Ghrelin/GHSR signaling in the lateral septum ameliorates chronic stress-induced depressive-like behaviors

Ghrelin is a gastrointestinal hormone on feeding and metabolism regulation, and acts through its receptor-growth hormone secretagogue receptor (GHSR), which is widely distributed throughout the central nervous system. Recent studies have suggested that ghrelin plays an important role in the regulation of depression, but the underlying mechanisms remain uncertain. Lateral septum (LS) is a critical brain region in modulating depression. Therefore, we investigated the role of ghrelin/GHSR signaling in the LS on the depressive-like behaviors of mice under conditions of chronic stress by using behavioral tests, neuropharmacology, and molecular biology techniques. We found that infusion of ghrelin into the LS produced antidepressant-like responses in mice. Activation of LS GABAergic neurons was involved in the antidepressant effect of ghrelin. Importantly, GHSR was highly expressed and distributed in the LS neurons. Blockade of GHSR in the LS reversed the ghrelin-induced antidepressant-like effects. Molecular knockdown of GHSR in the LS induced depressive-like symptoms in mice. Furthermore, administration of ghrelin into the LS alleviated depressive-like behaviors induced by chronic social defeat stress (CSDS). Consistent with the neuropharmacological results, overexpression of GHSR in the LS reversed CSDS-induced depressive-like behaviors. Our findings clarify a key role for ghrelin/GHSR signaling in the regulation of chronic stress-induced depressive-like behaviors, which could provide new strategies for the treatment of depression.

Synthesis and biological evaluation of novel 18F-labeled 2,4-diaminopyrimidine derivatives for detection of ghrelin receptor in the brain

The ghrelin receptor (GHSR) is known to regulate various physiological processes including appetite, food intake, and growth hormone release. Its expression is mainly observed in the brain, pancreas, stomach, and intestine. However, the functions of the receptor have not been fully elucidated. GHSR imaging with positron emission tomography (PET) is expected to further understanding of the functions and pathologies of the receptor. In this study, we newly designed and synthesized diaminopyrimidine derivatives ([18F]BPP-1 and [18F]BPP-2) and evaluated their utility as novel PET probes targeting GHSR. In in vitro competitive binding assays, the binding affinity of BPP-2 for GHSR (Ki = 274 nM) was comparable to that of the diaminopyimidine lead compound Abb8a (Ki = 109 nM). In a biodistribution study using normal mice, [18F]BPP-2 displayed low uptake in the brain and moderate uptake in the pancreas, but high radioactivity accumulation in bone was observed due to its defluorination in vivo. Taken together, although further improvement of the pharmacokinetics is needed, the diaminopyrimidine scaffold has potential for the development of useful GHSR-targeting PET probes.

Nutrient-sensing growth hormone secretagogue receptor in macrophage programming and meta-inflammation

OBJECTIVE

Obesity-associated chronic inflammation, aka meta-inflammation, is a key pathogenic driver for obesity-associated comorbidity. Growth hormone secretagogue receptor (GHSR) is known to mediate the effects of nutrient-sensing hormone ghrelin in food intake and fat deposition. We previously reported that global Ghsr ablation protects against diet-induced inflammation and insulin resistance, but the site(s) of action and mechanism are unknown. Macrophages are key drivers of meta-inflammation. To unravel the role of GHSR in macrophages, we generated myeloid-specific Ghsr knockout mice (LysM-Cre;Ghsrf/f).

METHODS

LysM-Cre;Ghsrf/f and control Ghsrf/f mice were subjected to 5 months of high-fat diet (HFD) feeding to induce obesity. In vivo, metabolic profiling of food intake, physical activity, and energy expenditure, as well as glucose and insulin tolerance tests (GTT and ITT) were performed. At termination, peritoneal macrophages (PMs), epididymal white adipose tissue (eWAT), and liver were analyzed by flow cytometry and histology. For ex vivo studies, bone marrow-derived macrophages (BMDMs) were generated from the mice and treated with palmitic acid (PA) or lipopolysaccharide (LPS). For in vitro studies, macrophage RAW264.7 cells with Ghsr overexpression or Insulin receptor substrate 2 (Irs2) knockdown were studied.

RESULTS

We found that Ghsr expression in PMs was increased under HFD feeding. In vivo, HFD-fed LysM-Cre;Ghsrf/f mice exhibited significantly attenuated systemic inflammation and insulin resistance without affecting food intake or body weight. Tissue analysis showed that HFD-fed LysM-Cre;Ghsrf/f mice have significantly decreased monocyte/macrophage infiltration, pro-inflammatory activation, and lipid accumulation, showing elevated lipid-associated macrophages (LAMs) in eWAT and liver. Ex vivo, Ghsr-deficient macrophages protected against PA- or LPS-induced pro-inflammatory polarization, showing reduced glycolysis, increased fatty acid oxidation, and decreased NF-κB nuclear translocation. At molecular level, GHSR metabolically programs macrophage polarization through PKA-CREB-IRS2-AKT2 signaling pathway.

CONCLUSIONS

These novel results demonstrate that macrophage GHSR plays a key role in the pathogenesis of meta-inflammation, and macrophage GHSR promotes macrophage infiltration and induces pro-inflammatory polarization. These exciting findings suggest that GHSR may serve as a novel immunotherapeutic target for the treatment of obesity and its associated comorbidity.

Hypobaric hypoxia induces iron mobilization from liver and spleen and increases serum iron via activation of ghrelin/GHSR1a/MAPK signalling pathway in mice

Hypobaric hypoxia (HH) exposure affects appetite and serum iron levels in both humans and animals. Thus, whether appetite-regulating ghrelin is involved in iron regulation under HH needs to be elucidated. In vivo, C57BL/6J mice were placed in a hypobaric chamber to establish a 6000-m-high altitude exposure animal model. In vitro, mouse primary hepatocytes and peritoneal macrophages were exposed to hypoxia (1% O2) to examine the effects of ghrelin on iron-regulating proteins. HH obviously reduced the body weight of mice and significantly increased the levels of erythrocytes, and also significantly enhanced the levels of serum iron and plasma ghrelin. However, iron content in the liver and spleen was decreased, while ferroportin (Fpn) expression was increased. Moreover, ghrelin significantly induced Fpn and pERK expression in both hepatocytes and macrophages under hypoxia, which were reversed by pretreatment with growth hormone secretagogue receptor 1a (GHSR1a) antagonist or pERK inhibitor. Our findings indicated that HH leads to decreased appetite and insufficient dietary intake, which may negatively regulate the levels of ghrelin. Furthermore, GHSR1a/ERK signalling pathway is further activated to upregulate the expression of Fpn, and then promoting iron mobilization both in the liver/hepatocytes and spleen/macrophages in mice. Thus, these results revealed that ghrelin may be a potential iron regulatory hormone, and raised the possibility of ghrelin as a promising therapeutic target against iron disorders under HH.

Desacylghrelin modulates GHS-R1 receptor expression and cell differentiation in placental BeWo cells

BACKGROUND

and purpose: Ghrelin is the endogenous ligand of the growth hormone secretagogue receptor (GHS-R1). Ghrelin, and GHS-R1, may have a role in placental growth and function, and its unacylated form desacylghrelin (DAG) could be involved in fetal growth. Nevertheless, the effects of DAG on placental function, and the receptor involved in its actions, remain to be determined. We aimed to investigate the effect of DAG in placental BeWo cells viability, proliferation, differentiation, and GSH-R1 expression.

METHODS

BeWo cells, a human trophoblast cell line, was cultured with 3 nM DAG during 12, 24, 48, and 72 h. Cell viability, proliferation, differentiation (assessed by human Chorionic Gonadotropin quantification), and GSH-R1 expression were analyzed. To evaluate the mechanism of DAG effect on GSH-R1, 30 nM receptor antagonist ([D-Lys3]-GHRP-6) was added alone or in combination with 3 nM DAG during 12 h and 24 h.

RESULTS

DAG has no effect on cell proliferation or viability, but it has an inhibitory effect on cell differentiation. DAG had a stimulatory effect on GSH-R1 expression at 12 and 24 h (p = 0.029 and p = 0.025, respectively). On the contrary, culture with 48 h DAG inhibits GSH-R1 expression compared to the control (p = 0.005), while GSH-R1 antagonist inhibited the effect of DAG on GSH-R1 expression. DAG also reduces intracellular (p = 0.020) and secreted (p = 0.011) hCG concentration in BeWo cells.

CONCLUSION

DAG increases GHS-R1 expression, potentially mediated through GHS-R1 itself. DAG may also inhibit placental BeWo cell differentiation, suggesting a possible role of DAG in placental and fetal physiology.

Ghrelin's orexigenic action in the lateral hypothalamic area involves indirect recruitment of orexin neurons and arcuate nucleus activation

OBJECTIVE

Ghrelin is a potent orexigenic hormone, and the lateral hypothalamic area (LHA) has been suggested as a putative target mediating ghrelin's effects on food intake. Here, we aimed to investigate the presence of neurons expressing ghrelin receptor (a.k.a. growth hormone secretagogue receptor, GHSR) in the mouse LHA (LHAGHSR neurons), its physiological implications and the neuronal circuit recruited by local ghrelin action.

