The orphan receptor Gpr83 regulates systemic energy metabolism via ghrelin-dependent and ghrelin-independent mechanisms. Nat Commun. 2013;4:1968. [PMID: 23744028 PMCID: PMC3709495 DOI: 10.1038/ncomms2968]

Understanding the genetic complexity of puberty timing across the allele frequency spectrum

Pubertal timing varies considerably and is associated with later health outcomes. We performed multi-ancestry genetic analyses on ~800,000 women, identifying 1,080 signals for age at menarche. Collectively, these explained 11% of trait variance in an independent sample. Women at the top and bottom 1% of polygenic risk exhibited ~11 and ~14-fold higher risks of delayed and precocious puberty, respectively. We identified several genes harboring rare loss-of-function variants in ~200,000 women, including variants in ZNF483, which abolished the impact of polygenic risk. Variant-to-gene mapping approaches and mouse gonadotropin-releasing hormone neuron RNA sequencing implicated 665 genes, including an uncharacterized G-protein-coupled receptor, GPR83, which amplified the signaling of MC3R, a key nutritional sensor. Shared signals with menopause timing at genes involved in DNA damage response suggest that the ovarian reserve might signal centrally to trigger puberty. We also highlight body size-dependent and independent mechanisms that potentially link reproductive timing to later life disease.

Sleep regulation and host genetics

Due to the multifactorial and complex nature of rest, we focus on phenotypes related to sleep. Sleep regulation is a multifactorial process. In this chapter, we focus on those phenotypes inherent to sleep that are highly prevalent in the population, and that can be modulated by lifestyle, such as sleep quality and duration, insomnia, restless leg syndrome and daytime sleepiness. We, therefore, leave in the background those phenotypes that constitute infrequent pathologies or for which the current level of scientific evidence does not favour the implementation of practical approaches of this type. Similarly, the regulation of sleep quality is intimately linked to the regulation of the circadian rhythm. Although this relationship is discussed in the sections that require it, the in-depth study of circadian rhythm regulation at the molecular level deserves a separate chapter, and this is how it is dealt with in this volume.

Nanomolar range of FAM237B can activate receptor GPR83

Our recent study confirmed that the mature neuropeptide FAM237A, also known as neurosecretory protein GL (NPGL), is an efficient agonist for GPR83. The paralog FAM237B was previously reported as a weak agonist for GPR83. In the present study, we prepared mature human FAM237B via an intein-fusion approach and demonstrated that it could cause a significant activation effect at the nanomolar range (1‒10 nM) in a NanoBiT-based β-arrestin recruitment assay. Thus, FAM237B appears to be another endogenous agonist for GPR83 and future in vivo studies will be required to confirm this.

Molecular and cellular mechanisms underlying the hepatoprotective role of ghrelin against NAFLD progression

The underlying mechanisms for the development and progression of nonalcoholic fatty liver disease (NAFLD) are complex and multifactorial. Within the last years, experimental and clinical evidences support the role of ghrelin in the development of NAFLD. Ghrelin is a gut hormone that plays a major role in the short-term regulation of appetite and long-term regulation of adiposity. The liver constitutes a target for ghrelin, where this gut-derived peptide triggers intracellular pathways regulating lipid metabolism, inflammation, and fibrosis. Interestingly, circulating ghrelin levels are altered in patients with metabolic diseases, such as obesity, type 2 diabetes, and metabolic syndrome, which, in turn, are well-known risk factors for the pathogenesis of NAFLD. This review summarizes the molecular and cellular mechanisms involved in the hepatoprotective action of ghrelin, including the reduction of hepatocyte lipotoxicity via autophagy and fatty acid β-oxidation, mitochondrial dysfunction, endoplasmic reticulum stress and programmed cell death, the reversibility of the proinflammatory phenotype in Kupffer cells, and the inactivation of hepatic stellate cells. Together, the metabolic and inflammatory pathways regulated by ghrelin in the liver support its potential as a therapeutic target to prevent NAFLD in patients with metabolic disorders.

Is the Neuropeptide PEN a Ligand of GPR83?

G protein-coupled receptor 83 (GPR83) is a class A G protein-coupled receptor with predominant expression in the cerebellum and proposed function in the regulation of food intake and in anxiety-like behavior. The neuropeptide PEN has been suggested as a specific GPR83 ligand. However, conflicting reports exist about whether PEN is indeed able to bind and activate GPR83. This study was initiated to evaluate PEN as a potential ligand of GPR83. Employing several second messenger and other GPCR activation assays as well as a radioligand binding assay, and using multiple GPR83 plasmids and PEN peptides from different sources, no experimental evidence was found to support a role of PEN as a GPR83 ligand.

Understanding the genetic complexity of puberty timing across the allele frequency spectrum

Pubertal timing varies considerably and has been associated with a range of health outcomes in later life. To elucidate the underlying biological mechanisms, we performed multi-ancestry genetic analyses in ~800,000 women, identifying 1,080 independent signals associated with age at menarche. Collectively these loci explained 11% of the trait variance in an independent sample, with women at the top and bottom 1% of polygenic risk exhibiting a ~11 and ~14-fold higher risk of delayed and precocious pubertal development, respectively. These common variant analyses were supported by exome sequence analysis of ~220,000 women, identifying several genes, including rare loss of function variants in ZNF483 which abolished the impact of polygenic risk. Next, we implicated 660 genes in pubertal development using a combination of in silico variant-to-gene mapping approaches and integration with dynamic gene expression data from mouse embryonic GnRH neurons. This included an uncharacterized G-protein coupled receptor GPR83, which we demonstrate amplifies signaling of MC3R, a key sensor of nutritional status. Finally, we identified several genes, including ovary-expressed genes involved in DNA damage response that co-localize with signals associated with menopause timing, leading us to hypothesize that the ovarian reserve might signal centrally to trigger puberty. Collectively these findings extend our understanding of the biological complexity of puberty timing and highlight body size dependent and independent mechanisms that potentially link reproductive timing to later life disease.

Quinpirole ameliorates nigral dopaminergic neuron damage in Parkinson's disease mouse model through activating GHS-R1a/D2R heterodimers

Growth hormone secretagogue receptor 1a (GHS-R1a) is an important G protein-coupled receptor (GPCR) that regulates a variety of functions by binding to ghrelin. It has been shown that the dimerization of GHS-R1a with other receptors also affects ingestion, energy metabolism, learning and memory. Dopamine type 2 receptor (D₂R) is a GPCR mainly distributed in the ventral tegmental area (VTA), substantia nigra (SN), striatum and other brain regions. In this study we investigated the existence and function of GHS-R1a/D₂R heterodimers in nigral dopaminergic neurons in Parkinson's disease (PD) models in vitro and in vivo. By conducting immunofluorescence staining, FRET and BRET analyses, we confirmed that GHS-R1a and D₂R could form heterodimers in PC-12 cells and in the nigral dopaminergic neurons of wild-type mice. This process was inhibited by MPP⁺ or MPTP treatment. Application of QNP (10 μM) alone significantly increased the viability of MPP⁺-treated PC-12 cells, and administration of quinpirole (QNP, 1 mg/kg, i.p. once before and twice after MPTP injection) significantly alleviated motor deficits in MPTP-induced PD mice model; the beneficial effects of QNP were abolished by GHS-R1a knockdown. We revealed that the GHS-R1a/D₂R heterodimers could increase the protein levels of tyrosine hydroxylase in the SN of MPTP-induced PD mice model through the cAMP response element binding protein (CREB) signaling pathway, ultimately promoting dopamine synthesis and release. These results demonstrate a protective role for GHS-R1a/D₂R heterodimers in dopaminergic neurons, providing evidence for the involvement of GHS-R1a in PD pathogenesis independent of ghrelin.

