A novel inhibitory gonadotropin-releasing hormone-related neuropeptide in the ascidian, Ciona intestinalis.

摘要

The gonadotropin-releasing hormone (GnRH) family peptides are most widely distributed neuropeptides and/or neurophysial hormones. GnRH is involved in diverse neuroendocrine, paracrine, autocrine, and neurotransmittereuromodulatory functions in the central and peripheral nervous system as well as peripheral tissues. In the present study, we show the identification of a novel GnRH-related peptide, Ci-GnRH-X, in the ascidian, Ciona intestinalis. Intriguingly, Ci-GnRH-X possesses a unique primary sequence consisting of 16 amino acids, although typical GnRH family peptides are composed of 10 amino acids. On the other hand, Ci-GnRH-X shares the GnRH consensus motifs, including the N-terminal pQHWS ('pQ' indicates a pyro-glutamic acid) and C-terminal Gly-amide. Reverse transcription (RT)-PCR analysis shows that the Ci-GnRH-X gene is expressed exclusively in the central nervous system. Moreover, in situ hybridization demonstrated that the Ciona GnRH-1 gene encoding Ciona GnRHs (t-GnRH-3, -5 and -6) was co-expressed with the Ci-GnRH-X gene in neurons of the cerebral ganglion. Of particular interest is that Ci-GnRH-X exhibited moderate (10-50%) inhibitory activity against t-GnRHs at their cognate receptors. Ci-GnRH-X repressed the elevation of the intracellular calcium and cAMP production by t-GnRH-6 at Ci-GnRHR-1, and cAMP production by t-GnRH-3, and t-GnRH-5 via Ci-GnRHR-3 was also inhibited by Ci-GnRH-X. In contrast, no inhibitory effect of Ci-GnRH-X at Ci-GnRHR-2 was observed. The localization and biochemical assays revealed that Ci-GnRH-X acts as an endogenous antagonist for the Ciona GnRHergic system. This is the first molecular and functional characterization of an endogenous inhibitor of GnRHs in an animal species.