Identification of sea lamprey GTHβ-like cDNA and its evolutionary implications

We have identified the first and perhaps only gonadotropin β-like protein by cDNA cloning in sea lamprey, a member of the oldest lineage of vertebrates, the agnathans. Two pituitary gonadotropins (GTHs: follicle-stimulating hormone (FSH) and luteinizing hormone (LH)) have been identified in representative species of all classes of vertebrates except the agnathans. The present study was undertaken to identify GTH in sea lamprey, Petromyzon marinus, to gain a further understanding of the origin and evolution of reproductive pituitary hormones and their respective genes in vertebrates. Sea lamprey preGTHβ-like cDNA was cloned from a plasmid cDNA library using an expressed sequence tag analysis. The preGTHβ-like cDNA encoded 150 amino acids, in which the GTHβ-like protein consisted of 134 amino acid residues. Sea lamprey GTHβ-like protein contained 12 Cys residues and two N-glycosylation sites at homologous positions to those of FSHβ and LHβ. The region of the molecule that has been proposed to control receptor binding specificity (i.e., the region between the 10th and 12th Cys residues) suggests that the proposed heterodimer would be more like a FSH than a LH. Sea lamprey GTHβ-like protein-producing cells were identified immunocytochemically in the ventral part of the proximal pars distalis of pituitary using antiserum prepared against a synthetic peptide of preGTHβ-like protein (52–68). Intraperitoneal administration of sea lamprey GnRH-I and -III at 100 μg/g body weight (twice at a 24 h interval) increased expression of GTHβ-like protein in the pituitary of adult female sea lamprey during the final maturational period. Thus, these results are the first to demonstrate the presence of a single GTH-like system in lampreys. Because the sea lamprey GTHβ-like protein is a clear out-group compared to those of the LH and FSH family based on phylogenic analysis, we propose that an ancestral glycoprotein hormone gave rise to only one GTH in lampreys and to the glycoprotein hormone family that gave rise to LH, FSH, and TSH during the early evolution of gnathostomes.