Ligand-Free Signaling of G Protein Coupled Receptors: Addressing Unresolved questions with Antagonist Probes and Genomics
Main Article Content
Abstract
G protein coupled receptors (GPCRs) exist each in multiple forms and aggregates distributed across various cell compartments, signaling along multiple pathways upon activation by agonists. In addition, pervasive ligand-free GPCR signaling also occurs with multiple functional states. I propose three distinct ligand-free receptor categories: first, low level slippage into active forms (‘spontaneous basal signaling’); second, ‘acutely activated ligand-free signaling’ which sustains signaling after the agonist has dissociated from the receptor; and third, ‘sustained ligand-free signaling’ that is regulated to sustain cellular responses. Studies of the µ opioid receptor differentiate these three receptor forms and suggest that continued agonist stimulation can lead to sustained ligand-free signaling with a role in opioid dependence. The serotonin 5HT2A receptor also appears to support ligand-free signaling of physiological and pharmacological relevance. Yet, systematic studies of distinct ligand-free receptor forms are scarce. Sustained ligand-free signaling can arise from various mechanisms, such as tethered extracellular peptide regions of GPCRs stabilizing an active receptor-G protein state, demonstrated with the glucagon-like peptide 1 receptor GLP1R. Accurate assays are needed to measure ligand-free signaling along specific pathways, localized to cellular sub-compartments. Neutral antagonists (no effect on ligand-free signaling) and inverse agonists (block ligand-free signaling) distinguish between the various forms of ligand-free signaling and can lead to distinct therapeutic applications, highlighted with opioid, serotonin, and peptide hormone receptors, targeting pain, depression and schizophrenia, and metabolic disorders. GPCR mutations that activate or suppress ligand-free signaling reveal (patho)physiological functions, but genetic effects on distinct ligand-free receptor signaling pathways are largely unresolved. Clarifying ligand-free signaling pathways of GPCRs has the potential to uncover hidden disease risk factors and novel targets for therapeutic interventions.
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How to Cite
SADEE, Wolfgang.
Ligand-Free Signaling of G Protein Coupled Receptors: Addressing Unresolved questions with Antagonist Probes and Genomics.
Medical Research Archives, [S.l.], v. 12, n. 9, sep. 2024.
ISSN 2375-1924.
Available at: <https://esmed.org/MRA/mra/article/view/5666>. Date accessed: 03 dec. 2024.
doi: https://doi.org/10.18103/mra.v12i9.5666.
Section
Review Articles
The Medical Research Archives grants authors the right to publish and reproduce the unrevised contribution in whole or in part at any time and in any form for any scholarly non-commercial purpose with the condition that all publications of the contribution include a full citation to the journal as published by the Medical Research Archives.
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3. Pándy-Szekeres W, Caroli J, Marmybekov A, et al. GPCRdb in 2023: State-specific structure models using AlphaFold2 and new ligand resources. Nucleic Acids Res. 2023, 51, D395–D402.
4. Sriram K, Insel PA. G protein-coupled receptors as targets for approved drugs: How many targets and how many drugs? Mol Pharmacol. 2018;93:251–258.
5. Perez DM, Karnik SS. Multiple signaling states of G-protein-coupled receptors. Pharmacol Rev. 2005;57:147–161.
6. Meye FJ, Ramakers GM, Adan RA. The vital role of constitutive GPCR activity in the mesolimbic dopamine system. Transl Psychiatry. 2014;4:e361. doi:10.1038/tp.2013.130.
7. Thompson MD, Percy ME, Cole DEC, Bichet DG, Hauser AS, Gorvin CM. G protein-coupled receptor (GPCR) gene variants and human genetic disease. Critic Rev Clinical Labor Sci. 2024; 1–30. https://doi.org/10.1080/10408363.2023.2286606.
8. Schöneberg T, Liebscher I. Inherited GPCR-related diseases. Pharmacol Rev. 2021;73(1):89-119.
9. Kern A, Grande C, Smith RG. apo-Ghrelin receptor (apo-GHSR1a) regulates dopamine signaling in the brain. Front Endocrinol. 2014;5:129. doi: 10.3389/fendo.2014.00129
10. Unal H, Karnik SS. Constitutive activity in the angiotensin II type 1 receptor: discovery and applications. Adv Pharmacol. 2014;70:155-174.
11. Sadee W, Oberdick J, Wang Z. Biased opioid antagonists as modulators of opioid dependence: Opportunities to improve pain therapy and opioid use management. Molecules. 2020;25:4163.