Gpr83 Tunes Nociceptor Function, Controlling Pain

The function of peripheral nociceptors is frequently tuned by the action of G protein-coupled receptors (GPRs) that are expressed in them, which contribute to pain alteration. Expanding new information on such GPRs and predicting their potential outcomes can help to construct new analgesic strategies based on their modulations. In this context, we attempted to present a new GPR not yet acknowledged for its pain association. Gpr83 exhibits relatively high expressions in the peripheral nervous system compared to other tissues when we mined and reconstructed Gene Expression Omnibus (GEO) metadata, which we confirmed using immunohistochemistry on murine dorsal root ganglia (DRG). When Gpr83 expression was silenced in DRG, neuronal and behavioral nociception were all downregulated. Pathologic pain in hind paw inflammation and chemotherapy-induced peripheral neuropathy were also alleviated by this Gpr83 knockdown. Dependent on exposure time, the application of a known endogenous Gpr83 ligand PEN showed differential effects on nociceptor responses in vitro. Localized PEN administration mitigated pain in vivo, probably following Gq/11-involved GPR downregulation caused by the relatively constant exposure. Collectively, this study suggests that Gpr83 action contributes to the tuning of peripheral pain sensitivity and thus indicates that Gpr83 can be among the potential GPR targets for pain modulation.

The oxytocin receptor represents a key hub in the GPCR heteroreceptor network: potential relevance for brain and behavior

In the last 10 years, it has become increasingly clear that large numbers of axon collaterals extend from the oxytocin (OXT) hypothalamic axons, especially the parvocellular components, to other brain regions. Consequently, the OXT signaling system forms, like other monoamine axons, a rich functional network across several brain regions. In this manuscript, we review the recently indicated higher order G-protein coupled heteroreceptor complexes of the oxytocin receptor (OXTR), and how these, via allosteric receptor-receptor interactions modulate the recognition, signaling, and trafficking of the participating receptor protomers and their potential impact for brain and behavior. The major focus will be on complexes of the OXTR protomer with the dopamine D2 receptor (D2R) protomer and the serotonin 2A (5-HT2AR) and 2C (5-HT2CR) receptor protomers. Specifically, the existence of D2R-OXTR heterocomplexes in the nucleus accumbens and the caudate putamen of rats has led to a postulated function for this heteromer in social behavior. Next, a physical interaction between OXTRs and the growth hormone secretagogue or ghrelin receptor (GHS-R1a) was demonstrated, which consequently was able to attenuate OXTR-mediated Gαq signaling. This highlights the potential of ghrelin-targeted therapies to modulate oxytocinergic signaling with relevance for appetite regulation, anxiety, depression, and schizophrenia. Similarly, evidence for 5-HT2AR-OXTR heteromerization in the pyramidal cell layer of CA2 and CA3 in the dorsal hippocampus and in the nucleus accumbens shell was demonstrated. This complex may offer new strategies for the treatment of both mental disease and social behavior. Finally, the 5-HT2CR-OXTR heterocomplexes were demonstrated in the CA1, CA2, and CA3 regions of the dorsal hippocampus. Future work should be done to investigate the precise functional consequence of region-specific OXTR heteromerization in the brain, as well across the periphery, and whether the integration of neuronal signals in the brain may also involve higher order OXTR-GHS-R1a heteroreceptor complexes including the dopamine (DA), noradrenaline (NA) or serotonin (5-HT) receptor protomers or other types of G-protein coupled receptors (GPCRs).

Mice lacking proSAAS display alterations in emotion, consummatory behavior and circadian entrainment

ProSAAS is a neuroendocrine protein that is cleaved by neuropeptide-processing enzymes into more than a dozen products including the bigLEN and PEN peptides, which bind and activate the receptors GPR171 and GPR83, respectively. Previous studies have suggested that proSAAS-derived peptides are involved in physiological functions that include body weight regulation, circadian rhythms and anxiety-like behavior. In the present study, we find that proSAAS knockout mice display robust anxiety-like behaviors in the open field, light-dark emergence and elevated zero maze tests. These mutant mice also show a reduction in cued fear and an impairment in fear-potentiated startle, indicating an important role for proSAAS-derived peptides in emotional behaviors. ProSAAS knockout mice exhibit reduced water consumption and urine production relative to wild-type controls. No differences in food consumption and overall energy expenditure were observed between the genotypes. However, the respiratory exchange ratio was elevated in the mutants during the light portion of the light-dark cycle, indicating decreased fat metabolism during this period. While proSAAS knockout mice show normal circadian patterns of activity, even upon long-term exposure to constant darkness, they were unable to shift their circadian clock upon exposure to a light pulse. Taken together, these results show that proSAAS-derived peptides modulate a wide range of behaviors including emotion, metabolism and the regulation of the circadian clock.

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.

PEN Receptor GPR83 in Anxiety-Like Behaviors: Differential Regulation in Global vs Amygdalar Knockdown

Anxiety disorders are prevalent across the United States and result in a large personal and societal burden. Currently, numerous therapeutic and pharmaceutical treatment options exist. However, drugs to classical receptor targets have shown limited efficacy and often come with unpleasant side effects, highlighting the need to identify novel targets involved in the etiology and treatment of anxiety disorders. GPR83, a recently deorphanized receptor activated by the abundant neuropeptide PEN, has also been identified as a glucocorticoid regulated receptor (and named GIR) suggesting that this receptor may be involved in stress-responses that underlie anxiety. Consistent with this, GPR83 null mice have been found to be resistant to stress-induced anxiety. However, studies examining the role of GPR83 within specific brain regions or potential sex differences have been lacking. In this study, we investigate anxiety-related behaviors in male and female mice with global knockout and following local GPR83 knockdown in female mice. We find that a global knockdown of GPR83 has minimal impact on anxiety-like behaviors in female mice and a decrease in anxiety-related behaviors in male mice. In contrast, a local GPR83 knockdown in the basolateral amygdala leads to more anxiety-related behaviors in female mice. Local GPR83 knockdown in the central amygdala or nucleus accumbens (NAc) showed no significant effect on anxiety-related behaviors. Finally, dexamethasone administration leads to a significant decrease in receptor expression in the amygdala and NAc of female mice. Together, our studies uncover a significant, but divergent role for GPR83 in different brain regions in the regulation of anxiety-related behaviors, which is furthermore dependent on sex.

Orphan GPR116 mediates the insulin sensitizing effects of the hepatokine FNDC4 in adipose tissue

The proper functional interaction between different tissues represents a key component in systemic metabolic control. Indeed, disruption of endocrine inter-tissue communication is a hallmark of severe metabolic dysfunction in obesity and diabetes. Here, we show that the FNDC4-GPR116, liver-white adipose tissue endocrine axis controls glucose homeostasis. We found that the liver primarily controlled the circulating levels of soluble FNDC4 (sFNDC4) and lowering of the hepatokine FNDC4 led to prediabetes in mice. Further, we identified the orphan adhesion GPCR GPR116 as a receptor of sFNDC4 in the white adipose tissue. Upon direct and high affinity binding of sFNDC4 to GPR116, sFNDC4 promoted insulin signaling and insulin-mediated glucose uptake in white adipocytes. Indeed, supplementation with FcsFNDC4 in prediabetic mice improved glucose tolerance and inflammatory markers in a white-adipocyte selective and GPR116-dependent manner. Of note, the sFNDC4-GPR116, liver-adipose tissue axis was dampened in (pre) diabetic human patients. Thus our findings will now allow for harnessing this endocrine circuit for alternative therapeutic strategies in obesity-related pre-diabetes.