METHODS

We investigated the distribution of LHAGHSR neurons using different histologic strategies, including the use of a reporter mice expressing enhanced green fluorescent protein under the control of the GHSR promoter. Also, we investigated the physiological implications of local injections of ghrelin within the LHA, and the extent to which the orexigenic effect of intra-LHA-injected ghrelin involves the arcuate nucleus (ARH) and orexin neurons of the LHA (LHAorexin neurons) RESULTS: We found that: 1) LHAGHSR neurons are homogeneously distributed throughout the entire LHA; 2) intra-LHA injections of ghrelin transiently increase food intake and locomotor activity; 3) ghrelin's orexigenic effect in the LHA involves the indirect recruitment of LHAorexin neurons and the activation of ARH neurons; and 4) LHAGHSR neurons are not targeted by plasma ghrelin.

CONCLUSIONS

We provide a compelling neuroanatomical and functional characterization of LHAGHSR neurons in male mice that indicates that LHAGHSR cells are part of a hypothalamic neuronal circuit that potently induces food intake.

Analysis of the Dynamics of the Human Growth Hormone Secretagogue Receptor Reveals Insights into the Energy Landscape of the Molecule

G protein-coupled receptors initiate signal transduction in response to ligand binding. Growth hormone secretagogue receptor (GHSR), the focus of this study, binds the 28 residue peptide ghrelin. While structures of GHSR in different states of activation are available, dynamics within each state have not been investigated in depth. We analyze long molecular dynamics simulation trajectories using "detectors" to compare dynamics of the apo and ghrelin-bound states yielding timescale-specific amplitudes of motion. We identify differences in dynamics between apo and ghrelin-bound GHSR in the extracellular loop 2 and transmembrane helices 5-7. NMR of the GHSR histidine residues reveals chemical shift differences in these regions. We evaluate timescale specific correlation of motions between residues of ghrelin and GHSR, where binding yields a high degree of correlation for the first 8 ghrelin residues, but less correlation for the helical end. Finally, we investigate the traverse of GHSR over a rugged energy landscape via principal component analysis.

Nutrient-Sensing Ghrelin Receptor in Macrophages Modulates Bisphenol A-Induced Intestinal Inflammation in Mice

Bisphenols are environmental toxins with endocrine disruptor activity, yet bisphenol A (BPA) and its analogs are still widely used in manufacturing plastic products. There is evidence showing that BPA elicits inflammation in humans and animals, but the target cell types of BPA are not well understood. In this study, we sought to determine BPA's direct effect on macrophages and BPA immunotoxicity in mouse intestine. Ghrelin is an important nutrient-sensing hormone, acting through its receptor growth hormone secretagogue receptor (GHSR) to regulate metabolism and inflammation. We found that BPA promotes intestinal inflammation, showing increased infiltrating immune cells in colons and enhanced expression of Ghsr and pro-inflammatory cytokines and chemokines, such as Il6 and Ccl2, in colonic mucosa. Moreover, we found that both long- and short-term BPA exposure elevated pro-inflammatory monocytes and macrophages in mouse peripheral blood mononuclear cells (PBMC) and peritoneal macrophages (PM), respectively. To determine the role of GHSR in BPA-mediated inflammation, we generated Ghsr deletion mutation in murine macrophage RAW264.7 using CRISPR gene editing. In wild-type RAW264.7 cells, the BPA exposure promotes macrophage pro-inflammatory polarization and increases Ghsr and cytokine/chemokine Il6 and Ccl2 expression. Interestingly, Ghsr deletion mutants showed a marked reduction in pro-inflammatory cytokine/chemokine expression in response to BPA, suggesting that GHSR is required for the BPA-induced pro-inflammatory response. Further understanding how nutrient-sensing GHSR signaling regulates BPA intestinal immunotoxicity will help design new strategies to mitigate BPA immunotoxicity and provide policy guidance for BPA biosafety.

Maternal acrylamide exposure changes intestinal epithelium, immunolocalization of leptin and ghrelin and their receptors, and gut barrier in weaned offspring

Acrylamide (ACR) is an amide formed as a byproduct in many heat-processed starchy-rich foods. In utero ACR exposure has been associated with restricted fetal growth, but its effects of postnatal functional development of small intestine is completely unknown. The current study investigated the time- and segment-dependent effects of prenatal ACR exposure on morphological and functional development of small intestine in weaned rat offspring. Four groups of pregnant female Wistar rats were exposed to ACR (3 mg/kg b.w./day) for 0, 5, 10 and 15 days during pregnancy. Basal intestinal morphology, immunolocalization of gut hormones responsible for food intake and proteins of intestinal barrier, activity of the intestinal brush border disaccharidases, apoptosis and proliferation in intestinal mucosa were analyzed in offspring at weaning (postnatal day 21). The results showed that in utero ACR exposure disturbs offspring gut structural and functional postnatal development in a time- and segment-depended manner and even a short prenatal exposure to ACR resulted in changes in intestinal morphology, immunolocalization of leptin and ghrelin and their receptors, barrier function, activity of gut enzymes and upregulation of apoptosis and proliferation. In conclusion, prenatal ACR exposure disturbed the proper postnatal development of small intestine.

Sodium is a negative allosteric regulator of the ghrelin receptor

The functional properties of G protein-coupled receptors (GPCRs) are intimately associated with the different components in their cellular environment. Among them, sodium ions have been proposed to play a substantial role as endogenous allosteric modulators of GPCR-mediated signaling. However, this sodium effect and the underlying mechanisms are still unclear for most GPCRs. Here, we identified sodium as a negative allosteric modulator of the ghrelin receptor GHSR (growth hormone secretagogue receptor). Combining 23Na-nuclear magnetic resonance (NMR), molecular dynamics, and mutagenesis, we provide evidence that, in GHSR, sodium binds to the allosteric site conserved in class A GPCRs. We further leveraged spectroscopic and functional assays to show that sodium binding shifts the conformational equilibrium toward the GHSR-inactive ensemble, thereby decreasing basal and agonist-induced receptor-catalyzed G protein activation. All together, these data point to sodium as an allosteric modulator of GHSR, making this ion an integral component of the ghrelin signaling machinery.

Hindbrain ghrelin and liver-expressed antimicrobial peptide 2, ligands for growth hormone secretagogue receptor, bidirectionally control food intake

Hindbrain growth hormone secretagogue receptor (GHSR) agonism increases food intake, yet the underlying neural mechanisms remain unclear. The functional effects of hindbrain GHSR antagonism by its endogenous antagonist liver-expressed antimicrobial peptide 2 (LEAP2) are also yet unexplored. To test the hypothesis that hindbrain GHSR agonism attenuates the food intake inhibitory effect of gastrointestinal (GI) satiation signals, ghrelin (at a feeding subthreshold dose) was administered to the fourth ventricle (4V) or directly to the nucleus tractus solitarius (NTS) before systemic delivery of the GI satiation signal cholecystokinin (CCK). Also examined, was whether hindbrain GHSR agonism attenuated CCK-induced NTS neural activation (c-Fos immunofluorescence). To investigate an alternate hypothesis that hindbrain GHSR agonism enhances feeding motivation and food seeking, intake stimulatory ghrelin doses were administered to the 4V and fixed ratio 5 (FR-5), progressive ratio (PR), and operant reinstatement paradigms for palatable food responding were evaluated. Also assessed were 4V LEAP2 delivery on food intake and body weight (BW) and on ghrelin-stimulated feeding. Both 4V and NTS ghrelin blocked the intake inhibitory effect of CCK and 4V ghrelin blocked CCK-induced NTS neural activation. Although 4V ghrelin increased low-demand FR-5 responding, it did not increase high-demand PR or reinstatement of operant responding. Fourth ventricle LEAP2 reduced chow intake and BW and blocked hindbrain ghrelin-stimulated feeding. Data support a role for hindbrain GHSR in bidirectional control of food intake through mechanisms that include interacting with the NTS neural processing of GI satiation signals but not food motivation and food seeking.

GHSR1a deficiency suppresses inhibitory drive on dCA1 pyramidal neurons and contributes to memory reinforcement

Growth hormone secretagogue receptor 1a (GHSR1a)-the receptor for orexigenic hormone ghrelin-is a G protein-coupled receptor that is widely distributed in the brain, including the hippocampus. Studies have demonstrated that genetic deletion of GHSR1a affects memory, suggesting the importance of ghrelin/GHSR1a signaling in cognitive control. However, current reports are controversial, and the mechanism underlying GHSR1a modulation of memory is uncertain. Here, we first report that global GHSR1a knockout enhances hippocampus-dependent memory, facilitates initial LTP in dorsal hippocampal Schaffer Collateral-CA1 synapses, and downregulates Akt activity in the hippocampus. Moreover, we show that the intrinsic excitability of GAD67+ interneurons-rather than neighboring pyramidal neurons in the dCA1-is suppressed by GHSR1a deletion, an effect that is antagonized by acute application of the Akt activator SC79. In addition, the inhibitory postsynaptic currents (IPSCs) on dCA1 pyramidal neurons are selectively reduced in mice with a GHSR1a deficiency. Finally, we demonstrate that selectively increasing the excitability of parvalbumin-expressing interneurons by hM3Dq-DREADDs increases IPSCs on dCA1 pyramidal neurons and normalizes memory in Ghsr1a KO mice. Our findings thus reveal a novel mechanism underlying memory enhancement of GHSR1a deficiency and herein support an adverse effect of GHSR1a signaling in hippocampus-dependent memory processes.

