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Urocortins of the South African Clawed Frog, Xenopus laevis: Conservation of Structure and Function in Tetrapod Evolution

Departments of Ecology and Evolutionary Biology (G.C.B., R.J.D.) and Molecular, Cellular and Developmental Biology (E.J.C., R.J.D.), The University of Michigan, Ann Arbor, Michigan 48109-1048; and Johnson & Johnson, Research & Development (F.M.D.), CNS Research, B-2340 Beerse, Belgium

Address all correspondence and requests for reprints to Dr. Robert J. Denver, 3065C Kraus Natural Science Building, University of Michigan, Ann Arbor, Michigan 48109-1048. E-mail: rdenver{at}umich.edu.

Several corticotropin-releasing factor (CRF) family genes have been identified in vertebrates. Mammals have four paralogous genes that encode CRF or the urocortins 1, 2, and 3. In teleost fishes, a CRF, urotensin I (a fish ortholog of mammalian urocortin 1) and urocortin 3 have been identified, suggesting that at least three of the four mammalian lineages arose in a common ancestor of modern bony fishes and tetrapods. Here we report the isolation of genes orthologous to mammalian urocortin 1 and urocortin 3 from the South African clawed frog, Xenopus laevis. We characterize the pharmacology of the frog peptides and show that X. laevis urocortin 1 binds to and activates the frog CRF₁ and CRF₂ receptors at picomolar concentrations. Similar to mammals, frog urocortin 3 is selective for the CRF₂ receptor. Only frog urocortin 1 binds to the CRF-binding protein, although with significantly lower affinity than frog CRF. Both urocortin genes are expressed in brain, pituitary, heart, and kidney of juvenile frogs; urocortin 1 is also expressed in skin. We also identified novel urocortin sequences in the genomes of pufferfish, zebrafish, chicken, and dog. Phylogenetic analysis supports the view that four paralogous lineages of CRF-like peptides arose before the divergence of the actinopterygian and sarcopterygian fishes. Our findings show that the functional relationships among CRF ligands and binding proteins, and their anorexigenic actions mediated by the CRF₂ receptor, arose early in vertebrate evolution.

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