Expression and Role of Gonadotropin-Releasing Hormone 2 and Its Receptor in Mammals Amy Desaulniers University of Nebraska-Lincoln, [email protected]
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University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Faculty Papers and Publications in Animal Science Animal Science Department 12-10-2017 Expression and Role of Gonadotropin-Releasing Hormone 2 and Its Receptor in Mammals Amy Desaulniers University of Nebraska-Lincoln, [email protected] Rebecca A. Cederberg University of Nebraska-Lincoln, [email protected] Clay A. Lents USDA, ARS, USMARC, [email protected] Brett R. White University of Nebraska-Lincoln, [email protected] Follow this and additional works at: http://digitalcommons.unl.edu/animalscifacpub Part of the Genetics and Genomics Commons, and the Meat Science Commons Desaulniers, Amy; Cederberg, Rebecca A.; Lents, Clay A.; and White, Brett R., "Expression and Role of Gonadotropin-Releasing Hormone 2 and Its Receptor in Mammals" (2017). Faculty Papers and Publications in Animal Science. 986. http://digitalcommons.unl.edu/animalscifacpub/986 This Article is brought to you for free and open access by the Animal Science Department at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Faculty Papers and Publications in Animal Science by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. REVIEW published: 11 December 2017 doi: 10.3389/fendo.2017.00269 Expression and Role of Gonadotropin-Releasing Hormone 2 and Its Receptor in Mammals Amy T. Desaulniers1, Rebecca A. Cederberg1, Clay A. Lents 2 and Brett R. White1* 1 Department of Animal Science, University of Nebraska-Lincoln, Lincoln, NE, United States, 2 USDA, ARS, USMARC, Clay Center, NE, United States Gonadotropin-releasing hormone 1 (GnRH1) and its receptor (GnRHR1) drive mammalian reproduction via regulation of the gonadotropins. Yet, a second form of GnRH (GnRH2) and its receptor (GnRHR2) also exist in mammals. GnRH2 has been completely conserved throughout 500 million years of evolution, signifying high selection pressure and a critical biological role. However, the GnRH2 gene is absent (e.g., rat) or inactivated (e.g., cow and sheep) in some species but retained in others (e.g., human, horse, and pig). Likewise, many species (e.g., human, chimpanzee, cow, and sheep) retain the GnRHR2 gene but lack the appropriate coding sequence Edited by: to produce a full-length protein due to gene coding errors; although production of Ivana Bjelobaba, University of Belgrade, Serbia GnRHR2 in humans remains controversial. Certain mammals lack the GnRHR2 gene Reviewed by: (e.g., mouse) or most exons entirely (e.g., rat). In contrast, old world monkeys, musk Emilie Rissman, shrews, and pigs maintain the coding sequence required to produce a functional North Carolina State University, GnRHR2. Like GnRHR1, GnRHR2 is a 7-transmembrane, G protein-coupled receptor United States Buffy Sue Ellsworth, that interacts with Gαq/11 to mediate cell signaling. However, GnRHR2 retains a cytoplas- Southern Illinois University mic tail and is only 40% homologous to GnRHR1. A role for GnRH2 and its receptor Carbondale, United States in mammals has been elusive, likely because common laboratory models lack both the *Correspondence: ligand and receptor. Uniquely, both GnRH2 and GnRHR2 are ubiquitously expressed; Brett R. White [email protected] transcript levels are abundant in peripheral tissues and scarcely found in regions of the brain associated with gonadotropin secretion, suggesting a divergent role from Specialty section: GnRH1/GnRHR1. Indeed, GnRH2 and its receptor are not physiological modulators This article was submitted to Neuroendocrine Science, of gonadotropin secretion in mammals. Instead, GnRH2 and GnRHR2 coordinate the a section of the journal interaction between nutritional status and sexual behavior in the female brain. Within Frontiers in Endocrinology peripheral tissues, GnRH2 and its receptor are novel regulators of reproductive organs. Received: 08 July 2017 GnRH2 and GnRHR2 directly stimulate steroidogenesis within the porcine testis. Accepted: 26 September 2017 Published: 11 December 2017 In the female, GnRH2 and its receptor may help mediate placental function, implanta- Citation: tion, and ovarian steroidogenesis. Furthermore, both the GnRH2 and GnRHR2 genes Desaulniers AT, Cederberg RA, are expressed in human reproductive tumors and represent emerging targets for can- Lents CA and White BR (2017) Expression and Role of cer treatment. Thus, GnRH2 and GnRHR2 have diverse functions in mammals which Gonadotropin-Releasing Hormone 2 remain largely unexplored. and Its Receptor in Mammals. Front. Endocrinol. 