Ghrelin receptor signaling in health and disease: a biased view

As allosteric complexes, G-protein-coupled receptors (GPCRs) respond to extracellular stimuli and pleiotropically couple to intracellular transducers to elicit signaling pathway-dependent effects in a process known as biased signaling or functional selectivity. One such GPCR, the ghrelin receptor (GHSR1a), has a crucial role in restoring and maintaining metabolic homeostasis during disrupted energy balance. Thus, pharmacological modulation of GHSR1a bias could offer a promising strategy to treat several metabolism-based disorders. Here, we summarize current evidence supporting GHSR1a functional selectivity in vivo and highlight recent structural data. We propose that precise determinations of GHSR1a molecular pharmacology and pathway-specific physiological effects will enable discovery of GHSR1a drugs with tailored signaling profiles, thereby providing safer and more effective treatments for metabolic diseases.

Molecular cloning and analysis of the ghrelin/GHSR system in Xenopus tropicalis

Ghrelin is a gut-derived peptide with several physiological functions, including feeding, gastrointestinal motility, and hormonal secretion. Recently, a host defense peptide, liver-expressed antimicrobial peptide-2 (LEAP2), was reported as an endogenous antagonist of growth hormone secretagogue receptor (GHS-R). The physiological relevance of the molecular LEAP2-GHS-R interaction in mammals has been explored; however, studies on non-mammals are limited. Here, we report the identification and functional characterization of ghrelin and its related molecules in Western clawed frog (Xenopus tropicalis), a known model organism. We first identified cDNA encoding X. tropicalis ghrelin and GHS-R. RT-qPCR revealed that ghrelin mRNA expression was most abundant in the stomach. GHS-R mRNA was widely distributed in the brain and peripheral tissues, and a relatively strong signal was observed in the stomach and intestine. In addition, LEAP2 was mainly expressed in intestinal tissues at higher levels than in the liver. In functional analysis, X. tropicalis ghrelin and human ghrelin induced intracellular Ca²⁺ mobilization with EC₅₀ values in the low nanomolar range in CHO-K1 cells expressing X. tropicalis GHS-R. Furthermore, ghrelin-induced GHS-R activation was antagonized with IC₅₀ values in the nanomolar range by heterologous human LEAP2. We also validated the expression of ghrelin and feeding-related factors under fasting conditions. After 2 days of fasting, no changes in ghrelin mRNA levels were observed in the stomach, but GHS-R mRNA levels were significantly increased, associated with significant downregulation of nucb2. In addition, LEAP2 upregulation was observed in the duodenum. These results provide the first evidence that LEAP2 functions as an antagonist of GHS-R in the anuran amphibian X. tropicalis. It has also been suggested that the ghrelin/GHS-R/LEAP2 system may be involved in energy homeostasis in X. tropicalis.

Elevated Ghrelin Promotes Hippocampal Ghrelin Receptor Defects in Humanized Amyloid-β Knockin Mice During Aging

BACKGROUND

Emerging evidence has revealed that dysregulation of the hormone ghrelin and its receptor, growth hormone secretagogue receptor (GHSR), contributes to the pathogenesis of Alzheimer's disease (AD). Specifically, defective GHSR function and resultant hippocampal ghrelin resistance are linked to hippocampal synaptic injury in AD paradigms. Also, AD patients exhibit elevated ghrelin activation. However, the detailed molecular mechanisms of hippocampal GHSR dysfunction and the relevance of ghrelin elevation to hippocampal ghrelin resistance in AD-relevant pathological settings are not fully understood.

OBJECTIVE

In the current study, we employed a recently established mouse line of AD risk [humanized amyloid beta knockin (hAβ KI mice), also referred to as a mouse model of late-onset AD in previous literature] to further define the role of ghrelin system dysregulation in the development of AD.

METHODS

We employed multidisciplinary techniques to determine the change of plasma ghrelin and the functional status of GHSR in hAβ KI mice as well as primary neuron cultures.

RESULTS

We observed concurrent plasma ghrelin elevation and hippocampal GHSR desensitization with disease progression. Further examination excluded the possibility that ghrelin elevation is a compensatory change in response to GHSR dysfunction. In contrast, further in vitro and in vivo results show that agonist-mediated overstimulation potentiates GHSR desensitization through enhanced GHSR internalization.

CONCLUSIONS

These findings suggest that circulating ghrelin elevation is a pathological event underlying hippocampal GHSR dysfunction, culminating in hippocampal ghrelin resistance and resultant synaptic injury in late-onset AD-related settings.

Toward a consensus nomenclature for ghrelin, its non-acylated form, liver expressed antimicrobial peptide 2 and growth hormone secretagogue receptor

The stomach-derived octanoylated peptide ghrelin was discovered in 1999 and recognized as an endogenous agonist of the growth hormone secretagogue receptor (GHSR). Subsequently, ghrelin has been shown to play key roles in controlling not only growth hormone secretion, but also a variety of other physiological functions including, but not limited to, food intake, reward-related behaviors, glucose homeostasis and gastrointestinal tract motility. Importantly, a non-acylated form of ghrelin, desacyl-ghrelin, can also be detected in biological samples. Desacyl-ghrelin, however, does not bind to GHSR at physiological levels, and its physiological role has remained less well-characterized than that of ghrelin. Ghrelin and desacyl-ghrelin are currently referred to in the literature using many different terms, highlighting the need for a consistent nomenclature. The variability of terms used to designate ghrelin can lead not only to confusion, but also to miscommunication, especially for those who are less familiar with the ghrelin literature. Thus, we conducted a survey among experts who have contributed to the ghrelin literature aiming to identify whether a consensus may be reached. Based on the results of this consensus, we propose using the terms "ghrelin" and "desacyl-ghrelin" to refer to the hormone itself and its non-acylated form, respectively. Based on the results of this consensus, we further propose using the terms "GHSR" for the receptor, and "LEAP2" for liver-expressed antimicrobial peptide 2, a recently recognized endogenous GHSR antagonist/inverse agonist.

Establishment of a Cell Line Stably Expressing the Growth Hormone Secretagogue Receptor to Identify Crocin as a Ghrelin Agonist

The growth hormone secretagogue receptor-1a (GHSR1a) is the endogenous receptor for ghrelin. Activation of GHSR1a participates in many physiological processes including energy homeostasis and eating behavior. Due to its transitory half-life, the efficacy of ghrelin treatment in patients is restricted; hence the development of new adjuvant therapy is an urgent need. This study aimed to establish a cell line stably expressing GHSR1a, which could be employed to screen potential ghrelin agonists from natural compounds. First, by means of lentiviral transduction, the genome of a human HEK293T cell was modified, and a cell platform stably overexpressing GHSR1a was successfully established. In this platform, GHSR1a was expressed as a fusion protein tagged with mCherry, which allowed the monitoring of the dynamic cellular distribution of GHSR1a by fluorescent microscopy. Subsequently, the authenticity of the GHSR1a mediated signaling was further characterized by using ghrelin and teaghrelin, two molecules known to stimulate GHSR1a. The results indicated that both ghrelin and teaghrelin readily activated GHSR1a mediated signaling pathways, presumably via increasing phosphorylation levels of ERK. The specific GHSR1a signaling was further validated by using SP-analog, an antagonist of GHSR1a as well as using a cell model with the knockdown expression of GHSR1a. Molecular modeling predicted that crocin might be a potential ghrelin agonist, and this prediction was further confirmed by the established platform.

The expression and function of growth hormone secretagogue receptor in immune cells: A current perspective

The orexigenic hormone ghrelin and its receptor, growth hormone secretagogue receptor (GHS-R), have been extensively studied in the last two decades, revealing that ghrelin signaling has important implications in health and disease. Metabolic diseases, such as obesity and diabetes, are often accompanied by low-grade chronic inflammation, that has been coined as "meta-inflammation." Immune cells are key cellular mediators of meta-inflammation, controlling both initiation and resolution of inflammation. Immune cells exhibit dynamic changes in cellular characteristics and functional output in response to the stimuli/insults from their surrounding microenvironment. Emerging evidence shows that ghrelin has an important effect on inflammation, in addition to its well-known effects on metabolism. However, the cellular/molecular mechanism of ghrelin signaling in immunity is largely unknown because the knowledge in regard to the expression and function of GHS-R in immune cells is currently sparse. In this review, we have accumulated the recent findings related to the expression and functions of GHS-R in various immune cells under different physiological and pathological states. This review aims to inspire further investigation of the immunological roles of ghrelin signaling and advance the therapeutic applications of ghrelin signaling in meta-inflammation.

