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Department of Organismal Animal Physiology, The Clayton Foundation Laboratories for Peptide Biology, Faculty of Science, Radboud University Nijmegen, Heyendaalseweg 135, 6525AJ Nijmegen, The Netherlands
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We identified orthologues of all mammalian Janus kinase (JAK) and signal transducer and activator of transcription (STAT) genes in teleostean fishes, indicating that these protein families were already largely complete before the teleost tetrapod split, 450 million years ago. In mammals, the STAT repertoire consists of seven genes (STAT1, -2, -3, -4, -5a, -5b, and -6). Our phylogenetic analyses show that STAT proteins that are recruited downstream of endocrine hormones (STAT3 and STAT5a and -5b) show a markedly higher primary sequence conservation compared with STATs that convey immune signals (STAT1-2, STAT4, and STAT6). A similar dichotomy in evolutionary conservation is observed for the JAK family of protein kinases, which activate STATs. The ligands to activate the JAK/STAT-signalling pathway include hormones and cytokines such as GH, prolactin, interleukin 6 (IL6) and IL12. In this paper, we examine the evolutionary forces that have acted on JAK/STAT signalling in the endocrine and immune systems and discuss the reasons why the JAK/STAT cascade that conveys classical immune signals has diverged much faster compared with endocrine JAK/STAT paralogues.
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Department of Animal Physiology, Department of Integrative Biology, Center for Molecular and Biomolecular Informatics (CMBI), The Clayton Foundation Laboratories for Peptide Biology, Faculty of Science, Radboud University Nijmegen, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands
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We describe duplicate leptin genes in zebrafish (Danio rerio) that share merely 24% amino acid identity with each other and only 18% with human leptin. We were also able to retrieve a second leptin gene in medaka (Oryzias latipes). The presence of duplicate leptin genes in these two distantly related teleosts suggests that duplicate leptin genes are a common feature of teleostean fishes. Despite low primary sequence conservation, we are confident in assigning orthology between mammalian and zebrafish leptins for several reasons. First, both zebrafish leptins share their characteristic gene structure and display key features of conserved synteny with mammalian leptin genes. Secondly, the cysteine residues that make up leptin's single disulphide bridge are equally spaced in mammalian and zebrafish leptins and are unique among all members of the class-I helical cytokine family. Thirdly, the zebrafish leptins cluster with other fish leptins and mammalian leptins in phylogenetic analysis, supported by high bootstrap values. Within the leptin cluster, leptin-b forms a separate clade with the leptin-b orthologue from medaka. Finally, our prediction of the tertiary structures shows that both leptins conform to the typical four α-helix bundle structure of the class-I α-helical cytokines. The zebrafish leptins are differentially expressed; the liver shows high leptin-a expression (in concordance with what we observed for carp leptins), while leptin-b is expressed at much lower levels, which are downregulated further upon fasting. The finding of duplicate leptin genes in teleosts adds to our understanding of the evolution of leptin physiology in the early vertebrate lineage.
