Submitted to: International Journal of Peptides
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 6, 2009
Publication Date: February 10, 2010
Citation: Richards, M.P., Mcmurtry, J.P. 2010. The avian proghrelin system. International Journal of Peptides. Volume 2010, Article ID 749401, 14 pages.
Interpretive Summary: Ghrelin is a peptide hormone that plays important roles in stimulating food intake and the release of growth hormone from the pituitary gland. Ghrelin is unique in several ways. It is the only known hormone produced by the gastrointestinal tissue that stimulates food intake and it is thought to represent a ‘hunger signal’. Also, ghrelin requires the attachment of a fatty acid for it to become active. In mammals, ghrelin is encoded by a single gene in the form of a precursor protein (proghrelin). That precursor protein also contains an additional peptide hormone, obestatin which exhibits physiological actions that are different from ghrelin. In fact, obestatin is thought to oppose the effects of ghrelin on food intake, although its exact function has not yet been clearly determined. The actions of ghrelin and obestatin are mediated by specific receptors present in the cell membranes of tissues that respond to these hormones. Collectively, the peptide hormones derived from the proghrelin precursor and the specific receptors that bind them constitute the proghrelin system which plays a vital role in regulating food intake, metabolism and body weight. Despite the fact that this system has been widely studied in a variety of animals and in humans, there is relatively little known about the proghrelin system in birds. Because birds regulate their food intake differently than mammals, it is important to understand the basis for this difference compared and how the proghrelin system might be involved. In birds, ghrelin has been found to stimulate growth hormone release but inhibit food intake. Therefore, the goal of this review was to summarize what is currently known about proghrelin, the receptors that bind ghrelin and obestatin, and the physiological actions of the proghrelin-derived peptide hormones specifically in birds. This information will help scientists better understand how the proghrelin system functions to control food intake, growth, metabolism and energy balance and provide useful background information for future investigations that will explore the nature and actions of this important hormonal system in other species.
To understand how the proghrelin system functions in regulating growth hormone release and food intake as well as defining its pleiotropic roles in such diverse physiological processes as energy homeostasis, gastrointestinal tract function and reproduction requires detailed knowledge of the structure and function of the components that comprise this system. These include the preproghrelin gene that encodes the proghrelin precursor protein from which two peptide hormones, ghrelin and obestatin, are derived and the cognate receptors that bind proghrelin-derived peptides to mediate their physiological actions in different tissues. Also key to the functioning of this system is the post-translational processing of the proghrelin precursor protein and the individual peptides derived from it. While this system has been intensively studied in a variety of animal species and humans over the last decade, there has been considerably less investigation of the avian proghrelin system which exhibits some unique differences compared to mammals. This review summarizes what is currently known about the proghrelin system in birds and offers new insights into the nature and function of this important endocrine system. Such information facilitates cross-species comparisons and contributes to our understanding of the evolution of the proghrelin system.