Ghrelin and ghrelin receptor (GHSR) in Chinese alligator, alligator sinensis: Molecular characterization, tissue distribution and mRNA expression changes during the active and hibernating periods

The Chinese alligator (Alligator sinensis) is a freshwater crocodilian endemic to China. So far, the endocrine regulation of feeding and growth in Chinese alligator is poorly understood. In this study, the molecular structure and tissue expression profiles of ghrelin and its receptor GHSR in the Chinese alligator were characterized for the first time. The full-length cDNA of ghrelin was 1770 bp, including a 37 bp 5 '-UTR (untranslated region), a 435 bp ORF (open reading frame) and a 1298 bp 3 '-UTR. The ORF encodes a ghrelin precursor, which consists of 145 amino acid residues, including a signal peptide with 52 amino acid residues at the N-terminus, a mature peptide with 28 amino acid residues, and a possibly obestain at the C-terminus. The full-length cDNA of GHSR was 3961 bp, including a 5'-UTR of 375-bp, an ORF of 1059-bp and a 3' -UTR of 2527-bp. The ORF encodes a protein of 352 amino acid residues containing seven transmembrane domains, with multiple N glycosylation modification sites and conserved cysteine residue sites. The active core "GSSF" of Chinese alligator ghrelin was identical to that of mammals and birds, and the ghrelin binding site of GHSR was similar to that of mammals. The amino acid sequences of both ghrelin and GHSR share high identity with American alligator (Alligator mississippiensis) and birds. Ghrelin was highly expressed in cerebrum, mesencephalon, hypothalamus and multiple peripheral tissues, including lung, stomach and intestine, suggesting that it could play functions in paracrine and/or autocrine manners in addition to endocrine manner. GHSR expression level was higher in hypothalamus, epencephalon and medulla oblongata, and moderate in multiple peripheral tissues including lung, kindey, stomach and oviduct, implicating that ghrelin/GHSR system may participate in the regulation of energy balance, food intake, water and mineral balance, gastrointestinal motility, gastric acid secretion and reproduction. During hibernation, the expression of ghrelin and GHSR in the brain was significantly increased, while ghrelin was significantly decreased in heart, liver, lung, stomach, pancreas and ovary, and GHSR was significantly decreased in heart, liver, spleen, lung, kindey, stomach, ovary and oviduct. These temporal changes in ghrelin and GHSR expression could facilitate the physiological adaption to the hibernation of Chinese alligator. Our study could provide basic data for further studies on the regulation of feeding, physiological metabolism and reproduction of Chinese alligator, which could also be useful for the improvement of artificial breeding of this endangered species.

Role of the Ghrelin System in Colitis and Hepatitis as Risk Factors for Inflammatory-Related Cancers

It is not known exactly what leads to the development of colorectal cancer (CRC) and hepatocellular carcinoma (HCC), but there are specific risk factors that increase the probability of their occurrence. The unclear pathogenesis, too-late diagnosis, poor prognosis as a result of high recurrence and metastasis rates, and repeatedly ineffective therapy of both cancers continue to challenge both basic science and practical medicine. The ghrelin system, which is comprised of ghrelin and alternative peptides (e.g., obestatin), growth hormone secretagogue receptors (GHS-Rs), and ghrelin-O-acyl-transferase (GOAT), plays an important role in the physiology and pathology of the gastrointestinal (GI) tract. It promotes various physiological effects, including energy metabolism and amelioration of inflammation. The ghrelin system plays a role in the pathogenesis of inflammatory bowel diseases (IBDs), which are well known risk factors for the development of CRC, as well as inflammatory liver diseases which can trigger the development of HCC. Colitis-associated cancer serves as a prototype of inflammation-associated cancers. Little is known about the role of the ghrelin system in the mechanisms of transformation of chronic inflammation to low- and high-grade dysplasia, and, finally, to CRC. HCC is also associated with chronic inflammation and fibrosis arising from different etiologies, including alcoholic and nonalcoholic fatty liver diseases (NAFLD), and/or hepatitis B (HBV) and hepatitis C virus (HCV) infections. However, the exact role of ghrelin in the progression of the chronic inflammatory lesions into HCC is still unknown. The aim of this review is to summarize findings on the role of the ghrelin system in inflammatory bowel and liver diseases in order to better understand the impact of this system on the development of inflammatory-related cancers, namely CRC and HCC.

Ablation of Ghrelin Receptor Mitigates the Metabolic Decline of Aging Skeletal Muscle

The orexigenic hormone ghrelin has multifaceted roles in health and disease. We have reported that ablation of the ghrelin receptor, growth hormone secretagogue receptor (GHS-R), protects against metabolic dysfunction of adipose tissues in aging. Our further observation interestingly revealed that GHS-R deficiency phenocopies the effects of myokine irisin. In this study, we aim to determine whether GHS-R affects the metabolic functions of aging skeletal muscle and whether GHS-R regulates the muscular functions via irisin. We first studied the expression of metabolic signature genes in gastrocnemius muscle of young, middle-aged and old mice. Then, old GHS-R knockout (Ghsr^−/−) mice and their wild type counterparts were used to assess the impact of GHS-R ablation on the metabolic characteristics of gastrocnemius and soleus muscle. There was an increase of GHS-R expression in skeletal muscle during aging, inversely correlated with the decline of metabolic functions. Remarkedly the muscle of old GHS-R knockout (Ghsr^−/−) mice exhibited a youthful metabolic profile and better maintenance of oxidative type 2 muscle fibers. Furthermore, old Ghsr^−/− mice showed improved treadmill performance, supporting better functionality. Also intriguing to note was the fact that old GHS-R-ablated mice showed increased expression of the irisin precursor FNDC5 in the muscle and elevated plasma irisin levels in circulation, which supports a potential interrelationship between GHS-R and irisin. Overall, our work suggests that GHS-R has deleterious effects on the metabolism of aging muscle, which may be at least partially mediated by myokine irisin.

Ghrelin-Ghrelin receptor (GSHR) pathway via endocannabinoid signal affects the expression of NPY to promote the food intake of Siberian sturgeon (Acipenser baerii)

Previous data suggested that activation of endocannabinoid receptor 1 (CB1) was necessary for the orexigenic effect of Ghrelin in rodents, but the information is limited in teleosts. To investigate the feeding regulation pathway of Ghrelin and CB1 in Siberian sturgeon (Acipenser baerii), this study first identified the Ghrelin (345 bp, complete coding sequence) and Ghrelin receptor (GHSR, 500 bp, partial coding sequence) sequences, and then detected their tissue distribution patterns, which showed that Ghrelin is mainly distribution in peripheral tissues, while GSHR is mainly in different brain divisions. Besides, the qPCR before and after feeding showed that the mRNA expressions of Ghrelin and GHSR were inhibited after feeding in telencephalon, diencephalon and mesencephalon. Subsequently, the food intake and appetite factor expressions were measured by i.c.v. co-injection of Ghrelin and GSHR antagonist. The results showed that Ghrelin promoted the food intake of Siberian sturgeon, which was reversed by its receptor antagonist. Besides, i.c.v. injection of Ghrelin decreased telencephalon CART expression while increased NPY expression in the three brain regions. In addition, to further explore the relationship of Ghrelin and CB1 signal regulating feeding, the co-injection of Ghrelin and CB1 antagonists was performed. The results showed that AM6545 (CB1 peripheral restricted antagonist) failed to affect the orexigenic effect of Ghrelin and the expression pattern of NPY mRNA in the telencephalon. While in the diencephalon, the increase of food intake and NPY mRNA expression induced by Ghrelin was completely reversed by Rimonabant (CB1 global antagonist). These results indicate Ghrelin-GSHR pathway promotes the food intake of Siberian sturgeon by inducing the expression of NPY in the diencephalon, and the stimulating effect will be reversed by cannabinoid receptor antagonism. This study provides a foundation for understanding the pathways Ghrelin and CB1 signals in appetite regulation of the teleost.

[Development of Therapy for Cancer Cachexia in Pancreatic Cancer]

Cancer cachexia is a metabolic disorder syndrome that causes skeletal muscle loss and progressive physical dysfunction, which is observed in more than half of patients with advanced pancreatic cancer. Cancer cachexia is considered to be associated with poor prognosis, worsening adverse events, decreased treatment compliance, and decreased treatment efficacy. The ghrelin receptor agonist leads skeletal muscle gain, weight gain, and the improvement of anorexia in patients with cancer cachexia, and has been approved as an anti-cachexia treatment in Japan. Cancer cachexia patients who present with cachexia symptoms such as appetite loss as well as weight loss are able to receive the treatment of the ghrelin receptor agonist, but the evidence for patients with advanced pancreatic cancer is limited. Further research is needed to evaluate the efficacy of the ghrelin receptor agonist for cancer cachexia in pancreatic cancer patients. Inhibition of inflammatory cytokine such as interleukin-1α and nutrition and exercise therapy are under development for anti-cachexia therapy.

Body mass variability in age-matched outbred male Swiss mice is associated to differential control of food intake by ghrelin

Swiss mice belong to an outbred strain of mice largely used as a model for experimental obesity induced by high fat diet (HFD). We have previously demonstrated that a given cohort of age-matched Swiss mice is hallmarked by heterogeneous changes in body weight when exposed to HFD. The reasons underlying such variability, however, are not completely understood. Therefore we aimed to clarify the mechanisms underlying the variability in spontaneous weight gain in age-matched male swiss mice. To achieve that, individuals in a cohort of age-matched male Swiss mice were categorized as prone to body mass gain (PBMG) and resistant to body mass gain (RBMG). PBMG animals had higher caloric intake and body mass gain. RBMG and PBMG mice had a similar reduction in food intake when challenged with leptin but only RBMG exhibited a drop in ghrelin concentrations after refeeding. PBMG also showed increased midbrain levels of ghrelin receptor (Ghsr) and Dopamine receptor d2 (Drd2) mRNAs upon refeeding. Pharmacological blockade of GHSR with JMV3002 failed to reduce food intake in PMBG mice as it did in RBMG. On the other hand, the response to JMV3002 seen in PBMG was hallmarked by singular transcriptional response in the midbrain characterized by a simultaneous increase in both tyrosine hydroxylase (Th) and Proopiomelanocortin (Pomc) expressions. In conclusion, our data show that differences in the expression of genes related to the reward system in the midbrain as well as in ghrelin concentrations in serum correlate with spontaneous variability in body mass and food intake seen in age-matched male Swiss mice.