8:269. Keywords: GnRH2, GnRH2 receptor, reproductive function, G protein-coupled receptor, G protein-coupled doi: 10.3389/fendo.2017.00269 receptor signal transduction, autocrine/paracrine mechanisms, testis, cancer Frontiers in Endocrinology | www.frontiersin.org 1 December 2017 | Volume 8 | Article 269 Desaulniers et al. GnRH2 and GnRHR2 in Mammals BACKGROUND ancient form of GnRH (18). In contrast, GnRH1 evolved 350 million years ago and its sequence varies greatly among verte- The Classical Form of Mammalian brates (19). Gonadotropin-Releasing Hormone The Gene for GnRH2 (GnRH1) GnRH2 is not merely a splice variant of the GnRH1 gene; instead, The classical, hypophysiotropic GnRH1 is hailed as the master it is produced from its own gene that encodes the peptide, prepro- regulator of reproduction in mammals. GnRH1 is a decapeptide GnRH2 (20). TheGnRH2 gene is located on chromosome 20 in (pGlu–His–Trp–Ser–Tyr–Gly–Leu–Arg–Pro–Gly–NH2) humans, chimpanzees, and orangutans, chromosome 13 in the produced by hypothalamic neurons and secreted in a pulsatile cow, chromosome 22 in the horse, chromosome 10 in the rhesus manner into hypophyseal portal capillaries where it travels to macaque, and chromosome 17 in the pig (21). The genomic orienta- the anterior pituitary gland. GnRH1 then binds to its receptor tion of the GnRH2 gene is highly conserved across species (21, 22). (GnRHR1) on gonadotrope cells, promoting the synthesis and It is flanked by the PTPRA and MRPS26 genes in all mammalian secretion of the gonadotropins, follicle-stimulating hormone and non-mammalian vertebrates examined to date (21, 22). The (FSH) and luteinizing hormone (LH), into peripheral circulation PTPRA gene resides about 5–6 kb upstream of the GnRH2 gene where they act on their target organs, the gonads. In females, FSH (21) and encodes the enzyme, receptor-type tyrosine-protein stimulates follicular development, whereas LH promotes ovula- phosphatase α, which is critical for neural development (23). The tion and maintenance of the corpus luteum. Within the testes, MRPS26 gene resides about 300 bp downstream of the GnRH2 FSH regulates spermatogenesis and LH elicits secretion of testos- gene (21), encoding mitochondrial ribosome protein S26, which terone. Ultimately, the gonads cease to function and reproduction assists in protein synthesis (24). A graphical representation of the is halted in the absence of GnRH1 (1–3). porcine GnRH2 gene is depicted in Figure 1A. GnRH Variants in Mammals The human GnRH2 gene has three coding exons like the GnRH1 gene; however, the GnRH2 gene is notably shorter Gonadotropin-releasing hormone 1 was first identified in the (2.1 versus 5.1 kb), primarily due to differences in intron length hypothalami of pigs and sheep (4–6) and was originally thought (20). Otherwise, organization of the GnRH1 and GnRH2 genes to be a novel peptide. However, 23 other forms of GnRH have remain similar (25). The first coding exon in humans encodes the since been discovered (7), all with 10 amino acids and at least signal sequence, mature decapeptide, and a portion of the GnRH- a 50% sequence identity (8). Within these forms, the sequences associated peptide (GAP). The second and third exons encode of both the N-terminus (pGlu–His–Trp–Ser) and C-terminus the remaining GAP (20). Likewise, porcine prepro-GnRH2 (Pro–Gly–NH2) are conserved (7, 9). The amino acid substitu- is encoded by 3 exons and yields a 110 amino acid product tions only occur between residues 5 and 8 (7, 9). In vertebrates, (Figure 1B) that must undergo post-translational proteolytic three forms of GnRH (GnRH1, GnRH2, and GnRH3) are the processing for functionality (20). most common. The third form of GnRH (GnRH3; pGlu–His– Trp–Ser–His–Asp–Trp–Lys–Pro–Gly–NH2) was first discovered Presence of the GnRH2 Gene in Mammals in lamprey (10) but the GnRH3 gene has only been confirmed in Although the GnRH2 gene was first identified in humans fish and amphibians to date (7, 11). Therefore, only GnRH1 and (20), Stewart et al. (21) examined the genomes of mammals GnRH2 are produced in mammals (7). encompassing 10 orders for the presence of the GnRH2 gene. GONADOTROPIN-RELEASING The GnRH2 gene was positively identified in 21 animals. Using bioinformatics, the authors concluded that gene coding errors HORMONE 2 likely prevent the successful production of GnRH2 in many species (21). A summary of the coding errors present in the The Second Form of Mammalian GnRH GnRH2 gene of mammals is available in Table 1. A premature (GnRH2) stop codon truncates the mature decapeptide in the chimpanzee, A second structural variant of GnRH, GnRH2, has been identi- orangutan, mouse lemur, sheep, and cat (21, 26), whereas the fied in mammals. Like GnRH1, GnRH2 is a decapeptide but it rabbit, pika, cow, dog, cat, and dolphin GnRH2 genes encode was first isolated from the hypothalami of 10,000 chickens and an inactive peptide (21, 26). Early evidence implied that the rat therefore named “chicken GnRH2” (12). It was later discovered and mouse also maintain a GnRH2 gene as immunoreactive in mammals, the first being marsupials (13), and renamed simply GnRH2 was detected in the rodent brain (27–29). Although it “GnRH2” to prevent confusion (14). Since then, GnRH2 has was later determined that the mouse genome only maintains a been found in animals of every vertebrate class including primi- fragment of the GnRH2 gene (exon 1) on chromosome 2 and it tive orders (e.g., bony fish) as well as complex mammals 15( ). is completely deleted from chromosome 3 in the rat (21, 22, 30).