The soluble bioactive form of the transmembrane protein fibronectin type III domain containing 4 (sFNDC4) has anti-inflammatory effects and improves insulin sensitivity. Here the authors show that liver derived sFNDC4 signals through adipose tissue GPCR GPR116 to promote insulin-mediated glucose uptake.

Neuroendocrine Peptides of the Gut and Their Role in the Regulation of Food Intake

The regulation of food intake encompasses complex interplays between the gut and the brain. Among them, the gastrointestinal tract releases different peptides that communicate the metabolic state to specific nuclei in the hindbrain and the hypothalamus. The present overview gives emphasis on seven peptides that are produced by and secreted from specialized enteroendocrine cells along the gastrointestinal tract in relation with the nutritional status. These established modulators of feeding are ghrelin and nesfatin-1 secreted from gastric X/A-like cells, cholecystokinin (CCK) secreted from duodenal I-cells, glucagon-like peptide 1 (GLP-1), oxyntomodulin, and peptide YY (PYY) secreted from intestinal L-cells and uroguanylin (UGN) released from enterochromaffin (EC) cells. © 2021 American Physiological Society. Compr Physiol 11:1679-1730, 2021.

Biased signaling: A viable strategy to drug ghrelin receptors for the treatment of obesity

Obesity is a global burden and a chronic ailment with damaging overall health effects. Ghrelin, an octanoylated 28 amino acid peptide hormone, is secreted from the oxyntic mucosa of the stomach. Ghrelin acts on regions of the hypothalamus to regulate feeding behavior and glucose homeostasis through its G protein-coupled receptor. Recently, several central pathways modulating the metabolic actions of ghrelin have been reported. While these signaling pathways can be inhibited or activated by antagonists or agonists, they can also be discriminatingly activated in a "biased" response to impart different degrees of activation in distinct pathways downstream of the receptor. Here, we review recent ghrelin biased signaling findings as well as characteristics of ghrelin hormone and its receptors pertinent for biased signaling. We then evaluate the feasibility for ghrelin receptor biased signaling as a strategy for the development of effective pharmacotherapy in obesity treatment.

A new understanding of GHSR1a--independent of ghrelin activation

Growth hormone secretagogue receptor 1a (GHSR1a), a member of the G protein-coupled receptor (GPCR) family, is a functional receptor of ghrelin. The expression levels and activities of GHSR1a are affected by various factors. In past years, it has been found that the ghrelin-GHSR1a system can perform biological functions such as anti-inflammation, anti-apoptosis, and anti-oxidative stress. In addition to mediating the effect of ghrelin, GHSR1a also has abnormally high constitutive activity; that is, it can still transmit intracellular signals without activation of the ghrelin ligand. This constitutive activity affects brain functions, growth and development of the body; therefore, it has profound impacts on neurodegenerative diseases and some other age-related diseases. In addition, GHSR1a can also form homodimers or heterodimers with other GPCRs, affecting the release of neurotransmitters, appetite regulation, cell proliferation and insulin release. Therefore, further understanding of the constitutive activities and dimerization of GHSR1a will enable us to better clarify the characteristics of GHSR1a and provide more therapeutic targets for drug development. Here, we focus on the roles of GHSR1a in various biological functions and provide a comprehensive summary of the current research on GHSR1a to provide broader therapeutic prospects for age-related disease treatment.

The wonderful and masterful G protein-coupled receptor (GPCR): A focus on signaling mechanisms and the neuroendocrine control of fertility

Human GnRH deficiency, both clinically and genetically, is a heterogeneous disorder comprising of congenital GnRH deficiency with anosmia (Kallmann syndrome), or with normal olfaction [normosmic idiopathic hypogonadotropic hypogonadism (IHH)], and adult-onset hypogonadotropic hypogonadism. Our understanding of the neural mechanisms underlying GnRH secretion and GnRH signaling continues to increase at a rapid rate and strikingly, the heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) continue to emerge as essential players in these processes. GPCRs were once viewed as binary on-off switches, where in the "on" state they are bound to their Gα protein, but now we understand that view is overly simplistic and does not adequately characterize GPCRs. Instead, GPCRs have emerged as masterful signaling molecules exploiting different physical conformational states of itself to elicit an array of downstream signaling events via their G proteins and the β-arrestins. The "one receptor-multiple signaling conformations" model is likely an evolved strategy that can be used to our advantage as researchers have shown that targeting specific receptor conformations via biased ligands is proving to be a powerful tool in the effective treatment of human diseases. Can biased ligands be used to selectively modulate signaling by GPCR regulators of the neuroendocrine axis in the treatment of IHH? As discussed in this review, the grand possibility exists. However, while we are still very far from developing these treatments, this exciting likelihood can happen through a much greater mechanistic understanding of how GPCRs signal within the cell.

Uterine Gpr83 mRNA is highly expressed during early pregnancy and GPR83 mediates the actions of PEN in endometrial and non-endometrial cells

OBJECTIVE

To characterize the expression and signaling of uterine GPR83 in vivo in the nonpregnant and pregnant mouse and in vitro in human endometrial and nonendometrial cells.

DESIGN

Controlled laboratory study.

SETTING

Not applicable.

PATIENTS

Not applicable.

INTERVENTIONS

None.

MAIN OUTCOME MEASURES

Expression of uterine Gpr83 was determined by quantitative polymerase chain reaction throughout the estrous cycle and during early pregnancy in ovarian-stimulated and non-ovarian-stimulated mice and pregnant and pseudopregnant mice. Expression was also determined in ovariectomized mice after the administration of oil, E2, P4, or E2 + P4 and in stromal cells following 6 days of in vitro decidualization. GPR83 signaling was studied in human endometrial and embryonic kidney cell lines. Cells were treated by PEN, a GPR83 ligand, and PEN-induced extracellular signal-regulated kinase (ERK) phosphorylation was assayed under conditions that blocked Gα_q/11 and/or β-arrestin signaling.

RESULTS

Uterine Gpr83 is expressed throughout the estrous cycle and during early pregnancy; expression increases dramatically at the time of uterine receptivity, embryo implantation, and stromal cell decidualization. In the ovariectomized mouse, hormone add-back reveals that Gpr83 expression is highly responsive to the combined treatment of E2 and P4, and studies in the ovarian-stimulated mouse show that expression is also very sensitive to changes in E2 and P4 and is therefore tightly regulated by E2 and P4. At the implantation site, expression is elevated up to D6 of pregnancy and then declines rapidly on D7 and D8, suggesting that if there is any involvement in decidualization, it is likely associated with primary but not secondary stromal cell decidualization. This premise was supported by the observation that stromal cell decidualization in vitro progresses with a decline in Gpr83 expression. In ERα/PR-expressing endometrial Ishikawa cells, GPR83 mediates PEN signals in a Gα_q/11-dependent manner, and studies conducted in HEK 293 cells lacking β-arrestin revealed that GPR83 also signals via a β-arrestin-dependent manner. When signaling by either one or both pathways is downregulated, cells exhibit a major reduction in responsiveness to PEN treatment, demonstrating that signaling by both pathways is significant.

