The Novel Actions of the Metabolite GnRH-(1-5) are Mediated by a G Protein-Coupled Receptor

Characterizing the SREB G protein-coupled receptor family in fish: Brain gene expression and genomic differences in upstream transcription factor binding sites

The SREB (Super-conserved Receptors Expressed in Brain) family of orphan G protein-coupled receptors is highly conserved in vertebrates and consists of three members: SREB1 (orphan designation GPR27), SREB2 (GPR85), and SREB3 (GPR173). SREBs are associated with processes ranging from neuronal plasticity to reproductive control. Relatively little is known about similarities across the entire family, or how mammalian gene expression patterns compare to non-mammalian vertebrates. In fish, this system may be particularly complex, as some species have gained a fourth member (SREB3B) while others have lost genes. To better understand the system, the present study aimed to: 1) use qPCR to characterize sreb and related gene expression patterns in the brains of three fish species with different systems, and 2) identify possible differences in transcriptional regulation among the receptors, using upstream transcription factor binding sites across 70 ray-finned fish genomes. Overall, regional patterns of sreb expression were abundant in forebrain-related areas. However, some species-specific patterns were detected, such as abundant expression of receptors in zebrafish (Danio rerio) hypothalamic-containing sections, and divergence between sreb3a and sreb3b in pufferfish (Dichotomyctere nigroviridis). In addition, a gene possibly related to the system (dkk3a) was spatially correlated with the receptors in all three species. Genomic regions upstream of sreb2 and sreb3b, but largely not sreb1 or sreb3a, contained many highly conserved transcription factor binding sites. These results provide novel information about expression differences and transcriptional regulation across fish that may inform future research to better understand these receptors.

Superconserved receptors expressed in the brain: Expression, function, motifs and evolution of an orphan receptor family

GPR27, GPR85 and GPR173 constitute a small family of G protein-coupled receptors (GPCR) that share the distinctive characteristics of being highly conserved throughout vertebrate evolution and predominantly expressed in the brain. Accordingly, they have been coined as "Superconserved Receptors Expressed in the Brain" (SREB), although their expression profile is more complex than what was originally thought. SREBs have no known validated endogenous ligands and are thus labeled as "orphan" receptors. The investigation of this particular category of uncharacterized receptors holds great promise both in terms of physiology and drug development. In the largest GPCR family, the Rhodopsin-like or Class A, around 100 receptors are considered orphans. Because GPCRs are the most successful source of drug targets, the discovery of a novel function or ligand most likely will lead to significant breakthroughs for the discovery of innovative therapies. The high level of conservation is one of the characteristic features of the SREBs. We propose herein a detailed analysis of the putative evolutionary origin of this family. We highlight the properties that distinguish SREBs from other rhodopsin-like GPCRs. We present the current evidence for these receptors downstream signaling pathways and functions. We discuss the pharmacological challenge for the identification of natural or synthetic ligands of orphan receptors like SREBs. The different SREB-related scientific questions are presented with a highlight on what should be addressed in the near future, including the confirmation of published evidence and their validation as drug targets. In particular, we discuss in which pathological conditions these receptors may be of great relevance to solve unmet medical needs.

New Aspects of Corpus Luteum Regulation in Physiological and Pathological Conditions: Involvement of Adipokines and Neuropeptides

The corpus luteum is a small gland of great importance because its proper functioning determines not only the appropriate course of the estrous/menstrual cycle and embryo implantation, but also the subsequent maintenance of pregnancy. Among the well-known regulators of luteal tissue functions, increasing attention is focused on the role of neuropeptides and adipose tissue hormones—adipokines. Growing evidence points to the expression of these factors in the corpus luteum of women and different animal species, and their involvement in corpus luteum formation, endocrine function, angiogenesis, cells proliferation, apoptosis, and finally, regression. In the present review, we summarize the current knowledge about the expression and role of adipokines, such as adiponectin, leptin, apelin, vaspin, visfatin, chemerin, and neuropeptides like ghrelin, orexins, kisspeptin, and phoenixin in the physiological regulation of the corpus luteum function, as well as their potential involvement in pathologies affecting the luteal cells that disrupt the estrous cycle.

