Molecular evolution of GPCRs: Ghrelin/ghrelin receptors

Evolution of Vertebrate Hormones and Their Receptors: Insights from Non-Osteichthyan Genomes

Homeostatic control and reproductive functions of humans are regulated at the molecular levels largely by peptide hormones secreted from endocrine and/or neuroendocrine cells in the central nervous system and peripheral organs. Homologs of those hormones and their receptors function similarly in many vertebrate species distantly related to humans, but the evolutionary history of the endocrine system involving those factors has been obscured by the scarcity of genome DNA sequence information of some taxa that potentially contain their orthologs. Focusing on non-osteichthyan vertebrates, namely jawless and cartilaginous fishes, this article illustrates how investigating genome sequence information assists our understanding of the diversification of vertebrate gene repertoires in four broad themes: (a) the presence or absence of genes, (b) multiplication and maintenance of paralogs, (c) differential fates of duplicated paralogs, and (d) the evolutionary timing of gene origins.

Hormone resistance and short stature: A journey through the pathways of hormone signaling

Hormone resistances have been described in association with growth disorders, the majority involving the growth hormone (GH)/insulin-like growth factor 1(IGF-1) axis or hormones with specific paracrine-autocrine actions in the growth plate. Defects in hormone receptors or in proteins involved in intracellular signal transduction (post-receptor defects) are the main mechanisms of hormone resistance leading to short stature. The characteristic phenotypes of each of these hormonal resistances are very distinct and bring with them important insights into the role of each hormone and its signaling pathway. In this review, we discuss the molecular and clinical aspects of the main hormone resistances associated with short stature in humans.

Identification of a Putative β-Arrestin Superagonist of the Growth Hormone Secretagogue Receptor (GHSR)

Ghrelin is a pleiotropic feeding hormone which also has a pivotal role in the central nervous system. Upon the activation of its receptor, growth hormone secretagogue receptor (GHSR), the Gα_q/11 -mediated and the β-arrestin-mediated signaling pathways are activated. As the β-arrestin pathway is a potential drug target, there is a strong need for β-arrestin-biased GHSR modulators. Activation of the β-arrestin pathway should inhibit the Gα_q/11 -mediated calcium flux through internalization of the receptor. Hence, we used the antagonistic activity in the calcium assay as the first screening for the β-arrestin activation. By conducting the second screening assay for the β-arrestin activation based on extracellular signal regulated kinase (ERK) 1/2 phosphorylation, we discovered a putative β-arrestin-biased superagonist. The activity of the compound was not completely blocked with the competitive antagonist, which implies that the effect is mediated, at least partly, by allosteric binding of the compound.

The Zinc-Sensing Receptor GPR39 in Physiology and as a Pharmacological Target

The G-protein coupled receptor GPR39 is abundantly expressed in various tissues and can be activated by changes in extracellular Zn²⁺ in physiological concentrations. Previously, genetically modified rodent models have been able to shed some light on the physiological functions of GPR39, and more recently the utilization of novel synthetic agonists has led to the unraveling of several new functions in the variety of tissues GPR39 is expressed. Indeed, GPR39 seems to be involved in many important metabolic and endocrine functions, but also to play a part in inflammation, cardiovascular diseases, saliva secretion, bone formation, male fertility, addictive and depression disorders and cancer. These new discoveries offer opportunities for the development of novel therapeutic approaches against many diseases where efficient therapeutics are still lacking. This review focuses on Zn²⁺ as an endogenous ligand as well as on the novel synthetic agonists of GPR39, placing special emphasis on the recently discovered physiological functions and discusses their pharmacological potential.

