Lost and Found: The Family of NF-κB Inhibitors Is Larger than Assumed in Salmonid Fish

NF-kappa B signalling is largely controlled by the family of 'inhibitors of NF-kappa B' (I kappa B). The relevant databases indicate that the genome of rainbow trout contains multiple gene copies coding for i kappa b alpha (nfkbia), i kappa b delta (nfkbie), i kappa b delta (nkfbid), i kappa b zeta (nfkbiz), and bcl3, but it lacks i kappa b beta (nfkbib) and i kappa b eta (ankrd42). Strikingly, three nfkbia paralogs are apparently present in salmonid fish, two of which share a high sequence identity, while the third putative nfkbia gene is significantly less like its two paralogs. This particular nfkbia gene product, i kappa b zeta, clusters with the human I kappa B alpha in a phylogenetic analysis, while the other two i kappa b alpha proteins from trout associate with their human I kappa B alpha counterpart. The transcript concentrations were significantly higher for the structurally more closely related nfkbia paralogs than for the structurally less similar paralog, suggesting that i kappa b beta probably has not been lost from the salmonid genomes but has been incorrectly designated as i kappa b alpha. In the present study, two gene variants coding for i kappa b alpha (nfkbia) and i kappa b epsilon (nfkbie) were prominently expressed in the immune tissues and, particularly, in a cell fraction enriched with granulocytes, monocytes/macrophages, and dendritic cells from the head kidney of rainbow trout. Stimulation of salmonid CHSE-214 cells with zymosan significantly upregulated the i kappa b alpha-encoding gene while elevating the copy numbers of the inflammatory markers interleukin-1-beta and interleukin-8. Overexpression of i kappa b alpha and i kappa b epsilon in CHSE-214 cells dose-dependently quenched both the basal and stimulated activity of an NF-kappa B promoter suggesting their involvement in immune-regulatory processes. This study provides the first functional data on i kappa b epsilon-versus the well-researched i kappa b alpha factor-in a non-mammalian model species.

Automated summary

NF-kappa B signaling is controlled by the family of 'inhibitors of NF-kappa B' (I kappa B). Rainbow trout genome contains multiple gene copies coding for different I kappa B proteins, except for two types. The expression levels of these genes vary, suggesting their involvement in immune-regulatory processes.