Zebrafish IκB kinase 1 negatively regulates NF-κB activity

The I kappa B kinase (IKK) activity is critical for processing I kappa B inhibitory proteins and activating the NF-kappa B signaling, which is involved in a series of physiological and developmental steps in vertebrates [1-4]. The IKK activity resides in two catalytic subunits, IKK1 and IKK2, and two regulatory subunits, NEMO and ELKS [5-8]. IKK2 is the major cytokine-responsive I kappa B kinase [9-11] because depletion of IKK1 does not interfere with the IKK activity [12-14]. In fact, IKK1(-/-) mice display morphological abnormalities that are independent of its kinase activity and NF-kappa B activation [12-14]. Hence, using zebrafish (Danio rerio) as a model, we examined the evolutionary role of IKK1 in modulating NF-kappa B. Ikk1(-/-) zebrafish embryos present head and tail malformations and, surprisingly, show upregulation of NF-kappa B-responsive genes and increased NF-kappa B-dependent apoptosis. Overexpression of ikk1 leads to midline structure defects that resemble NF-kappa B blockage in vivo [1]. Zebrafish Ikk1 forms complexes with NEMO that represses NF-kappa B in vertebrate cells. Indeed, truncation of its NEMO binding domain (NBD) restores NF-kappa B-dependent transcriptional activity and, consequently, the ikk1-overexpressing phenotype. Here, we report that Ikk1 negatively regulates NF-kappa B by sequestering NEMO from active IKK complexes, indicating that IKK1 can function as a repressor of NF-kappa B.