CONCLUSION

We hypothesize that PEN/GPR83 signaling regulates uterine receptivity, embryo implantation, and primary stromal cell decidualization by coupling to Gα_q/11- and β-arrestin-dependent pathways.

A Pilot Screen of a Novel Peptide Hormone Library Identified Candidate GPR83 Ligands

The identification of novel peptide hormones by functional screening is challenging because posttranslational processing is frequently required to generate biologically active hormones from inactive precursors. We developed an approach for functional screening of novel potential hormones by expressing them in endocrine host cells competent for posttranslational processing. Candidate preprohormones were selected by bioinformatics analysis, and stable endocrine host cell lines were engineered to express the preprohormones. The production of mature hormones was demonstrated by including the preprohormones insulin and glucagon, which require the regulated secretory pathway for production of the active forms. As proof of concept, we screened a set of G-protein-coupled receptors (GPCRs) and identified protein FAM237A as a specific activator of GPR83, a GPCR implicated in central nervous system and regulatory T-cell function. We identified the active form of FAM237A as a C-terminally cleaved, amidated 9 kDa secreted protein. The related protein FAM237B, which is 64% homologous to FAM237A, demonstrated similar posttranslational modification and activation of GPR83, albeit with reduced potency. These results demonstrate that our approach is capable of identifying and characterizing novel hormones that require processing for activity.

The role of the neuropeptide PEN receptor, GPR83, in the reward pathway: Relationship to sex-differences

GPR83, the receptor for the neuropeptide PEN, exhibits high expression in the nucleus accumbens of the human and rodent brain, suggesting that it plays a role in modulating the mesolimbic reward pathway. However, the cell-type specific expression of GPR83, its functional impact in the reward pathway, and in drug reward-learning has not been fully explored. Using GPR83/eGFP mice, we show high GPR83 expression on cholinergic interneurons in the nucleus accumbens and moderate expression on ventral tegmental area dopamine neurons. In GPR83 knockout mice, baseline dopamine release in the nucleus accumbens is enhanced which disrupts the ratio of tonic vs phasic release. Additionally, GPR83 knockout leads to changes in the expression of dopamine-related genes. Using the morphine conditioned place preference model, we identify sex differences in morphine reward-learning, show that GPR83 is upregulated in the nucleus accumbens following morphine conditioned place preference, and show that shRNA-mediated knockdown of GPR83 in the nucleus accumbens leads to attenuation morphine reward. Together, these findings detect GPR83 expression in the reward-pathway, and show its involvement in dopamine release and morphine reward-learning.

The Role of Ghrelin in Regulating Synaptic Function and Plasticity of Feeding-Associated Circuits

Synaptic plasticity of the neuronal circuits associated with feeding behavior is regulated by peripheral signals as a response to changes in the energy status of the body. These signals include glucose, free fatty acids, leptin and ghrelin and are released into circulation, being able to reach the brain. Ghrelin, a small peptide released from the stomach, is an orexigenic hormone produced in peripheral organs, and its action regulates food intake, body weight and glucose homeostasis. Behavioral studies show that ghrelin is implicated in the regulation of both hedonic and homeostatic feeding and of cognition. Ghrelin-induced synaptic plasticity has been described in neuronal circuits associated with these behaviors. In this review, we discuss the neuromodulatory mechanisms induced by ghrelin in regulating synaptic plasticity in three main neuronal circuits previously associated with feeding behaviors, namely hypothalamic (homeostatic feeding), ventral tegmental (hedonic and motivational feeding) and hippocampal (cognitive) circuits. Given the central role of ghrelin in regulating feeding behaviors, and the altered ghrelin levels associated with metabolic disorders such as obesity and anorexia, it is of paramount relevance to understand the effects of ghrelin on synaptic plasticity of neuronal circuits associated with feeding behaviors.

Neuropeptide PEN and Its Receptor GPR83: Distribution, Signaling, and Regulation

Neuropeptides are chemical messengers that act to regulate a number of physiological processes, including feeding, reward, pain, and memory, among others. PEN is one of the most abundant hypothalamic neuropeptides; however, until recently, its target receptor remained unknown. In this Review, we summarize recent developments in research focusing on PEN and its receptor GPR83. We describe the studies leading to the deorphanization of GPR83 as the receptor for PEN. We also describe the signaling mediated by the PEN-GPR83 system, as well as the physiological roles in which PEN-GPR83 has been implicated. As studies have suggested a role for the PEN-GPR83 system in food intake and body weight regulation, as well as in drug addiction and reward disorders, a thorough understanding of this novel neuropeptide-receptor system will help identify novel therapeutic targets to treat pathophysiological conditions involving PEN-GPR83.

Two Proximally Close Priority Candidate Genes for diplopodia-1, an Autosomal Inherited Craniofacial-Limb Syndrome in the Chicken: MRE11 and GPR83

Next-generation sequencing (NGS) and expression technologies were utilized to investigate the genes and sequence elements in a 586 kb region of chicken chromosome 1 associated with the autosomal recessive diplopodia-1 (dp-1) mutation. This mutation shows a syndromic phenotype similar to known human developmental abnormalities (e.g., cleft palate, polydactyly, omphalocele [exposed viscera]). Toward our goal to ascertain the variant responsible, the entire 586 kb region was sequenced following utilization of a specifically designed capture array and to confirm/validate fine-mapping results. Bioinformatic analyses identified a total of 6142 sequence variants, which included SNPs, indels, and gaps. Of these, 778 SNPs, 146 micro-indels, and 581 gaps were unique to the UCD-Dp-1.003 inbred congenic line; those found within exons and splice sites were studied for contribution to the mutant phenotype. Upon further validation with additional mutant samples, a smaller subset (of variants [51]) remains linked to the mutation. Additionally, utilization of specific samples in the NGS technology was advantageous in that fine-mapping methodologies eliminated an additional 326 kb of sequence information on chromosome 1. Predicted and confirmed protein-coding genes within the smaller 260 kb region were assessed for their developmental expression patterns over several stages of early embryogenesis in regions/tissues of interest (e.g., digits, craniofacial region). Based on these results and known function in other vertebrates, 2 genes within 5 kb of each other, MRE11 and GPR83, are proposed as high-priority candidates for the dp-1 mutation.

Ghrelin, a gastrointestinal hormone, regulates energy balance and lipid metabolism

Ghrelin, an acylated peptide hormone of 28 amino acids, is an endogenous ligand of the released growth hormone secretagogue receptor (GHSR). Ghrelin has been isolated from human and rat stomach and is also detected in the hypothalamic arcuate nucleus. Ghrelin receptor is primarily located in the neuropeptide Y and agouti-related protein neurons. Many previous studies have shown that ghrelin and GHSR are involved in the regulation of energy homeostasis, and its administration can increase food intake and body weight gain. AMP-activated protein kinase is activated by ghrelin in the hypothalamus, which contributes to lower intracellular long-chain fatty acid level. Ghrelin appears to modulate the response to food cues via a neural network involved in the regulation of feeding and in the appetitive response to food cues. It also increases the response of brain areas involved in visual processing, attention, and memory to food pictures. Ghrelin is also an important factor linking the central nervous system with peripheral tissues that regulate lipid metabolism. It promotes adiposity by the activation of hypothalamic orexigenic neurons and stimulates the expression of fat storage-related proteins in adipocytes. Meanwhile, ghrelin exerts direct peripheral effects on lipid metabolism, including increase in white adipose tissue mass, stimulation of lipogenesis in the liver, and taste sensitivity modulation.