Characterization of the G protein-coupled receptor family SREB across fish evolution

The SREB (Super-conserved Receptors Expressed in Brain) family of G protein-coupled receptors is highly conserved across vertebrates and consists of three members: SREB1 (orphan receptor GPR27), SREB2 (GPR85), and SREB3 (GPR173). Ligands for these receptors are largely unknown or only recently identified, and functions for all three are still beginning to be understood, including roles in glucose homeostasis, neurogenesis, and hypothalamic control of reproduction. In addition to the brain, all three are expressed in gonads, but relatively few studies have focused on this, especially in non-mammalian models or in an integrated approach across the entire receptor family. The purpose of this study was to more fully characterize sreb genes in fish, using comparative genomics and gonadal expression analyses in five diverse ray-finned (Actinopterygii) species across evolution. Several unique characteristics were identified in fish, including: (1) a novel, fourth euteleost-specific gene (sreb3b or gpr173b) that likely emerged from a copy of sreb3 in a separate event after the teleost whole genome duplication, (2) sreb3a gene loss in Order Cyprinodontiformes, and (3) expression differences between a gar species and teleosts. Overall, gonadal patterns suggested an important role for all sreb genes in teleost testicular development, while gar were characterized by greater ovarian expression that may reflect similar roles to mammals. The novel sreb3b gene was also characterized by several unique features, including divergent but highly conserved amino acid positions, and elevated brain expression in puffer (Dichotomyctere nigroviridis) that more closely matched sreb2, not sreb3a. These results demonstrate that SREBs may differ among vertebrates in genomic structure and function, and more research is needed to better understand these roles in fish.

Gonadotropin-Releasing Hormone and GnRH Receptor: Structure, Function and Drug Development

BACKGROUND

Gonadotropin-Releasing Hormone (GnRH) is a key element in sexual maturation and regulation of the reproductive cycle in the human organism. GnRH interacts with the pituitary cells through the activation of the Gonadotropin Releasing Hormone Receptors (GnRHR). Any impairments/dysfunctions of the GnRH-GnRHR complex lead to the development of various cancer types and disorders. Furthermore, the identification of GnRHR as a potential drug target has led to the development of agonist and antagonist molecules implemented in various treatment protocols. The development of these drugs was based on the information derived from the functional studies of GnRH and GnRHR.

OBJECTIVE

This review aims at shedding light on the versatile function of GnRH and GnRH receptor and offers an apprehensive summary regarding the development of different agonists, antagonists and non-peptide GnRH analogues.

CONCLUSION

The information derived from these studies can enhance our understanding of the GnRH-GnRHR versatile nature and offer valuable insight into the design of new more potent molecules.

Phoenixin-A Pleiotropic Gut-Brain Peptide

Phoenixin is a recently discovered brain peptide initially thought to be restricted to reproductive functions. The subsequent identification of phoenixin’s expression in peripheral tissues was accompanied by the description of several other actions of this hormone, such as effects on behavior, sensory perception, memory retention, the cardiovascular system as well as food intake, pointing towards a pleiotropic role of this peptide. The present review will discuss the present knowledge on phoenixin and the signaling involved as well as highlight gaps in knowledge to stimulate further research.

Effect of hypothyroidism on the hypothalamic-pituitary-ovarian axis and reproductive function of pregnant rats

BACKGROUND

This study aimed to detect changes in hormone levels in the hypothalamic-pituitary-ovarian axis in Sprague-Dawley (SD) rats with hypothyroidism, and identify differences in the pregnancy and abortion rates of female adult rats. The potential role of gonadotropin releasing hormone (GnRH) as the link between the hypothalamic-pituitary-ovarian axis and reproductive function regulated by thyroid hormones was also investigated.

METHODS

Female SD rats (n = 136) were causally classified into two groups: the normal-drinking-water group (n = 60) and the 0.05% propylthiouracil-drinking-water group (PTU 2 mg/kg/day, n = 76) to establish an adult rat model of hypothyroidism (6 weeks). Female and male rats at a ratio of 1:2 were used to establish a hypothyroidism pregnancy model. GnRH mRNA and GnRH receptor (GnRHR) expression in rats was detected using real time quantitative PCR(qRT-PCR) and immunohistochemistry, respectively.

RESULTS

The abortion rate differed significantly between the hypothyroidism pregnancy group and the normal pregnancy group (P < 0.05). No significant differences were found in the distribution of the GnRHR among the five nuclei (hypothalamic arcuate nucleus, hypothalamic ventromedial nucleus, hypothalamic anterior nucleus, paraventricular nucleus of the hypothalamus, and ventral premammillary nucleus) of the hypothalamus and ovary (P > 0.05). Hypothyroidism had no significant effect on GnRH mRNA expression in the hypothalamic-pituitary-ovarian axis in the four groups (normal control group, normal pregnancy group, hypothyroidism pregnancy group, and hypothyroidism group) (P > 0.05).

