Evidence for the ancient origin of the NF-κB/IκB cascade:: Its archaic role in pathogen infection and immunity

The evolutionary conservation of the NF-kappa B transcription factors, from Drosophila to humans, underscores its pivotal role in immune response. Unexpectedly, the canonical NF-kappa B signaling pathway is not functional in the immune system of Caenorhabditis elegans. Therefore, the ancient origin of the NF-kappa B signaling pathway is still unknown. Here, we report the discovery and characterization of a primitive and functional NF-kappa B/I kappa B pathway in the immune defense of a living fossil, the horseshoe crab, Carcinoscorpius rotundicauda. The ancient NF-kappa B/I kappa B homologues, CrNF kappa B, CrRelish, and CrI kappa B, share numerous signature motifs with their vertebrate orthologues. CrNF kappa B recognizes both horseshoe crab and mammalian KB response elements. CrI kappa B interacts with CrNF kappa B and inhibits its nuclear translocation and DNA-binding activity. The activation of the CrNF kappa B is autoregulated by a feedback mechanism mediated by CrI kappa B, the natural inhibitor of CrNF kappa B. We further show that Gram-negative bacteria infection causes rapid degradation of CrI kappa B and nuclear translocation of CrNF kappa B. Infection also leads to an increase in the kappa B-binding activity and up-regulation of immune-related gene expression, like inducible nitric oxide synthase and Factor C, an LPS-activated serine protease. Altogether, our study shows that, although absent in C. elegans, the NF-kappa B/I kappa B signaling cascade remains well conserved from horseshoe crab to humans, playing an archaic but fundamental role in regulating the expression of critical immune defense molecules.