational control; OAS proteins act as strict reporters of helix regularity; and TLR3 as well as NLRP1 respond to dsRNA in compartment- and context-dependent ways. Epitranscriptomic marks and chemical modifications-including 2'-O-methylation, N6-methyladenosine, pseudouridine, and ADAR1-mediated inosine-further refine sensing by modulating helical stability and end structure, establishing a biochemical 'self-code' that shapes RNA immunogenicity. Together, these pathways form an integrated network that distinguishes between viral and endogenous dsRNA and coordinates antiviral defence with immune tolerance.