Nearly 500 different chemical and biological entities target them, accounting for more than one-third of all FDA-approved drugs. In 2012, the study of their structure and function earned Robert Lefkowitz and Brian Kobilka the Nobel Prize in Chemistry.

But whom are we talking about?

G protein-coupled receptors (GPCRs), of course.

Members of the GPCR family form the largest family of membrane receptors, both in terms of functionality and number. Involved in homeostasis, cell growth or metabolic processes, more than 800 entities (4% of all genes) are constantly finding new alter egos, notably through the identification of orphan GPCRs.

When a GPCR is identified, but when its endogenous ligand is still unknown and its related pairing is not ratified by the International Union of Basic and Clinical Pharmacology (IUPHAR), the GPCR is referred as orphan GPCR. GPCR orphan receptors are usually given the name "GPR" followed by a dedicated number. Once the ligand of the orphan GPCR is discovered, the GPCR becomes adopted or deorphanized.

Since 1986 (1), thanks to the development of the genetic cloning and nucleic acid-based homology screening to known receptors (2), the class of orphan GPCRs has rapidly grown to represent more than 122 new members, with:

• 87 class A (rhodopsin-like GPCRs family);
• 8 class C (glutamate family); GPR156, GPR158, GPR179, GPRC5A, GPRC5B, GPRC5C,GPRC5D, GPRC6
• 27 adhesion members (large extracellular region, similar to the Class B GPCR, but which is linked to the 7TM region by a GPCR autoproteolysis-inducing domain).

In 1987, Fargin et al (3) were working on the cloning of guanine nucleotides regulatory proteins. They stumbled upon a specific clone, G-21, that contained an intronless gene with high similarities to β2-adrenergic receptors. Putting the clone in contact with serotonergic ligands, they revealed transfected COS-7 cell line was serotonin 1A receptor (or 5-HT1A receptor). The same year, Bunzow et al applied the identical methodology for RGB-2 rat clone and identify the D2 dopamine receptor (4).

Since then, expressing the orphan GPCR in a cellular system and testing a library of potential ligands in high-throughput screening (HTS) has become the routine procedure to deorphanize receptors. The technique is called reverse pharmacology.

Like any other unknown protein, the first step is to identify the function of the orphan GPCR. Elucidation of receptor function is traditionally performed by in vivo gene deletion, i.e. homologous recombination or CRISPR technology application to generate knockout rodents (KO). The generated species are then studied for observable phenotypic changes. Since 2010, whole-genome and exome sequencing of patients with suspect phenotypes are of great help in discovering human monogenic diseases caused by GPCR mutations.

• In december 2020, GSK licenses global rights to a portfolio of GPR35 agonists designed by Sosei Heptares using its StaR® technology
• Noor Abdulkareem demonstrated that ADGRF1 (GPR110) represents a novel drug target in HER2+ breast cancer, warranting discovery of novel ADGRF1 antagonists (5)
• Priyanka Karmokar showed that GPR40/120-mediated signaling may be involved in the regulation of renal cancer tumorigenesis and progression on the cellular level. Mechanistic studies are underway to assess the cellular cascades that are influenced by GPR40 and GPR120 (6)
• Le Mercier et al elucidated role of GPR182 as a new atypical chemokine receptor for CXCL10, CXCL12, and CXCL13, which is involved in the regulation of hematopoietic stem cell homeostasis (7)
• Stoo et al identified two new inverse agonists of the orphan GPCR GPR52, O-1918 and CBD (8)
• Recent studies have also revealed a significant role for GPR116 in renal physiology (9)
• GPRC5A is expressed in human ASM and increases with asthma, RA or cytokines relevant to asthma (10)
• The physiological role of Gpr52 has been recently clarified as it seems that GPR52 promotes the biosynthesis of fatty acid and cholesterol in a ligand-dependent and a constitutive manner, respectively, and GPR52 participates in HFD-induced fatty acid synthesis in liver (11).
• Rebeillard et al validated the interaction of GPR88 with nuclear proteins ATRX, TOP2B, and BAZ2B, all involved in chromatin remodeling (12)
• GPR56 seems to be a potential target to develop novel antidepressants (13)

(1) Dixon RA, Kobilka BK, Strader DJ, Benovic JL, Dohlman HG, Frielle T, Bolanowski MA, Bennett CD, Rands E, Diehl RE, Mumford RA, Slater EE, Sigal IS, Caron MG, Lefkowitz RJ, Strader CD. Cloning of the gene and cDNA for mammalian beta-adrenergic receptor and homology with rhodopsin.  Nature. 1986 May 1-7; 321(6065):75-9.

