Abstract

The class-I helical cytokines constitute a large group of signalling molecules that play key roles in a plethora of physiological processes including host defence, immune regulation, somatic growth, reproduction, food intake and energy metabolism, regulation of neural growth and many more. Despite little primary amino acid sequence similarity, the view that all contemporary class-I helical cytokines have expanded from a single ancestor is widely accepted, as all class-I helical cytokines share a similar three-dimensional fold, signal via related class-I helical cytokine receptors and activate similar intracellular signalling cascades. Virtually all of our knowledge on class-I helical cytokine signalling derives from research on primate and rodent species. Information on the presence, structure and function of class-I helical cytokines in non-mammalian vertebrates and non-vertebrates is fragmentary. Consequently, our ideas about the evolution of this versatile multigene family are often based on a limited comparison of human and murine orthologs. In the last 5 years, whole genome sequencing projects have yielded draft genomes of the early vertebrates, pufferfish (Takifugu rubripes), spotted green pufferfish (Tetraodon nigroviridis) and zebrafish (Danio rerio). Fuelled by this development, fish orthologs of a number of mammalian class-I helical cytokines have recently been discovered. In this review, we have characterised the mammalian class-I helical cytokine family and compared it with the emerging class-I helical cytokine repertoire of teleost fish. This approach offers important insights into cytokine evolution as it identifies the helical cytokines shared by fish and mammals that, consequently, existed before the divergence of teleosts and tetrapods. A ‘fish–mammalian’ comparison will identify the class-I helical cytokines that still await discovery in fish or, alternatively, may have been evolutionarily recent additions to the mammalian cytokine repertoire.