Targeting the Recently Deorphanized Receptor GPR83 for the Treatment of Immunological, Neuroendocrine and Neuropsychiatric Disorders

G-protein coupled receptors (GPCRs) are a superfamily of receptors responsible for initiation of a myriad of intracellular signaling cascades. Currently, GPCRs represent approximately 34% of marketed pharmaceuticals, a large portion of which have no known endogenous ligand. These orphan GPCRs represent a large pool of novel targets for drug development. Very recently, the neuropeptide PEN, derived from the proteolytic processing of the precursor proSAAS, has been identified as a selective, high-affinity endogenous ligand for the orphan receptor, GPR83. GPR83 is highly expressed in the brain, spleen and thymus, indicating that this receptor may be a target to treat neurological and immune disorders. In the brain GPR83 is expressed in regions involved in the reward pathway, stress/anxiety responses, learning and memory and metabolism. However, the cell type specific expression of GPR83 in these regions has only recently begun to be characterized. In the immune system, GPR83 expression is regulated by Foxp3 in T-regulatory cells that are involved in autoimmune responses. Moreover, in the brain this receptor is regulated by interactions with other GPCRs, such as the recently deorphanized receptor, GPR171, and other hypothalamic receptors such as MC4R and GHSR. The following review will summarize the properties of GPR83 and highlight its known and potential significance in health and disease, as well as its promise as a novel target for drug development.

Orphan neuropeptides and receptors: Novel therapeutic targets

Neuropeptides are the largest class of intercellular signaling molecules, contributing to a wide variety of physiological processes. Neuropeptide receptors are therapeutic targets for a broad range of drugs, including medications to treat pain, addiction, sleep disorders, and nausea. In addition to >100 peptides with known functions, many peptides have been identified in mammalian brain for which the cognate receptors have not been identified. Similarly, dozens of "orphan" G protein-coupled receptors have been identified in the mammalian genome. While it would seem straightforward to match the orphan peptides and receptors, this is not always easily accomplished. In this review we focus on peptides named PEN and big LEN, which are among the most abundant neuropeptides in mouse brain, and their recently identified receptors: GPR83 and GPR171. These receptors are co-expressed in some brain regions and are able to interact. Because PEN and big LEN are produced from the same precursor protein and co-secreted, the interaction of GPR83 and GPR171 is physiologically relevant. In addition to interactions of these two peptides/receptors, PEN and LEN are co-localized with neuropeptide Y and Agouti-related peptide in neurons that regulate feeding. In this review, using these peptide receptors as an example, we highlight the multiple modes of regulation of receptors and present the emerging view that neuropeptides function combinatorially to generate a network of signaling messages. The complexity of neuropeptides, receptors, and their signaling pathways is important to consider both in the initial deorphanization of peptides and receptors, and in the subsequent development of therapeutic applications.

The G protein-coupled receptors deorphanization landscape

G protein-coupled receptors (GPCRs) are usually highlighted as being both the largest family of membrane proteins and the most productive source of drug targets. However, most of the GPCRs are understudied and hence cannot be used immediately for innovative therapeutic strategies. Besides, there are still around 100 orphan receptors, with no described endogenous ligand and no clearly defined function. The race to discover new ligands for these elusive receptors seems to be less intense than before. Here, we present an update of the various strategies employed to assign a function to these receptors and to discover new ligands. We focus on the recent advances in the identification of endogenous ligands with a detailed description of newly deorphanized receptors. Replication being a key parameter in these endeavors, we also discuss the latest controversies about problematic ligand-receptor pairings. In this context, we propose several recommendations in order to strengthen the reporting of new ligand-receptor pairs.

Epistatic effects between pairs of the growth hormone secretagogue receptor 1a, growth hormone, growth hormone receptor, non-SMC condensin I complex, subunit G and stearoyl-CoA desaturase genes on carcass, price-related and fatty acid composition traits in Japanese Black cattle

Growth hormone secretagogue receptor 1a (GHSR1a), growth hormone (GH), growth hormone receptor (GHR), non-SMC condensin I complex, subunit G (NCAPG) and stearoyl-CoA desaturase (SCD), are known to play important roles in growth and lipid metabolisms. Single and epistatic effects of the five genes on carcass, price-related and fatty acid (FA) composition traits were analyzed in a commercial Japanese Black cattle population of Ibaraki Prefecture. A total of 650 steers and 116 heifers for carcass and price-related traits, and 158 steers for FA composition traits were used in this study. Epistatic effects between pairs of the five genes were found in several traits. Alleles showing strain-specific differences in the five genes had significant single and epistatic effects in some traits. The data suggest that a TG-repeat polymorphism of the GHSR1a.5'UTR-(TG)_n locus plays a central role in gene-gene epistatic interaction of FA composition traits in the adipose tissue of Japanese Black cattle.

Carboxypeptidase E and the Identification of Novel Neuropeptides as Potential Therapeutic Targets

Peptides and small molecules that bind to peptide receptors are important classes of drugs that are used for a wide variety of different applications. The search for novel neuropeptides traditionally involved a time-consuming approach to purify each peptide to homogeneity and determine its amino acid sequence. The discovery in the 1980s of enkephalin convertase/carboxypeptidase E (CPE), and the observation that this enzyme was involved in the production of nearly every known neuropeptide led to the idea for a one-step affinity purification of CPE substrates. This approach was successfully used to isolate hundreds of known neuropeptides in mouse brain, as well as over a dozen novel peptides. Some of the novel peptides found using this approach are among the most abundant peptides present in brain, but had not been previously identified by traditional approaches. Recently, receptors for two of the novel peptides have been identified, confirming their role as neuropeptides that function in cell-cell signaling. Small molecules that bind to one of these receptors have been developed and found to significantly reduce food intake and anxiety-like behavior in an animal model. This review describes the entire project, from discovery of CPE to the novel peptides and their receptors.

Evolutionary adaptation revealed by comparative genome analysis of woolly mammoths and elephants

Comparative genomics studies typically limit their focus to single nucleotide variants (SNVs) and that was the case for previous comparisons of woolly mammoth genomes. We extended the analysis to systematically identify not only SNVs but also larger structural variants (SVs) and indels and found multiple mammoth-specific deletions and duplications affecting exons or even complete genes. The most prominent SV found was an amplification of RNase L (with different copy numbers in different mammoth genomes, up to 9-fold), involved in antiviral defense and inflammasome function. This amplification was accompanied by mutations affecting several domains of the protein including the active site and produced different sets of RNase L paralogs in four mammoth genomes likely contributing to adaptations to environmental threats. In addition to immunity and defense, we found many other unique genetic changes in woolly mammoths that suggest adaptations to life in harsh Arctic conditions, including variants involving lipid metabolism, circadian rhythms, and skeletal and body features. Together, these variants paint a complex picture of evolution of the mammoth species and may be relevant in the studies of their population history and extinction.