(2) Libert, F., Vassart, G., & Parmentier, M. (1991). Current developments in G-protein-coupled receptors. Current Opinion in Cell Biology, 3(2), 218–223. doi:10.1016/0955-0674(91)90142-l

(3) Fargin A, Raymond JR, Lohse MJ, Kobilka BK, Caron MG, Lefkowitz RJ.

The genomic clone G-21 which resembles a beta-adrenergic receptor sequence encodes the 5-HT1A receptor. Nature. 1988 Sep 22; 335(6188):358-60.

(4) Bunzow JR, Van Tol HH, Grandy DK, Albert P, Salon J, Christie M, Machida CA, Neve KA, Civelli O. Cloning and expression of a rat D2 dopamine receptor cDNA. Nature. 1988 Dec 22-29; 336(6201):783-7

(5) Noor Abdulkareem. A novel role of ADGRF1 (GPR110) in promoting cellular quiescence and chemoresistance in human epidermal growth factor receptor 2-positive breast cancer. 2021 Apr.

(6) Priyanka Karmokar. Diverse Roles of G-Protein-Coupled Receptor 40 (GPR40/FFA1) and GPR120 (FFA4) in the Regulation of Cell Growth and Motile Activities in Renal Cell Carcinoma (RCC). 2021 Apr.

(7) GPR182 is an endothelium-specific atypical chemokine receptor that maintains hematopoietic stem cell homeostasis. 2021 Apr.

(8) Lisa A. Stott, Cheryl A. Brighton, Jason Brown, Richard Mould, Kirstie A.Bennett, Robert Newman, Heather Currinn, Flavia Autore, Alicia P.Higueruelo, Benjamin G.Tehan, Cliona Mac Sweeney. Characterisation of inverse agonism of the orphan-G protein-coupled receptor GPR52 by cannabinoid ligands Cannabidiol and O-1918. 2021 June.

(9) Zaidman NA, Tomilin VN, Hassanzadeh Khayyat N, Damarla M, Tidmore J, Capen DE, et al. Adhesion-gpcr gpr116 (adgrf5) expression inhibits renal acid secretion. Proc Natl Acad Sci U S A. 2020.

(10) B. Roos, J. J. Teske, S. Varadharajan, S. A. Wicher, R. D. Britt, C. M. Pabelick, S. Venkatachalem, Y. Prakash. Anesthesiology and Perioperative Medicine, Mayo Clinic, Rochester, MN, United States. Class C Orphan GPCR GPRC5A in Human Airway Smooth Muscle.

(11) Mitsuo Wada, Kayo Yukawa, Hiroyuki Ogasawara, Koichi Suzawa, Tatsuya Maekawa, Yoshihisa Yamamoto, Takeshi Ohta, Eunyoung Lee, and Takashi Mik. GPR52 accelerates fatty acid biosynthesis in a ligand-dependent manner in hepatocytes and in response to excessive fat intake in mice.

(12) Florian Rebeillard, Stéphanie De Gois, Nicolas Pietrancosta, Thi Hue Mai, René Lai-Kuen, Brigitte L Kieffer, Bruno Giros, Renaud Massart, Michèle Darmon, Jorge Diaz. The Orphan GPCR Receptor, GPR88, Interacts with Nuclear Protein Partners in the Cerebral Cortex

(13) Belzeaux R, Gorgievski V, Fiori LM, et al. GPR56/ADGRG1 is associated with response to antidepressant treatment. Nat Commun. 2020;11:1635.