Ghrelin and food acquisition in wild and cultured Japanese eel (Anguilla japonica)

To clarify the relationships between growth, endocrine status and habitat characteristics in Japanese eel (Anguilla japonica), plasma and stomach mRNA levels of ghrelin were examined in wild eels captured in the river and the bay, and in cultured eels during and after experimental fasting. Wild juvenile eels captured in freshwater habitats within the river showed significantly higher plasma and stomach mRNA levels of ghrelin than did fish obtained from brackish-water habitats within the bay. In cultured eels experimentally fasted for 4 weeks, plasma and stomach mRNA levels of ghrelin increased. After refeeding, the both parameters returned to the levels observed in continuously feeding control fish. In pigmented elvers, 2 months of feed restriction resulted in a significant increase in whole-body ghrelin mRNA. It is suggested that interaction between ghrelin and feeding is related to their habitats through differential food acquisition in fresh and brackish water environments.

Ghrelin and Its Receptors in Gilthead Sea Bream: Nutritional Regulation

Ghrelin is involved in the regulation of growth in vertebrates through controlling different functions, such as feed intake, metabolism, intestinal activity or growth hormone (Gh) secretion. The aim of this work was to identify the sequences of preproghrelin and Ghrelin receptors (ghsrs), and to study their responses to different nutritional conditions in gilthead sea bream (Sparus aurata) juveniles. The structure and phylogeny of S. aurata preproghrelin was analyzed, and a tissue screening was performed. The effects of 21 days of fasting and 2, 5, 24 h, and 7 days of refeeding on plasma levels of Ghrelin, Gh and Igf-1, and the gene expression of preproghrelin, ghsrs and members of the Gh/Igf-1 system were determined in key tissues. preproghrelin and the receptors are well conserved, being expressed mainly in stomach, and in the pituitary and brain, respectively. Twenty-one days of fasting resulted in a decrease in growth while Ghrelin plasma levels were elevated to decrease at 5 h post-prandial when pituitary ghsrs expression was minimum. Gh in plasma increased during fasting and slowly felt upon refeeding, while plasma Igf-1 showed an inverse profile. Pituitary gh expression augmented during fasting reaching maximum levels at 1 day post-feeding while liver igf-1 expression and that of its splice variants decreased to lowest levels. Liver Gh receptors expression was down-regulated during fasting and recovered after refeeding. This study demonstrates the important role of Ghrelin during fasting, its acute down-regulation in the post-prandial stage and its interaction with pituitary Ghsrs and Gh/Igf-1 axis.

Orphan GPR61, GPR62 and GPR135 receptors and the melatonin MT2 receptor reciprocally modulate their signaling functions

Understanding the function of orphan G protein-coupled receptors (GPCRs), whose cognate ligand is unknown, is of major importance as GPCRs are privileged drug targets for many diseases. Recent phylogenetic studies classified three orphan receptors, GPR61, GPR62 and GPR135 among the melatonin receptor subfamily, but their capacity to bind melatonin and their biochemical functions are not well characterized yet. We show here that GPR61, GPR62 and GPR135 do not bind [³H]-melatonin nor 2-[¹²⁵I]iodomelatonin and do not respond to melatonin in several signaling assays. In contrast, the three receptors show extensive spontaneous ligand-independent activities on the cAMP, inositol phosphate and ß-arrestin pathways with distinct pathway-specific profiles. Spontaneous ß-arrestin recruitment internalizes all three GPRs in the endosomal compartment. Co-expression of the melatonin binding MT₂ receptor with GPR61, GPR62 or GPR135 has several consequences such as (i) the formation of receptor heteromers, (ii) the inhibition of melatonin-induced ß-arrestin2 recruitment to MT₂ and (iii) the decrease of elevated cAMP levels upon melatonin stimulation in cells expressing spontaneously active GPR61 and GPR62. Collectively, these data show that GPR61, GPR62 and GPR135 are unable to bind melatonin, but show a reciprocal regulatory interaction with MT₂ receptors.

