Six Novel Gonadotropin-Releasing Hormones Are Encoded as Triplets on Each of Two Genes in the Protochordate,Ciona intestinalis

Abstract

GnRH is the key regulator of the reproductive axis in vertebrates, but little is known about GnRH before the origin of vertebrates. We have identified two genes encoding GnRH in a protochordate, Ciona intestinalis, thought to be related to the ancestral animal that gave rise to vertebrates. Each gene, Ci-gnrh1 and Ci-gnrh2, encodes in tandem three GnRH peptides, each of which is unique compared with known forms. Ci-gnrh1 encodes three peptides and contains no introns, whereas Ci-gnrh2 encodes three more peptides but has two introns. This is the first report in which more than one GnRH peptide is encoded on a single gene. The Ciona genes reveal consensus promoter elements that are conserved compared with human GNRH1. Both tunicate genes are expressed as mRNA early and throughout development, measured at the stages of four-cell, gastrulation, tail release, and tail resorption. In a closely related tunicate species, Ciona savignyi, we used in silico analysis to identify two similar genes encoding six peptides, only one of which is unique compared with C. intestinalis. Immunohistochemistry showed that at least one GnRH peptide was in the nerve net that surrounds the dorsal strand. Synthetic forms of the seven novel tunicate peptides induced release of gametes in adult tunicates. In contrast, the peptides did not activate the human GnRH-I receptor or cause release of LH in a rat pituitary cell assay. These data provide insight into the structural evolution of the GnRH peptides and their genes and show a functional role for GnRH in tunicate spawning.

Tachykinin and Tachykinin Receptor of an Ascidian, Ciona intestinalis

“…Recently, in addition to these advantages, the genome data base for C. intestinalis was constructed (35), and morpholino DNA gene silencing (41) and transgenic technology (42) for the ascidian have emerged, leading to expansion of the applicability of ascidians to other fields, including neuroscience and endocrinology, as the only invertebrate chordate model, which is more suitable than protostome models such as Drosophila and C. elegans in regard to the phylogenetic distance. However, only two peptides, GnRH and cionin (CCK/gastrin-related peptide), have so far been identified from ascidians (43)(44)(45), although the presence of TK-like compounds in ascidians was implied by previous immunohistochemical analyses and radioimmunoassays for several ascidian tissues (24 -28). Furthermore, TKRPs, but not TKs, have been found in invertebrates, which complicates the evolutionary aspect of the TK family (7,20).…”

Diversity of actions of GnRHs mediated by ligand-induced selective signaling

“…Thirteen structural variants of GnRH have now been identified from all the vertebrate classes, nine from protochordates and one from a species of octopus, see Refs. [2,9,95,129]. These diverse structures represent "experiments of nature" over some 600 million years and provide insight into conserved and functionally important residues.…”

“…In vertebrates, the GnRH sequence is represented as a single copy within a gene and the active peptide is cleaved from the precursor protein by processing enzymes [90,100,127]. In contrast, several GnRH sequences are encoded in tandem within single genes in protochordates [2].…”

Evolution of Constrained Gonadotropin-releasing Hormone Ligand Conformation and Receptor Selectivity

“…To date 13 structural variants of the GnRH decapeptide have been identified in vertebrates (8), 9 from protochordates, which are vertebrate progenitors (8,9), and a 12-amino acid homolog from an octopus species (10) (Fig. 1).…”