Characterization of a third ghrelin receptor, GHS-R3a, in channel catfish reveals novel expression patterns and a high affinity for homologous ligand.

A novel third channel catfish growth hormone secretagogue (ghrelin) receptor, GHS-R3a, gene was characterized. Identification and analysis of the genomic organization of channel catfish GHS-R3a revealed differences in exon/intron structure relative to the previously published GHS-R1a and GHS-R2a sequences. Amino acid sequence alignment of catfish GHS-R3a with -R1a and -R2a revealed 48 and 52% sequence identity, respectively. Phylogenetic analysis predicted a new clade of GHS-R3a receptors found only in fish, with representation in the teleost infradivisions Osteoglossomorpha, Clupeomorpha, and Euteleostei. In functional analyses, homologous catfish ghrelin increased intracellular Ca concentration in human embryonic kidney (HEK) 293 cells stably expressing catfish GHS-R3a. On the contrary, intracellular Ca concentration was unaffected by treatment with the synthetic growth hormone secretagogues GHRP-6 and hexarelin. Realtime PCR results indicated high expression of GHS-R3a in the brain and gonads, demonstrating tissue specificity among the catfish GHS-Rs. The effects of fasting and refeeding on all three ghrelin receptors were evaluated in catfish brain, pituitary, stomach, and Brockmann bodies. Most notably, GHS-R3a was the only receptor observed to significantly increase (2.9-6.3-fold) in brain, pituitary, and stomach within 4 days of fasting (P < .05). Stomach GHS-R1a also increased (P < .05) after 4 days; however, GHS-R2a was only elevated in brain and pituitary after refeeding for 1 week. Expression of all three ghrelin receptors were elevated (P < .05) in the Brockmann bodies after 2 weeks of fasting and returned to prefasting levels following refeeding. Together with the previously published characterization of GHS-R1a and -R2a, these results establish three ghrelin receptors, each altered by energy state, in channel catfish and add to the growing body of information on GHS-R evolution and function.