Molecular characterization and expression analysis of the putative interleukin 6 receptor (IL-6Rα and glycoprotein-130) in rainbow trout (Oncorhynchus mykiss): salmonid IL-6Rα possesses a polymorphic N-terminal Ig domain with variable numbers of two repeats

Interleukin (IL)-6, the founding member of IL-6 family cytokines, plays non-redundant roles in hematopoiesis and acute phase responses. IL-6 signals via a specific private IL-6Rα and a common beta chain gp130. In this study, we have cloned both the IL-6Rα and gp130 in rainbow trout. The trout gp130 cDNA encodes 906 aa and is similar in size, extracellular domain structure (D1-D6) and presence of intracellular motifs important for signal transduction to tetrapod gp130s. The trout IL-6Rα cDNA encodes for 834 aa and is larger compared to tetrapod IL-6Rαs, as are other fish IL-6Rα molecules due to a large D1 domain. However, the cytokine-binding domain is well conserved across vertebrates, with four conserved cysteine residues in the N-terminal FNIII domain and a WSXWS motif in the C-terminal FNIII domain. Furthermore, a phylogenetic tree analysis confirmed that the reported fish IL-6Rα and gp130 molecules are orthologues to their tetrapod counterparts. The extra large D1 domain of the salmonid IL-6Rα molecules results partially from the insertions of two repetitive sequences of [TS]-[TF]-VSTTT-[ND]-TTSNG and TTVS-[AT]-IKD-[DG]-S-[KD]-N-[GR], respectively. Furthermore the numbers of repetitions of the two motifs were variable in different individuals and cell lines, and even in the same fish allelic polymorphism exists. Trout IL-6Rα was expressed at higher levels than gp130 in a number of tissues examined and the expression of both IL-6Rα and gp130 could be modulated by LPS and Poly I:C in the cell lines studied. The expression patterns of the receptors suggest that high level expression of IL-6Rα is critical for IL-6 responsiveness.