Identification and expression analysis of two fish-specific IL-6 cytokine family members, the ciliary neurotrophic factor (CNTF)-like and M17 genes, in rainbow trout Oncorhynchus mykiss.

The mammalian interleukin (IL)-6 family of cytokines consist of eight class-I helical cytokines and are major players in hematopoiesis, as well as in neuroendocrine and immune systems, and have pro- and anti-inflammatory properties. We have identified a novel fish CNTF-like molecule, for the first time, as well as the trout M17 gene. The trout CNTF-like gene encodes a putative 191 amino acid peptide without a signal peptide, and shares high amino acid sequence identities (39-99%) within the fish CNTF-like molecules that we have identified, but only limited identities (21-24%) to higher vertebrate CNTF molecules among the IL-6 family members. The fish CNTF-like gene has two phase 0 introns in the coding region whilst the mammalian CNTF gene has only one phase 0 intron. The trout M17 encodes a peptide of 227 amino acid residues which has a predicted signal peptide of 35 aa and shares the highest identities (38-95%) to other fish M17 molecules, but limited homology to other IL-6 family members. Both of the fish CNTF-like and M17 proteins have four predicted helices and their tertiary structure could be stabilised by multiple conserved disulphide bonds. Phylogenetic analysis suggests that the fish CNTF-like and M17 genes may have arisen from ancestral genes that have given rise to mammalian LIF/OSM, and CNTF/CLC/CT-1/CT-2 of the IL-6 family. The trout CNTF-like gene is highly expressed in brain and muscle, whilst the M17 is highly expressed in immune tissues, including gills, spleen and head kidney. Furthermore, immune stimulation of a macrophage cell line and bacterial infection in vivo up-regulated M17 expression but had little effect on CNTF-like expression. These results suggest a major role of the CNTF-like molecule in fish nervous and muscular systems and of M17 in the immune system.

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