Submitted to: General and Comparative Endocrinology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/8/2005
Publication Date: 2/1/2006
Citation: Richards, M.P., Poch, S.M., McMurtry, J.P. 2006. Characterization of turkey and chicken ghrelin genes, and regulation of ghrelin and ghrelin receptor mRNA levels in broiler chickens. General and Comparative Endocrinology. 145(3):298-310. Interpretive Summary: Poultry producers have, over the years, intensively selected for commercial lines of poultry that grow faster and produce more meat than preceding generations. In doing so, there has also been an unintended selection for over consumption of feed driven by increased appetite such that birds do not eat appropriately to meet their basic energy requirements. In an attempt to address this problem, research interest has focused on endocrine factors that regulate feed intake and the genes that encode them. One such factor is the hormone ghrelin that is produced by the stomach in mammals and the proventriculus in birds. In mammals, ghrelin stimulates food intake and increases the release of growth hormone from the pituitary gland. In birds, ghrelin also stimulates growth hormone release, but it inhibits feed intake. Therefore, our objectives in this study were to further characterize the DNA sequence and structure of chicken and turkey ghrelin genes and to study the function of the ghrelin and ghrelin receptor genes in broiler chickens under different feeding states. We found that chicken and turkey ghrelin genes are similar in structure and function. However, a specific sequence difference was detected in the ghrelin gene that distinguished meat-type from egg-type chickens, birds with markedly different appetites and growth rates. We also found that the ghrelin gene appears to function differently in chickens as compared to mammals in response to changes in feeding state. Ghrelin receptor gene function was not affected by feeding. Plasma ghrelin showed no apparent relationship to other circulating hormones such as growth hormone, corticosterone or insulin. These findings could, in part, account for the apparent discrepancy between mammals and birds with respect to the action of ghrelin on food intake regulation. This information will be useful to researchers studying the role of endocrine factors in determining growth and body composition of different poultry species, as well as, to producers in formulating and implementing new genetic selection strategies for commercial poultry flocks.
Technical Abstract: Ghrelin, a peptide hormone produced by the stomach in mammals, stimulates growth hormone release and food intake. Recently, ghrelin was identified and characterized in chicken proventriculus and shown to stimulate growth hormone release but inhibit feed intake. The purpose of this work was to identify and further characterize the ghrelin gene in chickens and in turkeys. Using molecular cloning techniques we have sequenced cDNAs corresponding to chicken (Leghorn) and turkey ghrelin mRNAs (cDNAs). A total of 844 (chicken) or 869 (turkey) bases including the complete coding regions (CDS), and the 5'- and 3'-untranslated regions (UTRs) were determined. Nucleotide sequence (CDS) predicted a 116 amino acid precursor protein (preproghrelin) for both the chicken and the turkey that demonstrated complete conservation of an N-terminal ‘active core’ (GSSF) including a serine (position 3 of the mature hormone) known to be a modification (acylation) site important for ghrelin bioactivity. Additional nucleotide sequence was found in the 5’-UTRs of both Leghorn and turkey cDNAs that was not present in broilers or the red jungle fowl. The turkey ghrelin gene, sequenced from genomic DNA templates, contained 5 exons and 4 introns, a structure similar to mammalian and chicken ghrelin genes. Ghrelin was highly expressed in proventriculus with much lower levels of expression in other tissues such as pancreas, brain and intestine. RT-PCR was used to quantify ghrelin gene expression relative to 18S rRNA in 3 wk-old male broiler chickens. Ghrelin expression in proventriculus increased in response to fasting but did not decline with subsequent refeeding. Plasma ghrelin levels did not change significantly in response to fasting or refeeding and did not appear to reflect changes in proventriculus ghrelin gene expression. Ghrelin gene expression declined in broiler pancreas after a 48 hr fast and increased upon refeeding. Expression of the gene encoding the receptor for ghrelin (growth hormone secretagogue receptor, GHS-R) and a variant form was detected in a variety of tissues collected from 3-wk-old male broiler chickens possibly suggesting autocrine/paracrine effects. These results offer new information about the avian ghrelin and ghrelin receptor genes and the potential role that this system might play in regulating feed intake and energy balance in poultry.