Submitted to: General and Comparative Endocrinology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 16, 2008
Publication Date: April 1, 2008
Citation: Richards, M.P., McMurtry, J.P. 2008. Expression of proglucagon and proglucagon-derived peptide hormone receptor genes in the chicken. General and Comparative Endocrinology. 156(2):323-338. Interpretive Summary: Glucagon is a peptide hormone that plays a counter-regulatory role to insulin working to raise blood glucose levels. In mammals, glucagon is encoded by a single gene in the form of a precursor protein (proglucagon) that also contains additional peptide hormones including two glucagon-like peptide hormones (GLP-1 and GLP-2) which have different physiological functions. GLP-1 works to promote insulin production by the pancreas and thus raise blood glucose, whereas GLP-2 is thought to act as a growth factor for the intestine. The actions of glucagon, GLP-1 and GLP-2 are all mediated by specific receptors present in the cell membrane. Together, the peptide hormones derived from the proglucagon precursor and the specific receptors constitute the proglucagon system which plays an important role in regulating metabolism and energy balance in animals. Despite the fact that this system has been widely studied in animals and humans, there is relatively little known about the proglucagon system in birds. Therefore, the goal of this work was to: 1) clone, sequence and characterize the structure of the chicken proglucagon gene; 2) determine possible regulatory mechanisms for expression of this gene; and 3) study the expression of specific receptor genes for glucagon, GLP-1 and GLP-2 in different tissues from broiler chickens. We found that chickens have a single proglucagon gene that expresses four distinct mRNAs and two types of precursor proteins. An interesting finding was that the stomach (proventriculus) exhibits a high level of proglucagon expression. Receptor genes for glucagon, GLP-1 and GLP-2 were expressed in all tissues examined with some tissues showing high levels of expression for a particular receptor. Our results offer new insights into structure and function of the chicken proglucagon gene, processing of the precursor proteins produced from it and potential activity sites for proglucagon-derived peptide hormones mediated by their specific receptors. Together this information will help scientists better understand the proglucagon system in birds as it relates to control of metabolism and energy balance.
Technical Abstract: To better understand how the proglucagon system functions in birds, we utilized a molecular cloning strategy to sequence and characterize the chicken proglucagon gene that encodes glucagon, glucagon-like peptide(GLP)-1 and GLP-2. This gene has seven exons and six introns with evidence for an additional (alternate) first exon and two promoter regions. We identified two distinct classes of proglucagon mRNA transcripts (PGA and PGB) produced by alternative splicing at their 3’-ends. These were co-expressed in all tissues examined with pancreas and proventriculus showing the highest levels of each. Although both mRNA classes contained coding sequence for glucagon and GLP-1, class A mRNA lacked that portion of the coding region (CDS) containing GLP-2; whereas, class B mRNA had a larger CDS that included GLP-2. Both classes of mRNA transcripts exhibited two variants, each with a different 5’-end arising from alternate promoter and alternate first exon usage. Fasting and refeeding had no effect on proglucagon mRNA expression despite significant changes in plasma glucagon levels. To investigate potential differences in proglucagon precursor processing among tissues, mRNA expression for two prohormone convertase (PC) genes was analyzed. PC2 mRNA was predominantly expressed in pancreas and proventriculus, whereas PC1/3 mRNA was more highly expressed in duodenum and brain. We also determined mRNA expression of the specific receptor genes for glucagon, GLP-1 and GLP-2 to help define major sites of hormone action. Glucagon receptor mRNA was most highly expressed in liver and abdominal fat, whereas GLP-1 and GLP-2 receptor genes were highly expressed in the gastrointestinal tract, brain, pancreas and abdominal fat. These results offer new insights into structure and function of the chicken proglucagon gene, processing of the precursor proteins produced from it and potential activity sites for proglucagon-derived peptide hormones mediated by their cognate receptors.