Abstract
This paper reports the identification, expression, binding kinetics, and functional studies of two novel type III lamprey GnRH receptors (lGnRH-R-2 and lGnRH-R-3) in the sea lamprey, a basal vertebrate. These novel GnRH receptors share the structural features and amino acid motifs common to other known gnathostome GnRH receptors. The ligand specificity and activation of intracellular signaling studies showed ligands lGnRH-II and -III induced an inositol phosphate (IP) response at lGnRH-R-2 and lGnRH-R-3, whereas the ligand lGnRH-I did not stimulate an IP response. lGnRH-II was a more potent activator of lGnRH-R-3 than lGnRH-III. Stimulation of lGnRH-R-2 and lGnRH-R-3 testing all three lGnRH ligands did not elicit a cAMP response. lGnRH-R-2 has a higher binding affinity in response to lGnRH-III than lGnRH-II, whereas lGnRH-R-3 has a higher binding affinity in response to lGnRH-II than IGnRH-III. lGnRH-R-2 precursor transcript was detected in a wide variety of tissues including the pituitary whereas lGnRH-R-3 precursor transcript was not as widely expressed and primarily expressed in the brain and eye of male and female lampreys. From our phylogenetic analysis, we propose that lGnRH-R-1 evolved from a common ancestor of all vertebrate GnRH receptors and lGnRH-R-2 and lGnRH-R-3 likely occurred due to a gene duplication within the lamprey lineage. In summary, we propose from our findings of receptor subtypes in the sea lamprey that the evolutionary recruitment of specific pituitary GnRH receptor subtypes for particular physiological functions seen in later evolved vertebrates was an ancestral character that first arose in a basal vertebrate.