MRAP2 inhibits β-arrestin recruitment to the ghrelin receptor by preventing GHSR1a phosphorylation

The melanocortin receptor accessory protein 2 (MRAP2) is essential for several physiological functions of the ghrelin receptor growth hormone secretagogue receptor 1a (GHSR1a), including increasing appetite and suppressing insulin secretion. In the absence of MRAP2, GHSR1a displays high constitutive activity and a weak G-protein-mediated response to ghrelin and readily recruits β-arrestin. In the presence of MRAP2, however, G-protein-mediated signaling via GHSR1a is strongly dependent on ghrelin stimulation and the recruitment of β-arrestin is significantly diminished. To better understand how MRAP2 modifies GHSR1a signaling, here we investigated the role of several phosphorylation sites within the C-terminal tail and third intracellular loop of GHSR1a, as well as the mechanism behind MRAP2-mediated inhibition of β-arrestin recruitment. We show that Ser252 and Thr261 in the third intracellular loop of GHSR1a contribute to β-arrestin recruitment, whereas the C-terminal region is not essential for β-arrestin interaction. Additionally, we found that MRAP2 inhibits GHSR1a phosphorylation by blocking the interaction of GRK2 and PKC with the receptor. Taken together, these data suggest that MRAP2 alters GHSR1a signaling by directly impacting the phosphorylation state of the receptor and that the C-terminal tail of GHSR1a prevents rather than contribute to β-arrestin recruitment.

Beta-Hydroxybutyrate Suppresses Hepatic Production of the Ghrelin Receptor Antagonist LEAP2

INTRODUCTION

Liver-expressed antimicrobial peptide-2 (LEAP2) is an endogenous ghrelin receptor antagonist, which is upregulated in the fed state and downregulated during fasting. We hypothesized that the ketone body beta-hydroxybutyrate (BHB) is involved in the downregulation of LEAP2 during conditions with high circulating levels of BHB.

METHODS

Hepatic and intestinal Leap2 expression were determined in 3 groups of mice with increasing circulating levels of BHB: prolonged fasting, prolonged ketogenic diet, and oral BHB treatment. LEAP2 levels were measured in lean and obese individuals, in human individuals following endurance exercise, and in mice after BHB treatment. Lastly, we investigated Leap2 expression in isolated murine hepatocytes challenged with BHB.

RESULTS

We confirmed increased circulating LEAP2 levels in individuals with obesity compared to lean individuals. The recovery period after endurance exercise was associated with increased plasma levels of BHB levels and decreased LEAP2 levels in humans. Leap2 expression was selectively decreased in the liver after fasting and after exposure to a ketogenic diet for 3 weeks. Importantly, we found that oral administration of BHB increased circulating levels of BHB in mice and decreased Leap2 expression levels and circulating LEAP2 plasma levels, as did Leap2 expression after direct exposure to BHB in isolated murine hepatocytes.

CONCLUSION

From our data, we suggest that LEAP2 is downregulated during different states of energy deprivation in both humans and rodents. Furthermore, we here provide evidence that the ketone body, BHB, which is highly upregulated during fasting metabolism, directly downregulates LEAP2 levels. This may be relevant in ghrelin receptor-induced hunger signaling during energy deprivation.

Energetic demands of lactation produce an increase in the expression of growth hormone secretagogue receptor in the hypothalamus and ventral tegmental area of the rat despite a reduction in circulating ghrelin

Lactating rats show changes in the secretion of hormones and brain signals that promote hyperphagia and facilitate the production of milk. Little is known, however, about the role of ghrelin in the mechanisms sustaining lactational hyperphagia. Here, we used Wistar female rats that underwent surgery to sever the galactophores to prevent milk delivery (GC rats) and decrease the energetic drain of milk delivery. We compared plasma acyl-ghrelin concentrations and growth hormone secretagogue receptor (GHSR) mRNA expression in different brain regions of GC rats with those of sham operated lactating and nonlactating rats. Additional lactating and nonlactating rats were implanted with cannulae aimed at the lateral ventricles and were used to compare feeding responses to central ghrelin or GHSR antagonist infusions to those of nonlactating rats receiving similar infusions on day 14-16 postpartum (pp). Results show lower plasma acyl-ghrelin concentrations on day 15 pp sham operated lactating rats compared to GC or nonlactating rats. These changes occur in association with increased GHSR mRNA expression in the hypothalamic arcuate nucleus (ARC) and ventral tegmental area (VTA) of sham operated lactating rats. Despite lactational hyperphagia, infusions of ghrelin (0.25 or 1 μg) resulted in similar increases in food intake in lactating and nonlactating rats. In addition, infusions of the GHSR antagonist JMV3002 (4 μg in 1 μl of vehicle) produced greater suppression of food intake in lactating rats than in nonlactating rats. These data suggest that, despite lower plasma ghrelin, the energetic drain of lactation increases sensitivity to the orexigenic effects of ghrelin in brain regions important for food intake and energy balance, and these events are associated with lactational hyperphagia.

Novel Role of Ghrelin Receptor in Gut Dysbiosis and Experimental Colitis in Aging

Chronic low-grade inflammation is a hallmark of aging, which is now coined as inflamm-aging. Inflamm-aging contributes to many age-associated diseases such as obesity, type 2 diabetes, cardiovascular disease, and inflammatory bowel disease (IBD). We have shown that gut hormone ghrelin, via its receptor growth hormone secretagogue receptor (GHS-R), regulates energy metabolism and inflammation in aging. Emerging evidence suggests that gut microbiome has a critical role in intestinal immunity of the host. To determine whether microbiome is an integral driving force of GHS-R mediated immune-metabolic homeostasis in aging, we assessed the gut microbiome profiles of young and old GHS-R global knockout (KO) mice. While young GHS-R KO mice showed marginal changes in Bacteroidetes and Firmicutes, aged GHS-R KO mice exhibited reduced Bacteroidetes and increased Firmicutes, featuring a disease-susceptible microbiome profile. To further study the role of GHS-R in intestinal inflammation in aging, we induced acute colitis in young and aged GHS-R KO mice using dextran sulfate sodium (DSS). The GHS-R KO mice showed more severe disease activity scores, higher proinflammatory cytokine expression, and decreased expression of tight junction markers. These results suggest that GHS-R plays an important role in microbiome homeostasis and gut inflammation during aging; GHS-R suppression exacerbates intestinal inflammation in aging and increases vulnerability to colitis. Collectively, our finding reveals for the first time that GHS-R is an important regulator of intestinal health in aging; targeting GHS-R may present a novel therapeutic strategy for prevention/treatment of aging leaky gut and inflammatory bowel disease.

Status of ghrelin as an islet hormone and paracrine/autocrine regulator of insulin secretion

Ghrelin is expressed in the pancreatic islet cells as well as the stomach. In the perfused pancreas and isolated islets, GHS-R antagonism, ghrelin immunoneutralization and ghrelin-knockout (Ghr-KO) all increase glucose-induced insulin release. Thus, pharmacological, immunological and genetic blockades of ghrelin in the pancreatic islets all markedly augment glucose-induced insulin release, showing that islet-derived ghrelin physiologically restricts insulin release in rodents. In this review, we focus on the current understanding of the following key questions: 1) from which islet cells ghrelin is released, 2) on which islet cells ghrelin acts, and 3) mechanisms by which the islet-derived ghrelin inhibits insulin secretion.

The Role of Ghrelin/GHS-R1A Signaling in Nonalcohol Drug Addictions

Drug addiction causes constant serious health, social, and economic burden within the human society. The current drug dependence pharmacotherapies, particularly relapse prevention, remain limited, unsatisfactory, unreliable for opioids and tobacco, and even symptomatic for stimulants and cannabinoids, thus, new more effective treatment strategies are researched. The antagonism of the growth hormone secretagogue receptor type A (GHS-R1A) has been recently proposed as a novel alcohol addiction treatment strategy, and it has been intensively studied in experimental models of other addictive drugs, such as nicotine, stimulants, opioids and cannabinoids. The role of ghrelin signaling in these drugs effects has also been investigated. The present review aims to provide a comprehensive overview of preclinical and clinical studies focused on ghrelin's/GHS-R1A possible involvement in these nonalcohol addictive drugs reinforcing effects and addiction. Although the investigation is still in its early stage, majority of the existing reviewed experimental results from rodents with the addition of few human studies, that searched correlations between the genetic variations of the ghrelin signaling or the ghrelin blood content with the addictive drugs effects, have indicated the importance of the ghrelin's/GHS-R1As involvement in the nonalcohol abused drugs pro-addictive effects. Further research is necessary to elucidate the exact involved mechanisms and to verify the future potential utilization and safety of the GHS-R1A antagonism use for these drug addiction therapies, particularly for reducing the risk of relapse.

Effect of Growth Hormone Secretagogue Receptor Deletion on Growth, Pulsatile Growth Hormone Secretion, and Meal Pattern in Male and Female Mice

INTRODUCTION

While the vast majority of research investigating the role of ghrelin or its receptor, GHS-R1a, in growth, feeding, and metabolism has been conducted in male rodents, very little is known about sex differences in this system. Furthermore, the role of GHS-R1a signaling in the control of pulsatile GH secretion and its link with growth or metabolic parameters has never been characterized.

