Characterization of three corticotropin-releasing factor receptors in catfish: A novel third receptor is predominantly expressed in pituitary and urophysis

The present study reports the isolation of three complementary DNA (cDNA) c lones encoding distinct subtypes of CRF receptors from the diploid catfish (cf) species, Ameiurus nebulosus. The first clone encodes a 446-amino acid protein (cfCRF-R1) that is highly homologous to mouse (m) CRF-R1 (93% ident ical). The cfCRF-R1 messenger RNA is highly expressed in the brain, and its distribution pattern correlates well with that of mammalian CRF-R1, except for weak expression in the pituitary. When transiently expressed in COS-7 cells, cfCRF-R1 bound CRF, urotensin I, and sauvagine with similar affiniti es. The second full-length cDNA, which was cloned from catfish heart, encod es a 406-amino acid protein that showed homology to murine CRF-R2 (88%) and when expressed in COS-7 cells preferentially bound sauvagine. The highest level of cfCRF-R2 expression was observed in the heart. The third full-leng th cDNA clone, which encodes a 428-amino acid protein, is structurally clos er to cfCRF-R1 (85%) than to cfCRF-R2 (80%). This novel CRF receptor (cfCRF -R3) bound CRF with a 5-fold higher affinity than urotensin I and sauvagine and was expressed in the pituitary gland, urophysis, and brain. The presen ce of three different CRF receptors, each with distinct tissue distribution and ligand binding properties, suggests a complex CRF/urotensin I system.