Ghrelin Facilitates GLUT2-, SGLT1- and SGLT2-mediated Intestinal Glucose Transport in Goldfish (Carassius auratus)

Glucose homeostasis is an important biological process that involves a variety of regulatory mechanisms. This study aimed to determine whether ghrelin, a multifunctional gut-brain hormone, modulates intestinal glucose transport in goldfish (Carassius auratus). Three intestinal glucose transporters, the facilitative glucose transporter 2 (GLUT2), and the sodium/glucose co-transporters 1 (SGLT1) and 2 (SGLT2), were studied. Immunostaining of intestinal sections found colocalization of ghrelin and GLUT2 and SGLT2 in mucosal cells. Some cells containing GLUT2, SGLT1 and SGLT2 coexpressed the ghrelin/growth hormone secretagogue receptor 1a (GHS-R1a). Intraperitoneal glucose administration led to a significant increase in serum ghrelin levels, as well as an upregulation of intestinal preproghrelin, ghrelin O-acyltransferase and ghs-r1 expression. In vivo and in vitro ghrelin treatment caused a concentration- and time-dependent modulation (mainly stimulatory) of GLUT2, SGLT1 and SGLT2. These effects were abolished by the GHS-R1a antagonist [D-Lys3]-GHRP-6 and the phospholipase C inhibitor U73122, suggesting that ghrelin actions on glucose transporters are mediated by GHS-R1a via the PLC/PKC signaling pathway. Finally, ghrelin stimulated the translocation of GLUT2 into the plasma membrane of goldfish primary intestinal cells. Overall, data reported here indicate an important role for ghrelin in the modulation of glucoregulatory machinery and glucose homeostasis in fish.

The chicken is an interesting animal for study of the functional role of ghrelin in the gastrointestinal tract

Ghrelin has been identified in vertebrates from fish to mammals, and it has multiple biological activities including gastrointestinal (GI) motor-stimulating action. In some non-mammalian vertebrates, we examined the effects of ghrelin on contractility of the isolated GI tract as well as the mRNA expression of growth hormone secretagogue-receptor 1a (GHS-R1a) to determine whether the motor-stimulating action of ghrelin is common in vertebrates. The expression level of GHS-R1a mRNA differed depending on the species and on the GI region (stomach, small intestine, and colon). GI region-dependent expression of GHS-R1a mRNA was remarkable in chickens, and the expression levels changed depending on age. In a functional study, ghrelin did not cause contraction of unstimulated GI strips in fish (goldfish and rainbow trout) or amphibians (bullfrog and Japanese fire belly newts) even using their homologous ghrelin. In avian species, ghrelin caused contraction of the unstimulated GI tract of the chicken but not of the Japanese quail, and the responses to ghrelin in the chicken GI tract decreased with aging. Our in vitro studies show that the motor-stimulating action of ghrelin is not conserved across vertebrates and that the chicken is a unique animal species for evaluation of the GI-stimulating action of ghrelin of different age.

Current knowledge of ghrelin in amphibians

We are exploring physiological importance of the ghrelin system in vertebrates. This review summarizes current knowledge of the ghrelin system in amphibians. Our study on ghrelin precursor in various amphibians revealed that the third amino acid with acyl modification has changed to threonine (Thr-3) instead of serine (Ser-3) only in the genus, Rana. Functional analyses of the ghrelin receptor in three species of amphibians, Japanese fire belly newt, American bullfrog and Japanese tree frog revealed that ghrelin and GHS-R1a agonists increase intracellular Ca²⁺ concentration in HEK293 cells expressing each receptor, and that ligand selectivity of ghrelin with Ser-3 and Thr-3 that expected to see in the bullfrog receptor was not found in the two frog receptors, but in the newt receptor. The brain, gastrointestinal tract, kidney and gonad highly express GHS-R1a mRNA. In frogs and newt, fasting did not increase GHS-R1a mRNA expression in the brain, but in the stomach. However, intraperitoneal (IP) injection of ghrelin did not affect food intake. A dehydration treatment increased GHS-R1a mRNA expression in the brain, stomach and ventral skin in the tree frog. However, intracerebroventricular (ICV) injection of ghrelin did not affect water absorption. Ghrelin did not influence gastrointestinal motility in in vitro studies using smooth muscle strips of the bullfrog and newt in vitro. These results suggest that the ghrelin system is present in various amphibians, but little is known about the physiological functions except hypophyseal hormone secretion.