METHODS

We assessed the sex-specific contribution of GHS-R1a signaling in the activity of the GH/IGF-1 axis, metabolic parameters, and feeding behavior in adolescent (5-6 weeks old) or adult (10-19 weeks old) GHS-R KO (Ghsr-/-) and WT (Ghsr+/+) male and female mice.

RESULTS

Adult Ghsr-/- male and female mice displayed deficits in weight and linear growth that were correlated with reduced GH pituitary contents in males only. GHS-R1a deletion was associated with reduced meal frequency and increased meal intervals, as well as reduced hypothalamic GHRH and NPY mRNA in males, not females. In adult, GH release from Ghsr-/- mice pituitary explants ex vivo was reduced independently of the sex. However, in vivo pulsatile GH secretion decreased in adult but not adolescent Ghsr-/- females, while in males, GHS-R1a deletion was associated with reduction in pulsatile GH secretion during adolescence exclusively. In males, linear growth did not correlate with pulsatile GH secretion, but rather with ApEn, a measure that reflects irregularity of the rhythmic secretion. Fat mass, plasma leptin concentrations, or ambulatory activity did not predict differences in GH secretion.

DISCUSSION/CONCLUSION

These results point to a sex-dependent dimorphic effect of GHS-R1a signaling to modulate pulsatile GH secretion and meal pattern in mice with different compensatory mechanisms occurring in the hypothalamus of adult males and females after GHS-R1a deletion. Altogether, we show that GHS-R1a signaling plays a more critical role in the regulation of pulsatile GH secretion during adolescence in males and adulthood in females.

Involvement of POMC neurons in LEAP2 regulation of food intake and body weight

Liver-expressed antimicrobial peptide 2 (LEAP2) is a newly discovered antagonist of the growth hormone secretagogue receptor (GHSR) and is considered the first endogenous peptide that can antagonize the metabolic actions of ghrelin. The effects of ghrelin administration on feeding behavior, body weight, and energy metabolism involve the activation of orexigenic neurons in the arcuate nucleus (ARC) of the hypothalamus. It is unclear, however, if LEAP2 applied directly to the ARC of the hypothalamus affects these metabolic processes. Here, we show that overexpression of LEAP2 in the ARC through adeno-associated virus (AAV) reduced food intake and body weight in wild-type (WT) mice fed chow and a high-fat diet (HFD) and improved metabolic disorders. LEAP2 overexpression in the ARC overrides both central and peripheral ghrelin action on a chow diet. Interestingly, this AAV-LEAP2 treatment increased proopiomelanocortin (POMC) expression while agouti-related peptide (AGRP)/neuropeptide Y (NPY) and GHSR levels remained unchanged in the hypothalamus. Additionally, intracerebroventricular (i.c.v.) administration of LEAP2 decreased food intake, increased POMC neuronal activity, and repeated LEAP2 administration to mice induced body weight loss. Using chemogenetic manipulations, we found that inhibition of POMC neurons abolished the anorexigenic effect of LEAP2. These results demonstrate that central delivery of LEAP2 leads to appetite-suppressing and body weight reduction, which might require activation of POMC neurons in the ARC.

Circulating ghrelin crosses the blood-cerebrospinal fluid barrier via growth hormone secretagogue receptor dependent and independent mechanisms

Ghrelin is a peptide hormone mainly secreted from gastrointestinal tract that acts via the growth hormone secretagogue receptor (GHSR), which is highly expressed in the brain. Strikingly, the accessibility of ghrelin to the brain seems to be limited and restricted to few brain areas. Previous studies in mice have shown that ghrelin can access the brain via the blood-cerebrospinal fluid (CSF) barrier, an interface constituted by the choroid plexus and the hypothalamic tanycytes. Here, we performed a variety of in vivo and in vitro studies to test the hypothesis that the transport of ghrelin across the blood-CSF barrier occurs in a GHSR-dependent manner. In vivo, we found that the uptake of systemically administered fluorescent ghrelin in the choroid plexus epithelial (CPE) cells and in hypothalamic tanycytes depends on the presence of GHSR. Also, we detected lower levels of CSF ghrelin after a systemic ghrelin injection in GHSR-deficient mice, as compared to WT mice. In vitro, the internalization of fluorescent ghrelin was reduced in explants of choroid plexus from GHSR-deficient mice, and unaffected in primary cultures of hypothalamic tanycytes derived from GHSR-deficient mice. Finally, we found that the GHSR mRNA is detected in a pool of CPE cells, but is nearly undetectable in hypothalamic tanycytes with current approaches. Thus, our results suggest that circulating ghrelin crosses the blood-CSF barrier mainly by a mechanism that involves the GHSR, and also possibly via a GHSR-independent mechanism.

The emerging role of heterodimerisation and interacting proteins in ghrelin receptor function

Ghrelin is a peptide hormone secreted primarily by the stomach that acts upon the growth hormone secretagogue receptor (GHSR1), a G protein-coupled receptor whose functions include growth hormone secretion, appetite regulation, energy expenditure, regulation of adiposity, and insulin release. Following the discovery that GHSR1a stimulates food intake, receptor antagonists were developed as potential therapies to regulate appetite. However, despite reductions in signalling, the desired effects on appetite were absent. Studies in the past 15 years have demonstrated GHSR1a can interact with other transmembrane proteins, either by direct binding (i.e. heteromerisation) or via signalling cross-talk. These interactions have various effects on GHSR1a signalling including preferential coupling to one pathway (i.e. biased signalling), coupling to a unique G protein (G protein switching), suppression of GHSR1a signalling, and enhancement of signalling by both receptors. While many of these interactions have been shown in cells overexpressing the proteins of interest and remain to be verified in tissues, substantial evidence exists showing that GHSR1a and the dopamine receptor D1 (DRD1) form heteromers, which promote synaptic plasticity and formation of hippocampal memory. Additionally, a reduction in GHSR1a-DRD1 complexes in favour of establishment of GHSR1a-Aβ complexes correlates with Alzheimer's disease, indicating that GHSR1a heteromers may have pathological functions. Herein, we summarise the evidence published to date describing interactions between GHSR1a and transmembrane proteins, discuss the experimental strengths and limitations of these studies, describe the physiological evidence for each interaction, and address their potential as novel drug targets for appetite regulation, Alzheimer's disease, insulin secretion, and inflammation.

Synthesis of PF-6870961, a major hydroxy metabolite of the novel ghrelin receptor inverse agonist PF-5190457

Preclinical and human studies have indicated involvement of the ghrelin system in alcohol-related behaviors illuminating the possibility of using ghrelin receptor blockers as a pharmacological intervention for alcohol use disorder (AUD). Preliminary data from a recently conducted phase 1b human study with a ghrelin receptor inverse agonist, PF-5190457 (2-(2-methylimidazo[2,1-b][1,3thiazol-6-yl)-1-{2-(1R)-5-(6-methylpyrimidin-4-yl)-2,3-dihydro-1H-inden-1-yl]-2,7-diazaspiro[3.5]non-7-ylethanone), provided evidence on the safety and tolerability of this compound when co-administered with alcohol. Furthermore, the study revealed important information on the biotransformation pathways for this compound and prompted the discovery and then synthesis of a newly identified major metabolite, PF-6870961 ((R)-1-(2-(5-(2-hydroxy-6-methylpyrimidin-4-yl)-2,3-dihydro-1H-inden-1-yl)-2,7-diazaspiro[3.5]nonan-7-yl)-2-(2-methylimidazo[2,1-b]thiazol-6-yl)ethan-1-one). The metabolite was synthesized and fully characterized through a design that enabled it to be prepared in useful quantities. The synthesis provided direct access to the recently discovered PF-6870961 and is allowing researchers to conduct additional and deeper evaluation of its in vitro and in vivo properties.

Expression of the growth hormone secretagogue receptor 1a (GHS-R1a) in the brain

The review presents data on the expression of growth hormone secretagogue receptor 1a (GHS-R1a) in the brain regions in model animals (zebrafish, rodents, primates), and in the human brain. Studies show widespread distribution of the receptor in the brain, which evidences the involvement of the receptor in many physiological processes. Using various organisms, data have been obtained regarding the participation of the GHS-R1a in the regulation of the anti- and pro-inflammatory response, proliferation, and apoptosis. It is known that the receptor plays an important role in eating behavior and is also involved in the pathogenetic mechanisms of drug addiction, obesity, and chronic alcohol consumption. Based on this, research is underway with the use of various therapeutic agents that can be used for the pharmacological correction of these conditions. This review also presents hypothetical pathways of intracellular signaling, in which GHS-R1a may participate. A complete understanding of these mechanisms has not yet been reached. The ghrelin intracellular signaling seem to be specific to brain region and, probably, also depend on the metabolic or stress status of the organism.

