Chemical Probe Identification Platform for Orphan GPCRs Using Focused Compound Screening: GPR39 as a Case Example

Abstract

Orphan G protein-coupled receptors (oGPCRs) are a class of integral membrane proteins for which endogenous ligands or transmitters have not yet been discovered. Transgenic animal technologies have uncovered potential roles for many of these oGPCRs, providing new targets for the treatment of various diseases. Understanding signaling pathways of oGPCRs and validating these receptors as potential drug targets requires the identification of chemical probe compounds to be used in place of endogenous ligands to interrogate these receptors. A novel chemical probe identification platform was created in which GPCR-focused libraries were screened against sets of oGPCR targets, with a goal of discovering fit-for-purpose chemical probes for the more druggable members of the set. Application of the platform to a set of oGPCRs resulted in the discovery of the first reported small molecule agonists for GPR39, a receptor implicated in the regulation of insulin secretion and preservation of beta cells in the pancreas. Compound 1 stimulated intracellular calcium mobilization in recombinant and native cells in a GPR39-specific manner but did not potentiate glucose-stimulated insulin secretion in human islet preparations.

Novel Zn2+ Modulated GPR39 Receptor Agonists Do Not Drive Acute Insulin Secretion in Rodents

“…These findings indicate that GPR39 is permissive to binding a broad range of chemotypes for agonistic modulation. This is further supported by recently published Zn 2+ dependent agonists with yet different chemotypes [ 22 – 24 ]. The druggability of GPR39 is good news and provides an opportunity to further probe the pharmacology of this receptor.…”

“…Thus, there was no correlation between rat INS-1E and mouse islet GSIS data. A recent report on a Zn 2+ dependent GPR39 agonist with potent Ca 2+ /Gα q effects showed no effects on GSIS in human islets and in INS-1E cells [ 22 ]. The lack of correlations between assays on recombinant cells and assays on cells endogenously expressing GPR39 is a frustrating dilemma, both regarding drug discovery feasibility and regarding building a pharmacological understanding of the target.…”

Discovery and Characterization of Novel GPR39 Agonists Allosterically Modulated by Zinc

“…In a clinical study of LY2784544, diarrhea, nausea, anemia, and transient increases in serum creatinine, uric acid, and potassium have been reported and attributed to a "typical tumor lysis syndrome" (Tefferi, 2012); however, it seems conceivable that at least some of these effects might be due to activation of GPR39. Interestingly, GPR39 is highly expressed in human colorectal adenocarcinoma HT-29 cells, and Zn 21 and a GPR39 agonist stimulated Gq signaling and promoted survival in these cells (Cohen et al, 2012(Cohen et al, , 2014Boehm et al, 2013). By using the fluorescent imaging plate reader assay, we have shown that GPR39-C3, LY2784544, and GSK2636771 strongly activate the Gq pathway in HT-29 cells (Supplemental Fig.…”

“…Zn 21 stimulates signaling via the Gq and b-arrestin pathways, as well as the Gs pathway (Holst et al, 2007). Recently, three groups reported small-molecule agonists for GPR39 (Boehm et al, 2013;Peukert et al, 2014;Fjellström et al, 2015). However, the signaling pathways induced by these agonists, and their relationship to Zn 21 concentrations, remain to be elucidated.…”

What can we learn from molecular dynamics simulations for GPCR drug design?

“…In other words, there are still ample opportunities to exploit hitherto unexplored GPCR targets with known ligands and function, but also orphan receptors. Besides novel molecular and cell biology techniques such as specifically engineered receptors [2] or the systematic generation of chemical probes [8–10] recent breakthroughs in the elucidation of GPCR structural information [11–13] expand the toolbox to discover and optimize novel ligands with therapeutic potential for this target class.…”