Cloning, tissue distribution, and functional characterization of chicken glucagon receptor

Glucagon has been reported to play an important role in hepatic glucose metabolism of vertebrates including birds. However, the molecular mechanism of its actions in birds remains largely unknown. In present study, the full-length cDNA of chicken glucagon receptor (GCGR) was first cloned from brain tissue using reverse transcription-PCR. This putative chicken GCGR (named cGCGR-s) is 496 amino acids (AA) long and shares high AA sequence identity with that of human (70%), rat (69%), mouse (69%), and Xenopus (64%), and a comparatively lower identity with goldfish (53%). In addition, a full-length cDNA encoding a novel glucagon receptor variant (named cGCGR-v1) of 554 AA was identified in this study. Sequence analysis revealed that this receptor variant arises from the retention of intron 4 (174 bp) and thus causes a 58-AA insertion at the large N-terminal extracellular domain. Using the pGL3-CRE-luciferase reporter system, we demonstrated that human glucagon could potently activate chicken GCGR-s and GCGR-v1 expressed in Chinese hamster ovary cells, confirming that both cGCGR-s and cGCGR-v1 are functional and able to couple to the intracellular cyclic adenosine mono-phosphate-protein kinase A signaling pathway. Using a reverse transcription-PCR assay, we further examined the expression of glucagon receptor in adult chicken tissues, including different regions of the brain. Glucagon receptor was shown to be highly expressed in liver and moderately or weakly expressed in other tissues examined. In the central nervous system, the greatest expression was consistently detected in the hypothalamus. Taken together, our data not only suggest that glucagon receptor plays a critical role in mediating the actions of glucagon in liver, but also imply that glucagon may have important roles in nonhepatic tissues, such as in the hypothalamus of brain in chickens.