12. Wingler LM, Lefkowitz RJ. Conformational basis of G protein-coupled receptor signaling versatility. Trends Cell Biol. 2020;30(9):736–747.
13. Kurose H, Kim SG. Pharmacology of antagonism of GPCR. Biol Pharm Bul. 2022;45:669–674.
14. Zhou B, Giraldo J. An operational model for GPCR homodimers and its application in the analysis of biased signaling. Drug Discov Today. 2018;23:1591–1595.
15. Tóth AD, Szalai B, Kovács OT, et al. G protein-coupled receptor endocytosis generates spatiotemporal bias in β-arrestin signaling. Sci Signal. 2024;17(842):eadi0934. doi: 10.1126/scisignal.adi0934.
16. Anantakrishnan S, Naganathan AN. Thermodynamic architecture and conformational plasticity of GPCRs. Nat Commun. 2023;14(1):128. doi: 10.1038/s41467-023-35790-z.
17. Weis, W.I.; Kobilka, B.K. The molecular basis of G protein-coupled receptor activation. Annu. Rev. Biochem. 2018, 87, 897–919.
18. Schafer CT, Fay JF, Janz JM, Farrens DL. Decay of an active GPCR: Conformational dynamics govern agonist rebinding and persistence of an active, yet empty, receptor state. Proc Natl Acad Sci USA. 2016;113:11961–11966.
19. Wang D, Sun X, Sadee W. Wang D, Sun X, Sadee W. Different effects of opioid antagonists on mu-, delta-, and kappa-opioid receptors with and without agonist pretreatment. J Pharmacol Exp Ther. 2007;321(2):544-552.
20. Piñeyro G, Azzi M, deLean A, Schiller PW, Bouvier M. Reciprocal regulation of agonist and inverse agonist signaling efficacy upon short-term treatment of the human delta-opioid receptor with an inverse agonist. Mol. Pharmacol. 2005;67:336–348.
21. Kaneko S, Imai S, Uchikubo-Kamo T, Hisano T, Asao N, Shirouzu M, Shimada I. Structural and dynamic insights into the activation of the μ-opioid receptor by an allosteric modulator. Nat Commun. 2024;15(1):3544. doi: 10.1038/s41467-024-47792-6.
22. O’Brien, E.S., Rangari, V.A., El Daibani, A. et al. A µ-opioid receptor modulator that works cooperatively with naloxone. Nature. 2024;631:686-693.
23. Perry DC, Rosenbaum JS, Kurowski M, Sadée W. 3H-Etorphine receptor binding in vivo: small fractional occupancy elicits analgesia. Mo. Pharmacol. 1982:21,272–279.
24. Alhosan N, Cavallo D, Santiago M, Kelly E, Henderson G. Potency, dissociation kinetics and reversibility of fentanyls and nitazenes by naloxone at the μ opioid receptor. BioRX. 2024; doi: https://doi.org/10.1101/2024.06.04.597322 June 5, 2024.
25. Rosenbaum JS, Holford NHG, Richard ML, Aman RA, Sadee W. Discrimination of three types of opioid binding sites in rat brain in vivo. Mol Pharmacol. 1984;25:242–248.
26. Molla H, Lee R, Tare I, deWit H. Greater subjective effects of a low dose of LSD in participants with depressed mood. Neuropsychopharmacol. 2024;49:774–781 (2024).
27. Kim K, Che T, Panova O, DiBerto JF, et al. Structure of a hallucinogen-activated Gq-coupled 5-HT2A serotonin receptor. Cell. 2020;182(6):1574-1588.
28. Wacker D, Wang S, McCorvy, JD, et al. Crystal structure of an LSD-bound human serotonin receptor. Cell. 2017;168:377-389.
29. Cornejo MP, Mustafá ER, BarrileF, Cassano,D, et al.The intriguing ligand-dependent and -independent actions of the growth hormone secretagogue receptor on reward related behaviors. Neurosci Biobehav Rev. 2021;120: 401–416.
30. Canto I, Soh UJ, Trejo J. Allosteric modulation of protease-activated receptor signaling. Mini Rev. Med. Chem. 2012;12:804–811.
31. Wilde C, Fischer L, Lede V, et al. The constitutive activity of the adhesion GPCR GPR114/ADGRG5 is mediated by its tethered agonist. FASEB J. 2016;30:666–673.
32. Purgert CA, Izumi Y, Jong YJ, Kumar V, Zorumski CF, O’Malley KL. Intracellular mGluR5 can mediate synaptic plasticity in the hippocampus. J Neurosci. 2014;34:4589–4598.
