Biology:Growth hormone secretagogue receptor

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Short description: Protein-coding gene in the species Homo sapiens


A representation of the 3D structure of the protein myoglobin showing turquoise α-helices.
Generic protein structure example

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".[1][2] The role of GHS-R is thought to be in regulating energy homeostasis and body weight.[3] 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.[4] Outside the central nervous system, too, GSH-Rs are also found in the liver, in skeletal muscle, and even in the heart.[5]

Structure

Two identified transcript variants are expressed in several tissues and are evolutionarily 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.[6]

GHS-R1a is a member of the 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,[2] dopamine receptor type 2 (DRD2),[7] melanocortin-3 receptor (MC3R), and serotonin receptor type 2C (5-HT2c receptor).[7] 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, but not the synthesis, of growth hormone (GH) by the pituitary gland.[4][8][9]

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.[10] In fact, some GHS-R1a genetic variations, caused by single nucleotide polymorphisms (SNPs), have been found to be associated with hereditary obesity and others with hereditary short stature.[11] 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.[12][13]

Intracellular signaling mechanisms

When the 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)[5]), protein kinase A (PKA),[5] protein kinase B (PKB), also known as AKT[5]), and AMP Activated Protein Kinase (AMPK) cascades.[5]

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.[2] 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.[4] In fact, the more ghrelin administered, the more food the rodent consumed.[4] 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.[4] 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.[4]

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.[14] 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.[4] 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.[14]

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[16]

References

  1. ↑ "International Union of Pharmacology. LVI. Ghrelin receptor nomenclature, distribution, and function". Pharmacological Reviews 57 (4): 541–546. December 2005. doi:10.1124/pr.57.4.1. PMID 16382107. 
  2. ↑ 2.0 2.1 2.2 "Ghrelin: much more than a hunger hormone". Current Opinion in Clinical Nutrition and Metabolic Care 16 (6): 619–624. November 2013. doi:10.1097/mco.0b013e328365b9be. PMID 24100676. 
  3. ↑ "Basic aspects of ghrelin action". Ghrelin. Vitamins & Hormones. 77. 2008. pp. 89–119. doi:10.1016/S0083-6729(06)77005-4. ISBN 9780123736857. 
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 "The extra-hypothalamic actions of ghrelin on neuronal function". Trends in Neurosciences 34 (1): 31–40. January 2011. doi:10.1016/j.tins.2010.10.001. PMID 21035199. 
  5. ↑ 5.0 5.1 5.2 5.3 5.4 "The growth hormone secretagogue receptor: its intracellular signaling and regulation". International Journal of Molecular Sciences 15 (3): 4837–4855. March 2014. doi:10.3390/ijms15034837. PMID 24651458. 
  6. ↑ "Entrez Gene: GHS-R growth hormone secretagogue receptor". https://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=2693. 
  7. ↑ 7.0 7.1 "Taking two to tango: a role for ghrelin receptor heterodimerization in stress and reward". Frontiers in Neuroscience 7: 148. August 2013. doi:10.3389/fnins.2013.00148. PMID 24009547. 
  8. ↑ "The novel hypothalamic peptide ghrelin stimulates food intake and growth hormone secretion". Endocrinology 141 (11): 4325–4328. November 2000. doi:10.1210/endo.141.11.7873. PMID 11089570. 
  9. ↑ "Ghrelin: ghrelin as a regulatory Peptide in growth hormone secretion". Journal of Clinical and Diagnostic Research 8 (8): MC13–MC17. August 2014. doi:10.7860/JCDR/2014/9863.4767. PMID 25302229. 
  10. ↑ "Loss of constitutive activity of the growth hormone secretagogue receptor in familial short stature". The Journal of Clinical Investigation 116 (3): 760–768. March 2006. doi:10.1172/jci25303. PMID 16511605. 
  11. ↑ "Ghrelin Receptor Mutations and Human Obesity". Genetics of Monogenic and Syndromic Obesity. 140. 2016. 131–50. doi:10.1016/bs.pmbts.2016.02.001. ISBN 9780128046159. 
  12. ↑ "High constitutive signaling of the ghrelin receptor--identification of a potent inverse agonist". Molecular Endocrinology 17 (11): 2201–2210. November 2003. doi:10.1210/me.2003-0069. PMID 12907757. 
  13. ↑ "In vivo characterization of high Basal signaling from the ghrelin receptor". Endocrinology 150 (11): 4920–4930. November 2009. doi:10.1210/en.2008-1638. PMID 19819980. 
  14. ↑ 14.0 14.1 "Depression and metabolism: linking changes in leptin and ghrelin to mood". F1000 Biology Reports 1 (63): 63. August 2009. doi:10.3410/b1-63. PMID 20948621. 
  15. ↑ Brain Somatic Cross-Talk and the Central Control of Metabolism. Springer Science & Business Media. 6 December 2012. pp. 42–. ISBN 978-3-642-18999-9. https://books.google.com/books?id=Ml7vCAAAQBAJ&pg=PA42. 
  16. ↑ "Discovery of PF-5190457, a Potent, Selective, and Orally Bioavailable Ghrelin Receptor Inverse Agonist Clinical Candidate". ACS Medicinal Chemistry Letters 5 (5): 474–479. May 2014. doi:10.1021/ml400473x. PMID 24900864. 

Further reading

External links

This article incorporates text from the United States National Library of Medicine, which is in the public domain.



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