Novel GPCR Screening Approach: Indirect Identification of S1P Receptor Agonists in Antagonist Screening Using a Calcium Assay

An Overview of Cell-Based Assay Platforms for the Solute Carrier Family of Transporters

The solute carrier (SLC) superfamily represents the biggest family of transporters with important roles in health and disease. Despite being attractive and druggable targets, the majority of SLCs remains understudied. One major hurdle in research on SLCs is the lack of tools, such as cell-based assays to investigate their biological role and for drug discovery. Another challenge is the disperse and anecdotal information on assay strategies that are suitable for SLCs. This review provides a comprehensive overview of state-of-the-art cellular assay technologies for SLC research and discusses relevant SLC characteristics enabling the choice of an optimal assay technology. The Innovative Medicines Initiative consortium RESOLUTE intends to accelerate research on SLCs by providing the scientific community with high-quality reagents, assay technologies and data sets, and to ultimately unlock SLCs for drug discovery.

PRESTO-Tango as an open-source resource for interrogation of the druggable human GPCRome

G protein-coupled receptors (GPCRs) are essential mediators of cellular signaling and are important targets of drug action. Of the approximately 350 nonolfactory human GPCRs, more than 100 are still considered to be 'orphans' because their endogenous ligands remain unknown. Here, we describe a unique open-source resource that allows interrogation of the druggable human GPCRome via a G protein-independent β-arrestin-recruitment assay. We validate this unique platform at more than 120 nonorphan human GPCR targets, demonstrate its utility for discovering new ligands for orphan human GPCRs and describe a method (parallel receptorome expression and screening via transcriptional output, with transcriptional activation following arrestin translocation (PRESTO-Tango)) for the simultaneous and parallel interrogation of the entire human nonolfactory GPCRome.

Sphingosine-1-phosphate induced contraction of bladder smooth muscle

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid that contracts most smooth muscles. Although S1P has been shown to contract bladder smooth muscle, the mechanism(s) by which S1P initiates contraction has not been extensively investigated. The goal of this study was to determine if S1P-induced force generation and myosin light chain (MLC) phosphorylation are dependent on calcium sensitization pathways mediated by protein kinase C (PKC) and Rho kinase (ROCK) and which S1P receptor is important in this response. Bladder smooth muscle strips from rabbit and rat were mounted for isometric force recording and contracted in response to carbachol or S1P in the presence and absence of an inhibitor of PKC (3 µM Bisindolylmaleimide-1) or ROCK (1 µM H-1172). 10 µM S1P produced approximately 40% of the force generated in response to 110 mM KCl in rabbit bladder smooth muscle. S1P, up to 100 µM, did not produce a response in rat bladder smooth muscle, any response evoked was due to solvent (NaOH). S1P-dependent force development was associated with a concomitant increase in Ser(19), but not dual Thr(18)/Ser(19) MLC phosphorylation. Inhibition of PKC decreased force development, whereas inhibition of ROCK abolished S1P-induced force. An inhibitor of the S1P2 receptor, JTE-013, relaxed a S1P-induced contraction; whereas, an agonist with low affinity to the S1P2 receptor, dihydro-S1P, did not elicit a contraction. Our results suggest that S1P contracts rabbit, but not rat, bladder smooth muscle via the S1P2 receptor and is dependent on MLC phosphorylation and myofilament calcium sensitization primarily in response to ROCK activation.

Reversible translocation of p115-RhoGEF by G(12/13)-coupled receptors

G protein-coupled receptors (GPCRs) are important targets for medicinal agents. Four different G protein families, G(s), G(i), G(q), and G(12), engage in their linkage to activation of receptor-specific signal transduction pathways. G(12) proteins were more recently studied, and upon activation by GPCRs they mediate activation of RhoGTPase guanine nucleotide exchange factors (RhoGEFs), which in turn activate the small GTPase RhoA. RhoA is involved in many cellular and physiological aspects, and a dysfunction of the G(12/13)-Rho pathway can lead to hypertension, cardiovascular diseases, stroke, impaired wound healing and immune cell functions, cancer progression and metastasis, or asthma. In this study, regulator of G protein signaling (RGS) domain-containing RhoGEFs were tagged with enhanced green fluorescent protein (EGFP) to detect their subcellular localization and translocation upon receptor activation. Constitutively active Galpha(12) and Galpha(13) mutants induced redistribution of these RhoGEFs from the cytosol to the plasma membrane. Furthermore, a pronounced and rapid translocation of p115-RhoGEF from the cytosol to the plasma membrane was observed upon activation of several G(12/13)-coupled GPCRs in a cell type-independent fashion. Plasma membrane translocation of p115-RhoGEF stimulated by a GPCR agonist could be completely and rapidly reversed by subsequent application of an antagonist for the respective GPCR, that is, p115-RhoGEF relocated back to the cytosol. The translocation of RhoGEF by G(12/13)-linked GPCRs can be quantified and therefore used for pharmacological studies of the pathway, and to discover active compounds in a G(12/13)-related disease context.

Cell-based assays in GPCR drug discovery

G protein-coupled receptors (GPCRs) transmit extracellular signals into the intracellular space, and play key roles in the physiological regulation of virtually every cell and tissue. Characteristic for the GPCR superfamily of cell surface receptors are their seven transmembrane-spanning alpha-helices, an extracellular N terminus and intracellular C-terminal tail. Besides transmission of extracellular signals, their activity is modulated by cellular signals in an auto- or transregulatory fashion. The molecular complexity of GPCRs and their regulated signaling networks triggered the interest in academic research groups to explore them further, and their drugability and role in pathophysiology triggers pharmaceutical research towards small molecular weight ligands and therapeutic antibodies. About 30% of marketed drugs target GPCRs, which underlines the importance of this target class. This review describes current and emerging cellular assays for the ligand discovery of GPCRs.