The atypical antipsychotic, olanzapine, potentiates ghrelin-induced receptor signaling: An in vitro study with cells expressing cloned human growth hormone secretagogue receptor

The growth hormone secretagogue receptor (GHS-R) belongs to Gαq-coupled G protein-coupled receptor (GPCR) that mediates growth hormone release, food intake, appetite, glucose metabolism and body composition. Ghrelin has been identified as an endogenous ligand for GHS-R, and it is the only orexigenic peptide found in the peripheral organs. Olanzapine, an atypical antipsychotic agent that binds to and inhibits the activation of GPCR for several neurotransmitters, has metabolic side effects such as excessive appetite and weight gain. Recently, studies have revealed that the orexigenic mechanism of olanzapine is mediated via GHS-R signaling, although the precise mechanisms have not been clarified. In this study, we investigated the effect of olanzapine on ghrelin-mediated GHS-R signaling by using an electrical impedance-based receptor biosensor assay system (CellKey™). Olanzapine at concentrations of 10(-7) and 10(-6)mol/L enhanced ghrelin-induced (10(-10)-10(-8)mol/L) GHS-R activation. A Ca(2+) imaging assay revealed that olanzapine (10(-7) and 10(-6)mol/L) enhanced ghrelin (10(-7) M)-induced GHS-R activity. In contrast, haloperidol (an antipsychotic agent) failed to enhance this ghrelin-mediated GHS-R activation, as demonstrated by both the CellKey™ and Ca(2+) imaging assays. Together, these results suggest that olanzapine, but not haloperidol, promotes appetite by enhancing ghrelin-mediated GHS-R signaling.

Diversification and coevolution of the ghrelin/growth hormone secretagogue receptor system in vertebrates

The gut hormone ghrelin is involved in numerous metabolic functions, such as the stimulation of growth hormone secretion, gastric motility, and food intake. Ghrelin is modified by ghrelin O-acyltransferase (GOAT) or membrane-bound O-acyltransferase domain-containing 4 (MBOAT4) enabling action through the growth hormone secretagogue receptors (GHS-R). During the course of evolution, initially strong ligand/receptor specificities can be disrupted by genomic changes, potentially modifying physiological roles of the ligand/receptor system. Here, we investigated the coevolution of ghrelin, GOAT, and GHS-R in vertebrates. We combined similarity search, conserved synteny analyses, phylogenetic reconstructions, and protein structure comparisons to reconstruct the evolutionary history of the ghrelin system. Ghrelin remained a single-gene locus in all vertebrate species, and accordingly, a single GHS-R isoform was identified in all tetrapods. Similar patterns of the nonsynonymous (dN) and synonymous (dS) ratio (dN/dS) in the vertebrate lineage strongly suggest coevolution of the ghrelin and GHS-R genes, supporting specific functional interactions and common physiological pathways. The selection profiles do not allow confirmation as to whether ghrelin binds specifically to GOAT, but the ghrelin dN/dS patterns are more similar to those of GOAT compared to MBOAT1 and MBOAT2 isoforms. Four GHS-R isoforms were identified in teleost genomes. This diversification of GHS-R resulted from successive rounds of duplications, some of which remained specific to the teleost lineage. Coevolution signals are lost in teleosts, presumably due to the diversification of GHS-R but not the ghrelin gene. The identification of the GHS-R diversity in teleosts provides a molecular basis for comparative studies on ghrelin's physiological roles and regulation, while the comparative sequence and structure analyses will assist translational medicine to determine structure-function relationships of the ghrelin/GHS-R system.