Dynamic roles for the N-terminus of the yeast G protein-coupled receptor Ste2p

The Saccharomyces cerevisiae α-factor receptor Ste2p has been used extensively as a model to understand the molecular mechanism of signal transduction by G protein-coupled receptors (GPCRs). Single and double cysteine mutants of Ste2p were created and served as surrogates to detect intramolecular interactions and dimerization of Ste2p using disulfide cross-linking methodology. When a mutation was introduced into the phylogenetically conserved tyrosine residue at position 26 (Y26C) in the N-terminus of Ste2p, dimerization was increased greatly. The amount of dimer formed by this Y26C mutant was greatly reduced by ligand binding even though the ligand binding site is far removed from the N-terminus; the lowering of the dimer formation was consistent with a conformational change in the N-terminus of the receptor upon activation. Dimerization was decreased by double mutations Y26C/V109C or Y26C/T114C indicating that Y26 is in close proximity to V109 and T114 of extracellular loop 1 in native Ste2p. Combined with earlier studies, these results indicate previously unrecognized roles for the N-terminus of Ste2p, and perhaps of GPCRs in general, and reveal a specific N-terminus residue or region, that is involved in GPCR signaling, intrareceptor interactions, and receptor dimerization.

Impact of G protein-coupled receptor heteromers in endocrine systems

The fine-tuning of endocrine homeostasis is regulated by dynamic receptor mediated processes. The superfamily of G protein-coupled receptors (GPCRs) have diverse roles in the modulation of all endocrine axes, thus understanding the mechanisms underpinning their functionality is paramount for treatment of endocrinopathies. Evidence over the last 20 years has highlighted homo and heteromerization as a key mode of mediating GPCR functional diversity. This review will discuss the concept of GPCR heteromerization and its relevance to endocrine function, detailing in vitro and in vivo evidence, and exploring current and potential pharmacological strategies for specific targeting of GPCR heteromers in endocrine heath and disease.

Circadian neurons in the lateral habenula: Clocking motivated behaviors

The main circadian clock in mammals is located in the hypothalamic suprachiasmatic nucleus (SCN), however, central timing mechanisms are also present in other brain structures beyond the SCN. The lateral habenula (LHb), known for its important role in the regulation of the monoaminergic system, contains such a circadian clock whose molecular and cellular mechanisms as well as functional role are not well known. However, since monoaminergic systems show circadian activity, it is possible that the LHb-clock's role is to modulate the rhythmic activity of the dopamine, serotonin and norephinephrine systems, and associated behaviors. Moreover, the LHb is involved in different pathological states such as depression, addiction and schizophrenia, states in which sleep and circadian alterations have been reported. Thus, perturbations of circadian activity in the LHb might, in part, be a cause of these rhythmic alterations in psychiatric ailments. In this review the current state of the LHb clock and its possible implications in the control of monoaminergic systems rhythms, motivated behaviors (e.g., feeding, drug intake) and depression (with circadian disruptions and altered motivation) will be discussed.

Adipocyte Glucocorticoid Receptor Deficiency Attenuates Aging- and HFD-Induced Obesity and Impairs the Feeding-Fasting Transition

Glucocorticoids (GCs) are important regulators of systemic energy metabolism, and aberrant GC action is linked to metabolic dysfunctions. Yet, the extent to which normal and pathophysiological energy metabolism depend on the GC receptor (GR) in adipocytes remains unclear. Here, we demonstrate that adipocyte GR deficiency in mice significantly impacts systemic metabolism in different energetic states. Plasma metabolomics and biochemical analyses revealed a marked global effect of GR deficiency on systemic metabolite abundance and, thus, substrate partitioning in fed and fasted states. This correlated with a decreased lipolytic capacity of GR-deficient adipocytes under postabsorptive and fasting conditions, resulting from impaired signal transduction from β-adrenergic receptors to adenylate cyclase. Upon prolonged fasting, the impaired lipolytic response resulted in abnormal substrate utilization and lean mass wasting. Conversely, GR deficiency attenuated aging-/diet-associated obesity, adipocyte hypertrophy, and liver steatosis. Systemic glucose tolerance was improved in obese GR-deficient mice, which was associated with increased insulin signaling in muscle and adipose tissue. We conclude that the GR in adipocytes exerts central but diverging roles in the regulation of metabolic homeostasis depending on the energetic state. The adipocyte GR is indispensable for the feeding-fasting transition but also promotes adiposity and associated metabolic disorders in fat-fed and aged mice.

Evidence for three genetic loci involved in both anorexia nervosa risk and variation of body mass index

The maintenance of normal body weight is disrupted in patients with anorexia nervosa (AN) for prolonged periods of time. Prior to the onset of AN, premorbid body mass index (BMI) spans the entire range from underweight to obese. After recovery, patients have reduced rates of overweight and obesity. As such, loci involved in body weight regulation may also be relevant for AN and vice versa. Our primary analysis comprised a cross-trait analysis of the 1000 single-nucleotide polymorphisms (SNPs) with the lowest P-values in a genome-wide association meta-analysis (GWAMA) of AN (GCAN) for evidence of association in the largest published GWAMA for BMI (GIANT). Subsequently we performed sex-stratified analyses for these 1000 SNPs. Functional ex vivo studies on four genes ensued. Lastly, a look-up of GWAMA-derived BMI-related loci was performed in the AN GWAMA. We detected significant associations (P-values <5 × 10-5, Bonferroni-corrected P<0.05) for nine SNP alleles at three independent loci. Interestingly, all AN susceptibility alleles were consistently associated with increased BMI. None of the genes (chr. 10: CTBP2, chr. 19: CCNE1, chr. 2: CARF and NBEAL1; the latter is a region with high linkage disequilibrium) nearest to these SNPs has previously been associated with AN or obesity. Sex-stratified analyses revealed that the strongest BMI signal originated predominantly from females (chr. 10 rs1561589; Poverall: 2.47 × 10-06/Pfemales: 3.45 × 10-07/Pmales: 0.043). Functional ex vivo studies in mice revealed reduced hypothalamic expression of Ctbp2 and Nbeal1 after fasting. Hypothalamic expression of Ctbp2 was increased in diet-induced obese (DIO) mice as compared with age-matched lean controls. We observed no evidence for associations for the look-up of BMI-related loci in the AN GWAMA. A cross-trait analysis of AN and BMI loci revealed variants at three chromosomal loci with potential joint impact. The chromosome 10 locus is particularly promising given that the association with obesity was primarily driven by females. In addition, the detected altered hypothalamic expression patterns of Ctbp2 and Nbeal1 as a result of fasting and DIO implicate these genes in weight regulation.

Current Understanding of the Hypothalamic Ghrelin Pathways Inducing Appetite and Adiposity

Ghrelin is a multifaceted regulator of metabolism. Ghrelin regulates energy balance in the short term via induction of appetite and in the long term via increased body weight and adiposity. Recently, several central pathways modulating the metabolic actions of ghrelin were unmasked, and it was shown to act through different hypothalamic nuclei to induce feeding. Ghrelin also modulates glucose homeostasis, but the central mechanisms responsible for this action have not been studied in detail. Although ghrelin also acts through extrahypothalamic areas to promote feeding, this review specifically dissects hypothalamic control of ghrelin's orexigenic and adipogenic actions and presents current understanding of the intracellular ghrelin orexigenic pathways, including their dependence on other relevant systems implicated in energy balance.

