Growth hormone secretagogue receptor

Growth hormone secretagogue receptor(GHS-R), also known as ghrelin receptor, is a G protein-coupled receptor that binds growth hormone secretagogues (GHSs), such as ghrelin, the "hunger hormone".[5][6] The role of GHS-R is thought to be in regulating energy homeostasis and body weight.[7] In the brain, they are most highly expressed in the hypothalamus, specifically the ventromedial nucleus and arcuate nucleus. GSH-Rs are also expressed in other areas of the brain, including the ventral tegmental area, hippocampus, and substantia nigra.[8] Outside the central nervous system, too, GSH-Rs are also found in the liver, in skeletal muscle, and even in the heart.[9]

GHSR
Identifiers
AliasesGHSR, GHDP, growth hormone secretagogue receptor gene, growth hormone secretagogue receptor
External IDsOMIM: 601898 MGI: 2441906 HomoloGene: 57161 GeneCards: GHSR
Orthologs
SpeciesHumanMouse
Entrez

2693

208188

Ensembl

ENSG00000121853

ENSMUSG00000051136

UniProt

Q92847

Q99P50

RefSeq (mRNA)

NM_198407
NM_004122

NM_177330

RefSeq (protein)

NP_004113
NP_940799

NP_796304

Location (UCSC)Chr 3: 172.44 – 172.45 MbChr 3: 27.43 – 27.43 Mb
PubMed search[3][4]
Wikidata
View/Edit HumanView/Edit Mouse

Structure

Two identified transcript variants are expressed in several tissues and are evolutionary conserved in fish and swine. One transcript, 1a, excises an intron and encodes the functional protein; this protein is the receptor for the ghrelin ligand and defines a neuroendocrine pathway for growth hormone release. The second transcript (1b) retains the intron and does not function as a receptor for ghrelin; however, it may function to attenuate activity of isoform 1a.[10]

GHS-R1a falls into G-protein-coupled receptor (GPCR) family. Previous studies have shown that GPCRs can form heterodimers, or functional receptor pairs with other types of G-protein coupled receptors (GPCRs). Various studies suggest that GHS-R1a specifically forms dimers with the following hormone and neurotransmitter receptors: somatostatin receptor 5,[6] dopamine receptor type 2 (DRD2),[11] melanocortin-3 receptor (MC3R), and serotonin receptor type 2C (5-HT2c receptor).[11] See "Function" section below for details on the purported functions of these heterodimers.

Function

Growth hormone release

The binding of ghrelin to GHS-R1a in pituitary cells stimulates the secretion of growth hormone (GH) by the pituitary gland.[8][12]

Constitutive activity

One important feature of GHS-R1a is that there is still some activity in the receptor even when it is not actively being stimulated. This is called constitutive activity, and it means that the receptor is always "on," unless acted on by an inverse agonist. This constitutive activity seems to provide a tonic signal required for the development of normal height, probably through an effect on the GH axis.[13] In fact, some genetic variations, or single nucleotide polymorphisms (SNPs), in growth hormone secretagogue receptor, have been found to be associated with hereditary obesity and others with hereditary short stature.[14] It was also found that, when GHS-R1A constitutive activity, was diminished, there were decreased levels of hunger-inducing hormone neuropeptide Y (NPY) as well as in food intake and body weight.[15][16]

Intracellular signaling mechanisms

When growth hormone secretagogue receptor is activated, a variety of different intracellular signaling cascades can result, depending on the cell type in which the receptor is expressed. These intracellular signaling cascades include mitogen-activated protein kinase (MAPK)[9]), protein kinase A (PKA),[9] protein kinase B (PKB), also known as AKT[9]), and AMP Activated Protein Kinase (AMPK) cascades.[9]

Behavioral reinforcement of food intake

It is well-characterized that activating the growth hormone secretagogue receptor with ghrelin induces an orexigenic state, or general feeling of hunger.[6] However, ghrelin may also play a role in behavioral reinforcement. Studies in animal models, found that food intake increased when ghrelin was specifically administered to just the ventral tegmental area (VTA), a brain area that uses dopamine signaling to reinforce behavior.[8] In fact, the more ghrelin administered, the more food the rodent consumed.[8] This is called a dose-dependent effect. Building on this, it was found that there are growth hormone secretagogue receptors in the VTA and that ghrelin acts on the VTA through these receptors.[8] Current studies, furthermore, suggest that the VTA may contain dimers of GHS-R1a and dopamine receptor type 2 (DRD2). If these two receptors do indeed form dimers, this would somehow link ghrelin signaling to dopaminergic signaling.[8]

Enhancement of learning and memory

The growth hormone secretagogue receptor may also be linked to learning and memory. First of all, the receptor is found in the hippocampus, the brain region responsible for long-term memory.[17] Second, it was found that specifically activating the receptor in just the hippocampus increased both long-term potentiation (LTP) and dendritic spine density, two cellular phenomena thought to be involved in learning.[8] Third, short-term calorie restriction, defined as a 30% reduction in caloric intake for two weeks, which naturally increases ghrelin levels and thus activates the receptor, was found to increase both performance on spatial learning tasks as well as neurogenesis in the adult hippocampus.[17]

Selective ligands

A range of selective ligands for the GHS-R receptor are now available and are being developed for several clinical applications. GHS-R agonists have appetite-stimulating and growth hormone-releasing effects, and are likely to be useful for the treatment of muscle wasting and frailty associated with old-age and degenerative diseases. On the other hand, GHS-R antagonists have anorectic effects and are likely to be useful for the treatment of obesity.