Changes in energy metabolism and respiration in different tracheal narrowing in rats

Why obstructive sleep apnea (OSA) treatment does not completely restore healthy metabolic physiology is unclear. In rats, the need for respiratory homeostasis maintenance following airway obstruction (AO) is associated with a loss of thermoregulation and abnormal metabolic physiology that persists following successful obstruction removal. Here, we explored the effect of two different types of tracheal narrowing, i.e., AO and mild airway obstruction (mAO), and its removal on respiratory homeostasis and metabolic physiology. We show that after ten weeks, mAO vs. AO consumes sufficient energy that is required to maintain respiratory homeostasis and thermoregulation. Obstruction removal was associated with largely irreversible increased feeding associated with elevated serum ghrelin, hypothalamic growth hormone secretagogue receptor 1a, and a phosphorylated Akt/Akt ratio, despite normalization of breathing and energy requirements. Our study supports the need for lifestyle eating behavior management, in addition to endocrine support, in order to attain healthy metabolic physiology in OSA patients.

Discovery of a macromolecular complex mediating the hunger suppressive actions of cocaine: Structural and functional properties

Cocaine not only increases brain dopamine levels but also activates the sigma1 receptor (σ1 R) that in turn regulates orexigenic receptor function. Identification of interactions involving dopamine D1 (D1 R), ghrelin (GHS-R1a ), and σ1 receptors have been addressed by biophysical techniques and a complementation approach using interfering peptides. The effect of cocaine on receptor functionality was assayed by measuring second messenger, cAMP and Ca2+ , levels. The effect of acute or chronic cocaine administration on receptor complex expression was assayed by in situ proximity ligation assay. In silico procedures were used for molecular model building. σ1 R KO mice were used for confirming involvement of this receptor. Upon identification of protomer interaction and receptor functionality, a unique structural model for the macromolecular complex formed by σ1 R, D1 R, and GHS-R1a is proposed. The functionality of the complex, able to couple to both Gs and Gq proteins, is affected by cocaine binding to the σ1 R, as confirmed using samples from σ1 R-/- mice. The expression of the macromolecular complex was differentially affected upon acute and chronic cocaine administration to rats. The constructed 3D model is consistent with biochemical, biophysical, and available structural data. The σ1 R, D1 R, and GHS-R1a complex constitutes a functional unit that is altered upon cocaine binding to the σ1 R. Remarkably, the heteromer can simultaneously couple to two G proteins, thus allowing dopamine to signal via Ca2+ and ghrelin via cAMP. The anorexic action of cocaine is mediated by such complex whose expression is higher after acute than after chronic administration regimens.

Contribution of Ghrelin to the Pathogenesis of Growth Hormone Deficiency

In this review we described the interactions between ghrelin and the growth hormone (GH)-insulin-like growth factor 1 (IGF-1) axis in children and adults with growth hormone deficiency (GHD). A possible involvement of these interactions in the pathogenesis of unexplained cases of GHD was suggested. Current research provides more and more details to the knowledge on the circadian rhythm of ghrelin. We gathered reports on the decreasing effect of Helicobacter pylori-related chronic gastritis on the number of ghrelin immunopositive cells and the consequent decrease in ghrelin serum concentration. The gastrointestinal tract microflora modification of the ghrelin action, by the mechanism of molecular mimicry, was also stressed. Moreover, the mutual relationships between ghrelin and the TSH-FT4/FT3 axis in growth and metabolic processes are described. It is to be recalled that FT4 and FT3 exert a permissive impact on IGF-1 action and, in turn, GH, in reaction mediated by IGF-1, enhances the monodeiodination of FT4 to FT3. Finally, we discussed the latest attempts to use the GH secretagogue receptor (GHS-R) analogues for possible diagnostic and therapeutic purposes.

Identification of the Ghrelin and Cannabinoid CB2 Receptor Heteromer Functionality and Marked Upregulation in Striatal Neurons from Offspring of Mice under a High-Fat Diet

Cannabinoids have been reported as orexigenic, i.e., as promoting food intake that, among others, is controlled by the so-called “hunger” hormone, ghrelin. The aim of this paper was to look for functional and/or molecular interactions between ghrelin GHSR1a and cannabinoid CB₂ receptors at the central nervous system (CNS) level. In a heterologous system we identified CB₂-GHSR1a receptor complexes with a particular heteromer print consisting of impairment of CB₂ receptor/G_i-mediated signaling. The blockade was due to allosteric interactions within the heteromeric complex as it was reverted by antagonists of the GHSR1a receptor. Cannabinoids acting on the CB₂ receptor did not affect cytosolic increases of calcium ions induced by ghrelin acting on the GHSR1a receptor. In situ proximity ligation imaging assays confirmed the expression of CB₂-GHSR1a receptor complexes in both heterologous cells and primary striatal neurons. We tested heteromer expression in neurons from offspring of high-fat-diet mouse mothers as they have more risk to be obese. Interestingly, there was a marked upregulation of those complexes in striatal neurons from siblings of pregnant female mice under a high-fat diet.

Differential expression levels of the hippocampal ghrelin and its receptor mRNA during memory consolidation

Ghrelin is a peptide, secreted mainly from the stomach. But, it is also produced in the brain. Studies have confirmed the positive impact of ghrelin on memory formation. However, the expression levels of ghrelin or its receptors were not measured in the brain during the process of memory formation. The probable alteration in the expression levels of ghrelin or its receptors in the brain during memory formation can be a reason for the contribution of its signaling in this process. We quantified the gene expression levels of ghrelin and its receptors in the hippocampus during fear and spatial memory consolidation. Thirty- nine adult male Wistar rats weighing 180-220 g were utilized. Memory consolidation was evaluated using the inhibitory avoidance task and Morris water maze. Rats were euthanized at different times (1, 3, and 24 h) post-training and their hippocampi were removed and freezed directly in liquid nitrogen. Quantitative real-time polymerize chain reaction (PCR) was used to quantify the messenger ribonucleic acid (mRNA) expression levels of the hippocampal ghrelin and its receptors. The mRNA levels of ghrelin exhibited a significant increase, 24 h post-training in the inhibitory avoidance task, while its receptor levels were down-regulated. Also, the mRNA expression levels of the hippocampal ghrelin were not changed significantly during memory consolidation in the Morris water maze, while its receptor showed a significant increase, 24 h post-training. The results show a differential profile of the expression levels of the hippocampal ghrelin or its receptor mRNA during fear or spatial memory consolidation. This proposes that a local increase in the hippocampal ghrelin or its receptor levels might be crucial for fear, and spatial memory consolidation. However, due to the small sample sizes, it is worth noting the preliminary nature of the conclusions in the present study.

Allosteric modulation of ghrelin receptor signaling by lipids

The membrane is an integral component of the G protein-coupled receptor signaling machinery. Here we demonstrate that lipids regulate the signaling efficacy and selectivity of the ghrelin receptor GHSR through specific interactions and bulk effects. We find that PIP2 shifts the conformational equilibrium of GHSR away from its inactive state, favoring basal and agonist-induced G protein activation. This occurs because of a preferential binding of PIP2 to specific intracellular sites in the receptor active state. Another lipid, GM3, also binds GHSR and favors G protein activation, but mostly in a ghrelin-dependent manner. Finally, we find that not only selective interactions but also the thickness of the bilayer reshapes the conformational repertoire of GHSR, with direct consequences on G protein selectivity. Taken together, this data illuminates the multifaceted role of the membrane components as allosteric modulators of how ghrelin signal could be propagated.

A ghrelin receptor antagonist reduces the ability of ghrelin, alcohol or amphetamine to induce a dopamine release in the ventral tegmental area and in nucleus accumbens shell in rats

The orexigenic peptide ghrelin increases the release of dopamine in the nucleus accumbens (NAc) shell via central ghrelin receptors, especially those located in the ventral tegmental area (VTA). The activity of the VTA dopamine neurons projecting to NAc shell, involves somatodendritic dopamine release within the VTA. However, the effects of ghrelin on the concomitant dopamine release in the VTA and NAc shell is unknown. It is further unknown whether addictive drugs, such as alcohol and amphetamine, enhance the dopamine levels in both these areas via ghrelin receptor dependent mechanisms. Thus, the effects of a ghrelin receptor antagonist, JMV2959, on the ability of i) central ghrelin ii) systemic alcohol or iii) systemic amphetamine to increase the dopamine release in the VTA and in the NAc shell in rats by using in vivo microdialysis was explored. We showed that systemic administration of JMV2959 blocks the ability of central ghrelin to increases dopamine release in the VTA and the NAc shell, and reduces the alcohol- and amphetamine-induced dopamine release in both these areas. Locomotor activity studies was then conducted in an attempt to correlate the ghrelin-induced dopamine release in the VTA to a behavioural outcome. These revealed that local infusion of a dopamine D1 receptor antagonist into the VTA blocks the ability of central ghrelin to cause a locomotor stimulation in mice. Collectively, this study adds to the growing body of evidence indicating that ghrelin signalling modulates the ability of ghrelin, and addictive drugs, to activate the mesoaccumbal dopamine pathway.

Relationship between Orexigenic Peptide Ghrelin Signal, Gender Difference and Disease

Growth hormone secretagogue receptor 1a (GHS-R1a), which is one of the G protein-coupled receptors (GPCRs), is involved in various physiological actions such as energy consumption, growth hormone secretion promoting action, and cardiovascular protective action. The ligand was searched for as an orphan receptor for a while, but the ligand was found to be acylated ghrelin (ghrelin) discovered by Kangawa and Kojima et al. in 1999. Recently, it has also been reported that dysregulation of GHS-R1a mediates reduced feeding in various diseases. On the other hand, since the physiological effects of ghrelin have been studied exclusively in male mice, few studies have been conducted on gender differences in ghrelin reactivity. In this review, we describe (1) the characteristics of GHS-R1a, (2) the role of ghrelin in hypophagia due to stress or anticancer drugs, and (3) the gender differences in the physiological effects of GHS-R1a and the influence of stress on it.