An update on the physiological and therapeutic relevance of GPCR oligomers

The traditional view on GPCRs held that they function as single monomeric units composed of identical subunits. This notion was overturned by the discovery that GPCRs can form homo- and hetero-oligomers, some of which are obligatory, and can further assemble into receptor mosaics consisting of three or more protomers. Oligomerisation exerts significant impacts on receptor function and physiology, offering a platform for the diversification of receptor signalling, pharmacology, regulation, crosstalk, internalization and trafficking. Given their involvement in the modulation of crucial physiological processes, heteromers could constitute important therapeutic targets for a wide range of diseases, including schizophrenia, Parkinson's disease, substance abuse or obesity. This review aims at depicting the current developments in GPCR oligomerisation research, documenting various class A, B and C GPCR heteromers detected in vitro and in vivo using biochemical and biophysical approaches, as well as recently identified higher-order oligomeric complexes. It explores the current understanding of dimerization dynamics and the possible interaction interfaces that drive oligomerisation. Most importantly, it provides an inventory of the wide range of physiological processes and pathophysiological conditions to which GPCR oligomers contribute, surveying some of the oligomers that constitute potential drug targets. Finally, it delineates the efforts to develop novel classes of ligands that specifically target and tether to receptor oligomers instead of a single monomeric entity, thus ameliorating their ability to modulate GPCR function.

Hypothalamic regulation of body growth and appetite by ghrelin-derived peptides during balanced nutrition or undernutrition

Among the gastrointestinal hormones that regulate food intake and energy homeostasis, ghrelin plays a unique role as the first one identified to increases appetite and stimulate GH secretion. This review highlights the latest mechanism by which ghrelin modulates body growth, appetite and energy metabolism by exploring pharmacological actions of the hormone and consequences of genetic or pharmacological blockade of the ghrelin/GHS-R (Growth Hormone Secretagogue Receptor) system on physiological responses in specific nutritional situations. Within the hypothalamus, novel mechanisms of action of this hormone involve its interaction with other ghrelin-derived peptides, such as desacyl ghrelin and obestatin, which are thought to act as functional ghrelin antagonists, and possible modulation of the GHS-R with other G-protein coupled receptors. During chronic undernutrition such as anorexia nervosa, variations of ghrelin-derived peptides may be an adaptative metabolic response to maintain normal glycemic control. Interestingly, some of ghrelin's metabolic actions are thought to be relayed through modulation of GH, an anabolic and hyperglycemic agent.

Insights into Basal Signaling Regulation, Oligomerization, and Structural Organization of the Human G-Protein Coupled Receptor 83

The murine G-protein coupled receptor 83 (mGPR83) is expressed in the hypothalamus and was previously suggested to be involved in the regulation of metabolism. The neuropeptide PEN has been recently identified as a potent GPR83 ligand. Moreover, GPR83 constitutes functionally relevant hetero-oligomers with other G-protein coupled receptors (GPCR) such as the ghrelin receptor (GHSR) or GPR171. Previous deletion studies also revealed that the long N-terminal extracellular receptor domain (eNDo) of mGPR83 may act as an intra-molecular ligand, which participates in the regulation of basal signaling activity, which is a key feature of GPCR function. Here, we investigated particular amino acids at the eNDo of human GPR83 (hGPR83) by side-directed mutagenesis to identify determinants of the internal ligand. These studies were accompanied by structure homology modeling to combine functional insights with structural information. The capacity for hetero-oligomer formation of hGPR83 with diverse family A GPCRs such as the melanocortin-4 receptor (MC4R) was also investigated, with a specific emphasis on the impact of the eNDo on oligomerization and basal signaling properties. Finally, we demonstrate that hGPR83 exhibits an unusual basal signaling for different effectors, which also supports signaling promiscuity. hGPR83 interacts with a variety of hypothalamic GPCRs such as the MC4R or GHSR. These interactions are not dependent on the ectodomain and most likely occur at interfaces constituted in the transmembrane regions. Moreover, several amino acids at the transition between the eNDo and transmembrane helix 1 were identified, where mutations lead also to biased basal signaling modulation.

Genetic Associations with Obstructive Sleep Apnea Traits in Hispanic/Latino Americans

RATIONALE

Obstructive sleep apnea is a common disorder associated with increased risk for cardiovascular disease, diabetes, and premature mortality. Although there is strong clinical and epidemiologic evidence supporting the importance of genetic factors in influencing obstructive sleep apnea, its genetic basis is still largely unknown. Prior genetic studies focused on traits defined using the apnea-hypopnea index, which contains limited information on potentially important genetically determined physiologic factors, such as propensity for hypoxemia and respiratory arousability.

OBJECTIVES

To define novel obstructive sleep apnea genetic risk loci for obstructive sleep apnea, we conducted genome-wide association studies of quantitative traits in Hispanic/Latino Americans from three cohorts.

METHODS

Genome-wide data from as many as 12,558 participants in the Hispanic Community Health Study/Study of Latinos, Multi-Ethnic Study of Atherosclerosis, and Starr County Health Studies population-based cohorts were metaanalyzed for association with the apnea-hypopnea index, average oxygen saturation during sleep, and average respiratory event duration.

MEASUREMENTS AND MAIN RESULTS

Two novel loci were identified at genome-level significance (rs11691765, GPR83, P = 1.90 × 10-8 for the apnea-hypopnea index, and rs35424364; C6ORF183/CCDC162P, P = 4.88 × 10-8 for respiratory event duration) and seven additional loci were identified with suggestive significance (P < 5 × 10-7). Secondary sex-stratified analyses also identified one significant and several suggestive associations. Multiple loci overlapped genes with biologic plausibility.

CONCLUSIONS

These are the first genome-level significant findings reported for obstructive sleep apnea-related physiologic traits in any population. These findings identify novel associations in inflammatory, hypoxia signaling, and sleep pathways.

Oligomerization of GPCRs involved in endocrine regulation

More than 800 different human membrane-spanning G-protein-coupled receptors (GPCRs) serve as signal transducers at biological barriers. These receptors are activated by a wide variety of ligands such as peptides, ions and hormones, and are able to activate a diverse set of intracellular signaling pathways. GPCRs are of central importance in endocrine regulation, which underpins the significance of comprehensively studying these receptors and interrelated systems. During the last decade, the capacity for multimerization of GPCRs was found to be a common and functionally relevant property. The interaction between GPCR monomers results in higher order complexes such as homomers (identical receptor subtype) or heteromers (different receptor subtypes), which may be present in a specific and dynamic monomer/oligomer equilibrium. It is widely accepted that the oligomerization of GPCRs is a mechanism for determining the fine-tuning and expansion of cellular processes by modification of ligand action, expression levels, and related signaling outcome. Accordingly, oligomerization provides exciting opportunities to optimize pharmacological treatment with respect to receptor target and tissue selectivity or for the development of diagnostic tools. On the other hand, GPCR heteromerization may be a potential reason for the undesired side effects of pharmacological interventions, faced with numerous and common mutual signaling modifications in heteromeric constellations. Finally, detailed deciphering of the physiological occurrence and relevance of specific GPCR/GPCR-ligand interactions poses a future challenge. This review will tackle the aspects of GPCR oligomerization with specific emphasis on family A GPCRs involved in endocrine regulation, whereby only a subset of these receptors will be discussed in detail.

