Genome-wide identification, expression analysis and antiviral function of RIG-I-like receptors (RLRs) in basal...

The retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) serve as critical cytoplasmic RNA sensors and play key roles in antiviral innate immunity. However, the distribution, evolutionary dynamics, and antiviral functions of RLRs in basal chordates remain poorly understood. In this study, we investigated the distribution and evolutionary patterns of RLRs across species ranging from single-celled eukaryotes to mammals, and further examined their antiviral innate immune responses in the basal chordate Branchiostoma japonicum. Our results demonstrate that RLRs are widely present in metazoans from sponges to mammals: two subfamilies (RIG-I and Melanoma Differentiation-Associated protein 5, MDA5) in invertebrate and three subfamilies (RIG-I, MDA5 and Laboratory of Genetics and Physiology 2, LGP2) in vertebrate, with evolutionary features including gene duplication and loss, domain rearrangement, and positive selection in the evolutionary history. Acute immune challenge experiments revealed that expression of RLRs in B. japonicum was significantly altered following infection with poly(I:C), LPS and poly(dA:dT). Subcellular localization analysis indicated that both BjRIG-Ia and BjMDA5a are broadly distributed in the cytoplasm. Overexpression experiments showed that overexpression of BjMDA5a can upregulate the expression of several RLR signaling pathway genes, such as DHX58, TBK1, IFNα, IRF3, IRF7, ISG15, ATG12, and TNFα. In contrast, overexpression of BjRIG-Ia significantly downregulated the expression of DHX58, ATG12, IκBα, IRF3, IRF7, ISG15, p65, and p50. Given the established role of MDA5 in antiviral immunity, crystal violet staining assays demonstrated that BjMDA5a inhibited the proliferation of spring viremia of carp virus (SVCV) at the cellular level in a dose-dependent manner. This study provides valuable insights into the origin and evolution of antiviral regulatory networks in chordates.