Agonists

Antagonists
  • A-778,193
  • PF-5190457[19]

References
  1. GRCh38: Ensembl release 89: ENSG00000121853 - Ensembl, May 2017
  2. GRCm38: Ensembl release 89: ENSMUSG00000051136 - Ensembl, May 2017
  3. "Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  4. "Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  5. Davenport AP, Bonner TI, Foord SM, Harmar AJ, Neubig RR, Pin JP, Spedding M, Kojima M, Kangawa K (December 2005). "International Union of Pharmacology. LVI. Ghrelin receptor nomenclature, distribution, and function". Pharmacological Reviews. 57 (4): 541–6. doi:10.1124/pr.57.4.1. PMID 16382107. S2CID 11254096.
  6. Pradhan G, Samson SL, Sun Y (November 2013). "Ghrelin: much more than a hunger hormone". Current Opinion in Clinical Nutrition and Metabolic Care. 16 (6): 619–24. doi:10.1097/mco.0b013e328365b9be. PMC 4049314. PMID 24100676.
  7. Pazos Y, Casanueva FF, Camiña JP (2008). "Basic aspects of ghrelin action". Ghrelin. Vitamins & Hormones. Vol. 77. pp. 89–119. doi:10.1016/S0083-6729(06)77005-4. ISBN 9780123736857. PMID 17983854.
  8. Andrews ZB (January 2011). "The extra-hypothalamic actions of ghrelin on neuronal function". Trends in Neurosciences. 34 (1): 31–40. doi:10.1016/j.tins.2010.10.001. PMID 21035199. S2CID 42200775.
  9. Yin Y, Li Y, Zhang W (March 2014). "The growth hormone secretagogue receptor: its intracellular signaling and regulation". International Journal of Molecular Sciences. 15 (3): 4837–55. doi:10.3390/ijms15034837. PMC 3975427. PMID 24651458.
  10. "Entrez Gene: GHS-R growth hormone secretagogue receptor".
  11. Schellekens H, Dinan TG, Cryan JF (August 2013). "Taking two to tango: a role for ghrelin receptor heterodimerization in stress and reward". Frontiers in Neuroscience. 7: 148. doi:10.3389/fnins.2013.00148. PMC 3757321. PMID 24009547.
  12. Wren AM, Small CJ, Ward HL, Murphy KG, Dakin CL, Taheri S, Kennedy AR, Roberts GH, Morgan DG, Ghatei MA, Bloom SR (November 2000). "The novel hypothalamic peptide ghrelin stimulates food intake and growth hormone secretion". Endocrinology. 141 (11): 4325–8. doi:10.1210/endo.141.11.7873. PMID 11089570.
  13. Pantel J, Legendre M, Cabrol S, Hilal L, Hajaji Y, Morisset S, Nivot S, Vie-Luton MP, Grouselle D, de Kerdanet M, Kadiri A, Epelbaum J, Le Bouc Y, Amselem S (March 2006). "Loss of constitutive activity of the growth hormone secretagogue receptor in familial short stature". The Journal of Clinical Investigation. 116 (3): 760–8. doi:10.1172/jci25303. PMC 1386106. PMID 16511605.
  14. Wang W, Tao YX (2016). "Ghrelin Receptor Mutations and Human Obesity". Genetics of Monogenic and Syndromic Obesity. Progress in Molecular Biology and Translational Science. Vol. 140. pp. 131–50. doi:10.1016/bs.pmbts.2016.02.001. ISBN 9780128046159. PMID 27288828.
  15. Holst B, Cygankiewicz A, Jensen TH, Ankersen M, Schwartz TW (November 2003). "High constitutive signaling of the ghrelin receptor--identification of a potent inverse agonist". Molecular Endocrinology. 17 (11): 2201–10. doi:10.1210/me.2003-0069. PMID 12907757.
  16. Petersen PS, Woldbye DP, Madsen AN, Egerod KL, Jin C, Lang M, Rasmussen M, Beck-Sickinger AG, Holst B (November 2009). "In vivo characterization of high Basal signaling from the ghrelin receptor". Endocrinology. 150 (11): 4920–30. doi:10.1210/en.2008-1638. PMID 19819980.
  17. Lutter M, Elmquist J (August 2009). "Depression and metabolism: linking changes in leptin and ghrelin to mood". F1000 Biology Reports. 1 (63): 63. doi:10.3410/b1-63. PMC 2948264. PMID 20948621.
  18. Kordon C, Robinson I, Hanoune J, Dantzer R (6 December 2012). Brain Somatic Cross-Talk and the Central Control of Metabolism. Springer Science & Business Media. pp. 42–. ISBN 978-3-642-18999-9.
  19. Bhattacharya SK, Andrews K, Beveridge R, Cameron KO, Chen C, Dunn M, et al. (May 2014). "Discovery of PF-5190457, a Potent, Selective, and Orally Bioavailable Ghrelin Receptor Inverse Agonist Clinical Candidate". ACS Medicinal Chemistry Letters. 5 (5): 474–9. doi:10.1021/ml400473x. PMC 4027753. PMID 24900864.

Further reading

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