Abstract
The evolutionary conservation of the NF-kappaB transcription factors, from Drosophila to humans, underscores its pivotal role in immune response. Unexpectedly, the canonical NF-kappaB signaling pathway is not functional in the immune system of Caenorhabditis elegans. Therefore, the ancient origin of the NF-kappaB signaling pathway is still unknown. Here, we report the discovery and characterization of a primitive and functional NF-kappaB/IkappaB pathway in the immune defense of a "living fossil," the horseshoe crab, Carcinoscorpius rotundicauda. The ancient NF-kappaB/IkappaB homologues, CrNFkappaB, CrRelish, and CrIkappaB, share numerous signature motifs with their vertebrate orthologues. CrNFkappaB recognizes both horseshoe crab and mammalian kappaB response elements. CrIkappaB interacts with CrNFkappaB and inhibits its nuclear translocation and DNA-binding activity. The activation of the CrNFkappaB is autoregulated by a feedback mechanism mediated by CrIkappaB, the natural inhibitor of CrNFkappaB. We further show that Gram-negative bacteria infection causes rapid degradation of CrIkappaB and nuclear translocation of CrNFkappaB. Infection also leads to an increase in the kappaB-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-kappaB/IkappaB 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.