How to Contribute to the Progress of Neuroendocrinology: Discovery of GnIH and Progress of GnIH Research

Abstract

It is essential to discover novel neuropeptides that regulate the functions of pituitary, brain and peripheral secretory glands for the progress of neuroendocrinology. Gonadotropin-releasing hormone (GnRH), a hypothalamic neuropeptide stimulating gonadotropin release was isolated and its structure was determined by Schally's and Guillemin's groups at the beginning of the 1970s. It was subsequently shown that GnRH is highly conserved among vertebrates. GnRH was assumed the sole hypothalamic neuropeptide that regulates gonadotropin release in vertebrates based on extensive studies of GnRH over the following three decades. However, in 2000, Tsutsui's group isolated and determined the structure of a novel hypothalamic neuropeptide, which inhibits gonadotropin release, in quail, an avian species, and named it gonadotropin-inhibitory hormone (GnIH). Following studies by Tsutsui's group demonstrated that GnIH is highly conserved among vertebrates, from humans to agnathans, and acts as a key neuropeptide inhibiting reproduction. Intensive research on GnIH demonstrated that GnIH inhibits gonadotropin synthesis and release by acting on gonadotropes and GnRH neurons via GPR147 in birds and mammals. Fish GnIH also regulates gonadotropin release according to its reproductive condition, indicating the conserved role of GnIH in the regulation of the hypothalamic-pituitary-gonadal (HPG) axis in vertebrates. Therefore, we can now say that GnRH is not the only hypothalamic neuropeptide controlling vertebrate reproduction. In addition, recent studies by Tsutsui's group demonstrated that GnIH acts in the brain to regulate behaviors, including reproductive behavior. The 18 years of GnIH research with leading laboratories in the world have significantly advanced our knowledge of the neuroendocrine control mechanism of reproductive physiology and behavior as well as interactions of the HPG, hypothalamic-pituitary-adrenal and hypothalamic-pituitary-thyroid axes. This review describes how GnIH was discovered and GnIH research progressed in this new research era of reproductive neuroendocrinology.

Keywords: glucocorticoid; gonadotropin-inhibitory hormone (GnIH); gonadotropin-releasing hormone (GnRH); gonadotropins; melatonin; norepinephrine; reproduction; thyroid hormone.

Figure 1

Multiple sequence alignment and phylogenetic analysis of chordate GnIH (LPXRFa) and NPFF (PQRFa) precursor proteins as well as synteny analysis of human GnIH and NPFF genes. (A) Multiple sequence alignment of vertebrate GnIH (LPXRFa) and NPFF (PQRFa) precursor proteins highlighting the sequences of identified and predicted biologically active peptides. Precursor protein sequences were aligned by EMBL-EBI Clustal Omega Multiple Sequence Alignment software. Biochemically identified mature peptide sequences are shown in bold. Adapted from Ubuka and Tsutsui (35). (B) Phylogenetic tree of chordate GnIH (LPXRFa) and NPFF (PQRFa) precursor proteins. The evolutionary history was inferred using the Neighbor-Joining method. The percentage of replicate trees in which the associated taxa clustered together in the bootstrap test (1,000 replicates) are shown next to the branches. The tree is drawn to scale, with branch lengths in the same units as those of the evolutionary distances used to infer the phylogenetic tree. The evolutionary distances were computed using the Poisson correction method and are in the units of the number of amino acid substitutions per site. Evolutionary analyses were conducted in MEGA7 (36). Accession numbers are human (Homo sapiens) GnIH precursor (Human GnIH; NP_071433.3), Japanese quail (Coturnix japonica) GnIH precursor (Quail GnIH; XP_015709159.1), Japanese fire belly newt (Cynops pyrrhogaster) GnIH precursor (Newt LPXRFamide peptide; BAJ78290.1), West Indian Ocean coelacanth (Latimeria chalumnae) GnIH precursor (Coelacanth LPXRFamide peptide; XP_005993154.1), zebrafish (Danio rerio) GnIH precursor (Zebrafish LPXRFamide peptide, NP_001076418.1), spotted gar (Lepisosteus oculatus) GnIH precursor (Gar LPXRFamide peptide; XP_015213317.1), sea lamprey (Petromyzon marinus) GnIH precursor (Petromyzon marinus LPXRFamide peptide; BAL52329.1), Japanese amphioxus (Branchiostoma japonicum) GnIH precursor (Branchiostoma japonicum RFamide peptide; BAO77760.1), human NPFF precursor isoform 1 (Human NPFF isoform 1; NP_003708.1), human NPFF precursor isoform 2 (Human NPFF isoform 2; NP_001307225.1), Japanese quail NPFF precursor (Quail NPFF; XP_015705838.1), Western painted turtle (Chrysemys picta bellii) NPFF precursor (Turtle NPFF; XP_005307776.1), zebrafish NPFF precursor (Zebrafish NPFF; BAF34891.1), spotted gar NPFF precursor isoform X2 (Gar NPFF isoform 2; XP_015199730.1), sea lamprey NPFF precursor (Petromyzon marinus PQRFamide peptide; BAE79779.1), Florida lancelet (Branchiostoma floridae) RFamide precursor 1 (Branchiostoma floridae RFamide peptide 1; XP_002599251.1), Florida lancelet RFamide precursor 2 (Branchiostoma floridae RFamide peptide 2; XP_002609543.1). Fruit fly (Drosophila melanogaster) FMRFamide precursor (Fruit fly FMRFamide; NP_523669.2) served as outgroup (root) of the evolutionary tree. (C) Synteny analysis of human GnIH and NPFF genes. Paralogous genes are linked by dotted lines. Adapted from Osugi et al. (37).

