Identification of two corticotropin-releasing factor receptors from Xenopus laevis with high ligand selectivity: unusual pharmacology of the type 1 receptor.

Two cDNA clones encoding distinct members of the corticotropin-releasing factor (CRF) receptor family have been isolated from Xenopus laevis with PCR-based approaches. The first full-length cDNA amplified from Xenopus brain encoded a 415-amino acid protein with approximately 80% identity to mammalian CRF receptor type 1 (CRF-R1). The second full-length cDNA isolated from Xenopus brain and heart encoded a 413-amino acid protein with approximately 81% identity to the alpha-variant of mammalian CRF receptor, type 2 (CRF-R2). No evidence could be obtained that the beta-variant of CRF-R2 existed in Xenopus laevis. Binding studies using human embryonic kidney 293 (HEK 293) cells stably transfected with xenopus CRF-R2 showed that the CRF analogues urotensin I, urocortin, and sauvagine were bound with higher affinities than human/rat CRF, xenopus CRF, and ovine CRF. In contrast to human CRF-R1, xenopus CRF-R1 (xCRF-R1) was very selective for different CRF ligands. Urotensin I, urocortin, human/rat CRF, and xenopus CRF were bound with significantly (10-22-fold) higher affinities than ovine CRF (K(D) = 31.7 nM) and sauvagine (K(D) = 51.4 nM). In agreement with these binding data, EC50 values of 39.7 and 1.1 nM were found for sauvagine and for human/rat CRF or xenopus CRF, respectively, when the cyclic AMP production in HEK 293 cells stably transfected with xCRF-R1 was determined.