Ghrelin Receptors Enhance Fat Taste Responsiveness in Female Mice

Ghrelin is a major appetite-stimulating neuropeptide found in circulation. While its role in increasing food intake is well known, its role in affecting taste perception, if any, remains unclear. In this study, we investigated the role of the growth hormone secretagogue receptor's (GHS-R; a ghrelin receptor) activity in the peripheral taste system using feeding studies and conditioned taste aversion assays by comparing wild-type and GHS-R-knockout models. Using transgenic mice expressing enhanced green fluorescent protein (GFP), we demonstrated GHS-R expression in the taste system in relation phospholipase C ß2 isotype (PLCβ2; type II taste cell marker)- and glutamate decarboxylase type 67 (GAD67; type III taste cell marker)-expressing cells using immunohistochemistry. We observed high levels of co-localization between PLCβ2 and GHS-R within the taste system, while GHS-R rarely co-localized in GAD67-expressing cells. Additionally, following 6 weeks of 60% high-fat diet, female Ghsr^-/- mice exhibited reduced responsiveness to linoleic acid (LA) compared to their wild-type (WT) counterparts, while no such differences were observed in male Ghsr^-/- and WT mice. Overall, our results are consistent with the interpretation that ghrelin in the taste system is involved in the complex sensing and recognition of fat compounds. Ghrelin-GHS-R signaling may play a critical role in the recognition of fatty acids in female mice, and this differential regulation may contribute to their distinct ingestive behaviors.

The Complex World of Regulation of Pituitary Growth Hormone Secretion: The Role of Ghrelin, Klotho, and Nesfatins in It

The classic concept of how pituitary GH is regulated by somatostatin and GHRH has changed in recent years, following the discovery of peripheral hormones involved in the regulation of energy homeostasis and mineral homeostasis. These hormones are ghrelin, nesfatins, and klotho. Ghrelin is an orexigenic hormone, released primarily by the gastric mucosa, although it is widely expressed in many different tissues, including the central nervous system and the pituitary. To be active, ghrelin must bind to an n-octanoyl group (n = 8, generally) on serine 3, forming acyl ghrelin which can then bind and activate a G-protein-coupled receptor leading to phospholipase C activation that induces the formation of inositol 1,4,5-triphosphate and diacylglycerol that produce an increase in cytosolic calcium that allows the release of GH. In addition to its direct action on somatotrophs, ghrelin co-localizes with GHRH in several neurons, facilitating its release by inhibiting somatostatin, and acts synergistically with GHRH stimulating the synthesis and secretion of pituitary GH. Gastric ghrelin production declines with age, as does GH. Klotho is an anti-aging agent, produced mainly in the kidneys, whose soluble circulating form directly induces GH secretion through the activation of ERK1/2 and inhibits the inhibitory effect that IGF-I exerts on GH. Children and adults with untreated GH-deficiency show reduced plasma levels of klotho, but treatment with GH restores them to normal values. Deletions or mutations of the Klotho gene affect GH production. Nesfatins 1 and 2 are satiety hormones, they inhibit food intake. They have been found in GH3 cell cultures where they significantly reduce the expression of gh mRNA and that of pituitary-specific positive transcription factor 1, consequently acting as inhibitors of GH production. This is a consequence of the down-regulation of the cAMP/PKA/CREB signaling pathway. Interestingly, nesfatins eliminate the strong positive effect that ghrelin has on GH synthesis and secretion. Throughout this review, we will attempt to broadly analyze the role of these hormones in the complex world of GH regulation, a world in which these hormones already play a very important role.

Mice with an RGS-insensitive Gαi2 protein show growth hormone axis dysfunction

Mice carrying an RGS-insensitive Gαi2 mutation display growth retardation early after birth. Although the growth hormone (GH)-axis is a key endocrine modulator of postnatal growth, its functional state in these mice has not been characterized. The present study was undertaken to address this issue. Results revealed that pituitary mRNA levels for GH, prolactin (PRL), somatostatin (SST), GH-releasing-hormone receptor (GHRH-R) and GH secretagogue receptor (GHS-R) were decreased in mutants compared to controls. These changes were reflected by a significant decrease in plasma levels of GH, IGF-1 and IGF-binding protein-3 (IGFBP-3). Mutants were also less responsive to GHRH and ghrelin (GhL) on GH stimulation of release from pituitary primary cell cultures. In contrast, they were more sensitive to the inhibitory effect of SST. These data provide the first evidence for an alteration of the functional state of the GH-axis in Gαi2G184S mice that likely contributes to their growth retardation.

Unacylated ghrelin binds heparan-sulfate proteoglycans which modulate its function

Acylated ghrelin (AG) is a gut-derived peptide with growth hormone secretagogue (GHS), orexigenic and other physiological activities mediated by GHS receptor-1a (GHSR). Ghrelin occurs in unacylated form (UAG) with activities opposing AG, although its mechanism of action is unknown. UAG does not antagonize AG at GHSR, and has biological effects on cells that lack this receptor. Because UAG binds to cells, it has been hypothesized that UAG acts via a cell-surface receptor, although this has not been confirmed. This study aimed to identify cell surface proteins to which UAG binds that could modulate or mediate its biological effects. The MCF7 cell-line was used as a model because UAG induces ERK signaling in these cells in the absence of GHSR. Using ligand-receptor capture and LC-MS/MS we identified specific heparan-sulfate proteoglycans (HSPGs) to which UAG interacts on cell surfaces. In line with this, UAG, as well as AG, bind with high affinity to heparin, and heparin and heparinase treatment suppress, whereas HSPG overexpression increases, UAG binding to MCF7 cell surfaces. Moreover, heparin suppresses the ERK response to UAG. However, conversion of the lysines in UAG to alanine, which prevents its binding to heparin and cell surface HSPGs, does not prevent its activation of ERK. Our data show that the interaction of UAG with HSPGs modulates its biological activity in cells. More broadly, the interaction of UAG and AG with HSPGs could be important for the specificity and potency of their biological action in vivo.

Ghrelin as an Anti-Sepsis Peptide: Review

Sepsis continues to produce widespread inflammation, illness, and death, prompting intensive research aimed at uncovering causes and therapies. In this article, we focus on ghrelin, an endogenous peptide with promise as a potent anti-inflammatory agent. Ghrelin was discovered, tracked, and isolated from stomach cells based on its ability to stimulate release of growth hormone. It also stimulates appetite and is shown to be anti-inflammatory in a wide range of tissues. The anti-inflammatory effects mediated by ghrelin are a result of both the stimulation of anti-inflammatory processes and an inhibition of pro-inflammatory forces. Anti-inflammatory processes are promoted in a broad range of tissues including the hypothalamus and vagus nerve as well as in a broad range of immune cells. Aged rodents have reduced levels of growth hormone (GH) and diminished immune responses; ghrelin administration boosts GH levels and immune response. The anti-inflammatory functions of ghrelin, well displayed in preclinical animal models of sepsis, are just being charted in patients, with expectations that ghrelin and growth hormone might improve outcomes in patients with sepsis.

Mouse Spexin: (II) Functional Role as a Satiety Factor inhibiting Food Intake by Regulatory Actions Within the Hypothalamus

Spexin (SPX) is a pleiotropic peptide with highly conserved protein sequence from fish to mammals and its biological actions are mediated by GalR2/GalR3 receptors expressed in target tissues. Recently, SPX has been confirmed to be a novel satiety factor in fish species but whether the peptide has a similar function in mammals is still unclear. Using the mouse as a model, the functional role of SPX in feeding control and the mechanisms involved were investigated. After food intake, serum SPX in mice could be up-regulated with elevations of transcript expression and tissue content of SPX in the glandular stomach but not in other tissues examined. As revealed by immunohistochemical staining, food intake also intensified SPX signals in the major cell types forming the gastric glands (including the foveolar cells, parietal cells, and chief cells) within the gastric mucosa of glandular stomach. Furthermore, IP injection of SPX was effective in reducing food intake with parallel attenuation in transcript expression of NPY, AgRP, NPY type 5 receptor (NPY5R), and ghrelin receptor (GHSR) in the hypothalamus, and these inhibitory effects could be blocked by GalR3 but not GalR2 antagonism. In agreement with the central actions of SPX, similar inhibition on feeding and hypothalamic expression of NPY, AgRP, NPY5R, and GHSR could also be noted with ICV injection of SPX. In the same study, in contrast to the drop in NPY5R and GHSR, SPX treatment could induce parallel rises of transcript expression of leptin receptor (LepR) and melanocortin 4 receptor (MC4R) in the hypothalamus. These findings, as a whole, suggest that the role of SPX as a satiety factor is well conserved in the mouse. Apparently, food intake can induce SPX production in glandular stomach and contribute to the postprandial rise of SPX in circulation. Through GalR3 activation, this SPX signal can act within the hypothalamus to trigger feedback inhibition on feeding by differential modulation of feeding regulators (NPY and AgRP) and their receptors (NPY5R, GHSR, LepR, and MC4R) involved in the feeding circuitry within the CNS.