Endocannabinoid Regulation of Neuroendocrine Systems

The hypothalamus is a part of the brain that is critical for sustaining life through its homeostatic control and integrative regulation of the autonomic nervous system and neuroendocrine systems. Neuroendocrine function in mammals is mediated mainly through the control of pituitary hormone secretion by diverse neuroendocrine cell groups in the hypothalamus. Cannabinoid receptors are expressed throughout the hypothalamus, and endocannabinoids have been found to exert pronounced regulatory effects on neuroendocrine function via modulation of the outputs of several neuroendocrine systems. Here, we review the physiological regulation of neuroendocrine function by endocannabinoids, focusing on the role of endocannabinoids in the neuroendocrine regulation of the stress response, food intake, fluid homeostasis, and reproductive function. Cannabis sativa (marijuana) has a long history of recreational and/or medicinal use dating back to ancient times. It was used as an analgesic, anesthetic, and antianxiety herb as early as 2600 B.C. The hedonic, anxiolytic, and mood-elevating properties of cannabis have also been cited in ancient records from different cultures. However, it was not until 1964 that the psychoactive constituent of cannabis, Δ(9)-tetrahydrocannabinol, was isolated and its chemical structure determined (Gaoni & Mechoulam, 1964).

Ghrelin O-Acyl Transferase in Zebrafish Is an Evolutionarily Conserved Peptide Upregulated During Calorie Restriction

Ghrelin is a multifunctional orexigenic hormone with a unique acyl modification enabled by ghrelin O-acyl transferase (GOAT). Ghrelin is well-characterized in nonmammals, and GOAT sequences of several fishes are available in the GenBank. However, endogenous GOAT in non-mammals remains poorly understood. In this research, GOAT sequence comparison, tissue-specific GOAT expression, and its regulation by nutrient status and exogenous ghrelin were studied. It was found that the bioactive core of zebrafish GOAT amino acid sequence share high identity with that of mammals. GOAT mRNA was most abundant in the gut. GOAT-like immunoreactivity (i.r.) was found colocalized with ghrelin in the gastric mucosa. Food deprivation increased, and feeding decreased GOAT and preproghrelin mRNA expression in the brain and gut. GOAT and ghrelin peptides in the gut and brain showed corresponding decrease in food-deprived state. Intraperitoneal injection of acylated fish ghrelin caused a significant decrease in GOAT mRNA expression, suggesting a feedback mechanism regulating its abundance. Together, these results provide the first in-depth characterization of GOAT in a non-mammal. Our results demonstrate that endogenous GOAT expression is responsive to metabolic status and availability of acylated ghrelin, providing further evidences for GOAT in the regulation of feeding in teleosts.

Identification, tissue distribution and functional characterization of the ghrelin receptor in West African lungfish, Protopterus annectens

We identified two ghrelin receptor isoforms, the ghrelin receptor type-1a (GHS-R1a) and its alternative splice form (GHS-R1b) for West African lungfish, Protopterus annectens. Lungfish GHS-R1a and 1b comprised 361 and 281 amino acids, respectively. Lungfish GHS-R1a showed the highest identity to coelacanth GHS-R1a (80.4%). The highest expression of GHS-R1a mRNAs was seen in the brain, liver, ovary, heart, intestine, and gills. GHS-R1b mRNAs were also detected in the same tissues with GHS-R1a, but their expression level was 1/20 that of GHS-R1a. In human embryonic kidney 293 cells transiently expressing lungfish GHS-R1a, rat and bullfrog ghrelin, and two GHS-R1a agonists, GHRP-6 and hexarelin, increased intracellular Ca(2+) concentrations. The intensity of the Ca(2+) increases induced by GHS-R1a agonists was twice when compared to that induced by ghrelin, although the median effective doses (ED50) were similar, suggesting a long-lasting effect of GHS-R1a agonists with similar affinity. We also examined changes in the GHS-R gene expression during an eight-week estivation. Body weight was slightly lowered, but plasma sodium and glucose concentrations decreased; plasma urea concentration increased significantly 4weeks after the start of estivation. Overall, expression of GHS-R1a mRNA decreased, but changes in GHS-R1b mRNA expression were inconsistent with those of GHS-R1a during estivation, suggesting an involvement of GHS-R in energy homeostasis, as seen in mammals. Our results suggest that the ghrelin-GHS-R1a system is present in this lungfish although ghrelin has not yet been found. The structure of GHS-R1a is closer to that of tetrapods than Actinopterygian fish, indicating a process of evolution that follows the Crossopterygii such as coelacanth.