Tunicate gonadotropin-releasing hormone (GnRH) peptides selectively activate Ciona intestinalis GnRH receptors and the green monkey type II GnRH receptor.

In vertebrates, GnRH binds to its receptor and stimulates predominantly G(q/11)-mediated signal transduction in gonadotropes. However, little is known about the GnRH receptor and its signaling pathway in tunicates, a group that arose before the vertebrates. Although tunicates have had duplications of a few genes in the last 600 million years, the early vertebrates had duplications of the full genome. Also unknown is the nature of GnRH signaling in the tunicate, which lacks both a pituitary gland and sex steroids. However, we know that tunicates have GnRH peptides because we previously reported six GnRH peptides encoded within the tunicate genome of Ciona intestinalis. Here we clone and sequence cDNAs for four putative GnRH receptors from C. intestinalis. These are the only invertebrate GnRH receptors found to date. Each Ciona GnRH receptor was expressed in COS-7 cells, incubated with each of the six C. intestinalis GnRHs and assayed for a signaling response. GnRH receptors 1, 2, and 3 responded to Ciona GnRH peptides to stimulate intracellular cAMP accumulation. In contrast, only GnRH receptor 1 activated inositol phosphate turnover in response to one of the Ciona GnRHs. The green monkey type II GnRH receptor cDNA was tested as a comparison and a positive control. In conclusion, the four GnRH receptors encoded within the C. intestinalis genome were all transcribed into messenger RNA, but only three of the Ciona GnRH receptors were biologically active in our assays. The Ciona GnRH receptors almost exclusively activated the cAMP pathway.