33. Vargas MV, Dunlap LE, Dong C, et al. Psychedelics promote neuroplasticity through the activation of intracellular 5-HT2A receptors. Science. 2023;379:700–706.
34. Kanetsky PA, Swoyer J, Panossian S, Holmes R, Guerry D, Rebbeck TR. A polymorphism in the agouti signaling protein gene is associated with human pigmentation. Am J Hum Genet. 2002;70:770-775.
35. Lu X, Huang L, Huang Z, Feng D, Clark RJ, Chen C. LEAP-2: an emerging Endogenous Ghrelin Receptor Antagonist in the Pathophysiology of Obesity. Front Endocrinol. 2021;12:717544.
doi: 10.3389/fendo.2021.717544.
36. Liu JG, Prather PL. Chronic agonist treatment converts antagonists into inverse agonists at delta-opioid receptors. J Pharmacol Exp Ther. 2002;302:1070–1079.
37. Ko MC, Divin, MF, Lee H, Woods JH, Traynor JR. Differential in Vivo Potencies of Naltrexone and 6β-Naltrexol in the Monkey. J Pharmacol Exp Ther. 2006;316:772–779.
38. Soler-Cedeño O, Alton H, Bi GH, et al. AM6527, a neutral CB1 receptor antagonist, suppresses opioid taking and seeking, as well as cocaine seeking in rodents without aversive effects. Neuropsychopharmacol. 2024; https://doi.org/10.1038/s41386-024-01861-y
39. Kantrowitz JT. Targeting serotonin 5-HT2A receptors to better treat schizophrenia: Rationale and current approaches. CNS Drugs. 2020;34:947–959.
40. Muneta-Arrate I, Miranda-Azpiazu P, Horrillo I, Diez-Alarcia R, Meana JJ. Ligand bias and inverse agonism on 5-HT2A receptor-mediated modulation of G protein activity in post-mortem human brain. Br J Pharmacol. 2024; Apr 21. doi: 10.1111/bph.16368. Epub ahead of print.
41. Kossatz, E, Diez-Alarcia R, Gaitonde SA, et al. G protein-specific mechanisms in the serotonin 5-HT2A receptor regulate psychosis-related effects and memory deficits. Nat Commun. 15:4307 (2024). https://doi.org/10.1038/s41467-024-48196-2
42. Wallach J, Cao AB, Calkins MM, et al. Identification of 5-HT2A receptor signaling pathways associated with psychedelic potential. Nat Commun. 14, 8221 (2023). https://doi.org/10.1038/s41467-023-44016-1
43. Gaitonde, SA, Avet C, de la Fuente Revenga M, et al. Pharmacological fingerprint of antipsychotic drugs at the serotonin 5-HT2A receptor. Mol Psychiatry. 2024; doi: 10.1038/s41380-024-02531-7. Online ahead of print.
44. Müller A, Berkmann JC, Scheerer P, Biebermann H, Kleinau G. Insights into basal signaling regulation, oligomerization, and structural organization of the human G-protein coupled receptor 83. PLoS ONE. 2016;11, 68260.
45. Pantel J, Legendre M,Cabrol S, et al. Loss of constitutive activity of the growth hormone secretagogue receptor in familial short stature. J. Clin. Investig. 2006;116:760–768.
46. Wellman M, Abizaid A. Growth hormone secretagogue receptor dimers: A new pharmacological target. eNeuro. 2015:, ENEURO.0053-14.2015.
47. Sullivan LC, Chavera TS, Jamshidi RJ, Berg KA, Clarke WP. Constitutive desensitization of opioid receptors in peripheral sensory neurons. J Pharmacol Exp Ther. 2016;359:411–419.
48. Matrisciano F, Locci V, Dong E, Nicoletti F, Guidotti A, Grayson DR. Altered Expression and In Vivo Activity of mGlu5 Variant a Receptors in the Striatum of BTBR Mice: Novel Insights Into the Pathophysiology of Adult Idiopathic Forms of Autism Spectrum Disorders. Curr Neuropharmacol. 2022;20(12):2354-2368.
49. Hubert GW, Paquet M, Smith Y. Differential subcellular localization of mGluR1a and mGluR5 in the rat and monkey substantia nigra. J Neurosci. 2001;21(6):1838–1847.
50. Nash CA, Wei W, Irannejad R, Smrcka AV. Golgi localized β1-adrenergic receptors stimulate Golgi PI4P hydrolysis by PLCε to regulate cardiac hypertrophy. Elife. 2019,8:e48167.
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