Figure 2

GnIH actions and regulation of GnIH biosynthesis by environmental and internal factors. Cell bodies for GnIH neurons are located in the hypothalamus, paraventricular nucleus in birds and the dorsomedial hypothalamic area in mammals. GnIH neuronal terminals are located to the median eminence (ME) and GnRH1 neurons in the preoptic area in birds and mammals. GnIH receptor is expressed in gonadotropes in the pituitary and GnRH1 neurons in birds and mammals. Thus, GnIH inhibits gonadotropin synthesis and release by directly acting on gonadotropes in the pituitary and by inhibiting the activity of GnRH1 neurons via GnIH receptor in birds and mammals. GnIH neurons project not only to GnRH1 neurons but also to kisspeptin neurons in the hypothalamus in mammals. Kisspeptin neurons express GnIH receptor. GnIH and GnIH receptor are expressed in steroidogenic cells and germ cells in gonads, and GnIH acts in an autocrine/paracrine manner to suppress sex steroid production and germ cell differentiation and maturation in birds and mammals. GnIH participates not only in neuroendocrine functions but also in the control of behavior in birds and mammals. GnIH inhibits reproductive behaviors, such as sexual and aggressive behaviors, by acting within the brain. Furthermore, GnIH inhibits reproductive behaviors by stimulating the biosynthesis of neuroestrogen (E2) in the POA. GnIH neurons further project to many other neurons in the brain suggesting multiple actions of GnIH. Environmental factors, such as photoperiod, stress and social interaction, and internal factors, such as melatonin, glucocorticoid and norepinephrine, are important for the control of reproduction and reproductive behaviors. GnIH expression and release are modulated via a melatonin-dependent process. Melatonin increases GnIH expression in quail and rats, but melatonin decreases GnIH expression in hamsters and sheep. Stress increases GnIH expression by the actions of glucocorticoids in birds and mammals. Thus, GnIH is a mediator of stress-induced reproductive disruption. The social environment also changes GnIH expression and release mediated by the action of norepinephrine. Stimulatory regulations are shown by arrows, whereas inhibitory regulations are shown by blunt end lines. Lines with a question mark indicate morphological evidence without demonstration of physiological actions. ME, median eminence; POMC, proopiomelanocortin; MCH, melanin-concentrating hormone; CRH, corticotropin-releasing hormone.

Figure 3

The crosstalk of GnIH with internal factors of different endocrine axes. The interaction between the HPA axis and the axis is mediated by GC and GnIH. Stress suppresses gonadotropin secretion through the increase in GnIH expression in mammals and birds. In addition, the interaction between the axis and the axis was demonstrated. TH-mediated HPG regulation is initiated by inhibiting the expression of GnIH, which acts at the most upstream level of the HPG axis by inhibiting the activity of GnRH neurons to reduce circulating levels of gonadotropins (LH and FSH) and gonadal sex steroids. High concentrations of TH decrease GnIH expression, whereas a lower level of TH increases GnIH expression. The increased GnIH expression induced by hypothyroidism delays pubertal onset. Stimulatory regulations are shown by arrows, whereas inhibitory regulations are shown by blunt end lines. Lines with a question mark indicate morphological evidence without demonstration of physiological actions. HPA, hypothalamic-pituitary-adrenal; HPG, hypothalamic-pituitary-gonadal; HPT, hypothalamic-pituitary-thyroid; GC, glucocorticoid; TH, thyroid hormone.