CONCLUSIONS

Hypothyroidism had an adverse impact on pregnancy in rats and may affect the distribution of pituitary GnRHR, whereas it did not obviously affect the distribution of GnRHR in the nuclei of the hypothalamus and ovary. Hypothyroidism had no effect on GnRH mRNA expression.

Regulation of Gonadotropin-Releasing Hormone-(1-5) Signaling Genes by Estradiol Is Age Dependent

Gonadotropin-releasing hormone (GnRH) is a key regulatory molecule of the hypothalamus-pituitary (PIT)-gonadal (HPG) axis that ultimately leads to the downstream release of estradiol (E2) and progesterone (P). These gonadal steroids feed back to the hypothalamus and PIT to regulate reproductive function and behavior. While GnRH is thought to be the master regulator of reproduction, its metabolic product GnRH-(1-5) is also biologically active. Thimet oligopeptidase 1 (also known as EP24.15) cleaves GnRH to form GnRH-(1-5). GnRH-(1-5) is involved in regulation of the HPG axis, exerting its actions through a pair of orphan G protein-coupled receptors, GPR101 and GPR173. The physiological importance of GnRH-(1-5) signaling has been studied in several contexts, but its potential role during reproductive senescence is poorly understood. We used an ovariectomized (OVX) rat model of reproductive senescence to assess whether and how GnRH-(1-5) signaling genes in hypothalamic subnuclei change in response to aging and/or different estradiol replacement regimens designed to model clinical hormone replacement in women. We found that Gpr101 and Gpr173 mRNA expression was increased with age in the arcuate nucleus, while expression of Gpr173 and EP24.15 increased with age in the medial preoptic area. Treatment with E2 in younger OVX animals increased expression of Gpr101, Gpr173, and EP24.15. However, older animals treated with E2 showed decreased expression of these GnRH-(1-5) signaling genes, displaying an age-related decline in responsiveness to E2. To our knowledge, this is the first study to systematically assess the effects of age and different clinically relevant regimens of E2 replacement on GnRH-(1-5) signaling genes.

Paired Expression Analysis of Tumor Cell Surface Antigens

Adoptive immunotherapy with antibody-based therapy or with T cells transduced to express chimeric antigen receptors (CARs) is useful to the extent that the cell surface membrane protein being targeted is not expressed on normal tissues. The most successful CAR-based (anti-CD19) or antibody-based therapy (anti-CD20) in hematologic malignancies has the side effect of eliminating the normal B cell compartment. Targeting solid tumors may not provide a similar expendable marker. Beyond antibody to Her2/NEU and EGFR, very few antibody-based and no CAR-based therapies have seen broad clinical application for solid tumors. To expand the way in which the surfaceome of solid tumors can be analyzed, we created an algorithm that defines the pairwise relative overexpression of surface antigens. This enables the development of specific immunotherapies that require the expression of two discrete antigens on the surface of the tumor target. This dyad analysis was facilitated by employing the Hotelling’s T-squared test (Hotelling–Lawley multivariate analysis of variance) for two independent variables in comparison to a third constant entity (i.e., gene expression levels in normal tissues). We also present a unique consensus scoring mechanism for identifying transcripts that encode cell surface proteins. The unique application of our bioinformatics processing pipeline and statistical tools allowed us to compare the expression of two membrane protein targets as a pair, and to propose a new strategy based on implementing immunotherapies that require both antigens to be expressed on the tumor cell surface to trigger therapeutic effector mechanisms. Specifically, we found that, for MYCN amplified neuroblastoma, pairwise expression of ACVR2B or anaplastic lymphoma kinase (ALK) with GFRA3, GFRA2, Cadherin 24, or with one another provided the strongest hits. For MYCN, non-amplified stage 4 neuroblastoma, neurotrophic tyrosine kinase 1, or ALK paired with GFRA2, GFRA3, SSK1, GPR173, or with one another provided the most promising paired-hits. We propose that targeting these markers together would increase the specificity and thereby the safety of CAR-based therapy for neuroblastoma.

