Pharmacology and function of the orphan GPR139 G protein-coupled receptor.

G protein-coupled receptors (GPCRs) constitute the largest family of receptors and membrane proteins in the human genome with ~800 members of which half are olfactory. GPCRs are activated by a very broad range of endogenous signalling molecules and are involved in a plethora of physiological functions. All GPCRs contain a transmembrane domain, consisting of a bundle of seven α-helices spanning the cell membrane, and forming the majority of the known ortho- or allosteric ligand binding sites. Due to their many physiological functions and the accessible and druggable transmembrane pocket, GPCRs constitute the largest family of drug targets mediating the actions of 34% of currently marketed drugs. GPCRs activate one or more of the four G protein families (Gq/11 , Gi/o , Gs and G12/13 ) and/or ß-arrestin. About a third of the non-olfactory GPCRs are referred to as orphan receptors which means that their endogenous agonist(s) have not yet been found or firmly established. In this MiniReview, we focus on the orphan GPR139 receptor, for which the aromatic amino acids L-Trp and L-Phe as well as ACTH/α-MSH-related peptides have been proposed as endogenous agonists. GPR139 has been reported to activate several G protein pathways of which Gq/11 is the primary one. The receptor shows the highest expression in the striatum, thalamus, hypothalamus, pituitary and habenula of the human, rat and mouse CNS. We review the surrogate agonists and antagonists that have been published as well as the agonist pharmacophore and binding site. Finally, the putative physiological functions and therapeutic potential are outlined.

Identification of a novel scaffold for a small molecule GPR139 receptor agonist.

GPR139 is an orphan G protein-coupled receptor (GPCR) that is primarily expressed in the brain in regions known to regulate motor control and metabolism. Here, we screened a diverse 4,000 compound library in order to identify GPR139 agonists. We identified 11 initial hits in a calcium mobilization screen, including one compound, AC4, which contains a different chemical scaffold to what has previously been described for GPR139 agonists. Our mutagenesis data shows that AC4 interacts with the same hotspots in the binding site of GPR139 as those reported to interact with the reference agonists 1a and 7c. We additionally tested and validated 160 analogs in a calcium mobilization assay and found 5 compounds with improved potency compared to AC4. In total, we identified 36 GPR139 agonists with potencies in the nanomolar range (90-990 nM). The most potent compounds were confirmed as GPR139 agonists using an orthogonal ERK phosphorylation assay where they displayed a similar rank order of potency. Accordingly, we herein introduce multiple novel GPR139 agonists, including one with a novel chemical scaffold, which can be used as tools for future pharmacological and medicinal chemistry exploration of GPR139.

The GPR139 reference agonists 1a and 7c, and tryptophan and phenylalanine share a common binding site.

GPR139 is an orphan G protein-coupled receptor expressed in the brain, in particular in the habenula, hypothalamus and striatum. It has therefore been suggested that GPR139 is a possible target for metabolic disorders and Parkinson's disease. Several surrogate agonist series have been published for GPR139. Two series published by Shi et al. and Dvorak et al. included agonists 1a and 7c respectively, with potencies in the ten-nanomolar range. Furthermore, Isberg et al. and Liu et al. have previously shown that tryptophan (Trp) and phenylalanine (Phe) can activate GPR139 in the hundred-micromolar range. In this study, we produced a mutagenesis-guided model of the GPR139 binding site to form a foundation for future structure-based ligand optimization. Receptor mutants studied in a Ca2+ assay demonstrated that residues F1093×33, H1875×43, W2416×48 and N2717×38, but not E1083×32, are highly important for the activation of GPR139 as predicted by the receptor model. The initial ligand-receptor complex was optimized through free energy perturbation simulations, generating a refined GPR139 model in agreement with experimental data. In summary, the GPR139 reference surrogate agonists 1a and 7c, and the endogenous amino acids L-Trp and L-Phe share a common binding site, as demonstrated by mutagenesis, ligand docking and free energy calculations.

The orphan G protein-coupled receptor GPR139 is activated by the peptides: Adrenocorticotropic hormone (ACTH), α-, and ß-melanocyte stimulating hormone (α-MSH, and ß-MSH), and the conserved core motif HFRW.

GPR139 is an orphan G protein-coupled receptor that is expressed primarily in the brain. Not much is known regarding the function of GPR139. Recently we have shown that GPR139 is activated by the amino acids l-tryptophan and l-phenylalanine (EC50 values of 220 µM and 320 µM, respectively), as well as di-peptides comprised of aromatic amino acids. This led us to hypothesize that GPR139 may be activated by peptides. Sequence alignment of the binding cavities of all class A GPCRs, revealed that the binding pocket of the melanocortin 4 receptor is similar to that of GPR139. Based on the chemogenomics principle "similar targets bind similar ligands", we tested three known endogenous melanocortin 4 receptor agonists; adrenocorticotropic hormone (ACTH) and α- and ß-melanocyte stimulating hormone (α-MSH and ß-MSH) on CHO-k1 cells stably expressing the human GPR139 in a Fluo-4 Ca2+-assay. All three peptides, as well as their conserved core motif HFRW, were found to activate GPR139 in the low micromolar range. Moreover, we found that peptides consisting of nine or ten N-terminal residues of α-MSH activate GPR139 in the submicromolar range. α-MSH1-9 was found to correspond to the product of a predicted cleavage site in the pre-pro-protein pro-opiomelanocortin (POMC). Our results demonstrate that GPR139 is a peptide receptor, activated by ACTH, α-MSH, ß-MSH, the conserved core motif HFRW as well as a potential endogenous peptide α-MSH1-9. Further studies are needed to determine the functional relevance of GPR139 mediated signaling by these peptides.

Functional consequences of glucagon-like peptide-1 receptor cross-talk and trafficking.

The signaling capacity of seven-transmembrane/G-protein-coupled receptors (7TM/GPCRs) can be regulated through ligand-mediated receptor trafficking. Classically, the recycling of internalized receptors is associated with resensitization, whereas receptor degradation terminates signaling. We have shown previously that the incretin glucagon-like peptide-1 receptor (GLP-1R) internalizes fast and is primarily resensitized through recycling back to the cell surface. GLP-1R is expressed in pancreatic islets together with the closely related glucose-dependent insulinotropic polypeptide (GIPR) and glucagon (GCGR) receptors. The interaction and cross-talk between coexpressed receptors is a wide phenomenon of the 7TM/GPCR superfamily. Numerous reports show functional consequences for signaling and trafficking of the involved receptors. On the basis of the high structural similarity and tissue coexpression, we here investigated the potential cross-talk between GLP-1R and GIPR or GCGR in both trafficking and signaling pathways. Using a real-time time-resolved FRET-based internalization assay, we show that GLP-1R, GIPR, and GCGR internalize with differential properties. Remarkably, upon coexpression of the internalizing GLP-1R and the non-internalizing GIPR, GLP-1-mediated GLP-1R internalization was impaired in a GIPR concentration-dependent manner. As a functional consequence of such impaired internalization capability, GLP-1-mediated GLP-1R signaling was abrogated. A similar compromised signaling was found when GLP-1R internalization was abrogated by a dominant-negative version of dynamin (dynamin-1 K44E), which provides a mechanistic link between GLP-1R trafficking and signaling. This study highlights the importance of receptor internalization for full functionality of GLP-1R. Moreover, cross-talk between the two incretin receptors GLP-1R and GIPR is shown to alter receptor trafficking with functional consequences for GLP-1R signaling.