SILVER, M.R.*; SOWER, S.A.; University of New Hampshire; University of New Hampshire: Characterization of an ancestral type II GnRH receptor: function and pharmacological profile of the sea lamprey GnRH receptor

Neuroendocrine regulation of reproduction in vertebrates is regulated by gonadotropin-releasing hormone (GnRH). GnRH acts on gonadotropes located in the anterior pituitary via its receptor, a 7-transmembrane G-protein coupled receptor. The objective of this study was to describe the function and molecular evolution of the GnRH receptor through the characterization of the previously cloned sea lamprey GnRH receptor (Silver et al., 2005). The lamprey GnRH receptor was shown to stimulate both IP and cAMP when treated with either lamprey GnRH-I or -III; however lamprey GnRH-III is a far more potent activator of either system. Competitive binding analysis revealed that lamprey GnRH-III binds to the lamprey GnRH receptor with approximately 100 fold higher affinity compared to lamprey GnRH-I, suggesting lamprey GnRH-III is the native ligand for this receptor. Interestingly, chicken GnRH-II binds the receptor with equal affinity compared to lamprey GnRH-III. Truncation of the lamprey GnRH receptor C-terminal tail resulted in a decrease in second messenger production; however IP signaling was fully recovered by the tail-less mutant. Saturation binding assays using the wild type and mutant lamprey GnRH receptors indicated a decrease in Kd of the tail-less receptor, which is compensated by a significant decrease in ligand dependant internalization, contributing to the recovered signaling capacity. This unique lamprey GnRH receptor, with both high affinity for lamprey GnRH-III and chicken GnRH-II and ligand binding activity in the tail-less form may represent an important ancestral state which provides insight into the function and evolution of the vertebrate GnRH receptor family. (Supported by NSF 0421923 to SAS)