Peripheral and Central Mechanisms Involved in the Hormonal Control of Male and Female Reproduction

Reproduction involves the integration of hormonal signals acting across multiple systems to generate a synchronised physiological output. A critical component of reproduction is the luteinising hormone (LH) surge, which is mediated by oestradiol (E2 ) and neuroprogesterone interacting to stimulate kisspeptin release in the rostral periventricular nucleus of the third ventricle in rats. Recent evidence indicates the involvement of both classical and membrane E2 and progesterone signalling in this pathway. A metabolite of gonadotrophin-releasing hormone (GnRH), GnRH-(1-5), has been shown to stimulate GnRH expression and secretion, and has a role in the regulation of lordosis. Additionally, gonadotrophin release-inhibitory hormone (GnIH) projects to and influences the activity of GnRH neurones in birds. Stress-induced changes in GnIH have been shown to alter breeding behaviour in birds, demonstrating another mechanism for the molecular control of reproduction. Peripherally, paracrine and autocrine actions within the gonad have been suggested as therapeutic targets for infertility in both males and females. Dysfunction of testicular prostaglandin synthesis is a possible cause of idiopathic male infertility. Indeed, local production of melatonin and corticotrophin-releasing hormone could influence spermatogenesis via immune pathways in the gonad. In females, vascular endothelial growth factor A has been implicated in an angiogenic process that mediates development of the corpus luteum and thus fertility via the Notch signalling pathway. Age-induced decreases in fertility involve ovarian kisspeptin and its regulation of ovarian sympathetic innervation. Finally, morphological changes in the arcuate nucleus of the hypothalamus influence female sexual receptivity in rats. The processes mediating these morphological changes have been shown to involve the rapid effects of E2 controlling synaptogenesis in this hypothalamic nucleus. In summary, this review highlights new research in these areas, focusing on recent findings concerning the molecular mechanisms involved in the central and peripheral hormonal control of reproduction.

Identification and molecular docking studies for novel inverse agonists of SREB, super conserved receptor expressed in brain

The identification of novel synthetic ligands for G protein-coupled receptors (GPCRs) is important not only for understanding human physiology, but also for the development of novel drugs, especially for orphan GPCRs for which endogenous ligands are unknown. One of the orphan GPCR subfamilies, Super conserved Receptor Expressed in Brain (SREB), consists of GPR27, GPR85 and GPR173 and is expressed in the central nervous system. We report herein the identification of inverse agonists for the SREB family without their agonists. We carried out an in vitro screening of 5472 chemical compounds from the RIKEN NPDepo chemical library. The binding of [(35) S]GTPγS to the GPR173-Gsα fusion protein expressed in Sf9 cells was measured and resulted in the identification of 8 novel GPR173 inverse agonists. The most potent compound showed an IC50 of approximately 8 μm. The identified compounds were also antagonists for other SREB members, GPR27 and GPR85. These results indicated that the SREB family could couple Gs-type G proteins, and SREB-Gsα fusion proteins showed significant constitutive activities. Moreover, a molecular model of GPR173 was constructed using the screening results. The combination of computational and biological methods will provide a unique approach to ligand identification for orphan GPCRs and brain research.

β-Arrestin 2 is a mediator of GnRH-(1-5) signaling in immortalized GnRH neurons

We have previously demonstrated that the cleavage product of the full-length GnRH, GnRH-(1-5), is biologically active, binds G protein-coupled receptor 173 (GPR173), and inhibits the migration of cells in the immortalized GnRH-secreting GN11 cell. In this study, we attempted to characterize the GnRH-(1-5) intracellular signaling mechanism. To determine whether the signaling pathway mediating GnRH-(1-5) regulation of migration involves a G protein-dependent mechanism, cells were treated with a generic G protein antagonist in the presence and absence of GnRH-(1-5), and a wound-healing assay was conducted to measure migration. G Protein antagonist 2 treatment abolished the GnRH-(1-5) inhibition of migration, indicating that the mechanism of GnRH-(1-5) is G protein coupled. To identify the potential Gα-subunit recruited by GnRH-(1-5) binding GPR173, we measured the second messengers cAMP and inositol triphosphate levels. GnRH-(1-5) treatment did not alter cAMP levels relative to cells treated with vehicle or forskolin, suggesting that GnRH-(1-5) does not couple to the Gαs or Gαi subunits. Similarly, inositol triphosphate levels remained unchanged with GnRH-(1-5) treatment, indicating a mechanism not mediated by the Gαq/11 subunit. Therefore, we also examined whether GnRH-(1-5) activating GPR173 deviated from the canonical G protein-coupled receptor signaling pathway by coupling to β-arrestin 1/2 to regulate migration. Our coimmunoprecipitation studies indicate that GnRH-(1-5) induces the rapid interaction between GPR173 and β-arrestin 2 in GN11 cells. Furthermore, we demonstrate that this association recruits phosphatase and tensin homolog to mediate the downstream action of GnRH-(1-5). These findings suggest that the GnRH-(1-5) mechanism deviates from the canonical G protein-coupled receptor pathway to regulate cell migration in immortalized GnRH neurons.