Identification of GPR83 as the receptor for the neuroendocrine peptide PEN

PEN is an abundant peptide in the brain that has been implicated in the regulation of feeding. We identified a receptor for PEN in mouse hypothalamus and Neuro2A cells. PEN bound to and activated GPR83, a G protein (heterotrimeric guanine nucleotide)-binding protein)-coupled receptor (GPCR). Reduction of GPR83 expression in mouse brain and Neuro2A cells reduced PEN binding and signaling, consistent with GPR83 functioning as the major receptor for PEN. In some brain regions, GPR83 colocalized with GPR171, a GPCR that binds the neuropeptide bigLEN, another neuropeptide that is involved in feeding and is generated from the same precursor protein as is PEN. Coexpression of these two receptors in cell lines altered the signaling properties of each receptor, suggesting a functional interaction. Our data established PEN as a neuropeptide that binds GPR83 and suggested that these two ligand-receptor systems-PEN-GPR83 and bigLEN-GPR171-may be functionally coupled in the regulation of feeding.

Ring Finger Protein 11 Inhibits Melanocortin 3 and 4 Receptor Signaling

Intact melanocortin signaling via the G protein-coupled receptors (GPCRs), melanocortin receptor 4 (MC4R), and melanocortin receptor 3 (MC3R) is crucial for body weight maintenance. So far, no connection between melanocortin signaling and hypothalamic inflammation has been reported. Using a bimolecular fluorescence complementation library screen, we identified a new interaction partner for these receptors, ring finger protein 11 (RNF11). RNF11 participates in the constitution of the A20 complex that is involved in reduction of tumor necrosis factor α (TNFα)-induced NFκB signaling, an important pathway in hypothalamic inflammation. Mice treated with high-fat diet (HFD) for 3 days demonstrated a trend toward an increase in hypothalamic Rnf11 expression, as shown for other inflammatory markers under HFD. Furthermore, Gs-mediated signaling of MC3/4R was demonstrated to be strongly reduced to 20-40% by co-expression of RNF11 despite unchanged total receptor expression. Cell surface expression was not affected for MC3R but resulted in a significant reduction of MC4R to 61% by co-expression with RNF11. Mechanisms linking HFD, inflammation, and metabolism remain partially understood. In this study, a new axis between signaling of specific body weight regulating GPCRs and factors involved in hypothalamic inflammation is suggested.

The β-cell GHSR and downstream cAMP/TRPM2 signaling account for insulinostatic and glycemic effects of ghrelin

Gastric hormone ghrelin regulates insulin secretion, as well as growth hormone release, feeding behavior and adiposity. Ghrelin is known to exert its biological actions by interacting with the growth hormone secretagogue-receptor (GHSR) coupled to G_q/11-protein signaling. By contrast, ghrelin acts on pancreatic islet β-cells via G_i-protein-mediated signaling. These observations raise a question whether the ghrelin action on islet β-cells involves atypical GHSR and/or distinct signal transduction. Furthermore, the role of the β-cell GHSR in the systemic glycemic effect of ghrelin still remains to be defined. To address these issues, the present study employed the global GHSR-null mice and those re-expressing GHSR selectively in β-cells. We here report that ghrelin attenuates glucose-induced insulin release via direct interaction with ordinary GHSR that is uniquely coupled to novel cAMP/TRPM2 signaling in β-cells, and that this β-cell GHSR with unique insulinostatic signaling largely accounts for the systemic effects of ghrelin on circulating glucose and insulin levels. The novel β-cell specific GHSR-cAMP/TRPM2 signaling provides a potential therapeutic target for the treatment of type 2 diabetes.

The gut sensor as regulator of body weight

The gastrointestinal (GI) tract comprises a large endocrine organ that regulates not only nutrient sensing and metabolising but also satiety and energy homeostasis. More than 20 hormones secreted from the stomach, intestine, and pancreas as well as signaling mediators of the gut microbiome are involved in this process. A better understanding of how related pathways affect body weight and food intake will help us to find new strategies and drugs to treat obesity. For example, weight loss secondary to lifestyle intervention is often accompanied by unfavorable changes in multiple GI hormones, which may cause difficulties in maintaining a lower body weight status. Conversely, bariatric surgery favorably changes the hormone profile to support improved satiety and metabolic function. This partially explains stronger sustained body weight reduction resulting in better long-term results of improved metabolic functions. This review focuses on GI hormones and signaling mediators of the microbiome involved in satiety regulation and energy homeostasis and summarizes their changes following weight loss. Furthermore, the potential role of GI hormones as anti-obesity drugs is discussed.

Ghrelin

BACKGROUND

The gastrointestinal peptide hormone ghrelin was discovered in 1999 as the endogenous ligand of the growth hormone secretagogue receptor. Increasing evidence supports more complicated and nuanced roles for the hormone, which go beyond the regulation of systemic energy metabolism.

SCOPE OF REVIEW

In this review, we discuss the diverse biological functions of ghrelin, the regulation of its secretion, and address questions that still remain 15 years after its discovery.

MAJOR CONCLUSIONS

In recent years, ghrelin has been found to have a plethora of central and peripheral actions in distinct areas including learning and memory, gut motility and gastric acid secretion, sleep/wake rhythm, reward seeking behavior, taste sensation and glucose metabolism.

The extracellular N-terminal domain of G-protein coupled receptor 83 regulates signaling properties and is an intramolecular inverse agonist

Background

Recently, the orphan G-protein coupled receptor 83 (GPR83) was identified as a new participant in body weight regulation. This receptor is highly expressed in the hypothalamic arcuate nucleus and is regulated in response to nutrient availability. Gpr83 knock-out mice are protected from diet-induced obesity. Moreover, in a previous study, we designed and characterized several artificial constitutively activating mutations (CAMs) in GPR83. A particular CAM was located in the extracellular N-terminal domain (eNDo) that is highly conserved among GPR83 orthologs. This suggests the contribution of this receptor part into regulation of signaling, which needed a more detailed investigation.

Findings

In this present study, therefore, we further explored the role of the eNDo in regulating GPR83-signaling and demonstrate a proof-of-principle approach in that deletion mutants are characterized by a strong increase in basal Gq/11-mediated signaling, whilst none of the additionally characterized signaling pathways (Gs, Gi, G12/13) were activated by the N-terminal deletion variants. Of note, we detected basal GPR83 MAPK-activity of the wild type receptor, which was not increased in the deletion variants.

Conclusions

Finally, the extracellular portion of GPR83 has a strong regulatory function on this receptor. A suppressive - inverse agonistic - effect of the eNDo on GPR83 signaling activity is demonstrated here, which also suggests a putative link between extracellular receptor activation and proteolytic cleavage. These new insights highlight important aspects of GPR83-regulation and might open options in the development of tools to modulate GPR83-signaling.

Hunting for the function of orphan GPCRs - beyond the search for the endogenous ligand

Seven transmembrane-spanning proteins (7TM), also called GPCRs, are among the most versatile and evolutionary successful protein families. Out of the 400 non-odourant members identified in the human genome, approximately 100 remain orphans that have not been matched with an endogenous ligand. Apart from the classical deorphanization strategies, several alternative strategies provided recent new insights into the function of these proteins, which hold promise for high therapeutic potential. These alternative strategies consist of the phenotypical characterization of organisms silenced or overexpressing orphan 7TM proteins, the search for constitutive receptor activity and formation of protein complexes including 7TM proteins as well as the development of synthetic, surrogate ligands. Taken together, a variety of ligand-independent functions can be attributed to orphan 7TM proteins that range from constitutive activity to complex formation with other proteins and include 'true' orphans for which no ligand exist and 'conditional' orphans that behave like orphans in the absence of ligand and as non-orphans in the presence of ligand.