Two new forms of gonadotropin-releasing hormone in a protochordate and the evolutionary implications.

Abstract

The neuropeptide gonadotropin-releasing hormone (GnRH) is the major regulator of reproduction in vertebrates. Our goal was to determine whether GnRH could be isolated and identified by primary structure in a protochordate and to examine its location by immunocytochemistry. The primary structure of two novel decapeptides from the tunicate Chelyosoma productum (class Ascidiacea) was determined. Both show significant identity with vertebrate GnRH. Tunicate GnRH-I (pGlu-His-Trp-Ser-Asp-Tyr-Phe-Lys-ProGly-NH2) has 60% of its residues conserved, compared with mammalian GnRH, whereas tunicate GnRH-II (pGlu-HisTrp-Ser-Leu-Cys-His-Ala-Pro-Gly-NH2) is unusual in that it was isolated as a disulfide-linked dimer. Numerous immunoreactive GnRH neurons lie within blood sinuses close to the gonoducts and gonads in both juveniles and adults, implying that the neuropeptide is released into the bloodstream. It is suggested that in ancestral chordates, before the evolution of the pituitary, the hormone was released into the bloodstream and acted directly on the gonads.The origins of the vertebrate forebrain and pituitary gland are enigmatic. In the tunicates, a protochordate group representing an early offshoot from the main chordate lineage (1, 2), the brain is a single, simple ganglion, and there is no pituitary gland. As in amphioxus and the vertebrates, the nervous system develops from the neural tube, formed as an infolding of the dorsal embryonic ectoderm. Although not all the adult derivatives of the tunicate neural tube have homologs in vertebrates, the tunicate central nervous system can be said to "foreshadow" that of vertebrates (3). An important physiological function of the vertebrate central nervous system is the regulation of reproduction. In all classes of vertebrates, reproduction is mediated by the neuropeptide, gonadotropin-releasing hormone (GnRH; ref. 4). In some invertebrates, the nervous system controls reproduction (5), but GnRH-like molecules have not hitherto been implicated in their reproductive physiology. As shown here, however, GnRH is present in tunicates and is located in nerve cells along the dorsal strand. MATERIALS AND METHODSTissue Collection. GnRH was extracted from 1200 adult ascidians (Chelyosoma productum) that were obtained by divers at -"60 feet in the Strait of Juan de Fuca near Victoria, BC, Canada. The dorsal portion of the animal, including the cerebral ganglion, the neural gland, and the dorsal strand was dissected after removal of the tunic. The tissue was frozen and stored at -80°C.Peptide Extraction and Purification. The frozen tissue was powdered, extracted in an acetone/HCI mixture, and defatted with petroleum ether as described (6). Sequential HPLC procedures in which the ion pairing agents varied were used to purify the two GnRH peptides. Detection of GnRH in each step of the procedure was based on antisera raised against lamprey GnRH (Bla-5) or salmon GnRH (GF-4) in our laboratory. Characterization of the Primary Structure. The primary structure of the purified peptides...

Genomic Structure and Expression Sites of Three Gonadotropin-Releasing Hormone Genes in One Species

“…However, the function of 5His 5 Trp 7 Tyr 8 6GnRH remains unknown. Given that these three genes seem to have arisen before the origin of vertebrates (Powell et al, 1996;White et al, 1998), perhaps not all three genes are involved in reproductive behavior but may have been coopted for other functions.…”

Identification, localization, and regulation of passerine GnRH-I messenger RNA

“…One or more of these forms has been found in all vertebrates studied to date, regulating gonadotropin release and reproductive activity. Some forms of GnRH have also been isolated from invertebrates such as tunicates (Powell et al 1996) and a cephalopod (Iwakoshi et al 2002).…”

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

“…2 The abbreviations used are: GnRH, gonadotropin-releasing hormone; pGlu, pyroglutamic acid; IM-MS, ion mobility-mass spectrometry.…”

“…In vertebrates, gonadotropin-releasing hormone (GnRH) 2 is synthesized in hypothalamic neurones and conducted a few millimeters in the hypophysial portal system to the anterior pituitary, where it binds to high affinity receptors in gonadotrophs to stimulate the release of gonadotropins (1). The gonadotropins in turn stimulate hormone and gamete production by the testes and ovaries.…”

“…The gonadotropins in turn stimulate hormone and gamete production by the testes and ovaries. GnRHs have also been isolated from protochordate species (2,3) and are thought to be secreted from neurones to directly regulate the gonads (2-4) in these representatives of vertebrate progenitors. GnRHs and GnRH receptors have also been found to directly affect vertebrate gonadal function (5), possibly reflecting the earliest role of GnRH as exemplified in protochordates (6,7).…”